U.S. Department of Transportation Publication No. FHWA NHI-05-037 

Federal Highway Administration May 2006 

 

NHI Course No. 132040_______________________________ 

Geotechnical Aspects of Pavements 

 

Reference Manual / Participant Workbook 

Surface

Base

Subbase

Subgrade

Very Strong

Durable

Impermeable

Manufactured

Expensive

Strong

Free-Draining

Manufactured

Less Expensive

Moderate Strength

Free-Draining

Natural Material

Inexpensive

Weak

Moisture Sensitive

In Situ Soil

Surface

Base

Subbase

Subgrade

Very Strong

Durable

Impermeable

Manufactured

Expensive

Strong

Free-Draining

Manufactured

Less Expensive

Moderate Strength

Free-Draining

Natural Material

Inexpensive

Weak

Moisture Sensitive

In Situ Soil

 National Highway Institute
NOTICE 

The contents of this report reflect the views of the authors, who are responsible for 

the facts and the accuracy of the data presented herein. The contents do not 

necessarily reflect policy of the Department of Transportation. This report does not 

constitute a standard, specification, or regulation. The United States Government 

does not endorse products or manufacturers. Trade or manufacturer's names 

appear herein only because they are considered essential to the objective of this 

document. 
Technical Report Documentation Page

1. REPORT NO. 

FHWA NHI-05-037

2. GOVERNMENT 

ACCESSION NO. 

3. RECIPIENT'S CATALOG NO. 

5. REPORT DATE 

May 2006 

4. TITLE AND SUBTITLE 

Geotechnical Aspects of Pavements 

6. PERFORMING ORGANIZATION CODE 

7. AUTHOR(S): Barry R. Christopher, Ph.D., 

P.E., Charles Schwartz, Ph.D., P.E. and 

Richard Boudreau, P.E.

8. PERFORMING ORGANIZATION REPORT NO. 

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. WORK UNIT NO. 

Ryan R. Berg & Associates, Inc. 

2190 Leyland Alcove 

Woodbury, MN 55125

11. CONTRACT OR GRANT NO. 

DTFH61-02-T-63035

12. SPONSORING AGENCY NAME AND ADDRESS 13. TYPE OF REPORT & PERIOD COVERED 

National Highway Institute 

Federal Highway Administration 

U.S. Department of Transportation 

Washington, D.C.

14. SPONSORING AGENCY CODE 

15. SUPPLEMENTARY NOTES 

COTR - Larry Jones 

FHWA Technical Consultant: Sam Mansukhani, P.E. 

Technical Consultants: Ryan R. Berg, P.E. and Bruce Vandre, P.E. 

16. ABSTRACT 

This is the Reference Manual and Participants Workbook for the FHWA NHI’s Course No. 

132040 – Geotechnical Aspects of Pavements. The manual covers the latest methods and 

procedures to address the geotechnical issues in pavement design, construction and 

performance for new construction, reconstruction, and rehabilitation projects. The manual 

includes details on geotechnical exploration and characterization of in place and constructed 

subgrades as well as unbound base/subbase materials. The influence and sensitivity of 

geotechnical inputs are reviewed with respect to the requirements in past and current 

AASHTO design guidelines and the mechanistic-empirical design approach developed under 

NCHRP 1-37A, including the three levels of design input quality. Design details for drainage 

features and base/subbase material requirements are covered along with the evaluation and 

selection of appropriate remediation measures for unsuitable subgrades. Geotechnical aspects 

in relation to construction, construction specifications, monitoring, and performance 

measurements are discussed. 

17. KEY WORDS 

geotechnical, mechanistic-empirical, base, 

subgrades, subbase, construction, pavements

18. DISTRIBUTION STATEMENT 

No restrictions.

19. SECURITY CLASSIF. 

Unclassified

20. SECURITY CLASSIF. 

Unclassified

21. NO. OF PAGES 

888 

22. PRICE
FHWA NHI-05-037 

Geotechnical Aspects of Pavements i - 2 May 2006 
FHWA NHI-05-037 

Geotechnical Aspects of Pavements i - 3 May 2006 

TABLE OF CONTENTS

1.0 INTRODUCTION 

1.1 Introduction 1-1 

1.2 A Historical Perspective of Pavement Design 1-3 

1.3 The Pavement System and Typical Pavement Types 1-5 

1.3.1 Components of a Pavement System 1-5 

1.3.2 Alternate Types of Pavement 1-9 

1.3.3 Flexible Pavements 1-10 

1.3.4 Rigid Pavements 1-12 

1.3.5 Composite Pavements 1-15 

1.3.6 Unpaved Roads 1-15 

1.4 Pavement Performance with Ties to Geotechnical Issues 1-16 

1.5 Case Histories of Pavement Geotechnics 1-25 

 1.5.1 Drainage Failure 1-25 

 1.5.2 Collapsible Soils 1-26 

1.6 Concluding Remarks 1-27 

1.7 References 1-27 

2.0 STATE AGENCIES 

 2.1 Host Agency 2-1 

 2.2 Questionnaire on Geotechncial Practices in Pavement Design 2-2 

3.0 GEOTECHNICAL ISSUES IN PAVEMENT DESIGN AND PERFORMANCE 

3.1 Introduction 3-1 

3.2 Basic Concepts 3-2 

3.3 Key Geotechnical Issues 3-4 

 3.3.1 General Issues 3-4 

 New Construction vs. Rehabilitation vs. Reconstruction 3-4 

 Natural Subgrade vs. Cut vs. Fill 3-5 

 Environmental Effects 3-6 

 3.3.2 Specific Issues 3-7 

 Material Types and Properties 3-7 

 Drainage 3-9 

 Special Conditions 3-11 

 Soil Improvement 3-12 

3.4 Sensitivity of Pavement Design 3-12 

3.5 Incorporation of Geotechnical Factors in Pavement Design 3-16 

 3.5.1 Pavement Design Methodologies 3-16 

 Empirical Design 3-16 

 Mechanistic Design 3-17 

 Mechanistic-Empirical Design Approach 3-17 

 3.5.2 The AASHTO Pavement Design Guides 3-18 

 1961 Interim Guide 3-20 

 1972 Interim Guide 3-21 

 1986 Guide 3-26 

 1993 Guide 3-37 
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 1998 Guide Supplement 3-37 

 3.5.3 The NCHRP 1-37A Pavement Design Guide 3-39 

 3.5.4 Low Volume Roads 3-42 

3.6 Exercise 3-45 

3.7 References 3-45 

4.0 GEOTECHNICAL EXPLORATION AND TESTING 

 4.1 Introduction 4-1 

 4.2 Levels of Geotechnical Exploration for Different Types 

 of Pavement Projects 4-3 

 4.2.1 New Pavement Construction 4-5 

 4.2.2 Reconstruction 4-6 

 4.2.3 Rehabilitation 4-7 

 4.2.4 Subsurface Exploration Program Objectives 4-8 

 4.3 Search Available Information 4-10 

 4.4 Perform Site Reconnaissance 4-14 

 4.5 Plan and Perform the Subsurface Exploration Program 4-15 

 4.5.1 Depth of Influence 4-15 

 4.5.2 Subsurface Exploration Techniques 4-17 

 4.5.3 Remote Sensing 4-18 

 4.5.4 Geophysical Investigations 4-18 

 4.5.5 In-Situ Testing 4-33 

 4.5.6 Borings and Sampling 4-35 

 Frequency of Borings 4-43 

 Soil Sampling 4-46 

 Rock Sampling 4-47 

 Groundwater 4-48 

 Test Pits 4-51 

 Sampling for Fill/Borrow Materials 4-52 

 Standards and Guidelines 4-52 

 4.5.7 Guidelines for Idealized Subsurface Exploration Program 4-54 

 4.6 Identify Source for Other Geotechnical Components 4-57 

 4.7 Subgrade Characterization 4-57 

 4.7.1 Boring Logs 4-58 

 4.7.2 Soil Classification 4-61 

 4.7.3 Subsurface Profile 4-67 

 4.7.4 Select Samples for Laboratory Tests 4-69 

 4.8 References 4-71 

5.0 GEOTECHNICAL INPUTS FOR PAVEMENT DESIGN 

 5.1 Introduction 5-1 

 5.2 Required Geotechnical Inputs 5-3 

 5.2.1 1993 AASHTO Design Guide 5-3 

 5.2.2 NCHRP 1-37A Design Guide 5-5 

 Hierarchial Approach to Design Inputs 5-5 

 Required Geotechnical Inputs 5-8 

 5.2.3 Other Geotechnical Properties 5-12 
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Geotechnical Aspects of Pavements i - 5 May 2006 

 5.3 Physical Properties 5-12 

 5.3.1 Weight-Volume Relationships 5-12 

 5.3.2 Determination of Physical Properties 5-15 

 Volumetric Properties 5-15 

 Compaction 5-19 

 Gradation 5-24 

 Plasticity 5-28 

 5.3.3 Identification of Problem Soils 5-31 

 5.3.4 Other Aggregate Tests 5-33 

 5.4 Mechanical Properties 5-35 

 5.4.1 California Bearing Ratio (CBR) 5-37 

 5.4.2 Stabilometer (R-Value) 5-40 

 5.4.3 Elastic (Resilient) Modulus 5-42 

 Unbound Materials 5-42 

 Fractured PCC Slabs 5-55 

 Bedrock 5-56 

 5.4.4 Poisson’s Ratio 5-57 

 5.4.5 Structural Layer Coefficients 5-58 

 New Construction/Reconstruction 5-59 

 Rehabilitation 5-65 

 5.4.6 Modulus of Subgrade Reaction 5-67 

 1993 AASHTO Guide 5-69 

 NCHRP 1-37A Design Guide 5-77 

 5.4.7 Interface Friction 5-78 

 1993 AASHTO Guide 5-78 

 NCHRP 1-37A Procedure 5-79 

 5.4.8 Permanent Deformation Characteristics 5-80 

 5.4.9 Coefficient of Lateral Pressure 5-83 

 5.5 Thermo-Hydraulic Properties 5-85 

 5.5.1 1993 AASTHO Guide 5-86 

 Drainage Coefficients 5-86 

 Swelling Parameters 5-88 

 Frost Heave Parameters 5-90 

 5.5.2 NCHRP 1-37A Design Guide 5-93 

 Groundwater Depth 5-94 

 Infiltration and Drainage 5-94 

 Physical Properties 5-96 

 Soil Water Characteristic Curve 5-98 

 Hydraulic Conductivity (Permeability) 5-99 

 Thermal Conductivity 5-105 

 Heat Capacity 5-105 

5.6 Environmental/Climate Inputs 5-107 

5.6.1 1993 AASHTO Guide 5-107 

5.6.2 NCHRP 1-37A Design Guide 5-107 

 Climate Inputs 5-108 

 Groundwater Depth 5-108 

 Surface Shortwave Absorptivity 5-109 
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Geotechnical Aspects of Pavements i - 6 May 2006 

5.7 Development of Design Inputs 5-109 

5.8 Exercises 5-114 

5.8.1 1993 AASHTO Design Guide – Flexible Pavements 5-114 

5.8.2 1993 AASTHO Design Guide – Rigid Pavements 5-114 

5.9 References 5-115 

6.0 PAVEMENT STRUCTURAL DESIGN AND PERFORMANCE 

6.1 Introduction 6-1 

6.2 Baseline Designs 6-3 

 6.2.1 1993 AASHTO Design 6-4 

 Flexible Pavement 6-4 

 Rigid Pavement 6-6 

 6.2.2 NCHRP 1-37A Design 6-9 

 Flexible Pavement 6-9 

 Rigid Pavement 6-15 

 6.2.3 Summary 6-20 

6.3 Soft Subgrade 6-21 

 6.3.1 1993 AASHTO Design 6-22 

 Flexible Pavement 6-22 

 Rigid Pavement 6-22 

 6.3.2 NCHRP 1-37A Design 6-23 

 Flexible Pavement 6-23 

 Rigid Pavement 6-26 

 6.3.3 Summary 6-27 

6.4 Subgrade Stabilization 6-30 

6.5 Low Quality Base/Subbase 6-31 

 6.5.1 1993 AASHTO Design 6-31 

 Flexible Pavement 6-31 

 Rigid Pavement 6-32 

 6.5.2 NCHRP 1-37A Design 6-33 

 Flexible Pavement 6-33 

 Rigid Pavement 6-35 

 6.5.3 Summary 6-38 

6.6 Poor Drainage 6-41 

 6.6.1 1993 AASHTO Design 6-42 

 Flexible Pavement 6-42 

 Rigid Pavement 6-43 

 6.6.2 NCHRP 1-37A Design 6-44 

 Flexible Pavement 6-44 

 Rigid Pavement 6-44 

 6.6.3 Summary 6-46 

6.7 Shallow Bedrock 6-49 

 6.7.1 1993 AASHTO Design 6-49 

 Flexible Pavement 6-49 

 Rigid Pavement 6-49 

 6.7.2 NCHRP 1-37A Design 6-50 

 Flexible Pavement 6-50 
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Geotechnical Aspects of Pavements i - 7 May 2006 

 Rigid Pavement 6-51 

6.8 Concluding Comments 6-52 

6.9 Exercises 6-56 

6.10 References 6-56 

7.0 DESIGN DETAILS AND SUBGRADE CONDITIONS REQUIRING 

 SPECIAL DESIGN ATTENTION 

7.1 Introduction 7-1 

7.2 Subsurface Water and Drainage Requirements 7-1 

7.2.1 Moisture Damage Acceleration 7-2 

7.2.2 Approaches to Address Moisture in Pavements 7-7 

7.2.3 Drainage in Pavement Design 7-7 

7.2.4 Type of Subsurface Drainage 7-11 

7.2.5 Daylighted Base Sections 7-13 

7.2.6 Longitudinal Edgedrains 7-13 

7.2.7 Permeable Bases 7-17 

7.2.8 Dens-Graded Stabilized Base with Permeable Shoulders 7-18 

7.2.9 Horizontal Geocomposite Drains 7-19 

7.2.10 Separator Layers 7-19 

7.2.11 Performance of Subsurface Drainage 7-20 

7.2.12 Design of Pavement Drainage 7-21 

7.2.13 Hydraulic Requirements for the Permeable Layer(s) 7-22 

7.2.14 Edgedrain Pipe Size and Outlet Spacing Requirements 7-25 

7.2.15 Separator Layer 7-27 

7.2.16 Geotexile Separator and Filter Design 7-28 

7.3 Base Layer: Requirements, Stabilization and Reinforcement 7-31 

7.3.1 Erodibility of Bases 7-32 

7.3.2 Bound Bases 7-33 

7.3.3 Modified (or Treated) Bases 7-35 

7.3.4 Base Reinforcement 7-37 

7.4 Compaction 7-41 

7.4.1 Compaction Theory 7-41 

7.4.2 Effect on Soil Properties 7-44 

7.5 Subgrade Conditions Requiring Special Design Attention 7-46 

7.5.1 Problematic Soil Types 7-49 

7.5.2 Compressible Soils 7-52 

7.5.3 Collapsible Soils 7-53 

7.5.4 Swelling Soils 7-55 

7.5.5 Subsurface Water 7-57 

7.5.6 Frost-Susceptible Soils 7-58 

7.5.7 Summary 7-64 

7.6 Subgrade Improvement and Strengthening 7-64 

7.6.1 Objectives of Soil Stabilization 7-65 

7.6.2 Characteristics of Stabilized Soils 7-68 

7.6.3 Thick Granular Layers 7-69 

7.6.4 Geotextiles and Geogrids 7-72 

7.6.5 Admixture Stabilization 7-76 
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Geotechnical Aspects of Pavements i - 8 May 2006 

7.6.6 Soil Encapsulation 7-91 

7.6.7 Light-weight Fill 7-92 

7.6.8 Deep Foundations & Other Foundation Improvement Methods 7-95 

7.7 Recycle 7-100 

 7.7.1 Pavement Recycling 7-100 

 7.7.2 Recycled Waste 7-101 

7.8 References 7-102 

8.0 CONSTRUCTION AND DESIGN VERIFICATION FOR UNBOUND 

PAVEMENT MATRIALS 

8.1 Introduction 8-1 

8.2 Specifications 8-1 

8.3 Quality Control and Quality Assurance 8-4 

 8.3.1 QC/QA: Tradition Methods 8-5 

 8.3.2 QC/QA: Emerging Technologies 8-10 

 8.3.3 Risk Acceptance 8-16 

8.4 Construction Techniques 8-17 

 8.4.1 Drainage 8-19 

 8.4.2 Excavation 8-22 

 8.4.3 Hauling and Placement 8-22 

 8.4.4 Field Compaction 8-25 

 8.4.5 Stabilization 8-33 

 8.4.6 Base and Subbase Construction 8-35 

 8.4.7 Pavement Drainage Systems 8-36 

8.5 Performance Monitoring 8-40 

 8.5.1 Pavement Management Systems 8-40 

 8.5.2 Geophysical 8-40 

 8.5.3 Falling Weight Deflectometer 8-41 

 8.5.4 Drainage Inspection 8-42 

 8.5.5 Instrumented Geosynthetics 8-43 

8.6 Post-Construction Issues and Special Cases 8-43 

8.7 References 8-47 

APPENDICES 

A. Terminology 

B. Main Highway Project 

C. 1993 AASHTO Design Method 

D. NCHRP 1-37A Design Method 

E. Typical Key for Boring Log Preparation 

F. Determination of Admixture Content for Subgrade Stabilization 
FHWA NHI-05-037 

Geotechnical Aspects of Pavements i - 9 May 2006 

LIST OF FIGURES

1-1 Basic components of a typical pavement system 1-6 

1-2 Pavement system with representative alternative features 1-8 

1-3 Some common variations of flexible pavement sections 1-11 

1-4 Variations for rigid pavement section 1-13 

1-5 Typical composite pavement sections 1-15 

1-6 Normal rutting 1-23 

1-7 Examples of geotechnical related pavement failures 1-23 

3-1 Attenuation of load-induced stresses with depth 3-2 

3-2 Variation of material quality with depth in a pavement system with 

 ideal drainage characteristics 3-3 

3-3 Sources of moisture in pavement systems 3-11 

3-4 Impact of subgrade strength on pavement structural design 3-14 

3-5 Impact of base strength on pavement structural design 3-14 

3-6 Approximate pavement cost for varying subgrade support conditions 3-15 

3-7 Slope failure beneath road pavement 3-16 

3-8 Pavement serviceability in the AASHTO Design Guide 3-19 

3-9 Sensitivity of 1972 AASHTO flexible pavement design to 

 foundation support quality 3-25 

3-10 Sensitivity of 1972 AASHTO flexible pavement design to 

 environmental conditions 3-25 

3-11 Sensitivity of 1972 AASHTO rigid pavement design to 

 foundation stiffness 3-26 

3-12 Sensitivity of 1986 AASHTO flexible pavement design to 

 subgrade stiffness 3-32 

3-13 Sensitivity of 1986 AASHTO flexible pavement design to 

 drainage conditions 3-32 

3-14 Sensitivity of 1986 AASHTO rigid pavement design to 

 subgrade stiffness 3-34 

3-15 Sensitivity of 1986 AASHTO rigid pavement design to 

 subbase erodibility 3-34 

3-16 Sensitivity of 1986 AASHTO rigid pavement design to 

 drainage conditions 3-35 

3-17 Sensitivity of 1986 AASHTO flexible pavement design to 

 reliability level 3-36 

3-18 Sensitivity of 1986 AASHTO rigid pavement design to 

 reliability level 3-37 

3-19 Extrapolation of traffic levels in current AASHTO pavement 

 design procedures 3-41 
FHWA NHI-05-037 

Geotechnical Aspects of Pavements i - 10 May 2006 

4-1 Geotechnical exploration and testing for pavement design 4-4 

4-2 Soil survey information along the Main Highway pavement alignment 4-13 

4-3 Typical zone of influence for an asphalt pavement section 4-17 

4-4 FWD showing: a) typical equipment; b) schematic of procedure; and 

 sketch of deflection bowl for interpretation of results 4-23 

4-5 Two-dimensional cross-section resistivity profile for detection of sinkholes and 

 caves in limestone 4-26 

4-6 Representative values of resistivity for different soils 4-26 

4-7 Resistivity data showing stratigraphic changes 4-26 

4-8 Representative ground coupled GPR results showing buried utilities 

 and soil profile 4-28 

4-9 Conductivity results along a road in New Mexico 4-28 

4-10 Seismic refraction survey 4-31 

4-11 Rippability versus seismic velocity 4-31 

4-12 Common in-situ test for pavement evaluation 4-34 

4-13 CPT log in comparison to SPT data from several locations 4-41 

4-14 Excavation requirements for frost susceptible soils over undulating rock 4-49 

4-15 Qualitative relationship between relative subsurface exploration cost 

 & reliability 4-55 

4-16 Geophysical evaluation used by the Finnish National Road Administration 

 for rehabilitation and reconstruction projects showing a) results from road 

 analysis and b) moisture profile beneath the pavement 4-56 

4-17 Subsurface exploration log 4-60 

4-18 The AASHTO and the Unified Soil Classification System 4-62 

4-19 Particle size limit by different classification systems 4-63 

4-20 Subsurface profile based on boring data showing cross-sectional view 4-68 

4-21 Plan view of subsurface information 4-68 

5-1 Thermal crack prediction from NCHRP 1-37A Design Guide 

 using Level 1 material inputs 5-7 

5-2 Thermal crack prediction from NCHRP 1-37A Design Guide 

 using Level 3 material inputs 5-7 

5-3 Relationships between volume and weight/mass of bulk soil 5-13 

5-4 Typical moisture-density relationship from a standard compaction test 5-22 

5-5 Laboratory sieves for mechanical analysis of grain size distribution 5-27 

5-6 Soil hydrometer apparatus 5-27 

5-7 Representative grain size distribution for several soil types 5-28 

5-8 Variation of total soil volume and consistency with change in water 

 content for a fine-grained soil 5-29 

5-9 Liquid limit test device 5-31 

5-10 Influence of subgrade stiffness on critical pavement strains 5-36 

5-11 Influence of subgrade stiffness on critical pavement strains 5-37 

5-12 California Bearing Ratio test device 5-39 

5-13 Schematic of stabilometer test setup for measuring R-value 5-41 

5-14 Resilient modulus under cyclic load 5-47 

5-15 Definition of resilient modulus MR for cyclic triaxial loading 5-47 

5-16 Triaxial cell set-up for resilient modulus test 5-50 
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Geotechnical Aspects of Pavements i - 11 May 2006 

5-17 Correlations between subgrade resilient modulus and other soil properties 5-52 

5-18 Illustration of Poisson’s ratio 5-57 

5-19 Correlation between structural layer coefficients a2 and various 

 strength and stiffness parameters for unbound granular bases 5-61 

5-20 Correlation between structural layer coefficients a2 and various 

 strength and stiffness parameters for cement treated granular bases 5-62 

5-21 Correlation between structural layer coefficients a2 and various 

 strength and stiffness parameters for bituminous-treated granular bases 5-63 

5-22 Correlation between structural layer coefficients a2 and various 

 strength and stiffness parameters for unbound granular subbases 5-65 

5-23 Coefficients of subgrade reaction k 5-68 

5-24 Chart for estimating composite modulus of subgrade reaction, k∞, 

 Assuming a semi-infinite subgrade depth 5-71 

5-25 Chart to modify modulus of subgrade reaction to consider effects 

 of rigid foundation near surface 5-72 

5-26 Chart for estimating relative damage to rigid pavements based on slab 

 thickness and underlying support 5-73 

5-27 Correction of effective modulus of subgrade reaction for potential 

 loss of subbase support 5-74 

5-28 Effective dynamic k value determination from d0 and AREA 5-76 

5-29 Structural model for rigid pavement structural response computations 5-78 

5-30 Accumulation of permanent deformations with repeated cyclic loading 5-82 

5-31 Correlation between coefficient of lateral earth pressure and 

 overconsolidation ratio for clays of various plasticity indices 5-84 

5-32 Horizontal and vertical in-situ stresses during a load-unload-reload path 5-85 

5-33 Nomograph for estimating swell rate constant 5-89 

5-34 Chart for estimating potential vertical rise of natural soils 5-90 

5-35 Chart for estimating frost heave rate for subgrade soil 5-92 

5-36 Graph for estimating maximum serviceability loss due to frost heave 5-93 

5-37 Soil water characteristic curves 5-99 

5-38 Schematic of a constant head permeameter 5-102 

5-39 Schematic of a falling head permeameter 5-104 

5-40 Variation of pavement response variable versus distance for given project 5-112 

6-1 Predicted rutting performance for NCHRP 1-37A baseline flexible 

 pavement design 6-15 

6-2 Predicted faulting performance for NCHRP 1-37A baseline rigid 

 pavement design 6-20 

6-3 Predicted rutting performance for soft subgrade scenario 6-25 

6-4 Predicted faulting performance for soft subgrade scenario 6-26 

6-5 Summary of rigid pavement sections: soft subgrade scenario 6-28 

6-6 Example construction costs for rigid pavement sections: soft subgrade 

 scenario 6-28 

6-7 Summary of rigid pavement sections: soft subgrade scenario 6-29 

6-8 Example construction costs for rigid pavement sections: 

 soft subgrade scenario 6-29 

6-9 Effect of lime stabilization on predicted total rutting 6-31 
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Geotechnical Aspects of Pavements i - 12 May 2006 

6-10 Summary of flexible pavement sections: low quality base scenario 6-35 

6-11 Predicted faulting performance for low quality base scenario 6-36 

6-12 Summary of flexible pavement sections: low quality base scenario 6-39 

6-13 Example construction costs for flexible pavement sections: 

 low quality base scenario 6-39 

6-14 Summary of rigid pavement sections: low quality base scenario 6-40 

6-15 Example construction costs for rigid pavement sections: 

 low quality base scenario 6-41 

6-16 Predicted faulting performance for poor drainage scenario 6-45 

6-17 Summary of flexible pavement sections: poor drainage scenario 6-46 

6-18 Example construction costs for flexible pavement sections: 

 poor drainage scenario 6-47 

6-19 Summary of rigid pavement sections: poor drainage scenario 6-48 

6-20 Example construction costs for rigid pavement sections: 

 poor drainage scenario 6-48 

6-21 Summary of costs for example design scenarios: 

 flexible pavement designs 6-54 

6-22 Summary of costs for example design scenarios: 

 Rigid pavement designs 6-54 

6-23 Initial service lifes for example design scenarios: 

 flexible pavement designs 6-55 

6-24 Initial service lifes for example design scenarios: rigid pavement designs 6-55 

7-1 The influence of saturation on design life of a pavement system 7-4 

7-2 Design elements of a drainable pavement system 7-12 

7-3 Typical AC pavement with a daylighted base 7-14 

7-4 Typical AC pavement with pipe edgedrains 7-14 

7-5 Typical PCC pavement with geocomposite edgedrains 7-14 

7-6 Typical edgedrains for rehabilitation projects 7-15 

7-7 Dual pipe edgedrain systems showing alternate locations of the parallel 

 collector pipe, either adjacent to or beneath the drain line 7-18 

7-8 Recommended design of PCC pavement with a nonerodible 

 dense-graded base and permeable shoulder 7-19 

7-9 Time factor for 50 percent drainage 7-24 

7-10 Rainfall intensity in in./hr for a 2-year, 1-hour storm event 7-28 

7-11 Effect of compaction energy on compaction curves 7-42 

7-12 Laboratory compaction curves for different soil 7-43 

7-13 Effect of compacted water content on soil fabric for clays 7-44 

7-14 Strength as measured by CBR and dry density vs. water content for 

 laboratory impact compaction 7-45 

7-15 Stiffness as a function of compactive effort and water content 7-47 

7-16 Shrinkage as a function of water content and type of compaction 7-48 

7-17 Guide to collapsible sol behavior 7-54 

7-18 Estimated location of swelling soils 7-56 

7-19 Elements of frost heave 7-59 

7-20 Average rate of heave versus % fines for natural soil gradations 7-60 
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Geotechnical Aspects of Pavements i - 13 May 2006 

7-21 Thickness design curves with geosynthetics for a) single and b) 

 dual wheel loads 7-77 

7-22 Effect of lime content on engineering properties of a CH clay 7-82 

8-1 Nuclear densometer 8-7 

8-2 Process control, field density and moisture 8-8 

8-3 Lightweight deflectometer 8-15 

8-4 Underdrain installation 8-21 

8-5 Self-loading scraper 8-23 

8-6 Track-mounted excavator 8-23 

8-7 Articulated dump truck 8-24 

8-8 Bulldozer 8-25 

8-9 Motor grader 8-25 

8-10 Earth moving conveyor system 8-25 

8-11 Stress distribution under rollers over different foundations 8-28 

8-12 Smooth drum roller 8-30 

8-13 Sheepsfoot roller 8-31 

8-14 Impact roller 8-32 

8-15 Compaction efficiency 8-32 

8-16 Roadway stabilizer/mixer 8-34 

 
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Geotechnical Aspects of Pavements i - 14 May 2006 
FHWA NHI-05-037 

Geotechnical Aspects of Pavements i - 15 May 2006 

LIST OF TABLES

1-1 Geotechnical influences on major distresses in flexible pavements 1-18 

1-2 Geotechnical influences on major distresses in rigid pavements 1-18 

1-3 Geotechnical influences on major distresses in rehabilitated pavements 1-19 

3-1 Major material types in pavement systems 3-8 

3-2 Stabilization methods for pavements 3-13 

3-3 Recommended value for Regional Factor R 3-22 

3-4 Ranges of structural coefficients from agency survey 3-22 

3-5 Flexible pavement baseline conditions for 1986 AASHTO 

 sensitivity study 3-31 

3-6 Rigid pavment baseline conditions for 1986 AASHTO 

 sensitivity study 3-33 

3-7 Suggested levels of reliability for various functional classifications 3-36 

3-8 Reliability factors for use in Equation 3.10 3-45 

4-1 Sources of topographic and geological data for identifying 

 landform boundaries 4-12 

4-2 Falling weight deflectometer 4-21 

4-3 Surface resistivity 4-25 

4-4 Ground penetrating radar 4-27 

4-5 Electromagnetic conductivity 4-29 

4-6 Mechanical wave using seismic refraction 4-30 

4-7 In-situ for subsurface exploration in pavement design and construction 4-36 

4-8 Standard penetration test 4-37 

4-9 Dynamic cone penetrometer 4-39 

4-10 Cone penetrometer 4-40 

4-11 Subsurface exploration-exploratory boring methods 4-42 

4-12 Frequently-used standards for field investigations 4-53 

4-13 Classification of soils 4-63 

4-14 Summary of soil characteristics as a pavement material 4-65 

4-15 Minimum laboratory testing requirements for pavement designs 4-69 

5-1 Required geotechnical inputs for flexible pavement design 

 using the 1993 AASHTO Guide 5-4 

5-2 Required geotechnical inputs for rigid pavement design 

 using the 1993 AASHTO Guide 5-4 

5-3 Geotechnical mechanical property inputs required for the flexible 

 pavement design procedure in the NCHRP 1-37A Design Guide 5-9 

5-4 Geotechnical mechanical property inputs required for the rigid 

pavement design procedure in the NCHRP 1-37A Design Guide 5-10 

5-5 Thermo-hydraulic inputs required for the NCHRP 1-37A Design Guide 5-11 

5-6 Distress model material properties required for the NCHRP 1-37A 

 Design Guide 5-11 

5-7 Terms in weight-volume relations 5-13 
FHWA NHI-05-037 

Geotechnical Aspects of Pavements i - 16 May 2006 

5-8 Unit weight-volume relationships 5-14 

5-9 Typical porosity, void ratio, and unit weight values for soils in 

 their natural state 5-14 

5-10 Specific gravity of soil and aggregate solids 5-16 

5-11 Moisture content 5-17 

5-12 Unit weight 5-18 

5-13 Compaction characteristics 5-21 

5-14 Consistency of granular soils at various relative densities 5-22 

5-15 Principal differences between standard and modified Proctor 

 compaction tests 5-22 

5-16 Recommended minimum requirements for compaction of 

 Embankment and subgrades 5-23 

5-17 Typical compacted densities and optimum moisture content for 

 USCS soil types 5-23 

5-18 Typical compacted densities and optimum moisture contents for 

 AASHTO soil types 5-24 

5-19 Grain size distribution of coarse particles 5-25 

5-20 Grain size distribution of fine particles 5-26 

5-21 Plasticity of fine-grained soils 5-30 

5-22 Characteristics of soils with different plasticity indices 5-31 

5-23 Swell potential of clays 5-32 

5-24 Estimation of swell potential 5-33 

5-25 Collapse potential of soils 5-33 

5-26 Other tests for aggregate quality and durability 5-34 

5-27 California Bearing Ratio 5-38 

5-28 Typical CBR values 5-39 

5-29 Stabilometer or R-Value 5-41 

5-30 Typical ranges for k1 and k2 coefficient in Equation 5.9 5-46 

5-31 Resilient modulus 5-48 

5-32 Average backcalculated to laboratory determined elastic modulus ratios 5-51 

5-33 Hierarchical input levels for unbound material stiffness in the 

 NCHRP 1-37A Design Guide 5-51 

5-34 Correlations between resilient modulus and various material strength 

 and index properties 5-53 

5-35 Default MR values for unbound granular and subgrade materials 

 at unsoaked optimum moisture content and density conditions 5-54 

5-36 Recommended fractured slab design modulus values for 

 Level 1 characterization 5-55 

5-37 Recommended fractured slab design modulus values for 

 Level 3 characterization 5-56 

5-38 Typical Poisson’s ratio values for geomaterials in pavements 5-58 

5-39 Typical values for k1 and k2 for use in Equation 5.17 for unbound 

 base and subbase materials 5-60 

5-40 Granular base resilient modulus ESB vlues from AASHO Road Test 5-60 

5-41 Suggested bulk stress θ values for use in design of granular base layers 5-60 

5-42 Granular subbase resilient modulus ESB values from AASHO Road Test 5-64 

5-43 Suggested bulk stress θ values for use in design of granular subbase layers 5-64 
FHWA NHI-05-037 

Geotechnical Aspects of Pavements i - 17 May 2006 

5-44 Suggested layer coefficients for existing flexible pavement layer materials 5-66 

5-45 Typical ranges of loss support, LS factors for various types of materials 5-75 

5-46 Suggested range for modulus of subgrade reaction for design 5-75 

5-47 Recommended friction factor values 5-79 

5-48 Typical values of base/slab friction coefficient recommended 

 for CRCP design in the NCHRP 1-37A procedure 5-80 

5-49 Recommended mi values for modifying structural layer coefficients 

 of untreated base and subbase materials in flexible pavements 5-87 

5-50 Recommended values of drainage coefficient Cd values for 

 rigid pavement design 5-87 

5-51 Infiltration categories in the NCHRP 1-37A Design Guide 5-95 

5-52 Physical properties for unbound materials required for EICM calculations 5-97 

5-53 Options for estimating the SWCC parameters 5-100 

5-54 Options for determining the saturated hydraulic conductivity for 

 unbound materials 5-102 

5-55 Saturated hydraulic conductivity 5-103 

5-56 Typical values of saturated hydraulic conductivity for soils 5-104 

5-57 Typical values of saturated hydraulic conductivity for highway materials 5-105 

5-58 Options for determining the dry thermal conductivity and heat capacity 

 for unbound materials 5-106 

5-59 Typical values for thermal conductivity and heat capacity of 

 unbound materials 5-107 

5-60 Summary of typical pavement parameter variability 5-113 

5-61 Resilient modulus versus CBR testing for fine grained subgrade soil 5-114 

6-1 Typical in-place unit material costs for use in example design problems 6-4 

6-2 Input parameters for 1993 AASHTO flexible pavement baseline design 6-6 

6-3 Input parameters for 1993 AASHTO rigid pavement baseline design 6-8 

6-4 Input parameters for NCHRP 1-37A flexible pavement baseline design 6-10 

6-5 AADTT distribution by truck class 6-13 

6-6 Hourly truck traffic distribution 6-13 

6-7 Truck axle load distributions: Percentage of axle loads by truck class 6-14 

6-8 Truck axle distribution 6-15 

6-9 Input parameters for NCHRP 1-37A rigid pavement baseline design 6-17 

6-10 Wheelbase spacing distribution 6-20 

6-11 Modified input parameters for NCHRP 1-37A flexible pavement design: 

 soft subgrade scenario 6-24 

6-12 Trial cross sections for NCHRP 1-37A flexible pavement design: soft 

 subgrade scenario 6-25 

6-13 Trial cross sections for NCHRP 1-37A rigid pavement design: soft 

 subgrade scenario 6-27 

6-14 Modified input parameters for NCHRP 1-37A flexible pavement design: 

 low quality base scenario 6-34 

6-15 Trial cross sections for NCHRP 1-37A flexible pavement design: low 

 quality base scenario 6-35 

6-16 NCHRP 1-37A recommendations for assessing erosion potential of 

 base material 6-37 
FHWA NHI-05-037 

Geotechnical Aspects of Pavements i - 18 May 2006 

6-17 Trial cross sections for NCHRP 1-37A rigid pavement design: low 

 quality base scenario 6-37 

6-18 Trial cross sections for NCHRP 1-37A rigid pavement design: poor 

 drainage scenario 6-45 

6-19 Design pavement sections for 1993 AASHTO rigid pavement design: 

 shallow bedrock effects 6-49 

6-20 Influence of bedrock depth on predicted total rutting: NCHRP 1-37A 

 design method 6-50 

6-21 Trial cross sections for NCHRP 1-37A flexible pavement design: 

 baseline conditions with shallow bedrock at 3 ft depth 6-50 

6-22 Influence of bedrock depth on predicted joint faulting: NCHRP 1-37A 

 design methodology 6-51 

6-23 Trial cross sections for NCHRP 1-37A rigid pavement design: baseline 

 conditions with shallow bedrock at 3 ft depth 6-51 

7-1 Moisture-related distresses in flexible (AC) pavements 7-5 

7-2 Moisture-related distresses in rigid (PCC) pavements 7-6 

7-3 Assessment of need for subsurface drainage in new or reconstructed 

 pavements 7-8 

7-4 AASHTO definitions for pavement drainage recommended for use 

 in both flexible and rigid pavement design 7-9 

7-5 Geotextile survivability requirements 7-30 

7-6 Level 1 recommendation for assessing erosion potential of base material 7-33 

7-7 Design Level 2 recommendations for assessing erosion potential 

 of base material 7-34 

7-8 Design Level 3 recommendations for assessing erosion potential 

 of base material description only 7-35 

7-9 Recommended asphalt stabilizer properties for asphalt-treated 

 permeable base/subbase materials 7-36 

7-10 Recommended Portland cement stabilizer properties for cement 

 treated permeable base/subbase materials 7-37 

7-11 Qaulitative review of reinforcement application potential for 

 paved permanent roads 7-38 

7-12 Frost susceptibility classification of soils 7-60 

7-13 Stabilization methods for pavements 7-66 

7-14 Transportation uses of geosynthetic materials 7-73 

7-15 Appropriate subgrade conditions for stabilization using geosynthetics 7-74 

7-16 Guide for selection of admixture stabilization method(s) 7-78 

7-17 Swell potential of soils 7-79 

7-18 Examples of the effects of lime stabilization on various soils 7-81 

7-19 Recommended gradations for bituminous stabilized subgrade materials 7-88 

7-20 Recommended gradations for bituminous-stabilized base and 

 subbase materials 7-89 

7-21 Densities and approximate costs for various lightweight fill materials 7-94 

7-22 Ground improvement categories, functions, methods and applications 7-98 

7-23 Comparative costs of ground improvement methods 7-99 
FHWA NHI-05-037 

Geotechnical Aspects of Pavements i - 19 May 2006 

8-1 Typical material properties measured for construction 8-4 

8-2 Field monitoring checklist 8-6 

8-3 Summary of design soil properties for example problem (pre-construction) 8-11 

8-4 Recommended field compaction equipment for different soils 8-27 

8-5 Geotechnical related post-construction problems in flexible pavements 8-45 

8-6 Geotechnical related post-construction problems in rigid pavements 8-46 
FHWA NHI-05-037 

Geotechnical Aspects of Pavements i - 20 May 2006 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 1 May 2006 

CHAPTER 1.0 INTRODUCTION 

1.1 INTRODUCTION 

This is the reference manual for FHWA NHI’s Course #132040 – Geotechnical Aspects of 

Pavements. Many groups within an agency are involved with different aspects of definition, 

design use and construction verification of pavement geomaterials. These groups include 

• pavement design engineers, 

• geotechnical engineers, 

• specification writers, and 

• construction engineers. 

The three-day training course was developed as a format for these various personnel to meet 

and together develop a better understanding of the geotechnical aspects of pavements. The 

overall goal is for this group of personnel to work together to enhance current procedures to 

build and maintain more cost-efficient pavement structures. The geotechnical aspects are 

particularly important today, as longer pavement performance (analysis) periods are being 

used in design. The maintenance and rehabilitation activities used in the pavement 

management process to achieve the design performance period require a competent support 

from the underlying geomaterials. 

Thus, this manual has been prepared to assist pavement design engineers, geotechnical 

engineers, specification writers, and construction engineers in understanding the geotechnical 

aspects of pavement design, and as a common tool for future reference. This manual covers 

the latest methods and procedures to address the geotechnical issues in pavement design, 

construction, and performance, including 

• a review of the geotechnical parameters of interest in pavement design, construction, 

and performance of different types of pavements. 

• the influence of climate, moisture, and drainage on pavement performance. 

• the impact of unsuitable subgrades on pavement performance. 

• the determination of the geotechnical inputs needed for the design, construction, and 

performance of pavements. 

• evaluation and selection of appropriate remediation measures for unsuitable pavement 

subgrades. 

• the geotechnical aspects of pavement construction specifications and inspection 

requirements. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 2 May 2006 

• a review of typical subgrade problems during construction and the recommended 

solutions. 

This manual covers the latest methods and procedures for 

• new construction, 

• reconstruction, and 

• rehabilitation projects (e.g., widenings, overlays, and treatments). 

The manual covers designing and constructing pavement subgrades and unbound materials 

for paved and unpaved roads with emphasis on 

• the current AASHTO, 1993 design guidelines, and 

• the mechanistic-empirical design approach, including the three levels of design inputs 

being developed under the NCHRP 1-37A. 

Previous AASHTO design methods are also reviewed in relation to the sensitivity of 

geotechnical inputs. The design details section also provides a review of the overall 

geotechnical and drainage aspects of bases, subbases, and subgrades. 

The manual is divided into modules, with each chapter representing a module for specific 

geotechnical aspects of pavement system design, including 

• introduction to geotechnical aspects of pavements, 

• basic concepts and special conditions, 

• subsurface exploration, 

• determination of key geotechnical inputs, 

• actual design (and sensitivity of design/performance to geotechnical inputs), 

• design details and special problems, and 

• construction and QC/QA issues. 

Each module contains a short background section, which provides an overview of the 

specific design or construction element. Following the background section, the design 

element and, subsequently, the construction process, is presented. Examples used to 

demonstrate the method and case histories of both successes and failures are presented to 

support the use of the design and construction concepts. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 3 May 2006 

1.2 A HISTORICAL PERSPECTIVE OF PAVEMENT DESIGN 

Pavements with asphalt or concrete surface layers have been used in the United States since 

the late 1880s. The historical development of asphalt and concrete pavement design is 

discussed in more detail in Chapter 3. Although pavement materials and construction 

methods have advanced significantly over the past century, until the last decade, pavement 

design has been largely empirical, based on regional experience. Even the empirically based 

designs of the 1980s and 1990s, as expressed in the AASHTO 1986, 1993, and 1998, 

guidelines have been, for most cases, modified by state agencies, based on regional 

experience. For example, several agencies still use their own modifications of the 1972 

AASHTO design guidelines. Currently, approximately one-half of the state agencies are 

using the 1993 guide, albeit usually with some modification. This close reliance on empirical 

evidence makes it difficult to adopt new design concepts. Empirical designs are significantly 

challenged by constantly changing design considerations (e.g., traffic loads and number of 

applications, types of pavements, road base aggregate supply, etc.). An additional change is 

the type of pavement construction, which has shifted over the past several decades from new 

construction to rehabilitation. Recycled materials now often replace new construction 

materials. During the past ten years, a major thrust has been to develop a more scientific 

explanation of the interaction between the pavement structure, the materials, the environment 

and the wheel loading. The need for a more sophisticated design method becomes even more 

apparent when considering the number of variables, with more than twenty just for the 

geotechnical features (e.g., unit weight, moisture content, gradation, strength, stiffness, and 

hydraulic conductivity, as described in Chapter 6) that influence the design in a modern 

pavement system. 

Fortunately, the tools available for design have also significantly advanced over the past 

several decades. Specifically, computerized numerical modeling techniques (i.e., mechanistic 

models) are now available that can accommodate the analysis of these complex interaction 

issues and, at the same time, allow the models to be modified based on empirical evidence. 

The development of mechanistic-empirical models is described in Chapter 3 and their use in 

design of unbound pavement materials is detailed in Chapter 6. The new national Pavement 

Design Guide development under NCHRP Project 1-37A (NCHRP 1-37A Design Guide) 

provides the basis for the information in these chapters. Several agencies have already 

adopted mechanistic-empirical analysis, at least as a secondary method for flexible pavement 

design (Newcomb and Birgisson, 1999). The newer, more sophisticated design models for 

flexible and rigid pavements rely heavily on accurate characterization of the pavement 

materials and supporting conditions for design input. As a result, there is a greater reliance on 

geotechnical inputs in the design models. Geotechnical exploration and testing programs are 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 4 May 2006 

essential components in the reliability of pavement design and have also advanced 

significantly in the past several decades. 

Better methods for subsurface exploration and evaluation have been developed over time. 

Standard penetration tests (SPT), where a specified weight is dropped from a specific height 

on a thick-walled tube sampler to obtain an index strength value and disturbed sample of the 

subgrade, was developed in the 1920s. A typical practice is to locate the sampling intervals at 

a standard spacing along the roadway alignment. However, subgrade conditions can vary 

considerably both longitudinally and transversely to the alignment. This approach evaluates 

and samples less than a billionth of the soil along the roadway alignment, often missing 

critical subsurface features and/or variations. In addition, the SPT value itself has a 

coefficient of variation of up to 100% (Orchant et al., 1988). Based on these considerations, 

one must question the use of this approach as the sole method for subsurface evaluation. As 

discussed in Chapter 4, geophysical methods (e.g., ground penetrating radar (GPR) and 

falling weight deflectometer (FWD)) and rapid in-situ testing (e.g., cone penetration test - 

CPT) now allow for economical spatial characterization of subsurface conditions such that 

soil borings for sampling can be optimally located. The use of FWD to directly evaluate the 

dynamic response of existing pavement materials and support conditions for reconstruction 

and/or rehabilitation – now a standard of practice by a number of agencies – is also reviewed. 

Empirical design methods of the past often relied on index tests such as CBR or R-value for 

characterizing the supporting aggregate and subgrade materials. Just as the design methods 

were modified for local conditions, agencies have modified the test methods to the extent that 

there are currently over ten index methods used across the United States to characterize these 

materials. Tests include the IBR (Illinois), the LBR (Florida), the Washington R-value, the 

California R-value, the Minnesota R-value, and the Texas triaxial, to name a few. Rough 

correlations between many of these methods are reviewed in Chapter 5. Considering that 

both the design and the input values may rely upon local knowledge, it is not surprising that 

comparison of test sections constructed by different agencies is often difficult. A method that 

allows for direct modeling of the dynamic response of subsurface soils and base course 

aggregate materials is the resilient modulus test. Advancements in the resilient modulus 

equipment and test procedures are reviewed in Chapter 5. 

Throughout the history of pavement design, special problems have been encountered in 

relation to pavement support. These include expansive soils, frost susceptible materials, 

caliche, karst topography, pumping soils and highly fluctuating groundwater conditions. The 

solution to these problems is often to remove and replace these materials, often at great 

expense to the project. Today there are a number of alternate techniques available to resolve 

these issues, as discussed in Chapter 7. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 5 May 2006 

Finally, verification of construction over the past century has used an array of methods for 

spot-check detection techniques, including evaluation of density using sand cone, balloon 

and, more recently, nuclear gauge techniques and/or load tests using plate load, field CBR, 

and drop cone methods. As with many of the older laboratory testing techniques, these field 

quality control (QC) methods are usually index tests and do not directly measure the dynamic 

response of the inplace materials. Many of these methods are reviewed in Chapter 8, along 

with some newer rapid assessment techniques (e.g., the Geogage) that provide a direct 

assessment of the dynamic support conditions. While most of the index methods, as well as 

the dynamic response methods, usually produce reliable results, the small sample area 

evaluated does not allow an assessment of the overall project uniformity. Also, construction 

methods have developed to the extent that it is often impossible to keep up with the 

placement of materials. One of the oldest methods that does provide good area coverage and 

which is still widely used in current practice is proof rolling. However, this method is often 

subjective. Improvements in evaluation of proof rolling through the use of modern survey 

techniques now allow for a more quantitative, less subjective evaluation of the subgrade 

conditions, and are discussed in Chapter 8. The other disadvantage of all these methods is 

that the work is checked after the fact, often requiring rework and slowing the project. A 

new, more rapid technique that allows for real time evaluation of each pavement layer as it is 

constructed is now available, and is reviewed in Chapter 8. These “intelligent compaction” 

methods can also be directly tied into pavement guarantee and warranty programs, also 

discussed in Chapter 8. 

1.3 THE PAVEMENT SYSTEM AND TYPICAL PAVEMENT TYPES 

The purpose of the pavement system is to provide a smooth surface over which vehicles may 

safely pass under all climatic conditions for the specific performance period of the pavement. 

In order to perform this function, a variety of pavement systems have been developed, the 

components of which are basically the same. 

1.3.1 Components of a Pavement System 

The pavement structure is a combination of subbase, base course, and surface course placed 

on a subgrade to support the traffic load and distribute it to the roadbed. Figure 1-1 presents 

a cross section of a basic modern pavement system, showing the primary components. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 6 May 2006 

Figure 1-1. Basic components of a typical pavement system. 

The subgrade is the top surface of a roadbed upon which the pavement structure and 

shoulders are constructed. The purpose of the subgrade is to provide a platform for 

construction of the pavement and to support the pavement without undue deflection that 

would impact the pavement’s performance. For pavements constructed on-grade or in cuts, 

the subgrade is the natural in-situ soil at the site. The upper layer of this natural soil may be 

compacted or stabilized to increase its strength, stiffness, and/or stability. 

For pavements constructed on embankment fills, the subgrade is a compacted borrow 

material. Other geotechnical aspects of the subgrade of interest in pavement design include 

the depth to rock and the depth to the groundwater table, especially if either of these is close 

to the surface. The actual thickness of the subgrade is somewhat nebulous, and the depth of 

consideration will depend on the design method, as discussed in Chapter 3. 

The subbase is a layer or layers of specified or selected materials of designed thickness 

placed on a subgrade to support a base course. The subbase layer is usually of somewhat 

lower quality than the base layer. In some cases, the subbase may be treated with Portland 

cement, asphalt, lime, flyash, or combinations of these admixtures to increase its strength and 

stiffness. A subbase layer is not always included, especially with rigid pavements. A subbase 

layer is typically included when the subgrade soils are of very poor quality and/or suitable 

material for the base layer is not available locally, and is, therefore, expensive. Inclusion of a 

subbase layer is primarily an economic issue, and alternative pavement sections with and 

without a subbase layer should be evaluated during the design process. 

Subbase

Compacted/Natural Subgrade

Surface Course (AC, PCC)

Base Course

Embankment/Natural Soil
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 7 May 2006 

In addition to contributing to the structural capacity of flexible pavement systems, subbase 

layers have additional secondary functions: 

• Preventing the intrusion of fine-grained subgrade soils into the base layer. Gradation 

characteristics of the subbase relative to those of the subgrade and base materials are 

critical here. 

• Minimizing the damaging effects of frost action. A subbase layer provides insulation 

to frost-susceptible subgrades and, in some instances, can be used to increase the 

height of the pavement surface above the groundwater table. 

• Providing drainage for free water that may enter the pavement system. The subbase 

material must be free draining for this application, and suitable features must be 

included in the pavement design for collecting and removing any accumulated water 

from the subbase. 

• Providing a working platform for construction operations in cases where the subgrade 

soil is very weak and cannot provide the necessary support. 

The base is a layer or layers of specified or select material of designed thickness placed on a 

subbase or subgrade (if a subbase is not used) to provide a uniform and stable support for 

binder and surface courses. The base layer typically provides a significant portion of the 

structural capacity in a flexible pavement system and improves the foundation stiffness for 

rigid pavements, as defined later in this section. The base layer also serves the same 

secondary functions as the subbase layer, including a gradation requirement that prevents 

subgrade migration into the base layer in the absence of a subbase layer. It usually consists of 

high quality aggregates, such as crushed stone, crushed slag, gravel and sand, or 

combinations of these materials. The specifications for base materials are usually more 

stringent than those for the lower-quality subbase materials. 

High quality aggregates are typically compacted unbound – i.e., without any stabilizing 

treatments – to form the base layer. Materials unsuitable for unbound base courses can 

provide satisfactory performance when treated with stabilizing admixtures, such as Portland 

cement, asphalt, lime, flyash, or a combination of these treatments, to increase their strength 

and stiffness. These stabilizing admixtures are particularly attractive when suitable untreated 

materials are in short supply local to the project site. Base layer stabilization may also reduce 

the total thickness of the pavement structure, resulting in a more economical overall design. 

Finally, the surface course is one or more layers of a pavement structure designed to 

accommodate the traffic load, the top layer of which resists skidding, traffic abrasion, and the 

disintegrating effects of climate. The surface layer may consist of asphalt (also called 

bituminous) concrete, resulting in “flexible” pavement, or Portland cement concrete (PCC), 

resulting in “rigid” pavement. The top layer of flexible pavements is sometimes called the 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 8 May 2006 

"wearing" course. The surface course is usually constructed on top of a base layer of 

unbound coarse aggregate, but often is placed directly on the prepared subgrade for lowvolume roads. In addition to providing a significant fraction of the overall structural capacity 

of the pavement, the surface layer must minimize the infiltration of surface water, provide a 

smooth, uniform, and skid-resistant riding surface, and offer durability against traffic 

abrasion and the climate. 

Figure 1-2 expands the basic components, showing other important features (e.g., drainage 

systems) that are often included in a pavement design. The permeable base drainage layer in 

Figure 1-2 is provided to remove infiltrated water quickly from the pavement structure. 

Suitable features, including edgedrains and drain outlets, must be included in the pavement 

design for collecting and removing any accumulated water from the drainage layer. In order 

to function properly, the layer below the drainage layer must be constructed to grades 

necessary to promote positive subsurface drainage (i.e., no undulations and reasonable crown 

or cross slope). Filter materials (e.g., geotextiles) may also be required to prevent clogging of 

the drainage layer and collector system. Pavement drainage is discussed in more detail in 

Chapters 3, 6, and 7. 

The geotechnical components of a pavement system as covered in this manual include 

surfacing aggregate, unbound granular base, unbound granular subbase, the subgrade or 

roadbed (either mechanically or chemically stabilized, or both), aggregate and geosynthetics 

used in drainage systems, graded granular aggregate and geosynthetic used as separation and 

filtration layers, and the roadway embankment foundation. These and other terms related to 

the components of the pavement system are defined in Appendix A. 

Figure 1-2. Pavement system with representative alternative features. 

Edgedrain Outlet 

Surface Course 

Permeable base 

Subbase layer 

Shoulder
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 9 May 2006 

1.3.2 Alternate Types of Pavement 

The most common way of categorizing pavements is by structural type: rigid, flexible, 

composite and unpaved. 

• Rigid pavements in simplest terms are those with a surface course of Portland cement 

concrete (PCC). The Portland cement concrete slabs constitute the dominant loadcarrying component in a rigid pavement system. 

• Flexible pavements, in contrast, have an asphaltic surface layer, with no underlying 

Portland cement slabs. The asphaltic surface layer may consist of high quality, hot 

mix asphalt concrete, or it may be some type of lower strength and stiffness asphaltic 

surface treatment. In either case, flexible pavements rely heavily on the strength and 

stiffness of the underlying unbound layers to supplement the load carrying capacity of 

the asphaltic surface layer. 

• Composite pavements combine elements of both flexible and rigid pavement systems, 

usually consisting of an asphaltic concrete surface placed over PCC or bound base. 

• Unpaved roads or naturally surfaced roads simply are not paved, relying on granular 

layers and the subgrade to carry the load. Seal coats are sometimes applied to 

improve their resistance to environmental factors. 

Pavements can also be categorized based on type of construction: 

• New construction: The design and construction of a pavement on a previously 

unpaved alignment. All pavements start as new construction. 

• Rehabilitation: The restoration or addition of structural capacity to a pavement. 

Overlays (either asphalt or Portland cement concrete), crack and seat and full or 

partial depth reclamation are examples of rehabilitation construction. 

• Reconstruction: The complete removal of an existing pavement and construction of a 

new pavement on the same alignment. Except for the demolition of the existing 

pavement (usually done in stages, i.e., one lane at a time) and traffic control during 

construction, reconstruction is very similar to new construction in terms of design. 

Categorization of pavements by structural type is generally the more useful approach for the 

overall pavement design, as well as performance monitoring and management of the 

pavement structure. The material types and structural behavior of flexible versus rigid 

pavements are sufficiently different to require fundamentally different design approaches. 

Unpaved roads also provide a unique set of challenges and correspondingly unique design 

requirements. Key features of flexible, rigid, composite pavement systems, and unpaved 

roads are described in the following subsections. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 10 May 2006 

1.3.3 Flexible Pavements (Adapted from AASHTO 1993) 

As was described in Figure 1-1, flexible pavements in general consist of an asphalt-bound 

surface course or layer on top of unbound base and subbase granular layers over the subgrade 

soil. In some cases, the subbase and/or base layers may be absent (e.g., full-depth asphalt 

pavements), while in others the base and/or subbase layers may be stabilized using 

cementitious or bituminous admixtures. Drainage layers may also be provided to remove 

water quickly from the pavement structure. Some common variations of flexible pavement 

systems are shown in Figure 1-3. Full depth asphalt pavements (Figure 1-3 upper right 

corner) are used primarily for flexible pavements subjected to very heavy traffic loadings. 

Hot mix asphalt concrete produced by an asphalt plant is the most common surface layer 

material for flexible pavements, especially for moderately to heavily trafficked highways. 

Dense-graded (i.e., well-graded with a low void ratio) aggregates with a maximum aggregate 

size of about 25 mm (1 in.) are most commonly used in hot mix asphalt concrete, but a wide 

variety of other types of gradations (e.g., gap-graded) have also been used successfully for 

specialized conditions. The Superpave procedure has become the standard for asphalt 

mixture design, although county and local government agencies may still use the older 

Marshall and Hveem mix design procedures (Asphalt Institute MS-2, 1984). 

The asphalt surface layer in a flexible pavement may be divided into sub-layers. Typical sublayers, proceeding from the top downward, are as follows: 

• Seal coat: A thin asphaltic surface treatment used to increase (or restore) the water 

and skid resistance of the road surface. Seal coats may be covered with aggregate 

when used to increase skid resistance. 

• Surface course (also called the wearing course): The topmost sublayer (in the 

absence of a seal coat) of the pavement. This is typically constructed of dense graded 

asphalt concrete. The primary design objectives for the surface course are 

waterproofing, skid resistance, rutting resistance, and smoothness. 

• Binder course (also called the asphalt base course): The hot mix asphalt layer 

immediately below the surface course. The base course generally has a coarser 

aggregate gradation and often a lower asphalt content than the surface course. A 

binder course may be used as part of a thick asphalt layer either for economy (the 

lower quality asphalt concrete in the binder course has a lower material cost than the 

higher-asphalt content concrete in the surface course) or if the overall thickness of the 

surface layer is too great to be paved in one lift. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 11 May 2006 

Conventional Flexible Deep Strength Full Depth

Asphalt Concrete

Cement Treated Base

Unbound Sub-Base

Compacted Subgrade

Natural Subgrade

Asphalt Concrete

Asphalt Treated Base

Unbound Sub-Base

Compacted Subgrade

Natural Subgrade

Asphalt Concrete

Unbound Base

Asphalt Treated or

Cement Treated Layer

Compacted Subgrade

Natural Subgrade

Asphalt Concrete

Unbound Base

Unbound Sub-Base

Compacted Subgrade

Natural Subgrade

Asphalt Concrete

Compacted Subgrade

Natural Subgrade

Unbound Base

Asphalt Surface

Compacted Subgrade

Natural Subgrade

Asphalt Binder

Asphalt Base

Semi-Rigid with ATB Semi-Rigid with CTB Inverted Section

Figure 1-3. Some common variations of flexible pavement sections (NCHRP 1-37A, 2002). 

Additionally, thin liquid bituminous coatings may be used in the pavement, as follows: 

• Tack coat: Applied on top of stabilized base layers and between lifts in thick asphalt 

concrete surface layers to promote bonding of the layers. 

• Prime coat: Applied on the untreated aggregate base layer to minimize flow of 

asphalt cement from the asphalt concrete to the aggregate base and to promote a good 

interface bond. Prime coats are often used to stabilize the surface of the base to 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 12 May 2006 

support the paving construction activities above. Cutback asphalt (asphalt cement 

blended with a petroleum solvent) is typically used because of its greater depth 

penetration. 

Proper compaction of the asphalt concrete during construction is critical for satisfactory 

pavement performance. Improper compaction can lead to excessive rutting (permanent 

deformations) in the asphalt concrete layer due to densification under traffic; cracking or 

raveling of the asphalt concrete due to embrittlement of the bituminous binder from exposure 

to air and water; and failure of the underlying unbound layers due to excessive infiltration of 

surface water. Typical construction specifications require field compaction levels of 92% or 

more of the theoretical maximum density for the mixture. Layers of unbound material below 

the asphalt concrete layers must be constructed properly in order to achieve the overall 

objectives of pavement performance. 

1.3.4 Rigid Pavements 

Rigid pavements in general consist of Portland cement pavement slabs constructed on a 

granular base layer over the subgrade soil. The base layer serves to increase the effective 

stiffness of the slab foundation. The base layer also provides the additional functions listed in 

Section 1.3.1, plus the base must also prevent pumping of fine-grained soils at joints, cracks, 

and edges of the slab. Gradation characteristics of the base and/or subbase are critical here. 

The base may also be stabilized with asphalt or cement to improve its ability to perform this 

function. A subbase layer is occasionally included between the base layer and the subgrade. 

The effective foundation stiffness will be a weighted average of the subbase and subgrade 

stiffnesses. For high quality coarse subgrades (e.g., stiffness equal to that of the base) or low 

traffic volumes (less than 1 million 80-kN (18-kip) ESALs), the base and subbase layer may 

be omitted. 

Because of the low stresses induced by traffic and environmental effects (e.g., thermal 

expansion and contraction) relative to the tensile strength of Portland cement concrete, most 

rigid pavement slabs are unreinforced or only slightly reinforced. When used, the function of 

reinforcement is to eliminate or lengthen spacing of joints, which fault and infiltrate water. 

Reinforcement in the concrete does not influence subgrade support requirements. The 

subbase layer may be omitted if there is low truck traffic volume or good subgrade 

conditions. For high traffic volumes and/or poor subgrade conditions, the subbase may be 

stabilized using cementitious or bituminous admixtures. Drainage layers can and should be 

included to remove water quickly from the pavement structure, similar to flexible pavements. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 13 May 2006 

A geotextile layer may be used to control migration of fines into the open graded base layer. 

Some common variations of rigid pavement systems are shown in Figure 1-4. 

Rigid pavement systems are customarily divided into four major categories: 

• Jointed Plain Concrete Pavements (JPCP). These unreinforced slabs require a 

moderately close spacing of longitudinal and transverse joints to maintain thermal 

stresses within acceptable limits. Longitudinal joint spacing typically conforms to the 

lane width (around 3.7 m (12 ft)), and transverse joint spacing typically ranges 

between 4.5 – 9 m (15 – 30 ft). Aggregate interlock, often supplemented by steel 

dowels or other load transfer devices, provides load transfer across the joints. 

• Jointed Reinforced Concrete Pavements (JRCP). The light wire mesh or rebar 

reinforcement in these slabs is not designed to increase the load capacity of the 

pavement, but rather to resist cracking under thermal stresses and, thereby, permit 

longer spacings between the transverse joints between slabs. Transverse spacing 

typically ranges between 9 – 30 m (30 - 100 ft) in JRCP pavements. Dowel bars or 

other similar devices are required to ensure adequate load transfer across the joints. 

Figure 1-4. Variations for rigid pavement section (NCHRP 1-37A, 2002). 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 14 May 2006 

• Continuously Reinforced Concrete Pavements (CRCP). Transverse joints are not 

required in CRCP pavements. Instead, the pavement is designed so that transverse 

thermal cracks develop at very close spacings, with typical spacings on the order of a 

meter (a few feet). The continuous conventional reinforcement bars are designed to 

hold these transverse thermal cracks tightly together and to supplement the aggregate 

interlock at the cracks to provide excellent load transfer across the cracks. In addition 

to the benefit of no transverse joints, CRCP pavement designs are typically 25 – 50 

mm (1 – 2 in.) thinner than conventional JPCP or JRCP systems. 

• Prestressed Concrete Pavements (PCP). PCP designs are similar to CRCP, except 

that the longitudinal reinforcement now consists of continuous steel strands that are 

prestressed prior to placing the concrete (or post-tensioned after the concrete has 

hardened). The initial tensile stress in the reinforcement counteracts the load- and 

thermal-induced tensile stresses in the concrete and, therefore, permits thinner slabs. 

Prestressed concrete pavements are more commonly used for airfield pavements than 

for highway pavements because of the greater benefit from the reinforcement in the 

comparatively thicker airfield slabs. Precast, prestressed concrete sections are also 

being used for pavement rehabilitation. 

As suggested above, the basic components in rigid pavement slabs are Portland cement 

concrete, reinforcing steel, joint load transfer devices, and joint sealing materials. The 

AASHTO Guide Specifications for Highway Construction and the Standard Specifications 

for Transportation Materials provide guidance on mix design and material specifications for 

rigid highway pavements. These specifications can be modified based on local conditions 

and experience. Pavement concrete tends to have a very low slump, particularly for use in 

slip-formed paving. Air-entrainment is used to provide resistance to deterioration from 

freezing and thawing and to improve the workability of the concrete mix. Joint sealing 

materials must be sufficiently pliable to seal the transverse and longitudinal joints in JPCP 

and JRCP pavements against water intrusion under conditions of thermal expansion and 

contraction of the slabs. Commonly used joint sealing materials include liquid sealants 

(asphalt, rubber, elastomeric compounds, and polymers), preformed elastomeric seals, and 

cork expansion filler. 

Load transfer devices in JPCP and JRCP pavements are designed to spread the traffic load 

across transverse joints to adjacent slabs and correspondingly reduce or eliminate joint 

faulting. The most commonly used load transfer device is a plain, round steel dowel; these 

are typically 450 mm (18 in.) long, 25 mm (1 in.) in diameter, and spaced at approximately 

every 300 mm (12 in.) along transverse joints. Tie bars, typically deformed steel rebars, are 

often used to hold the faces of abutting slabs in firm contact, but tie bars are not designed to 

act as load transfer devices. 
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Geotechnical Aspects of Pavements 1 - 15 May 2006 

1.3.5 Composite Pavements 

Composite pavements consist of asphaltic concrete surface course over PCC or treated bases 

as shown in Figure 1-5. Composite pavements with PCC over asphalt are also being used. 

The treated bases may be either asphalt-treated base (ATB) or cement-treated base (CTB). 

The base layers are treated to improve stiffness and, in the case of permeable base, stability 

for construction. The composite pavement type shown in Figure 1-5 of an AC overlay on top 

of a PCC rigid pavement system is a very common rehabilitation scenario. 

1.3.6 Unpaved Roads (Naturally Surfaced) 

Why use a paved surface? Over one-half of the roads in the United States are unpaved. In 

some cases, these roads are simply constructed with compacted or modified subgrade. In 

most cases, a gravel (base) layer is used as the wearing surface. Because of sparse population 

and low volumes of traffic, these roads will remain unpaved long into the future. 

Consideration for the subgrade are, again, the same as with flexible pavement, albeit the load 

levels are generally much higher. The subgrade should also be crowned to a greater extent 

than paved sections to promote drainage of greater quantities of infiltration surface water. 

The function of the gravel surfaced is now to carry the load and to provide adequate service. 

The problems with this approach include roughness, lateral displacement of surface gravel, 

traction, and dust. Maintenance of ditch lines is also problematic, due to continuous infilling, 

but open ditches are critical to long-term performance. 

Existing

Rigid

Pavement

AC Overlay AC Overlay AC Overlay AC Overlay

ATB Unbound Base ATB

Unbound Base ATB CTB

Existing

Pavement

Existing

Pavement

Existing

Pavement

Figure 1-5. Typical composite pavement sections. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 16 May 2006 

 

1.4 PAVEMENT PERFORMANCE WITH TIES TO GEOTECHNICAL ISSUES 

Regardless of which pavement type is used, all of the components make up the pavement 

system, and failure to properly design or construct any of these components often leads to 

reduced serviceability or premature failure of the system. 

Distress refers to conditions that reduce serviceability or indicate structural deterioration. 

Failure is a relative term. In the context of this manual, failure denotes a pavement section 

that experiences excessive rutting or cracking that is greater than anticipated during the 

performance period, or that a portion of the pavement is structurally impaired at any time 

during the performance period with incipient failure anticipated from the local distress. There 

are a number of ways that a pavement section can fail, and there are many reasons for 

pavement distress and failure. 

Yoder and Witczak (1975) define two types of pavement distress, or failure. The first is a 

structural failure, in which a collapse of the entire structure or a breakdown of one or more of 

the pavement components renders the pavement incapable of sustaining the loads imposed on 

its surface. The second type of failure is a functional failure; it occurs when the pavement, 

due to its roughness, is unable to carry out its intended function without causing discomfort 

to drivers or passengers or imposing high stresses on vehicles. The cause of these failure 

conditions may be due to inadequate maintenance, excessive loads, climatic and 

environmental conditions, poor drainage leading to poor subgrade conditions, and 

disintegration of the component materials. Excessive loads, excessive repetition of loads, and 

high tire pressures can cause either structural or functional failures. 

Pavement failures may occur due to the intrusion of subgrade soils into the granular base, 

which results in inadequate drainage and reduced stability. Distress may also occur due to 

excessive loads that cause a shear failure in the subgrade, base course, or the surface. Other 

causes of failures are surface fatigue and excessive settlement, especially differential of the 

subgrade. Volume change of subgrade soils due to wetting and drying, freezing and thawing, 

or improper drainage may also cause pavement distress. Inadequate drainage of water from 

the base and subgrade is a major cause of pavement problems (Cedergren, 1987). If the 

subgrade is saturated, excess pore pressures will develop under traffic loads, resulting in 

subsequent softening of the subgrade. Under dynamic loading, fines can be literally pumped 

up into the subbase and/or base. 

Improper construction practices may also cause pavement distress. Wetting of the subgrade 

during construction may permit water accumulation and subsequent softening of the 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 17 May 2006 

subgrade in the rutted areas after construction is completed. Use of dirty aggregates or 

contamination of the base aggregates during construction may produce inadequate drainage, 

instability, and frost susceptibility. Reduction in design thickness during construction due to 

insufficient subgrade preparation, may result in undulating subgrade surfaces, failure to place 

proper layer thicknesses, and unanticipated loss of base materials due to subgrade intrusion. 

Yoder and Witczak (1975) state that a major cause of pavement deterioration is inadequate 

observation and field control by qualified engineers and technicians during construction. 

After construction is complete, improper or inadequate maintenance may also result in 

pavement distress. Sealing of cracks and joints at proper intervals must be performed to 

prevent surface water infiltration. Maintenance of shoulders will also affect pavement 

performance. 

Nearly all measures of pavement performance are based upon observations at the surface of 

the pavement – e.g., surface rutting, cracking of the asphalt or PCC, ride quality, and others. 

In some cases, these surface distresses are due directly to deficiencies in the asphalt or PCC 

surface layers, but in many other cases these distresses are caused at least in part by 

deficiencies from the underlying unbound layers. Since pavement design is ultimately an 

attempt to minimize pavement distresses and, thereby, maximize pavement performance, it is 

important to understand how geotechnical factors impact these distresses. 

Table 1-1, Table 1-2, and Table 1-3 summarize the geotechnical influences on the major 

distresses for flexible, rigid, and composite pavements, respectively. The composite 

pavement type considered in Table 1-3 is an AC overlay on top of a PCC rigid pavement 

system and a very common rehabilitation scenario. 

The dominant geotechnical factor(s) for many pavement distresses is/are the stiffness and/or 

strength of the unbound materials. In reality, the stresses that develop in any well-designed 

in-service pavement are well below the failure strength of the unbound materials. As a 

consequence, the true strength parameters (i.e., the cohesion and friction angle from triaxial 

tests) are not typically needed or measured for unbound pavement materials. Strength indices 

like the California Bearing Ratio1

 (CBR) have been commonly measured in the past as an 

overall indication of the material quality in terms of stiffness and resistance to permanent 

deformation. More recent trends have been to replace these quality indices with direct 

stiffness testing via the resilient modulus2

 (MR). Fortunately, strength and stiffness are 

 

1

 California Bearing Ratio is described in more detail in Chapters 3 and 5. 

2

 Resilient Modulus is described in more detail in Chapters 3, 4, and 5. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 18 May 2006 

usually closely correlated in most geomaterials (see, for example, the correlations between 

MR and CBR described in Chapter 5). 

Table 1-1. Geotechnical influences on major distresses in flexible pavements. 

Insufficient Base Stiffness/Strength Insufficient Subgrade Stiffness/Strength Moisture/Drainage Problems Freeze/Thaw Swelling Contamination Erosion Spatial Variability 

Fatigue Cracking X X X X X 

Rutting X X X X X 

Corrugations X 

Bumps X X X 

Depressions X X X X X 

Potholes X X X 

Roughness X X X X X X X 

Table 1-2. Geotechnical influences on major distresses in rigid pavements. 

Insufficient Base Stiffness/Strength Insufficient Subgrade Stiffness/Strength 

Moisture/Drainage Problems Freeze/Thaw Swelling Contamination Erosion Spatial Variability 

Fatigue Cracking X X X X X X 

Punchouts (CRCP) X X X X X X 

Pumping X X 

Faulting X X X X X X 

Roughness X X X X X X X 
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Geotechnical Aspects of Pavements 1 - 19 May 2006 

Table 1-3. Geotechnical influences on major distresses in rehabilitated pavements 

(AC overlay over PCC). 

Insufficient Base 

Stiffness/Strength 

Insufficient Subgrade 

Stiffness/Strength Moisture/Drainage Problems Freeze/Thaw Swelling Contamination Erosion Spatial Variability 

Reflection Cracking X X X 

Roughness X X X X X X 

A major effect of the moisture/drainage, freeze/thaw, and contamination (material from one 

layer intermixing with another) factors listed in Table 1-1 through 1-3 is to degrade the 

stiffness and strength of the affected unbound materials. Moisture and drainage are combined 

here because excessive moisture in the pavement system is often the result of inadequate or 

malfunctioning drainage systems. Freeze/thaw and swelling can cause heaving of the 

pavement surface. Erosion can produce voids beneath the surface layers, causing a complete 

loss of foundation support. The spatial variability factor represents the nonuniformity of the 

geotechnical factors along the pavement and will, in general, apply to all of the other 

geotechnical factors. 

Note that there are many other important pavement distresses, like thermal cracking, low skid 

resistance, and others, that are not included in Tables 1-1 through 1-3. The influence of 

geotechnical factors on these other distresses is generally quite small. 

Some further comments on the major distress types are given in the following paragraphs: 

Permanent Deformations (Rutting, Bumps, Corrugations, and Depressions). Surface 

rutting is often the controlling stress mode for flexible pavements. It is sometimes caused by 

an unstable asphalt concrete mixture that deforms plastically within the first few inches 

beneath the wheel paths. For a well-designed mixture, however, any rutting observed at the 

surface will be only partly due to permanent deformations in the asphalt layer, with the 

remainder due to accumulated permanent deformations in the underlying unbound layers and 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 20 May 2006 

the subgrade. For example, at the AASHO road test, the percent of final total surface rutting 

attributable to the asphalt layer averaged 32%, versus 18% for the granular base layer, 39% 

for the granular subbase, and 11% for the subgrade. In other words, two-thirds of the rutting 

observed at the surface was due to accumulated permanent deformations in the geomaterials 

in the pavement structure. Potential causes for excessive permanent deformations in the 

pavement geomaterials include 

• inadequate inherent strength and stiffness of the material. 

• degradation of strength and stiffness due to moisture effects (including freeze/thaw); 

inadequate or clogged drainage systems will contribute to this degradation. 

• contamination of base and subbase materials by subgrade fines (i.e., inadequate 

separation of layer materials). 

The shape of the rut trough is usually a good indicator of the source of the permanent 

deformations. Permanent deformations concentrated in the surface asphalt layers tend to give 

a narrow rut trough (individual wheel tracks may even be evident), while deep seated 

permanent deformations from the underlying unbound layers and subgrade typically give a 

much broader rut trough at the surface. 

Nonuniform geotechnical conditions along the pavement can contribute to local permanent 

deformations in the form of bumps, corrugations, and depressions. 

Fatigue Cracking. This form of distress is the cracking of the pavement surface as a result of 

repetitive loading. It may be manifested as longitudinal or alligator cracking (interconnected 

or interlaced cracks forming a pattern that resembles an alligator's hide) in the wheel paths 

for flexible pavement and transverse cracking (and sometimes longitudinal cracking) for 

jointed concrete pavement. Fatigue cracking in both flexible and rigid pavements is governed 

by two factors: the inherent fatigue resistance of the surface layer material, and the 

magnitude of the cyclic tensile strains at the bottom of the layer. The inherent fatigue 

resistance is clearly dependent only on the properties of the asphalt or PCC. The magnitude 

of the cyclic tensile strain, on the other hand, is a function of the composite response of the 

entire pavement structure. Low stiffness in the base, subbase, or subgrade materials – 

whether due to deficient material quality and/or thickness, moisture influences, or 

freeze/thaw effects – will all raise the magnitude of the tensile strains in the bound surface 

layer and increase the potential for fatigue cracking. Localized fatigue cracking may also be 

caused by nonuniformities in the geomaterials along the pavement alignment – e.g., voids, 

local zones of low stiffness material, etc. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 21 May 2006 

Reflective Cracking. Reflective cracking in asphalt or concrete surfaces of pavements occurs 

over joints or cracks in the underlying layers. Like fatigue cracking, reflection cracking of 

asphalt overlays on top of rigid pavements is governed by the inherent fatigue resistance of 

the asphalt concrete and the magnitude of the tensile and shear strains in the overlay above 

the joint in the underlying rigid pavement. Inadequate foundation support (e.g., voids) at the 

joint will allow differential movement between slabs under a passing vehicle, producing 

large strains in the overlay above. Intrusion of water, inadequate drainage, and erosion of the 

unbound base material beneath a joint are all major geotechnical factors influencing 

reflection cracking. 

Potholes. Potholes are formed due to a localized loss of support for the surface course though 

either a failure in the subgrade or base/subbase layers. Potholes are often associated with 

frost heave, which pushes the pavement up due to ice lenses forming in the subgrade during 

the freeze. During the thaw, voids (often filled with water) are created in the soil beneath the 

pavement surface due to the melting ice and/or gaps beneath the surface pavement resulting 

from heave. When vehicles drive over this gap, high hydraulic pressure is created in the void, 

which further weakens the surrounding soil. The road surface cracks and falls into the void, 

leading to the birth of another pothole. Potholes can also occur as a result of pumping 

problems. 

Punchouts. Punchouts are identified as a broken area of a CRCP bounded by closely spaced 

cracks usually spaced less than 1 m (3 ft).

Pumping. Pumping is the ejection of foundation material, either wet or dry, through joints or 

cracks, or along edges of rigid slabs, resulting from vertical movements of the slab under 

traffic, or from cracks in semi-rigid pavements. 

Faulting. Faulting appears as an elevation or depression of a PCC slab in relation to an 

adjoining slab, usually at transverse joints and cracks. 

Roughness. Surface roughness is due in large measure to nonuniform permanent 

deformations and cracking along the wheel path. Consequently, all of the geotechnical 

factors influencing permanent deformations and cracking will also impact roughness. 

Nonuniformity of the stiffness/strength of the geomaterials along the pavement, in particular, 

can be a major contributor to surface roughness. Nonuniform swelling of subgrade soils 

along the pavement alignment provides a classic example of extreme pavement roughness in 

some areas of the country. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 22 May 2006 

Liquefaction. The process of transforming any soil from a solid state to a liquid state, usually 

as a result of increased pore pressure and reduced shearing resistance (ASTM, 2001) is called 

liquefaction. Spontaneous liquefaction may be caused by a collapse of the structure by shock 

or other type of strain, and is associated with a sudden, but temporary, increase of the 

prefluid pressure. 

Thermal Cracking. Thermal cracks appear in an asphalt pavement surface, usually full width 

transverse, as a result of seasonal or diurnal volume changes of the pavement restrained by 

friction with an underlying layer. 

By now it should be apparent that there are a number of ways that a pavement may become 

impaired to the extent that it is no longer serviceable. In designing a pavement section, the 

pavement is anticipated to deform over its service life so that at a period in time it will need 

to be repaired or replaced. Normal failure is defined by rutting of the pavement section, as 

shown in Figure 1-6, and usually consists of no more than 20 – 25 mm (3⁄4 – 1 in.) within the 

anticipated performance period. However, as previously reviewed in this section, there are a 

number of factors that may result in premature failure, long before the performance period, 

most of which are related to geotechnical issues. Specifically, geotechnical failures, as shown 

in Figure 1-7, are generally related to excessive subgrade rutting, aggregate contamination or 

degeneration, subgrade pumping, poor drainage, frost action, and swelling soils. There are 

other ancillary geotechnical issues, which will impact pavement performance, but are usually 

addressed in roadway design (i.e., not by the pavement group). These include differential 

embankment settlement, embankment and cut slope stability, liquefaction, collapsing soils, 

and karstic (sinkhole) formations. Design methods to evaluate these specific issues, along 

with procedures to mitigate potential problems, can be found in reference manuals for NHI 

132012 Soils and Foundations Workshop (FHWA NHI-00-045 (Cheney & Chassie, 2000)) 

and NHI 132034 on Ground Improvement Methods (FHWA NHI-04-001 (Elias et al., 

2004)). 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 23 May 2006 

Figure 1-6. Normal rutting. 

 

a) Excessive rutting b) Aggregate contamination or degeneration 

Figure 1-7. Examples of geotechnical related pavement failures. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 24 May 2006 

c) Subgrade pumping d) Drainage problems 

e) Frost action f) Swelling soils 

g) Differential settlement h) Collapsing soils, karst conditions, or 

 liquefaction 

Figure 1-7. Examples of geotechnical related pavement failures (continued). 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 25 May 2006 

1.5 CASE HISTORIES OF PAVEMENT GEOTECHNICS (Failure Examples) 

Geotechnical failures are often the result of not recognizing or adequately evaluating 

conditions prior to construction of the road. The following section provides several case 

histories of pavement failures that occurred due to inadequate geotechnical information.

1.5.1 Drainage Failure 

The existing pavement along a 3 km (2 mile) portion of U.S. Route 1A in a northern state had 

been plagued by cracking, rutting, and potholes. The highway is a major transport route for 

tanker trucks that transport oil from the port to a major city. This particular roadway section 

required frequent maintenance to maintain a trafficable pavement surface, and recently had 

received a 100-mm (4-in.) overlay. However, within two years of construction, the overlay 

was badly cracked and rutted, again needing repair. These conditions prompted the 

reconstruction project. A subsurface investigation encountered moist clay soils (locally 

known as the Presumpscot Formation) along the entire length of the project. These soils are 

plastic and moisture sensitive, with water contents greater than 20%. Borings indicated up to 

300 mm (12 in.) of asphalt in some sections, and an extensively contaminated base. During 

the investigation, water was observed seeping out of pavement sections, even though this had 

been the second driest summer on record in the state. Water in the pavement section was 

obviously one of the existing pavement section failure mechanisms. Based on soil conditions 

and past roadway construction experiences, designers initially recommended that the 

subgrade soils be undercut by 150 mm (6 in.) – with a greater depth of undercut anticipated 

in some areas – and replaced with granular soil to create a stable working surface prior to 

placing the overlying subbase course. However, this approach would not solve the drainage 

problem. Roadway drainage was not conventionally used in this state due to concerns that 

outlet freezing would prevent effectiveness. 

In order to evaluate the most effective repair methods, test sections were established along 

the alignment consisting of alternate stabilization methods and drainage sections. The test 

sections were fully instrumented. Monitoring included FWD testing performed prior to 

reconstruction, after construction and periodically (e.g., before and after the spring thaw) 

since the project was completed in 1997. An indication of the poor subgrade condition on this 

project was encountered during construction, when a control section (no stabilization lift) 

failed and required a 600 mm (24 in.) undercut and gravel replacement to allow construction 

over the section. A 820 mm (32 in.) pavement section was then constructed over the 

undercut. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 26 May 2006 

The roadway is performing well in all sections, and at this time (five years after construction) 

it is too early to determine which stabilization method proved most effective. Minimal frost 

heave has been observed thus far in all of the test sections, and it may take several additional 

seasons to provide discernible results. In the drainage section, water flows from the drains 

and corresponds strongly with precipitation events and water table levels. One surprising 

result occurs in the spring of each year. More water flows from the drains during the month 

of spring thaw than all of the other months combined. Over the long-term it is anticipated 

that this drainage will prove very beneficial to the performance of the pavement system. 

1.5.2 Collapsible Soils 

Sections of Interstate 15 within a 27 km (17-mi) length of roadway in a western state have 

been experiencing considerable distress since construction. Maintenance costs have been 

significant, and it appears that distress may not simply be due to an inadequate pavement 

section. The problems associated with bumps, cracks, and edge failures were likely 

associated with troubles in the subgrade soils along the alignment. Potential causes could 

have included collapsible soil, expansive soil, compressible soil, poorly compacted fill, and 

poor drainage. A study was performed with the objectives of determining the causes for the 

problems and developing potential solutions prior to design and reconstruction of the area in 

question. Based on surficial geology and borehole data, zones were identified where 

collapsible soils were likely the culprit. Because the zone of collapsible soil extends to depths 

of up to 6 m (20 ft) below the ground surface, deep dynamic compaction was recommended 

over excavation and replacement as a treatment method in these zones. Distress related to 

expansive soils exists throughout the study area, but significant damage concentrations are 

located in a cut section between mileposts 208 and 207 along I-15. This area is long enough 

to propose treatments for the area, in order to improve ride quality throughout the cut section. 

This study recommends a combination of methods to improve the odds of success. Because 

of the potential for differential settlement on the roadway, asphalt pavement should be used 

in reconstructing the roadway in the study area. A lack of adequate surface drainage is 

another critical factor leading to problems with both collapsible and expansive subgrade soils 

in this area. Deep dynamic compaction was found not to be feasible during construction, 

most likely due to an intervening fine-grained layer in the deposit. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 27 May 2006 

1.6 CONCLUDING REMARKS 

All pavement systems are constructed on earth and practically all components are constructed 

with earth materials. When these materials are bound with asphalt or cement to form surface 

layers, they take on a manufactured structural component that is relatively well understood 

by pavement designers. However, in their unbound state, the properties of these 

“geotechnical” materials are extremely variable and are the results of the natural processes 

that have formed them, and natural or man-made events following their formation. Often the 

earth provides inferior foundation materials in their natural state, but replacement is often 

impractical and uneconomical. As a result, the design engineer is often faced with the 

challenge of using the foundation and construction materials available on or near the project 

site. Therefore, designing and building pavement systems requires a thorough understanding 

of the properties of available soils and rocks that will constitute the foundation and other 

components of the pavement system. 

1.7 REFERENCES 

AASHTO (1972). AASHTO Interim Guide for Design of Pavement Structures, American 

Association of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (1986). AASHTO Guide for Design of Pavement Structures, American Association 

of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (1993). AASHTO Guide for Design of Pavement Structures, American Association 

of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (1998). AASHTO Interim Guide for Design of Pavement Structures, American 

Association of State and Highway Transportation Officials, Washington, D.C.

The Asphalt Institute (1984). MS-2, Mix Design Methods for Asphalt Concrete and Other 

Hot Mix Types. 

ASTM (2001). Annual Book of ASTM Standards, Section 4 Construction, Volume 04.03 

Road and Paving Materials; Vehicle-Pavement Systems, American Society for Testing and 

Materials, West Conshohocken, PA. 

Cedergren, H.R. (1987). Drainage of Highway and Airfield Pavements, Robert E. Krieger 

Publishing Co. Inc., Malabar, FL. 
FHWA NHI-05-037 Chapter 1 – Introduction 

Geotechnical Aspects of Pavements 1 - 28 May 2006 

Cheney, R. and Chassie, R.G. (2000). Soils and Foundations Workshop, Reference Manual 

for NHI Course No. 132012, U.S. Department of Transportation, Federal Highway 

Administration, Washington D.C., FHWA NHI-00-045, 358 p. 

Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J.G., and Berg, R.R. (2004). Ground 

Improvement Methods, Reference Manual for NHI Course No. 132034, U.S. Department of 

Transportation, Federal Highway Administration, Washington, D.C., FHWA-NHI-04-001, 

884 p. 

NCHRP 1-37A Design Guide (2002). 2002 Design Guide – Design of New and Rehabilitated 

Pavement Structures, Draft Final Report, Part 1 – Introduction and Part 2 – Design Inputs, 

Prepared for the National Cooperative Highway Research Program by ERES Division of 

ARA. 

Newcombe, D.A., and Birgisson B. (1999). “Measuring In-Situ Mechanical Properties of 

Pavement Subgrade Soils,” NCHRP Synthesis 278, Transportation Research Board, 

Washington, D.C. 

Orchant, C.J., Kulhawy, F.H., and Trautmann, C.H. (1988). Reliability Based Foundation 

Design for Transmission Line Structures: Critical Evaluation of In-Situ Methods, Report 

EL-5507(2), Electric Power Research Institute, Palo Alto California et al., 1988 

Schultze, E. (1972). “Frequency Distributions and Correlations Of Soil Properties,” in 

Statistics and Probability in Civil Engineering, Hong Kong University Press (Hong Kong 

International Conference). Ed. P. Lumb, distributed by Oxford University Press, London. 

Yoder, E. J., and Witczak, M.W. (1975). Principles of Pavement Design, Wiley, New York.
FHWA NHI-05-037 Chapter 2 – Host State 

Geotechnical Aspects of Pavement Page 2 - 1 May 2006 

CHAPTER 2.0 STATE AGENCIES 

2.1 HOST AGENCY AND OTHER IDENTIFIED GUESTS 

Various state agencies take different approaches to pavement design. Local practices often 

vary in terms of 1) the methods for obtaining subsurface information and laboratory testing in 

relation to pavement design; 2) design guides followed by the agency (usually a variation of 

current or previous AASHTO guidelines); and 3) field construction monitoring for subgrade 

approval and pavement component approval, as well as contractors’ quality control 

requirements for pavement component construction. This chapter presents issues related to 

each of these items, discussed by a representative of the agency conducting this course. A 

questionnaire for an agency to document and evaluate their own practice in relation to 

geotechnical aspects of pavement design is provided. 

Issues that should be specifically addressed by each agency include: 

• Current subsurface investigation practices and procedures for new construction, 

reconstruction, and rehabilitation pavement projects, including in-house facilities 

(field and laboratory) and outsourced capabilities, noting prequalification and QC/QA 

requirements for outsourced programs. 

• Special or complex subsurface conditions (e.g., soft soils, frost susceptible soils, 

swelling soils, collapsible soils, caliche, karst topography, etc.) encountered in this 

state. 

• The standard pavement systems (including pavement type, base and subbase layers, 

drainage requirements, and subgrade treatments) currently used by the agency. 

• Current design approach (i.e., AASHTO referenced to year of provision, state agency 

procedure, or other) and implementation status of empirical-mechanistic design 

approach. 

• Current pavement projects requiring special or complex procedures. 

• Construction and design verification procedures, including determination of subgrade 

stabilization requirements during construction (e.g., undercut, use of geosynthetics, 

lime, etc.). 

• Agency organizational structure, as it relates to personnel involved with pavement 

material evaluation, design, construction, and maintenance. 

There are also many impediments such as time, money, and personnel to performing an 

adequate subsurface investigation program for pavement design. Agencies should always be 

aware of these issues and the continual work to remove such impediments. The cost benefit 
FHWA NHI-05-037 Chapter 2 – Host State 

Geotechnical Aspects of Pavement Page 2 - 2 May 2006 

of performing an adequate subsurface program will be discussed in Chapter 4. However, a 

cost-benefit analysis as evaluated in Pavement Management Programs could substantially 

assist individual states in assessing their own priorities. 

2.2. QUESTIONAIRE ON GEOTECHNICAL PRACTICES IN PAVEMENT 

 DESIGN 

 

1. Which of the following pavement design methods (or modification thereof) is currently 

used by your agency? (Please circle appropriate method and provide details of any 

modifications.) 

− AASHTO 1972 

− AASHTO 1986 

− AASHTO 1993 with 1998 Supplement 

− Mechanistic-Empirical design (please identify reference method) ______________ 

− Other (please identify or describe) ____________________ 

2. What are the design performance periods (a.k.a. design life) assigned to each of the 

following type of roads in your state? 

Performance Period (# or years) Type Asphalt PCC 

I Secondary 

II Primary 

III Interstate and Freeway 

 Comments: 

3. Does your current design achieve the performance period? (If no, what is the typical 

actual performance period, or range?) 
FHWA NHI-05-037 Chapter 2 – Host State 

Geotechnical Aspects of Pavement Page 2 - 3 May 2006 

4. What method(s) or test(s) do(es) your state use to evaluate subgrades for inputs values 

(e.g., CBR, R-value, resilient modulus, etc.) to pavement design? 

5. Which group within your agency (i.e., pavement design, geotechnical, hydrology, or 

other) is responsible for design of pavement drainage? 

 

6. Of the following, which method(s) do(es) your state perform for evaluating subgrade 

conditions in the field? 

Method Frequently Sometimes Never 

Remote sensing* (e.g., air photo, landsat 

photos, etc.) 

Geophysical Non-destructive Tests* 

 Falling Weight Deflectometer, FWD 

 Ground Penetrating Radar, GPR 

 Surface Resistivity, SR 

 Seismic Refraction 

 

 In-situ Investigation* 

 (Cone Penetration Test, CPT) 

 (Dynamic Cone Penetration Test, DCP) 

 (Standard Penetration Test, SPT) 

 

Disturbed sampling (usually with borings) 

Undisturbed sampling (usually with borings) 

 * Please list the equipment that you have available and identify it as 1) in-house or 2) outsourced. 
FHWA NHI-05-037 Chapter 2 – Host State 

Geotechnical Aspects of Pavement Page 2 - 4 May 2006 

7. How is the frequency and spacing determined for borings along the alignment (e.g.,

standard spacing – provide, available info, site recognizance, etc.) and where are the 

borings located (e.g., centerline, wheel path, shoulder, other)? 

8. What method(s) or test(s) do(es) your state use to evaluate/control subgrade construction? 

9. Which of the following stabilization methods are used in your state? 

Stabilization Method Yes No 

Undercut and Backfill 

Thicker Aggregate 

Geotextiles and Aggregate 

Geogrids and Aggregate 

Cement 

Lime 

Lime-Flyash 

Lime-Cement 

Lime-Cement-Flyash 

Bitumen Modification 

Other – Please provide details 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 1 May 2006 

CHAPTER 3.0 GEOTECHNICAL ISSUES IN 

PAVEMENT DESIGN AND PERFORMANCE 

3.1 INTRODUCTION 

Satisfactory pavement performance depends upon the proper design and functioning of all of 

the key components of the pavement system. These include: 

1. A wearing surface that provides sufficient smoothness, friction resistance, and sealing 

or drainage of surface water (i.e., to minimize hydroplaning). 

2. Bound structural layers (i.e., asphalt or Portland cement concrete) that provide 

sufficient load-carrying capacity, as well as barriers to water intrusion into the 

underlying unbound materials. 

3. Unbound base and subbase layers that provide additional strength – especially for 

flexible pavement systems – and that are resistant to moisture-induced deterioration 

(including swelling and freeze/thaw) and other degradation (e.g., erodibility, intrusion 

of fines). 

4. A subgrade that provides a uniform and sufficiently stiff, strong, and stable 

foundation for the overlying layers. 

5. Drainage systems that quickly remove water from the pavement system before the 

water degrades the properties of the unbound layers and subgrade. 

6. Remedial measures, in some cases, such as soil improvement/stabilization or 

geosynthetics to increase strength, stiffness, and/or drainage characteristics of various 

layers or to provide separation between layers (e.g., to prevent fines contamination). 

Traditionally, these design issues are divided among many groups within an agency. The 

geotechnical group is typically responsible for characterizing the foundation characteristics 

of the subgrade. The materials group may be responsible for designing a suitable asphalt or 

Portland cement concrete mix and unbound aggregate blend for use as base course. The 

pavement group may be responsible for the structural ("thickness") design. The construction 

group may be responsible for ensuring that the pavement structure is constructed as designed. 

Nonetheless, the overall success of the design – i.e., the satisfactory performance of the 

pavement over its design life – is the holistic consequence of the proper design of all of these 

key components. 

Keeping this holistic view in mind, this chapter builds upon the introduction from Chapter 1 

and expands upon the major geotechnical considerations in pavement design (i.e., the factors 

influencing items 3-6 above). The emphasis is on the "big picture," on identifying the key 

geotechnical issues and describing their potential impact on the pavement design and 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 2 May 2006 

performance. Most of the issues introduced here are elaborated in subsequent chapters, and 

forward references to these later sections are given as appropriate. A brief history of the 

AASHTO highway pavement design techniques is also included to illustrate how 

geotechnical design considerations have grown in importance and prominence over time. 

3.2 BASIC CONCEPTS 

Pavements are layered systems designed to meet the following objectives: 

• to provide a strong structure to support the applied traffic loads (structural capacity). 

• to provide a smooth wearing surface (ride quality). 

• to provide a skid-resistant wearing surface (safety). 

Additionally, the system must have sufficient durability so that it does not deteriorate 

prematurely due to environmental influences (water, oxidation, temperature effects). 

The unbound soil layers in a pavement provide a substantial part of the overall structural 

capacity of the system, especially for flexible pavements (often more than 50 percent). As 

shown in Figure 3-1, the stresses induced in a pavement system by traffic loads are highest in 

the upper layers and diminish with depth. Consequently, higher quality – and generally more 

expensive – materials are used in the more highly stressed upper layers of all pavement 

systems, and lower quality and less expensive materials are used for the deeper layers of the 

pavement (Figure 3-2). This optimization of material usage minimizes construction costs and 

maximizes the ability to use locally available materials. However, this approach also 

requires greater attention to the lower quality layers in the design (i.e., the subgrade) in order 

to reduce life-cycle pavement costs. Good long-term performance of lower layers means that 

upper layers can be maintained (rehabilitated) while avoiding the more costly total 

reconstruction typically associated with foundation failures. 

Figure 3-1. Attenuation of load-induced stresses with depth. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 3 May 2006 

Surface

Base

Subbase

Subgrade

Very Strong

Durable

Impermeable

Manufactured

Expensive

Strong

Free-Draining

Manufactured

Less Expensive

Moderate Strength

Free-Draining

Natural Material

Inexpensive

Weak

Moisture Sensitive

In Situ Soil

Surface

Base

Subbase

Subgrade

Very Strong

Durable

Impermeable

Manufactured

Expensive

Strong

Free-Draining

Manufactured

Less Expensive

Moderate Strength

Free-Draining

Natural Material

Inexpensive

Weak

Moisture Sensitive

In Situ Soil

Figure 3-2. Variation of material quality with depth in a pavement system with ideal drainage 

characteristics. 

As is the case for all geotechnical structures, pavements will be strongly influenced by 

moisture and other environmental factors. Water migrates into the pavement structure 

through combinations of surface infiltration (e.g., through cracks in the surface layer), edge 

inflows (e.g., from inadequately drained side ditches or inadequate shoulders), and from the 

underlying groundwater table (e.g., via capillary potential in fine-grained foundation soils). 

In cold environments, the moisture may undergo seasonal freeze/thaw cycles. Moisture 

within the pavement system nearly always has detrimental effects on pavement performance. 

It reduces the strength and stiffness of the unbound pavement materials, promotes 

contamination of coarse granular material due to fines migration, and can cause swelling 

(e.g., frost heave and/or soil expansion) and subsequent consolidation. Moisture can also 

introduce substantial spatial variability in the pavement properties and performance, which 

can be manifested either as local distresses, like potholes, or more globally as excessive 

roughness. The design of the geotechnical aspects of pavements must consequently focus on 

the selection of moisture-insensitive free-draining base and subbase materials, stabilization of 

moisture-sensitive subgrade soils, and adequate drainage of any water that does infiltrate into 

the pavement system. Material selection and characterization is described more fully later in 

Chapter 5, and pavement drainage design is covered in Chapter 7. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 4 May 2006 

3.3 KEY GEOTECHNICAL ISSUES 

The geotechnical issues in pavement design can be organized into two categories: (1) general 

issues that set the entire tone for the design – e.g., new versus rehabilitation design; and (2) 

specific technical issues – e.g., subgrade stiffness and strength determination. The 

geotechnical considerations in each of these categories are briefly introduced in the following 

subsections. Again, the intent here is to provide an overview of the broad range of 

geotechnical issues in pavement design. More detailed treatment of each of these issues will 

be provided in subsequent chapters. 

3.3.1 General Issues 

New Construction vs. Rehabilitation vs. Reconstruction 

The first issue to be confronted in any pavement design is whether the project involves new 

construction, rehabilitation, or reconstruction. As defined in Chapter 1, new construction is 

the construction of a pavement system on a new alignment that has not been previously 

constructed. Rehabilitation is defined as the repair and upgrading of an existing in-service 

pavement. Typically, this involves repair/removal and construction of additional bound 

pavement layers (e.g., asphalt concrete overlays) and could include partial-depth or full-depth 

recycling or reclamation. Reconstruction is defined as the complete removal of an existing 

pavement system, typically down to and including the upper portions of the foundation soil, 

and the replacement with a new pavement structure. New construction has been the 

traditional focus of most pavement design procedures, although this focus has shifted to 

rehabilitation and reconstruction over recent years, as highway agencies have switched from 

system expansion to system maintenance and preservation. 

New construction vs. rehabilitation vs. reconstruction has a significant impact on several key 

geotechnical aspects of pavement design. As described more fully in Chapter 4, new 

construction typically requires substantial “conventional” site characterization work – e.g.,

examination of geological and soil maps, boring programs, laboratory testing of borehole 

samples, geophysical subsurface exploration, etc. Little will be known in advance of the soil 

profiles and properties along the new alignment, so a comparatively comprehensive 

subsurface exploration and material characterization program is required. Exploration also 

usually involves evaluation of both cut and fill conditions along the alignment. Access is 

often limited due to adverse terrain conditions. 

For rehabilitation projects, on the other hand, original design documents and as-built 

construction records are often available to provide substantial background information about 

the subsurface conditions along the project alignment. The material properties (e.g., subbase 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 5 May 2006 

stiffness) determined during the initial design may no longer be relevant (e.g., because of 

contamination from subgrade fines), so new tests may be required, either from laboratory 

tests on samples extracted from borings through the existing pavement or from in-situ tests 

like the Dynamic Cone Penetrometer (DCP—see Chapter 4), again via boreholes through the 

existing pavement structure. Nondestructive evaluation via falling weight deflectometers 

(FWD—see Chapter 4) is very commonly used to determine in-place material properties for 

rehabilitation design. Forensic evaluation of the distresses in the existing pavement can also 

help identify deficiencies in the underlying unbound layers. However, since the underlying 

unbound layers are not exposed or removed in typical rehabilitation projects, any deficiencies 

in these layers must be compensated by increased structural capacity, etc., in the added 

surface layers. 

Original design documents and as-built construction records are also often available for 

reconstruction projects. Information on the original subsurface profile will generally remain 

relevant for the reconstruction design. However, detailed material characterization from the 

original design documents will generally not be useful, since the original pavement materials 

down to and often including the upper portion of the foundation are completely removed and 

replaced during reconstruction. Although direct testing of the newly exposed foundation soil 

is theoretically possible in reconstruction projects, this will occur only once construction has 

begun and, thus, will be too late for design purposes. Consequently, foundation soil 

properties for reconstruction projects must typically be determined from original design 

records, borehole sampling and testing, and FWD testing, similar to rehabilitation design. 

The characterization of the new or recycled unbound subbase and base materials in 

reconstruction projects will typically be performed via laboratory tests, similar to new 

construction design. 

The influence of new construction vs. rehabilitation vs. reconstruction on site 

characterization and subsurface exploration is described in detail in Chapter 4. The different 

methods for characterizing the geotechnical materials in these different types of projects are 

detailed in Chapter 5. 

Natural Subgrade vs. Cut vs. Fill 

Pavement construction on a natural subgrade is the classic “textbook” case for pavement 

design. The subsurface profile (including depth to bedrock and groundwater table) are 

determined directly from the subsurface exploration program, and subgrade properties 

needed for the design can be taken from tests on the natural foundation soil in its in-situ 

condition and in its compacted state, if the upper foundation layer is to be processed and 

recompacted or removed and replaced during construction. This issue is discussed in greater 

detail in Chapter 4. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 6 May 2006 

However, the alignment for most highway projects does not always follow the site 

topography, and consequently a variety of cuts and fills will be required. The geotechnical 

design of the pavement will involve additional special considerations in cut and fill areas. 

Attention must also be given to transition zones – e.g., between a cut and an at-grade 

section—because of the potential for nonuniform pavement support and subsurface water 

flow. 

The main additional concern for cut sections is drainage, as the surrounding site will be 

sloping toward the pavement structure and the groundwater table will generally be closer to 

the bottom of the pavement section in cuts. Stabilization of moisture-sensitive natural 

foundation soils may also be required. Stability of the cut slopes adjacent to the pavement 

will also be an important design issue, but one that is typically treated separately from the 

pavement design itself. 

The embankments for fill sections are constructed from well compacted material, and, in 

many cases, this results in a subgrade that is of higher quality than the natural foundation 

soil. Drainage and groundwater issues will, in general, be less critical for pavements on 

embankments, although erosion of side slopes from pavement runoff may be a problem, 

along with long-term infiltration of water. The principal additional concerns for pavements 

in fill sections will be the stability of the embankment slopes and settlements, either due to 

compression of the embankment itself or due to consolidation of soft foundation soils 

beneath the embankment (again, usually evaluated by the geotechnical unit as part of the 

roadway embankment design). 

Information on soil slope and embankment design can be found in the reference manual for 

FHWA NHI 132033 (FHWA NHI-01-028). Reinforced slope design (often an alternative 

where steep embankment slopes are required) is addressed in the reference manual for 

FHWA NHI 132042 (FHWA NHI-00-043). Rock slope design is covered in the FHWA NHI 

132035 reference manual (FHWA NHI-99-007). 

Environmental Effects 

Environmental conditions have a significant effect on the performance of both flexible and 

rigid pavements. Specifically, moisture and temperature are the two environmentally driven 

variables that can significantly affect the pavement layer and subgrade properties and, thus, 

the performance of the pavement. Some of the effects of environment on pavement materials 

include the following: 

• Asphalt bound materials exhibit varying modulus values depending on temperature. 

Modulus values can vary from 2 to 3 million psi (14,000 to 20,000 MPa) or more 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 7 May 2006 

during cold winter months to about 100,000 psi (700 MPa) or less during hot summer 

months. 

• Although cementitious material properties like flexural strength and modulus are not 

significantly affected by normal temperature changes, temperature and moisture 

gradients can induce significant stresses and deflections—and consequently pavement 

damage and distresses—in rigid pavement slabs. 

• At freezing temperatures, water in soil freezes and the resilient modulus of unbound 

pavement materials can rise to values 20 to 120 times higher than the values before 

freezing. 

• The freezing process may be accompanied by the formation and subsequent thawing 

of ice lenses. This creates zones of greatly reduced strength in the pavement structure. 

• The top down thawing in spring traps water above the still frozen zone; this can 

greatly reduce strength of geomaterials. 

• All other conditions being equal, the stiffness of unbound materials decreases as 

moisture content increases. Moisture has two separate effects: 

o First, it can affect the state of stress through suction or pore water pressure. 

Coarse grained and fine-grained materials can exhibit more than a fivefold 

increase in modulus as they dry. The moduli of cohesive soils are affected by 

complex clay-water-electrolyte interactions. 

o Second, it can affect the structure of the soil through destruction of the 

cementation between soil particles. 

• Bound materials are not directly affected by the presence of moisture. However, 

excessive moisture can lead to stripping in asphalt mixtures or can have long-term 

effects on the structural integrity of cement bound materials. 

• Cement bound materials may also be damaged during freeze-thaw and wet-dry 

cycles, which causes reduced modulus and increased deflections. 

All pavement distresses are affected by environmental factors to some degree. However, it is 

often very difficult to include these effects in pavement design procedures. 

3.3.2 Specific Issues 

Material Types and Properties 

The major material types encountered in pavement systems are listed in Table 3-1. The 

geotechnical materials that are the focus of this manual include non-stabilized granular 

base/subbase materials (including recycled materials), nonstabilized subgrade soils, 

mechanically and chemically stabilized subgrade soils, and bedrock groups. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 8 May 2006 

Table 3-1. Major material types in pavement systems (NCHRP 1-37A). 

Asphalt Materials 

 Hot Mix AC—Dense Graded 

 Central Plant Produced 

 In-Place Recycled 

 Hot Mix AC—Open Graded Asphalt 

 Hot Mix AC—Sand Asphalt Mixtures 

 Cold Mix AC 

 Central Plant Processed 

 In-Place Recycled 

PCC Materials 

 Intact Slabs 

 Fractured Slabs 

 Crack/Seat 

 Break/Seat 

 Rubblized 

Cementitiously Stabilized Materials 

 Cement Stabilized Materials 

 Soil Cement 

 Lime Cement Flyash 

 Lime Flyash 

 Lime Stabilized/Modified Soils 

 Open Graded Cement Stabilized Materials 

Non-Stabilized Granular Base/Subbase 

 Granular Base/Subbase 

 Sandy Subbase 

 Cold Recycled Pavement (used as aggregate) 

 RAP (includes millings) 

 Pulverized In-Place 

 

Subgrade Soils 

 Gravelly Soils (A-1; A-2; GW; GP; GM; GC) 

 Sandy Soils 

 Loose Sands (A-3; SW; SP) 

 Dense Sands (A-3; SW; SP) 

 Silty Sands (A-2-4;A-2-5; SM) 

 Clayey Sands (A-2-6; A-2-7; SC) 

 Silty Soils (A-4;A-5; ML; MH) 

 Clayey Soils 

 Low Plasticity Clays (A-6; CL) 

 Dry-Hard 

 Moist Stiff 

 Wet/Sat-Soft 

 High Plasticity Clays (A-7; CH) 

 Dry-Hard 

 Moist Stiff 

 Wet/Sat-Soft 

Bedrock 

 Solid, Massive and Continuous 

 Highly Fractured, Weathered 

The material properties of interest in pavement design can be organized into the following 

categories: 

• Physical properties (e.g., soil classification, density, water content) 

• Stiffness and/or strength (e.g., resilient modulus, modulus of subgrade reaction, CBR) 

• Thermo-hydraulic properties (e.g., drainage coefficients, permeability, coefficient of 

thermal expansion) 

• Performance-related properties (e.g., repeated load permanent deformation 

characteristics) 

Details of the procedures for determining the geotechnical properties required for pavement 

design are given in Chapter 5. Note that not all material properties will be equally important 

in terms of their impact on pavement design and performance, and not all properties are 

required in all pavement design procedures. Stiffness, usually quantified in terms of the 

resilient modulus (see Chapter 5), is the most important geotechnical property in pavement 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 9 May 2006 

design and is incorporated explicitly in most current pavement design procedures (e.g., the 

1993 AASHTO Pavement Design Guide). Newer mechanistic-empirical design procedures, 

such as developed in the recently-completed NCHRP Project 1-37A, require more 

information regarding material properties, particularly in relation to thermo-hydraulic 

behavior and performance. 

Bedrock is worth a brief special mention here because its presence at shallow depths beneath 

the pavement structure may have a significant impact on pavement construction (Chapter 8), 

design (Chapters 5 and 6), and performance (Chapter 6). While the precise measurement of 

bedrock properties like stiffness is seldom if ever warranted, the effect of shallow (less than 3 

m (10 ft) depth) bedrock on pavement analyses must be considered. This is especially true for 

FWD backcalculation procedures used to estimate in-situ material stiffnesses in rehabilitation 

design (see Chapter 4). 

Drainage 

As early as 1820, John McAdam noted that, regardless of the thickness of the structure, many 

roads in Great Britain deteriorated rapidly when the subgrade was saturated: 

“The roads can never be rendered thus perfectly secure until the following principles be fully 

understood, admitted and acted upon: namely, that it is the native soil which really supports 

the weight of traffic: that while it is preserved in a dry state, it will carry any weight without 

sinking, and that it does in fact carry the road and the carriages also; that this native soil must 

previously be made quite dry, and a covering impenetrable to rain must then be placed over 

it, to preserve it in that dry state; that the thickness of a road should only be regulated by the 

quantity of material necessary to form such impervious covering, and never by any reference 

to its own power of carrying weight. 

The erroneous opinion so long acted upon and so tenaciously adhered to, that by placing a 

large quantity of stone under the roads, a remedy will be found for the sinking into wet clay, 

or other soft soils, or in other words, that a road may be made sufficiently strong artificially, 

to carry heavy carriages, though the subsoil be in a wet state, and by such means to avert the 

inconveniences of the natural soil receiving water from rain or other causes, has produced 

most of the defects of the roads of Great Britain.” (McAdam, 1820)

It is widely recognized today that excess moisture in pavement layers, when combined with 

heavy traffic and moisture-susceptible materials, can reduce service life. Freezing of this 

moisture often causes additional performance deterioration. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 10 May 2006 

Moisture in the subgrade and pavement structure can come from many different sources 

(Figure 3-3). Water may seep upward from a high groundwater table, or it may flow laterally 

from the pavement edges and shoulder ditches. However, the most significant source of 

excess water in pavements is typically infiltration through the surface. Joints, cracks, 

shoulder edges, and various other defects in the surface provide easy access paths for water. 

A major objective in pavement design is to prevent the base, subbase, subgrade, and other 

susceptible paving materials from becoming saturated or even exposed to constant high 

moisture levels in order to minimize moisture-related problems. The three main approaches 

for controlling or reducing moisture problems follow below: 

• Prevent moisture from entering the pavement system. Techniques for preventing 

moisture from entering the pavement include providing adequate cross slopes and 

longitudinal slopes for rapid surface water runoff and sealing all cracks, joints, and 

other discontinuities to minimize surface water infiltration. 

• Use materials and design features that are insensitive to the effects of moisture.

Materials that are relatively insensitive to moisture effects include granular materials 

with few fines, cement-stabilized and lean concrete bases, and asphalt stabilized base 

materials.1

 Appropriate design features for rigid pavements include dowel bars and 

widened slabs to reduce faulting and inclusion of a subbase between the base and 

subgrade to reduce erosion and promote bottom drainage. Design features for flexible 

pavements include full width paving to eliminate longitudinal joints, asphalt 

stabilized base layers, and use of a subbase to reduce erosion and promote drainage.

• Quickly remove moisture that enters the pavement system. A variety of different 

drainage features are available for removing excess moisture. Features such as 

underdrains and ditches are designed to permanently lower the water table under the 

pavement, whereas other features, such as permeable bases and edge drains, are 

designed to remove surface infiltration water. 

Pavement drainage design is described in more detail in Chapter 7. Additional detail can be 

found in Christopher and McGuffey (1997) and in the reference manual for FHWA NHI 

Course 131026 Pavement Subsurface Drainage Design. 

 

1

 Moisture-induced stripping of asphalt stabilized materials may be a problem for some aggregates and some 

asphalt cements. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 11 May 2006 

From edge

Seepage from

high ground

Through surface discontinuities

Water table

Capillary 

action Vapor

movements

Rising water table

Figure 3-3. Sources of moisture in pavement systems (NHI 13126). 

Special Conditions 

Special problem soil conditions include frost heave, swelling or expansive soils, and 

collapsible soils. 

Freeze/thaw: The major effect is the weakening that occurs during the spring thaw period. 

Frost heave during the winter can also cause a severe reduction in pavement serviceability 

(increased roughness). The requirements for freeze/thaw conditions are (a) a frost-susceptible 

soil; (b) freezing temperatures; and (c) availability of water. 

Swelling or expansive soils: Swelling refers to the localized volume changes in expansive 

roadbed soils as they absorb moisture. It is estimated that the damage to pavements caused by 

expansive soils is well over $1 billion each year. 

 

Collapsible soils: Collapsible soils have metastable structures that exhibit large volume 

decreases when saturated. Silty loess deposits are the most common type of collapsible soil. 

Native subgrades of collapsible soils must be soaked with water prior to construction and 

rolled with heavy compaction equipment. If highway embankments are to be constructed 

over collapsible soils, special remedial measures may be required to prevent large-scale 

cracking and differential settlement. 

Identification of potential problem soils is a primary objective of the pavement geotechnical 

design. Design approaches and mitigation measures for these special conditions are detailed 

in Chapter 7. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 12 May 2006 

Soil Improvement 

The natural soils at a project site are often unsuitable for use in the pavement structure. They 

may have inappropriate gradation, inadequate strength and/or stiffness, or insufficient 

stability against swelling. Some of these deficiencies can be addressed by blending two or 

more soils and/or providing adequate mechanical stabilization (compaction). Other 

deficiencies, particularly for subgrades, may require the mixing of stabilizing admixtures 

such as bituminous binders or lime, Portland cement, or other pozzolanic materials with the 

natural soil. Although the primary purpose of these admixtures is usually to improve the 

strength and stiffness of the soil, they can also be used to improve workability, reduce 

swelling, and provide a suitable construction platform. Geosynthetic products can also be 

used as soil reinforcement and as filter and drainage layers. 

In extreme soft soil conditions, special ground improvement techniques may be required, 

such as wick drains, piled embankments, surcharge, lightweight fill (e.g., geofoam), etc. 

These techniques are typically evaluated by the geotechnical unit as part of the roadway 

design. The methods are discussed briefly in Chapter 7. 

A summary of the stabilization methods most commonly used in pavements, the types of 

soils for which they are most appropriate, and their intended effects on soil properties is 

provided in Table 3-2. Design inputs for improved soils will be covered in Chapter 5 and 

details for selection and implementation of treatment techniques for specific problems will be 

covered in Chapter 7. Compaction, one of the key geotechnical issues in pavement design 

and construction, is covered in Chapter 5 (determination of design inputs) and Chapter 8 

(construction issues). 

3.4 SENSITIVITY OF PAVEMENT DESIGN TO GEOTECHNICAL FACTORS 

While the most significant layer for pavement performance is the surface course, the 

geotechnical layers are intimately intertwined in the pavement design. For example, the 

stiffness or strength of the subgrade soil is a direct input to most pavement design 

procedures, and its impact on the structural design can thus be evaluated quantitatively. 

Figure 3-4 shows the influence of the subgrade California Bearing Ratio (CBR—see Chapter 

5) on the required thickness and structural capacity contribution for the unbound granular 

base layer in a flexible pavement designed according to the 1993 AASHTO procedures (see 

Section 3.5.2). The contribution of the granular base to the overall structural capacity varies 

from 50% for a low subgrade CBR value of 2 to essentially zero at a high CBR value of 50. 

The influence of base layer quality on the pavement structural design is similarly shown in 

Figure 3-5. Additional examples of the sensitivity of pavement design to various 

geotechnical factors are provided in Chapter 5. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 13 May 2006 

A good indicator of the overall sensitivity of pavement design to geotechnical inputs. is the 

impact of subgrade support on the cost of the pavement, as shown in Figure 3-6. For 

example, at a traffic loading of 10 million ESALs and a subgrade CBR of 8, the cost per 

1000 square yards (850 m2) of surface area is approximately $9,800 for the asphalt layer and 

$3,000 dollars for the underlying base and granular borrow, for a total pavement cost of 

$12,800 per 1000 square yards of surface area. If the subgrade CBR value were only 4, the 

same area of pavement section would cost $15,600, or more than 20% more. 

It is also important to recognize at the outset that while many of the geotechnical factors 

influencing pavement performance can be incorporated explicitly in the design process, other 

important considerations can not. For example, the potential for a slope failure beneath a 

pavement constructed on a side hill cut is not generally considered as part of “pavement 

design,” even though such a failure can be much more devastating to the pavement than 

inadequate subgrade stiffness (see Figure 3-7). 

Table 3-2. Stabilization methods for pavements (from Rollings and Rollings, 1996). 

Method Soil Effect Remarks 

Blending Moderately plastic None Too difficult to mix 

 

 Others Improve gradation 

 Reduce plasticity 

 Reduce breakage 

Lime Plastic Drying Rapid 

 Immediate strength gain Rapid 

 Reduce plasticity Rapid 

 Coarsen texture Rapid 

 Long-term pozzolanic cementing Slow 

 Coarse with fines Same as with plastic soils Dependent on quantity 

of plastic fines 

 Nonplastic None 

Cement Plastic Similar to lime Less pronounced 

 Cementing of grains Hydration of cement 

 Coarse Cementing of grains Hydration of cement 

Bituminous Coarse Strengthen/bind, waterproof Asphalt cement or liquid 

asphalt 

 Some fines Same as coarse Liquid asphalt 

 Fine None Can’t mix 

Pozzolanic and slags Silts and coarse Acts as a filler Denser and stronger 

 Cementing of grains Slower than cement 

Misc. methods Variable Variable Depends on mechanism 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 14 May 2006 

0

5

10

15

20

25

30

0 20 40 60

Subgrade CBR

Req'd Base Thickness (in)

0%

10%

20%

30%

40%

50%

60%

Base % of Overall Structural

Capacity

Figure 3-4. Impact of subgrade strength on pavement structural design (AASHTO 93 Design 

Guide: W18=10M, 85% reliability, So=0.4, ∆PSI=1.7, a1=0.44, a2=0.14, m2=1). 

6

7

8

9

0 50 100 150

Base CBR

Required AC Thickness (in)

30%

40%

50%

Base % of Overall Structural

Capacity

Figure 3-5. Impact of base strength on pavement structural design design (AASHTO 93 

Design Guide: W18=10M, 85% reliability, So=0.4, ∆PSI=1.7, a1=0.44, m2=1, 

subgrade CBR=4). 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 15 May 2006 

Notes: 

• Assumed unit costs are: asphalt - $1.25/inch thickness; untreated base - $0.30/inch thickness; granular 

borrow - $0.20/inch thickness. 

• Thicknesses used in cost estimating are based on 90% reliability. 

• Minimum granular borrow or base thickness is 6 in. 

• Thickness/cost of asphalt only varies with ESALs because base support value is constant. 

• Units: 1 inch = 25 mm; 1 yd2

 = 0.85 m2.

Figure 3-6. Approximate pavement cost for varying subgrade support conditions (B.Vandre, 

personal communication). 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 16 May 2006 

Figure 3-7. Slope failure beneath road pavement (www.geoengineer.com). 

3.5 INCORPORATION OF GEOTECHNICAL FACTORS IN PAVEMENT DESIGN 

3.5.1 Pavement Design Methodologies 

The terms empirical design, mechanistic design, and mechanistic-empirical design are 

frequently used to identify general approaches toward pavement design. The key features of 

these design methodologies are described in the following subsections. 

Empirical Design 

An empirical design approach is one that is based solely on the results of experiments or 

experience. Observations are used to establish correlations between the inputs and the 

outcomes of a process – e.g., pavement design and performance. These relationships 

generally do not have a firm scientific basis, although they must meet the tests of engineering 

reasonableness (e.g., trends in the correct directions, correct behavior for limiting cases, etc.). 

Empirical approaches are often used as an expedient when it is too difficult to define 

theoretically the precise cause-and-effect relationships of a phenomenon. 

The principal advantages of empirical design approaches are that they are usually simple to 

apply and are based on actual real-world data. Their principal disadvantage is that the 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 17 May 2006 

validity of the empirical relationships is limited to the conditions in the underlying data from 

which they were inferred. New materials, construction procedures, and changed traffic 

characteristics cannot be readily incorporated into empirical design procedures. 

Mechanistic Design 

The mechanistic design approach represents the other end of the spectrum from the empirical 

methods. The mechanistic design approach is based on the theories of mechanics to relate 

pavement structural behavior and performance to traffic loading and environmental 

influences. The mechanistic approach for rigid pavements has its origins in Westergaard's 

development during the 1920s of the slab on subgrade and thermal curling theories to 

compute critical stresses and deflections in a PCC slab. The mechanistic approach for 

flexible pavements has its roots in Burmister’s development during the 1940s of multilayer 

elastic theory to compute stresses, strains, and deflections in pavement structures. 

A key element of the mechanistic design approach is the accurate prediction of the response 

of the pavement materials – and, thus, of the pavement itself. The elasticity-based solutions 

by Boussinesq, Burmister, and Westergaard were an important first step toward a theoretical 

description of the pavement response under load. However, the linearly elastic material 

behavior assumption underlying these solutions means that they will be unable to predict the 

nonlinear and inelastic cracking, permanent deformation, and other distresses of interest in 

pavement systems. This requires far more sophisticated material models and analytical tools. 

Much progress has been made in recent years on isolated pieces of the mechanistic 

performance prediction problem. The Strategic Highway Research Program during the early 

1990s made an ambitious but, ultimately, unsuccessful attempt at a fully mechanistic 

performance system for flexible pavements. To be fair, the problem is extremely complex; 

nonetheless, the reality is that a fully mechanistic design approach for pavement design does 

not yet exist. Some empirical information and relationships are still required to relate theory 

to the real world of pavement performance. 

Mechanistic-Empirical Design Approach 

As its name suggests, a mechanistic-empirical approach to pavement design combines 

features from both the mechanistic and empirical approaches. The mechanistic component is 

a mechanics-based determination of pavement responses, such as stresses, strains, and 

deflections due to loading and environmental influences. These responses are then related to 

the performance of the pavement via empirical distress models. For example, a linearly 

elastic mechanics model can be used to compute the tensile strains at the bottom of the 

asphalt layer due to an applied load; this strain is then related empirically to the accumulation 

of fatigue cracking distress. In other words, an empirical relationship links the mechanistic 

response of the pavement to its expected or observed performance. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 18 May 2006 

The development of mechanistic-empirical design approaches dates back at least four 

decades. Huang (1993) notes that Kerkhoven and Dormon (1953) were the first to use the 

vertical compressive strain on top of the subgrade as a failure criterion for permanent 

deformation in flexible pavement systems, while Saal and Pell (1960) recommended the use 

of horizontal tensile strain at the bottom of the AC layer to minimize fatigue cracking. 

Likewise, Barenberg and Thompson (1990) note that mechanistic-based design procedures 

for concrete pavements have also been pursued for many years. Several design 

methodologies based on mechanistic-empirical concepts have been proposed over the years, 

including the Asphalt Institute procedure (Shook et al., 1982) for flexible pavements, the 

PCA procedure for rigid pavements (PCA, 1984), the AASHTO 1998 Supplemental Guide 

(AASHTO, 1998) for rigid pavements, and the NCHRP 1-26 procedures (Barenberg and 

Thompson, 1990, 1992) for both flexible and rigid pavements. Some mechanistic-empirical 

design procedures have also been implemented at the state level (e.g., Illinois, Kentucky, 

Washington, and Minnesota; see also Newcomb and Birgisson, 1999). 

3.5.2 The AASHTO Pavement Design Guides 

The AASHTO Guide for Design of Pavement Structures is the primary document used to 

design new and rehabilitated highway pavements. The Federal Highway Administration’s 

1995-1997 National Pavement Design Review found that some 80 percent of states use the 

1972, 1986, or 1993 AASHTO Guides2

 (AASHTO, 1972; 1986; 1993). Of the 35 states that 

responded to a 1999 survey by Newcomb and Birgisson (1999), 65% reported using the 1993 

AASHTO Guide for both flexible and rigid pavement designs. 

All versions of the AASHTO Design Guide are empirical methods based on field 

performance data measured at the AASHO Road Test in 1958-60, with some theoretical 

support for layer coefficients and drainage factors. The overall serviceability of a pavement 

during the original AASHO Road Test was quantified by the Present Serviceability Rating 

(PSR; range = 0 to 5), as determined by a panel of highway raters. This qualitative PSR was 

subsequently correlated with more objective measures of pavement condition (e.g., cracking, 

patching, and rut depth statistics for flexible pavements) and called the Pavement 

Serviceability Index (PSI). Pavement performance was represented by the serviceability 

history of a given pavement – i.e., by the deterioration of PSI over the life of the pavement 

(Figure 3-8). Roughness is the dominant factor in PSI and is, therefore, the principal 

component of performance under this measure. 

 

2

 A 1998 supplement to the 1993 AASHTO Guide (AASHTO, 1998) provides optional alternative methods for 

rigid pavement and rigid pavement joint design procedures based on recommendations from NCHRP Project 

1-30 and verification studies conducted using the LTPP database. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 19 May 2006 

 

Figure 3-8. Pavement serviceability in the AASHTO Design Guides (AASHTO, 1993). 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 20 May 2006 

Each successive version of the AASHTO Design Guide has introduced more and more 

sophisticated geotechnical concepts into the pavement design process. The 1986 Guide in 

particular introduced important refinements for materials input parameters, design reliability, 

and drainage factors, as well as empirical procedures for rehabilitation design. Enhancements 

were made to both the flexible and rigid design methodologies, although the impact is 

perhaps more significant for flexible pavements because of the greater contribution of the 

unbound layers to the structural capacity of these systems. The evolution of geotechnical 

considerations in the various versions of the AASHTO Design Guides is highlighted in the 

following sections. 

1961 Interim Guide 

The 1961 Interim AASHO Pavement Design Guide contained the original empirical 

equations relating traffic, pavement performance, and structure, as derived from the data 

measured at the AASHO Road Test (HRB, 1962). These equations were specific to the 

particular foundation soils, pavement materials, and environmental conditions at the test site 

in Ottawa, Illinois. The empirical equation for the flexible pavements at the AASHO Road 

Test is 

( ) ( )

( ) 18 5.19

log 4.2 / 4.2 1.5 log 9.36log( 1) 0.20

0.4 1094 / 1

t p W SN

SN

 − −    = +− +

+ +

 (3.1) 

in which W18 = number of 18 kip equivalent single axle loads (ESALs) 

 pt = terminal serviceability at end of design life 

 SN = structural number 

Equation (3.1) must be solved implicitly for the structural number SN as a function of the 

other input parameters. The structural number SN is defined as 

11 2 2 33 SN a D a D a D =+ + (3.2) 

in which D1, D2, and D3 are the thicknesses (inches) of the surface, base, and subbase layers, 

respectively, and a1, a2, and a3 are the corresponding layer coefficients. For the materials 

used in the majority of the flexible pavement sections at the AASHO Road Test, the values 

for the layer coefficients were determined as a1=0.44, a2=0.14, and a3=0.11. Note that there 

may be many combinations of layer thicknesses that can provide satisfactory SN values; cost 

and other issues must be considered as well to determine the final design layer structure. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 21 May 2006 

The corresponding empirical design equation relating traffic, performance, and structure for 

the rigid pavements at the AASHO Road Test is 

( )

( ) 18 8.46 7

log 4.5 /(4.5 1.5) log 7.35log( 1) 0.06

1 1.624 10 / 1

t p W D

D

 − −    = +− +

+× +

 (3.3) 

in which D is the pavement slab thickness (inches) and the other terms are as defined 

previously. Equation (3.3) must be solved implicitly for the slab thickness D as a function of 

the other input parameters. 

Since Eqs. (3.1) through (3.3) are for the specific foundation soils, materials, and 

environmental conditions at the AASHO Road Test site, there are no geotechnical or 

environmental inputs to determine. This clearly limited the applicability of these design 

equations to other sites and other conditions and was the primary motivation behind the 

development of the 1972 Interim Guide. 

1972 Interim Guide 

The 1972 Interim Design Guide (AASHTO, 1972) was the first attempt to extend the 

findings from the AASHO Road Test to foundation, material, and environmental conditions 

different from those at the test site. This was done through the introduction of several new 

features for the flexible and rigid pavement design. A rudimentary overlay design procedure 

was also included in the 1972 Interim Guide. 

Flexible Pavements

The major new features added to the 1972 Interim Guide to extend its flexible pavement 

design methodology to conditions other than those at the AASHO Road Test were: 

• An empirical soil support scale to reflect the influence of local foundation soil 

conditions in Equation (3.1). This soil support scale ranged from 1 to 10, with a soil 

support value Si of 3 corresponding to the silty clay foundation soils at the AASHO 

Road Test site and the upper value of 10 corresponding to crushed rock base 

materials. All other points on the scale were assumed from experience, with some 

limited checking through theoretical computations. It is important to note that “the 

units of soil support, represented by the soil support scale, have no direct relationship 

to any procedure for testing soils” (AASHTO, 1972) and that it was left up to each 

agency to determine correlations between soil support and material testing 

procedures. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 22 May 2006 

• An empirical regional factor R to provide an adjustment to the structural number SN

in Equation (3.2) for local environmental and other considerations. Values for the 

regional factor were estimated from serviceability reduction rates in the AASHO 

Road Test. These estimates varied between 0.1 and 4.8, with an annual average value 

of about 1.0. Recommended values for the regional factor based on the AASHO Road 

Test results are summarized in Table 3-3. However, the Guide cautions that “the 

regional factor may not adjust for special conditions, such as serious frost conditions, 

or other local problems” and that “considerable judgment must still be exercised in 

evaluating [environmental] effects and in selecting an appropriate regional factor for 

design” (AASHTO, 1972). 

• Guidelines for estimating structural layer coefficients a1, a2, and a3 in Equation (3.2) 

for materials other than those at the AASHO Road Test. These guidelines were based 

primarily on a survey of state highway agencies regarding the values for the layer 

coefficients that they were currently using in design for various materials. Ranges of 

layer coefficient values reported in this survey are summarized in Table 3-4. The 

Guide recommends that “Because of widely varying environments, traffic, and 

construction practices, it is suggested that each design agency establish layer 

coefficients applicable to its own experience. Careful consideration should be given 

before adoption of values developed by others” (AASHTO, 1972). 

Table 3-3. Recommended values for Regional Factor R (AASHTO, 1972). 

Roadbed Material Condition R 

Frozen to depth of 5” (130 mm) or more (winter) 0.2 to 1.0 

Dry (summer and fall) 0.3 to 1.5 

Wet (spring thaw) 4.0 to 5.0 

Table 3-4. Ranges of structural layer coefficients from agency survey (AASHTO, 1972). 

Coefficient Low Value High Value 

a1 (surface) 0.17 0.45 

a2 (untreated base) 0.05 0.18 

a3 (subbase) 0.05 0.14 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 23 May 2006 

The modified version of Equation (3.1) for flexible pavements implemented in the 1972 

Interim Guide is as follows: 

( ) ( )

( )

18 5.19

log 4.2 / 4.2 1.5 log 9.36log( 1) 0.20 0.40 1094 /( 1)

1 log 0.372 3.0

t

i

p W SN

SN

S

R

 − −    = +− +

+ +

++ −

 (3.4) 

in which R is the regional factor, Si is the soil support value, and the other terms are as 

defined previously. As in the 1961 Interim Guide, the thicknesses for each pavement layer 

are determined as functions of the structural layer coefficients using Equation (3.2) and the 

required SN determined from Equation (3.4). The principal geotechnical inputs in the design 

procedure are thus the soil support value Si for the subgrade and the structural layer 

coefficients a2, a3 and thicknesses D2, D3 for the base and subbase layers, respectively. 

Rigid Pavements

Only one major new feature was added to the 1972 Interim Guide to extend its rigid 

pavement design methodology to conditions other than those at the AASHO Road Test. This 

was the use of the Spangler/Westergaard theory for stress distributions in rigid slabs to 

incorporate the effects of local foundation soil conditions. The foundation soil conditions are 

characterized by the overall modulus of subgrade reaction k, which is a measure of the 

stiffness of the foundation soil.3

 

Interestingly, the modifications made to the rigid pavement design procedure in the 1972 

Interim Guide do not include a regional factor for local environmental conditions similar to 

that implemented in the flexible design procedure. The explanation offered for this was that 

“it was not possible to measure the effect of variations in climate conditions over the twoyear life of the pavement at the Road Test site” (AASHTO, 1972). 

The modified version of Equation (3.3) for rigid pavements implemented in the 1972 Interim 

Guide is as follows: 

 

3

 Although the 1972 Guide does not state this explicitly, it is presumed that the k value for design includes the 

influence of the subbase layer, if present, as well as the subgrade soil. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 24 May 2006 

( ) ( )

( )

( ) ( )

18 8.46 7

0.75

0.25 0.75

log 4.5 / 4.5 1.5 log 7.3log( 1) 0.06 1 1.624 10 / 1

1.132 4.22 0.32 log 215.63 18.42 / /

t

c

t

c

p W D

D

S D

p J D Ek

  − −   = +− +

+× +

     − + −         −    

 (3.5) 

in which Sc is the modulus of rupture and Ec is the modulus of elasticity for the concrete 

(psi), J is an empirical joint load transfer coefficient, k is the modulus of subgrade reaction 

(pci), and all other terms are as defined previously. Note that k, the principle geotechnical 

input in the 1972 rigid pavement design procedure, is a “gross” k defined as load (stress) 

divided by deflection, and as such it includes both elastic and inelastic response of the 

foundation soil. 

For the design of reinforcement in jointed reinforced concrete pavements (JRCP), one 

additional geotechnical design input is required: the friction coefficient between the slab and 

the subbase/subgrade. 

Sensitivity to Geotechnical Inputs

The sensitivity of the pavement design to the new geotechnical properties in the 1972 

AASHTO Guide can be illustrated via some simple examples. Figure 3-9 shows the variation 

of the required structural number SN with the soil support factor Si for a three-layer (asphalt, 

base, subgrade) flexible pavement system with design traffic W18 = 10 million, regional 

factor R = 1 (i.e., the environmental conditions at the AASHO Road Test), and terminal 

serviceability pt = 2.5. Also shown in the figure is the pavement cost index as a function of 

soil support, assuming that asphalt is twice as expensive per inch of thickness than crushed 

stone base and that the cost index equals 1 at Si = 3 (i.e., the foundation conditions in the 

AASHO Road Test). Figure 3-10 shows similar variations of SN and cost index with the 

regional factor R for the same three-layer flexible pavement and Si = 3. The results for this 

example suggest that the pavement design and cost is quite sensitive to soil support (cost 

index varying between 0.3 and 1.3 over the range of valid Si values), but only moderately 

sensitive to the regional factor (cost index varying by about +20% over the range of valid R

values). 

The sensitivity of rigid pavement slab thickness to the modulus of subgrade reaction k is 

summarized in Figure 3-11 for three different concrete compressive strength values. The 

results confirm the conventional wisdom that rigid pavement designs are relatively 

insensitive to foundation stiffness. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 25 May 2006 

1

2

3

4

5

6

0 2 4 6 8 10 12

Soil Support Si

SN

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Cost Index

SN

Cost Index

Figure 3-9. Sensitivity of 1972 AASHTO flexible pavement design to foundation support 

quality. 

3

4

5

6

0246

Regional Factor R

SN

0.8

0.9

1.0

1.1

1.2

1.3

Cost Index

SN

Cost Index

Figure 3-10. Sensitivity of 1972 AASHTO flexible pavement design to environmental 

conditions. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 26 May 2006 

7.0

7.5

8.0

8.5

9.0

9.5

10.0

0 100 200 300 400 500 600

Modulus of Subgrade Reaction k (pci)

Slab Thickness D (in)

4000 psi

AASHO Road Test (5430 psi)

6000 psi

Figure 3-11. Sensitivity of 1972 AASHTO rigid pavement design to foundation stiffness 

(1 in = 25 mm; 1 pci = 284 MN/m3

). 

1986 Guide 

The 1986 AASHTO Design Guide (AASHTO, 1986) retained the basic approach from the 

1972 Interim Guide but added several new features. Key among these are a more rational 

characterization of subgrade and unbound materials in terms of the resilient modulus, the 

explicit consideration of the benefits of pavement drainage (and conversely the consequences 

of poor drainage), and better treatment of environmental influences on pavement 

performance. Additional significant enhancements in the 1986 Guide include the 

incorporation of a reliability factor into the design, expanded treatment of rehabilitation (both 

with and without overlays), and life-cycle cost analysis. 

The geotechnical-related enhancements in the 1986 Guide include the following: 

Flexible and Rigid Pavements

• Use of the resilient modulus MR (AASHTO T272) as a stiffness parameter for 

characterizing the soil support provided by the subgrade. The resilient modulus MR is 

a measure of the elastic stiffness of the soil recognizing certain nonlinear 

characteristics. It is a basic material property that can be measured directly using 

established laboratory test protocols, evaluated in-situ from nondestructive tests, or 

estimated using various empirical relations as detailed later in Chapter 5. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 27 May 2006 

• Improvements in incorporating the effects of environment on pavement performance. 

Specific emphasis is given to frost heave, thaw-weakening, and swelling of subgrade 

soils. The enhancements in the 1986 Guide for environmental effects include 

o The explicit separation of total serviceability loss ∆PSI into load- and 

environment-related components: 

∆ =∆ +∆ +∆ PSI PSI PSI PSI TR SW FH (3.6) 

in which ∆PSITR, ∆PSISW and ∆PSIFH are the components of serviceability loss 

attributable to traffic, swelling, and frost heave, respectively. 

o Estimation of an effective resilient modulus for the roadbed that reflects the 

seasonal variations in subgrade stiffness. 

• Incorporation of reliability considerations to reflect the inevitable uncertainty and 

variability in the design inputs and the importance of the project. Reliability is 

incorporated in the design through factors that increase the design traffic level. 

Flexible Pavements

The geotechnical-related enhancements to the flexible pavement design procedures in the 

1986 AASHTO Guide included the following: 

• Use of the resilient modulus for determining the structural layer coefficients for both 

stabilized and unstabilized unbound materials in flexible pavements. The structural 

layer coefficients a2 and a3 for base and subbase materials are estimated via 

correlations with resilient modulus; these regressions are detailed later in Chapter 5, 

Section 5.4.5. Nomographs that relate layer coefficients for unstabilized and 

stabilized base and subbase materials to other strength and stiffness properties are 

also provided in the 1993 Guide. It is important to remember, however, that these 

relations for the structural layer coefficients are largely empirical and are based 

primarily on engineering judgment with only limited amounts of data. 

• Guidance for the design of subsurface drainage systems and modifications to the 

flexible pavement design equations to take advantage of improvements in 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 28 May 2006 

performance due to good drainage. The benefits of drainage are incorporated into the 

structural number via empirical drainage coefficients: 

11 2 22 333 SN a D a D m a D m =+ + (3.7) 

in which m2 and m3 are the drainage coefficients for the base and subbase layers, 

respectively, and all other terms are as defined previously. The empirical values for 

mi, which are specified in terms of quality of drainage and the estimated percentage of 

time the layer will be near saturation, range from 0.4 to 1.4. Section 5.5.1 in Chapter 

5 provides the details for estimating the mi input values for design. The development 

of these values can be found in Appendix DD of the 1986 AASHTO Guide. 

The modified version of Equation (3.4) for flexible pavements implemented in the 

1986 Guide is as follows: 

( ) ( )

( )

( )

log 9.36log 1 0.20 10 18 0 10

log10 4.2 1.5 2.32log 8.07 1094 10 0.40 5.19 1

W Z S SN R

PSI

MR

SN

= + +−

  ∆

    − + +−

+

+

 (3.8) 

in which ZR is a function of the design reliability level, S0 is a measure of the overall 

uncertainty or variability of the design inputs and performance prediction, MR is the 

subgrade resilient modulus, and the other terms are as defined previously. Equation 

(3.7) is used to determine the layer thicknesses required to achieve the total SN value 

required by Equation (3.8). 

In summary, the explicit geotechnical inputs in the 1986 flexible design procedure are the 

• seasonally adjusted subgrade resilient modulus MR, 

• base and subbase resilient moduli EBS and ESB (used to determine the a2 and a3

structural layer coefficients), 

• base and subbase drainage coefficients m2 and m3, and 

• base and subbase layer thicknesses D2 and D3. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 29 May 2006 

Rigid Pavements

The geotechnical-related enhancements to the rigid pavement design procedures in the 1986 

AASHTO Guide included the following: 

• Guidance for the design of subsurface drainage systems and modifications to the rigid 

pavement design procedure to take advantage of improvements in performance due to 

good drainage. The benefits of drainage are incorporated in the rigid pavement design 

equation via an empirical drainage coefficient Cd. The empirical values for Cd, which 

are specified in terms of quality of drainage and the estimated percentage of time the 

pavement will be near saturation, range from 0.7 to 1.25. Section 5.5.1 in Chapter 5 

provides the details for estimating the Cd input values for design. 

• Enhancements to the procedures for estimating a composite modulus of subgrade 

reaction that explicitly incorporate the influence of subbase type and thickness, the 

presence of shallow bedrock, and seasonal variations in subgrade and subbase 

resilient moduli. 

• Adjustment of the design equations to account for the potential loss of support arising 

from subbase erosion and/or differential vertical soil movements. A loss of support 

factor LS is used to determine the effective k value for the foundation soil. Section 

5.4.6 in Chapter 5 summarizes the recommended values for LS in the 1986 AASHTO 

Guide for various subbase material types. 

The modified version of Equation (3.5) for rigid pavements implemented in the 1986 Guide 

is as follows: 

( ) ( )

( )

( ) ( )

( )

10 18 10

0.75 10

7 10

0.75

8.46 0.25

log 7.35log 1 0.06

log 1.132 4.5 1.5 4.22 0.32 log 1.64x10 18.42 1 215.63 1 /

R o

c d

t

c

W ZS D

PSI

SC D

p

J D D E k

= + +−

    ∆     −  −    + +−     +     − +     

 (3.9) 

in which Cd is the drainage coefficient and the other terms are as defined previously. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 30 May 2006 

In summary, the explicit geotechnical inputs in the 1986 rigid pavement design procedure 

are: 

• The seasonally adjusted effective modulus of subgrade reaction k. This in turn is a 

function of the seasonally adjusted values for the subgrade and subbase resilient 

moduli MR and ESB, the thickness of the subbase DSB, the subgrade depth to rigid 

foundation DSG, and the loss of support factor LS. 

• The drainage coefficient Cd. 

• A friction factor related to the frictional resistance between the slab and 

subbase/subgrade for reinforcement design in JRCP pavements. 

Sensitivity to Geotechnical Inputs

The key geotechnical inputs in the 1986 AASHTO design procedure for flexible pavements 

are 

• foundation stiffness, as characterized by the subgrade resilient modulus (MR), and 

• moisture and drainage, as characterized by the layer drainage coefficients (mi). 

For rigid pavements, the key geotechnical inputs are 

• foundation stiffness, as characterized by the resilient moduli of the subgrade (MR) and 

granular subbase (ESB) and the thickness of the subbase (DSB). 

• erodibility of the granular subbase, as characterized by the Loss of Support factor

(LS). 

• moisture and drainage, as characterized by the drainage coefficient (Cd). 

The sensitivity of the pavement design to the geotechnical inputs in the 1986 AASHTO 

Guide can be illustrated via some simple examples. Table 3-5 summarizes assumed baseline 

design inputs for a typical flexible pavement section. These values (except for traffic) 

generally conform to those at the AASHO Road Test. The variation of required pavement 

structure with subgrade stiffness and drainage for these conditions are summarized in Figure 

3-12 and Figure 3-13, respectively. Also shown in these figures is a pavement cost index, 

which is based on the assumption that asphalt concrete is twice as expensive as crushed stone 

base per inch of thickness; the cost index is normalized to 1.0 at baseline conditions (i.e.,

values in Table 3-5). The vertical cost axes in Figure 3-12 and Figure 3-13 have been kept 

constant in order to highlight the relative sensitivities of cost to subgrade stiffness and 

drainage conditions. The horizontal axes in the figures span the full range of stiffness and 

drainage conditions for flexible pavements. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 31 May 2006 

Both the structural number and pavement cost are highly sensitive to foundation stiffness. As 

shown in Figure 3-12, reducing MR from 20,000 psi (138 MPa, corresponding to a CBR of 

about 30) to 2000 psi (13.8 MPa, corresponding to a CBR value of about 2) results in a 115% 

increase in required total structural number. This translates to a corresponding 170% increase 

in cost. 

From Equation (3.8), it is clear that changing the drainage coefficient m2 for the base layer 

will not affect the total required structural number SN (nor will it directly affect the required 

structural number for each of the layers). However, changes in drainage do directly affect the 

structural effectiveness of the granular material in the base layer and, thus, its thickness and 

cost. As shown in Figure 3-13, reducing m2 from its maximum value of 1.4 to its minimum 

value of 0.4 requires more than a 3-fold increase in required base thickness. This translates to 

a 150% increase in overall pavement structural cost for these example conditions. 

A similar sensitivity analysis can be performed for the rigid pavement design procedure in 

the 1986 AASHTO Guide. Table 3-6 summarizes assumed design inputs for a typical rigid 

pavement section. Again, these values (except for traffic) generally conform to those at the 

AASHO Road Test. The variations of required slab thickness with foundation stiffness, base 

erodibility, and drainage conditions are summarized in Figure 3-14, Figure 3-15, and Figure 

3-16, respectively. The vertical axes in Figure 3-14 through Figure 3-16 have been kept 

constant in order to highlight the relative sensitivities of slab thickness to the respective 

geotechnical inputs. Since rigid pavement cost essentially varies directly with slab thickness, 

a cost index is not included in the figures. The horizontal axes in the figures span the full 

range of stiffness, erodibility, and drainage conditions for rigid pavements. 

Table 3-5. Flexible pavement baseline conditions for 1986 AASHTO sensitivity study.

Input Parameter Design Value 

Traffic (W18) 10x106 ESALs

Reliability 90% 

Reliability factor (ZR) -1.282 

Overall standard error (So) 0.45 

Allowable serviceability deterioration (∆PSI) 1.7 

Subgrade resilient modulus (MR) 3,000 psi (20.7 MPa) 

Granular base resilient modulus (EBS) 30,000 psi (207 MPa) 

Granular base layer coefficient (a2) 0.14

Granular base drainage coefficient (m2) 1.0 

Asphalt concrete layer coefficient (a1) 0.44 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 32 May 2006 

2

3

4

5

6

7

8

0 5000 10000 15000 20000 25000 30000

Resilient Modulus MR (psi)

Structural Number SN

0.0

0.5

1.0

1.5

2.0

2.5

Cost Index

SN

Cost Index

Figure 3-12. Sensitivity of 1986 AASHTO flexible pavement design to subgrade stiffness (1 

psi = 6.9 kPa). 

10

20

30

40

50

60

70

0.4 0.6 0.8 1.0 1.2 1.4

Base Layer Drainage Coefficient m2

Base Thickness D2

0.0

0.5

1.0

1.5

2.0

2.5

Cost Index

Base Thickness

Cost Index

Figure 3-13. Sensitivity of 1986 AASHTO flexible pavement design to drainage conditions 

(1 inch = 25 mm). 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 33 May 2006 

Figure 3-14 clearly shows that slab thickness is quite insensitive to foundation stiffness. This 

conforms to conventional wisdom, and in fact is one of the reasons that rigid pavements are 

often considered when foundation soils are very poor. Erodibility of the granular subbase is 

somewhat more important. As shown in Figure 3-15, increasing LS from 0 (least erodible) to 

3 (most erodible) results in an additional 1.0 inch (25 mm) of required slab thickness. By far 

the most important rigid pavement geotechnical input is the moisture/drainage condition. As 

shown in Figure 3-16, decreasing the drainage coefficient Cd from its maximum value of 1.25 

to its minimum value of 0.7 results in a 3.5 inch (87.5 mm) or 35% increase in required slab 

thickness for these example conditions. 

Table 3-6. Rigid pavement baseline conditions for 1986 AASHTO sensitivity study. 

Input Parameter Design Value 

Traffic (W18) 10x106 ESALs

Reliability 90% 

Reliability factor (ZR) -1.282 

Overall standard error (So) 0.35 

Allowable serviceability deterioration (∆PSI) 1.9 

Terminal serviceability level (pt) 2.5 

Subgrade resilient modulus (MR) 3,000 psi (20.7 MPa) 

Granular subbase resilient modulus (ESB) 30,000 psi (207 MPa)

Drainage coefficient (Cd) 1.0 

Loss of Support (LS) 1.0 

PCC modulus of rupture (Sc') 690 psi (4.8 MPa) 

PCC modulus of elasticity (Ec) 4.2x106

 psi (29 GPa) 

Joint load transfer coefficient (J) 4.1 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 34 May 2006 

9

10

11

12

13

14

0 10000 20000 30000

Resilient Modulus MR (psi)

Slab Thickness D (in)

0

100

200

300

400

Subgrade Reaction kdesign (pci)

D

kdesign

Figure 3-14. Sensitivity of 1986 AASHTO rigid pavement design to subgrade stiffness 

(1 inch = 25 mm; 1 psi = 6.9 kPa; 1 pci = 284 MN/m3

). 

9

10

11

12

13

14

0123

Loss of Support LS

Slab Thickness D (in)

0

100

200

300

400

Subgrade Reaction kdesign (pci)

D

kdesign

Figure 3-15. Sensitivity of 1986 AASHTO rigid pavement design to subbase erodibility 

(1 inch = 25 mm; 1 pci = 284 MN/m3

). 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 35 May 2006 

9

10

11

12

13

14

0.7 0.9 1.1 1.3

Drainage Coefficient Cd

Slab Thickness D (in)

Figure 3-16. Sensitivity of 1986 AASHTO rigid pavement design to drainage conditions 

(1 inch = 25 mm). 

Another of the new parameters introduced in the 1986 Design Guide is design reliability. The 

target reliability level is set by agency policy; Table 3-7 summarizes common 

recommendations for design reliability for different road categories. Although reliability is 

not strictly a geotechnical parameter, it is useful to examine the sensitivity of pavement 

designs to the target reliability level. Figure 3-17 and Figure 3-18 summarize the sensitivity 

of the example flexible and rigid pavement designs (design inputs in Tables 3-5 and 3-6) to 

the design reliability level. It is clear from these figures that the required pavement structure 

is quite sensitive to the design reliability level, especially for the higher reliability levels. 

Increasing the design reliability level from 50% to 99.9% increases both the required SN and 

cost for flexible pavements by approximately 50% for these example conditions. The 

increase in required slab thickness for rigid pavements is of a similar magnitude. These 

increases in design structure in essence correspond to a safety factor based on agency policy 

for the design reliability level. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 36 May 2006 

Table 3-7. Suggested levels of reliability for various functional classifications 

(AASHTO 1986). 

Recommended level 

Functional classification of reliability (%) 

Urban Rural 

Interstate and other freeways 85-99.9 80-99.9 

Principal arterials 80-99 75-95 

Collectors 80-95 75-95 

Local 50-80 50-80 

Note: Results based on a survey of AASHTO Pavement Design Task 

Force. 

2

3

4

5

6

7

8

9

50 60 70 80 90 100

Reliability (%)

Structural Number SN

0.0

0.5

1.0

1.5

2.0

2.5

Cost Index

SN

Cost Index

Figure 3-17. Sensitivity of 1986 AASHTO flexible pavement design to reliability level. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 37 May 2006 

9

10

11

12

13

14

15

50 60 70 80 90 100

Reliability (%)

Slab Thickness D (in)

Figure 3-18. Sensitivity of 1986 AASHTO rigid pavement design to reliability level 

(1 inch = 25 mm). 

1993 Guide 

The major additions to the 1993 version of the AASHTO Pavement Design Guide 

(AASHTO, 1993) were in the areas of rehabilitation designs for flexible and rigid pavement 

systems using overlays. The only significant change to the geotechnical aspects of pavement 

design was the increased emphasis on nondestructive deflection testing for evaluation of the 

existing pavement and backcalculation of layer moduli. All other geotechnical aspects are 

identical to those in the 1986 Guide. 

A summary of the design procedures for flexible and rigid pavements in the 1993 AASHTO 

Guide is provided in Appendix C. A detailed discussion of the key geotechnical inputs in the 

1993 AASHTO Guide is presented in Chapter 5. Examples of the sensitivity of the pavement 

structural design to the various geotechnical factors included in the 1993 AASHTO Guide 

are the focus of Chapter 6. 

1998 Guide Supplement 

The 1998 supplement to the 1993 AASHTO Pavement Design Guide (AASHTO, 1998) 

provided an alternate method for rigid pavement design. The main changes from the 

procedures in the 1993 Guide included the following: 

• The modulus of subgrade reaction k is now defined as the elastic value on the top of 

the subgrade (or embankment, if present). When measured in a plate loading test, 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 38 May 2006 

only the elastic (i.e., recoverable) deformation is now used to compute k, and all 

permanent deformation is neglected. This is in contrast to previous versions of the 

Guide which defined k as a gross value that included both the elastic and permanent 

deformations from plate loading tests. Recommended procedures in the 1998 Guide 

Supplement for determining k are (a) correlations with soil type and other soil 

properties or tests; (b) deflection testing and backcalculation (most highly 

recommended); and (c) plate bearing tests. 

• The design k value is still modified for the influence of shallow bedrock, as in the 

1993 Guide. A new modification is also included for the effects of embankments. 

• The effective k value for design is no longer modified for the stiffness and thickness 

of the base4

 layer, as in the 1993 Guide. Instead, the base layer thickness and resilient 

modulus are included explicitly in the revised rigid pavement design equations. 

• The drainage factor Cd is no longer included in the design equations. 

• The loss of support factor LS is no longer included in the design procedure. 

• Both load and temperature stresses are included in the design calculations. 

A set of revised design equations for the alternate rigid pavement design method are provided 

in the 1998 supplement. The principal geotechnical parameters in these equations are: 

effective elastic modulus of subgrade support (k); modulus of elasticity of the base (Eb); and 

thickness of the base layer (Hb). The coefficient of friction between the slab and the 

base/subgrade is also required for reinforcement design in JRCP systems. 

 

4

 The granular layer between the slab and the subgrade is termed the base layer in the 1998 supplement. In 

earlier versions of the AASHTO Design Guides, this layer was termed the subbase. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 39 May 2006 

3.5.3 The NCHRP 1-37A Pavement Design Guide5

The various editions of the AASHTO Guide for Design of Pavement Structures have served 

well for several decades. These procedures are all based on performance data from the 

original AASHO Road Test (HRB, 1962). However, the range of conditions considered in 

the AASHO Road Test were quite limited, and these increasingly serious deficiencies limit 

the continued use of the AASHTO Design Guide as the nation's primary pavement design 

procedure: 

• Traffic loading: Heavy truck traffic levels have increased tremendously. The original 

Interstate pavements were designed in the 1960s for 5 – 10 million equivalent single-axle 

loads, whereas today these same pavements must be designed for 50 – 200 million axle 

loads, and sometimes more. It is unrealistic to expect that the existing AASHTO Guide 

based on the data from the original AASHO Road Test can be used reliably to design for 

this level of traffic. The pavements in the AASHO Road Test sustained slightly over 1 

million axle load applications—less than the traffic carried by many modern pavements 

within the first few years of their use. When applying these procedures to modern traffic 

streams, the designer must extrapolate the design methodology far beyond the original 

field data (Figure 3-19). Such highly-trafficked projects are likely either under-designed 

or over-designed to an unknown degree, with significant economic inefficiency in either 

case. 

• Rehabilitation limitations: Pavement rehabilitation design procedures were not 

considered at the AASHO Road Test. The rehabilitation design recommendations in the 

1993 Guide are completely empirical and very limited, especially under heavy traffic 

conditions. Improved capabilities for rehabilitation design are vital to today’s highway 

designs, as most projects today involve rehabilitation rather than new construction. 

• Climatic conditions: Because the AASHO Road Test was conducted at one geographic 

location, the effects of different climatic conditions can only be included in a very 

approximate manner in the AASHTO Design Guides. A significant amount of distress at 

the original AASHO Road Test occurred in the pavements during the spring thaw, a 

condition that does not exist in a large portion of the country. Direct consideration of sitespecific climatic effects will lead to improved pavement performance and reliability. 

 

5

 The official name for the NCHRP 1-37A project is the “2002 Guide for the Design of New and Rehabilitated 

Pavement Structures.” However, since official AASHTO approval of this guide is still in process, it will be 

referred to in this report simply as the “NCHRP 1-37A Pavement Design Guide.” 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 40 May 2006 

• Subgrade types: One type of subgrade−and a poor one at that (AASHTO A-6/A-7-

6)−existed at the Road Test, but many other types exist nationally. The significant 

influence of subgrade support on the performance of highway pavements can only be 

included very approximately in the current AASHTO design procedures. 

• Surfacing materials: Only a single asphalt concrete and Portland cement concrete mixture 

were used at the Road Test. The HMAC and PCC mixtures in common use today (e.g.,

Superpave, stone-mastic asphalt, high-strength PCC) are significantly different and better 

than those at the Road Test, but the benefits from these improved materials cannot be 

fully considered in the existing AASHTO Guide procedures. 

• Base materials: Only two unbound dense granular base/subbase materials were included 

in the main flexible and rigid pavement sections of the AASHO Road Test (limited 

testing of stabilized bases was included for flexible pavements). These exhibited 

significant loss of modulus due to frost and erosion. Today, various stabilized types are 

used routinely, especially for heavier traffic loadings. 

• Traffic: Truck suspension, axle configurations, and tire types and pressures were 

representative of the types used in the late 1950s. Many of these are outmoded (tire 

pressures of 80 psi versus 115 psi today), and pavement design procedures based on the 

older, lower tire pressures may be deficient for today’s higher values. 

• Construction and drainage: Pavement designs, materials, and construction were 

representative of those used at the time of the Road Test. No subdrainage was included 

in the Road Test sections, but positive subdrainage has become common in today’s 

highways. 

• Design life: Because of the short duration of the Road Test, the long-term effects of 

climate and aging of materials were not addressed. The AASHO Road Test was 

conducted over 2 years, while the design lives for many of today’s pavements are 20 to 

50 years. Direct consideration of the cyclic effect on materials response and aging are 

necessary to improve design life reliability. 

• Performance deficiencies: Earlier AASHTO procedures relate the thickness of the 

pavement surface layers (asphalt layers or concrete slab) to serviceability. However, 

research and observations have shown that many pavements need rehabilitation for 

reasons that are not related directly to pavement thickness (e.g., rutting, thermal cracking, 

faulting). These failure modes are not considered directly in the current AASHTO Guide. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 41 May 2006 

• Reliability: The 1986 AASHTO Guide included a procedure for considering design 

reliability that has never been fully validated. The reliability multiplier for design traffic 

increases rapidly with reliability level and may result in excessive layer thicknesses for 

heavily trafficked pavements that may not be warranted. 

Figure 3-19. Extrapolation of traffic levels in current AASHTO pavement design procedures 

(NHI Course 131064). 

The latest step forward in mechanistic-empirical design is the recently-completed NCHRP 

Project 1-37A Development of the 2002 Guide for the Design of New and Rehabilitated 

Pavement Structures (NCHRP, 2004). NCHRP Project 1-37A was a multi-year effort to 

develop a new national pavement design guide based on mechanistic-empirical principles. A 

key distinction of the models developed under NCHRP Project 1-37A is their calibration and 

validation using data from the FHWA Long Term Pavement Performance Program national 

database in a well-balanced experiment design representing all regions of the country. The 

NCHRP 1-37A models also include flexibility for re-calibration and validation using local or 

regional databases, if desired, by individual agencies. The mechanistic-empirical design 

approach as implemented in the NCHRP 1-37A Pavement Design Guide will allow pavement 

designers to: 

• evaluate the impact of new load levels and conditions, 

• better utilize current and new materials, 

• incorporate daily, seasonal, and yearly changes in materials, climate, and traffic, 

• better characterize seasonal/drainage effects, 

• improve rehabilitation design, 

• • • • • • •

Data 

Limits

(AASHO 

Road 

Test)

Current Current

Designs Designs

AXLE LOAD REPETITIONS

PAVEMENT THICKNESS

Projection A Projection A

Projection Projection B

Projection

Projection C

0 2 > 100 Million

• • • • • • •

Data 

Limits

(AASHO 

Road 

Test)

Current Current

Designs Designs

AXLE LOAD REPETITIONS

PAVEMENT THICKNESS

Projection A Projection A

Projection Projection B

Projection

Projection C

0 2 > 100 Million
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 42 May 2006 

• predict/minimize specific failure modes, 

• understand/minimize premature failures (forensics), 

• extrapolate from limited field and laboratory data, 

• reduce life cycle costs, 

• rationalize cost allocation, and 

• create more efficient, reliable, and cost-effective designs. 

 

Of course, benefits do not come without a cost. There are some drawbacks to mechanisticempirical design methodologies like those in the NCHRP 1-37A procedure: 

• Substantially more input data are required for design. Detailed information is required 

for traffic data, project environmental conditions, and material properties. 

• Most of the required material properties are fundamental engineering properties that 

should be measured via laboratory and field testing, as opposed to empirical 

properties that can be estimated qualitatively. 

• The design calculations are no longer amenable to hand computation. Sophisticated 

software is generally required. The execution time for this software is generally 

longer than that required for the DarWIN software commonly used for the current 

AASHTO design procedures. 

• Many agencies will need to upgrade their technical capabilities. This may include 

laboratory upgrades, new and faster computers, training for personnel, and changes in 

operational procedures. 

An extended summary of the NCHRP 1-37A methodology is provided in Appendix D. A 

detailed discussion of the key geotechnical inputs in the NCHRP 1-37A Pavement Design 

Guide is presented in Chapter 5. Examples using the NCHRP 1-37A Design Guide, including 

comparisons with the current AASTHO Design Guide, are the focus of Chapter 6. 

3.5.4 Low-Volume Roads 

Pavement structural design for low-volume roads is divided into four categories: 

1. Flexible pavements 

2. Rigid pavements 

3. Aggregate surfaced roads 

4. Natural surface roads 

The traffic levels on low-volume roads are significantly lower than those for which pavement 

structural design methods like the empirical 1993 AASHTO Guide and the mechanisticempirical NCHRP 1-37A procedure are intended. Consequently, these methods are generally 

not applied directly to the design of low-volume roads. Instead, both the 1993 AASHTO and 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 43 May 2006 

NCHRP 1-37A Design Guides provide catalogs of typical flexible pavement, rigid pavement, 

and aggregate surfaced designs for low-volume roads as functions of traffic category, 

subgrade quality, and climate zone. The 1993 AASHTO Guide also provides a simple 

separate design procedure for aggregate surfaced roads. Refer to the 1993 AASHTO Design 

Guide for additional details. 

Rutting is the primary distress for aggregate or natural surfaced roads. Vehicles traveling 

over aggregate or natural surfaced roads generate significant compressive and shear stresses 

that can cause failure of the soil. An acceptable rutting depth for aggregate surfaced roads 

can be estimated considering aggregate thickness and vehicle travel speed. A 2-inch (50 

mm) rut depth in a 4-inch-thick (100 mm) aggregate layer probably will result in mixing of 

the soil subgrade with the aggregate, which will destroy the paving function of the aggregate. 

Rutting depths greater than 2 to 3 inches (50 to 75 mm) in either aggregate or natural surface 

roads can be expected to significantly reduce vehicle speeds. 

Note that rutting may not be the only design consideration. Poor traction or dust conditions 

may dictate a hard surface. Traction characteristics may be indicated by the soil plasticity 

index, and dust potential may be indicated by the percent fines. 

The depth of rutting in aggregate or natural surfaced roads will depend upon the soil support 

characteristics and magnitude and number of repetitions of vehicle loads. The most common 

measure of rutting susceptibility is the California Bearing Ratio (CBR – see Section 5.4.1). 

Both the CBR test and rutting involve penetration of the soil surface due to a vertical loading. 

Although the CBR test does not measure compressive or shear strength values, it has been 

empirically correlated to rut depth for a range of vehicle load magnitudes and repetitions. 

The U.S. Forest Service (USDA, 1996) uses the following relationship for designing 

aggregate thickness in aggregate surfaced roads: 

( )

0.2476

.002 0.9335 0.2848

1 2

Rut Depth (inches) = 5.833 (log )

F Rr

tC C (3.10) 

in which 

R = number of Equivalent Single Axle Loads (ESALs) at a tire pressure of 

 80 psi 

t = thickness of top layer (inches) 

C1 = CBR of top layer 

C2 = CBR of subgrade 

Fr = reliability factor applied to R—see Table 3-8 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 44 May 2006 

Equation (3.10) is based upon an algorithm developed by the U.S. Army Corps of Engineers 

(Barber et al., 1978). Consult the U.S. Forest Service Earth and Aggregate Surfacing Design 

Guide (USDA, 1996) for more details on the design procedure. 

The allowable ESALs R in Equation (3.10) will vary depending upon the pavement materials 

and tire pressure. ESAL equivalency factors are defined in terms of pavement damage or 

reduced serviceability. The Forest Service Design Guide suggests that the ESAL equivalency 

factor for a 34-kip tandem axle be between 0.09 and 2.15 for tire pressures varying between 

25 – 100 psi (172 – 690 kPa). According to the AASHTO Design Guide, this same axle has 

equivalency factors of between 1.05 and 1.1 for flexible pavements (SN between 1 and 6) and 

between 1.8 and 2 for rigid pavements (slab thickness D between 6 and 14 inches). Rut 

depth can be managed by limiting tire pressures. Rut depth can decrease by more than 50% 

for aggregate surfaced roads if the tire pressure for a 34-kip tandem axle is reduced from 100 

to 25 psi (690 to 172 kPa). The Forest Service has partnered with industry to develop 

equipment that will centrally adjust tire pressures of log-hauling vehicles. 

Equation (3.10) can also be used to estimate rut depth for naturally surfaced roads. The upper 

layer of soil is expected to be compacted by traffic. Values must therefore be assigned to the 

compacted surface CBR (C1), the underlying soil CBR (C2), and the compacted thickness (t). 

Values of C1 at 90% relative compaction, C2 at 85% relative compaction, and t = 6 inches 

(150 mm) are reasonable values for typical conditions. 

The South Dakota Gravel Roads Maintenance and Design Manual (Skorseth and Selim, 

2000) discusses two additional design approaches for aggregate surfaced roads. One 

approach consists of design catalogs based on traffic categories, soil support classes, and 

climatic region. The more analytical approach considers ESALs, subgrade resilient modulus, 

seasonal variations of subgrade stiffness, the elastic moduli of the other pavement materials, 

allowable serviceability loss, allowable rutting depth, and allowable aggregate loss. The loss 

of pavement thickness due to traffic is unique to aggregate surfacing and must be considered 

by all thickness design methods for these types of roads. The hardness and durability of the 

aggregate may also require evaluation. 

For low-volume road surface layers that are stiffer than aggregate – e.g., hot mix asphalt and 

concrete – the recoverable strain within the subgrade can be used to calculate deflections in 

the soil that can cause fatigue damage in the material above. The use of unconfined 

compressive strength or unconsolidated-undrained shear strength is a reasonable approach for 

identifying pavement sections that have a potential for subgrade rutting. Intuitively, if the 

computed stresses within the pavement section are substantially less that the measured 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 45 May 2006 

strength, rutting is less likely. It has been proposed that the unconfined compressive strength 

(psi) is equal to approximately 4.5 times the CBR value (IDOT, 1995). 

Table 3-8. Reliability factors for use in Equation (3.10). 

Reliability Level (%) Reliability Factor Fr

50 1.00 

70 1.44 

90 2.32 

3.6 EXERCISE 

The Main Highway project is described in Appendix B. Working in groups, participants 

should read through this description and summarize in order of importance the key 

geotechnical issues that will influence the pavement design for this project. Each group will 

list its key geotechnical issues on the blackboard/flip chart, and all groups will then discuss 

the commonalities and discrepancies between the individual groups’ assessments. 

3.7 REFERENCES 

AASHTO (1972). AASHTO Interim Guide for Design of Pavement Structures, American 

Association of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (1986). Guide for Design of Pavement Structures, American Association of State 

Highway and Transportation Officials, Washington, D.C. 

AASHTO (1993). AASHTO Guide for Design of Pavement Structures, American Association 

of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (2003) Standard Specifications for Transportation Materials and Methods of 

Sampling and Testing (23rd ed.), American Association of State Highway and Transportation 

Officials, Washington, D.C. 

AASHTO (1998). Supplement to the AASTHO Guide for Design of Pavement Structures. 

Part II—Rigid Pavement Design and Rigid Pavement Joint Design, American Association of 

State Highway and Transportation Officials, Washington, D.C. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 46 May 2006 

Barber, V.C. et al. (1978). The Deterioration and Reliability of Pavements, Technical Report 

S-78-8, U.S. Army Engineering Waterways Experiment Station, Vicksburg, MS, July. 

Barenberg, E.J., and M.R. Thompson (1990). Calibrated Mechanistic Structural Analysis 

Procedures for Pavements, Phase 1 NCHRP Project 1-26. National Cooperative Highway 

Research Program. Transportation Research Board, Washington, D.C. 

Barenberg, E.J., and M.R. Thompson (1992). Calibrated Mechanistic Structural Analysis 

Procedures for Pavements, Phase 2 NCHRP Project 1-26. National Cooperative Highway 

Research Program. Transportation Research Board, Washington, D.C. 

Christopher, B.R. and V.C. McGuffey (1997). Synthesis of Highway Practice 239: Pavement 

Subsurface Drainage Systems, National Cooperative Highway Research Program, 

Transportation Research Board, National Academy Press, Washington, D.C. 

FHWA NHI Course 131026 reference manual (1999) Pavement Subsurface Drainage 

Design, Participants Reference Manual, prepared by ERES Consultants, Inc, National 

Highway Institute, Federal Highway Administration, Washington, D.C. 

FHWA NHI-00-043 (2000) Mechanically Stabilized Earth Walls and Reinforced Soil Slopes 

Design and Construction Guidelines, authors: Elias, V, Christopher, B.R. and Berg, R.R., 

[Technical Consultant - DiMaggio, J.A.], U.S. Department of Transportation, Federal Highway 

Administration, Washington DC, 418 p. 

FHWA NHI-01-028 (2001) Soil Slope and Embankment Design, authors: Collin, J.G., Hung, 

J.C., Lee, W.S. and Munfakh, G., National Highway Institute, Federal Highway 

Administration, Washington, D.C. 

FHWA NHI-99-007 (1998) Rock Slope Design, authors: Munfakh, G., Wyllie, D. and Mah, 

C.W., National Highway Institute, Federal Highway Administration, Washington, D.C. 

HRB (1962). The AASHO Road Test. Report 5: Pavement Research; Report 6: Special 

Studies; Report 7: Summary Report; Special Reports 61E, 61F, and 61G, Highway Research 

Board, Washington, D.C. 

Huang, Y.H. (1993). Pavement Analysis and Design, Prentice-Hall, Englewood Cliffs, NJ. 

IDOT (1995). Subgrade Inputs for Local Pavements, Design Procedures No. 95-11, Illinois 

Department of Transportation, Springfield, IL. 
FHWA NHI-05-037 Chapter 3 – Geotechnical Issues 

Geotechnical Aspects of Pavements 3 - 47 May 2006 

Kerkhoven, R.E., and G.M. Dormon (1953). “Some Considerations on the California Bearing 

Ratio Method for the Design of Flexible Pavement,” Shell Bitumen Monograph No. 1. 

McAdam, J.L. (1820). Remarks on the Present System of Road Making; With Observations, 

deduced from Practice and Experience, with a View to a Revision of the Existing Laws, and 

the Introduction of Improvement in the Method of Making, Repairing, and Preserved Roads, 

and Defending the Road Funds from Misapplication, Longman, Hurst, Rees, Orme, and 

Brown, London. 

NCHRP (2004). Mechanistic-Empirical Design of New and Rehabilitated Pavement 

Structures, draft report, NCHRP Project 1-37A, National Cooperative Highway Research 

Program, National Research Council, Washington, D.C. 

Newcombe, D.A., and B. Birgisson (1999). “Measuring In-situ Mechanical Properties of 

Pavement Subgrade Soils,” NCHRP Synthesis 278, Transportation Research Board, 

Washington, D.C. 

PCA (1984). Thickness Design for Concrete Highway and Street Pavements, Portland 

Cement Association, Skokie, IL. 

Rollings, M.P., and R.S. Rollings (1996). Geotechnical Materials in Construction, McGrawHill, NY. 

Saal, R.N.J., and P.S. Pell (1960). Kolloid-Zeitschrift MI, Heft 1, pp. 61-71. 

Shook, J.F., F.N. Finn, M.W. Witczak, and C.L. Monismith (1982). “Thickness Design of 

Asphalt Pavements—The Asphalt Institute Method,” Proceedings, 5th International 

Conference on the Structural Design of Asphalt Pavements, Vol. 1, pp. 17-44. 

Skorseth, K, and A.A. Selim (2000). Gravel Roads Maintenance and Design Manual, 

Federal Highway Administration, U.S. Department of Transportation, Washington, D.C. 

(available in pdf format at http://www.epa.gov/owow/nps/gravelroads/). 

USDA (1996). Earth and Aggregate Surfacing Design Guide for Low Volume Roads, EM7170-16, FHWA-FLP-96,001, U.S. Forest Service, U.S. Department of Agriculture, 

September. 
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Geotechnical Aspects of Pavements 3 - 48 May 2006 

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FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 1 May 2006 

CHAPTER 4.0 GEOTECHNICAL EXPLORATION AND TESTING 

4.1 INTRODUCTION 

The purpose of the geotechnical subsurface investigation program for pavement design and 

construction is to obtain a thorough understanding of the subgrade conditions along the 

alignment that will constitute the foundation for support of the pavement structure. The 

specific emphasis of the subsurface investigation is to identify the impact of the subgrade 

conditions on the construction and performance of the pavement, characterize material from 

cut sections that may be used as subgrade fill, and to obtain design input parameters. The 

investigation may be accomplished through a variety of techniques, which may vary with 

geology, design methodology and associated design requirements, type of project and local 

experience. To assist agencies in achieving the stated purpose of subsurface investigation, 

this chapter presents the latest methodologies in the planning and execution of the various 

exploratory investigation methods for pavement projects. It is understood that the procedures 

discussed in this chapter are subject to local variations. Users are also referred to AASHTO 

R 13 and ASTM D 420, Conducting Geotechnical Subsurface Investigations and FHWA 

NHI-01-031 Subsurface Investigations, for additional guidance. 

In Chapter 1, a simplistic subsurface exploration program consisting of uniformly spaced soil 

borings (i.e., systematic sampling) with SPT testing was mentioned as an antiquated method 

for determining the subsurface characteristics for pavement design. “Adequate for design and 

low cost” are often used in defense of this procedure. The cost-benefit of additional 

subsurface exploration is a subject that is often debated. This subject is now addressed in the 

new NCHRP 1-37A Design Guide. The guide allows for use of default values in the absence 

of sufficient data for characterizing the foundation, thus minimizing agency design costs, but 

at the increased risk of over- or under-designing the pavement structure. 

In evaluating the cost-benefit of the level of subsurface investigation, all designers must 

recognize that the reliability and quality of the design will be directly related to the 

subsurface information obtained. The subsurface exploration program indeed controls the 

quality of the roadway system. A recent FHWA study indicated that a majority of all 

construction claims were related to inadequate subsurface information. With great certainty, 

inadequate information will lead to long-term problems with the roadway design. The cost of 

a subsurface exploration program is a few thousand dollars, while the cost of overconservative designs or costly failures in terms of construction delays, construction extras, 

shortened design life, increased maintenance, and public inconvenience is typically in the 

hundreds of thousands of dollars. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 2 May 2006 

Engineers should also consider that the actual amount of subgrade soil sampled and tested is 

typically on the order of one-millionth to one-billionth of the soil being investigated. 

Compare this with sampling and testing of other civil engineering materials. Sampling and 

testing of concrete is on the order of 1 sample (3 test specimens, or about 1⁄4 cubic meter) 

every 40 cubic meters, which leads to 1 test in 100,000. Sampling and testing of asphalt is on 

the same order as concrete. Now consider that the variability in properties of these wellcontrolled, manufactured materials is much less than the properties of the subgrade, which 

often have coefficients of variation of well over 100% along the alignment. Again cost, not 

quality is usually the deciding factor. The quality of sampling can be overcome with 

conservative designs (as is often the case; e.g., AASHTO 1972). For example, laboratory 

tests are often run on soil samples in a weaker condition than in the ground, rather than 

running more tests on the full range of conditions that exist in the field. While this approach 

may provide a conservative value for design purposes, there are hidden costs in both 

conservatism and questionable reliability. Modern pavement design uses averages with 

reliability factors to account for uncertainty (AASHTO, 1993 and NCHRP 1-37A). However, 

sufficient sampling and testing are required to check the variability of design parameters to 

make sure that they are within the bounds of reliability factors; otherwise, on highly variable 

sites designs, they will not be conservative and on very uniform sites, they will still be over 

conservative. 

The expense of conducting soil borings is certainly a detriment to obtaining subsurface 

information. However, exploration itself is not just doing borings. There is usually a 

significant amount of information available from alternate methods that can be performed 

prior to drilling to assist in optimizing boring and sampling locations (i.e., representative 

sampling). This is especially the case for reconstruction and rehabilitation projects. 

Significant gains in reliability can be made by investigating subgrade spatial variability in a 

pavement project and often at a cost reduction due to decreased reliance on samples. This 

chapter provides guidelines for a well-planned exploration program for pavement design, 

with alternate methods used to overcome sampling and testing deficiencies. Geotechnical 

exploration requirements for borrow materials (base, subbase, and subgrades) are also 

reviewed. 

Figure 4-1 provides a flow chart of the process for performing a geotechnical exploration and 

testing program. As shown in the flow chart, the steps for planning and performing a 

complete geotechnical and testing program include 
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Geotechnical Aspects of Pavement 4 - 3 May 2006 

Subsurface Exploration Steps Relevance to Pavement Design

1) Establish the type of pavement 

construction. 

 Whether new construction, reconstruction, or 

rehabilitation. 

2) Search available information. To identify anticipated subsurface conditions 

at the vertical and horizontal location of the 

pavement section. 

3) Perform site reconnaissance. To identify site conditions requiring special 

consideration. 

4) Plan the exploration program for 

evaluation of the subsurface conditions and 

identification of the groundwater table, 

including methods to be used with 

consideration for using 

• remote sensing, 

• geophysical investigations, 

• in-situ testing, 

• disturbed sampling, and 

• undisturbed sampling. 

 

To identify and obtain 

- more information on site conditions, 

- spacial distribution of subsurface conditions, 

- rapid evaluation of subsurface condition, 

- subgrade soils & classification test samples, 

- samples for resilient modulus tests and 

 calibration of in-situ results. 

5) Evaluate conceptual designs, examine 

subsurface drainage and determine sources 

for other geotechnical components (e.g.,

base and subbase materials). 

 Identify requirements for subsurface drainage 

and subgrade stabilization requirements, as 

well as construction material properties. 

6) Examine the boring logs, classification 

tests, soil profiles and plan view, then 

select representative soil layers for 

laboratory testing. 

 Use the soil profile and plan view along the 

roadway alignment to determine resilient 

modulus or other design testing requirements 

for each influential soil strata encountered. 

Each of these steps will be reviewed in the following sections of this chapter. 

4.2 LEVELS OF GEOTECHNICAL EXPLORATION FOR DIFFERENT TYPES OF 

 PAVEMENT PROJECTS 

There are three primary types of pavement construction projects. They are 

• new construction, 

• reconstruction, and 

• rehabilitation. 

Each of these pavement project types requires different considerations and a corresponding 

level of effort in the geotechnical exploration program. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 4 May 2006 

Figure 4-1. Geotechnical exploration and testing for pavement design. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 5 May 2006 

4.2.1 New Pavement Construction 

For new construction, the exploration program will require a complete evaluation of the 

subgrade, subbase, and base materials. Sources of materials will need to be identified and a 

complete subsurface exploration program will need to be performed to evaluate pavement 

support conditions. Prior to planning and initiating the investigation, the person responsible 

for planning the subsurface exploration program (i.e., the geotechnical engineer or engineer 

with geotechnical training) needs to obtain from the designers the type, load, and 

performance criteria, location, geometry and elevations of the proposed pavement sections. 

The locations and dimensions of cuts and fills, embankments, retaining structures, and 

substructure elements (e.g., utilities, culverts, storm water detention ponds, etc.) should be 

identified as accurately as practicable. 

Also, for all new construction projects, samples from the subgrade soils immediately beneath 

the pavement section and from proposed cut soils to be used as subgrade fill will be required 

to obtain the design-input parameters for the specific design method used by the agency. 

Available site information (e.g., geological maps and United States Department of 

Agriculture Natural Resources Conservation Service’s soil survey reports) as discussed in 

Section 4.3, site reconnaissance (see Section 4.4), air photos (see Section 4.5.3) and 

geophysical tests (see Section 4.5.4) can all prove beneficial in identify representative and 

critical sampling locations. 

For all designs using AASHTO 1993 or NCHRP 1-37A, particularly for critical projects, 

repeated load resilient modulus tests are needed to evaluate the support characteristics and 

the effects of moisture changes on the resilient modulus of each supporting layer. The 

procedures, sample preparation and interpretation of the resilient modulus test are discussed 

in Chapter 5. For designs based on subgrade strength, either lab tests (e.g., CBR) as discussed 

in Chapter 5 or in-situ tests (e.g., DCP) as discussed later in Section 4.5.5 of this chapter can 

be used to determine the support characteristics of the subgrade. 

Another key part of subsurface exploration is the identification and classification (through 

laboratory tests) of the subgrade soils in order to evaluate the vertical and horizontal 

variability of the subgrade and select appropriate representative design tests. Field 

identification along with classification through laboratory testing also provides information 

to determine stabilization requirements to improve the subgrade should additional support be 

required, as discussed in Chapter 7. 

Location of the groundwater table is also an important aspect of the subsurface exploration 

program for new construction to evaluate water control issues (e.g., subgrade drainage 
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Geotechnical Aspects of Pavement 4 - 6 May 2006 

requirements) with respect to both design and construction. Methods for locating the 

groundwater level are discussed in Section 4.5.6. Other construction issues include the 

identification of rock in the construction zone, rock rippability, and identification of soft or 

otherwise unsuitable materials to be removed from the subgrade. The location and rippability 

of rock can be determined by geophysical methods (e.g., seismic refraction), as discussed in 

section 4.5.4 and/or borings and rock core samples. 

4.2.2 Reconstruction 

For pavement reconstruction projects, such as roadway replacement, full depth reclamation, 

or road widening, information may already exist on the subgrade support conditions from 

historical subsurface investigations. Existing borings should be carefully evaluated with 

respect to design elevation of the new facility. A survey of the type, severity, and amount of 

visible distress on the surface of the existing pavement (i.e., a condition survey as described 

in the NHI, 1998, “Techniques for Pavement Rehabilitation” Participant’s Manual) can also 

indicate local issues that need a more extensive evaluation. However, an additional limited 

subsurface investigation is usually advisable to validate the pavement design calculations and 

design for weak subgrade conditions, if present. It is also likely that resilient modulus, CBR 

or other design input values used by agencies would need to be obtained for the existing 

materials using current procedures. Test methods used by the agency often change over time 

(e.g., lab CBR versus field CBR). Previous data may also not be valid for current conditions 

(e.g., traffic). Water in old pavements can often result in poorer subgrade conditions than 

originally encountered. Drainage features, or lack thereof, in the existing pavement and their 

functionality should be examined. Again, subgrade soil identification and classification will 

be required to provide information on subgrade variability and assist in selection of soils to 

be tested. 

It is possible to determine the value of reworking the subgrade (i.e., scarifying, drying, and 

recompacting) if results indicate stiffness and/or subgrade strength values are below expected 

or typical values. This comparison can be made by examining the resilient modulus of 

undisturbed tube samples obtained to verify backcalculated moduli to that of a recompacted 

specimen remolded to some prescribed level of density and moisture content. For example, 

this comparison may ultimately lead to the need for underdrain installation in order to reduce 

and maintain lower moisture levels in the subgrade. 

 

Subsurface investigation on reconstruction projects can usually be facilitated by using nondestructive tests (NDT) (a.k.a. geophysical methods) performed over the old pavement (or 

shoulder section for road widening) with one or more of the variety of methods presented in 

Section 4.5. For example, resilient modulus properties can best be obtained from non-
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Geotechnical Aspects of Pavement 4 - 7 May 2006 

destructive geophysical methods, such as falling weight deflectometer (FWD) tests and back 

calculating elastic moduli to characterize the existing structure and foundation soils needed 

for design. This approach is suggested because it provides data on the response 

characteristics of the in-situ soils and conditions. Back calculation of layer elastic moduli 

from deflection basin data is discussed later in Section 4.5.4 of this chapter. These results can 

be supported by laboratory tests on samples obtained from a minimal subsurface exploration 

program (described in Section 4.5). Old pavement layer thickness (i.e., asphalt or concrete, 

base and/or subbase) should also be obtained during sampling to provide information for 

back-calculation of the modulus values. 

For designs based on subgrade strength (e.g., CBR), in-situ tests (e.g., Dynamic Cone 

Penetrometer (DCP), field CBR, and other methods as described in Section 4.7) can be 

performed to obtain a rapid assessment of the variability in subgrade strength and to 

determine design strength values via correlations. Some samples should still be taken to 

perform laboratory tests and confirm in-situ test correlation values. Geophysical test results 

(e.g., FWD, Ground Penetrating Radar (GPR), and others described in Section 4.5.4) can also 

be used to assist in locating borings. 

The potential sources of new base and subbase materials will need be identified and 

laboratory tests performed to obtain resilient modulus, CBR or other design values, unless 

catalogued values exist for these engineered materials. For pavement reclamation or 

recycling projects, composite samples should be obtained from the field and test specimens 

constituted following the procedures outlined in Chapter 5 to obtain design input values. The 

subgrade soils will also need to be evaluated for their ability to support construction 

activities, such as rubblize-and-roll type construction. 

4.2.3 Rehabilitation 

As discussed in Chapter 3, rehabilitation projects include a number of strategies, including 

overlays, rubbilization, and crack and seat. The details required for the subsurface 

investigation of pavement rehabilitation projects depends on a number of variables: 

• The condition of the pavement to be rehabilitated (e.g., pavement rutting, cracking, 

riding surface uniformity and roughness, surface distress, surface deflection under 

traffic, presence of water, etc., as described in the condition survey section of NHI, 

1998, “Techniques for Pavement Rehabilitation” Participants Manual.) 

• If the facility is distressed, the type, severity and extent of distress (pavement distress, 

pavement failures, crack-type pattern, deep-seated failures, settlement, drainage and 

water flow, and collapse condition) (see NHI, 1998, “Techniques for Pavement 

Rehabilitation” Participants Manual) should be quantified. Rutting and fatigue 
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Geotechnical Aspects of Pavement 4 - 8 May 2006 

cracking are often associated with subgrade issues and general require coring, 

drilling, and sampling to diagnose the cause of these conditions. 

• Techniques to be considered for rehabilitation. 

• Whether the facility will be returned to its original and as-built condition, or whether 

it will be upgraded, for example, by adding another lane to a pavement. If facilities 

will be upgraded, the proposed geometry, location, new loads and structure changes 

(e.g., added culverts) must be considered in the investigation. 

• The required performance period of the rehabilitated pavement section. 

Selection of the rehabilitation alternative will partly depend on the condition assessment. 

NHI, 1998, “Techniques for Pavement Rehabilitation” Participants Manual covers condition 

surveys and selection of techniques for pavement rehabilitation. Information from the 

subsurface program performed for the original pavement design should also be reviewed. 

However, as with reconstruction projects, some additional corings and borings will need to 

be performed to evaluate the condition and properties of the of the pavement surface and 

subgrade support materials. Pavements are frequently cored at 150 – 300 m (500 – 1000 ft) 

intervals for rehabilitation projects. The core holes in the pavement also provide access to 

investigate the in-situ and disturbed properties of the base, subbase, and subgrade materials. 

Samples can be taken and/or in-situ tests (e.g., DCP) can be used to indicate structural 

properties, as well as layer thickness. 

Geophysical tests (e.g., FWD, GPR, and others described in Section 4.5.4) can be used to 

assist in locating coring and boring locations, especially if the base is highly contaminated or 

there are indications of subgrade problems. Otherwise, the frequency of corings and borings 

should be increased. As with reconstruction projects, rehabilitation projects can use FWD 

methods and associated back-calculated elastic modulus to characterize the existing structure 

and foundation. Again, the FWD method is covered in Section 4.5.4 and back-calculation of 

layer elastic moduli from deflection basin data is discussed in Chapter 5. FWD results can 

also be correlated with strength design values (e.g., CBR). A limited subsurface drilling and 

sampling program can then be used to confirm the back-calculated resilient modulus values 

and/or correlation with other strength design parameters. The layer thickness of each 

pavement component (i.e., surface layer, base, and or subbase layer) is critical for backcalculation of modulus values. 

4.2.4 Subsurface Exploration Program Objectives 

As stated in the NCHRP 1-37A Design Guide, the objective of subsurface investigation or 

field exploration is to obtain sufficient subsurface data to permit the selection of the types, 

locations, and principal dimensions of foundations for all roadways comprising the proposed 
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Geotechnical Aspects of Pavement 4 - 9 May 2006 

project, thus providing adequate information to estimate their costs. More importantly, these 

explorations should identify the site in sufficient detail for the development of feasible and 

cost-effective pavement design and construction. 

As outlined in the FHWA Soils and Foundation Workshop manual (FHWA NHI-00-045), the 

subsurface exploration program should obtain sufficient subsurface information and samples 

necessary to define soil and rock subsurface conditions as follows: 

1) Statigraphy (for evaluating the areal extent of subgrade features) 

a) Physical description and extent of each stratum 

b) Thickness and elevation of various locations of top and bottom of each stratum 

2) For cohesive soils (identify soils in each stratum, as described in Section 4.7, to assess the 

relative value for pavement support and anticipated construction issues, e.g., stabilization 

requirements) 

a) Natural moisture contents 

b) Atterberg limits 

c) Presence of organic materials 

d) Evidence of desiccation or previous soil disturbance, shearing, or slickensides 

e) Swelling characteristics 

f) Shear strength 

g) Compressibility 

3) For granular soils (identify soils in each stratum, as described in Section 4.7, to assess the 

relative value for pavement support and use in the pavement structure) 

a) In-situ density (average and range) 

b) Grain-size distribution (gradations) 

c) Presence of organic materials 

4) Groundwater (for each aquifer within zone of influence on construction and pavement 

support, especially in cut sections as detailed in Section 4.5) 

a) Piezometric surface over site area, existing, past, and probable range in future 

b) Perched water table 

5) Bedrock (and presence of boulders) (within the zone of influence on construction and 

pavement support as detailed in Section 4.5) 

a) Depth over entire site 

b) Type of rock 

c) Extent and character of weathering 

d) Joints, including distribution, spacing, whether open or closed, and joint infilling 

e) Faults 

f) Solution effects in limestone or other soluble rocks 

g) Core recovery and soundness (RQD) 

h) Ripability 
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4.3 SEARCH AVAILABLE INFORMATION

The next step in the investigation process is to collect and analyze all existing data. A 

complete and thorough investigation of the topographic and subsurface conditions must be 

made prior to planning the field exploration program so that it is clear where the pavement 

subgrade will begin and to identify the type of soils anticipated within the zone of influence 

of the pavement. The extent of the site investigation and the type of exploration required will 

depend on this information. (“If you do not know what you should be looking for in a site 

investigation, you are not likely to find much of value.” Quote from noted speaker at the 8th

Rankine Lecture). Simply locating borings without this information is like sticking a needle 

in your arm blindfolded and hoping to hit the vein. A little sleuthing can greatly assist in 

gaining an understanding of the site and planning the appropriate exploration program. 

An extensive amount of information can be obtained from a review of literature about the 

site. There are a number of very helpful sources of data that can and should be used in 

planning subsurface investigations. Review of this information can often minimize surprises 

in the field, assist in determining boring locations and depths, and provide very valuable 

geologic and historical information, which may have to be included in the exploration report. 

The first information to obtain is prior agency subsurface investigations (historical data) at or 

near the project site, especially for rehabilitation and reconstruction projects. To determine its 

value, this data should be carefully evaluated with respect to location, elevation, and site 

variability. Also, in review of data, be aware that test methods change over time. For 

example, SPT values 20 to 30 years ago were much less efficient than today, as discussed in 

Section 4.5.5. Prior construction and records of structural performance problems at the site 

(e.g., excessive seepage, unpredicted settlement, and other information) should also be 

reviewed. Some of this information may only be available in anecdotal forms. For 

rehabilitation and reconstruction projects, contact agency maintenance personnel and discuss 

their observations and work along the project alignment. The more serious construction 

and/or maintenance problems should be investigated, documented if possible, and evaluated 

by the engineer. 

In this initial stage of site exploration, for new pavement projects, the major geologic 

processes that have affected the project site should also be identified. Geology will be a key 

factor to allow the organization and interpretation of findings. For example, if the pavement 

alignment is through an ancient lakebed, only a few representative borings will be required to 

evaluate the pavement subgrade. However, in highly variable geologic conditions, additional 

borings (i.e., in excess of the normal minimum) should be anticipated. Geological 

information is especially beneficial in pavement design and construction to identify the 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 11 May 2006 

presence and types of shallow rock, rock outcrops, and rock excavation requirements. 

Geological information can readily be obtained from U.S. Geological Survey (USGS) maps, 

reports, publications and websites (www.usgs.gov), and State Geological Survey maps and 

publications. 

Soils deposited by a particular process assume characteristic topographic features, called 

landforms, which can be readily identified by a geotechnical specialist or geologist. A 

landform contains soils with generally similar engineering properties and typically extends 

irregularly over wide areas of a project alignment. The soil may be further described as a 

residual or transported soil. A residual soil has been formed at a location by the in-place 

decomposition of the parent material (sedimentary, igneous, or metamorphic rock). Residual 

soils often contain a structure and lose strength when disturbed. A transported soil was 

formed at one location and has been transported by exterior forces (e.g., water, wind, or 

glaciers). Alluvium soils are transported by water, loess type soils are transported by wind, 

and tills are transported by ice. Transported soils (especially alluvium and loess) are often 

fine grained and are usually characterized as poorly draining, compressible when saturated, 

and frost susceptible (i.e., not the most desirable soils for supporting pavement systems). 

Sources of information for determining landform boundaries and their functional uses are 

given in Table 4-1. 

One of the more valuable sources of landform information for pavement design and 

construction are soil survey maps produced by the U.S. Department of Agriculture, Natural 

Resources Conservation Service, in cooperation with state agricultural experiment stations 

and other Federal and State agencies. The county soil maps provide an overview of the 

spatial variability of the soil series within a county. These are well-researched maps and 

provide detailed information for shallow surficial deposits, especially valuable for pavements 

at or near original surface grade. They may also show frost penetration depths, drainage 

characteristics, and USCS soil types. Knowledge of the regional geomorphology (i.e., the 

origin of landforms and types of soils in the region and the pedologic soil series definitions) 

is required to take full advantage of these maps. Such information will be of help in planning 

soil exploration activities. Plotting the pavement alignment on a USDA map and/or a USGS 

map can be extremely helpful. Figure 4-2 shows an example for a section of the Main 

Highway project. 

The majority of the above information can be obtained from commercial sources (i.e.,

duplicating services) or U.S. and state government, local or regional offices. Specific sources 

(toll-free phone numbers, addresses, etc.) for flood and geologic maps, aerial photographs, 

USDA soil surveys, can be very quickly identified through the Internet (e.g., at the websites 

listed in Table 4-1). 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 12 May 2006 

Table 4-1. Sources of topographic & geologic data for identifying landform boundaries. 

Source Functional Use 

Topographic maps prepared by the United 

States Coast and Geodetic Survey (USCGS). 

Determine depth of borings required to evaluate 

pavement subgrade; determine access for 

exploration equipment; identify physical 

features, and find landform boundaries. 

County agricultural soil maps and reports 

prepared by the U.S. Department of 

Agriculture’s Natural Resources 

Conservation Service (a list of published soil 

surveys is issued annually, some of which are 

available on the web at 

http://soils.usda.gov/survey/online_surveys/). 

Provide an overview of the spatial variability of 

the soil series within a county. 

U.S. Geological Survey (USGS) maps, 

reports, publications and websites 

(www.usgs.gov), and State Geological 

Survey maps and publications. 

Type, depth, and orientation of rock formations 

that may influence pavement design and 

construction. 

State flood zone maps prepared by state or 

U.S. Geological Survey or the Federal 

Emergency Management Agency (FEMA: 

www.fema.gov) can be obtained from local 

or regional offices of these agencies. 

Indicate deposition and extent of alluvial soils, 

natural flow of groundwater, and potential high 

groundwater levels (as well as danger to crews in 

rain events). 

Groundwater resource or water supply 

bulletins (USGS or State agency). 

Estimate general soils data shown, and indicate 

anticipated location of groundwater with respect 

to pavement grade elevation. 

Air photos prepared by the United States 

Geologic Survey (USGS) and others (e.g.,

state agencies). 

Detailed physical relief shown; flag major 

problems. By studying older maps, reworked 

landforms from development activities can be 

identified along the alignment, e.g., buried 

streambed or old landfill. 

Construction plans for nearby structures 

(public agency). 

Foundation type and old borings shown. 
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Figure 4-2. Soil Survey information along the Main Highway pavement alignment. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 14 May 2006 

4.4 PERFORM SITE RECONNAISSANCE 

A very important step in planning the subsurface exploration program is to visit the site with 

the project plans (i.e., a plan-in-hand site visit). It is imperative that the engineer responsible 

for exploration, and, if possible, the project design engineer, conduct a reconnaissance visit 

to the project site to develop an appreciation of the geotechnical, topographic, and geological 

features of the site and become knowledgeable of access and working conditions. A plan-inhand site visit is a good opportunity to learn about 

• design and construction plans. 

• general site conditions including special issues and local features, such as lakes and 

streams, exploration and construction equipment accessibility. 

• surficial geologic and geomorphologic reconnaissance for mapping stratigraphic 

exposures and outcrops and identifying problematic surficial features, such as organic 

deposits. 

• type and condition of existing pavements at or in the vicinity of the project. 

• traffic control requirements during field investigations (a key factor in the type of 

exploration, especially for reconstruction and rehabilitation projects). 

• location of underground and overhead utilities for locating in-situ tests and borings. 

(For pavement rehab projects, the presence of underground utilities may also support 

the use of non-destructive geophysical methods to assist in identifying old utility 

locations. 

• adjacent land use (schools, churches, research facilities, etc.). 

• restrictions on working hours (e.g., noise issues), which may affect the type of 

exploration, as well as the type of construction. 

• right-of-way constraints, which may limit boring locations. 

• environmental issues (e.g., old service stations for road widening projects).

• escarpments, outcrops, erosion features, and surface settlement. 

• flood levels (as they relate to the elevation of the pavement and potential drainage 

issues. 

• benchmarks and other reference points to aid in the location of borehole. 

• subsurface soil and rock conditions from exposed cuts in adjacent works. 

For reconstruction or rehabilitation projects, the site reconnaissance should include a 

condition survey of the existing pavement as detailed in NHI (1998) “Techniques for 

Pavement Rehabilitation.” During this initial inspection of the project, the design engineer, 

preferably accompanied by the maintenance engineer, should determine the scope of the 

primary field survey, begin to assess the potential distress mechanisms, and identify the 
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Geotechnical Aspects of Pavement 4 - 15 May 2006 

candidate rehabilitation alternatives. As part of this activity, subjective information on 

distress, road roughness, surface friction, and moisture/drainage problems should be 

gathered. Unless traffic volume is a hazard, this type of data can be collected without any 

traffic control, through both “windshield” and road shoulder observations. In addition, an 

initial assessment of traffic control options (both during the primary field survey and during 

rehabilitation construction), obstructions, and safety aspects should be made during this visit. 

4.5 PLAN AND PERFORM THE SUBSURFACE EXPLORATION PROGRAM 

Following the collection and evaluation of available information from the above sources, the 

geotechnical engineer (or engineer or geologist with geotechnical training) is ready to plan 

the field exploration program. The field exploration methods, sampling requirements, and 

types and frequency of field tests to be performed will be determined based on the existing 

subsurface information obtained from the available literature and site reconnaissance, project 

design requirements, the availability of equipment, and local practice. A geologist can often 

provide valuable input regarding the type, age, and depositional environment of the geologic 

formations present at the site for use in planning and interpreting the site conditions. 

The subsurface investigation for any pavement project should be sufficiently detailed to 

define the depth, thickness, and area of all major soil and rock strata that will affect 

construction and long-term performance of the pavement structure. The extent of the 

exploration program depends on the nature of both the project and the site-specific 

subsurface conditions. To acquire reliable engineering data, each job site must be explored 

and analyzed according to its subsurface conditions. The engineer in charge of the subsurface 

exploration must furnish complete data so that an impartial and thorough study of practical 

pavement designs can be made. 

4.5.1 Depth of Influence 

Planning the subsurface exploration program requires a basic understanding of the depth to 

which subsurface conditions will influence the design, construction, and performance of the 

pavement system. For pavement design, the depth of influence is usually assumed to relate 

only to the magnitude and distribution of the traffic loads imposed on the pavement structure 

under consideration. Current AASHTO (1993) describes this depth at 1.5 m (5 ft) below the 

proposed subgrade elevation with this depth increased for special circumstances (e.g., deep 

deposits of very soft soils). In this section, support for the recommended depth is provided, 

and special circumstances where this depth should be extended are reviewed. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 16 May 2006 

The zone of influence under the completed pavement varies with the pavement section, but 

typically 80 – 90 percent of the applied stress is dissipated within 1 m (3 ft) below the asphalt 

section as shown in Figure 4-3. However, consideration must also be given to the roadway 

section (i.e., height and width of the roadway embankment for fill application), the nature of 

the subsurface conditions, and consideration for construction (e.g., depth of soils that may 

require stabilization to allow for construction). A common rule of thumb in geotechnical 

engineering is that the depth of influence is on the order of two times the width of the load. 

This adage is also true for pavement sections during construction and for unpaved roads. 

Considering a dual wheel is about 1 m (3 ft) in width, subsurface investigations for shallow 

cut and fill with no special problems should generally extend to 1.5 – 2 m (5 – 7 ft) below the 

proposed subgrade level to account for construction conditions. Special problems requiring 

deeper exploration may include deep highly compressible deposits (e.g., peat or marsh areas) 

or deep deposits of frost-susceptible soils in cold regions. Greater depths may also be 

required for embankment design. 

From a pavement design perspective, the critical layers are in the upper meter of the 

subgrade. This understanding is especially critical for rehabilitation projects. Mechanistic 

design is based upon the critical horizontal tensile strain at the base of an asphalt layer or the 

critical vertical compressive strain at the surface of the subgrade (and within the other 

pavement layers) under repetitions of a specific wheel or axle load (Huang, 1993). Subgrade 

strain often controls the pavement design except for very thick asphalt layers or overlays. For 

rehabilitation projects and in consideration of sampling for roadway design, the depth of 

influence should be evaluated based on the type of pavement and the reconstruction layering. 

The subsurface investigations should focus on these depths (typically the upper 1 to 2 

meters). However, as discussed in Chapter 3, groundwater and bedrock at depths of less than 

3 m (10 ft) beneath the pavement can have an influence on pavement design. In addition, 

location of the groundwater level within 3 m (10 ft) of the pavement will influence decisions 

of frost susceptibility, as discussed later in Chapter 7, and the presence of bedrock within 

6 – 9 m (20 – 30 ft) can influence deflections of pavement layers and FWD results. 

Therefore, in order to confirm that there are no adverse deeper deposits, to identify 

groundwater conditions, and to locate bedrock within the influence zone, a limited amount of 

exploration should always be performed to identify conditions in the subgrade to depths of 6 

m (20 ft). However, as discussed in the next section, this does not necessarily mean borings 

to that depth. 
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Geotechnical Aspects of Pavement 4 - 17 May 2006 

Figure 4-3. Typical zone of influence for an asphalt pavement section (Vandre et al., 1998). 

4.5.2 Subsurface Exploration Techniques 

Generally, there are four types of field subsurface investigation methods, best conducted in 

this order: 

1. Remote sensing 

2. Geophysical investigations 

3. In-situ investigation 

4. Borings and sampling 

All of these methods are applicable for pavement design. For example, in new pavement 

construction projects, the location of old streambeds, usually containing soft, organic 

deposits that will require removal or stabilization, can usually be identified by remote 

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

Percent of Subgrade Surface Value

Deflection

Normal Pressure

DCP Investigation Depth

(~ 9.5 psi normal pressure)

Typical Boring Investigation Depth

 Normal Pressure 

 Deflection 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 18 May 2006 

sensing. Once identified, the vertical and horizontal extent of the streambed deposit can be 

explored by using geophysics to determine the horizontal extent, followed by in-situ tests to 

quantify the vertical extent and qualitatively evaluate soil properties, and borings with 

samples to quantify soil properties. The extent of use for a specific exploration method will 

be dependent upon the type of pavement project (i.e., new construction, reconstruction, or 

rehabilitation), as discussed in the following subsections. 

4.5.3 Remote Sensing 

Remote sensing data from satellite and aircraft imagery can effectively be used to identify 

terrain conditions, geologic formations, escarpments and surface reflection of faults, buried 

stream beds, site access conditions, and general soil and rock formations that may impact 

new pavement design and construction. Infrared imagery can also be used to identify locally 

wet areas. 

While remote sensing methods are most valuable for new construction, this information may 

also be used to explain poor performance of existing pavements in rehabilitation and 

reconstruction projects. 

Remote sensing data from satellites (e.g., LANDSAT images from NASA), aerial 

photographs from the USGS or state geologists, U.S. Corps of Engineers, commercial aerial 

mapping service organizations) can be easily obtained. State DOTs also use aerial 

photographs for right-of-way surveys and road and bridge alignments, and they can make 

them available for use by the engineer responsible for exploration. Especially valuable are 

old air photos compared to new ones in developed areas, which often identify buried features, 

such as old streambeds. Some ground control (e.g., borings) is generally required to verify 

the information derived from remote sensing data. 

4.5.4 Geophysical Investigations 

Geophysical survey methods can be used to selectively identify boring locations, supplement 

borehole data, and interpolate between borings. There are several kinds of geophysical tests 

that can be used for stratigraphic profiling and delineation of subsurface geometries. These 

include the measurement of mechanical waves (deflection response, seismic refraction 

surveys, crosshole, downhole, and spectral analysis of surface wave tests), as well as 

electromagnetic techniques (resistivity, EM, magnetometer, and radar). Mechanical waves 

are additionally useful for the determination of elastic properties of subsurface media, 

primarily the small-strain shear modulus. Electromagnetic methods can help locate 

anomalous regions, such as underground cavities, buried objects, and utility lines. The 
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Geotechnical Aspects of Pavement 4 - 19 May 2006 

geophysical tests do not alter the soil conditions and, therefore, classify as non-destructive. 

Several are performed at the surface level (termed non-invasive). The advantages of 

performing geophysical methods include 

• nondestructive and/or non-invasive, 

• fast and economical testing, 

• provide theoretical basis for interpretation, and are 

• applicable to soils and rocks. 

The primary disadvantage is that no samples or direct physical penetration tests are taken. 

Models are also assumed for interpretation, which sometimes appears to be an art. The results 

are also affected by cemented layers or inclusions, and are influenced by water, clay, and 

depth. 

The most common geophysical methods used for pavement evaluation is deflection response 

testing, with a majority of the agencies using the falling weight deflectometer (FWD) 

impulse type method (as previously mentioned in Sections 4.2.2 and 4.2.3 for rehabilitation 

and reconstruction projects). In rehabilitation and reconstruction projects, this method 

provides a direct evaluation of the stiffness of the existing pavement layers under simulated 

traffic loading. FWD, especially the newer lightweight deflectometers (LWD), can also be 

used during the construction of new pavements to confirm subgrade stiffness characteristics, 

either for verifying design assumptions or providing a quality control (QC) tool. LWD, along 

with other methods used for evaluating the stiffness of natural or compacted subgrades for 

construction control, are discussed in Chapter 8. 

Other deflection methods include steady-state dynamic methods, which produce a sinusoidal 

vibration in the pavement, and quasi-static devices, which measure pavement deflections 

from a slow, rolling load (e.g., the Benkelman beam). The most promising development in 

deflection methods is the high-speed deflectometers, which measure deflections while 

continuously moving. While these methods increase the complexity of measurement, they 

offer significant advantages in terms of safety, through reduced traffic control requirements, 

productivity (typically 3 – 20 km/hr {2 – 12 mph}), and increased volume of information. A 

detailed review of each of these deflection methods in provided in NCHRP Synthesis 278 

(Newcomb and Birgisson, 1999), and guidelines for deflection measurements are provided in 

ASTM, one on general dynamic deflection equipment (ASTM D 4695) and one on fallingweight-type impulse load devices (ASTM D 4602). 

Electrical-type geophysical tests may also be used in pavement design and construction, 

including surface resistivity (SR), ground penetrating radar (GPR), electromagnetic 

conductivity (EM), and magnetic survey (MS). These electrical methods are based on the 

resistivity or, conversely, the conductivity of pore water in soil and rock materials. Mineral 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 20 May 2006 

grains comprised of soils and rocks are essentially nonconductive, except in some exotic 

materials, such as metallic ores, so the resistivity of soils and rocks is governed primarily by 

the amount of pore water, its resistivity, and the arrangement of the pores. These techniques 

allow for mapping of the entire surface area of the site, making them useful in imaging the 

generalized subsurface conditions and detecting utilities, hidden objects, boulders, and other 

anomalies. The mapping is conducted on a relative scale of measurements that reflect 

changes across the property. For rehabilitation projects and reconstruction projects, GPR is 

often used for mapping the thickness of existing pavement layers. Electrical-type methods 

may also aid in finding underground cavities, caves, sinkholes, and erosion features in 

limestone and dolomite terrain. In developed areas, they may be used to detect underground 

utility lines, buried tanks and drums, and objects of environmental concern. Additional 

details on SR, EM, GPR, and MS can be found in Greenhouse, et al. (1998), FHWA manual 

on Application of Geophysical Methods to Highway Related Problems (Wightman et al., 

2003), and in the geophysical information portion of the Geoforum website at 

http://www.geoforum.com/info/geophysical/. 

Mechanical wave geophysical methods are also used in pavement design and construction, 

including seismic refraction, seismic reflection, and, most recently, spectral analysis of 

surface waves (SASW). Both methods can be used to locate the depth to bedrock. Seismic 

refraction is also a key method for estimating rippability of rock. The use of the SASW 

mechanical wave method for determining subgrade modulus values for pavement design has 

recently been demonstrated in field trails. An automated device has been developed and is 

being tested by the Texas DOT. However, the testing and interpretation time is still 

somewhat long for use in pavement applications (Newcomb and Birgisson, 1999, and 

Wightman et al., 2003). 

A general summary for each of the more common geophysical methods used in pavement 

design is outlined in Tables 4-2 through 4-7. Application examples are provided following 

the tables. 
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Table 4-2. Falling weight deflectometer (FWD). 

Reference 

Procedures 

ASTM D4694 (Deflections with a Falling Weight Type Impulse Load Device); LTPP 

Manual for Falling Weight Deflectometer Measurement: Operational Field Guidelines

(August 2000). 

Purpose Used to determine the variation of pavement layer and subgrade stiffness along a length of 

pavement. For geotechnical features, can be used to backcalculate resilient modulus of 

subgrade and previously constructed base layers and to identify areas (e.g., weak subgrade 

conditions) requiring boring and sampling. 

Procedure As described in ASTM D4694, the FWD method consist of applying an impulse load to 

the paved or unpaved road surface using a falling weight, typically between 4 – 107 kN 

(1,000 – 24,000 lbs), dropped on a plate resting on the pavement surface, as shown in 

Figure 4-4 below. The peak force at impact is measured by a load cell and can be recorded 

as the impact force or the mean stress (by dividing the load by the plate area). The vertical 

deflection of the pavement surface is measured at the center of the applied load and at 

various distances (up to eight locations are typical) away from the load, as shown in 

Figure 4-4. The method usually uses a vehicle or a trailer that is brought to a stop with the 

loading plate positioned over the desired test location. Several tests may be performed at 

the same location and at the same or different heights. By measuring the deflection 

response at the same location under different loads (drop heights), the linear or non-linear 

characteristics of the pavement system and individual layers can be evaluated. The vehicle 

is then moved to the next location. The plate and deflection sensors are lowered to the 

pavement surface. For routine surveys, the tests are typically performed on a spacing of 

20 – 50 m (70 – 160 ft) along the road. The deflection at the center gives an idea of the 

overall pavement performance and the difference in deflection between deflections at 

various distances indicates the conditions of the pavement layers (bound, unbound, and 

subgrade). Profiles of the deflections can then be plotted over the length of the pavement 

(Figure 4.4c), in order to show the variation of pavement layer and subgrade stiffness. 

The deflection bowls obtained from the FWD data can be analyzed to back calculate the 

effective stiffness (or load spreading ability) of the various pavement and subgrade layers, 

by matching measured deflections to computed values. Back calculation is most 

commonly performed using a multi-layer linear elastic model for the pavement layers. For 

example, the effective pavement modulus, which is a measure of the effective or 

combined stiffness of all layers above the subgrade, can be determined from the center 

deflection as follows (AASHTO, 1993): 

















































 



 +

−

+

















+

=

P

R

P

R

E

a

D

M

E

a

D M

d pa

2

2

3

0

1

1 1

1

1 1.5

where, 

d0 = deflection measured at the center of the load plate (and adjusted to a standard 

temperature of 20°C {68°F} for hot mix asphalt), inches 

p = load plate pressure, psi 
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a = load plate radius, inches 

D = total thickness of pavement layers above the subgrade, inches 

Ep = effective modulus of all pavement layers above the subgrade, psi 

1 in = 25.4 mm, 1 psi = 6.7 kPa 

Outer deflection reading of the deflection basin primarily reflects the in-situ modulus of 

the lower soil (or subgrade). The subgrade resilient modulus can be calculated as follows: 































 = +

≥

=

2

3

2

0.7

0.24

R

P

e

e

r

R

M

E

a a D

r a

d r

P M

where, 

MR = back-calculated subgrade resilient modulus, psi 

P = applied load, pound 

dr = deflection at a distance r from the center of the load, inches 

r = distance from center of load, inches 

ae = radius of the stress bulb at subgrade-pavement interface, inches 

a = load plate radius, inches 

D = total pavement thickness above subgrade, inches 

1 in = 25.4 mm, 1 psi = 6.7 kPa, 

Subgrade resilient modulus for design purposes is usually less than the value directly from 

FWD data. The AASTHO design guide (1993) recommends a design subgrade resilient 

modulus equal to 33% of that back calculated from FWD data for flexible pavement and 

25% of the back-calculated value for PCC pavement. 

 
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Geotechnical Aspects of Pavement 4 - 23 May 2006 

 a) 

 b) c) 

Figure 4.4. FWD showing: a) typical equipment; b) schematic of procedure; and, c) sketch of 

deflection bowl for interpretation of results. 

D2 

D D5

D 4 3 

D6 D7
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 24 May 2006 

Commentary The FWD produces a dynamic impulse load that simulates a moving wheel load, rather 

than a static, semi-static, or vibratory load. FWD tests can be used for all construction 

types (i.e., new construction, rehabilitation, or reconstruction). For new construction, 

testing can be performed directly on cleared subgrade, or done during construction, after 

placement of the subbase, base, or pavement surface layer. The method can also be used 

to evaluate the effectiveness of subgrade improvements (for weak subgrades). Based on 

the results of FWD, the roadway section can be delineated into design sections with 

similar properties and the intrusive explorations methods (i.e., in-situ testing and borings) 

located accordingly to obtain the thickness of layers, confirm subgrade stiffness values, 

and obtain other characteristics of the subgrade (e.g., soil type and moisture conditions). 

The deflection profile along the project may be examined to determine if changes exist in 

the pavement’s structural response. The profile can be used to assist in locating areas 

where more intensive sampling and testing will be required, greatly improving the 

efficiency of laboratory evaluation. The profile may also be used to divide the project into 

design sections. For example, in rehabilitation projects, FWD results can be used to 

optimize the overlay design and/or subdrainage design in each of the design sections. 

It should be noted that the influence depth for elastic deflections measured with FWD 

may extend more than 9 m (30 ft) and, as a result, may miss near-surface critical features. 

Also, the results may be influenced by deep and often unknown conditions. FWD results 

are also affected by temperature and freezing conditions. Thus, it is important to consider 

the time of day and the season when scheduling the FWD program. As previously 

indicated, deflection measurements are corrected to a standard temperature, typically 

20°C (68°F), and critical season equivalent deflections based on locally-developed 

procedures. 

 ADVANTAGES DISADVANTAGES

- Speed, repeatability & equipment robustness - Static method requires stopping 

 between readings 

- Easily transported - Traffic control required 

- Simulates moving traffic loads - Deep features (e.g., water table and 

 bedrock) and temperature affect results

- Direct evaluation of design MR values - MR over predicted 

- Non-destructive - Requires well-defined layer thickness 

 
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Geotechnical Aspects of Pavement 4 - 25 May 2006 

Table 4-3. Surface resistivity (SR). 

Reference 

Procedures 

ASTM G57 (Field Measurement of Soil Resistivity [Wenner Array]) 

Purpose Resistivity is used to locate bedrock, stratigraphy, wet regions, compressible soils, map 

faults, karstic features, contamination plumes, buried objects, and other uses. 

Procedure Resistivity is a fundamental electrical property of geomaterials and can be used to evaluate 

soil types and variations of pore fluid and changes in subsurface media (Santamarina et al., 

2001). The resistivity (ρR) is measured in ohm-meters and is the reciprocal of electrical 

conductivity (kE = 1/ρR). Conductivity is reported in siemens per meter (S/m), where S = 

amps/volts. Electrical current is put into the ground using two electrodes, and the resulting 

voltage is measured using two other electrodes. Using pairs or arrays of electrodes 

embedded into the surface of the ground, a surface resistitivity survey can be conducted to 

measure the difference in electrical potential of an applied current across a site. The 

spacing of the electrodes governs the depth of penetration by the resistivity method and the 

interpretation is affected by the type of array used (Wenner, dipole-dipole, Schlumberger). 

The entire site is gridded and subjected to parallel arrays of SR-surveys, if a complete 

imaging map is desired. Mapping allows for relative variations of soil types to be 

discerned, as well as unusual features. Recently developed automated resistivity systems 

collects much more data than simple SR and combines resistivity sounding and traverse 

data to form a resistivity section with detailed interpretation, as shown in Figure 4-5. 

Commentary In general, resistivity values increase with soil grain size. Figure 4-6 presents some 

illustrative values of bulk resistivity for different soil and rock types. Figure 4-7 shows the 

field resistivity illustrative showing stratigraphic changes. Downhole resistivity surveys 

can also be performed using electronic probes that are lowered vertically down boreholes, 

or are direct-push placed. The latter can be accomplished using a resistivity module that 

trails a cone penetrometer, termed a resistivity piezocone (RCPTu). Downhole resistivity 

surveys are particularly advantageous in distinguishing the interface between upper 

freshwater and lower saltwater zones in coastal regions. 

For new pavement design, surface resistivity can be used to evaluate the areal extent of soil 

deposits and assist in identifying sample locations. Resistivity is also related to moisture 

content and can be used to map variations in moisture, and, thus, regions of compressible 

soils can be delineated. This moisture relation can also be valuable for rehabilitation and 

reconstruction projects, indicating areas requiring special considerations, such as improved 

drainage. SR may also be useful in determining the depth of rock, which as previously 

indicated may have an influence on FWD results and is a design input for ME design. SR 

can also be used in construction to assist in locating prospective sand, gravel, or other 

sources of borrow material. 

 ADVANTAGES DISADVANTAGES

- Moderately fast: ~150 m/hr (500 ft/hr) - Requires coring concrete or asphalt to 

 insert electrodes 

- Fairly simple - Traffic control required 

- Can evaluate significant depths - Lateral resistivity variations affect results 

- Works for higher- or lower-resistance - Nearby grounded metal objects affect 

 sublayers data 

- Automation improves interpretation - Wetting electrodes required in dry 

 ground 
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Geotechnical Aspects of Pavement 4 - 26 May 2006 

Figure 4-5. Two-dimensional cross-section resistivity profile for detection of sinkholes and 

caves in limestone (from Schnabel Engineering Associates). 

Figure 4-6. Representative values of resistivity for different soils (Mayne et al., 2002).

E 

 l 

 e 

 v. 

 (m)

 

Figure 4-7. Resistivity data showing stratigraphic changes (Advanced Geosciences, Inc.).
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Geotechnical Aspects of Pavement 4 - 27 May 2006 

Table 4-4. Ground-penetrating radar (GPR). 

Reference 

Procedures 

Wightman et al., (2003) Application of Geophysical Methods to Highway Related 

Problems, FHWA Contract Number DTFH68-02-P-00083. 

Purpose GPR can be a valuable tool used to define subsoil strata, moisture variation, depth to rock, 

voids beneath pavement, buried pipes, cables, as well as to characterize archaeological 

sites before soil borings, probes, or excavation operations. It can also be utilized to 

determine the thickness of pavement layers, thus complementing FWD evaluation, and 

mapping reinforcing steel in concrete surface pavements. 

Procedure Short impulses of a high-frequency electromagnetic waves are transmitted into the ground 

using a pair of transmitting & receiving antennae. The reflected signals, which occur at 

dielectric discontinuities in the pavement system and subgrade, are recorded. Thus 

changes in the dielectric properties (permittivity) of the soil reflect relative changes in the 

subsurface environment. The GPR surveys are made by driving over the surface with aircoupled antennas mounted on the vehicle or pulling a tracking cart with ground-coupled 

antenna mounted on a sled across the ground surface. Air-coupled antennas are used to 

evaluate shallow depths (e.g., thickness of pavement layers) at highway speeds. Groundcoupled antennas are used to evaluate greater depths (up to 18 m (60 ft)). The EM 

frequency and electrical conductivity of the ground control the depth of penetration of the 

GPR survey. Many commercial systems come with several sets of paired antennas to 

allow variable depths of exploration, as well as accommodate different types of ground. 

Commentary The GPR surveys provide a quick imaging of the subsurface conditions, leaving 

everything virtually unchanged and undisturbed. In pavement engineering practice, GPR 

using air-coupled antenna is most commonly used to identify layer thickness of pavement 

materials and perform condition evaluation of pavement surface materials. Methods for 

improving the accuracy of thickness measurements are reported by Al-Qadi et al., 2003. 

For subsurface evaluation, ground-coupled GPR is required. The GPR subsurface surveys 

are particularly successful in deposits of dry sands with depths of penetration up to 20 m 

or more (65 ft). In wet, saturated clays, GPR is limited to shallow depths of only 1 – 3 m 

(3 – 10 ft) (still adequate for pavement subgrade evaluation. Searches below the water 

table are difficult and, in some cases, not possible. Several illustrative examples of GPR 

surveys are shown in Figure 4-8. Additional information on current usage of GPR by state 

agencies is contained in NCHRP Synthesis 255 on Ground Penetrating Radar for 

Evaluating Subsurface Conditions for Transportation Facilities (Morey 1998). 

A recent development (GeoRadar) uses a variably-sweeping frequency to capture data at a 

variety of depths and soil types. Other developments include combining the use of aircoupled antenna with ski-mounted, ground-coupled antenna to allow for surface and 

subsurface evaluation at highway speeds. 

 ADVANTAGES DISADVANTAGES

- Fast: 2 – 80 km/h (1 – 50 mph) & easy to use - Perception of difficult interpretation 

- Different antenna provide different penetration - May require traffic control 

 depths and resolution - Restricted depth in saturated clay soils

- Produces real-time, continuous subsurface data 

- Non-destructive 
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Geotechnical Aspects of Pavement 4 - 28 May 2006 

Figure 4-8. Representative ground-coupled GPR results showing buried utilities and soil 

profile (from EKKO Sensors & Software: www.sensoft.on.ca). 

Figure 4-9. Conductivity results along a road in New Mexico (Blackhawk GeoServices, Inc). 

 Note: Plan view 0.75 m (2.5 ft) below road surface
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Geotechnical Aspects of Pavement 4 - 29 May 2006 

Table 4-5. Electromagnetic conductivity (EM). 

Reference 

Procedures 

Wightman et al., (2003) Application of Geophysical Methods to Highway Related 

Problems, FHWA Contract Number DTFH68-02-P-00083. 

Purpose The EM methods provide a very good tool for identifying areas of clay beneath existing 

pavements in rehabilitation and road widening projects, or in the subgrade for new 

construction. These methods are also excellent at tracking buried metal objects and are 

well known in the utility locator industry. They can also be used to detect buried tanks, 

map geologic units, and groundwater contaminants, generally best within the upper 1 or 2 

m (3 or 6 ft), yet can extend to depths of 5 m (16 ft) or more. 

Procedure Several types of electromagnetic (EM) methods can be used to image the ground and 

buried features, including: induction, frequency domain, low frequency, and time domain 

systems. Ground conductivity methods can rapidly locate conductive areas in the upper 

few meters of the ground surface. These measurements are recorded using several 

instruments that use electromagnetic methods. Electromagnetic instruments that measure 

ground conductivity use two coplanar coils, one for the transmitter and the other for the 

receiver. The transmitter coil produces an electromagnetic field, oscillating at several kHz, 

that produces secondary currents in conductive material in the ground. The amplitude of 

these secondary currents depends on the conductivity of the material. These secondary 

currents then produce secondary electromagnetic fields that are recorded by the receiver 

coil. Surveys are best handled by mapping the entire site to show relative variations and 

changes. Areas of high electrical conductivity are likely places to find clay. 

The choice of which instrument to use generally depends on the depth of investigation 

desired. Instruments commonly used include the EM38, EM31, EM34 (Geonics Ltd, 

Canada), and GEM2 (Geophex, USA). The EM38 is designed to measure soil 

conductivities and has a maximum depth of investigation of about 1.5 m (5 ft). The EM31 

has a depth of investigation to about 6 m (20 ft), and the EM34 has a maximum depth of 

investigation to about 60 m (200 ft). The depth of investigation of the GEM2 is advertised 

to be 30 – 50 m (100 – 165 ft) in resistive terrain (>1000 ohm-m) and 20 – 30 m (65 – 100 

ft) in conductive terrain (<100 ohm-m). 

Commentary Clay is almost always electrically conductive, and areas of high conductivity have a 

reasonable chance that they will contain clay (e.g., see Figure 4-9). However, estimating 

the amount of clay from conductivity measurements alone is generally not possible. 

Conductivity is influenced by many factors including the degree of saturation, porosity, 

and salinity of the pore fluids. Conductivity measurements taken with instruments that 

investigate to depths greater than the upper layer are also influenced by other layers. 

 ADVANTAGES DISADVANTAGES

- Moderately fast: 2 – 8 km/hr (1 – 5 mph) - Influenced by above- and below-ground 

 metal (e.g., fence post, utilities, rebar). 

- Data is easy and efficient to record - Several passes may be required 

- Instruments used in different modes to - Traffic control required 

 maximize information at tailored depths 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 30 May 2006 

Table 4-6. Mechanical wave using seismic refraction. 

Reference 

Procedures 

ASTM D5777 . 

Wightman et al., (2003) Application of Geophysical Methods to Highway Related 

Problems, FHWA Contract Number DTFH68-02-P-00083. 

Purpose Seismic refraction surveys are used to locate depth and characteristics (e.g., rippability) of 

bedrock, as well as evaluate dynamic elastic properties of the soil and rock . 

Procedure Seismic refraction involves placing a line of regularly spaced sensors (geophones) on the 

surface and measuring the relative arrival time of seismic energy transmitted from a 

specified source location. Seismic waves produced by the energy source penetrate the 

overburden and refract along the bedrock surface, continually radiating seismic waves 

back to the ground surface, as shown in Figure 4-10. Refraction data are recorded in the 

field using a portable seismograph, multiple (generally 12 per line) geophones (generally 

<15 Hz), a repeatable seismic source (e.g., sledgehammer striking a metal plate or light 

explosive charges), and a power source. Sledgehammer sources are generally used for 

depths less than 10 – 15 m (30 – 50 ft) and explosives for greater depths up to 30 m (100 

ft). Mechanical waves generated by the seismic source include the compression (P-wave) 

and shear (S-wave) wave types that are measured. P waves are the first arrival waves, are 

the easiest to measure, and are not absorbed by saturated soil units (i.e., shear waves 

cannot transmit through water). Seismic energy travels with a compression velocity that is 

characteristic of the density, porosity, structure, and water content of each geologic layer. 

The seismic refraction survey is planned with respect to anticipated soil/rock velocities to 

be encountered, the approximate depth to rock, and the end-use of the data (e.g.,

rippability of the rock). Multiple seismic source points permit improved delineation of 

soil/rock interfaces. 

Commentary Seismic surveys are not intended to supplant subsurface sampling investigations, but aid 

in quickly and economically extending subsurface characterization over large areas, filling 

in the gaps between discrete borings. 

Although a number of parameters (e.g., uniaxial strength, degree of weathering, 

abrasivenesss, frequency of planes of weakness) relate to rippability of rock, seismic 

refraction has historically been the geophysical method utilized to predetermine the degree 

of rippability. Correlations of rock rippability as published by the Caterpillar Company are 

shown in Figure 4-11. 

 ADVANTAGES DISADVANTAGES

- Lightweight equipment, 2-person crew - Slow (but faster than borings) 

- Very effective at locating bedrock - Traffic control required 

- Well-established correlation with rippability - Velocities must increase in successively 

 deeper strata. 

- Can be used where drilling is physically - Water has a higher velocity than soil 

 or economically limited and some weak or highly jointed rock 

- Background seismic noise may interfere 

 with data refinement and interpretation 

- Lateral deposition may influence results 
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Geotechnical Aspects of Pavement 4 - 31 May 2006 

Figure 4-10. Seismic refraction survey (Blackhawk GeoServices, Inc.).

Figure 4-11. Rippability (using a D9 Caterpillar tractor) versus seismic velocity (Caterpillar 

Handbook of Ripping, 8th Edition). 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 32 May 2006 

Figure 4-9 shows an example of using a geophysical method (i.e., EM) for locating clay 

seams on a project in New Mexico. This project demonstrates the ability to correlate the data 

with soil type and clay content. For this project survey, data were recorded with one of the 

EM instruments mounted on a trailer constructed primarily from non-conductive materials. 

The trailer was towed by an All Terrain Vehicle (ATV). A GPS receiver was also mounted 

on the trailer to provide position information. Data were recorded automatically at halfsecond intervals with the EM31 and EM38. Recordings with the EM31 were made at two 

different instrument heights above the ground, giving two different penetration depths. This 

procedure required several passes along the road. Having obtained conductivity 

measurements at a number of different depths at each recording location, the data were 

modeled and provided the interpreted vertical distribution of conductivity with depth. This 

interpretation is shown in the lower plot in Figure 4-9. The upper plot shows the ground 

conductivity measured with the EM38 at a depth of 0.75 m (2.5 ft) below the road surface. 

This data clearly shows the location of clay materials and provides a clear road map for 

planning additional exploration and sampling (Wightman et al., 2003). 

Another example of a project that effectively incorporated geophysical testing into the 

investigation program has been reported by the Missouri DOT. Geophysical surveys were 

conducted for the Missouri Department of Transportation (MoDOT) by the Department of 

Geology and Geophysics at the University of Missouri-Rolla to determine the most probable 

cause or causes of ongoing subsidence along a distressed section of Interstate 44 in 

Springfield, Missouri. Ground penetrating radar (GPR) and reflection seismic survey quickly 

assessed roadway and subsurface conditions with non-destructive, continuous profiles. The 

GPR proved to be of useful utility in defining upward-propagating voids in embankment fill 

material. The reflection seismic survey established the presence of reactivated paleosinkholes 

in the area. These were responsible for swallowing the fill material as water drained through 

the embankment. On the basis of interpretation of these data, MoDOT personnel were able to 

drill into the voids that had developed beneath the pavement (as a result of washing out of the 

fine-grained material of the embankment fill), and to devise an effective grouting plan for 

stabilization of the roadway (Newton, et al., 1998). 

Geophysical data was used to preclude additional subsurface exploration on a rehabilitation 

project by the Texas DOT. The project consisted of a 16.9 km (10.5 mile) section of road, 

which was exhibiting substantial alligator cracking and potholes in the southbound lane, as 

observed in 1999. The project was constructed in 1979 with 152 mm (6 in.) of lime stabilized 

subgrade where clay subgrade was present, 254 mm (10 in.) of granular base, and a 54-mm 

(2-in.) ACP surface. ACP level up courses and two open graded friction courses were then 

placed in 1988 and 1992, respectively. Maintenance forces had subsequently placed several 

seal coat patches, AC patches, and ACP overlays. FWD and GPR data were taken in the 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 33 May 2006 

outside wheelpath at 160 m (0.1 mile) and 3 m (10 ft), respectively. Cores were taken at 

select locations based on the GPR data analysis. The data indicated that the open graded 

friction courses were holding water where the maintenance forces had placed the ACP 

overlays. Cores indicated that the open graded friction course was disintegrating; however, 

the original ACP layer underneath the friction course was in good shape. FWD data analysis 

indicated that the base material was structurally in good shape and no base repair and, 

consequently, no additional subsurface exploration were needed. The overlay and friction 

course were removed and replaced with a 127 mm (5 in.) ACP overlay in 2001, and was 

reported to be performing well (Wimsatt, A.J. and Scullion, T., 2003). 

4.5.5 In-Situ Testing 

In-situ testing can also be used to supplement soil borings and compliment geophysical 

results. In-situ geotechnical tests include penetration-type and probing-type methods, in most 

cases without sampling, to directly obtain the response of the geomaterials under various 

loading situations and drainage conditions. In-situ methods can be particularly effective when 

they are used in conjunction with conventional sampling to reduce the cost and the time for 

field work. These tests provide a host of subsurface information, in addition to developing 

more refined correlations between conventional sampling, testing, and in-situ soil 

parameters. 

With respect to pavement design, in-situ tests can be used to rapidly evaluate the variability 

of subgrade support conditions, locate regions that require sampling, identify the location of 

rock and groundwater with some methods, and, with correlation, provide estimates of design 

values. Design values should always be confirmed through sampling and testing. Table 4-7 

provides a summary of in-situ subsurface exploration tests that have been used for design of 

pavements and evaluation of pavement construction considerations. 

For new pavement design, the most utilized in-situ method is the standard penetration test 

(SPT); however, the dynamic cone penetrometer (DCP) and/or electronic cone penetrometer 

test (CPT) (see Figure 4-12) should be given special consideration for pavement design and 

evaluation (as many agencies are currently doing). DCP and CPT offer a more efficient and 

rapid method for subgrade characterization and have a significantly greater reliability than 

SPT, as explained in Table 4-8 on the SPT, Table 4-9 on the DCP, and Table 4-10 on the 

CPT. 
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Geotechnical Aspects of Pavement 4 - 34 May 2006 

Figure 4-12. Common in-situ tests for pavement evaluation (Mayne et al., 2002). 

The CPT and DCP provide information on subsurface soils, without sampling disturbance 

effects, with data collected continuously on a real-time basis. Stratigraphy and strength 

characteristics are obtained as the CPT or DCP progresses. Since all measurements are taken 

during the field operations and there are no laboratory samples to be tested, considerable time 

and cost savings may be appreciated. DCP is more qualitative than CPT, and is only useful 

for identifying variation in the upper meter of soil; however, it is performed with low cost, 

lightweight equipment, with a one- to two-person crew. DCP offers an excellent tool to 

perform initial exploration through core holes in the surface pavement in rehabilitation 

projects. Results of DCP tests through the pavement can be compared to test in the shoulders 

for road widening projects. DCP can also be an effective tool in the construction of pavement 

to evaluate the suitability of the subgrade after cut, fill, or stabilization operations, as 

discussed in Chapter 8, and the requirements for stabilization (as discussed in Chapter 7). 

The CPT provides more quantitative results, can be correlated directly to design properties 

and types of subgrade, and is useful to greater depths than the DCP in fine grained and sand 

type soils. Use of CPT and these correlations are detailed in the FHWA Subsurface 

Investigation Manual (FHWA NHI-01-031). 

Other in-situ tests, such as pressuremeter (PM), dilatometer test (DMT) and vane shear test 

(VST), are also useful in obtaining in-situ design properties, as outline in Table 4-7 but 

require special skilled personnel and are time intensive. Thus, they are not often used for 

pavement design. There are also a number of static load tests (e.g., plate load and field CBR) 

that can be used to assess stiffness and/or strength of the subgrade surface. These tests are 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 35 May 2006 

most valuable for reconstruction and rehabilitation projects Additional information on in-situ 

testing can be found on the website http://www.ce.gatech.edu/~geosys/misc/links.htm. 

The relevance of each test also depends on the project type and its requirements. The general 

applicability of the test method depends in part on the geomaterial types encountered during 

the site investigation, as shown in Table 4-7. 

4.5.6 Borings and Sampling 

The final exploration method includes drilling bore holes or, in some cases, making 

excavations to obtain samples. This is the most complex and expensive part of the 

exploration program, and requires a great degree of care. Disturbed and undisturbed samples 

of the subsurface materials must be obtained for laboratory analyses (and/or tested in the 

field) to determine their engineering properties and verify geophysical and in-situ exploration 

results. 

Disturbed samples are generally obtained to determine the soil type, gradation, classification, 

consistency, moisture-density relations (Proctor), CBR, presence of contaminants, 

stratification, etc. The methods for obtaining disturbed samples vary from hand or 

mechanical excavation of test pits using truck-mounted augers and other rotary drilling 

techniques. These samples are considered “disturbed,” since the sampling process modifies 

their natural structure. 

Undisturbed samples are obtained where necessary to determine the in-place stiffness and 

strength, compressibility (settlement), natural moisture content, unit weight, permeability, 

discontinuities, fractures, and fissures of subsurface formations. Even though such samples 

are designated as “undisturbed,” in reality they are disturbed to varying degrees. The degree 

of disturbance depends on the type of subsurface materials, type and condition of the 

sampling equipment used, the skill of the drillers, and the storage and transportation methods 

used. Serious and costly inaccuracies may be introduced into the design if proper 

protocol and care is not exercised during recovery, transporting, or storing of the 

samples. 

Table 4-11 provides a summary of the use and limitation of boring methods using disturbed 

and undisturbed sampling equipment. Additional information on each of these methods is 

contained in FHWA NHI-01-031. 
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Geotechnical Aspects of Pavement 4 - 36 May 2006 

Table 4-7. In-situ tests for subsurface exploration in pavement design and construction. 

Type of Test Best 

Suited 

For 

Not 

Applicable 

Properties That Can be 

Determined for 

Pavement Design and 

Construction 

Remarks 

Standard 

Penetration Test 

(SPT)* 

AASHTO T 206 

& ASTM D1586 

Sand & 

Silt 

Gravel, 

questionable 

results in 

saturated 

Silt 

Crude estimate of modulus 

in sand. Disturbed samples 

for identification and 

classification. Evaluation 

of density for 

classification. 

Test best suited for sands. 

Estimated clay shear 

strengths are crude & should 

not be used for design. See 

Table 4-8 and FHWA 

NHI-01-031. 

Dynamic Cone 

Test (DCP)* 

ASTM D6951 

Sand, 

Gravel, 

& Clay 

Clay with 

varying 

gravel 

content 

Qualitative correlation to 

CBR. Identify spatial 

variation in subgrade soil 

and stratification. 

See Table 4-9 and FHWA 

TS-78-209. 

Static Piezocone 

Test (CPT)* 

ASTM D3441 

Sand, 

Silt, Clay 

 Undrained shear strength 

and correlation to CBR in 

clays, density & strength 

of sand & gravel. 

Evaluation of subgrade 

soil type, vertical strata 

limits, and groundwater 

level. 

Use piezocone for pore 

pressure data. Tests in clay 

are reliable only when used 

in conjunction with other 

calibration tests (e.g., vane 

tests). See Table 4-10 and 

FHWA NHI-01-031. 

Field CBR Sand, 

Gravel, 

Silt, Clay 

Granular 

Soils (Lab 

and field 

correlations 

erratic.) 

Load-deflection test 

providing direct evaluation 

of CBR and can be 

correlated with subgrade 

modulus k-value. 

Slow, and field moisture 

may not represent worst-case 

condition. 

Plate Load Test 

AASHTO T222 

& ASTM D1196 

Sand, 

Gravel, 

Silt, Clay 

 Subgrade modulus 

k-value. 

Slow and labor intensive. 

Vane Shear Test 

(VST) 

AASHTO T-223 

Clay Silt, Sand, 

Gravel 

Undrained shear strength, 

Cu with correlation to 

CBR. 

Test should be used with 

care, particularly in fissured, 

varved, & highly plastic 

clays. See FHWA 

NHI-01-031. 

Permeability 

Test 

ASTM D51216 

& ASTM D6391 

Sand, 

Gravel 

Clay Evaluation of coefficient 

of permeability in base 

and subbase for 

rehabilitation projects. 

Variable head tests in 

boreholes have limited 

accuracy. See FHWA 

NHI-01-031. 

Pressuremeter 

Test (PMT) 

ASTM D4719 

Soft 

rock, 

Sand, 

Silt, Clay 

 Subgrade modulus k-value 

& undrained shear strength 

with correlation to CBR. 

Requires highly skilled field 

personnel. See FHWA 

IP-89-008 and FHWA 

NHI-01-031. 

Dilatometer Test 

(DMT) 

Sand, 

Clay 

 Soil stiffness can be 

related to subgrade 

modulus k and 

compressibility. 

Limited database and 

requires highly skilled field 

personnel. See FHWA NHI01-031. 

* These tests can be used in pavement design to qualitatively evaluate subgrade stratification and determine 

optimum undisturbed sample locations required to obtain design property values.
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Geotechnical Aspects of Pavement 4 - 37 May 2006 

Table 4-8. Standard penetration test (SPT). 

Reference 

Procedures 

AASHTO T 206 and ASTM D 1586. 

Standard Penetration Test and Split-Barrel Sampling of Soils. 

Purpose A quick means to evaluate the variability of the subgrade with correlation to density of 

granular soils and to obtain disturbed samples. 

Procedure The SPT involves the driving of a hollow, thick-walled tube into the ground and 

measuring the number of blows to advance the split-barrel sampler a vertical distance of 

300 mm (1 ft). A drop weight system is used for the pounding where a 63.5-kg (140-lb) 

hammer repeatedly falls from 0.76 m (30 in.) to achieve three successive increments of 

150-mm (6-in.) each. The first increment is recorded as a “seating,” while the number of 

blows to advance the second and third increments are summed to give the N-value 

("blow count") or SPT-resistance (reported in blows/0.3 m or blows per foot). If the 

sampler cannot be driven 450 mm, the number of blows per each 150-mm increment and 

per each partial increment is recorded on the boring log. For partial increments, the depth 

of penetration is recorded in addition to the number of blows. In current U.S. practice, 

three types of drop hammers (donut, safety, and automatic) and four types of drill rods 

(N, NW, A, and AW) are used in the conduct of the SPT. The test, in fact, is highly 

dependent upon the equipment used and the operator performing the test. Most important 

factor is the energy efficiency of the system. The range of energy efficiency for the 

current US standard of practice varies from 35 – 85% with cathead equipment, and 

80 – 100% with automated trip Hammer equipment. A calibration of energy efficiency 

for a specific drill rig & operator is recommended by ASTM D-4633 using instrumented 

strain gages and accelerometer measurements in order to better standardize the energy 

levels. If the efficiency is measured (Ef), then the energy-corrected N-value (adjusted to 

60% efficiency) is designated N60 and given by 

 N60 = (Ef/60) Nmeas 

 (5-1) 

The measured N-values should be corrected to N60 for all soils, if possible. 

Commentary The test can be performed in a wide variety of soil types, as well as weak rocks, yet is 

not particularly useful in the characterization of gravel deposits nor soft clays. The fact 

that the test provides both a sample and a number is useful, yet problematic, as one 

cannot do two things well at the same time. SPT correlations exist with angle of internal 

friction, undrained shear strength, and modulus. However, the SPT value and these 

correlations have large scatter, and should not be used alone for design. 

For pavement design and construction, SPT provides a measure of subgrade variability. 

In granular soils, the method provides an evaluation of relative density, which can be 

correlated to CBR. In addition, disturbed samples are obtained for identification of 

subgrade materials and for classification tests. SPT results can be sued to identify 
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Geotechnical Aspects of Pavement 4 - 38 May 2006 

locations where undisturbed samples should be taken. SPT data can also be compared 

with FWD results to confirm reasonableness (i.e., low resilient modulus values should 

compare to low SPT values). The following relationships have been suggested by 

Kulhowey and Mayne (1990) as a first order estimate of Young’s modulus (E/Pa): 

E/Pa ~ 5 N60 (sands with fines) 

E/Pa ~ 10 N60 (clean, normally consolidated sands) 

E/Pa ~ 15 N60 (clean, over consolidated sands) 

Where, Pa = atmospheric pressure 

Correlations have been attempted for estimating undrained shear strength and 

correspondingly CBR values in cohesive soils from N values. These relationships are 

extremely widespread in terms of interpretations, soil types, and testing conditions, such 

that a universal relationship cannot be advanced. In cohesive subgrades, SPT is better 

used to evaluate the variability of the subgrade (e.g., based on identification and 

classification of soil types encountered) and identify locations where proper samples 

(e.g., undisturbed tube samples for resilient modulus tests or bulk samples for CBR tests) 

should be taken. Alternatively, drill crews could be instructed to switch to tube samples 

when cohesive soils are encountered. 

 ADVANTAGES DISADVANTAGES

- Obtain both a sample & a number - Obtain both a sample & a number* 

- Simple & rugged - Disturbed sample (index tests only) 

- Suitable in many soil types - Crude number for analysis 

- Can perform in weak rocks - Not applicable in soft clays & silts 

- Available throughout the U.S. - High variability and uncertainty 

 (COV of N =15 to 100%)** 

Note: *Collection simultaneously results in poor quality for both the sample and the 

number. 

**COV, coefficient of variation, as reported by Kulhawy and Mayne, 1990. 
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Table 4-9. Dynamic cone penetrometer (DCP). 

Reference 

Procedures 

ASTM D 6951. 

Standard Test Method for Use of the Dynamic Core Penetrometer in Shallow Pavement 

Applications. 

The method is also described in FHWA TS-78-209. 

Purpose Another type of test that can be performed in the field to measure the strength of soils inplace, and is being used more commonly for pavement design purposes to estimate the inplace strength of both fine- and coarse-grained soils. 

Procedure The principle behind the DCP is that a direct correlation exists between the strength of a 

soil and its resistance to penetration by solid objects, such as cones (Newcombe and 

Birgisson, 1999). The DCP consists of a cone attached to a rod that is driven into soil by 

means of a drop hammer that slides along the penetrometer shaft. The mass of the hammer 

can be adjusted to 4.6 and 8 kg (10 and 18 lbs) with the lighter weight applicable for 

weaker soils. According to NCHRP Synthesis 278 (Newcomb and Birgisson, 1999), more 

recent versions of the DCP have a cone angle of 60 degrees, with a diameter of 20 mm 

(0.8 in.). 

A number of empirical correlations exist to relate the DCP penetration index (DPI) to 

subgrade strength parameters required for pavement design. The most widely used is 

(Webster et al., 1994): 

 CBR = 292/(DPI)1.12 for gravel, sand, and silt 

 CBR = 1/0.002871 DCP for highly plastic clays 

 CBR = 1/(0.017 DCP)2

 for low plasticity clays 

The above methods were based on a database of field CBR versus DCP penetration rate 

values collected for many sites and different soil types, and correlated to test results by 

others (e.g., log CBR = 2.61 – 1.26 log DCP as developed by Kleyn, 1975, and currently 

used by the Illinois DOT). For DCPs with automatic release hammers (e.g., the Israeli 

automated DCP), CBR values are about 15% greater than the above correlations for 

manual hammers (after Newcomb and Birgisson, 1999). 

ADVANTAGES

- Can be operated by one or two people 

- Site access for testing not a problem 

- Equipment is simple, rugged, and 

inexpensive 

- Continuous record of soil properties 

with depth & immediate results 

- Can be used in pavement core holes 

- Suitable in many soil types & can 

perform in weak rocks 

- Fair reliability (COV ~ 15 – 22 %)* 

- Available throughout the U.S. 

DISADVANTAGES

- Does not obtain a sample 

- Index tests only 

- High variability and uncertainty in 

gravelly soils 

- Limited depth to 1 m (3.3 ft) (however, 

adequate for most rehab projects and good 

for rapid surficial characterization). 

- Extraction of cone can be difficult. 

Commentary 

Note: *COV, coefficient of variation as reported by Kulhawy and Mayne, 1990. 
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Table 4-10. Cone penetrometer test (CPT). 

Reference 

Procedures 

ASTM D-3441 (mechanical systems) and ASTM D 5778 (electric and electronic systems). 

Test Method for Electronic Cone Penetration Testing of Soils. 

Purpose Fast, economical, and provides continuous profiling of geostratigraphy and soil properties 

evaluation. 

Procedure The test consists of pushing a cylindrical steel probe into the ground at a constant rate of 

20 mm/s (0.8 in/s) and measuring the resistance to penetration. The standard penetrometer 

has a conical tip with 60o

 angle apex, 35.7-mm (1.4 in.) diameter body (10-cm2 (1.6-in2) 

projected area), and 150-cm2

 (23-in2) friction sleeve. The measured point or tip resistance 

is designated qc and the measured side or sleeve resistance is fs. The ASTM standard also 

permits a larger 43.7-mm (1.72-in.) diameter shell (15-cm2

 (2.3-in2) tip and 200-cm2 (31-

in2

) sleeve). Piezocones are cone penetrometers with added transducers to measure 

penetration porewater pressures during the advancement of the probe. Most 

electric/electronic cones require a cable that is threaded through the rods to connect with 

the power supply and data acquisition system at the surface. An analog-digital converter 

and Pentium notebook are sufficient for collecting data at approximate 1-sec intervals. 

Depths are monitored using either a potentiometer (wire-spooled LVDT), depth wheel that 

the cable passes through, or ultrasonic sensor. Systems can be powered by voltage using 

either generator (AC) or battery (DC), or alternatively run on current. New developments 

include (1) the use of audio signals to transmit digital data up the rods without a cable and 

(2) memocone systems where a computer chip in the penetrometer stores the data 

throughout the sounding. 

Commentary The CPT can be used in very soft clays to dense sands, yet is not particularly appropriate 

for gravel or rocky terrain. The pros and cons are listed below. As the test provides more 

accurate and reliable numbers for analysis, yet no soil sampling, it provides an excellent 

complement to the more conventional soil test boring with SPT measurements. 

Figure 4-13 provides a comparison of CPT and SPT logs. 

For pavement design, the CPT provides a continuous log of the vertical variability of the 

subgrade. CPT can be used to identify soil types and soil consistency, which in turn can be 

used to determine appropriate type(s) and location(s) for sampling. Empirical relations 

have been developed for undrained shear strength and elastic modulus, as reviewed in 

FHWA-NHI-01-031 (Mayne et al., 2002). The Lousiana Tansportation Research Center in 

cooperation with FHWA has recently developed a correlation between cone parameters 

and resilient modulus (Muhammad et al., 2002). 

 ADVANTAGES DISADVANTAGES____________ 

- Fast and continuous profiling - High capital investment 

- Economical and productive - Requires skilled operator to run 

- Results not operator-dependent - Electronic drift, noise, and calibration 

- Strong theoretical basis in interpretation - No soil samples are obtained 

- Particularly suitable for soft soils - Unsuitable for gravel or boulder deposits*

- Good reliability (COV ~ 7 – 12 %)** 

Note: *Except where special rigs are provided and/or additional drilling support is available. 

**COV, coefficient of variation as reported by Kulhawy and Mayne, 1990.
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Figure 4-13. CPT log in comparison to SPT data from several locations. 

To begin the boring and sampling exploration process, a boring layout and sampling plan 

should be established to ensure that the vertical and horizontal profile of the different soil 

conditions can be prepared. A typical design practice for pavements is to assign one subgrade 

support value to long roadway lengths, i.e., 1 – 16 km (0.6 – 10 mi). This approach may be 

reasonable for uniform soil deposits, especially considering the construction advantage of 

maintaining a uniform pavement cross section. However, for highly variable sites, this 

approach is questionable, as it invariably leads to either an overly conservative design or 

premature pavement distress in some sections. Significant local variations can best be 

handled as special design features. There may be more variation of soil properties vertically 

(drill holes) than horizontally at shallow depths; however, again, only one value is assigned. 
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Thus, one of the primary sampling issues is how best to sample such that appropriate values 

can be assigned to long sections of roadway. Two sampling options are available: systematic 

or representative. 

Systematic sampling is a common agency practice. It is done at uniform horizontal and/or 

vertical intervals. Intermediate locations are sampled when varying conditions are 

encountered. A large number of samples can be obtained, but the testing may either be on a 

random basis to obtain an average value for similar materials or a representative basis for 

variable conditions. 

Representative sampling and testing consists of taking samples that are believed to be 

representative of the typical or conservative soil support values. This type of sampling is 

based primarily on engineering judgment based on other information about the site (i.e.,

evaluation of available information, site reconnaissance, remote sensing, and geophysical 

testing) and involves fewer samples. 

Table 4-11. Subsurface exploration-exploratory boring methods.

Method Use Limitations 

Auger Boring 

ASTM D – 1452 

Obtain samples and identify changes in soil texture 

above water table. Locate groundwater. 

Grinds soft particles – 

stopped by rocks, etc. 

Test Boring 

ASTM D – 1586 

Obtain disturbed split spoon samples for soil 

classification. Identify texture and structures; 

estimate density or consistency in soil or soft rock 

using SPT (N). 

Poor results in gravel, hard 

seams. 

Thin Wall Tube 

ASTM D – 1587 

Obtain 51 – 86 mm (2 – 3-3/8 in.) diameter 

undisturbed samples of soft-firm clays and silts for 

later lab testing (e.g., resilient modulus tests). 

Cutting edge wrinkled in 

gravel. Samples lost in 

very soft clays and silts 

below water table. 

Stationary Piston 

Sampler 

Obtain undisturbed 51 – 86 mm (2 – 3-3/8 in.) 

diameter samples in very soft clays. Piston set 

initially at top of tube. After press is completed, 

any downward movement of the sample creates a 

partial vacuum, which holds the sample in the tube. 

In pavement design, these samples can be used for 

evaluating pavement settlement and/or treatability 

studies. 

Cutting edge wrinkled in 

gravel. 

Pits, Trenches Visual examination of shallow soil deposits and 

man-made fill above water table. Disturbed 

samples for density and CBR tests, or undisturbed 

block samples for resilient modulus tests, may be 

extracted. 

Caving of walls, 

groundwater. Requires 

careful backfill and 

compaction. 
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AASHTO 1993 requires the use of average subgrade support values along the alignment, and 

uses reliability to account for variation in subgrade strength along the alignment. To obtain a 

true statistical average, random sampling would be appropriate, provided the soil conditions 

are rather homogeneous. Systematic is not random, but it may be close with respect to 

averaging. Unfortunately, with systematic, additional borings are often not performed in 

areas where varying conditions are encountered. So while an average may be achieved, 

localized conditions along the alignment that could significantly impact performance are 

often missed. Statistically, the objective is to delineate locations with similar properties 

(origins and moisture conditions) and assign design values using random methods for the 

defined population. This is best accomplished by a combination of methods, as outlined in 

the following subsections. 

Frequency (number/spacing) of Borings 

The design engineer should prescribe the spacing and depth of the borings based on an 

evaluation of available information. As indicated in the previous section, only limited 

representative borings and sampling are required if geophysical and in-situ testing have been 

performed. Again, some borings should be performed at several cone locations for calibration 

and at critical locations identified by the preceding methods. A more extensive program is 

required in the absence of this alternative exploration information. 

The spacing and depth of these borings depend on the variability of the existing soil 

conditions, both vertically and horizontally, and the type of pavement project. Spacing of 

borings vary considerably among agencies, on the order of 12 per km (20 per mi) to as few as 

2 per km (3 per mi), with spacing generally decreased with high-volume roads and finegrained soils, as reported by Newcomb and Birgisson, 1999. Considering the variability of 

soils and the tests used to evaluate geotechnical materials, even the high number appears 

relatively low. The following provides a review of recommended practice from a 

geotechnical perspective based on guidelines from textbooks, several state agencies, and the 

FHWA. 

The spacing of borings along the roadway alignment generally should not exceed 60 m (200 

ft) for a fully invasive program. Where subsurface conditions are known to be uniform, a 

minimum spacing of 120 m (400 ft) is generally recommended. In a program supported by 

geophysical and in-situ tests, such as recommended in Sections 4.5.4 and 4.5.5, a spacing of 

150 – 450 m (500 – 1500 ft) as indicated in NCHRP 1-37A may be all that is necessary, 

depending on the uniformity of site conditions. For new pavement projects, most agencies 

locate borings along the centerline, unless conditions are anticipated to be variable. Borings 

should be located to disclose the nature of subsurface materials at the deepest points of cuts, 

areas of transition from cut to fill, and subgrade areas beneath the highest points of 
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embankments. The spacing and location of the borings should be selected considering the 

geologic complexity and soil/rock strata continuity within the project area, with the objective 

of defining the vertical and horizontal boundaries of distinct soil and rock units within the 

project limits. It should be noted that the cost for a few extra borings is insignificant in 

comparison to the cost of unanticipated field conditions or premature pavement failure. 

The spacing of borings for rehabilitation and reconstruction projects will depend on the 

condition of the existing pavement, the performance of non-destructive geophysical tests, and 

the availability of previous subsurface information. As indicated in the NHI (1998) 

“Techniques for Pavement Rehabilitation” Participants Manual, drilling and sampling is 

performed on three levels: 1) a high level in the absence of non-destructive geophysical tests, 

2) a low level to complement geophysical tests, and 3) at a diagnostic level to evaluate 

mechanisms of distress where it occurs. In the absence of non-destructive geophysical tests, 

spacing on the order of one boring every 150 m (500 ft) would appear to be a minimum for 

pavements with no unusual distressed conditions. Additional borings should be located in 

problem areas (e.g., areas of rutting or fatigue cracking, which are often associated with 

subgrade issues) identified in the condition survey as discussed in Sections 4.2.2 and 4.2.3. 

The number of borings should be increased to the level of new pavement projects when 

rehabilitation projects include substantial pavement removal and replacement. Again, 

performance of geophysical tests (e.g., FWD) and/or in-situ tests (e.g., DCP) tests could be 

used to supplement borings, in which case, sampling at a minimum of every 450 m (1500 ft) 

may be adequate to complement the geophysical or non-destructive test results, provided 

there are no areas of significant distress that require special attention. Spacing of borings 

should be decreased as the variability of the geophysical or in-situ results increase to verify 

those results via laboratory testing. 

For pavement rehabilitation projects, borings should be located in the wheel path to evaluate 

performance of existing unbound materials, as well as the subgrade. Borings should also be 

specifically located (and the number increased as required) to investigate the presence of wet 

or soft subgrade conditions indicated by site reconnaissance and/or maintenance records. If 

the project involves replacing or rubbilizing the existing pavement, all borings would be 

drilled through the existing pavement. If the project involves adding a lane, plus replacing or 

rubbilizing the existing pavement, half the borings should be in the new lane and half in the 

existing pavement. 

 

As previously indicated in the introduction of Section 4.5.6, borings should be taken to a 

minimum depth of 1.5 – 2 m (5 – 7 ft) below the proposed pavement subgrade elevation, 

with at least a few borings taken to 6 m (20 ft) below the grade line. These deeper borings 

should also be used to determine the water table depth and occurrence of bedrock. Deeper 
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borings are not generally required for rehabilitation projects, unless the previous section 

experienced premature failure due to subgrade conditions or there is a change in vertical 

alignment. All borings should extend through unsuitable foundation strata (for example, 

unconsolidated fill, highly organic materials, or soft, fine-grained soils) to reach relatively 

hard or compact materials of suitable bearing capacity to support the pavement system. 

Borings should extend a minimum of 1.5 m (5 ft) into relatively stiff or dense soils beneath 

soft deposits. Borings in potentially compressible fine-grained strata of great thickness 

should extend to a depth where the stress from superimposed traffic loads or a thick 

embankment is so small (less than 10% of the applied surface stress) that consideration will 

not significantly influence surface settlement. 

Greater depth of borings may be required where deep cuts are to be made, side hill cuts are 

required, large embankments are to be constructed, or subsurface information indicates the 

presence of weak (or water-saturated) layers. In those cases, the borings should be deep 

enough to provide information on any materials that may cause problems with respect to 

stability, settlement, and drainage. For side hill cuts, additional borings should be performed 

on the uphill side in uniform soil conditions and on the uphill and downhill side for 

nonuniform conditions. Additional borings may be required for slope stability considerations 

and analysis. 

Where stiff or compact soils are encountered at the surface and the general character and 

location of rock are known, borings should extend into sound rock. Where the location and 

character of rock are unknown or where boulders or irregularly weathered materials are 

likely to be found, the boring penetration into rock should be increased (NCHRP 1-37A, 

2003), as discussed later in this section. 

Take sufficient and appropriate auger, split tube, or undisturbed samples of all representative 

subsoil layers, as discussed in the next section. The soil samples must be properly sealed and 

stored to prevent moisture loss prior to laboratory testing. Prepare boring logs and soil 

profiles from this data. 

Subsurface investigation programs, regardless of how well they may be planned, must be 

flexible to adjust to variations in subsurface conditions encountered during drilling. The 

project engineer should, at all times, be available to confer with the field inspector. On 

critical projects, the engineer responsible for the exploration program should be present 

during the field investigation. He/she should also establish communication with the design 

engineer to discuss unusual field observations and changes to be made in the investigation 

plans. 
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Soil Sampling (after NCHRP 1-37A) 

Sampling will vary with the type of pavement project. For new construction projects, a 

majority of the samples taken will most likely be the disturbed type, such as those obtained 

by split barrel samplers. This will permit visual identification and classification of the soils 

encountered, as well as identification by means of grain size, water content, and Atterberg 

limit tests. In rehabilitation projects, sampling to determine the potential of full depth 

reclamation or the potential for rubbilization of asphalt pavements is somewhat different, 

requiring the sampling of the in-place base, subbase, and surface pavement to determine its 

suitability for reuse and/or rubblizing. The condition survey, as discussed in section 4.2.3, 

will help in identifying areas requiring sampling and the types of samples required. In 

general, sampling of the subgrade is not as intensive as is needed for new pavements. 

Detailed sampling of the base, subbase, and surface pavement will be required to determine if 

there is a large amount of variability in materials along the project and the condition of those 

materials for reuse (e.g., base and subbase that has been contaminated with large quantities of 

fines would not be desirable). 

Sampling at each boring location may be either continuous or intermittent. In the former 

case, samples are obtained throughout the entire length of the hole; in the latter (primarily 

used in areas of deep cuts), samples are taken about every 1.5 m (5 ft) and at every change in 

material. Initially, it is preferable to have a few holes with continuous sampling so that all 

major soil strata present can be identified. Every attempt should be made to obtain 100 

percent recovery where conditions warrant. The horizontal and vertical extent of these strata 

can then be established by intermittent sampling in later borings, if needed. 

To obtain a basic knowledge of the engineering properties of the soils that will have an effect 

on the design, undisturbed samples (such as those obtained with thin-wall samplers or double 

tube core barrel rock samplers) should be taken, if possible. The actual number taken should 

be sufficient to obtain information on the shear strength, consolidation characteristics, and 

resilient modulus of each major soil stratum. Undisturbed samples should comply with the 

following criteria: 

1. The samples should contain no visible distortion of strata, or opening or 

softening of materials. 

2. Specific recovery ratio (length of undisturbed sample recovered divided by 

length of sampling push) should exceed 95 percent. 

3. The samples should be taken with a sampler with an area ratio (cross sectional 

area of sampling tube divided by full area or outside diameter of sampler) less 

than 15 percent. 
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At least one representative undisturbed sample should be obtained in cohesive soil strata, in 

each boring for each 1.5-m (5-ft) depth interval, or just below the planned surface elevation 

of the subgrade. Recommended procedures for obtaining undisturbed samples are described 

in AASHTO Standard T207, Thin-Walled Tube Sampling of Soils. If undisturbed samples 

cannot be recovered, disturbed samples should be taken. 

All samples (disturbed and undisturbed) and cores should be wrapped or sealed to prevent 

any moisture loss, placed in protective core boxes, and transported to the laboratory for 

testing and visual observations. Special care is required for undisturbed tube samples. When 

additional undisturbed sample borings are taken, the undisturbed samples are sent to a soils 

laboratory for testing. Drilling personnel should exercise great care in extracting, handling, and 

transporting these samples to avoid disturbing the natural soil structure. Tubes should only be 

pressed, not driven with a hammer. The length of press should be 100 – 150 mm (4 – 6 in.) less 

than the tube length (DO NOT OVERPRESS). A plug composed of a mixture of bees’ wax 

and paraffin should be poured to seal the tube against moisture loss. The void at the upper tube 

end should be filled with sawdust, and then both ends capped and taped before transport. The 

most common sources of disturbance are rough, careless handling of the tube (such as dropping 

the tube samples in the back of a truck and driving 50 km (30 mi) over a bumpy road), or 

temperature extremes (leaving the tube sample outside in below zero weather or storing in front 

of a furnace). Proper storage and transport should be done with the tube upright and encased in 

an insulated box partially filled with sawdust or expanded polystryrene to act as a cushion. 

Each tube should be physically separated from adjacent tubes, like bottles in a case. A detailed 

discussion of sample preservation and transportation is presented in ASTM D 4220, Practice 

for Preserving and Transporting Soil Samples, along with a recommended transportation 

container design. 

Rock Sampling 

The need for sampling rock will depend on the location of bedrock with respect to the design 

subgrade elevation, geology of the region, the availability of geophysical data and local 

experience. The transition from soil to weathered rock to sound rock can be erratic and 

highly variable, often causing major geotechnical construction problems (i.e., claims). Rock 

above the subgrade elevation will need to be removed by ripping or blasting. Considering 

blasting typically cost 4 to 20 times more than ripping, in addition to the noise and vibration 

problems associated with blasting, a determination of ripability is an important part of the 

subsurface exploration program. As previously discussed in Section 4.5.4, ripability can be 

determined by refraction survey methods, and should be confirmed by coring a sampling of 

the rock. SPT values have also been used to assess ripability, with values or 80 to 100 

typically assumed to be the demarcation between ripping and blasting (Rolling and Rolling). 

However, there do not appear to be any hard-and-fast rules. The regional geology and the 
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local ability of the contractor are both significant factors. Considering the determination of 

ripping versus basting is not an exact science, test pits are recommended to confirm the 

exploration results. 

If the bedrock is near the subgrade level, then the pavement design will dictate requirements 

for additional samples. Technically pavements can be located directly above competent, 

intact rock with only a cushion/drainage layer, generally consisting of 150 mm (6 in.) of 

gravel required between flexible or rigid pavement and the rock. The rock surface should be 

sloped to promote drainage. It is imperative that the rock surface be level to provide a 

uniform bearing surface and prevent water from being trapped in local depressions. 

Undulating rock may therefore require additional excavation, especially if pockets contain 

poor quality materials, such as frost susceptible soils. For example, Figure 4-14 shows 

representative excavation requirements where frost susceptible soils exist over undulating 

rock. 

Highly weathered rock and deleterious rock (i.e., rock that degrades easily when exposed to 

the environment) such as shale, will be required to be removed to a greater depth, on the 

order of 0.6 – 1 m (2 – 3 ft) based on local experience. In either case, the reason for sampling 

is to determine the competency of the rock and the amount of excavation required. 

It is generally recommended that a minimum 1.5-m (5-ft) length of rock core be obtained to 

verify that the boring has indeed reached bedrock and not terminated on the surface of a 

boulder (Mayne et al., 2002). Coring methods and evaluation of rock quality is covered in 

FHWA NHI-01-031. This rock core depth should be followed if rock is encountered within 

1 m (3 ft) of pavement subgrade level, and could be reduced if rock is located at greater 

depths. 

Cores should be used to identify the rock, determine the quality of the rock, and evaluate its 

durability. Evaluation of durability should be based on a review of past performance, slaking 

tests and physical degradation tests (Rollins and Rollins, 1996). Many problems with 

deleterious rocks have been regionally identified across the U.S. Durability tests are reviewed 

in Chapter 5. 

Groundwater 

Observations of the groundwater level and pressure are an important part of geotechnical 

explorations for pavement design and construction, and the identification of groundwater 

conditions should receive the same level of care given to soil descriptions and samples. The 

water level is part of the input in the mechanistic-empirical design approach. Also, as 

mention is Section 4.5.4, the location of the water level will influence interpretation of FWD 
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Geotechnical Aspects of Pavement 4 - 49 May 2006 

and other geophysical results. The water level is also critical to determine the drainage 

requirements for construction and long-term performance of the pavement. In addition, the 

water level will influence the selection of appropriate stabilization methods, as discussed in 

Chapter 7. 

Figure 4-14. Excavation requirements for frost susceptible soils over undulating rock. 
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Measurements of water entry during drilling, and measurements of the groundwater level at 

least once following drilling, should be considered a minimum effort to obtain water level 

data, unless alternate methods, such as installation of observation wells or piezometers, are 

defined by the geotechnical engineer. Detailed information regarding groundwater 

observations can be obtained from ASTM D 4750, Standard Test Method For Determining 

Subsurface Liquid Levels in a Borehole or Monitoring Well and ASTM D 5092, Design and 

Installation of Groundwater Wells in Aquifers. 

The water level in the boring is not the only indication of the groundwater level. If the 

borehole has caved, the depth to the collapsed region should be recorded and reported on the 

boring record, as this may have been caused by groundwater conditions. The elevations of 

the caved depths of certain borings may be consistent with groundwater table elevations at 

the site, and this may become apparent once the subsurface profile is constructed. Drilling 

mud obscures observations of the groundwater level owing to filter cake action and the 

higher specific gravity of the drilling mud compared to that of the water. If drilling fluids are 

used to advance the borings, the drill crew should be instructed to bail and flush the hole 

prior to making groundwater observations. 

Unless the soils are granular with little or no fines (i.e., clay and/or silt size particles), the 

water level in the boring may take days or weeks to rise to the actual groundwater level. 

Considering the potential for cave-in and infiltration of surface water during this period and 

with consideration for the potential for seasonal changes in the groundwater level, a bore 

hole is usually not the best means to get a true picture of the long-term water conditions at a 

site. For accurate measures of groundwater, observation wells or piezometers should be 

installed in the borehole. An “observation well” measures the level in a water table aquifer, 

while a “piezometer” measures the pressure in a confined aquifer, or at a specific horizon of 

the geologic profile (Powers, 1992). 

The simplest type of observation well is formed by a small-diameter polyvinyl chloride 

(PVC) pipe set in an open hole. The bottom of the pipe is slotted and capped, and the annular 

space around the slotted pipe is backfilled with clean sand. The area above the sand is sealed 

with bentonite, and the remaining annulus is filled with grout, concrete, or soil cuttings. A 

surface seal, which is sloped away from the pipe, is commonly formed with concrete in order 

to prevent the entrance of surface water. The top of the pipe should also be capped to prevent 

the entrance of foreign material; a small vent hole should be placed in the top cap. 

Piezometers are available in a number of designs. Commonly used piezometers are of the 

pneumatic and the vibrating wire type. Interested readers are directed to the reference 
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manuals of the FHWA NHI course on Geotechnical Instrumentation (FHWA-NHI-98-034), 

FHWA NHI course on Subsurface Investigation (FHWA-NHI-01-031), or Dunnicliff (1988) 

for a detailed discussion of the various types of piezometers. 

Permeability of the subgrade is rarely an issue for pavement design, but may be of interest in 

terms of dewatering requirements for excavations or installation of interceptor drains to 

lower groundwater. For rehabilitation projects, permeability of existing base and subbase 

may be of interest in order to evaluate drainage characteristics (e.g., time to drain) of in-place 

materials. Field permeability tests may be conducted on natural soils (and rocks) by a number 

of methods, including simple falling head, packer (pressurized tests), pumping (drawdown), 

slug tests (dynamic impulse), and dissipation tests. Simple falling head tests are typically 

used for evaluating the permeability of in-place base and subbase materials. A brief listing of 

the field permeability methods is given in Table 4-12. 

Test Pits 

Exploration pits and trenches, excavated by hand, a backhoe, or bulldozer, permit detailed 

examination of the soil and rock conditions at shallow depths and relatively low cost. 

Exploration pits can be an important part of geotechnical explorations where significant 

variations in soil conditions occur (vertically and horizontally), large soil and/or non-soil 

materials exist (boulders, cobbles, debris) that cannot be sampled with conventional methods, 

or buried features must be identified and/or measured. 

The depth of the exploration pit is determined by the exploration requirements, but is 

typically about 2 – 3 m (6.5 – 10 ft). In areas with high groundwater level, the depth of the 

pit may be limited by the water table. Exploration pit excavations are generally unsafe and/or 

uneconomical at depths greater than about 5 m (16 ft), depending on the soil conditions. The 

U.S. Department of Labor's Construction Safety and Health Regulations, as well as 

regulations of any other governing agency, must be reviewed and followed prior to 

excavation of the exploration pit, particularly in regard to shoring requirements.

During excavation, the bottom of the pit should be kept relatively level so that each lift 

represents a uniform horizon of the deposit. At the surface, the excavated material should be 

placed in an orderly manner adjoining the pit with separate stacks to identify the depth of the 

material. The sides of the pit should be cleaned by chipping continuously in vertical bands, or 

by other appropriate methods, so as to expose a clean face of rock or soil. Survey control at 

exploration pits should be done using optical survey methods to accurately determine the 

ground surface elevation and plan locations of the exploration pit. Measurements should be 

taken and recorded documenting the orientation, plan dimensions and depth of the pit, and 

the depths and the thicknesses of each stratum exposed in the pit. In logging the exploration 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 52 May 2006 

pit, a vertical profile should be made parallel with one pit wall. After the pit is logged, the 

shoring will be removed and the pit may be photographed or video logged at the discretion of 

the geotechnical engineer. Photographs and/or video logs should be located with reference to 

project stationing and baseline elevation. A visual scale should be included in each photo or 

video. 

Exploration pits can, generally, be backfilled with the spoils generated during the excavation. 

The backfilled material should be compacted to avoid excessive settlements. Tampers or 

rolling equipment may be used to facilitate compaction of the backfill. 

Sampling for Fill/Borrow Materials 

Samples are also required to determine the suitability of cut materials to be used as fill and to 

evaluate suitable borrow sources for additional fill, as required, and for base and subbase 

materials. Many different soils may be suitable for use in the construction of the roadway 

embankment or fill. The fill for the subgrade material must be of high quality and, preferably, 

granular material. Silt and clay type soils are less desirable for subgrade, as they will dictate a 

thicker pavement section. Bulk samples should be obtained in order to determine the 

moisture-density relations (Proctor) of each soil type encountered. Moisture-density tests 

should be used to determine the compaction characteristics for embankment and/or surface 

soils and untreated pavement materials. AASHTO T99 should be used for medium to high 

plasticity fine-grained soils, whereas AASHTO T180 should be used for coarse-grained and 

low plasticity fine-grained soils. The degree of compaction required for the in-place density 

should be expressed as a percentage of the maximum density from the specified test 

procedure. Design tests (e.g., resilient modulus, CBR, etc.) are also required on the 

compacted subgrade material. 

Standards and Guidelines 

Field exploration by borings should be guided by local practice, by applicable FHWA and 

state agency procedures, and by the AASHTO and ASTM standards listed in Table 4-12. 

Current copies of these standards and manuals should be maintained in the engineer’s office 

for ready reference. The geotechnical engineer and field inspector should be thoroughly 

familiar with the contents of these documents, and should consult them whenever unusual 

subsurface situations arise during the field investigation. The standard procedures should 

always be followed; improvisation of investigative techniques may result in erroneous or 

misleading results that may have serious consequences on the interpretation of the field data. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 53 May 2006 

Table 4-12. Frequently-used standards for field investigations. 

Standard Title

AASHTO ASTM

M 146 C 294 Descriptive Nomenclature for Constituents of Natural Mineral 

Aggregates 

T 86 D 420 Guide for Investigating and Sampling Soil and Rock 

- D 1195 Test Method for Repetitive Static Plate Load Tests of Soils and Flexible 

Pavement Components, for Airport and Highway Pavements 

- D 1196 

Test Method for Nonrepetitive Static Plate Load Tests of Soils and 

Flexible Pavement Components, for Use in Evaluation and Design of 

Airport and Highway Pavements 

T 203 D 1452 Practice for Soil Investigation and Sampling by Auger Borings 

T 206 D 1586 Standard Penetration Test and Split-Barrel Sampling of Soils 

T 207 D 1587 Practice for Thin-Walled Tube Sampling of Soils 

T 225 D 2113 Practice for Diamond Core Drilling for Site Investigation 

M 145 D 2487 Test Method for Classification of Soils for Engineering Purposes 

- D 2488 Practice for Description and Identification of Soils (Visual-Manual 

Procedure) 

T 223 D 2573 Test Method for Field Vane Shear Test in Cohesive Soil 

- D 3385 Infiltration Rate of Soils in Field Using Double-Ring Infiltrometer 

- D 3550 Practice for Ring-Lined Barrel Sampling of Soils 

- D 4220 Practice for Preserving and Transporting Soil Samples 

- D 4428 Test Method for Crosshole Seismic Test 

- D 4544 Practice for Estimating Peat Deposit Thickness 

- D 4694 Test Method for Deflections with a falling-Weight-Type Impulse Load 

Device 

- D 4700 General Methods of Augering, Drilling, & Site Investigation 

- D 4719 Test Method for Pressuremeter Testing in Soils 

- D 4750 Test Method for Determining Subsurface Liquid Levels in a Borehole or 

Monitoring Well (Observation Well) 

- D 5079 Practices for Preserving and Transporting Rock Core Samples 

- D 5092 Design and Installation of Ground Water Monitoring Wells in Aquifers 

 D 5126 Guide for Comparison of Field Methods for Determining Hydraulic 

Conductivity in the Vadose Zone 

- D 5777 Guide for Seismic Refraction Method for Subsurface Investigation 

- D 5778 Test Method for Electronic Cone Penetration Testing of Soils 

- D 6391 Field Measurement of Hydraulic Conductivity Limits of Porous 

Materials Using Two Stages of Infiltration from a Borehole 

- D 6635 Procedures for Flat Plate Dilatometer Testing in Soils 

- D 6951 Test Method for Use of Dynamic Cone Penetrometer in Shallow 

Pavement Applications 

- G 57 Field Measurement of Soil Resistivity (Wenner Array) 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 54 May 2006 

4.5.7 Guidelines for Idealized Subsurface Exploration Program 

The ideal exploration program would begin with remote sensing to survey the area for site 

access issues and to identify geologic formations and other features that would guide the 

selection and suitability of geophysical test methods. Next, geophysical testing would be 

performed using FWD as the principle tool, where possible, for back-calculation of resilient 

modulus values and/or profiling the site, thus, potentially reducing the number of borings 

required and the cost of laboratory testing. Resistivity would be used in conjunction with 

FWD to evaluate the extent of significant soil strata, and ground probing radar could be used 

to provide continuous thickness profiles for the pavement layers, as well as the location of 

groundwater. CPT or DCP would then be used to classify soil strata, obtain characteristic 

strength values, and confirm thickness profiles. This would be followed by limited borings 

and sampling, with some borings performed at several cone locations for 

calibration/verification, and at critical locations identified by the preceding methods. Again, 

disturbed samples are generally obtained to determine the soil type, gradation, classification, 

consistency, moisture-density relations (Proctor), CBR, presence of contaminants, 

stratification, etc. Undisturbed samples are obtained where necessary to determine the inplace stiffness and strength, compressibility (settlement), natural moisture content, unit 

weight, permeability, discontinuities, fractures, and fissures of subsurface formations. 

The primary reason for following this idealized program is to develop a detailed 

understanding of subgrade and/or the existing unbound pavement layers that will impact 

design, construction, and the long-term performance of the pavement structure. There is also 

a cost implication for this program. Figure 4-15 provides an indication of relative cost for 

each phase. However, the reduced number of borings and sampling, and the improved 

reliability of the pavement system, should more than offset the cost of this program. 

The Finnish roadway authority has fully integrated this approach into their pavement design. 

For example, to obtain an initial evaluation of the existing pavement section in rehabilitation 

and reconstruction projects, they use 1) GPR to provide an evaluation of the thickness of 

existing pavement components (using air-coupled antenna) and subgrade quality information 

(using ground-coupled antenna); 2) FWD to obtain the existing roadway support conditions; 

3) roughness and rutting measurements; 4) pavement distress mapping; 5) GPS positioning; 

and reference drilling based on GPR results. The collected road survey data is processed, 

interpreted, analyzed, and classified, using Road DoctorTM software specifically developed 

for this purpose, as shown in Figure 4-16a. Most recently, they have added resistivity surveys 

to evaluate moisture content. By combining technologies, they are able to develop a complete 

map of the subgrade system, including moisture (Figure 4-16b) and corresponding settlement 

profiles (Dumas et al., 2003). The analysis includes a classification of the critical elements 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 55 May 2006 

affecting the lifetime of the road, including 1) overall pavement condition, 2) condition 

assessment of the unbound pavement structure, 3) road fatigue related to subgrade frostaction, 4) drainage condition, and 5) local damages, such as settlement of the road (Roimela 

et al, 2000). This information provides a better understanding of the causes of pavement 

distress and more precise rehabilitation measures for problem layers in the existing pavement 

system. Similar combinations of technology are used for the evaluation of subgrade 

conditions for new pavement design. This approach supports Finnish philosophy in pavement 

design, which presumes that any treatment to the subgrade should last from 60 – 100 years, 

the base and subbase should last from 30 – 50 years, and the surface should have a life of 

from 15 – 20 years. This sound philosophy is based on the relative cost of rehabilitation 

associated with each of these layers, and the importance of engineering in characterizing the 

soil and selecting material of the lower pavement layers. 

0

1

2

3

4

5

6

7

8

9

10

Heel Print

Geostick

Hand

Penetrometer

Geophysical

DCP

SPT

CPT

DMT

VST

Borehole

Shear

Menard

Pressuremeter

Borings & lab

test

0.5 1 1.5 2 3 3 5 6 7 8 9 10

Relative Test Reliability

Relative Cost per Test

Figure 4-15. Qualitative relationship between relative subsurface exploration cost and 

reliability (after Handy, 1980). 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 56 May 2006 

a) 

Water content 

image for 

settlement 

calculations 

b) 

Figure 4-16. Geophysical evaluation used by the Finnish National Road Administration for 

rehabilitation and reconstruction projects showing a) results from road analysis and b) 

moisture profile beneath the pavement (Tolla, 2002). 

Video image Video image of the

road under survey road under survey

(.avi or .mpg) GPR profile profile

IRI profile profile

FWD deflection bowls deflection bowls

Drill core Drill core data

Road Analysis Road Analysis 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 57 May 2006 

Texas DOT has recently developed a guideline, which supports the approach of using GPR 

and FWD data supported by DCP testing in the rehabilitation/reconstruction project 

evaluation process, as reported by Wimsatt and Scullion (2003). Computer programs have 

been developed to analyze the GPR and FWD data. GPR data is processed specifically to 

determine pavement layer thicknesses and the presence of excessive moisture or excessive air 

voids in pavement layers. FWD data is processed to generate remaining life estimates and 

pavement and subgrade layer moduli values. The DCP data is then used as required to verify 

the results of FWD data analysis, such as measuring base, subbase, and stiffness, or 

determining the depth to a stiff layer. Cores are generally collected at locations based on the 

GPR results (e.g., in suspect areas). 

4.6 IDENTIFY SOURCE FOR OTHER GEOTECHNICAL COMPONENTS 

As indicated in section 4.1, the next subsurface exploration step is to evaluate conceptual 

designs and determine sources for other geotechnical components (e.g., base and subbase 

materials). The requirements for subsurface drainage and subgrade stabilization, as well as 

construction material properties, should also be determined. Sampling of construction 

materials was discussed briefly in Section 4.5.6. The detailed requirements for these 

components will be covered in Chapter 7. 

4.7 SUBGRADE CHACTERIZATION 

The last step in the exploration process is to characterize the subgrade through 1) an 

evaluation of the field data, 2) performance of classification tests to support the fieldidentified subsurface stratigraphy, 3) develop stratigrahic profiles of the site, and 4) use that 

information to select representative soil layers for laboratory testing. Evaluation of the field 

data includes compiling and examining the stratigraphic information from the field 

investigation steps (i.e., existing information, geophysical results, in-situ tests and borings), 

and the generation of final boring logs. The final logs are generated using classification tests 

to establish and support stratigraphy in relation to the design parameters. Soil profiles and 

plan views along the roadway alignment can then be created and examined to determine 

resilient modulus or other design testing requirements for each influential soil strata 

encountered. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 58 May 2006 

4.7.1 Boring Logs 

The boring log is the basic record of almost every geotechnical exploration and provides a 

detailed record of the work performed and the findings of the investigation. A boring log is a 

description of exploration procedures and subsurface conditions encountered during drilling, 

sampling, and coring. The field log should be written or printed legibly, and should be kept 

as clean as is practical. 

Boring logs provide the basic information for the selection of test specimens. They provide 

background data on the natural condition of the formation, on the groundwater elevation, 

appearance of the samples, and the soil or rock stratigraphy at the boring location, as well as 

areal extent of various deposits or formations. The subsurface conditions observed in the soil 

samples and drill cuttings or perceived through the performance of the drill rig (for example, 

rig chatter in gravel, or sampler rebounding on a cobble during driving) should be described 

in the wide central column on the log labeled “Material Description,” or in the remarks 

column, if available. The driller's comments are valuable and should be considered as the 

boring log is prepared. All appropriate portions of the logs should be completed in the field 

prior to completion of the field exploration. Following is a brief list of items, which should 

be included in the logs. 

• Topographic survey data, including boring location and surface elevation, and bench 

mark location and datum, if available. 

• An accurate record of any deviation in the planned boring locations. 

• Identification of the subsoils and bedrock, including density, consistency, color, 

moisture, structure, geologic origin. 

• For rehabilitation and reconstruction projects, an accurate thickness (+/- 2 mm {0.1 

in.}) of each existing pavement layer should be carefully documented. 

• The depths of the various generalized soil and rock strata encountered. 

• Sampler type, depth, penetration, and recovery. 

• Sampling resistance in terms of hydraulic pressure or blows per depth of sampler 

penetration. Size and type of hammer. Height of drop. 

• Soil sampling interval and recovery. 

• Rock core run numbers, depths/lengths, core recovery, and Rock Quality Designation 

(RQD). 

• Type of drilling operation used to advance and stabilize the hole. 

• Comparative resistance to drilling. 

• Loss of drilling fluid. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 59 May 2006 

• Water level observations with remarks on possible variations due to tides and river 

levels. 

• The date/time that the borings are started, completed, and of water level 

measurements. 

• Closure of borings. 

A wide variety of drilling forms are used by various agencies, with some agencies using 

computerized logs entered on hand-held computers in the field. The specific forms to be used 

for a given type of boring will depend on local practice. A typical boring log is presented in 

Figure 4-17. A key or legend should be established by the agency for use by either in-house 

or outsource drilling in order to maintain uniformity in boring log preparation. A 

representative legend for soil boring logs and for core boring logs is included in Appendix D. 

In addition to the description of individual samples, the boring log should also describe 

various strata. The record should include a description of each soil layer, with solid 

horizontal lines drawn to separate adjacent layers. Soil description/identification is the 

systematic, precise, and complete naming of individual soils in both written and spoken 

forms (ASTM D-2488, AASHTO M 145). During progression of a boring, the field 

personnel should only describe the soils encountered. Group symbols associated with 

classification should not be used in the field. Samples are later returned to the lab where 

samples may be classified. Soil classification is the grouping of the soil with similar 

engineering properties into a category based on index test results; e.g., group name and 

symbol (ASTM D-2487, AASHTO M 145). A key part of classification of soil classification 

is the assignment of group symbols, which should only be assigned after supporting 

laboratory tests have been performed. 

It is important to distinguish between visual identification (i.e., a general visual 

evaluation of soil samples in the field) versus classification (i.e., a more precise 

laboratory evaluation supported by index tests) in order to minimize conflicts between 

field and final boring logs. Some agencies have assigned symbols in the field based on 

visual observation and later corrected them on final boring logs based on lab tests. This 

practice leads to discrepancies between the field logs and the final logs that have on several 

occasions been successfully used to support contractor’s claims in litigation procedures. In 

order to avoid these problems, it is recommended that group symbols not be included on field 

logs, but be reserved only for classification based on lab tests. Some states have avoided 

these problems by using lower-case symbols for field logs and upper-case symbols for labsupported classification results, with the lower-case symbols clearly defined on the logs as 

based on visual observation only. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 60 May 2006 

Figure 4-17. Subsurface exploration log. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 61 May 2006 

 The stratigraphic observations should include identification of existing fill, topsoil, and 

pavement sections. Visual descriptions in the field are often subjected to outdoor elements, 

which may influence results. It is important to send the soil samples to a laboratory for 

accurate verification of visual identification, classification tests, and the assignment of 

appropriate group symbols, as discussed in the next section. 

Data from the boring logs are combined with laboratory test results and other field 

information (i.e., historical logs, soil survey and geological information, geophysical and insitu tests) to identify subgrade profiles showing the extent and depth of various materials 

along the roadway alignment. Detailed boring logs, including the results of laboratory tests, 

are included in the geotechnical investigation report. Guidelines for completion of the boring 

log forms, preparation of soil descriptions and classifications, and preparation of rock 

descriptions and classifications are covered in detail in FHWA NHI-01-031, Subsurface 

Investigation manual.

4.7.2 Soil Classification 

All soils should be taken to the laboratory and classified using the AASHTO (or Unified) soil 

classification system (see Figure 4-18). As previously indicated, final identification with 

classification can only be appropriately performed in the laboratory. This will lead to more 

consistent final boring logs and will avoid conflicts with field descriptions. The Unified Soil 

Classification System (USCS) Group Name and Symbol (in parenthesis) appropriate for the 

soil type in accordance with AASHTO M 145, ASTM D 3282, or ASTM D 2487 is the most 

commonly used system in geotechnical work and, more recently, highway subgrade material. 

It is covered in detail in this section. The AASHTO classification system has been often used 

for classification of highway subgrade material, and is shown in comparison with the USCS 

in Figures 4-18 and 4-19. While both methods are based on grain size and plasticity, USCS 

groups soils with similar engineering properties. 

Table 4-13 provides an outline of the laboratory classification method. Table 4-14 relates the 

Unified soil classification of a material to the relative value of a material for use in a 

pavement structure. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 62 May 2006 

Figure 4-18. The AASHTO and the Unified Soil Classification System (after Utah DOT, 

1998). 

0

10

20

30

40

70

60

50

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

70

60

50

0 10 20 30 40 50 60 70 80 90 100

Liquid Limit Liquid Limit

 

 

4

7

A-6

A-2-6

A-7-6

A-7-5

A-2-7

A-5

A-2-5

A-4

A-2-4

PI = LL-30

Pla

stic

nI d

e

x

Pla

stic

nI d

e

x

CL or OL

CH or OH

MH or OH

CL-ML ML or OL

A Line: PI = 0.73(LL-20)

AASHTO Criteria Unified Criteria

,

AASHTO

A-1-a: mostly gravel or coarser with or without a well graded binder

A-1-b: coarse sand with or without a well graded binder

A-2: granular soils borderline between A-1 and A-3

A-3: fine sand with a small aount of nonplastic silt

A-4: silty soils

A-5: silty soils with high liquid limit

A-6: clayey soils

Unified

GW: well graded gravel

GP: poorly graded gravel

GM: silty gravel

GC: clayey gravel

SW: well graded sand

SP: poorly graded sand

SM: silty sand

SC: clayey sand

ML: silt

MH: silt with high liquid limit

CL: clay

CH: clay with high liquid limit

ML-CL: silty clay

OL: organic silt or clay with low liquid limi

OH: organic silt or clay with high liquid lim

S - sand

G = gravel

M = silt

C = clay

W = well graded

L = low liquid limit (<50)

H = high liquid limit (>50)

% passing No. 200 sieve % passing N0. 40 sieve

% passing No. 4 sieve of portion

retained on No. 200 sieve

% passing No. 10

Fine Grained:

ML, MH,

CL, CH,

OL, OH

Coarse Grained:

GM

SM

GC

SC

GW-GM, GW-GC, GP-GM, GP-GC

SW-SM , SW-SC, Sp-SM, SP-SC

SW, SP, GW, GP

Fine Grained:

A-4

A-5

A-6

A-7

SW

SP

SM

GW

GP

GM

100%

50%

12%

5%

0%

35%

25%

15%

10%

A-2

A-1-b

A-1-a

A-3

Granular:

A-3

A-1-b

A-1-a

30%

A-1-a

OL: organic silt or clay with low liquid limit 

OH: organic silt or clay with high liquid limit 

A-7: clayey soils 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 63 May 2006 

Silt and Clay Sand Gravel

0.01 No. 200 0.1 No. 40 1 No. 10 No. 4 10 3/4" 3" 100

Gravel

fine medium coarse fine coarse

Silt and Clay Sand Gravel

0.01 0.1 1 10 100

Gravel

fine coarse

Grain Size (mm)

Unified

AASHTO

Figure 4–19. Particle size limit by different classifications systems. 

Table 4-13. Classification of soils. 

Soil Classification Criteria for Assigning Group Symbols and Group Names Using 

Laboratory Testsa Group 

Symbol Group Nameb

CU $4 and 1#CC #3e CLEAN GW Well-graded Gravel 

GRAVELS 

Less than 5% 

fines 

CU #4 and 1$CC $3e GP Poorly-graded Gravelf

Fines classify as ML or MH GM Silty Gravelf,g,h

GRAVELS 

More than 

50% of coarse 

fraction 

retained on 

No. 4 sieve 

GRAVELS 

WITH FINES 

More than 

12% of finesc

Fines classify as CL or CH GC Clayey Gravelf,g,h

CU$6 and 1#CC #3e SW Well-graded Sandi CLEAN 

SANDS 

Less than 5% 

finesd

CU #6 and 1$CC $3e SP Poorly-graded Sandi

Fines classify as ML or MH SM Silty Sandg,h,i

SANDS 

50% or more 

of coarse 

fraction 

retained on 

No. 4 sieve 

SANDS WITH 

FINES 

More than 

12% finesd

Fines classify as CL or CH SC Clayey Sandg,h,i

PI > 7 and plots on or above "A" 

linej

CL Lean Clayk,l,m SILTS AND 

CLAYS 

Liquid limit 

less than 50% 

Inorganic 

PI < 4 or plots below "A" linej

 ML Siltk,l,m

Organic Clayk,l,m,n Organic Liquid limit - ovendried 

Liquid limit – not dried <0.75 OL Organic Siltk,l,m,o

PI plots on or above "A" line CH Fat Clayk,l,m SILTS AND 

CLAYS 

Liquid limit 

more than 50% 

Inorganic 

PI plots below "A" line MH Elastic Siltk,l,m

Organic Siltk,l,m,p Organic Liquid limit - ovendried 

Liquid limit – not dried <0.75 OH Organic Siltk,l,m,q

Highly fibrous 

organic soils 

Primary organic matter, dark in color, and organic 

odor 

Pt Peat and 

Muskeg 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 64 May 2006 

NOTES: 

a. Based on the material passing the 75-mm sieve. 

b. If field sample contained cobbles and/or boulders, add “with cobbles and/or boulders” to group 

 name. 

c. Gravels with 5 – 12% fines require dual symbols: 

 GW-GM well-graded gravel with silt 

 GW-GC well-graded gravel with clay 

 GP-GM poorly graded gravel with silt 

 GP-GC poorly graded gravel with clay 

d. Sands with 5 – 12% fines require dual symbols: 

 SW-SM well-graded sand with silt 

 SW-SC well-graded sand with clay 

 SP-SM poorly graded sand with silt 

 SP-SC poorly graded sand with clay 

e. 

C

D

D U = = Uniformity Coefficient alsoUC 60

10

( )

C

D

D D Coefficient of Curvature C = = ( )

( )( )

30

2

10 60

f. If soil contains $ 15% sand, add “with sand” to group name. 

g. If fines classify as CL-ML, use dual symbol GC-GM, SC-SM. 

h. If fines are organic, add “with organic fines” to group name. 

i. If soil contains $ 15% gravel, add “with gravel” to group name. 

j. If the liquid limit & plasticity index plot in hatched area on plasticity chart, soil is a CL-ML, silty 

 clay. 

k. If soil contains 15 – 29% plus No. 200, add “with sand” or “with gravel”, whichever is 

 predominant. 

l. If soil contains $ 30% plus No. 200, predominantly sand, add “sandy” to group name. 

m. If soil contains $ 30% plus No. 200, predominantly gravel, add “gravelly” to group name. 

n. Pl $ 4 and plots on or above “A” line. 

o. Pl < 4 or plots below “A” line. 

p. Pl plots on or above “A” line. 

q. Pl plots below “A” line. 

FINE-GRAINED SOILS (clays & silts): 50% or more passes the No. 200 sieve 

COARSE-GRAINED SOILS (sands & gravels): more than 50% retained on No. 200 sieve 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 65 May 2006 

Table 4-14. Summary of soil characteristics as a pavement material. 

(from NCHRP 1-37A Pavement Design Guide) 

Major Divisions Name 

Subgrade 

Strength when 

Not Subject to 

Frost Action 

Potential Frost 

Action 

Compressibility 

& Expansion 

Drainage 

Characteristics 

GW Well-graded gravels or gravelsand mixtures, little or no fines Excellent None to very slight Almost none Excellent GP Poorly graded gravels or gravelsand mixtures, little or no fines Good to excellent None to very slight Almost none Excellent *d GM ---  u 

Silty gravels, gravel-sand silt mixtures 

Good to excellent Slight to medium Very slight Fair to poor 

 Good Slight to medium Slight Poor to practically impervious 

Gravel And Gravelly Soils 

GC Clayey gravels, gravel-sand-clay mixture Good Slight to medium Slight Poor to practically impervious 

SW Well-graded sands or gravelly sands, little or no fines Good None to very slight Almost none Excellent SP Poorly graded sands or gravelly sands, little or no fines Fair to good None to very slight Almost none Excellent *d SM --- u 

Silty sands, sand-silt mixtures Fair to good Slight to high Very slight Fair to poor 

 Fair Slight to high Slight to medium Poor to practically impervious 

Sand and Sandy Soils 

SC Clayey sands, sand-clay mixtures Poor to fair Slight to high Slight to medium Poor to practically impervious 

*Division of GM and SM groups is based on Atterberg Limits (See Chapter 6) with suffix d used when L.L. is 28 or less and the PI is 6 or less. The suffix u is 

used when L.L. is greater that 28. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

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Table 4-14. Summary of soil characteristics as a pavement material (continued). 

Major Divisions Name 

Subgrade Strength 

when Not Subject 

to Frost Action 

Potential Frost 

Action 

Compressibility & 

Expansion 

Drainage 

Characteristics 

ML 

Inorganic silts & very fine sand, rock flour, silty or clayey fine sand or clayey silts with slight plasticity 

Poor to Fair Medium to Very High Slight to medium Fair to Poor 

CL 

Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays 

Poor to Fair Medium to High Slight to medium Practically Impervious 

Silts & Clays with Liquid Limit Less Than 50 

OL Organic silts & organic silt- clays or low plasticity Poor Medium to High Medium to high Poor 

MH Inorganic silts, micaceous or diatomaceous fine sand or silty 

soils, elastic silts 

Poor Medium to Very High High Fair to Poor 

CH Inorganic clays of high plasticity, fat clays Poor to Fair Medium to Very High High Practically Impervious 

Silts & Clays with Liquid Limit Greater Than 50 OH Organic clays of medium to high plasticity, organic silts Poor to Very Poor Medium High Practically Impervious Highly Organic Soils 

(Pt) Peat & other highly organic soils Not Suitable Slight Very high Fair to Poor 
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4.7.3 Subsurface Profile

On the basis of all subsurface information (i.e., from the literature review, geophysical 

evaluation, in-situ testing, soil borings, and laboratory test data), a subsurface profile can be 

developed. Longitudinal profiles are typically developed along the roadway alignment, and a 

limited number of transverse profiles may be included for key locations, such as at major 

bridge foundations, cut slopes, or high embankments. The subsurface information should also 

be presented in plan view, providing a map of general trends and changes in subsurface 

conditions. Vertical and plan view profiles provide an effective means of summarizing 

pertinent subsurface information and illustrating the relationship of the various investigation 

sites. By comparing the vertical profiles with the plan view, the subsurface conditions can be 

related to the site’s topography and physiography, providing a sense of lateral distribution 

over a large horizontal extent. Subsurface profiles should be developed by a geotechnical 

engineer, as the preparation requires geotechnical judgement and a good understanding of the 

geologic setting for accurate interpretation of subsurface conditions between the investigation 

sites. 

In developing a two-dimensional subsurface profile, the profile line (typically the roadway 

centerline) needs to be defined on the base plan, and the relevant borings, projected to this 

line. Judgment should be exercised in the selection of the borings since projection of the 

borings, even for short distances, may result in misleading representation of the subsurface 

conditions in some situations. Due to the subjective nature of the interpretation required, 

subsurface profiles and plan views should not be included in either the subsurface 

investigation report or the construction bid documents. 

The subsurface profile should be presented at a scale appropriate to the depth of the borings, 

frequency of the borings and soundings, and overall length of the cross-section. Generally, an 

exaggerated scale of 1(V):10(H) or 1(V):20(H) should be used. A representative example of 

an interpreted subsurface profile is shown in Figure 4-20, and a plan view profile is shown in 

Figure 4-21. The subsurface profile can be presented with reasonable accuracy and 

confidence at the locations of the borings. Generally, however, owners and designers would 

like the geotechnical engineer to present a continuous subsurface profile that shows an 

interpretation of the location, extent, and nature of subsurface formations or deposits between 

borings. At a site where rock or soil profiles vary significantly between boring locations, the 

value of such presentations becomes questionable. The geotechnical engineer must be very 

cautious in presenting such data. Such presentations should include clear and simple caveats 

explaining that the profiles, as presented, cannot be fully relied upon. Should there be a need 

to provide highly reliable, continuous subsurface profiles, the geotechnical engineer should 
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increase the frequency of borings and/or utilize geophysical methods to determine the 

continuity, or the lack of it, of subsurface conditions. 

Figure 4-20. Subsurface profile based on boring data showing cross-sectional view. 

Figure 4-21. Plan view of subsurface information. 
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4.7.4 Select Samples for Laboratory Testing

A program of laboratory tests will be required on representative samples of the foundation 

soils or soils to be used as construction materials so that pertinent properties can be 

determined. The extent of the laboratory program depends on the criticality of the design and 

on the complexity of the soil conditions. Those laboratory tests and analyses that are 

typically performed or required for an analysis and selection of the pavement type and 

thickness are listed in Table 4-15. A deep cut or high embankment, as used in the table, 

general implies greater than a few meters (6 ft or more). 

Table 4-15. Minimum laboratory testing requirements for pavement designs 

(NCHRP 1-37A Pavement Design Guide). 

Type of Laboratory Test Deep Cuts High 

Embankments At-Grade 

Moisture Content and 

Dry Unit Weight X X 

Atterberg Limits X X X 

Gradation X X 

Shrink Swell X X 

Permeability X 

Consolidation X 

Shearing and 

Bearing Strength X X X 

Resilient Modulus X X X 

Representative soil layers are selected for laboratory testing by examining the boring logs, 

soil profiles, and classification tests. The primary test for design will be either resilient 

modulus tests, CBR, or other agency-specific design value, as outlined in Chapter 5, along 

with other properties required for each design level. Where possible, resilient modulus tests 

should be performed on undisturbed specimens that represent the natural conditions 

(moisture content and density) of the subgrade. For disturbed or reconstituted specimens, 

bulk materials should be recompacted to as close to the natural conditions as possible. For 

rehabilitation projects, the type of distress is also an important consideration, with 

engineering properties required for structural design of the selected rehabilitation strategy. 

These tests must also indicate the existing condition of the pavement and highlight any 

degradation that has taken place during the life of the pavement. Geophysical tests will 

significantly help in this effort. Tests to evaluate stabilization alternatives typically can be 

performed on material from disturbed, undisturbed, or bulk samples, prepared and compacted 
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to the field requirements, as detailed in Chapter 7. Tests will also be required for 

constructability and performance. These tests can usually be performed on disturbed 

specimens and/or bulk samples. 

The number of test specimens depends on the number of different soils identified from the 

borings, as well as the condition of those soils. The availability of geophysical and/or in-situ 

tests will also affect the number and type of tests. Most of the subgrade test specimens should 

be taken from as close to the top of the subgrade as possible, extending down to a depth of 

0.6 m (2 ft) below the planned subgrade elevation. However, some tests should be performed 

on the soils encountered at a greater depth, especially if those deeper soils are softer or 

weaker. No guidelines are provided regarding the number of tests, except that all of the major 

soil types encountered near the surface should be tested with replicates, if possible. Stated 

simply, resilient modulus tests or other design tests (e.g., CBR) should be performed on any 

soil type that may have a detrimental impact on pavement performance (NCHRP 1-37A 

Pavement Design Guide). Other properties, such as shrink/swell and consolidation, will be 

required for evaluating stabilization requirements and long-term performance (e.g., potential 

deformation). 

For construction, as was discussed in section 4.5.6, moisture-density tests will be required on 

each soil type that will be used as fill in the pavement section, as well as the roadway 

embankment. AASHTO T99 should be used for medium to high plasticity fine-grained soils, 

whereas AASHTO T180 should be used for coarse-grained and low plasticity fine-grained 

soils. The degree of compaction required for the in-place density should be expressed as a 

percentage of the maximum density from the specified test procedure. Design tests (e.g.,

resilient modulus, CBR, etc.) are also required on the compacted subgrade material. 

For rehabilitation projects, the number of tests will depend on the condition of the existing 

pavement. The condition survey as discussed in section 4.2.3, should be analyzed to show 

where problems may exist and require detailed material property information. 

Another important point to remember in selecting the number of specimens to be tested is 

that the resilient modulus or other design value measured on different soils and soil structures 

(density, moisture) from repeated load tests can be highly variable. A coefficient of variation 

exceeding 25 percent for the resilient modulus on similar soils measured at the same stressstate is not uncommon. Repeatability studies indicate that coefficients of variation below 5 

percent are not uncommon when testing replicated soil specimens (Boudreau, 2003). The 

potential high variability in test results requires increased testing frequencies (i.e., many 

more than two or three resilient modulus tests along a project). As a general guide and 

suggested testing frequency, three resilient modulus tests should be performed on each major 
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Geotechnical Aspects of Pavement 4 - 71 May 2006 

subgrade soil found along the highway alignment. If the variability of test results (resilient 

modulus measured at the same stress-state) exceeds a coefficient of variation of 25 percent, 

then additional resilient modulus tests should be performed to obtain a higher confidence in 

the data (NCHRP 1-37A). 

Student Exercise 4-1. 

Boring logs and a stratagraphic profile from a proposed roadway alignment will be provided 

and the teams will be asked to 1) determine if the information is adequate, 2) evaluate 

method(s) for obtaining additional subsurface information, and 3) develop a laboratory 

testing program. One team will be randomly selected to present the results, followed by a 

solution discussion with the entire class.

Student Exercise 4-2. 

The students will be asked to provide considerations regarding selection, assignment, and 

number of laboratory tests. Each item will be noted on a flip chart and, upon completion, 

reviewed with the list in the Reference Manual. The class will discuss the implications of not 

running the right test, running too many tests, and incorrectly running tests. 

4.8 REFERENCES 

AASHTO (1972). AASHTO Interim Guide for Design of Pavement Structures, American 

Association of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (1986). AASHTO Guide for Design of Pavement Structures, American Association 

of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (1988). Manual on Subsurface Investigations, Developed by the Subcommittee on 

Materials, American Association of State Highway and Transportation Officials, 

Washington, D.C. 

AASHTO (1993). AASHTO Guide for Design of Pavement Structures, American Association 

of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (2002). Standard Specifications for Transportation Materials and Methods of 

Sampling and Testing: Part II: Tests, American Association of State Highway and 

Transportation Officials Sixteenth Edition, Washington, D.C. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 72 May 2006 

Al-Qadi, I.L., Lahouar, S. and Loulizi, A (2003). “Ground Penetrating Radar Evaluation for 

Flexible Pavement Thickness Estimation,” Journal of Transportation Engineering, ASCE, 

Vol. 129, No.1, Jan./Feb., pp. 93-99. 

American Society for Testing & Materials (2003). ASTM Book of Standards, Vol. 4, 

Section 03, Road and Paving Materials; Vehicle-Pavement Systems, ASTM International, 

West Conshohocken, PA. 

American Society for Testing & Materials (2000). ASTM Book of Standards, Vol. 4, 

Section 08 and 09, Construction Materials: Soils & Rocks, ASTM International, West 

Conshohocken, PA. 

Boudreau, R. L. (2003). “Repeatability of the Resilient Modulus Test Procedure,” ASTM STP 

1437, Resilient Modulus Testing for Pavement Components, G.N. Durham, A.W. Marr, and 

W.L. DeGroff, Eds., American Society for Testing and Materials, West Conshohocken, PA. 

Caterpillar Handbook of Ripping, 8th Edition, Caterpillar Company, Peoria, IL. 

Chassie, R.G. (2000) Soils and Foundations Workshop Manual, FHWA NHI-00-045, 

National Highway Institute. Federal Highway Administration, Washington, D.C. 

Cheney, R. and Chassie, R.G. (2000). Soils and Foundations Workshop, FHWA NHI-00-

045, NHI Course No. 132012, Federal Highway Administration, Washington, D.C. 

Dumas, C., Cannon, R.R., Lwin, M.M., Mansukhani, S., McLain, K.W., Pelnik, T.W., 

Porbaha, A., Putcha, S., Brown, D.A., Short, R.D., Macnab, A., and Christopher, B.R.

(2003). FHWA Scan Tour Report on The Innovative Technology for Accelerated 

Construction of Bridge and Embankment Foundations, FHWA International Technology 

Scanning Program, Federal Highway Administration, Washington, D.C. 

Dunnicliff, J. (1988). Geotechnical Instrumentation for Monitoring Field Performance, John 

Wiley & Sons, Inc., New York. 

Dunnicliff, J. (1998). Geotechnical Instrumentation Reference Manual, FHWA-NHI-98-034, 

National Highway Institute, Washington D.C. 

FHWA-NHI-98-034 (1998) Geotechnical Instrumentation, author: Dunnicliff, J., National 

Highway Institute, 238 p. 
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Geotechnical Aspects of Pavement 4 - 73 May 2006 

FHWA TS-78-209 (1978). Guidelines for Cone Penetration Test: Performance and Design, 

prepared by J.H. Schmertmann for the Federal Highway Administration, Washington, D.C. 

FHWA NHI-01-031 (2001) Subsurface Investigations, authors: Mayne, P.W., Christopher, 

B.R., DeJong, J., National Highway Institute, 394 p. 

Greenhouse, J.P., Slaine, D.D., and Gudjurgis, P. (1998). Application of Geophysics in 

Environmental Investigations, Matrix Multimedia Publishing, Toronto. 

Handy, R.L. (1980). “Realism in Site Exploration: Past, Present, Future, and Then Some – 

All Inclusive,” Proceedings, Symposium on Site Exploration in Soft Ground Using In-Situ 

Techniques, Report FHWA-TS-80-202, Federal highway Administration, Washington D.C. 

Huang, Y.H. (1993). Pavement Analysis and Design. Englewood Cliffs, NJ: Prentice Hall. 

Kleyn, E.G., “The Use of the Dynamic Cone Penetrometer (DCP),” Transvaal Roads 

Department, Report Number L2/74, Pretoria, 1975. 

Kulhawy, F.H. and Mayne, P.W. (1990). Manual on Estimating Soil Properties for 

Foundation for Design, Prepared by Cornell University for the Electric Power Research 

Institute, Report Number EPRI EL-6800, Electric Power Research Institute, Palo Alto, CA. 

LTPP Manual for Falling Weight Deflectometer Measurement: Operational Field 

Guidelines, Version 3.1 (August 2000) Prepared by: LAW PCS, A Division of Law 

Engineering and Environmental Services, Inc., Office of Infrastructure R&D, Long-Term 

Pavement Performance Team, HRDI-13. Federal Highway Administration, McLean, 

Virginia. 

Mayne, P.W., Christopher, B.R., Dejong, J. (2002). Subsurface Investigations, FHWA-NHI01-031, Federal Highway Administration, Washington, D.C. 

Morey, R.M. (1998). Ground Penetrating Radar for Evaluating Subsurface Conditions for 

Transportation Facilities, NCHRP Synthesis 278, Transportation Research Board, 

Washington, D.C. 

Muhammad, L.N., Titi, H.H., and Herath, A. (2002). Effect of Moisture Content and Dry 

Unit Weight on the Resilient Modulus of Subgrade Predicted by Cone Penetration Test, 

FHWA/LA.00/355, Louisiana Transportation Research Center, Baton Rouge, LA. 
FHWA NHI-05-037 Chapter 4 – Geotechnical Exploration & Testing 

Geotechnical Aspects of Pavement 4 - 74 May 2006 

NCHRP 1-37A Design Guide (2002). 2002 Design Guide – Design of New and Rehabilitated 

Pavement Structures, Draft Final Report, Part 1 – Introduction and Part 2 – Design Inputs, 

Prepared for the National Cooperative Highway Research Program by ERES Division of 

ARA. 

Newton, T., Anderson, N., and Hatheway, A. (1998). “Application of Innovative 

Geophysical Methods for Subgrade Investigations in Karst Terrain,” Report published by the 

University of Missouri, Rolla, for Missouri Department of Transportation and the Federal 

Highway Administration 

Newcombe, D.A., and B. Birgisson (1999). “Measuring In-situ Mechanical Properties of 

Pavement Subgrade Soils,” NCHRP Synthesis 278, Transportation Research Board, 

Washington, D.C. 

NHI (1998). Techniques for Pavement Rehabilitation; A Training Course, Participant’s 

Manual, National Highway Institute course number 13108, Federal Highway Administration, 

Washington D.C. 

Powers, J.P. (1992). Construction Dewatering, John Wiley & Sons, Inc., New York. 

Roimela, P., Salmenkaita, S., Maijala, P., Saarenketo, T. (2000). “Road Analysis: a Tool for 

Cost-effective Rehabilitation Measures for Finnish Roads,” Presented at GPR 2000, the 

Eighth International Conference on Ground Penetrating Radar, May 23-26, 2000, Gold 

Coast, Australia. 

Rollings, M.P., and R.S. Rollings (1996). Geotechnical Materials in Construction, McGrawHill, NY. 

Santamarina, J.C., Klein, K. and Fam, M.A. (2001). Soils and Waves, Particulate Materials 

Behavior Characterization, & Process Monitoring, John Wiley & Sons, Ltd., N.Y., 488 p. 

Scullion, T. (2004). Implementation of Ground Penetrating Radar Technology, AASHTO’s 

Technical Implementation Group, Demo CD Distributed at TRB 2004 Annual Meeting, 

Texas Transportation Institute. 

Tolla, P. (2002). Presentation to the FHWA Scan Team on “Innovative Technology for 

Accelerated Construction of Bridge and Embankment Foundations.” 
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Geotechnical Aspects of Pavement 4 - 75 May 2006 

U.S. Department of Labor Construction Safety and Health Regulations (current) Standard – 

29 CFR, Part 1926: http://www.osha.gov/pls/oshaweb/owares.do_search?p_doc_type= 

STANDARDS&p_search_type=StandTextPolicy&p_search_str=PART+1926+ 

Utah D.O.T. (1998). “Pavement Management and Pavement Design Manual.” 

Vandre, B., Budge, A., Nussbaum, S. (1998). “DCP- A Useful Tool for Characterizing 

Engineering Properties of Soils at Shallow Depths,” Proceedings of 34th Symposium on 

Engineering Geology and Geotechnical Engineering, Utah State University, Logan, UT. 

Webster, S.L., Brown, R.W. and Porter, J.R. (1994). Force Projection Site Evaluation Using 

the Electric Cone Penotrometer (ECP) and the Dynamic Cone Penetrometer (DCP), 

Technical Report GL-94-17, U.S. Air Force, Tyndall AFB, FL. 

Wightman, W.E., Jalinoos, F., Sirles, P., Hanna, K. (2003). Application of Geophysical 

Methods to Highway Related Problems, Report developed by Blackhawk GeoSciences, A 

Division Of Blackhawk GeoServices for FHWA under FHWA Contract Number DTFH68-

02-P-00083, Federal Highway Administration, Washington D.C. 

Wimsatt, A.J. and Scullion, T. (2003). “Selecting Rehabilitation Strategies for Flexible 

Pavement in Texas,” presented and published at the annual meeting of the Transportation 

Research Board, TRB, Washington D.C. 

www.fema.gov 

www.ce.gatech.edu/~geosys/misc/links.htm 

www.geoforum.com/info/geophysical/.

http://soils.usda.gov/survey/online_surveys/ 

www.usgs.gov 
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FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 1 May 2006 

CHAPTER 5.0 GEOTECHNICAL INPUTS FOR PAVEMENT DESIGN 

5.1 INTRODUCTION 

This chapter describes the determination of the specific geotechnical inputs required for the 

design of flexible and rigid pavements. Although the focus here is strictly on geotechnical 

inputs, there is obviously much other important information required for pavement design, 

including traffic characteristics, material properties for the bound asphalt and/or Portland 

cement concrete layers, desired reliability, and other details. These inputs are usually 

provided by agency units other than the geotechnical group. 

Most of the inputs described in this chapter relate to the material properties of the unbound 

pavement layers and subgrade soil. Other required inputs include geometric information like 

layer thickness, but these are generally self-explanatory and are not discussed here. 

Environmental/climate inputs are also covered in this chapter. Although these inputs are not 

“geotechnical” per se, they directly influence the behavior of the unbound materials through 

their effects on moisture content and freeze/thaw cycles. In addition, in many agencies, the 

group responsible for determining the environmental inputs is poorly defined, and thus this 

responsibility may end up with the geotechnical group. 

The coverage of the material in this chapter is guided by several considerations: 

• Only the explicit design inputs are treated. As described in Chapter 3, there may be 

other geotechnical issues (e.g., embankment slope stability) that can have a 

significant impact on pavement performance but that are not considered explicitly in 

the pavement design process. 

• Project-specific measured input parameters are often unavailable at design time, 

particularly for preliminary design. This is especially true for material properties. 

Consequently, much emphasis is placed in this chapter on “typical” values and/or 

empirical correlations that can be used to estimate the design inputs. These estimates 

can be used for preliminary design, sensitivity studies, and other purposes. Clearly, 

though, project-specific measured values are preferred for final design. 

• Many material property inputs can either be determined from laboratory or field tests. 

Field testing is covered in Chapter 4, and appropriate links to the Chapter 4 material 

are included here where appropriate. 

• The treatment in this chapter attempts to balance coverage between the current 

empirical 1993 AASHTO Design Guide and the forthcoming mechanistic-empirical 

NCHRP 1-37A design approach (hereafter referred to as the NCHRP 1-37A Design 

Guide). Although there is some overlap in the geotechnical inputs required by these 
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Geotechnical Aspects of Pavements 5 - 2 May 2006 

two design approaches (e.g., subgrade resilient modulus), there are substantial 

differences. The inputs to the 1993 AASHTO Guide are fewer in number and mostly 

empirical (e.g., layer drainage coefficients), while the inputs to the NCHRP 1-37A 

Guide are more numerous and fundamental (e.g., hydraulic conductivity vs. moisture 

content relations). 

• Only design inputs are described in this chapter. In cases where some intermediate 

analysis is required to determine the design input (e.g., for effective modulus of 

subgrade reaction in the 1993 Guide—see Section 5.4.6), the analysis methodology is 

described here, as well. The usage of the design inputs in the overall design 

calculations is described separately in Appendices C and D for the 1993 and NCHRP 

1-37A Design Guides, respectively. 

One consequence of all of the above is that this chapter is quite long; this is necessary to give 

sufficient coverage to all of the diverse geotechnical inputs required by the two design 

procedures. First, the geotechnical inputs required by the 1993 AASHTO and NCHRP 1-37A 

Design Guides are summarized (Section 5.2). Then, the geotechnical inputs are described in 

detail by category. The following is a road map of the sections in this chapter that describe 

the various categories of geotechnical design inputs: 

5.2 REQUIRED GEOTECHNICAL INPUTS 

5.2.1 1993 AASHTO Design Guide 

5.2.2 NCHRP 1-37A Design Guide 

5.2.3 Other Geotechnical Properties 

5.3 PHYSICAL PROPERTIES 

5.3.1 Weight-Volume Relationships 

5.3.2 Physical Property Determination 

5.3.3 Problem Soil Identification 

5.3.4 Other Aggregate Tests 

5.4 MECHANICAL PROPERTIES 

5.4.1 California Bearing Ratio (CBR) 

5.4.2 Stabilometer (R-Value) 

5.4.3 Elastic (Resilient) Modulus 

5.4.4 Poisson’s Ratio 

5.4.5 Structural Layer Coefficients 

5.4.6 Modulus of Subgrade Reaction 

5.4.7 Interface Friction 

5.4.8 Permanent Deformation Characteristics 

5.4.9 Coefficient of Lateral Pressure 

5.5 THERMO-HYDRAULIC PROPERTIES 

5.5.1 1993 AASHTO Guide 

5.5.2 NCHRP 1-37A Design Guide 

5.6 ENVIRONMENT/CLIMATE INPUTS 
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5.6.1 1993 AASHTO Guide 

5.6.2 NCHRP 1-37A Design Guide 

The chapter concludes with a section describing the development of final design values for 

each input when there are several estimates, e.g., material properties measured both in the 

field and in the laboratory. Most of the design inputs also exhibit significant spatial, 

temporal, and inherent variability. All of these issues must be reconciled to develop 

defensible input values for use in the final pavement design. 

5.2 REQUIRED GEOTECHNICAL INPUTS 

5.2.1 1993 AASHTO Design Guide 

As described previously in Chapter 3, the AASHTO Pavement Design Guide has evolved 

through several versions over the 40+ years since the AASHO Road Test. The current 

version is the 1993 Guide. The geotechnical inputs required for flexible pavement design 

using the 1993 Guide are summarized in Table 5-1. Also shown are cross references to the 

sections in this manual in which the determination of the respective geotechnical inputs are 

described. As previously described in Chapter 3, the geotechnical inputs for the 1986 Guide 

are identical to those for the 1993 Guide. Note that the thicknesses Di for the unbound layers 

are included as flexible pavement geotechnical inputs in Table 5-1; although these would 

typically be considered outputs from the design (i.e., determined from SN and the other 

defined inputs), there may be cases where the layer thicknesses are fixed and for which the 

design then focuses on selecting layer materials having sufficient structural capacity. 

The geotechnical inputs required for rigid pavement design using the 1993 Guide are 

summarized in Table 5-2. Again, these inputs are identical to those for the 1986 Guide. The 

first five properties in Table 5-2 are used to determine the effective modulus of subgrade 

reaction k in the 1993 Guide procedure. The geotechnical inputs required for rigid pavement 

design using the optional alternate approach in the 1998 supplement are the same as for the 

1993 approach; only the analysis procedure changed in the 1998 supplement. 

The last six parameters in both tables are the environmental parameters required by the 1993 

Guide for determining the serviceability loss due to swelling of expansive subgrade soils and 

frost heave. Although these are not geotechnical parameters in the strictest sense, the 

detrimental effects of swelling and frost heave are concentrated in the subgrade and other 

unbound layers and thus are important geotechnical aspects of pavement design. 
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Geotechnical Aspects of Pavements 5 - 4 May 2006 

Table 5-1. Required geotechnical inputs for flexible pavement design 

using the 1993 AASHTO Guide. 

Property Description Section 

MR Resilient modulus of subgrade 5.4.3 

EBS Resilient modulus of base (used to determine structural layer 

coefficient) 

5.4.3 

m2 Moisture coefficient for base layer 5.5.1 

D2 Thickness of base layer

ESB Resilient modulus of subbase (used to determine structural 

layer coefficient) 

5.4.3 

m3 Moisture coefficient for subbase layer 5.5.1 

D3 Thickness of subbase layer 

θ Swell rate 5.6.1 

VR Maximum potential swell 5.6.1 

PS Probability of swelling 5.6.1 

φ Frost heave rate 5.6.1 

∆PSIMAX Maximum potential serviceability loss from frost heave 5.6.1 

PF Probability of frost heave 5.6.1 

Note: Additional sets of layer properties (Ei, mi, Di) are required if there are more than two unbound layers in 

the pavement structure (exclusive of the natural subgrade). 

Table 5-2. Required geotechnical inputs for rigid pavement design 

using the 1993 AASHTO Guide. 

Property Description Section 

MR Resilient modulus of subgrade 5.4.3 

ESB Resilient modulus of subbase 5.4.3 

DSB Thickness of subbase 

DSG Depth from top of subgrade to rigid foundation 

LS Loss of Support factor 5.4.6 

Cd Drainage factor 5.5.1 

F Friction factor (for reinforcement design in JRCP) 5.4.7 

θ Swell rate 5.6.1 

VR Maximum potential swell 5.6.1 

PS Probability of swelling 5.6.1 

φ Frost heave rate 5.6.1 

∆PSIMAX Maximum potential serviceability loss from frost heave 5.6.1 

PF Probability of frost heave 5.6.1 
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Geotechnical Aspects of Pavements 5 - 5 May 2006 

5.2.2 NCHRP 1-37A Design Guide 

The mechanistic-empirical methodology that is the basis of the NCHRP 1-37A Design Guide 

requires substantially more input information than needed by the empirical design procedures 

in the 1993 AASHTO Guide. These inputs also tend to be more fundamental quantities, as 

compared to the often empirical inputs in the 1993 Guide. This is understandable given the 

inherent differences between mechanistic-empirical and empirical design methodologies. 

Hierarchical Approach to Design Inputs 

The level of design effort in any engineering design should be commensurate with the 

significance of the project being designed. Low-volume secondary road pavements do not 

require—and most agencies do not have the resources to provide—the same level of design 

effort as high-volume urban primary roads. 

In recognition of this reality, a hierarchical approach has been developed for determining the 

pavement design inputs in the NCHRP 1-37A Design Guide. The hierarchical approach is 

based on the philosophy that the level of engineering effort exerted in determining the design 

inputs, including the material property values, should be consistent with the relative 

importance, size, and cost of the design project. Three levels are provided for the design 

inputs in the NCHRP 1-37A Guide: 

Level 1 inputs provide the highest level of accuracy and the lowest level of 

uncertainty. Level 1 inputs would typically be used for designing heavily trafficked 

pavements or wherever there are serious safety or economic consequences of early 

failure. Level 1 material inputs require laboratory or field evaluation, such as resilient 

modulus testing or non-destructive deflection testing. Level 1 inputs require more 

resources and time to obtain than the other lower levels. 

Level 2 inputs provide an intermediate level of accuracy and are closest to the typical 

procedures used with earlier editions of the AASHTO Pavement Design Guides. This 

level could be used when resources or testing equipment are not available for Level 1 

characterization. Level 2 inputs would typically be derived from a limited testing 

program or estimated via correlations or experience (possibly from an agency 

database). Resilient modulus estimated from correlations with measured CBR values 

is one example of a Level 2 material input. 

Level 3 inputs provide the lowest level of accuracy. This level might be used for 

designs in which there are minimal consequences of early failure (e.g., low-volume 

roads). Level 3 material inputs typically are default values that are based on local 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 6 May 2006 

agency experience. A default resilient modulus based on AASHTO soil class is an 

example of a Level 3 material input. 

Although it is intuitively clear that higher level (i.e., higher quality) design inputs will 

provide more precise estimates of pavement performance, the current state-of-the-art of 

pavement design and the limited availability of Level 1 input data make it difficult to 

quantify these benefits at present. One exception to this is thermal cracking prediction in the 

NCHRP 1-37A Design Guide. Complete Level 1 material property and environmental data 

were available from the U.S. and Canadian Strategic Highway Research Programs for 

approximately 35 pavement sites in the northern United States and Canada. Predictions of 

thermal cracking were made based on these Level 1 material inputs as well as on Level 3 

default material properties. Figure 5-1 summarizes the differences between predicted and 

observed thermal cracking in units of lineal feet of cracking per 500 feet of pavement length 

for each of the field sites based on the Level 1 material inputs; Figure 5-2 summarizes the 

same results based on the Level 3 material inputs. The comparison of these two figures 

clearly shows that the higher quality Level 1 material inputs dramatically reduce the 

variability between predicted and observed cracking. 

Design inputs in the NCHRP 1-37A methodology may be specified using a mix of levels for 

any given project. For example, the modulus of rupture of a concrete surface layer may be 

specified as a Level 1 input, while the traffic load spectra are determined using a Level 2 

approach, and the subgrade resilient modulus via a Level 3 estimate based on subgrade soil 

class. The computational algorithms and distress models in the NCHRP 1-37A Design Guide 

(see Appendix D) are applied in the same way regardless of the input levels. However, the 

higher level inputs implicitly increase the accuracy and reliability of the predicted pavement 

performance. 

In summary, the advantages of the hierarchical approach for the material and other design 

inputs are as follows: 

• It provides the engineer with great flexibility in selecting an engineering approach 

consistent with the size, cost, and overall importance of the project. 

• It allows each agency to develop an initial design methodology consistent with its 

internal technical capabilities. 

• It provides a very convenient method for gradually increasing over time the technical 

skills and sophistication within the organization. 

• In concept, it provides the most accurate and cost-efficient design consistent with 

agency financial and technical resources. 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 7 May 2006 

-200

-150

-100

-50

0

50

100

150

200

13579

11131517192123252729313335

Number of Observations

Predicted - Observed

Figure 5-1. Thermal crack prediction from NCHRP 1-37A Design Guide using Level 1 

material inputs. 

-200

-150

-100

-50

0

50

100

150

200

13579

11131517192123252729313335

Number of Observations

Predicted - Observed

Figure 5-2. Thermal crack prediction from NCHRP 1-37A Design Guide using Level 3 

material inputs. 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 8 May 2006 

Required Geotechnical Inputs 

The geotechnical inputs for the NCHRP 1-37A Design Guide are organized into the 

following categories: 

• Mechanical properties that are used in an analysis model to relate applied structural 

loads to structural response (Table 5-3 and Table 5-4). 

• Thermo-hydraulic inputs that are used to relate environmental influences to the 

thermal and hydraulic state of the system (Table 5-5). 

• Distress model properties that enter directly in the empirical models for pavement 

performance (Table 5-6). 

As described previously, the NCHRP 1-37A Design Guide provides for three different 

hierarchical levels of input quality: Level 1 (highest), Level 2 (intermediate), and Level 3 

(lowest). For any given input parameter, different properties may be required for Level 1 vs. 

Level 2 vs. Level 3 inputs. For example, a Level 1 estimate of subgrade resilient modulus for 

new construction requires laboratory-measured properties, while Level 2 instead requires 

CBR or other similar index properties, and Level 3 requires only the AASHTO or USCS soil 

class. The hierarchical levels for each of the geotechnical inputs are included in Table 5-3 

through Table 5-6. The NCHRP 1-37A Guide recommends that the best available data (the 

highest level of inputs) be used for design. However, it does not require the same quality 

level for all inputs in the design. 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 9 May 2006 

Table 5-3. Geotechnical mechanical property inputs required for the flexible pavement 

design procedure in the NCHRP 1-37A Design Guide. 

Level Property Description 1 2 3 Section 

General 

 Material type 3 3 3 3.3.2 

γt In-situ total unit weight 3 3 3

K0 Coefficient of lateral earth pressure 3 3 3 5.4.9 

Stiffness/Strength of Subgrade and Unbound Layersa 

k1, k2, k3 Nonlinear resilient modulus parameters 3b 5.4.3 

MR Backcalculated resilient modulus 3c 5.4.3 

MR Estimated resilient modulus 3d 3 5.4.3 

CBR California Bearing Ratio 3d 5.4.1 

R R-Value 3d 5.4.2 

ai Layer coefficient 3d,e 5.4.5 

DCP Dynamic Cone Penetration index 3d 4.5.5 

PI Plasticity Index 3d 5.3.2 

P200 Percent passing 0.075 mm (No. 200 sieve) 3d 5.3.2 

 AASHTO soil class 3 4.7.2 

 USCS soil class 3 4.7.2 

ν Poisson’s ratio 3 3 3 5.4.4 

Interface friction 3 3 3 5.4.7 a

Estimates of MR andν are also required for shallow bedrock. 

b

For new construction/reconstruction designs only. c

Primarily for rehabilitation designs. 

d

For level 2, MR may be estimated directly or determined from correlations with one of the following: 

CBR; R; ai; DCP; or PI and P200. e

For unbound base and subbase layers only. 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 10 May 2006 

Table 5-4. Geotechnical mechanical property inputs required for the rigid pavement 

design procedure in the NCHRP 1-37A Design Guide. 

Level Property Description 1 2 3 Section 

General 

 Material type 3 3 3 3.3.2 

γt In-situ total unit weight 3 3 3 5.3.2 

K0 Coefficient of lateral earth pressure 3 3 3 5.4.9 

Stiffness/Strength of Subgrade and Unbound Layersa 

kdynamic Backcalculated modulus of subgrade reaction 3b 5.4.3 

MR Estimated resilient modulus 3c 3 5.4.3 

CBR California Bearing Ratio 3c 5.4.1 

R R-Value 3c 5.4.2 

ai Layer coefficient 3c 5.4.5 

DCP Dynamic Cone Penetration index 3c 4.5.5 

PI Plasticity Index 3c 5.3.2 

P200 Percent passing 0.075 mm (No. 200 sieve) 3c 5.3.2 

 AASHTO soil class 3 4.7.2 

 USCS soil class 3 4.7.2 

ν Poisson’s ratio 3 3 3 5.4.4 

Interface friction 3 3 3 5.4.7 a

Estimates of MR andν are also required for shallow bedrock in new/reconstruction designs. 

b

From FWD testing for rehabilitation designs. For new/reconstruction designs, kdynamic is determined 

from Level 2 estimates of MR. c

For Level 2, MR may be estimated directly or determined from correlations with one of the 

following: CBR; R; ai; DCP; or PI and P200. 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 11 May 2006 

Table 5-5. Thermo-hydraulic inputs required for the NCHRP 1-37A Design Guide. 

Level Property Description 1 2 3 Section 

 Groundwater depth 3 3 3 5.5.2 

Infiltration and Drainage 

Amount of infiltration 3 3 3 5.5.2 

Pavement cross slope 3 3 3 5.5.2 

Drainage path length 3 3 3 5.5.2 

Physical Properties 

Gs Specific gravity of solids 3 5.3.2 

γd max Maximum dry unit weight 3 5.3.2 

wopt Optimum gravimetric water content 3 5.3.2 

PI Plasticity Index 3 5.3.2 

D60 Gradation coefficient 3 5.3.2 

P200 Percent passing 0.075 mm (No. 200 sieve) 3 5.3.2 

Hydraulic Properties 

af, bf, cf, 

hr 

Soil water characteristic curve parameters 3 5.5.2 

ksat Saturated hydraulic conductivity (permeability) 3 5.5.2 

PI Plasticity Index 3 3 5.3.2 

D60 Gradation coefficient 3 3 5.3.2 

P200 Percent passing 0.075 mm (No. 200 sieve) 3 3 5.3.2 

Thermal Properties 

K Dry thermal conductivity 3 5.5.2 

Q Dry heat capacity 3 5.5.2 

AASHTO soil class 3 4.7.2 

Table 5-6. Distress model material properties required 

for the NCHRP 1-37A Design Guide. 

Level Property Description 1 2 3 Section 

k1 Rutting parameter (Tseng and Lytton model) 3 3 3 5.4.8 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 12 May 2006 

5.2.3 Other Geotechnical Properties 

In addition to the explicit design inputs listed in Table 5-1 and Table 5-2 for the 1993 

AASHTO Guide and Table 5-3 through Table 5-6 for the NCHRP 1-37A Guide, other 

geotechnical properties are typically required during pavement design and construction. 

These include standard properties required for soil identification and classification, 

compaction control, and field QC/QA. 

5.3 PHYSICAL PROPERTIES 

Physical properties provide the most basic description of unbound materials. These properties 

are also often used in correlations for more fundamental engineering properties, such as 

stiffness or permeability. The principal physical properties of interest are specific gravity of 

solids, water content, unit weight (density), gradation characteristics, plasticity (Atterberg 

limits), classification, and compaction characteristics. 

5.3.1 Weight-Volume Relationships 

It is useful to review some common soil mechanics terminology and fundamental weight and 

volume relationships before describing the various soil test methods. Basic soil mechanics 

textbooks should be consulted for further explanation. 

A sample of soil is a multi-phase material composed of solid soil grains, water, and air 

(Figure 5-3). The weight and volume of a soil sample depends on the specific gravity of the 

soil grains (solids), the size of the space between soil grains (voids and pores), and the 

amount of void space filled with water (moisture content and degree of saturation). Common 

terms associated with weight-volume relationships are shown in Table 5-7. Of particular 

note is the void ratio e, which is a general indicator of the relative strength and 

compressibility of a soil sample; i.e., low void ratios generally indicate strong soils of low 

compressibility, while high void ratios are often indicative of weak and highly compressible 

soils. Selected weight-volume (unit weight) relations are presented in Table 5-8. Typical 

values for porosity, void ratio, water content, and unit weight are presented in Table 5-9 for a 

range of soil types. 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 13 May 2006 

Figure 5-3. Relationships between volume and weight/mass of bulk soil (McCarthy, 2002). 

Table 5-7. Terms in weight-volume relations (after Cheney and Chassie, 1993). 

Property Symbol Units1 How obtained 

(AASHTO/ASTM) Direct Applications 

Moisture 

Content w D By measurement (T 265/ D 4959) 

Classification and 

weight-volume relations 

Specific 

Gravity Gs D By measurement (T 100/D 854) 

Volume computations 

Unit Weight γ FL-3

By measurement or 

from weight-volume 

relations 

Classification and 

pressure computations 

Porosity n D From weight-volume 

relations 

Defines relative volume of 

solids to total volume of soil

Void Ratio e D From weight-volume 

relations 

Defines relative volume of 

voids to volume of solids 

1

 F = Force or weight; L = Length; D = Dimensionless. Although by definition, 

moisture content is a dimensionless fraction (ratio of weight of water to weight of 

solids), it is commonly reported in percent by multiplying the fraction by 100. 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 14 May 2006 

Table 5-8. Unit weight-volume relationships. 

Case Relationship Applicable Geomaterials

Soil Identities: 

1. Gs w = S e

2. Total Unit Weight:

(1 )

(1 ) t sw

w G

e

γ γ

+ = + 

All types of soils & rocks 

Limiting Unit 

Weight 

Solid phase only: w = e = 0: 

 γrock = Gs γw

Maximum expected value for 

solid silica is 27 kN/m3

Dry Unit Weight For w = 0 (all air in void space): 

 γd = Gs γw/(1+e) 

Use for clean sands and soils 

above groundwater table 

Moist Unit Weight 

(Total Unit 

Weight) 

Variable amounts of air & water: 

 γt = Gs γw (1+w)/(1+e) 

 with e = Gs w/S 

Partially-saturated soils above 

water table; depends on degree 

of saturation (S, as decimal). 

Saturated Unit 

Weight 

Set S = 1 (all voids with water): 

 γsat = γw (Gs+e)/(1+e) 

All soils below water table; 

Saturated clays & silts above 

water table with full capillarity.

Hierarchy: γd < γt < γsat < γrock Check on relative values 

Note: γw = 9.8 kN/m3 (62.4 pcf) for fresh water. 

Table 5-9. Typical porosity, void ratio, and unit weight values for soils in 

their natural state (after Peck, Hanson, and Thornburn, 1974). 

Unit Weight 

kN/m3 Soil Type lb/cu ft Porosity 

n

Void 

Ratio 

e

Water 

Content 

w γd γsat γd γsat

Uniform sand (loose) 0.46 0.85 32% 14.1 18.5 90 118 

Uniform sand (dense) 0.34 0.51 19% 17.1 20.4 109 130 

Well-graded sand (loose) 0.40 0.67 25% 15.6 19.5 99 124 

Well-graded sand (dense) 0.30 0.43 16% 18.2 21.2 116 135 

Windblown silt (loess) 0.50 0.99 21% 13.4 18.2 85 116 

Glacial till 0.20 0.25 9% 20.7 22.8 132 145 

Soft glacial clay 0.55 1.2 45% 11.9 17.3 76 110 

Stiff glacial clay 0.37 0.6 22% 16.7 20.3 106 129 

Soft slightly organic clay 0.66 1.9 70% 9.1 15.4 58 98 

Soft very organic clay 0.75 3.0 110% 6.8 14.0 43 89 

Soft montmorillonitic clay 0.84 5.2 194% 4.2 12.6 27 80 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 15 May 2006 

5.3.2 Physical Property Determination 

Laboratory and field methods (where appropriate) for determining the physical properties of 

unbound materials in pavement systems are described in the following subsections and 

tables. Typical values for each property are also summarized. The soil physical properties are 

organized into the following categories: 

• Volumetric properties 

o Specific gravity (Table 5-10) 

o Moisture content (Table 5-11) 

o Unit weight (Table 5-12) 

• Compaction 

o Proctor compaction tests (Table 5-13) 

• Gradation 

o Mechanical sieve analysis (Table 5-19) 

o Hydrometer analysis (Table 5-20) 

• Plasticity 

o Atterberg limits (Table 5-21) 

Gradation and plasticity are the principle determinants for engineering soil classification 

using either the AASHTO or Unified soil classification systems. Soil classification is 

described as part of subsurface exploration in Section 4.7.2. 

The identification of problem soils (e.g., expansive clays) is typically based on their physical 

properties; this topic is addressed at the end of this section. Other additional tests commonly 

used for quality control of aggregates used in base and subbase layers and in asphalt and 

Portland cement concrete are also briefly summarized. 

Volumetric Properties 

The volumetric properties of most interest in pavement design and construction are 

• Specific gravity (Table 5-10) 

• Moisture content (Table 5-11) 

• Unit weight (Table 5-12) 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 16 May 2006 

Table 5-10. Specific gravity of soil and aggregate solids. 

Description The specific gravity of soil solids Gs is the ratio of the weight of a given 

volume of soil solids at a given temperature to the weight of an equal 

volume of distilled water at that temperature 

Uses in 

Pavements 

• Calculation of soil unit weight, void ratio, and other volumetric 

 properties (see Section 5.3.1). 

• Analysis of hydrometer test for particle distribution of fine-grained 

 soils (Table 5-20). 

Laboratory 

Determination 

AASHTO T 100 or ASTM D 854. 

Field 

Measurement 

Not applicable. 

Commentary Some qualifying words like true, absolute, apparent, bulk or mass, etc. 

are sometimes added to "specific gravity." These qualifying words 

modify the sense of specific gravity as to whether it refers to soil grains 

or to soil mass. The soil grains have permeable and impermeable voids 

inside them. If all the internal voids of soil grains are excluded for 

determining the true volume of grains, the specific gravity obtained is 

called absolute or true specific gravity (also called the apparent specific 

gravity). If the internal voids of the soil grains are included, the specific 

gravity obtained is called the bulk specific gravity. 

Complete de-airing of the soil-water mix during the test is imperative 

while determining the true or absolute value of specific gravity. 

Typical Values 

(Coduto ,1999)

Soil Type GS 

Clean, light colored sand (quartz, 

feldspar) 2.65 

Dark colored sand 2.72 

Sand-silt-clay mixtures 2.72 

Clay 2.65 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 17 May 2006 

Table 5-11. Moisture content. 

Description The moisture content expresses the amount of water present in a 

quantity of soil. The gravimetric moisture or water content w is defined 

in terms of soil weight as / wWW = w s , where Ww is the weight of water 

and Ws is the weight of the soil solids in the sample. 

Uses in 

Pavements 

• Calculation of soil total unit weight, void ratio, and other volumetric 

 properties (see Section 5.3.1). 

• Correlations with soil behavior, other soil properties. 

Laboratory 

Determination 

Drying of the soil in a conventional (temperature of 110±5oC) or 

microwave oven to a constant weight (AASHTO T 265, ASTM D 

2216/conventional oven, or ASTM D 4643/microwave). 

Field 

Measurement 

Nuclear gauge (ASTM D2922). 

Commentary Determination of the moisture or water content is one of the most 

commonly performed laboratory procedures for soils. The water content 

of soils, when combined with data obtained from other tests, produces 

significant information about the characteristics of the soil. For 

example, when the in-situ water content of a sample retrieved from 

below the groundwater table approaches its liquid limit, it is an 

indication that the soil in its natural state is susceptible to larger 

consolidation settlement. 

For fluid flow applications, the moisture content is often expressed as 

the volumetric moisture content / V V w t θ = , where Vw is the volume of 

water and Vt is the total volume of the sample. Volumetric moisture 

content can also be determined as θ = Sn , where S is the saturation and 

n is the porosity. 

Typical 

Values 

See Table 5-9. For dry soils, w ≅ 0 . For most natural soils, 

3 70%. ≤ ≤ w Saturated fine-grained and organic soils may have 

gravimetric moisture contents in excess of 100%.
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Geotechnical Aspects of Pavements 5 - 18 May 2006 

Table 5-12. Unit weight. 

Description The unit weight is the total weight divided by total volume for a soil 

sample. 

Uses in 

Pavements 

• Calculation of in-situ stresses. 

• Correlations with soil behavior, other soil properties. 

• Compaction control (see Compaction subsection). 

Laboratory 

Determination 

The unit weight for undisturbed fine-grained soil samples is measured in 

the laboratory by weighing a portion of a soil sample and dividing by its 

volume. This can be done with thin-walled tube (Shelby) samples, as 

well as piston, Sherbrooke, Laval, and NGI samplers. Where 

undisturbed samples are not available (e.g., for coarse grained soils), the 

unit weight must be evaluated from weight-volume relationships (see 

Table 5-8). 

Field 

Measurement 

Nuclear gauge (ASTM D2922), sand cone (ASTM D1556). 

Commentary Unit weight is also commonly termed density. 

The total unit weight is a function of the moisture content of the soil 

(Table 5-8). Distinctions must be maintained between dry (γd), saturated 

(γsat), and moist or total (γt) unit weights. The moisture content should 

therefore be obtained at the same time as the unit weight to allow 

conversion from total to dry unit weights. 

Typical 

Values 

See Table 5-9. 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 19 May 2006 

Compaction 

Soil compaction is one of the most important geotechnical concerns during the construction 

of highway pavements and related fills and embankments. Compaction improves the 

engineering properties of soils in many ways, including 

• increased elastic stiffness, which reduces short-term resilient deformations during 

cyclic loading. 

• decreased compressibility, which reduces the potential for excessive long-term 

settlement. 

• increased strength, which increases bearing capacity and decreases instability 

potential (e.g., for slopes). 

• decreased hydraulic conductivity (permeability), which inhibits flow of water through 

the soil. 

• decreased void ratio, which reduces the amount of water that can be held in the soil 

and, thus, helps maintain desired strength and stiffness properties. 

• dncreased erosion resistance. 

Compaction is usually quantified in terms of the equivalent dry unit weight γd of the soil as a 

measure of the amount of solid materials present in a unit volume. The higher the amount of 

solid materials, the stronger and more stable the soil will be. Standard laboratory testing 

(Table 5-13) involves compacting several specimens at different water contents (w). The total 

unit weight (γt) and water content are measured for each compacted specimen. The 

equivalent dry unit weight is then computed as 

 

1

t

d

w

γ γ = +

 (5.1) 

If the specific gravity of solids Gs is known, the saturation level (S) and void ratio (e) can 

also be determined using the following two identities: 

Gs w = S e (5.2)

(1 )

(1 )

s w

t

G w

e

γ γ+ = +

 (5.3) 

The pairs of equivalent dry weight vs. water content values are plotted as a moisture-density 

of compaction curve, as in Figure 5-4. Compaction curves will typically exhibit a well 

defined peak corresponding to the maximum dry unit weight ((γd)max) at an optimum moisture 

content (wopt). It is good practice to plot the zero air voids (ZAV) curve corresponding to 100 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 20 May 2006 

percent saturation on the moisture-density graph (see Figure 5-4). The measured compaction 

curve cannot fall above the ZAV curve if the correct specific gravity has been used. The 

peak or maximum dry unit weight usually corresponds to saturation levels of between 70 – 

85 percent. 

Relative compaction (CR) is the ratio (expressed as a percentage) of the density of compacted 

or natural in-situ soils to the maximum density obtainable in a specified compaction test: 

max

100%

( )

d

R

d

C γ

γ

= × (5.4) 

Specifications often require a minimum level of relative compaction (e.g., 95%) in the 

construction or preparation of foundations, subgrades, pavement subbases and bases, and 

embankments. Requirements for compacted moisture content relative to the optimum 

moisture content may also be included in compaction specifications. The design and 

selection of methods to improve the strength and stiffness characteristics of deposits depend 

heavily on relative compaction. 

Relative density (DR) (ASTM D 4253) is often a useful parameter in assessing the 

engineering characteristics of granular soils. It is defined as 

max

max min

r 100% e e D

e e

− = × − (5.5) 

in which emin and emax are the minimum and maximum void ratio values for the soil. Relative 

density can also be expressed in terms of dry unit weights: 

min max

max min

() () 100%

() ()

dd d

r

dd d

D γγ γ

γγ γ

   − = ×       − (5.6) 

Table 5-14 presents a classification of soil consistency based on relative density for granular 

soils. 

 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 21 May 2006 

Table 5-13. Compaction characteristics. 

Description Compaction characteristics are expressed as the equivalent dry unit weight vs. 

moisture content relationship for a soil at a given compaction energy level. Of 

particular interest are the maximum equivalent dry unit weight and 

corresponding optimum moisture content at a given compaction energy level. 

Uses in 

Pavements 

• In conjunction with other tests (e.g., resilient modulus), determines 

 influence of soil density on engineering properties. 

• Field QC/QA for compaction of natural subgrade, placed subbase and base 

 layers, and embankment fills. 

Laboratory 

Determination 

Two sets of test protocols are most commonly used: 

• AASHTO T 99 (Standard Proctor), T 180 (Modified Proctor) 

• ASTM D 698 (Standard Proctor), D 1557 (Modified Proctor) 

Compaction tests are performed using disturbed, prepared soils with or without 

additives. Normally, soil passing the No. 4 sieve is mixed with water to form 

samples at various moisture contents ranging from the dry state to wet state. 

These samples are compacted in layers in a mold by a hammer at a specified 

nominal compaction energy that is a function of the number of layers, hammer 

weight, drop height, and number of blows (see Table 5-15). Equivalent dry unit 

weight is determined based on the moisture content and the unit weight of 

compacted soil. A curve of dry unit weight versus moisture content is plotted 

(Figure 5-4) and the maximum ordinate on this curve is referred to as the 

maximum dry unit weight ((γd)max). The water content at which this maximum 

occurs is termed as the optimum moisture content (wopt) or OMC. 

Field 

Measurement 

Field determination of moisture content (Table 5-11) and unit weight (Table 

5-12) is used to check whether field-compacted material meets construction 

specifications. 

Commentary Where a variety of soils are to be used for construction, a moisture-density 

relationship for each major type of soil or soil mixture anticipated at the site 

should be established. 

When additives such as Portland cement, lime, or flyash are used to determine 

the maximum density of mixed compacted soils in the laboratory, care should 

be taken to duplicate the expected delay period between mixing and 

compaction in the field. It should be kept in mind that these chemical additives 

start reacting as soon as they are added to the wet soil. They cause substantial 

changes in soil properties, including densities achievable by compaction. The 

period between mixing and compaction in the field is expected to be three 

hours, for example, then in the laboratory the compaction of the soil should 

also be delayed three hours after mixing the stabilizing additives. 

Typical Values See Table 5-16 for AASHTO recommended minimum compaction levels. 

Typical ranges for compacted unit weight and optimum moisture content for 

USCS and AASHTO soil classes are summarized in Table 5-17 and 

Table 5-18, respectively. 
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Geotechnical Aspects of Pavements 5 - 22 May 2006 

Compaction Curve

0

5

10

15

20

25

30

0 5 10 15 20 25

Water Content, w (%)

Dry

U

nit W

eig

ht, γd (k

N/m3

)

ZAV = zero air void curve

 (G s = 2.70)

S = 

 100%

 80%

 70%

Measured

γd at varying 

moisture 

contents

Optimum

Moisture

Content, w opt

Max. Dry 

Unit Weight

Figure 5-4. Typical moisture-density relationship from a standard compaction test. 

Table 5-14. Consistency of granular soils at various relative densities. 

Relative Density Dr (%) Description 

85 – 100 Very dense 

65 – 85 Dense 

35 – 65 Medium dense 

15 – 35 Loose 

0 - 15 Very loose 

Table 5-15. Principal differences between standard and modified Proctor tests. 

 Standard Proctor Modified Proctor 

Standards AASHTO T 99 

ASTM D 698 

AASHTO T 180 

ASTM D 1557 

Hammer weight 5.5 lb (24.4 kN) 10.0 lb (44.5 kN) 

Hammer drop height 12 in (305 mm) 18 in (457 mm) 

Number of soil layers 3 5 

Hammer blows per layer 25 25 

Total compaction energy 12,400 ft-lb/ft3 

(600 kN-m/m3) 

56,000 ft-lb/ft3 

(2,700 kN-m/m3) 
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Table 5-16. Recommended minimum requirements for compaction 

of embankments and subgrades (AASHTO, 2003). 

Minimum Percent Compaction (%)a 

 Embankments 

AASHTO 

Soil Class 

< 50 ft. high > 50 ft. high Subgrades 

A-1, A-3 > 95 > 95 100 

A-2-4, A-2-5 > 95 > 95 100 

A-2-6, A-2-7 > 95 --b > 95c 

A-4, A-5, A-6, A-7 > 95 --b > 95c 

a

Based on standard Proctor (AASHTO T 99). 

b

Special attention to design and construction is required for these materials. 

c

Compaction at within 2% of the optimum moisture content. 

Table 5-17. Typical compacted densities and optimum moisture contents 

for USCS soil types (after Carter and Bentley, 1991). 

Compacted Dry Unit 

Weight Soil Description USCS 

Class 

(lb/ft3

) (kN/m3) 

Optimum 

Moisture 

Content 

(%) 

Gravel/sand mixtures: 

 well-graded, clean GW 125-134 19.6-21.1 8-11 

 poorly-graded, clean GP 115-125 18.1-19.6 11-14 

 well-graded, small silt content GM 119-134 18.6-21.1 8-12 

 well-graded, small clay content GC 115-125 18.1-19.6 9-14 

Sands and sandy soils: 

 well-graded, clean SW 109-131 17.2-20.6 9-16 

 poorly-graded, small silt content SP 94-119 15.7-18.6 12-21 

 well-graded, small silt content SM 109-125 17.2-19.6 11-16 

 well-graded, small clay content SC 106-125 16.7-19.6 11-19 

Fined-grained soils of low plasticity: 

 silts ML 94-119 14.7-18.6 12-24 

 clays CL 94-119 14.7-18.6 12-24 

 organic silts OL 81-100 12.7-15.7 21-33 

Fine-grained soils of high plasticity: 

 silts MH 69-94 10.8-14.7 24-40 

 clays CH 81-106 12.7-18.6 19-36 

 organic clays OH 66-100 10.3-15.7 21-45 
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Geotechnical Aspects of Pavements 5 - 24 May 2006 

Table 5-18. Typical compacted densities and optimum moisture contents 

for AASHTO soil types (after Carter and Bentley, 1991). 

Compacted Dry Unit 

Weight Soil Description AASHTO

Class 

(lb/ft3

) (kN/m3) 

Optimum 

Moisture 

Content 

(%) 

Well-graded gravel/sand mixtures A-1 115-134 18.1-21.1 5-15 

Silty or clayey gravel and sand A-2 109-134 17.2-21.1 9-18 

Poorly-graded sands A-3 100-119 15.7-18.6 5-12 

Low plasticity silty sands and 

gravels A-4 94-125 14.7-19.6 10-20 

Diatomaceous or micaceous silts A-5 84-100 13.2-15.7 20-35 

Plastic clay, sandy clay A-6 94-119 14.7-18.6 10.30 

Highly plastic clay A-7 81-115 12.7-18.1 15-35 

Gradation 

Gradation, or the distribution of particle sizes within a soil, is an essential descriptive feature 

of soils. Soil textural (e.g., gravel, sand, silty clay, etc.) and engineering (see Section 4.7.2) 

classifications are based in large part on gradation, and many engineering properties like 

permeability, strength, swelling potential, and susceptibility to frost action are closely 

correlated with gradation parameters. Gradation is measured in the laboratory using two 

tests: a mechanical sieve analysis for the sand and coarser fraction (Table 5-19), and a 

hydrometer test for the silt and finer clay material (Table 5-20). 

 

Gradation is quantified by the percentage (most commonly by weight) of the soil that is finer 

than a given size (“percent passing”) vs. grain size. Gradation is occasionally expressed 

alternatively in terms of the percent coarser than a given grain size. Gradation characteristics 

are also expressed in terms of Dn parameters, where D is the largest particle size in the n

percent finest fraction of soil. For example, D10 is the largest particle size in the 10% finest 

fraction of soil; D60 is the largest particle size in the 60% finest fraction of soil. 
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Table 5-19. Grain size distribution of coarse particles (mechanical sieve analysis). 

Description The grain size distribution is the percentage of soil finer than a given 

size vs. grain size. Coarse particles are defined as larger than 0.075 mm 

(0.0029 in, or No. 200 sieve). 

Uses in 

Pavements 

• Soil classification (see Section 4.7.2) 

• Correlations with other engineering properties 

Laboratory 

Determination 

The grain size distribution of coarse particles is determined from a 

mechanical washed sieve analysis (AASHTO T 88, ASTM D 422). A 

representative sample is washed through a series of sieves (Figure 5-5). 

The amount retained on each sieve is collected, dried, and weighed to 

determine the percentage of material passing that sieve size. Figure 5-7 

shows example grain size distributions for sand, silt, and clay soils as 

obtained from mechanical sieve and hydrometer (Table 5-20) tests. 

Field 

Measurement 

Not applicable. 

Commentary Obtaining a representative specimen is an important aspect of this test. 

When samples are dried for testing or “washing,” it may be necessary to 

break up the soil clods. Care should be taken to avoid crushing of soft 

carbonate or sand particles. If the soil contains a substantial amount of 

fibrous organic materials, these may tend to plug the sieve openings 

during washing. The material settling over the sieve during washing 

should be constantly stirred to avoid plugging. 

Openings of fine mesh or fabric are easily distorted as a result of normal 

handling and use. They should be replaced often. A simple way to 

determine whether sieves should be replaced is the periodic examination 

of the stretch of the sieve fabric on its frame. The fabric should remain 

taut; if it sags, it has been distorted and should be replaced. 

A common cause of serious errors is the use of “dirty” sieves. Some 

soil particles, because of their shape, size or adhesion characteristics, 

have a tendency to be lodged in the sieve openings. 

Typical 

Values 

Typical particles size ranges for various soil textural categories are as 

follows (ASTM D 2487): 

• Gravel: 4.75 – 75 mm (0.19 – 3 in; No. 4 to 3-inch sieves) 

• Sand: 0.075 – 4.75 mm (0.0029 – 0.19 in; No. 200 to No. 4 sieves) 

• Silt and clay: < 0.075 mm (0.0029 in; No. 200 sieve) 
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Table 5-20. Grain size distribution of fine particles (hydrometer analysis). 

Description The grain size distribution is the percentage of soil finer than a given 

size vs. grain size. Fine particles are defined as smaller than 0.075 mm 

(0.0029 in, or No. 200 sieve). 

Uses • Soil classification (see Section 4.7.2) 

• Correlations with other engineering properties 

Laboratory 

Determination 

The grain size distribution of fine particles is determined from a 

hydrometer analysis (AASHTO T 88, ASTM D 422). Soil finer than 

0.075 mm (0.0029 in or No. 200 sieve) is mixed with a dispersant and 

distilled water and placed in a special graduated cylinder in a state of 

liquid suspension (Figure 5-6). The specific gravity of the mixture is 

periodically measured using a calibrated hydrometer to determine the 

rate of settlement of soil particles. The relative size and percentage of 

fine particles are determined based on Stoke’s law for settlement of 

idealized spherical particles. Figure 5-5 shows example grain size 

distributions for sand, silt, and clay soils as obtained from mechanical 

sieve (Table 5-19) and hydrometer tests. 

Field 

Measurement 

Not applicable. 

Commentary The principal value of the hydrometer analysis is in obtaining the clay 

fraction (percent finer than 0.002 mm). This is because the soil behavior 

for a cohesive soil depends principally on the type and percent of clay 

minerals, the geologic history of the deposit, and its water content, 

rather than on the distribution of particle sizes. 

Repeatable results can be obtained when soils are largely composed of 

common mineral ingredients. Results can be distorted and erroneous 

when the composition of the soil is not taken into account to make 

corrections for the specific gravity of the specimen. 

Particle size of highly organic soils cannot be determined by the use of 

this method. 

Typical 

Values 

• Silt: 0.075 – 0.002 mm (0.0029 – 0.000079 in.) 

• Clay: < 0.002 mm (0.000079 in.) 

 
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Figure 5-5. Laboratory sieves for mechanical analysis of grain size distribution. Shown 

(right to left) are sieve Nos. 3/8-in. (9.5-mm), No. 10 (2.0-mm), No. 40 (250-μm) and 

No. 200 (750-μm) and example soil particle sizes including (right to left): medium 

gravel, fine gravel, medium-coarse sand, silt, and dry clay (kaolin). 

Figure 5-6. Soil hydrometer apparatus (http://www.ce.siue.edu/). 
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Geotechnical Aspects of Pavements 5 - 28 May 2006 

0

10

20

30

40

50

60

70

80

90

100

0.0001 0.001 0.01 0.1 1 10

Grain Size (m

Perc

e

nt Pa

s

sin

g (b

y

weig

ht)

Silica Sand

Piedmont Silt

Plastic Kaolin

CLAY SIZE SILT SIZE SAND SIZE GRAVEL 

0.075 mm 

Fine-Grained Soils Coarse-Grained Soils 

Figure 5-7. Representative grain size distributions for several soil types. 

Plasticity 

Plasticity describes the response of a soil to changes in moisture content. When adding water 

to a soil changes its consistency from hard and rigid to soft and pliable, the soil is said to 

exhibit plasticity. Clays can be very plastic, silts are only slightly plastic, and sands and 

gravels are non-plastic. For fine-grained soils, engineering behavior is often more closely 

correlated with plasticity than gradation. Plasticity is a key component of AASHTO and 

Unified soil classification systems (Section 4.7.2). 

Soil plasticity is quantified in terms of Atterberg limits. As shown in Figure 5-8, the 

Atterberg limit values correspond to values of moisture content where the consistency of the 

soil changes as it is progressively dried from a slurry: 

• The liquid limit (LL), which defines the transition between the liquid and plastic 

states. 

• The plastic limit (PL), which defines the transition between the plastic and semi-solid 

states. 
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Geotechnical Aspects of Pavements 5 - 29 May 2006 

• The shrinkage limit (SL), which defines the transition between the semi-solid and 

solid states. 

• Note in Figure 5-8 that the total volume of the soil changes as it is dried until the 

shrinkage limit is reached; drying below the shrinkage limit does not cause any 

additional volume change. 

It is important to recognize that Atterberg limits are not fundamental material properties. 

Rather, they should be interpreted as index values determined from standardized test methods 

(Table 5-21). 

Figure 5-8. Variation of total soil volume and consistency with change in water content for a 

fine-grained soil (from McCarthy, 2002). 
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Table 5-21. Plasticity of fine-grained soils (Atterberg limits). 

Description Plasticity describes the response of a soil to changes in moisture content. 

Plasticity is quantified by Atterberg limits. 

Uses in 

Pavements 

• Soil classification (see Section 4.7.2) 

• Correlations with other engineering properties 

Laboratory 

Determination 

Atterberg limits are determined using test protocols described in AASHTO 

T89 (liquid limit), AASHTO T90 (plastic limit), AASHTO T 92 (shrinkage 

limit), ASTM D 4318 (liquid and plastic limits), and ASTM D 427 (shrinkage 

limit). A representative sample is taken of the portion of the soil passing the 

No. 40 sieve. The moisture content is varied to identify three stages of soil 

behavior in terms of consistency: 

• The liquid limit (LL) is defined as the water content at which 25 blows of 

the liquid limit machine (Figure 5-9) closes a standard groove cut in the soil 

pat for a distance of 12.7 cm (1/2 in.). An alternate procedure in Europe and 

Canada uses a fall cone device to obtain better repeatability. 

• The plastic limit (PL) is as the water content at which a thread of soil, 

when rolled down to a diameter of 3 mm (1/8 in.), will crumble. 

• The shrinkage limit (SL) is defined as that water content below which no 

further soil volume change occurs with additional drying. 

Field 

Measurement 

Not applicable. 

Commentary The Atterberg limits provide general indices of moisture content relative to the 

consistency and behavior of soils. The LL defines the lower boundary for the 

liquid state, while the PL defines the upper bound of the solid state The 

difference is termed the plasticity index (PI = LL - PL). The liquidity index 

(LI), defined as LI = (w-PL)/PI, where w is the natural moisture content, is an 

indicator of soil consistency in its natural in-situ conditions. 

It is important to recognize that Atterberg limits are approximate and empirical 

values. They were originally developed for agronomic purposes. Their 

widespread use by engineers has resulted in the development of a large number 

of empirical relationships for characterizing soils. 

Considering the somewhat subjective nature of the test procedure, Atterberg 

limits should only be performed by experienced technicians. Lack of 

experience and care can introduce serious errors in the test results. 

The optimum compaction moisture content is often in the vicinity of the plastic 

limit. 

Typical Values See Table 5-22. 
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Geotechnical Aspects of Pavements 5 - 31 May 2006 

Figure 5-9. Liquid limit test device. 

Table 5-22. Characteristics of soils with different plasticity indices (after Sowers, 1979). 

Plasticity 

Index Classification Dry Strength Visual-Manual 

Identification of Dry Sample 

0 – 3 Nonplastic Very low Falls apart easily 

3 – 15 Slightly plastic Slight Easily crushed with fingers 

15 – 30 Medium plastic Medium Difficult to crush with fingers 

> 30 Highly plastic High Impossible to crush with fingers 

5.3.3 Problem Soil Identification 

Two special conditions that often need to be checked for natural subgrade soils are the 

potential for swelling clays (Table 5-23) or collapsible silts (Table 5-25). 

Swelling soils exhibit large changes in soil volume with changes in soil moisture. The 

potential for volumetric swell of a soil depends on the amount of clay, its relative density, the 

compaction moisture and density, permeability, location of the water table, presence of 

vegetation and trees, and overburden stress. Swell potential also depends on the 

mineralogical composition of fine-grained soils. Montmorillonite (smectite) exhibits a high 

degree of swell potential, illite has negligible to moderate swell characteristics, and kaolinite 

exhibits almost none. A one-dimensional swell potential test is used to estimate the percent 

swell and swelling pressures developed by the swelling soils (Table 5-23). 
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Geotechnical Aspects of Pavements 5 - 32 May 2006 

Collapsible soils exhibit abrupt changes in strength at moisture contents approaching 

saturation. When dry or at low moisture content, collapsible soils give the appearance of a 

stable deposit. At high moisture contents, these soils collapse and undergo sudden decreases 

in volume. Collapsible soils are found most commonly in loess deposits, which are composed 

of windblown silts. Other collapsible deposits include residual soils formed as a result of the 

removal of organics by decomposition or the leaching of certain minerals (calcium 

carbonate). In both cases, disturbed samples obtained from these deposits will be classified 

as silt. Loess, unlike other non-cohesive soils, will stand on almost a vertical slope until 

saturated. It has a low relative density, a low unit weight, and a high void ratio. A onedimensional collapse potential test is used to identify collapsible soils (Table 5-25). 

Table 5-23. Swell potential of clays. 

Description Swelling is a large change in soil volume induced by changes in moisture 

content. 

Uses in 

Pavements 

Swelling subgrade soils can have a seriously detrimental effect on pavement 

performance. Swelling soils must be identified so that they can be either 

removed, stabilized, or accounted for in the pavement design. 

Laboratory 

Determination 

Swell potential is measured using either the AASHTO T 258 or ASTM D 4546 

test protocols. The swell test is typically performed in a consolidation 

apparatus. The swell potential is determined by observing the swell of a 

laterally-confined specimen when it is surcharged and flooded. Alternatively, 

after the specimen is inundated, the height of the specimen is kept constant by 

adding loads. The vertical stress necessary to maintain zero volume change is 

the swelling pressure. 

Field 

Measurement 

Not applicable. 

Commentary This test can be performed on undisturbed, remolded, or compacted specimens. 

If the soil structure is not confined (i.e., a bridge abutment) such that swelling 

may occur laterally and vertically, triaxial tests can be used to determine three 

dimensional swell characteristics. 

Typical Values Swell potential can be estimated in terms of soil physical properties; see Table 

5-24. 
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Table 5-24. Estimation of swell potential (Holtz and Gibbs, 1956). 

% finer than Atterberg Limits 

0.001mm PI (%) SL (%) 

Probable expansion, 

% total volume change* 

Potential for 

expansion 

> 28 > 35 < 11 > 30 Very high 

20-31 25-41 7-12 20-30 High 

13-23 15-28 10-16 10-30 Medium 

< 15 < 18 >15 < 10 Low 

 *Based on a loading of 6.9 kPa (1 psi). 

Table 5-25. Collapse potential of soils. 

Description Collapsible soils exhibit large decreases in strength at moisture contents 

approaching saturation, resulting in a collapse of the soil skeleton and large 

decreases in soil volume. 

Uses in 

Pavements 

Collapsible subgrade soils can have a seriously detrimental effect on pavement 

performance. Collapsible soils must be identified so that they can be either 

removed, stabilized, or accounted for in the pavement design. 

Laboratory 

Determination 

Collapse potential is measured using the ASTM D 5333 test protocol. The 

collapse potential of suspected soils is determined by placing an undisturbed, 

compacted, or remolded specimen in a consolidometer ring. A load is applied 

and the soil is saturated to measure the magnitude of the vertical displacement. 

Field 

Measurement 

Not applicable. 

Commentary The collapse during wetting occurs due to the destruction of clay binding, 

which provides the original strength of these soils. Remolding and compacting 

may also destroy the original structure. 

Typical Values None available. 

5.3.4 Other Aggregate Tests 

There is a wide range of other mechanical property tests that are performed to measure the 

quality and durability of aggregates used as subbases and bases in pavement systems and as 

constituents of asphalt and Portland cement concrete. These other aggregate tests are 

summarized in Table 5-26. Additional information can be found in The Aggregate Handbook

published by the National Stone Association (Barksdale, 2000). A recent NCHRP study 

provides additional useful information on performance-related tests of aggregates used in 

unbound pavement layers (Saeed, Hall, and Barker, 2001). 
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Geotechnical Aspects of Pavements 5 - 34 May 2006 

Table 5-26. Other tests for aggregate quality and durability. 

Property Use AASHTO 

Specification 

ASTM 

Specification 

Fine Aggregate Quality 

Sand Equivalent 

Measure of the relative 

proportion of plastic fines and 

dust to sand size particles in 

material passing the No. 4 sieve 

T 176 D 2419 

Fine Aggregate 

angularity (also 

termed 

Uncompacted Air 

Voids) 

Index property for fine aggregate 

internal friction in Superpave 

asphalt mix design method T 304 C 1252 

Coarse Aggregate Quality 

Coarse Aggregate 

Angularity 

Index property for coarse 

aggregate internal friction in 

Superpave asphalt mix design 

method 

 D 5821 

Flat, Elongated 

Particles 

Index property for particle shape 

in Superpave asphalt mix design 

method 

 D 4791 

General Aggregate Quality 

Absorption Percentage of water absorbed 

into permeable voids T 84/T 85 C 127/C 128 

Particle Index Index test for particle shape D 3398 

Los Angeles 

degradation 

Measure of coarse aggregate 

resistance to degradation by 

abrasion and impact 

T 96 C 131 or 

 C 535 

Soundness 

Measure of aggregate resistance 

to weathering in concrete and 

other applications 

T 104 C 88 

Durability Index of aggregate durability T 210 D 3744 

Expansion Index of aggregate suitability D 4792 

Deleterious 

Materials 

Describes presence of 

contaminants like shale, clay 

lumps, wood, and organic 

material 

T 112 C 142 
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Geotechnical Aspects of Pavements 5 - 35 May 2006 

5.4 MECHANICAL PROPERTIES 

Stiffness is the most important mechanical characteristic of unbound materials in pavements. 

The relative stiffnesses of the various layers dictate the distribution of stresses and strains 

within the pavement system. Figure 5-10 and Figure 5-11 illustrate respectively how the 

stiffnesses of the subgrade and the unbound base layer influence the horizontal tensile strain 

at the bottom of the asphalt and the compressive vertical strain at the top of the subgrade for 

a simple three-layer flexible pavement system. These pavement response parameters are 

directly related to asphalt fatigue cracking and subgrade rutting performance, respectively. 

It may seem odd that stiffness rather than strength is considered the most important unbound 

material property for pavements. Pavement structural design is usually viewed as ensuring 

sufficient load-carrying capacity for the applied traffic – i.e., providing sufficient pavement 

strength. However, the stress levels in well-designed asphalt or PCC-surfaced pavement are 

well below the strength of the unbound materials, and thus failure under any given load 

application is not an issue. The situation for aggregate-surfaced roads is, of course, a bit 

different: strength of the aggregate surface will directly influence the road’s durability and 

performance. Subgrade strength is also an important issue during pavement construction. 

The preferred method for characterizing the stiffness of unbound pavement materials is the 

resilient modulus MR (Section 5.4.3), which is defined as the unloading modulus in cyclic 

loading. The AASTHO Design Guides beginning in 1986 have recommended the resilient 

modulus for characterizing subgrade support for flexible and rigid pavements and for 

determining structural layer coefficients for flexible pavements. The resilient modulus is also 

the primary material property input for unbound materials in the NCHRP 1-37A Design 

Guide for both flexible and rigid pavements. 

Both the AASHTO and NCHRP 1-37A design procedures recognize the need for backward 

compatibility with other properties used in the past to characterize unbound materials, in 

particular the California Bearing Ratio and the Stabilometer R-value. These index material 

properties continue to be used by many highway agencies. Correlations are provided in both 

design procedures for relating CBR and R-values to MR (or, in the case of the AASHTO 

Guides, to the structural layer coefficients ai). The modulus of subgrade reaction (k) used in 

the AASHTO Guides is also correlated to MR. 

Laboratory and field methods (where appropriate) for determining the stiffness and other 

relevant mechanical properties of unbound materials in pavement systems are described in 

the following subsections and tables. Typical values for each property are also summarized. 

The soil mechanical properties described here are: 
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Geotechnical Aspects of Pavements 5 - 36 May 2006 

• Index properties 

o California Bearing Ratio (Section 5.4.1) 

o Stabilometer R-Value (Section 5.4.2) 

o Structural Layer Coefficients (Section 5.4.5) 

• Stiffness properties 

o Resilient Modulus (Section 5.4.3) 

o Poisson’s Ratio (Section 5.4.4) 

o Modulus of Subgrade Reaction (Section 5.4.6) 

• Other properties 

o Interface Friction (Section 5.4.7) 

o Permanent Deformation (Section 5.4.8) 

o Coefficient of Lateral Pressure (Section 5.4.96) 

Figure 5-10. Influence of subgrade stiffness on critical pavement strains. (Elastic solution, 

6 in./150 mm AC over 18 in./450 mm granular base. Reference elastic moduli: EAC = 

500,000 psi/3450 MPa; EBS = 30,000 psi/207 Mpa; ESG = 3000 psi/20.7 MPa. Load: 

10 kip/44.5 kN single-wheel load, 100 psi/690 kPa contact pressure.) 

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

0 5000 10000 15000

Subgrade Stiffness ESG

Strain Ratio

AC tensile strain

SG comp strain
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Geotechnical Aspects of Pavements 5 - 37 May 2006 

Figure 5-11. Influence of granular base stiffness on critical pavement strains. (Elastic 

solution, 6 in./150 mm AC over 18 in./450 mm granular base. Reference elastic 

moduli: EAC = 500,000 psi/3450 MPa; EBS = 30,000 psi/207 Mpa; ESG = 3000 

psi/20.7 MPa. Load: 10 kip/44.5 kN single wheel load, 100 psi/690 kPa contact 

pressure.)

5.4.1 California Bearing Ratio (CBR) 

The California Bearing Ratio or CBR test (Table 5-27) is an indirect measure of soil strength 

based on resistance to penetration by a standardized piston moving at a standardized rate for 

a prescribed penetration distance (Figure 5-12). CBR values are commonly used for highway, 

airport, parking lot, and other pavement designs based on empirical local or agency-specific 

methods (i.e., FHWA, FAA, AASHTO). CBR has also been correlated empirically with 

resilient modulus and a variety of other engineering soil properties. 

CBR is not a fundamental material property and thus is unsuitable for direct use in 

mechanistic and mechanistic-empirical design procedures. However, it is a relatively easy 

and inexpensive test to perform, it has a long history in pavement design, and it is reasonably 

well correlated with more fundamental properties like resilient modulus. Consequently, it 

continues to be used in practice. 

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

0 10000 20000 30000 40000 50000

Granular Base Stiffness EBS

Strain Ratio

AC tensile strain

SG comp strain
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Table 5-27. California Bearing Ratio (CBR). 

Description The California Bearing Ratio or CBR is an indirect measure of soil strength 

based on resistance to penetration. 

Uses in 

Pavements 

• Direct input to some empirical pavement design methods 

• Correlations with resilient modulus and other engineering properties 

Laboratory 

Determination 

AASTHO T 193 or ASTM D 1883. CBR is based on resistance to penetration 

by a standardized piston moving at a standardized rate for a prescribed 

penetration distance (Figure 5-12). CBR is defined as the ratio of the load 

required to cause a certain depth of penetration of a piston into a compacted 

specimen of soil at some water content and density, to the standard load

required to obtain the same depth of penetration on a standard sample of 

crushed stone (usually limestone). Typically soaked conditions are used to 

simulate anticipated long-term conditions in the field. 

The CBR test is run on three identically compacted samples. Each series of the 

CBR test is run for a given relative compaction and moisture content. The 

geotechnical engineer must specify the conditions (dry, at optimum moisture, 

after soaking, 95% relative compaction, etc.) under which each test should be 

performed. 

Field 

Measurement 

ASTM D 4429. Test procedure is similar to that for laboratory determination. 

Commentary Most CBR testing is laboratory-based; thus, the results will be highly 

dependent on the representativeness of the samples tested. It is also important 

that the testing conditions be clearly stated: CBR values measured from ascompacted samples at optimum moisture and density conditions can be 

significantly greater than CBR values measured from similar samples after 

soaking, for example. 

For field measurement, care should be taken to make certain that the deflection 

dial is anchored well outside the loaded area. Field measurement is made at the 

field moisture content while laboratory testing is typically performed for 

soaked conditions, so soil-specific correlations between field and laboratory 

CBR values are often required. 

Typical Values See Table 5-28. For AASHO Road Test, CBR ≅ 100 for the granular base layer 

and about 30 for the granular subbase. 
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Figure 5-12. California Bearing Ratio test device (http://www.ele.com/geot/cali.htm). 

Table 5-28. Typical CBR values (after U.S. Army Corps of Engineers, 1953). 

USCS Soil Class Field CBR

GW 60 – 80 

GP 35 – 60 

GM 40 – 80 

GC 20 – 40 

SW 20 – 40 

SP 15 – 25 

SM 20 – 40 

SC 10 – 20 

ML 5 – 15 

CL 5 – 15 

OL 4 – 8 

MH 4 – 8 

CH 3 – 5 

OH 3 – 5 
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Geotechnical Aspects of Pavements 5 - 40 May 2006 

5.4.2 Stabilometer (R-Value) 

The Stabilometer or R-Value test (Table 5-29) was developed by the California Division of 

Highways for use in their in-house empirical pavement design method. The R-value 

measured in this test is a measure of the resistance to deformation and is expressed as a 

function of the ratio of the induced lateral pressure to the applied vertical pressure as 

measured in a triaxial-type loading device (Figure 5-13): 

 

2 [ ]

100 100

(2.5/ ) ( / ) 1 1 v h

R

D PP = − − +

 (5.7) 

in which R = resistance value 

 Pv = applied vertical pressure (160 psi) 

 Ph = transmitted horizontal pressure 

 D2 = displacement of stabilometer fluid necessary to increase 

 horizontal pressure from 5 to 100 psi, measured in revolutions 

 of a calibrated pump handle 

A kneading compactor is used to prepare the test samples, as specimens fabricated by this 

method are thought to develop internal structures most similar to those in actual field 

compacted materials. 

The R-Value is used either directly or translated into more common factors (i.e., CBR) 

through correlation charts to be used with other more common design methods (i.e.,

AASHTO). Like CBR, however, it is not a fundamental material property and thus is 

unsuitable for use in mechanistic and mechanistic-empirical design procedures. 
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Geotechnical Aspects of Pavements 5 - 41 May 2006 

Table 5-29. Stabilometer or R-Value. 

Description The R-value is a measure of the ability of a soil to resist lateral deformation 

under vertical load. 

Uses in 

Pavements 

• Direct input to some empirical pavement design methods 

• Correlations with other properties (e.g., CBR, resilient modulus) 

Laboratory 

Determination 

Measurement of the R-value of a soil is done with a stabilometer (AASHTO T 

190 or ASTM D 2844). A stabilometer (Figure 5-13) is similar to a triaxial 

device consisting of a metal cylinder in which there is a rubber membrane; the 

annular space between the two is filled with oil that transmits lateral pressure 

to the specimen. 

Compacted, unstabilized, or stabilized soils and aggregates can be tested. 

Samples are compacted using a special kneading compaction device. When the 

specimen is vertically loaded, a lateral pressure is transmitted to the soil, which 

can be measured on a pressure gage. The R-value is determined for the vertical 

to lateral pressure ratio and the displacement. 

Field 

Measurement 

Not applicable. 

Commentary The test also allows the measurement of swell pressure of expansive soils (see 

Section 5.3.3). The swell pressure or expansion pressure data is used in 

determining the suitability of expansive soils for use under pavements and the 

magnitude of overburden pressure needed to control the expansion of these 

soils. 

Typical Values Dense graded crushed stone: 80+ 

High compressibility silts: 15 – 30 

For the AASHO Road Test, R ≅ 85 for the granular base layer and about 60 for 

the granular subbase. 

Figure 5-13. Schematic of stabilometer test setup for measuring R-value (Yoder and 

Witczak, 1975). 
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Geotechnical Aspects of Pavements 5 - 42 May 2006 

5.4.3 Elastic (Resilient) Modulus 

Pavement thickness design prior to the 1986 AASHTO Design Guide was based on 

experience, soil classification, and the plastic response of pavement materials to static load, 

e.g., Marshall stability for asphalt concrete and CBR for unbound materials. The potential for 

fatigue cracking of asphalt concrete and the accumulation of permanent deformations in the 

unbound materials in flexible pavements under essentially elastic deformation conditions 

were not considered. Many expressed concerns about this approach, including Professor A. 

Casagrande (Burmeister, 1943): 

“Irrespective of the theoretical method of evaluation of load tests, there remains the important 

question as to what extent individual static load tests reflect the results of thousands of 

dynamic load repetitions under actual traffic. Experience and large-scale traffic tests have 

already indicated that various types of soils react differently, and that the results of static load 

tests by no means bear a simple relation to pavement behavior.” 

Investigators in the 1950s began using repeated load triaxial tests in the laboratory to 

evaluate the stiffness and other behavior of pavement materials under conditions that more 

closely simulated real traffic loadings in the field. Substantial pioneering contributions in this 

area were made by Seed, Chan, and Monismith (1955), Seed and McNeill (1956), and Seed, 

Chan, and Lee (1963) in their work on the deformation characteristics and resilient modulus 

of compacted subgrades. They found significant differences between values of initial tangent 

modulus measured from single-cycle unconfined compression tests as compared to values of 

resilient modulus as determined from repeated cyclic unconfined compression loading. The 

conclusion from this work was that the behavior of soils under traffic loading should be 

obtained from repeated load tests whenever possible. This conclusion was substantiated by 

field data obtained by the California Department of Highways that showed the marked 

difference in pavement deflections occurring under standing and slowly moving wheel loads. 

The culmination of this work was the adoption of resilient modulus testing by AASHTO in 

1982. The AASHTO T274 standard was the first modern test protocol for resilient 

modulus. The concept of resilient modulus was subsequently incorporated into the 1986 and 

AASHTO Guide for Design of Pavement Structures. 

Unbound Materials 

The elastic modulus for unbound pavement materials is most commonly characterized in 

terms of the resilient modulus, MR. The resilient modulus is defined as the ratio of the applied 

cyclic stress to the recoverable (elastic) strain after many cycles of repeated loading (Figure 

5-14) and thus is a direct measure of stiffness for unbound materials in pavement systems. It 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 43 May 2006 

is the single most important unbound material property input in most current pavement 

design procedures. Beginning in 1986, the AASTHO Design Guides have recommended use 

of resilient modulus for characterizing subgrade support for flexible and rigid pavements and 

for determining structural layer coefficients for flexible pavements. The resilient modulus is 

also the primary material property input for unbound materials in the NCHRP 1-37A Design 

Guide for both flexible and rigid pavements. It is an essential input to mechanistic pavement 

response models used to compute stresses, strains, and deformations induced in the pavement 

structure by the applied traffic loads. 

The definition of the resilient modulus as measured in a standard resilient modulus cyclic 

triaxial test is shown in Figure 5-15, in which σa and εa are the stress and strain in the axial 

(i.e., cyclic loading) direction. The sample is initially subjected to a hydrostatic confining 

pressure (σc), which induces an initial strain (εc). This initial strain is unmeasured in the test, 

but it is assumed the same in all directions for isotropic material behavior. The axial stress is 

then cycled at a constant magnitude (∆σ), which during unloading induces the cyclic resilient 

axial strain (∆ε). The resilient modulus (MR) is defined simply as the ratio of the cyclic axial 

stress to resilient axial strain: 

R

a

M σ

ε

∆ = ∆ (5.8) 

Although resilient modulus of unbound pavement materials is most commonly evaluated in 

the laboratory using a conventional triaxial cell, other test equipment/methods include the 

simple shear test, torsional resonant column testing, hollow cylinders, and true (cubical) 

triaxial tests. The pros and cons of these less-commonly employed testing procedures are 

described in Barksdale et al. (1996) and in LTPP (2003). The reasons that the triaxial device 

is most commonly used for resilient modulus testing include the following: 

• Equipment availability. Resilient modulus testing can be performed using triaxial 

testing equipment commonly found in many pavement materials laboratory. This 

equipment is virtually identical to that found in most geotechnical laboratories except 

for the requirement of larger specimen sizes (up to 6 in./150 mm diameter by 12 

in./300 mm tall) for coarse-grained base and subbase materials. 

• Stress state. The stress conditions within the specimen on any plane are defined 

throughout the triaxial test. The stress conditions applied in resilient modulus testing 

are similar in magnitude to those that occur when an isolated wheel loading is applied 

to the pavement directly above the element of material simulated in the test. 
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Geotechnical Aspects of Pavements 5 - 44 May 2006 

• Specimen drainage. The triaxial test permits relatively simple, controlled drainage of 

the specimen in the axial and/or radial directions. Pore pressures can also be easily 

measured at the ends of the specimen, or, with more difficulty, within the specimen. 

• Strain measurement. Axial, radial, and volumetric strains can all be measured 

relatively easily in the triaxial test. 

• Availability and robustness of test protocols. The testing protocols for triaxial 

resilient modulus have been improved steadily over the years. Good summaries of the 

evolution of the various protocols and their advantages and disadvantages can be 

found in Andrei (1999) and Witczak (2004). 

In addition to the above advantages, undisturbed tube samples of the subgrade obtained from 

the field can be extruded and tested with a minimum amount of specimen preparation. 

Finally, the triaxial cell used for the repeated load triaxial test can also be employed in static 

testing. 

The most severe limitation of the triaxial cell is its ability to simulate rotation of the principal 

stress axes and shear stress reversal. Both of these phenomena apply when a wheel load 

moves across the pavement. Additionally, the intermediate principal stress applied to a 

specimen cannot be controlled in the triaxial test. 

The laboratory-measured resilient modulus for most unbound pavement materials is stress 

dependent. The dominant effect for coarse-grained materials is an increase in MR with 

increasing confining stress, while the dominant effect for fine-grained soils is a decrease in 

MR with increasing shear stress. Many nonlinear MR models have been proposed over the 

years for incorporating the effects of stress level on the resilient modulus (Andrei, 1999; 

Witczak, 2004). The stress-dependent MR model implicitly included in the 1993 AASHTO 

Guide for granular base and subbase materials is (see Section 5.4.5 for more details) 

2

1

k MR = k θ (5.9) 

in which 

θ = bulk stress = σ1+σ2+σ3 (psi) 

k1,k2 = material properties 
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Geotechnical Aspects of Pavements 5 - 45 May 2006 

Guidance is provided in the 1993 AASHTO Guide for estimating the values of k1 and k2 for 

unbound base and subbase layers. Typical ranges of k1 and k2 are given in Table 5-30. 

The more general stress-dependent MR model adopted in the NCHRP 1-37A Design Guide is 

 

2 3

1 1

k k

oct

R a

a a

M kp

p p

   θ τ

= +       (5.10) 

in which 

θ = bulk stress = σ1+σ2+σ3 (same units as pa) 

τoct = octahedral shear stress = ( )( )( ) 2 22

12 23 13

1

3

σσ σσ σσ − +− +− 

 (same units as pa) 

pa = atmospheric pressure (to make equation dimensionless) 

k1,k2,k3 = material properties with constraints k1>0, k2 >0, k3 <0 

Equation (5.10) combines both the stiffening effect of the confinement or bulk stress (the 

term under the k2 exponent) for coarse-grained materials and the softening effect of shear 

stress (the term under the k3 exponent) for fine-grained soils. 

The seasonal variation of unbound material properties is often significant, particularly for 

moisture-sensitive fine-grained soils or for locations with significant freeze-thaw cycles. 

Both the 1993 AASHTO Guide and the NCHRP 1-37A design procedures include provisions 

for including seasonal variations of unbound material properties in the design. The procedure 

in the 1993 AASHTO Guide for incorporating seasonal variations into the effective subgrade 

(MR) can be briefly summarized as follows: 

• Determine an MR value for each time interval during a year. Typically, time intervals 

of two weeks or one month duration are used for this analysis. Methods for 

determining the MR value for each time interval include 

o laboratory measurement at the estimated in-situ water content for the time 

interval. 

o backcalculation from FWD tests performed during each season. Mohammad 

et al. (2002) and Andrei (2003) provide some useful correlations between MR, 

moisture content, and other soil parameters. 
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Geotechnical Aspects of Pavements 5 - 46 May 2006 

• Estimate a relative damage uf corresponding to each seasonal modulus value using the 

empirical relationship 

( ) 2.32 8 1.18 10 f R u M −

= × (5.11) 

• Compute the average relative damage f u as the sum of the relative damage values for 

each season divided by the number of seasons. 

• Determine the effective subgrade MR from using the inverse of Eq. (5.11): 

0.431 3015( )− MR = uf (5.12) 

This procedure can also be used to incorporate seasonal variations into the effective base and 

subbase MR values used to estimate structural layer coefficients in the 1993 AASHTO Guide 

(see Section 5.4.5). 

There are two options for incorporating the seasonal variation of unbound material properties 

in the NCHRP 1-37A design procedure. The first is the direct input of monthly MR values. 

The second method combines moisture and freeze/thaw predictions from the Enhanced 

Integrated Climate Model (EICM) with models relating MR to environmental conditions. The 

EICM and MR environment models are built into the NCHRP 1-37A Design Guide software; 

details are provided in Appendix D. 

Details of the procedures for determining MR for unbound paving materials are given in 

Table 5-31. Laboratory determination of MR is recommended for new construction and 

reconstruction projects. For rehabilitation projects, backcalculation of layer and subgrade MR

from FWD testing is the preferred approach (see Section 4.5.4), although calibrating 

backcalculated estimates with laboratory-measured values is a good practice (see Table 5-

32). 

Table 5-30. Typical ranges for k1 and k2 coefficients in Eq. (5.9) (AASHTO, 1993). 

Material k1 (psi) k2 

Granular base 3000 – 8000 0.5 – 0.7

Granular subbase 2500 - 7000 0.4 – 0.6
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Geotechnical Aspects of Pavements 5 - 47 May 2006 

Figure 5-14. Resilient modulus under cyclic loading. 

M R

σ

ε

∆

=

∆

ε

c

σ

c

∆ε

∆σ

MR

ε

a

σ

a

Figure 5-15. Definition of resilient modulus MR for cyclic triaxial loading. 

... ...

Strain ε

Stress ∆σ

MR

N= 1 2 3 4 ... 
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Geotechnical Aspects of Pavements 5 - 48 May 2006 

Table 5-31. Resilient modulus (MR). 

Description The resilient modulus (MR) is the elastic unloading modulus after many cycles 

of cyclic loading. 

Uses in 

Pavements 

• Characterization of subgrade stiffness for flexible and rigid pavements 

 (AASHTO 1986/1993; NCHRP 1-37A) 

• Determination of structural layer coefficients in flexible pavements 

 (AASHTO 1986/1993) 

• Characterization of unbound layer stiffness (NCHRP 1-37A) 

Laboratory 

Determination 

There currently are five test protocols in use for resilient modulus testing in the 

laboratory: 

• AASHTO T 292-91 

• AASHTO T 294-92 

• AASHTO T 307-99 (supersedes AASHTO 7 247) 

• AASHTO T P46-94 

• NCHRP 1-28 Appendix E 

• NCHRP 1-28A (“harmonized” protocol) 

The harmonized protocol developed in NCHRP Project 1-28A attempts to 

combine the best features from all of the earlier test methods with a new 

loading sequence that minimizes the potential for premature failure of the test 

specimen. All of the test procedures employ a closed loop electro-hydraulic 

testing machine to apply repeated cycles of a haversine shaped load-pulse. 

Load pulses are typically a 0.1 second loading time followed by a 0.9 second 

rest time for base/subbase materials, and a 0.2 second loading time followed by 

an 0.8 second rest time for subgrade materials. A triaxial set-up for the 

resilient modulus test is shown in Figure 5-16. Axial deformation is best 

measured on the sample using clamps positioned one quarter and three quarters 

from the base of the test specimen. For very soft specimens, the displacement 

may be measured between the top and bottom plates. 

Different specimen sizes, compaction procedures, and loading conditions are 

usually recommended for granular base/subbase materials, coarse-grained 

subgrades, and fine-grained subgrades. These different procedures reflect the 

different particle sizes of the materials, the state of stress specific to each layer 

in the pavement structure, and the mechanical behavior of the material type. 

Detailed comparisons of the various resilient modulus test protocols is 

presented in Witczak (2004). 

Field 

Measurement 

In-situ resilient modulus values can be estimated from backcalculation of 

falling weight deflectometer (FWD) test results (Section 4.5.4) or correlations 

with Dynamic Cone Penetrometer (DCP) values (Section 4.5.5; see also Table 

5-34). 

Commentary No definitive studies have been conducted to date to provide guidance on 

differences between measured MR from the various laboratory test protocols. 

Field MR values determined from FWD backcalculation are often significantly 

higher than design MR values measured from laboratory tests because of 

differences in stress states. The 1993 AASHTO Guide recommends for 
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Geotechnical Aspects of Pavements 5 - 49 May 2006 

subgrade soils that field MR values be multiplied by a factor of up to 0.33 for 

flexible pavements and up to 0.25 for rigid pavements to adjust to design MR

values. NCHRP 1-37A recommends adjustment factors of 0.40 for subgrade 

soils and 0.67 for granular bases and subbases under flexible pavements. More 

detailed guidance for adjusting backcalculated modulus values to design MR

values is given in Table 5-32. 

The 1993 AASHTO Guide includes procedures for incorporating seasonal 

variations into an effective MR for the subgrade. Seasonal variations of material 

properties are included directly in the NCHRP 1-37A M-E design 

methodology. 

The Level 1, 2, and 3 MR inputs in the NCHRP 1-37A design methodology are 

functions of pavement and construction type, as summarized in Table 5-33. 

Typical Values Correlations between MR and other soil properties include the following: 

AASHTO 1993 Guide 

• Granular base and subbase layers:

θ (psi) MR (psi) 

100 100 740 × CBR 

1000 + 780 × R 

30 440 × CBR 

1000 + 450 × R 

20 340 × CBR 

1000 + 350 × R 

10 250 × CBR 

1000 + 250 × R 

• Subgrade (roadbed) soils

 (psi) 1500 MR = ×CBR for CBR < 10 (5.13)

(Heukelom and Klomp, 1962) 

 (psi) R-value ( ) M AB R =+× (5.14)

with A= 772 to 1,155; B = 369 to 555 (Asphalt Institute, 1982) 

 (psi) 1000 555 R-value ( ) MR = +× (recommended values) (5.15)

Additional useful correlations for subgrade MR are provided in Figure 5-17. 

NCHRP 1-37A (Level 2 Inputs) 

See Table 5-34 for correlations between MR and various material strength and 

index properties. The correlations in Table 5-34 are in rough order of 

preference; correlations of MR with CBR have the longest history and most 

supporting data and thus are most preferable. 

NCHRP 1-37A (Level 3 Inputs)

See Table 5-35 for typical ranges and default values as functions of AASHTO 

and USCS soil class. Note that these values are for soils compacted at optimum 

moisture and density conditions; the NCHRP 1-37A analysis software adjusts 

these for in-situ moisture and density conditions. 
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Geotechnical Aspects of Pavements 5 - 50 May 2006 

Figure 5-16. Triaxial cell set-up for resilient modulus test. 
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Geotechnical Aspects of Pavements 5 - 51 May 2006 

Table 5-32. Average backcalculated to laboratory-determined elastic modulus ratios 

(Von Quintus and Killingsworth, 1997a; 1997b; 1998). 

Layer Type and Location Mean ER/MR Ratio 

Granular base/subbase between 

two stabilized layers 

(cementitious or asphalt 

stabilized materials). 

1.43 

Granular base/subbase under a 

PCC layer. 1.32 

Unbound Granular Base and 

Subbase Layers 

Granular base/subbase under an 

HMA surface or base layer. 0.62 

Embankment or subgrade soil 

below a stabilized subbase layer 

or stabilized soil. 

0.75 

Embankment or subgrade soil 

below a flexible or rigid 

pavement without a granular 

base/subbase layer. 

0.52 Embankment and Subgrade Soils 

Embankment or subgrade soil 

below a flexible or rigid 

pavement with a granular base or 

subbase layer. 

0.35 

ER = Elastic modulus backcalculated from deflection basin measurements. 

MR = Elastic modulus of the in-place materials determined from laboratory repeated load resilient 

 modulus test.

Table 5-33. Hierarchical input levels for unbound material stiffness in the 

NCHRP 1-37A Design Guide (NCHRP 1-37A, 2004). 

Project Type Level 1 Level 2 Level 3 

Flexible Pavements 

New/reconstruction Laboratory-measured 

MR with stress 

dependence—Eq. 

(5.10) 

MR correlations with 

other properties 

(Table 5-34) 

Default MR based on 

soil type (Table 5-35) 

Rehabilitation Backcalculated MR

from FWD deflections

MR correlations with 

other properties 

(Table 5-34) 

Default MR based on 

soil type (Table 5-35) 

Rigid Pavements 

New/reconstruction Not available MR correlations with 

other properties 

(Table 5-34) 

Default MR based on 

soil type (Table 5-35) 

Rehabilitation Backcalculated 

modulus of subgrade 

reaction (k) from 

FWD deflections (see 

Section 5.4.6)

MR correlations with 

other properties 

(Table 5-34) 

Default MR based on 

soil type (Table 5-35) 
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Geotechnical Aspects of Pavements 5 - 52 May 2006 

Figure 5-17. Correlations between subgrade resilient modulus and other soil properties 

(1 psi = 6.9 kPa; from Huang, 1993, after Van Til et al., 1972). 
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Geotechnical Aspects of Pavements 5 - 53 May 2006 

Table 5-34. Correlations between resilient modulus and various material strength 

and index properties (NCHRP 1-37A, 2004). 

Strength/Index 

Property 

Modela Comments Test Standard 

California 

Bearing Ratiob 

MR (psi) = 2555(CBR)

0.64

MR (MPa) = 17.6(CBR)

0.64

CBR = California 

Bearing Ratio (%) 

AASHTO T193—The 

California Bearing Ratio 

Stabilometer 

R-value 

MR (psi) = 1155 + 555R

MR (MPa) = 8.0 + 3.8R R = R-value 

AASHTO T190—

Resistance R-Value and 

Expansion Pressure of 

Compacted Soils 

AASHTO layer 

coefficient 

MR (psi) = 30,000 (ai/0.14)3 

MR (MPa) = 207 (ai/0.14)3

 

ai = AASHTO layer 

coefficient 

AASHTO Guide for the 

Design of Pavement 

Structures (1993) 

Plasticity index 

and gradation 

75

1 0.728( )

CBR

wPI = +

wPI = P200*PI

P200 = % passing 

No. 200 sieve size 

PI = plasticity index 

(%) 

AASHTO T27—Sieve 

Analysis of Coarse and Fine 

Aggregates 

AASHTO T90—

Determining the Plastic 

Limit and Plasticity Index of 

Soils 

Dynamic Cone 

Penetrationc CBR= 292/(DCP1.12) 

CBR = California 

Bearing Ratio (%) 

DCP =Penetration 

index, in./blow 

ASTM D6951—Standard 

Test Method for Use of the 

Dynamic Cone 

Penetrometer in Shallow 

Pavement Applications a

Correlations should be applied to similar conditions – i.e., CBR measured at optimum moisture and density vs. 

soaked conditions correlates to MR at corresponding moisture and density conditions. 

b

NCHRP 1-37A strongly recommends against use of the older Heukelom and Klomp (1962) correlation Eq. 

(5.13) between MR and CBR specified in the 1993 AASHTO Design Guide. c

Estimates of CBR are used to estimate MR. 
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Geotechnical Aspects of Pavements 5 - 54 May 2006 

Table 5-35. Default MR values for unbound granular and subgrade materials at 

unsoaked optimum moisture content and density conditions (NCHRP 1-37A, 2004). 

Material Classification MR Range (psi)* Typical MR (psi)* 

AASHTO Soil Class 

A-1-a 38,500 – 42,000 40,000 

A-1-b 35,500 – 40,000 38,000 

A-2-4 28,000 – 37,500 32,000 

A-2-5 24,000 – 33,000 28,000 

A-2-6 21,500 – 31,000 26,000 

A-2-7 21,500 – 28,000 24,000 

A-3 24,500 – 35,500 29,000 

A-4 21,500 – 29,000 24,000 

A-5 17,000 – 25,500 20,000 

A-6 13,500 – 24,000 17,000 

A-7-5 8,000 – 17,500 12,000 

A-7-6 5,000 – 13,500 8,000 

USCS Soil Class 

GW 39,500 – 42,000 41,000 

GP 35,500 – 40,000 38,000 

GM 33,000 – 42,000 38,500 

GC 24,000 – 37,500 31,000 

GW-GM 35,500 – 40,500 38,500 

GP-GM 31,000 – 40,000 36,000 

GW-GC 28,000 – 40,000 34,500 

GP-GC 28,000 – 39,000 34,000 

SW 28,000 – 37,500 32,000 

SP 24,000 – 33,000 28,000 

SM 28,000 – 37,500 32,000 

SC 21,500 – 28,000 24,000 

SW-SM 24,000 – 33,000 28,000 

SP-SM 24,000 – 33,000 28,000 

SW-SC 21,500 – 31,000 25,500 

SP-SC 21,500 – 31,000 25,500 

ML 17,000 – 25,500 20,000 

CL 13,500 – 24,000 17,000 

MH 8,000 – 17,500 11,500 

CH 5,000 – 13,500 8,000 

 *Multiply by 0.069 to convert to MPa. 
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Geotechnical Aspects of Pavements 5 - 55 May 2006 

Fractured PCC Slabs 

Rehabilitation designs for AC overlays over badly damaged PCC existing pavement 

frequently require fracturing (crack and seat, etc.) or rubblizing of the existing concrete slabs. 

The net effect of the fracturing or rubblization process is to turn the slabs into a very coarse 

unbound granular material. Table 5-36 summarizes recommended design values for the 

modulus of the fractured slab, Efs, for Level 1 characterization in the NCHRP 1-37A Design 

Guide. These recommended design values, which are functions of the anticipated variability 

of the slab fracturing process, were developed based on NDT data on fractured slab projects 

contained in NAPA IS-117 (NAPA, 1994). When using these design values, NDT of the 

fractured slab must be performed to ensure that not more than 5 percent of the in-situ 

fractured slab modulus values exceed 1000 ksi. The Level 1 design values may be used for 

all methods of fracture (crack and seat or rubblize for JPCP, break and seat or rubblize for 

JRCP, or rubblize for CRCP). 

Table 5-37 summarizes recommended design values for the modulus of the fractured slab, 

Efs, for Level 3 characterization in the NCHRP 1-37A Design Guide. These values, which are 

functions of the fracture method used and the nominal fragment size, were developed by 

applying conservatism to the relationship of Efs versus nominal fragment size published in 

the 1986 AASHTO Design Guide and NAPA IS-117. Level 3 should not be used with JRCP 

unless it is certain that full debonding of the steel and concrete occurs. 

Table 5-36. Recommended fractured slab design modulus values for 

Level 1 characterization (NCHRP 1-37A, 2004). 

Expected Control on 

Slab Fracture Process 

Anticipated Coefficient of Variation 

for the Fractured Slab Modulus, % Design Modulus 

Good to Excellent 25 600 ksi (4.1 GPa) 

Fair to Good 40 450 ksi (3.1 GPa) 

Poor to Fair 60 300 (2.1 GPa) 
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Geotechnical Aspects of Pavements 5 - 56 May 2006 

Table 5-37. Recommended fractured slab design modulus values for 

Level 3 characterization (NCHRP 1-37A, 2004). 

Type of Fracture Design Modulus

Rubbilization 150 ksi (1.0 GPa) 

Crack and Seat 

 12-in crack spacing 200 ksi (1.4 GPa) 

 24-in crack spacing 250 ksi (1.7 GPa) 

 36-in crack spacing 300 ksi (2.1 GPa) 

Note: For JRCP, Level 1 should be used unless agency experience dictates otherwise. 

Bedrock 

Shallow bedrock under an alignment can have a significant impact on the pavement’s 

mechanical responses and thus needs to be considered in mechanistic-empirical design. 

Shallow bedrock is also an important factor in the backcalculation of layer moduli for 

rehabilitation design. While a precise value of bedrock stiffness is seldom required, the effect 

of high bedrock stiffness must nonetheless be incorporated into the analysis. Recommended 

values from NCHRP 1-37A for the elastic modulus of bedrock are as follows: 

• Solid, massive bedrock: 

E = 750 – 2,000 ksi (5.2 – 13.8 GPa) 

Default = 1,000 ksi (6.9 GPa) 

• Highly fractured/weathered bedrock: 

E = 250 – 1,000 ksi (1.7 – 6.9 GPa) 

Default = 500 ksi (3.4 GPa) 
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Geotechnical Aspects of Pavements 5 - 57 May 2006 

5.4.4 Poisson’s Ratio 

Description Poisson’s ratio ν is defined as the ratio of the lateral strain εx to the axial strain 

εy due to an axial loading (Figure 5-18). 

Uses in 

Pavements 

• Direct input to pavement response models in M-E design procedure. 

• Estimation of in-situ lateral stresses (see Section 5.4.9). 

Laboratory 

Determination 

Determined as part of resilient modulus test (see Section 5.4.3.). 

Field 

Measurement 

Not applicable. 

Commentary The influence of ν on computed pavement response is normally quite small. 

Consequently, use of assumed values for ν often gives satisfactory results, and 

direct measurement in the laboratory is usually unnecessary. 

Typical Values Poisson’s ratio for isotropic elastic materials must be between 0 and 0.5. 

Typical values of ν for pavement geomaterials are given in Table 5-30. 

ε

x

y ε

Figure 5-18. Illustration of Poisson’s ratio. 
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Geotechnical Aspects of Pavements 5 - 58 May 2006 

Table 5-38. Typical Poisson’s ratio values for geomaterials in pavements 

(NCHRP 1-37A, 2004). 

Material Description ν Range ν Typical 

Clay (saturated) 0.4 – 0.5 0.45 

Clay (unsaturated) 0.1 – 0.3 0.2 

Sandy clay 0.2 – 0.3 0.25 

Silt 0.3 – 0.35 0.325 

Dense sand 0.2 – 0.4 0.3 

Coarse-grained sand 0.15 0.15 

Fine-grained sand 0.25 0.25 

Bedrock 0.1 – 0.4 0.25 

5.4.5 Structural Layer Coefficients 

The material quality of granular base and subbase layers is characterized in the AASHTO 

flexible pavement design procedures in terms of structural layer coefficients ai (see Section 

3.5.2). These coefficients were entirely empirical through the 1972 version of the Guide. 

Beginning with the 1986 Guide, the recommended procedure for estimating structural layer 

coefficients is through correlations with resilient modulus. 

It must be emphasized that structural layer coefficients are not fundamental engineering 

properties for a material. There are no laboratory or field procedures for measuring structural 

layer coefficients directly. The structural layer coefficients were originally defined as simple 

substitution ratios – i.e., how much additional thickness of granular base at a given reference 

stiffness must be added if a unit thickness of asphalt concrete of a given stiffness is removed 

in order to maintain the same surface deflection under a standardized load? These 

substitution ratios were evaluated in the 1986 AASHTO Guide1

 via a parametric analytical 

study for a limited range of flexible pavement geometries and layer stiffnesses. In this 

approach, the value of the structural layer coefficient for a given material also depends not 

only on its inherent stiffness, but also upon the material's location within the pavement 

structure (e.g., the a2 value for a given material when used in a base layer is different from 

the a3 value for that same material when used as a subbase). Subsequent correlations between 

structural layer coefficients and other engineering properties such as resilient modulus and 

CBR are entirely empirical. Structural layer coefficients are not used in mechanisticempirical design procedures like the NCHRP 1-37A Design Guide. 

 

1

 See Appendix GG in Volume 2 of AASHTO (1986). 
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Geotechnical Aspects of Pavements 5 - 59 May 2006 

New Construction/Reconstruction 

The relationship in the 1993 AASHTO Guide between the structural layer coefficient a2 and 

resilient modulus EBS (in psi) for granular base materials is given as 

2 10 0.249log 0.977 BS a E = − (5.16) 

The value for EBS in Eq. (5.16) will be a function of the stress state within the layer. The 

relationship suggested in the 1993 AASHTO Guide is 

2

1

k E k S = θ (5.17) 

in which 

θ = sum of principal stresses = σ1+σ2+σ3 (psi) 

k1,k2 = material properties 

Typical values for the material properties are (see also Table 5-39) 

k1 = 3000 to 8000 psi 

k2 = 0.5 to 0.7 

The values of EBS from the base layers in the original AASHO Road Test are summarized in 

Table 5-40. Note that the EBS values are not only functions of moisture, but also of stress 

state θ, which in turn is a function of the pavement structure – i.e., subgrade modulus and 

thickness of the surface layer. Typical values of θ recommended in the 1993 AASHTO 

Guide for use in base design are summarized in Table 5-41. 

Figure 5-19 summarizes correlations between the a2 structural layer coefficient for 

nonstabilized granular base layers and corresponding values of CBR, R-Value, Texas triaxial 

strength, and resilient modulus. Similar correlations between a2 and various strength and 

stiffness measures for cement- and bituminous-treated granular bases are given in Figure 5-

20 and Figure 5-21. 
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Table 5-39. Typical values for k1 and k2 for use in Eq. (5.17) for unbound base 

and subbase materials (AASHTO, 1993). 

Moisture Condition k1

*

 (psi)** k2

* 

(a) Base 

 Dry 6,000 – 10,000 0.5 – 0.7 

 Damp 4,000 – 6,000 0.5 – 0.7 

 Wet 2,000 – 4,000 0.5 – 0.7 

(b) Subbase 

 Dry 6,000 – 8,000 0.4 – 0.6 

 Damp 4,000 – 6,000 0.4 – 0.6 

 Wet 1,500 – 4,000 0.4 – 0.6 

*

Range in k1 and k2 is a function of the material quality **1 psi = 6.9 kPa 

Table 5-40. Granular base resilient modulus ESB values (psi) from AASHO Road Test 

(AASHTO, 1993). 

Stress State (psi* Moisture ) 

State Equation θ = 5 θ = 10 θ = 20 θ = 30 

Dry 8,000θ0.6 21,012 31,848 48,273 61,569 

Damp 4,000θ0.6 10,506 15,924 24,136 30,784 

Wet 3,200θ0.6 8,404 12,739 19,309 24,627 

*

1 psi = 6.9 kPa 

Table 5-41. Suggested bulk stress θ (psi) values for use in design of granular base layers 

(AASHTO, 1993). 

Roadbed Soil Resilient Modulus (psi* Asphalt Concrete ) 

Thickness (inches*

) 3,000 7,500 15,000 

< 2 20 25 30 

2 – 4 10 15 20 

4 – 6 5 10 15 

> 6 5 5 5 

*

1 inch = 25.4 mm; 1 psi = 6.9 kPa 
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Geotechnical Aspects of Pavements 5 - 61 May 2006 

Figure 5-19. Correlations between structural layer coefficient a2 and various strength and 

stiffness parameters for unbound granular bases (AASHTO, 1993). 
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Geotechnical Aspects of Pavements 5 - 62 May 2006 

Figure 5-20. Correlations between structural layer coefficient a2 and various strength and 

stiffness parameters for cement-treated granular bases (AASHTO, 1993). 
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Geotechnical Aspects of Pavements 5 - 63 May 2006 

Figure 5-21. Correlations between structural layer coefficient a2 and various strength and 

stiffness parameters for bituminous-treated granular bases (AASHTO, 1993). 
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Geotechnical Aspects of Pavements 5 - 64 May 2006 

The relationship in the 1993 AASHTO Guide between the structural layer coefficient a3 and 

resilient modulus ESB (in psi) for granular subbase materials is given as 

3 10 0.227log 0.839 SB a E = − (5.18) 

 

The resilient modulus ESB for granular subbase layers is influenced by stress state in a 

manner similar to that for the base layer, as given in Eq. (5.17). Typical values for the k1 and 

k2 material properties for granular subbases are 

k1 = 1500 to 6000 

k2 = 0.4 to 0.6 

The values of ESB from subbase layers in the original AASHO Road Test are summarized in 

Table 5-42. Note that the ESB values are not only functions of moisture, but also of stress 

state θ, which in turn is a function of the pavement structure – i.e., thickness of the asphalt 

concrete surface layer. Typical values of θ recommended in the 1993 AASHTO Guide for 

use in subbase design are summarized in Table 5-43. Figure 5-22 summarizes relationships 

between the a3 structural layer coefficient for granular subbase layers and corresponding 

values of CBR, R-Value, Texas Triaxial strength, and resilient modulus. 

Table 5-42. Granular subbase resilient modulus ESB values (psi) from 

AASHO Road Test (AASHTO, 1993). 

Stress State (psi*

) Moisture State Equation θ = 5 θ = 7.5 θ = 10 

Damp 5,400θ0.6 14,183 18,090 21,497 

Wet 4,600θ0.6 12,082 15,410 18,312 

*

1 psi = 6.9 kPa 

Table 5-43. Suggested bulk stress θ (psi) values for use in design of 

granular subbase layers (AASHTO, 1993). 

Asphalt Concrete 

Thickness 

(inches*

) 

Stress State 

(psi*

) 

< 2 10.0 

2 – 4 7.5 

> 4 4.0 

*

1 inch = 25.4 mm; 1 psi = 6.9 kPa 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 65 May 2006 

Figure 5-22. Correlations between structural layer coefficient a3 and various strength and 

stiffness parameters for unbound granular subbases (AASHTO, 1993). 

Rehabilitation 

Depending on the types and amounts of deterioration present, the layer coefficient values 

assigned to materials in in-service existing pavements should in most cases be less than the 

values that would be assigned to the same materials for new construction. Exceptions to this 

general rule would include unbound granular materials that show no sign of degradation or 

contamination. 
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Geotechnical Aspects of Pavements 5 - 66 May 2006 

Limited guidance is available for the selection of layer coefficients for in-service pavement 

materials. Recommendations from the 1993 AASHTO Pavement Design Guide are provided 

in Table 5-44. In addition to evidence of pumping noted during a visual condition survey, 

samples of base and subbase materials should be obtained and examined for evidence of 

erosion, degradation, and contamination by fines, as well as evaluated for drainability, and 

layer coefficients should be reduced accordingly. Coring and testing are recommended for 

evaluation of all materials and are strongly recommended for evaluation of stabilized layers. 

Table 5-44. Suggested layer coefficients for existing flexible pavement layer materials 

(AASHTO, 1993). 

Material Surface Condition Coefficient 

Little or no alligator cracking and/or only low-severity transverse 

cracking 0.35 – 0.40 

<10% low-severity alligator cracking and/or 

<5% medium- and high-severity transverse cracking 0.25 – 0.35 

>10% low-severity alligator cracking and/or 

<10 percent medium-severity alligator cracking and/or 

>5-10% medium- and high-severity transverse cracking 

0.20 – 0.30 

>10% medium-severity alligator cracking and/or 

<10% high-severity alligator cracking and/or 

>10% medium- and high-severity transverse cracking 

0.14 – 0.20 

AC Surface 

>10% high-severity alligator cracking and/or 

>10% high-severity transverse cracking 0.08 – 0.15 

Little or no alligator cracking and/or only low-severity transverse 

cracking 0.20 – 0.35 

<10% low-severity alligator cracking and/or 

<5% medium- and high-severity transverse cracking 0.15 – 0.25 

>10% low-severity alligator cracking and/or 

<10% medium-severity alligator cracking and/or 

>5-10% medium- and high-severity transverse cracking 

0.15 – 0.20 

>10% medium-severity alligator cracking and/or 

<10% high-severity alligator cracking and/or 

>10% medium- and high-severity transverse cracking 

0.10 – 0.20 

Stabilized Base 

>10% high-severity alligator cracking and/or 

>10% high-severity transverse cracking 0.08 – 0.15 

Granular No evidence of pumping, degradation, or contamination by fines 0.10 – 0.14 

Base/Subbase Some evidence of pumping, degradation, or contamination by fines 0.00 – 0.10 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 67 May 2006 

5.4.6 Modulus of Subgrade Reaction 

Mechanistic solutions for the stresses and strains in rigid pavements have historically 

characterized the stiffness of the foundation soil in terms of the modulus of subgrade reaction 

k (Figure 5-23). However, the modulus of subgrade reaction is not a true engineering 

property for the foundation soil because it depends not only upon the soil stiffness, but also 

upon the slab (or footing) size and stiffness. For an example of a square footing on a 

homogeneous isotropic elastic foundation soil, k can be expressed as 

 ( )

4

12 2

0.65

1 f

E EB k

B ν E I = − (5.19) 

in which 

B = width of footing 

E = elastic modulus of soil 

ν = Poisson's ratio of soil 

Ef = elastic modulus of footing 

I = moment of inertia of footing = 

3

12

Bt , t = footing thickness 

For a given slab/footing size and stiffness, k is directly proportional to the effective elastic 

modulus of the foundation soil in Eq. (5.19). 

The effective modulus of subgrade reaction is a direct input in the AASHTO design 

procedures for rigid pavements (see Section 3.5.2). The modulus of subgrade reaction was 

first introduced in the 1972 version of the Guide, with the recommendation that its value be 

determined from plate loading tests. Beginning with the 1986 Guide, the recommended 

procedure for estimating k for new/reconstruction designs is through correlations with 

subgrade MR plus various adjustments for base layer stiffness and thickness, presence of 

shallow rock, potential loss of slab support due to erosion, and seasonal variations.2

 The 

recommended procedure for determining k for rehabilitation designs is backcalculation from 

FWD test results. 

 

2

 The 1998 supplement to the 1993 AASHTO Guide provides an alternate approach for determining the 

effective modulus of subgrade reaction for rigid pavements. 
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Geotechnical Aspects of Pavements 5 - 68 May 2006 

Figure 5-23. Coefficient of subgrade reaction k (Yoder and Witczak, 1975). 

The subgrade, base, and subbase resilient moduli values are the direct inputs in the NCHRP 

1-37A design methodology. These values are adjusted internally within the NCHRP 1-37A 

Design Guide software for environmental effects and then converted into an average monthly 

effective k-value for structural response calculation and damage analysis. 

The detailed procedures used in the 1993 AASHTO and NCHRP 1-37A Design Guides to 

determine k for new/reconstruction and rehabilitation designs are described in the following 

subsections. 
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Geotechnical Aspects of Pavements 5 - 69 May 2006 

1993 AASHTO Guide 

New Construction/Reconstruction

The steps recommended in the 1993 AASHTO Design Guide for determining the effective 

modulus of subgrade reaction for new/reconstruction designs are as follows: 

1. Identify the subgrade and subbase type(s), thicknesses, and other properties. 

2. Determine the subgrade resilient modulus MR values for each season. Appropriate 

techniques for this are similar to those described earlier in Section 5.4.3. 

3. Determine the subbase3

 resilient modulus ESB for each season (similar to step 2). The 

1993 AASHTO Guide recommends the following limits for the subbase resilient 

modulus: 

15,000 (spring thaw) < ESB (psi) < 50,000 (winter freeze) (5.20) 

ESB < 4MR (psi) (5.21) 

4. Using Figure 5-24, determine a composite k value for each season that represents the 

combined stiffness of the subgrade and subbase. Figure 5-24 is based on the following 

model (see Appendix LL in Volume 2 of AASHTO, 1986): 

( ) ( )( )

( ) ( ) ( )( )

2 ln 2.807 0.1253(ln ) 1.062 ln 0.1282 ln ln

0.4114 ln 0.0581 ln 0.1317 ln ln

SB R SB SB

SB SB SB R

k D M DE

 D E D M

∞ =− + + +

− −− (5.22) 

 in which 

 

k∞ = composite modulus of subgrade reaction (pci) assuming a semi-infinite 

 roadbed soil 

DSB = subbase thickness (inches) 

ESB = subbase elastic modulus (psi) 

MR = subgrade resilient modulus (psi) 

5. Using Figure 5-25, correct the composite k values from step 4 for any effects of shallow 

bedrock. Figure 5-25 is based on the following model (see Appendix LL in Volume 2 of 

AASHTO, 1986): 

 

3

 In the 1993 AASHTO Guide terminology, the subbase is defined as the granular layer between the PCC slab 

and the roadbed (subgrade) soil. This layer is termed the base layer in the 1998 supplement to the 1993 Guide. 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 70 May 2006 

( )( ) ( )

() ()

ln 5.303 0.0710 ln ln 1.366 ln

0.9187 ln 0.6837 ln

rf SB R

SG R

k DM k

 D M

=+ + ∞

− − (5.23) 

in which 

krf = composite modulus of subgrade reaction (pci) considering the effect of a 

 rigid foundation near the surface 

DSG = depth to rigid foundation (inches) 

 and the other terms are as defined previously in Eq. (5.22). 

6. Determine the seasonal average composite k value using the following procedure: 

• Estimate the design thickness of the slab and use Figure 5-26 to determine the relative 

damage uri for each season. Figure 5-26 is based on the following simplified damage 

model (see Appendix HH in Volume 2 of AASHTO, 1986): 

 3.42 0.75 0.25 0.39 ri i uD k = −     (5.24) 

in which D is the slab thickness (inches) and ki is the modulus of subgrade reaction 

for each season. 

• Compute the average relative damage r u as the sum of the relative damage values for 

each season divided by the number of seasons. 

• Determine the seasonally averaged composite k from Figure 5-26 using r u and the 

estimated slab thickness. This seasonally averaged composite k is termed the effective 

modulus of subgrade reaction keff. 

7. Using Figure 5-27 and Table 5-45, correct the effective modulus of subgrade reaction keff

for loss of support due to subbase erosion. This corrected keff is the value to be used for 

design. Table 5-46 summarizes the recommended design values for the modulus of 

subgrade reaction from the low-volume road section of the 1993 Design Guide. 
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Geotechnical Aspects of Pavements 5 - 71 May 2006 

Figure 5-24. Chart for estimating composite modulus of subgrade reaction k∞, assuming a 

semi-infinite subgrade depth (AASHTO, 1993). 
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Geotechnical Aspects of Pavements 5 - 72 May 2006 

Figure 5-25. Chart to modify modulus of subgrade reaction to consider effects of rigid 

foundation near surface (within 10 ft) (AASHTO, 1993). 
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Geotechnical Aspects of Pavements 5 - 73 May 2006 

Figure 5-26. Chart for estimating relative damage to rigid pavements based on slab thickness 

and underlying support (AASHTO, 1993). 
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Geotechnical Aspects of Pavements 5 - 74 May 2006 

Figure 5-27. Correction of effective modulus of subgrade reaction for potential loss of 

subbase support (AASHTO, 1993). 
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Geotechnical Aspects of Pavements 5 - 75 May 2006 

Table 5-45. Typical ranges of loss of support LS factors for various types of materials 

(AASHTO, 1993). 

Type of Material Loss of Support 

(LS) 

Cement treated granular base 

(E = 1,000,000 to 2,000,000 psi) 0.0 to 1.0 

Cement aggregate mixtures 

(E = 500,000 to 1,000,000 psi) 0.0 to 1.0 

Asphalt treated base 

(E = 350,000 to 1,000,000 psi) 0.0 to 1.0 

Bituminous stabilized mixtures 

(E = 40,000 to 300,000 psi) 0.0 to 1.0 

Lime stabilized 

(E = 20,000 to 70,000) 1.0 to 3.0 

Unbound granular materials 

(E = 15,000 to 45,000 psi) 1.0 to 3.0 

Fine grained or natural subgrade materials 

(E = 3,000 to 40,000 psi) 2.0 to 3.0 

Table 5-46. Suggested ranges for modulus of subgrade reaction for design 

(AASHTO, 1993). 

Roadbed Soil Quality Range for keff (pci) 

Very Good > 550 

Good 400 – 500 

Fair 250 – 350 

Poor 150 – 250 

Very Poor < 150 

Rehabilitation

For rehabilitation projects, the modulus of subgrade reaction k can be determined from FWD 

deflection testing of the existing PCC pavement. An FWD with a load plate radius of 5.9 

inches and a load magnitude of 9000 pounds is recommended, with deflections measured at 

sensors located at 0, 12, 24, and 36 inches from the center of the load along the outer wheel 

path. For each slab tested, a dynamic kdynamic value (pci) can be determined from Figure 5-28 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 76 May 2006 

based on the deflection at the center of the loading plate, d0 (inches) and the AREA of the 

deflection basin as computed by4

12 24 36

0 00

61 2 2 d dd AREA

d dd

     

=+ + +            

 (5.25) 

in which the di values are the deflections at i inches from the plate center. The static keff value 

for design is then determined as: 

2

dynamic

eff

k

k = (5.26) 

As is the case for new/reconstruction, this keff value may need to be adjusted for seasonal 

effects. 

Figure 5-28. Effective dynamic k value determination from d0 and AREA (AASHTO, 1993). 

 

4

 For loads within 2000 pounds more or less of the 9000 pound desired value, deflections may be linearly scaled 

to equivalent 9000-pound deflections. 
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Geotechnical Aspects of Pavements 5 - 77 May 2006 

NCHRP 1-37A Design Guide 

New Construction/Reconstruction

All subgrade and unbound pavement layers for all pavement types are characterized using MR

in the NCHRP 1-37A design methodology. The pavement response model for rigid pavement 

design, however, is based on a Winkler-spring foundation model that requires a value for the 

modulus of subgrade reaction kdynamic (see Appendix D for more details on the rigid pavement 

response model). The modulus of subgrade reaction is obtained from the subgrade and 

subbase MR values and the subbase thickness through a conversion process that transforms 

the actual multilayer pavement structure into an equivalent three-layer structure consisting of 

the PCC slab, base, and an effective dynamic k, as shown in Figure 5-29. This conversion is 

performed internally in the NCHRP 1-37A Design Guide software as a part of input 

processing. 

The procedure to obtain the effective value of kdynamic for each time increment in the analysis 

can be summarized in the following steps: 

1. Assign initial estimates of the stiffness parameters MR and ν to each unbound layer in 

the pavement structure. 

2. Using multilayer elastic theory, simulate an FWD load and compute the stresses in 

the subgrade and subbase. 

3. Adjust the subgrade and subbase MR values to account for the stress states determined 

in Step 2. 

4. Using multilayer elastic theory, again simulate an FWD load using the updated 

subgrade and subbase MR values from Step 3. Calculate the PCC surface deflections 

at specified radii from the center of the load plate. 

5. Using the rigid pavement response model, determine the kdynamic value that gives the 

best fit to the PCC surface deflections from Step 4. 

The kdynamic value represents the compressibility of all layers beneath the base layer. 

It is a computed quantity and not a direct input to the NCHRP 1-37A design procedure for 

new/reconstruction. Note also that kdynamic is a dynamic value, which should be distinguished 

from the traditional static k values used in previous design procedures. 

The kdynamic value is calculated for each month of the year. It is used directly in the 

computation of the critical stresses, strains, and deflections for the incremental damage 

accumulation algorithms in the NCHRP 1-37A performance forecasting procedure. 

Environmental factors like water table depth, depth to bedrock, and freeze/thaw that can 

significantly affect the value for kdynamic are all considered in the NCHRP 1-37A calculations 
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Geotechnical Aspects of Pavements 5 - 78 May 2006 

via the Enhanced Integrated Climate Model (EICM). Additional details of these algorithms 

are provided in Appendix D. 

Rehabilitation

The modulus of subgrade reaction is a direct input for rigid pavement rehabilitation designs 

in the NCHRP 1-37A procedure. Measured surface deflections from FWD testing are used to 

backcalculate a kdynamic for design. The mean backcalculated kdynamic for a given month is 

input to the NCHRP 1-37A Design Guide software, and the kdynamic values for the remaining 

months of the year are seasonal adjustment factors computed by the EICM. 

5.4.7 Interface Friction 

1993 AASHTO Guide

The reinforcement design of jointed reinforced concrete pavements (JCRP) is dependent 

upon the frictional resistance between the bottom of the slab and the top of the underlying 

subbase or subgrade. This frictional resistance is characterized in the 1993 AASHTO Guide 

by a friction factor F that is related (but not equal) to the coefficient of friction between the 

slab and the underlying material. Recommended values for natural subgrade and a variety of 

subbase materials are presented in Table 5-47. The friction factor is required only for JCRP 

design. 

Concrete Slab

(JPCP, CRCP)

Bedrock

Base Course

(Unbound, Asphalt, Cement)

Subbase Course

(Unbound, Stabilized)

Compacted Subgrade

Natural Subgrade

Ebase

Effective k-value 

obtained through 

backcalculation

E

c

Concrete Slab

(JPCP, CRCP)

Base Course

(Unbound, Asphalt, Cement)

Concrete Slab

(JPCP, CRCP)

Bedrock

Base Course

(Unbound, Asphalt, Cement)

Subbase Course

(Unbound, Stabilized)

Compacted Subgrade

Natural Subgrade

Concrete Slab

(JPCP, CRCP)

Bedrock

Base Course

(Unbound, Asphalt, Cement)

Subbase Course

(Unbound, Stabilized)

Compacted Subgrade

Natural Subgrade

Ebase

Effective k-value 

obtained through 

backcalculation

E

c

Ebase

Effective k-value 

obtained through 

backcalculation

E

c

E Concrete Slab

(JPCP, CRCP)

Base Course

(Unbound, Asphalt, Cement)

Figure 5-29. Structural model for rigid pavement structural response computations. 
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Table 5-47. Recommended friction factor values (AASHTO, 1993). 

Type of Material 

Beneath Slab 

Friction Factor 

F

Surface treatment 2.2 

Lime stabilization 1.8 

Asphalt stabilization 1.8 

Cement stabilization 1.8 

River gravel 1.5 

Crushed stone 1.5 

Sandstone 1.2 

Natural subgrade 0.9 

NCHRP 1-37A Procedure

The NCHRP 1-37A procedure for flexible pavements permits specification of the degree of 

bonding between each layer and the layer immediately beneath. The degree of bonding is 

characterized by an interface coefficient that varies between the limits of 1 for fully bonded 

conditions and 0 for a full slip interface. No guidance is provided at present in the NCHRP 

1-37A procedure for specifying intermediate values for the interface coefficient to represent 

partial bond conditions between layers in flexible pavements. 

The NCHRP 1-37A procedure for jointed plain concrete pavements (JPCP) requires 

specification of fully bonded for fully unbonded interface conditions between the bottom of 

the slab and the underlying layer. No provision is provided for intermediate bond conditions. 

The friction conditions at the bottom of continuously reinforced concrete pavements (CRCP) 

are specified in terms of a base/slab friction coefficient. Guidelines for specifying this 

coefficient are provided in Table 5-48. Jointed reinforced concrete pavement design is not 

included in the NCHRP 1-37A design procedures. 
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Geotechnical Aspects of Pavements 5 - 80 May 2006 

Table 5-48. Typical values of base/slab friction coefficient recommended for CRCP 

design in the NCHRP 1-37A procedure (NCHRP 1-37A, 2004). 

Subbase/Base Type Friction Coefficient 

(low – medium – high) 

Fine grained soil 0.5 – 1.1 – 2.0 

Sand* 0.5 – 0.8 – 1.0 

Aggregate 0.5 – 2.5 – 4.0 

Lime stabilized clay* 3.0 – 4.1 – 5.3 

Asphalt treated base 2.5 – 7.5 – 15 

Cement treated base 3.5 – 8.9 – 13 

Soil cement 6.0 – 7.9 – 23 

Lime-cement-flyash 3.0 – 8.5 - 20 

Lime-cement-flyash, not cured* > 36 *

Base type did not exist or was not considered in the NCHRP 1-

37A calibration process. 

5.4.8 Permanent Deformation Characteristics 

The permanent deformation characteristics of unbound materials are used in the empirical 

rutting distress models in the NCHRP 1-37A design methodology. This information is not 

required for rigid pavement design in the NCHRP 1-37A Design Guide or at all in the 1993 

AASHTO design procedure. Permanent deformation characteristics are measured via triaxial 

repeated load tests conducted for many cycles of loading; Figure 5-30 shows schematically 

the typical behavior measured in this type of test. The repeated load permanent deformation 

tests are very similar to the cyclic triaxial tests used to measure resilient modulus (see 5.4.3), 

except that the cyclic deviator stress magnitude is kept constant throughout the test. There are 

at present no ASTM or AASHTO test specifications for repeated load permanent 

deformation testing. However, the first 1000 conditioning cycles of the AASHTO T307-99 

resilient modulus testing procedure are often used for permanent deformation modeling. 

The NCHRP 1-37A design methodology characterizes the permanent deformation behavior 

of unbound base, subbase, and subgrade materials using a model based on work by Tseng 

and Lytton (1989): 

 

2

1 ( ) o N

a v

r

N eh

ξ β ρ

ε δ ξ ε

ε

  −      =    

 (5.27) 
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 in which 

 δa = Permanent deformation for the layer/sublayer 

 N = Number of traffic repetitions 

 εo, β, ρ = Material properties 

 εr = Resilient strain imposed in laboratory test to obtain material 

 properties εo, β, and ρ

 εv = Average vertical resilient strain in the layer/sublayer as obtained 

 from the primary response model 

 h = Thickness of the layer/sublayer 

 ξ1, ξ2 = Field calibration functions 

Tseng and Lytton provide regression equations for the εo/εr, ρ, and β terms. These regression 

equations were substantially revised during development of the NCHRP 1-37A design 

methodology. The revised equations implemented in the NCHRP 1-37A procedure are as 

follows: 

 log 0.74168 0.08109 0.000012157 o

c R

r

W M ε

ε

    =+ −

 

 (5.28) 

 log 0.61119 0.017638 β =− − Wc (5.29) 

 log 0.622685 0.541524 ρ = + Wc (5.30) 

In Eq. (5.28) through Eq. (5.30), MR is the resilient modulus in psi, and Wc is an estimate of 

the average in-situ gravimetric water content in percent. The NCHRP 1-37A procedure 

proposes the following equation for determining Wc in the absence of measured values: 

 ( ) ( )0.1192 0.3586 51.712 GWT W CBR W W c c sat

−

= ≤ (5.31) 

In Eq. (5.31), GWT is the depth to the groundwater table in feet, and CBR can be estimated 

from resilient modulus using 

 

(1/ 0.64)

2555

MR CBR   =     (5.32) 
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The Wsat limit in Eq. (5.31) can be determined from 

 2.75 Wsat 1 100 / 2.75

SPG

  = −∗     (5.33) 

where SPG is the saturated specific gravity of the soil. For laboratory test conditions, Wc is 

presumably equal to the tested water content. 

Although fine-tuning of the calibration is still underway and therefore the expressions for ξ1,

ξ2 may yet change, the current best estimates are as follows: 

 0.058 /1000 ( ) 7

1 1.2 1.39 1 10 MR ξ e

− − = − ≤× (5.34) 

 2 ξ = 0.7 (5.35) 

In Eq. (5.34), a lower bound of 2.6 is set for MR/1000. 

Figure 5-30. Accumulation of permanent deformations with repeated cyclic loading 

(LTPP, 2003). 
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5.4.9 Coefficient of Lateral Pressure 

The coefficient of lateral earth pressure K0 is defined as the ratio of the horizontal to vertical 

in-situ effective stress: 

 0

ho

vo

K σ

σ = (5.36) 

The coefficient of lateral earth pressure is an input in the NCHRP 1-37A design procedure. It 

is used to compute the combined in-situ and induced stress states within the pavement 

system. 

Elasticity theory can be used to estimate K0 based on the confined Poisson expansion: 

 0 1

K ν

ν = − (5.37) 

in which ν is Poisson’s ratio. Values of K0 predicted by Eq. (5.37) for typical geomaterials 

range between 0.4 and 0.6. 

A common empirical correlation for K0 for cohesionless and normally consolidated cohesive 

soils is the Jaky relationship: 

 0 K = −1 sinφ (5.38) 

in which φ is the friction angle. Overconsolidation in cohesive soils will increase the value 

for K0 above that given in Eq. (5.38). Figure 5-31 shows the typical relationship between K0, 

the overconsolidation ratio OCR, and the plasticity index PI. 

Loading followed by unloading and reloading, such as occurs during compaction of unbound 

materials in pavements, often results in an increase in K0. The relative magnitudes of 

horizontal and vertical stress during a load-unload-reload path are shown schematically in 

Figure 5-32. Mayne and Kulhawy (1982) proposed the following model for K0 after loadingunloading-reloading: 

( ) 0 ( ) 1 sin

max max

3 1 sin

4

OCR OCR K

OCR OCR φ φ −

    =− +        

 (5.39) 

in which OCRmax is the maximum overconsolidation ratio achieved in the load path and the 

other terms are as defined previously. 
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Figure 5-31. Correlation between coefficient of lateral earth pressure and overconsolidation 

ratio for clays of various plasticity indices (Carter and Bentley, 1991). 
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Figure 5-32. Horizontal and vertical in-situ stresses during a load-unload-reload path 

(Mayne and Kulhawy, 1982). 

5.5 THERMO-HYDRAULIC PROPERTIES 

Thermo-hydraulic material properties are required to evaluate the temperature and moisture 

conditions in a pavement system and their effects on the material behavior. Temperature has 

significant effects on the stiffness of asphalt concrete, and temperature gradients can induce 

thermal curling and stresses in rigid pavement slabs. Moisture content influences the stiffness 

and strength of unbound materials, and moisture gradients can induce warping of rigid 

pavement slabs. Combined temperature and moisture effects can cause detrimental 

freeze/thaw cycles in unbound materials.5

 

The empirical 1993 AASHTO Design Guide and the mechanistic-empirical NCHRP 1-37A 

design procedure have drastically different input requirements for thermo-hydraulic 

properties. The thermo-hydraulic design inputs in the 1993 AASHTO Guide are largely 

empirical coefficients grouped in the following categories: 

• Drainage coefficients (for unbound layers) 

• Swelling parameters (for expansive subgrade soils) 

• Frost heave parameters (for frost-susceptible subgrade soils) 

 

5

 Moisture and freeze/thaw are also important factors behind stripping of asphalt concrete, but this material 

phenomenon is beyond our scope. 
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These empirical properties in the 1993 Guide often mix material property and climate factors. 

For example, drainage coefficients are functions of both climate-determined moisture 

conditions and material-related drainage quality. 

The thermo-hydraulic properties required as input to the NCHRP 1-37A Design Guide tend 

to be more fundamental material properties. These include 

• Groundwater table depth 

• Infiltration and drainage properties 

• Physical properties 

• Soil water characteristic curve 

• Hydraulic conductivity (permeability) 

• Thermal conductivity 

• Heat capacity 

These thermo-hydraulic properties are used in the mechanistic Enhanced Integrated Climate 

Model (EICM) along with climate inputs (discussed separately in Section 5.6) to predict 

temperature and moisture distributions in the pavement as functions of depth and time. 

Appendix D provides details on algorithms embedded in the EICM. 

Because of the substantial differences in these thermo-hydraulic inputs to the two design 

methods, each design method is discussed separately in the following subsections. 

5.5.1 1993 AASHTO Guide 

The environment-related aspects in the 1993 AASHTO Design Guide are grouped into two 

general categories: drainage and subgrade swelling/frost heave. As described in Section 3.5.2 

in Chapter 3, drainage is incorporated via adjustment to the unbound structural layer 

coefficients for flexible pavements or via a drainage factor in the design equation for rigid 

pavements. Swelling and/or frost heave, on the other hand, is incorporated via a partitioning 

of the total allowable serviceability loss ∆PSI; part of ∆PSI is allocated to environmentinduced deterioration due to swelling and/or frost heave, and the remainder of ∆PSI is 

allocated to traffic-induced deterioration. 

Drainage Coefficients

The 1993 AASHTO Guide provides guidance for the design of subsurface drainage systems 

and modifications to the flexible and rigid pavement design procedure to take advantage of 

improvements in performance due to good drainage. For flexible pavements, the benefits of 

drainage are incorporated into the structural number via empirical drainage coefficients: 
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11 2 22 333 SN a D a D m a D m =+ + (5.40) 

in which m2 and m3 are the drainage coefficients for the base and subbase layers, 

respectively, and all other terms are as defined previously. Table 5-49 summarizes the 

recommended values for mi in the 1993 AASHTO Guide as functions of qualitative 

descriptions of drainage quality and climate conditions. 

For rigid pavements, the benefits of drainage are incorporated via an empirical drainage 

coefficient Cd in the rigid pavement design equation. Table 5-50 summarizes the 

recommended values for Cd in the 1993 AASHTO Guide as a function of qualitative 

descriptions of drainage quality and climate conditions. 

Table 5-49. Recommended mi values for modifying structural layer coefficients of 

untreated base and subbase materials in flexible pavements (AASHTO, 1993). 

Quality of 

Drainage 

Water 

Removed 

Within 

Percent of Time Pavement is Exposed to Moisture Levels 

Approaching Saturation 

 <1% 1-5% 5-25% >25% 

Excellent 2 hours 1.40-1.35 1.35-1.30 1.30-1.20 1.20 

Good 1 day 1.35-1.25 1.25-1.15 1.15-1.00 1.00 

Fair 1 week 1.25-1.15 1.15-1.05 1.00-0.80 0.80 

Poor 1 month 1.05-0.80 1.05-0.80 0.80-0.60 0.60 

Very Poor no drainage 0.95-0.75 0.95-0.75 0.75-0.40 0.40 

Table 5-50. Recommended values of drainage coefficient Cd values 

for rigid pavement design (AASHTO, 1993). 

Quality of 

Drainage 

Water 

Removed 

Within 

Percent of Time Pavement is Exposed to Moisture Levels 

Approaching Saturation 

 <1% 1-5% 5-25% >25% 

Excellent 2 hours 1.25-1.20 1.20-1.15 1.15-1.10 1.10 

Good 1 day 1.20-1.15 1.15-1.10 1.10-1.00 1.00 

Fair 1 week 1.15-1.10 1.10-1.00 1.00-0.90 0.90 

Poor 1 month 1.10-1.00 1.00-0.90 0.90-0.80 0.80 

Very Poor no drainage 1.00-0.90 0.90-0.80 0.80-0.70 0.70 
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Swelling Parameters

The 1993 AASHTO Guide includes three empirical parameters for estimating potential 

serviceability loss due to swelling: 

• Swell rate constant θ

• Potential vertical rise VR

• Swell probability PS

The swell rate constant θ is used to estimate the rate at which swelling will take place. It 

varies between 0.04 and 0.20, with higher values appropriate for soils exposed to a large 

moisture supply either due to high rainfall, poor drainage, or some other source. Figure 5-33 

provides a nomograph for subjectively estimating the rate of subgrade soil swelling based 

upon qualitative descriptions of moisture supply and soil fabric. Little guidance beyond that 

in Figure 5-33 is provided in the 1993 Guide for estimating the values for moisture supply 

and soil fabric. 

The potential vertical rise VR is a measure of the vertical expansion that may occur in the 

subgrade soil under extreme swell conditions. Although it is possible to measure VR from 

laboratory swell tests, this is not commonly done in practice. Instead, VR is estimated using 

the chart in Figure 5-34 based on the soil’s plasticity index, moisture condition, and overall 

thickness of the layer. The moisture condition is a subjective estimate of the difference 

between the in-situ moisture conditions during construction and moisture conditions at a later 

date. 

The swell probability (PS) is a measure of the proportion (percent) of the project length that is 

subject to swell. The probability of swelling at a given location is assumed to be 100% if the 

subgrade soil plasticity index is greater than 30 and the layer thickness is greater than 2 feet 

(or if VR is greater than 0.20 inches). These criteria can be used to separate the project length 

into swelling and nonswelling sections, from which a length-averaged estimate of PS can be 

determined. 

These three swelling parameters are used in a nomograph (see Appendix C) along with the 

design life to determine the expected serviceability loss due to swelling ∆PSISW. However, it 

should be clear from the empirical and highly subjective procedures used to determine the 

input parameters that the predicted ∆PSISW will be only a very approximate estimate. 
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Figure 5-33. Nomograph for estimating swell rate constant (AASHTO, 1993). 
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Figure 5-34. Chart for estimating potential vertical rise of natural soils (AASHTO, 1993). 

Frost Heave Parameters

The 1993 AASHTO Guide includes three empirical parameters for estimating potential 

serviceability loss due to frost heave: 

• Frost heave rate φ

• Maximum potential serviceability loss ∆PSIMAX

• Frost heave probability PF

The frost heave rate φ is a measure of the rate of increase of frost heave in millimeters per 

day. The rate of frost heave depends on the type of subgrade material, in particular the 

percentage of fine-grained material. Figure 5-35 can be used to estimate the rate of frost 

heave based on the USCS class for the subgrade and the percentage of material finer than 

0.02 mm. 

The maximum potential serviceability loss ∆PSIMAX due to frost heave is dependent on the 

quality of drainage and the depth of frost penetration. Figure 5-36 can be used to estimate 

the maximum potential serviceability loss due to these two factors. The drainage quality 

parameter in Figure 5-36 is the same as that used to define the drainage coefficients in Table 
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Geotechnical Aspects of Pavements 5 - 91 May 2006 

5-49 and Table 5-50. See Yoder and Witczak (1975) for methods for determining the depth 

of frost penetration. 

The frost heave probability PF is the designer’s estimate of the percentage length of the 

project that will experience frost heave. This estimate will depend upon the extent of frostsusceptible subgrade material, moisture availability, drainage quality, number of freeze-thaw 

cycles during the year, and the depth of frost penetration. Past experience is valuable here, as 

there is no clear method for approximating the frost heave probability. 

These three frost heave parameters are used in a nomograph (see Appendix C) along with the 

design life to determine the expected serviceability loss due to frost heave ∆PSIFH. However, 

it should be clear from the empirical and highly subjective procedures used to determine the 

input parameters that the predicted ∆PSIFH will be only a very approximate estimate. 
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Figure 5-35. Chart for estimating frost heave rate for subgrade soil (AASHTO, 1993). 
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Figure 5-36. Graph for estimating maximum serviceability loss due to frost heave 

(AASHTO, 1993). 

5.5.2 NCHRP 1-37A Design Guide 

The thermo-hydraulic properties required as input to the NCHRP 1-37A Design Guide can be 

grouped into the following categories: 

• Groundwater depth 

• Infiltration and drainage properties 

• Physical/index properties 

• Soil water characteristic curve 

• Hydraulic conductivity (permeability) 

• Thermal conductivity 

• Heat capacity 
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Methods for determining the design inputs in each of these categories are described in the 

following subsections. In some cases, the design inputs are determined by direct 

measurement in the laboratory or the field. However, other design inputs (e.g., soil water 

characteristic curve) are much less commonly measured in geotechnical practice. 

Recognizing this, the NCHRP 1-37A project team expended substantial effort to develop 

robust correlations between these properties and other more conventional soil properties 

(e.g., gradation and plasticity). These correlations are also detailed in the following 

subsections as appropriate. 

Groundwater Depth 

The groundwater depth plays a significant role in the NCHRP 1-37A Design Guide 

predictions of moisture content distributions in the unbound pavement materials and thus on 

the seasonal resilient modulus values. The input value is intended to be the best estimate of 

the annual average groundwater depth. Groundwater depth can be determined from profile 

characterization borings during design (see Section 4.7.1) or estimated. The county soil 

reports produced by the National Resources Conservation Service can often be used to 

develop estimates of groundwater depth. 

Infiltration and Drainage 

Three input parameters related to infiltration and drainage are required in the NCHRP 1-37A 

design methodology: 

• Amount of infiltration 

• Pavement cross slope 

• Drainage path length 

Amount of Infiltration

The amount of infiltration will be a function of rainfall intensity and duration (determined 

from the climate inputs, see Section 5.6), pavement condition, shoulder type, and drainage 

features. The NCHRP 1-37A Design Guide qualitatively divides infiltration into four 

categories, as summarized in Table 5-51. These categories are used at all hierarchical input 

levels. The infiltration category is based upon shoulder type, generally the largest single 

source of moisture entry into the pavement structure, and edge drains, since these shorten the 

drainage path and provide a positive drainage outlet. Note that if a drainage layer is present 

in addition to edge drains, its influence is automatically accounted for within the EICM 

moisture calculations. 
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Table 5-51. Infiltration categories in the NCHRP 1-37A Design Guide 

(NCHRP 1-37A, 2004). 

Infiltration 

Category Conditions 

% Precipitation 

Entering 

Pavement 

None 0 

Minor This option is valid when tied and sealed concrete 

shoulders (rigid pavements), widened PCC lanes, or fullwidth AC paving (monolithic main lane and shoulder) are 

used or when an aggressive policy is pursued to keep the 

lane-shoulder joint sealed. This option is also applicable 

when edge drains are used. 

10 

Moderate This option is valid for all other shoulder types, PCC 

restoration, and AC overlays over old and cracked 

existing pavements where reflection cracking will likely 

occur. 

50 

Extreme Generally not used for new or reconstructed pavement 

levels. 

100 

Most designs and maintenance activities, especially for higher functional class pavements, 

should strive to achieve zero infiltration or reduce it to a minimum value. This can be done 

by proper design of surface drainage elements (cross slopes, side ditches, etc.), adopting 

construction practices that reduce infiltration (e.g., eliminating cold lane/shoulder joints, use 

of tied joints for PCC pavements, etc.), proactive routine maintenance activities (e.g., crack 

and joint sealing, surface treatments, etc.), and providing adequate subsurface drainage (e.g.,

drainage layers, edge drains). Chapter 7 provides more information on pavement drainage 

systems. 

Pavement Cross Slope

The pavement cross slope is the slope of the surface perpendicular to the direction of traffic. 

This input is used in computing the drainage path length, as described in the next subsection. 
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Drainage Path Length

The drainage path length is the resultant of the cross and longitudinal slopes of the pavement. 

It is measured from the highest point in the pavement cross section to the drainage outlet. 

This input is used in the EICM’s infiltration and drainage model to compute the time 

required to drain an unbound base or subbase layer from an initially wet condition. 

The DRIP computer program (Mallela et al., 2002) can be used to compute the drainage path 

length based on pavement cross and longitudinal slopes, lane widths, edge drain trench 

widths (if applicable, and cross section crown and superelevation). The DRIP program is 

provided as part of the NCHRP 1-37A Design Guide software. 

Physical Properties 

Several physical properties are required for the internal calculations in the EICM. For 

unbound materials, these are 

• Specific gravity of solids Gs (see Table 5-10) 

• Maximum dry unit weight γd max (see Table 5-13) 

• Optimum gravimetric moisture content wopt (see Table 5-13) 

Table 5-52 describes the procedures to obtain these physical property inputs for hierarchical 

input levels 1 and 2 (level 3 inputs are not applicable for this input category). From these 

properties, all other necessary weight and volume properties required in the EICM can be 

computed. These include 

• Degree of saturation at optimum compaction (Sopt) 

• Optimum volumetric moisture content (θopt) 

• Saturated volumetric water content (θsat) 

For rehabilitation designs only, the equilibrium or in-situ gravimetric water content is also a 

required input. NCHRP 1-37A recommends that this value be estimated from direct testing of 

bulk samples retrieved from the site, or through other appropriate means. 

Although the material properties of the lower natural subgrade layers are important to the 

overall response of the pavement, a lower level of effort is generally sufficient to characterize 

these deeper layers as compared to the overlying compacted materials. Level 1 inputs are 

thus generally not necessary for in-situ subgrade materials. NCHRP 1-37A recommends that 

only gradation properties and Atterberg limits be measured for the in-situ subgrade materials. 
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Table 5-52. Physical properties for unbound materials required for EICM calculations 

(NCHRP 1-37A, 2004). 

Material Property Input Level Description 

1 A direct measurement using AASHTO T100 

(performed in conjunction with consolidation tests – 

T180 for bases or T 99 for other layers). 

See Table 5-10. 

2 Determined from P200

1

 and PI 2 of the layer as below: 

1. Determine P200 and PI. 

2. Estimate Gs: 

Gs = 0.041(P200 * PI)0.29 + 2.65 (5.41) 

Specific gravity, Gs

3 Not applicable. 

1 Typically, AASHTO T180 compaction test for base 

layers and AASHTO T99 compaction test for other 

layers. See Table 5-13. 

2 Estimated from D60

1

, P200

1

 and PI 2 of the layer 

following these steps: 

1. Determine PI, P200, and D60. 

2. Estimate Sopt: 

 Sopt = 6.752 (P200 * PI)

0.147 + 78 (5.42)

3. Estimate wopt: 

 If P200 * PI > 0 

 wopt = 1.3 (P200 * PI)

0.73 + 11 (5.43)

 If P200 * PI = 0 

 wopt (T99) = 8.6425 (D60)

-0.1038 (5.44)

 If layer is not a base course 

 wopt = wopt (T99) (5.45) 

 If layer is a base course 

 ∆wopt = 0.0156[wopt(T99)]

2

 – 0.1465wopt(T99) 

 + 0.9 (5.46) 

 wopt = wopt (T99) - ∆wopt (5.47) 

4. Determine Gs using the level 2 procedure 

 described in this table above. 

5. Compute (γd)max comp at optimum moisture and 

 maximum compacted density: 

1

s water

d max comp

opt s

opt

G

w G

S

γ γ =

+

 (5.48) 

6. Determine (γd)max: 

 If layer is a compacted material: 

 d max d max comp γ = γ (5.49) 

 If layer is a natural in-situ material: 

 0.9 d max d max comp γ = γ (5.50) 

Optimum gravimetric 

water content, wopt, and 

maximum dry unit weight 

of solids, (γd)max

3 Not applicable. 1

 P200 and D60 can be obtained from a grain-size distribution test (AASHTO T 27)—see Table 5-19. 2

 PI can be determined from an Atterberg limit test (AASHTO T 90)—see Table 5-21.
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Soil Water Characteristic Curve 

The soil water characteristic curve (SWCC) defines the relationship between water content 

and matric suction h for a given soil. Matric suction is defined as the difference between the 

pore air pressure ua and pore water pressure uw in a partially saturated soil: 

 ( ) a w huu = − (5.51) 

This relationship is usually plotted as the variation of water content (gravimetric w, 

volumetric θ, or degree of saturation S) vs. soil suction (Figure 5-37). The SWCC is one of 

the primary material inputs used in the EICM to compute moisture distributions with depth 

and time. Although the SWCC can be measured in the laboratory (e.g., see Fredlund and 

Rahardjo, 1993), this is quite uncommon and rather difficult. Instead, empirical models are 

used to express the SWCC in terms of other, more easily measurable parameters. The EICM 

algorithms in the NCHRP 1-37A analysis procedure are based on a SWCC model proposed 

by Fredlund and Xing (1994): 

( )

ln (1)

w c f bf

sat

f

C h

h EXP

a

θ θ

 

 

           

         

= ×

+

 (5.52) 

5

ln 1

() 1 1.45 10 ln 1

r

r

h

h C h

h

         

   

   

+

= − ×

+

 (5.53) 

in which h = matric suction (units of stress) 

 θsat = volumetric moisture content at saturation 

 af, bf, cf, and hr = model parameters (af, hr in units of stress) 

Table 5-53 summarizes the NCHRP 1-37A recommended approach for estimating the 

parameters of the SWCC at each of the three hierarchical input levels. 
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0.0

0.2

0.4

0.6

0.8

1.0

1.2

1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6

Matric Suction (psi)

Degree of Saturation

D60=1 mm

P200PI=0.1

P200PI = 50

3 5

10 15 20 30 40

D60=0.1 mm

P200PI = P200 x PI

1.5E-2 1.5E-1 1.5E+0 1.5E+1 1.5E+2 1.5E+3 1.5E+4 1.5E+5

Figure 5-37. Soil water characteristic curves (NCHRP 1-37A, 2004). 

Hydraulic Conductivity (Permeability) 

Hydraulic conductivity (or permeability) k describes the ability of a material to conduct fluid 

(water). It is defined as the quantity of fluid flow through a unit area of soil under a unit 

pressure gradient. Hydraulic conductivity is one of the primary material inputs to the 

environment model in the NCHRP 1-37A analysis procedure, where it is used to determine 

the transient moisture profiles in unbound materials and to estimate their drainage 

characteristics. 

The unsaturated flow algorithms in the EICM require a complete specification of the 

unsaturated hydraulic conductivity as a function of matric suction h. Although the 

unsaturated hydraulic conductivity vs. matric suction relationship can be measured in the 

laboratory (e.g., see Fredlund and Rahardjo, 1993), this is uncommon and difficult. At best, 

only the saturated hydraulic conductivity ksat is measured in practice. Consequently, within 

the EICM an empirical model proposed by Fredlund et al. (1994) is used to express the 

unsaturated hydraulic conductivity k(h) vs. matric suction h relationship in terms of the 

Fredlund and Xing (1994) SWCC model, Eqs. (5.52) and (5.53). The k(h) model is expressed 

in terms of a relative hydraulic conductivity: 

( ) ( )/ r sat k h kh k ≡ (5.54) 
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Table 5-53. Options for estimating the SWCC parameters (NCHRP 1-37A, 2004). 

Input 

Level Procedure to Determine SWCC parameters Required Testing 

1 

1) Direct measurement of suction (h) in psi, and volumetric water 

 content (θw) pairs of values. 

2) Direct measurement of optimum gravimetric water content, 

 wopt and maximum dry unit weight, γd max. 

3) Direct measurement of the specific gravity of the solids, Gs. 

4) Compute opt d max

opt

water

w γ θ

γ = (5.55) 

5) Compute 

1

opt

opt

d max

water s

S

G

θ

γ

γ

=

−

 (5.56) 

6) Compute opt

sat

opt S

θ

θ = (5.57) 

7) Using non-linear regression analysis, compute the SWCC 

 model parameters af, bf, cf, and hr from Eqs. (5.52) and (5.53) 

 and the (h, θw) pairs of values obtained in Step 1. 

Pressure plate, filter 

paper, and/or Tempe 

cell testing. 

AASHTO T180 or 

T99 for γd max (see 

Table 5-13). 

AASHTO T100 for 

Gs (see Table 5-10). 

2 

1) Direct measurement of optimum gravimetric water content, 

 wopt and maximum dry unit weight, γd max. 

2) Direct measurement of the specific gravity of the solids, Gs. 

3) Direct measurement of P200, D60, and PI. 

 The EICM will then internally do the following: 

a) Calculate P200 * PI. 

b) Calculate θopt, Sopt, and θsat, as described for level 1. 

c) Determine the SWCC model parameters af, bf, cf, and hr in 

 Eqs. (5.52) and (5.53) via correlations with P200PI and D60. 

If P200 PI > 0 3.35

200 0.00364( ) 4( ) 11

6.895

200

f

P PI P PI

a

+ + = (psi) (5.58) 

 0.14

200 2.313( ) 5 f

f

b

P PI

c

=− + (5.59)

 0.465

200 0.0514( ) 0.5 f c P PI = + (5.60) 

 200 0.0186( ) 32.44 r P PI

f

h

e

a = (5.61) 

If P200 PI = 0 

 

0.751

60 0.8627( )

6.895 f

D

a

−

= (psi) (5.62) 

 7.5 f b = (5.63)

 

AASHTO T180 or 

T99 for γd max 

(see Table 5-13). 

T100 for Gs 

(see Table 5-10). 

AASHTO T88 for 

P200 and D60 

(see Table 5-19). 

AASTHO T90 for PI

(see Table 5-21). 
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60 0.1772 ln( ) 0.7734 f c D = + (5.64) 

 4

60

1

9.7

r

f

h

aD e− = +

 (5.65) 

3 

Direct measurement and input of P200, PI, and D60, after which the 

EICM uses correlations with P200PI and D60 to automatically 

generate the SWCC parameters for each soil as follows: 

1) Compute Gs, as outlined in Table 5-52 for level 2. 

2) Compute P200 * PI 

3) Estimate Sopt, wopt, and γd max, as shown Table 5-52 for level 2. 

4) Determine the SWCC model parameters af, bf, cf, and hr via 

correlations with P200PI and D60, as shown in this table for level 2. 

AASHTO T88 for 

P200 and D60 

(see Table 5-19). 

AASHTO T90 for PI 

(see Table 5-21). 

Recommendations from NCHRP 1-37A for determining the ksat value needed in Eq. (5.54) 

are summarized in Table 5-54. The Fredlund et al. (1994) model for kr(h) is then expressed 

in integral form as 

2

2

() () ( )

( ) ( ) ( )

r

r

aev

h

h

r h

s

h

x h

x dx

x k h

x

x dx

x

θ θ θ

θ θ θ

− ′

= − ′

∫

∫

 (5.66) 

in which: θ (h) = volumetric water content as a function of matric suction, from 

 the SWCC Eqs. (5.52) and (5.53) 

 θs = saturated volumetric water content 

 θ′( ) h = derivative of the SWCC 

 x = dummy integration variable corresponding to water content 

 hr = matric suction corresponding to the residual water content 

 (i.e., the water content below which a large increase in suction 

 is required to remove additional water) 

 have = the air-entry matric suction (i.e., the suction where air starts to 

 enter the largest pores in the soil) 

The procedures described in Table 5-53 are used in the EICM to determine the SWCC via 

Eqs. (5.52) and (5.53), which in turn is then used to determine the unsaturated hydraulic 

conductivity via Eq. (5.66). These calculations are performed internally within the EICM 

software. 
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Table 5-54. Options for determining the saturated hydraulic conductivity 

for unbound materials (NCHRP 1-37A, 2004). 

Material Property Input 

Level Description 

1 Direct measurement using a permeability test (AASHTO 

T215)—see Table 5-55. 

2 Determined from P200

1

, D60

1

, and PI2 of the layer as below: 

1. Determine P200PI = P200 * PI

2. If 0 ≤ P200PI < 1 

( )( ) 2

60 60 1.1275 log 2 7.2816 log 2 11.2891 118.11 10 D D

sat k − + + +−    = × (5.67) 

 Units: ft/hr 

 Valid for D60 < 0.75 in 

 If D60 > 0.75 in, set D60 = 0.75 mm 

3. If P200PI ≥ 1 

() () 2

200 200 0.0004 0.0929 6.56 118.11 10 P PI P PI

sat k  − −    = × (ft/hr) (5.68) 

Saturated hydraulic 

conductivity, ksat

3 Not applicable. 1

 P200 and D60 can be obtained from a grain-size distribution test (Table 5-19) 2

 PI can be determined from an Atterberg limit test (Table 5-21). 

Figure 5-38. Schematic of a constant head permeameter (Coduto, 1999).
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Table 5-55. Saturated hydraulic conductivity. 

Description Quantity of fluid flow through a unit area of soil under a unit pressure gradient. 

Uses in 

Pavements 

Used in the EICM for predicting distributions of moisture with depth and time 

in the NCHRP 1-37A Design Guide. 

Laboratory 

Determination 

AASHTO T 215; ASTM D 2434 (Granular Soils), ASTM D 5084 (All Soils). 

There are two basic standard types of test procedures to directly determine 

permeability: (1) the constant-head test, normally used for coarse materials 

(Figure 5-38); and (2) the falling-head test, normally used for clays (Figure 

5-39). Undisturbed, remolded, or compacted samples can be used in both 

procedures. 

Field 

Measurement 

Pumping tests can be used to measure hydraulic conductivity in-situ. 

Commentary Both test procedures determine permeability of soils under specified conditions. 

The geotechnical engineer must establish which test conditions are 

representative of the problem under consideration. As with all other laboratory 

tests, the geotechnical engineer has to be aware of the limitations of this test. 

The process is sensitive to the presence of air or gases in the voids and in the 

permeant or water. Prior to the test, distilled, de-aired water should be run 

through the specimen to remove as much of the air and gas as practical. It is a 

good practice to use de-aired or distilled water at temperatures slightly higher 

than the temperature of the specimen. As the water permeates through the voids 

and cools, it will have a tendency to dissolve the air and some of the gases, thus 

removing them during this process. The result will be a more representative, 

albeit idealized, permeability value. 

The type of permeameter, (i.e., flexible wall - ASTM D 5084 -versus rigid - 

ASTM D 2434 and AASHTO T215) may also affect the final results. For 

testing of fine-grained low-permeability soils, the use of flexible-wall 

permeameters is recommended, which are essentially very similar to the triaxial 

test apparatus. When rigid wall units are used, the permeant may find a route at 

the sample-permeameter interface. This will produce erroneous results. 

It should be emphasized that permeability is sensitive to viscosity. In computing 

permeability, correction factors for viscosity and temperatures must be applied. 

The temperature of the permeant and the laboratory should be kept constant 

during testing. 

Laboratory permeability tests produce reliable results under ideal conditions. 

Permeability of fine-grained soils can also be computed from one-dimensional 

consolidation test results, although these results are not as accurate as direct ksat

measurements. 

Typical 

Values 

See Table 5-56 and Table 5-57. Saturated hydraulic conductivity for loose 

clean sands can also be estimated using the Hazen relationship: 

2

sat 10 k CD = ∗ (5.69)

in which ksat is the saturated hydraulic conductivity in cm/sec; C is Hazen’s 

coefficient ranging between 0.8 and 1.2 (a value of 1.0 is commonly used); and 

D10 is the effective particle size, defined as the largest particle diameter in the 

finest 10% fraction of the soil. 
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Figure 5-39. Schematic of a falling head permeameter (Coduto, 1999). 

Table 5-56. Typical values of saturated hydraulic conductivity for soils (Coduto, 1999). 

Hydraulic Conductivity k Soil Description (cm/s) (ft/s) 

Clean gravel 1 – 100 3x10-2 – 3 

Sand-gravel mixtures 10-2 – 10 3x10-4 – 0.3 

Clean coarse sand 10-2 – 1 3x10-4 – 3x10-2

Fine sand 10-3 – 10-1 3x10-5 – 3x10-3

Silty sand 10-3 – 10-2 3x10-5 – 3x10-4

Clayey sand 10-4 – 10-2 3x10-6 – 3x10-4

Silt 10-8 – 10-3 3x10-10 – 3x10-5

Clay 10-10 – 10-6 3x10-12 – 3x10-8
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Table 5-57. Typical values of saturated hydraulic conductivity for highway materials 

(Carter and Bentley, 1991). 

Material Hydraulic Conductivity k (m/s)* 

Uniformly graded coarse aggregate 0.4 – 4x10-3 

Well-graded aggregate without fines 4x10-3 – 4x10-5 

Concrete sand, low dust content 7x10-4 – 7x10-6 

Concrete sand, high dust content 7x10-6 – 7x10-8 

Silty and clayey sands 10-7 – 10-9

Compacted silt 7x10-8 – 7x10-10 

Compacted clay < 10-9 

Bituminous concrete** 4x10-5 – 4x10-8 

Portland cement concrete < 10-10 

*

1 m/s = 3.25 ft/s 

**New pavements; values as low as 10-10 have been reported for sealed, trafficcompacted highway pavements. 

Thermal Conductivity 

Thermal conductivity K is defined as the ability of a material to conduct heat. Typical units 

are BTU/ft-hr-o

F or W/m- oK. Thermal conductivity is used in the EICM algorithms for the 

computation of temperature distributions with depth and time in the NCHRP 1-37A analysis 

methodology. 

Table 5-58 outlines the NCHRP 1-37A recommended approach for characterizing the dry 

thermal conductivity K for unbound materials. Note that thermal conductivity is not 

commonly measured for unbound pavement materials, and consequently the level 3 inputs 

will be used for nearly all designs. The EICM automatically adjusts the dry thermal 

conductivity for the influence of moisture during the calculations.

Heat Capacity 

Heat capacity Q is defined as the amount of heat required to raise by one degree the 

temperature of a unit mass of soil. Typical units are BTU/lb-o

F or J/kg-oK. Heat capacity is 

used in the EICM algorithms for the computation of temperature distributions with depth and 

time in the NCHRP 1-37A analysis methodology. 
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Table 5-58 outlines the NCHRP 1-37A recommended approached for characterizing the dry 

heat capacity Q for unbound materials. Note that heat capacity is not commonly measured 

for unbound pavement materials, and consequently the level 3 inputs will be used for nearly 

all designs. The EICM automatically adjusts the dry heat capacity for the influence of 

moisture content during the calculations. 

Table 5-58. Options for determining the dry thermal conductivity and heat capacity 

for unbound materials (NCHRP 1-37A, 2004). 

Material 

Property 

Input 

Level Description 

1 Direct measurement (ASTM E 1952). 

2 Not applicable. 

Dry Thermal 

Conductivity, K 

3 Soil Type Range Recommended 

 BTU/ft-hr-o

F*

 A-1-a 0.22 – 0.44 0.30 

 A-1-b 0.22 – 0.44 0.27 

 A-2-4 0.22 – 0.24 0.23 

 A-2-5 0.22 – 0.24 0.23 

 A-2-6 0.20 – 0.23 0.22 

 A-2-7 0.16 – 0.23 0.20 

 A-3 0.25 – 0.40 0.30 

 A-4 0.17 – 0.23 0.22 

 A-5 0.17 – 0.23 0.19 

 A-6 0.16 – 0.22 0.18 

 A-7-5 0.09 – 0.17 0.13 

 A-7-6 0.09 – 0.17 0.12 

Additional typical values are given in Table 5-59. 

1 Direct measurement (ASTM D 2766). 

2 Not applicable. 

Dry Heat 

Capacity, Q 

3 Typical values range from 0.17 to 0.20 BTU/lb-o

F. 

Additional typical values are given in Table 5-59. *

1 BTU/ft-hr-oF = 1.73 W/m-oK; 1 BTU/lb-oF = 4187 J/kg-oK 

 
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Table 5-59. Typical values for thermal conductivity and heat capacity of 

unbound materials (adapted from Sundberg, 1988). 

Soil Type Thermal Conductivity 

(W/m-o

K)* 

Heat Capacity 

(J/kg-o

K)* 

Clay with high clay content 0.85 - 1.1 1700 – 2050 

Silty clay/silt 1.1 - 1.5 1650 – 1900 

Silt 1.2 – 2.4 1400 – 1900 

Sand, gravel below GWT 1.5 – 2.6 

(1.6 – 2.0) 

1450 – 1850 

(1700) 

Sand, gravel above GWT 0.4 – 1.1 

(0.7 – 0.9) 

700 – 1000 

(800) 

Till below GWT 1.5 – 2.5 1350 – 1700 

Sandy till above GWT 0.6 – 1.8 750 – 1100 

Peat below GWT 0.6 2300 

Peat above GWT 0.2 – 0.5 400 - 1850 

*

1 W/m-oK = 0.578 BTU/ft-hr-oF; 1 J/kg-oK = 2.388E-4 BTU/lb-oF 

5.6 ENVIRONMENT/CLIMATE INPUTS 

5.6.1 1993 AASHTO Guide 

There are only four environmental inputs in the 1993 AASHTO Guide: 

• Estimated seasonal variation of the subgrade resilient modulus MR (Section 5.4.3) 

• The category for the percentage of time that the unbound pavement materials are 

exposed to moisture conditions near saturation (Section 5.5.1) 

• The qualitative description of moisture supply for expansive subgrades (Section 

5.5.1) 

• The depth of frost penetration (Section 5.5.1) 

These environmental factors are intertwined with their associated material property inputs 

and have already been described in this chapter in the sections noted above. 

5.6.2 NCHRP 1-37A Design Guide 

Three sets of environmental inputs are required in the NCHRP 1-37A design methodology: 

• Climate, defined in terms of histories of key weather parameters 

• Groundwater depth 
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Geotechnical Aspects of Pavements 5 - 108 May 2006 

• Surface shortwave absorptivity 

These parameters are the inputs/boundary conditions for the calculation of climate-specific 

temperature and moisture distributions with depth and time in the EICM (see Appendix D). 

These distributions, in turn, are used to determine seasonal moisture contents and freeze-thaw 

cycles for the unbound pavement materials. 

Climate Inputs 

The seasonal damage and distress accumulation algorithms in the NCHRP 1-37A design 

methodology require hourly history data for five weather parameters: 

• Air temperature 

• Precipitation 

• Wind speed 

• Percentage sunshine (used to define cloud cover) 

• Relative humidity. 

The NCHRP 1-37A Design Guide recommends that the weather inputs be obtained from 

weather stations located near the project site. At least 24 months of actual weather station 

data are required for the computations. The Design Guide software includes a database of 

appropriate weather histories from nearly 800 weather stations throughout the United States. 

This database is accessed by specifying the latitude, longitude, and elevation of the project 

site. The Design Guide software locates the six closest weather stations to the site; the user 

selects a subset of these to create a virtual project weather station via interpolation of the 

climatic data from the selected physical weather stations. 

Specification of the weather inputs is identical at all the three hierarchical input levels in the 

NCHRP 1-37A Design Guide. 

Groundwater Depth 

The groundwater table depth is intended to be the best estimate of the annual average depth. 

Level 1 inputs are based on soil borings, while level 3 inputs are simple estimates of the 

annual or seasonal average values. A potential source for level 3 groundwater depth estimates 

is the county soil reports produced by the National Resources Conservation Service. There is 

no level 2 approach for this design input. 

It is important to recognize that groundwater depth can play a significant role in the overall 

accuracy of the foundation/pavement moisture contents and, hence, the seasonal modulus 

values. This is explored further in Chapter 6. Every attempt should be made to characterize 

groundwater depth as accurately as possible. 
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Surface Shortwave Absorptivity 

This last environmental input is a property of the AC or PCC surface layer. The 

dimensionless surface short wave absorptivity defines the fraction of available solar energy 

that is absorbed by the pavement surface. It depends on the composition, color, and texture of 

the surface layer. Generally speaking, lighter and more reflective surfaces tend to have lower 

short wave absorptivity. 

The NCHRP 1-37A recommendations for estimating surface shortwave absorptivity at each 

hierarchical input level are as follows: 

• Level 1—Determined via laboratory testing. However, although laboratory 

procedures exist for measuring shortwave absorptivity, there currently are no 

AASHTO protocols for this for paving materials. 

• Level 2—Not applicable. 

• Level 3—Default values as follows: 

o Weathered asphalt (gray) 0.80 – 0.90 

o Fresh asphalt (black) 0.90 – 0.98 

o Aged PCC layer 0.70 – 0.90 

Given the lack of suitable laboratory testing standards, level 3 values will typically be used 

for this design input. 

5.7 DEVELOPMENT OF DESIGN INPUTS 

Myth has it that an unknown structural engineer offered the following definition of his 

profession (Coduto, 2001): 

“Structural engineering is the art and science of molding materials we do not fully 

understand into shapes we cannot precisely analyze to resist forces we cannot 

accurately predict, all in such a way that the society at large is given no reason to 

suspect the extent of our ignorance.” 

This definition applies even more emphatically to pavement engineering. In spite of our 

many technical advances, there are still great gaps in our understanding. Often the greatest 

uncertainties in an individual project are with site conditions and materials—the types and 

conditions of materials encountered along the highway alignment, their spatial, temporal, and 

inherent variability, and their complex behavior under repeated traffic loading and 

environmental cycles. 
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Site investigation and testing programs often generate large amounts of data that can be 

difficult to synthesize. Real soil profiles are nearly always very complex, so borings often do 

not correlate and results from different tests may differ enormously. The development of a 

simplified representation of the soils and geotechnical conditions at a project site requires 

much interpolation and extrapolation of data, combined with sound engineering judgment. 

But what is engineering judgment? Ralph Peck suggested several alternative definitions 

(Dunnicliff and Deere, 1984): 

“To the engineering student, judgment often appears to be an ingredient said to be 

necessary for the solution of engineering problems, but one that the student can 

acquire only later in his career by some undefined process of absorption from his 

experience and his colleagues. 

“To the engineering scientist, engineering judgment may appear to be a crutch used 

by practicing engineers as a poor substitute for sophisticated analytical procedures. 

“To the practicing engineer, engineering judgment may too often be an impressive 

name for guessing rather than for rational thinking.”

Perhaps Webster’s New Collegiate Dictionary offers the definitive statement: 

Judgment: The operation of the mind, involving comparison and discrimination, 

by which knowledge of values and relations is mentally formulated. 

But when confronted with voluminous quantities of inconsistent—and often contradictory—

information, how does the pavement engineer compare and discriminate? What tools (or 

tricks) of the trade are available? This is a difficult process to describe. However, some 

common techniques for determining design values from site exploration and other 

geotechnical data are as follows: 

• Find and remove any obvious outliers in the data. Although there are statistical 

techniques for doing this (e.g., McCuen, 1993), in practice, detailed knowledge of the data 

plus engineering reasoning is usually sufficient for removing data outliers for cause. Table 

5-60 summarizes some typical ranges of variability for pavement design inputs; additional 

information on measured variability of geotechnical parameters can be found in Baecher and 

Christian (2003). However, it is important that outliers (e.g., a single low stiffness value) not 

be arbitrarily removed without fully evaluating the data for an explanation. A local anomaly 

may exist in the field, for example, that requires remediation. 
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Geotechnical Aspects of Pavements 5 - 111 May 2006 

• Examine spatial (and in some cases, temporal) trends in the data. Look at both the 

subsurface stratagraphic profiles and plan view “map” of subsurface conditions. Refer to the 

1993 AASHTO Design Guide for resolving spatial variations in pavement design data by 

defining homogeneous analysis units based on a “cumulative difference” approach (Figure 

5-40). A separate set of design inputs can then be developed for each homogeneous analysis 

unit, reducing the variability of measured vs. design input values within each unit. 

• Check whether the magnitudes and trends in the data pass the test of “engineering 

reasonableness” – e.g., are the values of the right order of magnitude? Are the trends in the 

data in the intuitively correct directions? 

• Examine the internal consistency of the data – e.g., are the phase relationships by volume 

consistent with the phase relationships by weight? 

• Use correlations among different types of data to strengthen data interpretation – e.g.,

statistical correlations between resilient modulus and CBR can be used to supplement a 

limited set of measured MR values (although today, laboratory resilient modulus tests can 

often be performed more quickly and less expensively than laboratory CBR tests—see Table 

5-61). 

• Be clear on what is needed for a design value. The value of a material property used for 

specification purposes may be different from the value of that same material property when 

used for design. For example, a conservative value (mean plus one or two standard 

deviations) may be specified for the minimum compressive strength of a lime stabilized 

subgrade for construction quality control specifications; the mean value would be more 

appropriate for design applications where overall reliability (e.g., factor of safety) is 

considered explicitly, as is the case in both the AASHTO and NCHRP design procedures. 

• Evaluate the sensitivity of the design to the inputs! This is perhaps the most important—

and often the most overlooked—aspect of design. Evaluating sensitivity to design inputs can 

have several benefits. First, it will categorize which inputs are most important and which are 

less important to the design. There is no need to expend large effort determining the precise 

design values for inputs that have little impact on the final outcome. More resources can then 

be allocated to determining the inputs that have significant impact on the outcome once they 

have been identified. Second, design sensitivity analyses can indicate the potential 

consequences of incorrect judgments of the design inputs. For example, if the subgrade 

resilient modulus is underestimated by 50%, will this reduce the expected useful life of the 

pavement by 1 year or 10 years? How does the increased cost of reduced pavement life 
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compare with the cost of additional exploration in order to establish the subgrade resilient 

modulus value more robustly? 

• When in doubt run more tests (a single test is often worth a thousand guesses). 

Figure 5-40. Variation of pavement response variable versus distance for given project 

(NCHRP 1-37A, 2004). 
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Table 5-60. Summary of typical pavement parameter variability (AASHTO, 1993). 

Property Standard Deviation 

 Low Average High 

Thickness (inches) 

 Portland cement concrete 0.1 0.3 0.5 

 Asphalt concrete 0.3 0.5 0.7 

 Cement treated base 0.5 0.6 0.7 

 Granular base 0.6 0.8 1.0 

 Granular subbase 1.0 1.2 1.5 

Strength 

 CBR (%) 

 Subgrade (4 – 7) 0.5 1.0 2.0 

 Subgrade (7 – 13) 1.0 1.5 2.5 

 Subgrade (13 – 20) 2.5 4.0 6.0 

 Granular subbase (20 – 30) 5.0 8.0 12.0 

 Granular base (80+) 10.0 15.0 30.0 

Percent compaction (%) 

 Embankment/subgrade 2.0 4.5 7.0 

 Subbase/base 2.0 2.8 3.5 

Portland cement concrete properties 

 Air content (%) 0.6 1.0 1.5 

 Slump (inches) 0.6 1.0 1.4 

 28-day compressive strength (psi) 400 600 800 

Asphalt concrete properties 

 Gradation (%) 

 3/4 or 1/2 inch 1.5 3.0 4.5 

 3/8 inch 2.5 4.0 6.0 

 No. 4 3.2 3.8 4.2 

 No. 40 or No. 50 1.3 1.5 1.7 

 No. 200 0.8 0.9 1.0 

 Asphalt content (%) 0.1 0.25 0.4 

 Percent compaction (%) 0.75 1.0 1.5 

 Marshall mix properties 

 Stability (lbs) 200 300 400 

 Flow (in./in.) 1.0 1.3 2.0 

 Air voids (%) 0.8 1.0 1.4 

 AC consistency 

 Pen @ 77o

F 2 10 18 

 Viscosity @ 149o

F (kilopoise) 2 25 100 

 

 Coefficient of Variation (%) 

 Low Average High 

Pavement deflection 15 30 45 
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Table 5-61. Resilient modulus versus CBR testing for fine grained subgrade soil 

(Boudreau Engineering, 2004, personal communication). 

Property CBR Resilient Modulus 

Sample size required 60 lbs (27 kg) 5 lbs (2.3 kg) 

Turnaround time 10 days 4 days 

Data value Empirical Mechanistic 

In-situ testing Field test Shelby tube - lab 

Unit price $365 $300 

5.8 EXERCISES 

Depending upon the number of groups in the class, one or more of the following exercises 

may be assigned. 

5.8.1 1993 AASHTO Design Guide—Flexible Pavements 

Small group exercise: Given the pavement information for the Main Highway in Appendix 

B, estimate appropriate material property inputs for the unbound materials in a flexible 

pavement structure as required for the 1993 AASHTO Design Guide. (A worksheet will be 

distributed to guide this exercise.) 

5.8.2 1993 AASHTO Design Guide—Rigid Pavements 

Small group exercise: Given the pavement information for the Main Highway in Appendix 

B, estimate appropriate material property inputs for the unbound materials in a rigid 

pavement structure as required by the 1993 AASHTO Design Guide. (A worksheet will be 

distributed to guide this exercise.) 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 115 May 2006 

5.9 REFERENCES 

AASHTO (1972). AASHTO Interim Guide for Design of Pavement Structures, American 

Association of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (1986). Guide for Design of Pavement Structures, American Association of State 

Highway and Transportation Officials, Washington, D.C. 

AASHTO (1993). AASHTO Guide for Design of Pavement Structures, American Association 

of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (2003). Standard Specifications for Transportation Materials and Methods of 

Sampling and Testing (23rd ed.), American Association of State Highway and Transportation 

Officials, Washington, D.C. 

Andrei, D. (1999). Development of a Harmonized Test Protocol for the Resilient Modulus of 

Unbound Materials Used in Pavement Design, Master of Science thesis, University of 

Maryland, College Park, MD.

Andrei, D. (2003). Development of a Predictive Model for the Resilient Modulus of Unbound 

Materials, Ph.D. dissertation, Arizona State University, Tempe, AZ.

Asphalt Institute (1982). “Research and Development of the Asphalt Institute’s Thickness 

Design Manual, Ninth Edition,” Research Report No. 82-2. 

Baecher, G.B., and Christian, J.T. (2003). Reliability and Statistics in Geotechnical 

Engineering, John Wiley and Sons, New York, NY. 

Barksdale, R.D., Alba, J., Khosla, P.N., Kim, R., Lambe, P.C. and Rahman, M.S. (1996). 

Laboratory Determination of Resilient Modulus for Flexible Pavement Design, Final Report, 

NCHRP Project 1-28, National Cooperative Highway Research Program, Transportation 

Research Board, National Research Council, Washington, D.C. 

Barksdale, R.D. (ed.) (2000). The Aggregate Handbook Plus Supplement, National Stone 

Association, Washington, D.C. 

Boudreau Engineering (2004) personal communication. 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 116 May 2006 

Burmister, D.M. (1943). “The Theory of Stresses and Displacements in Layered Systems and 

Applications to the Design of Airport Runways,” Highway Research Board, Vol. 23, pp. 

126-144. 

Carter, M., and Bentley, S.P. (1991). Correlations of Soil Properties, Pentech Press, London. 

Cheney, R.S., and Chassie, R.G. (1993). Soils and Foundations Workshop Manual. Federal 

Highway Administration, Washington, D.C.

Coduto, D.P. (2001). Foundation Design Principles and Practices (2nd ed.), Prentice-Hall, 

Englewood Cliffs, NJ. 

Coduto, D.P. (1999). Geotechnical Engineering Principles and Practices, Prentice-Hall, 

Englewood Cliffs, NJ. 

Dunnicliff, D., and Deere, D.U. (1984). Judgment in Geotechnical Engineering: The 

Professional Legacy of Ralph B. Peck, John Wiley and Sons, New York, NY. 

Fredlund, D.G, and Rahardjo, H. (1993). Soil Mechanics for Unsaturated Soils, John Wiley 

and Sons, New York, NY. 

Fredlund, D.G., and Xing, A. (1994). “Equations for the Soil Water Characteristic Curve,” 

Canadian Geotechnical Journal, Vol. 31, No. 4, pp. 521-532. 

Fredlund, D.G., Xing, A. and Huang, S. (1994). “Predicting the Permeability Function for

Unsaturated Soils Using the Soil-Water Characteristic Curve,” Canadian Geotechnical 

Journal, Vol 3., No. 4, pp. 533 - 546. 

Heukelom, W., and Klomp, A.J.G. (1962). “Dynamic Testing as a Means of Controlling 

Pavements During and After Construction,” Proceedings, First International Conference on 

the Structural Design of Asphalt Pavements, Ann Arbor, MI. 

Holtz, W.G., and Gibbs, H.J. (1956). “Engineering Properties of Expansive Clays,” 

Transactions ASCE, Vol. 121, pp. 641-677. 

Huang, Y.H. (1993). Pavement Design and Analysis, Prentice-Hall, Englewood Cliffs, NJ. 

LTTP (2003). Guide for Determining Design Resilient Modulus Values for Unbound 

Materials, interactive CD ROM (Version 1.1), Long Term Pavement Performance Program, 
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 117 May 2006 

Federal Highway Administration, U.S. Department of Transportation, Washington, D.C. 

Mallela, J., Larson, G.E., Wyatt, T., Hall, J.P. and Barker W. (2002). User’s Guide for 

Drainage Requirements in Pavements – DRIP 2.0 Microcomputer Program, FHWA Contract 

No. DTFH61-00-F-00199, July. 

Mayne, P.W. and Kulhawy, F.H. (1982). “Ko-OCR Relationships in Soil,” Journal of the 

Geotechnical Engineering Division, ASCE, Vol. 108, GT6, pp. 851-872. 

McCarthy, D.F. (2002). Essentials of Soil Mechanics and Foundations, 6th Edition, PrenticeHall, Englewood Cliffs, NJ. 

McCuen, R.H. (1993). Microcomputer Applications in Statistical Hydrology, Prentice-Hall, 

Inc., Englewood Cliffs, NJ. 

Mohammad, L.N., Titi, H.H. and Herath, A. (2002). Effect of Moisture Content and Dry Unit 

Weight on the Resilient Modulus of Subgrade Soils Predicted by Cone Penetration Test, 

Report No. FHWA/LA.00/355, Federal Highway Administration, U.S. Department of 

Transportation, Washington, D.C.

NAPA (1994). “Guidelines for Use of HMA Overlays to Rehabilitate PCC Pavements,”

Publication IS 117, National Asphalt Paving Association, Lanham, MD. 

NCHRP 1-37A (2004). Mechanistic-Empirical Design of New and Rehabilitated Pavement 

Structures, Final Report, NCHRP Project 1-37A, Transportation Research Board, National 

Research Council, Washington, D.C. 

Peck, R.B., Hansen, W.E. and Thornburn, T.H. (1974). Foundation Engineering, Wiley. 

Saeed, A., Hall, J.W. and Barker, W. (2001). Performance-Related Tests of Aggregates for 

Use in Unbound Pavement Layers, NCHRP Report 453, Transportation Research Board, 

National Research Council, Washington, D.C. 

Seed, H.B., Chan, C.K. and Lee, C.E. (1963). “Resilience Characteristics of Subgrade Soils 

and Their Relation to Fatigue Failures in Asphalt Pavements,” 1st International Conference 

on the Structural Design of Asphalt Pavements, Ann Arbor, MI. 

Seed, H.B., Chan, C.K. and Monismith, C.L. (1955). “Effects of Repeated Loading on the 

Strength and Deformation of Compacted Clay,” HRB Proceedings, No. 34, pp. 541-558.
FHWA NHI-05-037 Chapter 5 – Geotechnical Inputs 

Geotechnical Aspects of Pavements 5 - 118 May 2006 

Seed, H.B., and McNeill, R.L. (1956). “Soils Deformation in Normal Compression and 

Repeated Loading Tests,” HRB Bulletin 141.

Sowers, G.F. (1979). Introductory Soil Mechanics and Foundations, Macmillan, New York, 

NY. 

Sundberg, J. (1988) Thermal Properties of Soils and Rocks, Report No. 35, Swedish 

Geotechnical Institute, Linkoping, Sweden. 

Tseng, K. and Lytton, R. (1989). “Prediction of Permanent Deformation in Flexible 

Pavement Material,” Implication of Aggregates in the Design, Construction, and 

Performance of Flexible Pavements, ASTM STP 1016, ASTM, pp. 154-172.

U.S. Army Corps of Engineers (1953). “The Unified Soil Classification System,” Technical 

Memorandum 3-357, Waterways Experiment Station, Vicksburg, MS. 

Van Til, C.J., McCullough, B.F., Vallerga, B.A. and Hicks, R.G. (1972). Evaluation of 

AASHO Interim Guides for Design of Pavement Structures, NCHRP 128, Highway Research 

Board, Washington, D.C. 

Von Quintus, H.L. and Killingsworth, B. (1997a). Design Pamphlet for the Determination of 

Design Subgrade in Support of the 1993 AASHTO Guide for the Design of Pavement 

Structures, Report No. FHWA-RD-97-083, FHWA, Washington, D.C. 

Von Quintus, H.L. and Killingsworth, B. (1997b). Design Pamphlet for the Determination of 

Layered Elastic Moduli for Flexible Pavement Design in Support of the 1993 AASHTO 

Guide for the Design of Pavement Structures, Report No. FHWA-RD-97-077, Federal 

Highway Administration, Department of Transportation, Washington, DC. 

Von Quintus, H.L. and Killingsworth, B. (1998). Analysis Relating to Pavement Material 

Characterizations and Their Effects of Pavement Performance, Report No. FHWA-RD-97-

085, Federal Highway Administration, Department of Transportation, Washington, DC. 

Witczak, M.W. (2004). Harmonized Test Methods for Laboratory Determination of Resilient 

Modulus for Flexible Pavement Design, NCHRP Project 1-28A Final Report, Transportation 

Research Board, National Research Council, Washington, D.C. 

Yoder, E.J., and Witczak, M.W. (1975). Principles of Pavement Design, John Wiley and 

Sons, New York, NY. 
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Geotechnical Aspects of Pavements 6 - 1 May 2006

CHAPTER 6.0 

PAVEMENT STRUCTURAL DESIGN AND PERFORMANCE 

6.1 INTRODUCTION 

Previous chapters have described in qualitative terms the many geotechnical factors 

influencing pavement design and performance, the wide range of geotechnical properties 

required as input to the design procedures, and the various methods for determining the 

values of these geotechnical inputs. Now it is time to evaluate quantitatively the importance 

of these factors and properties. These are the primary objectives of the present chapter: 

1. To illustrate via examples how the geotechnical properties described in Chapter 5 are 

incorporated in the pavement design calculations; and 

2. To highlight the effects of the geotechnical factors and inputs on pavement design 

and performance. 

These objectives will be met through a series of design scenarios. First, a set of reference or 

baseline flexible and rigid pavement designs are developed for a hypothetical and simple 

project scenario. Then, the effects of various deviations from the baseline conditions will be 

investigated and quantified. These include 

• Soft/weak subgrade conditions 

• Subgrade stabilization 

• Low quality base/subbase material 

• Drainage and water conditions 

• Shallow bedrock conditions 

The design scenarios are intentionally highly idealized and simplified. Their point is to 

emphasize in quantitative terms how changes in geotechnical inputs affect the overall 

pavement design and performance. In a sense, these design scenarios are examples of the 

types of sensitivity studies one should perform during design to evaluate the importance of 

the various design inputs, especially with reference to the quality of the information used in 

their estimation. 

All of the design scenarios described in this chapter are for new construction (or 

reconstruction). This is not to minimize the importance of rehabilitation design; as described 

in Chapter 1, most pavement design today is in fact for rehabilitation and not new 

construction. However, most structural rehabilitation designs focus on restoration of the 

surface layer, either through asphalt concrete overlays, concrete pavement restoration, or a 
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combination of the two. For these types of scenarios, the geotechnical inputs are essentially 

the same as for new construction design – e.g., a subgrade is a subgrade whether it is beneath 

a new or an existing pavement. The principal things that change are the methods by which 

the geotechnical inputs are determined – e.g., MR backcalculated from FWD tests instead of 

measured in the laboratory. The ways that these geotechnical inputs are used in the design 

calculations and the effects that they have on the design pavement structure are similar for 

new construction, rehabilitation, and reconstruction designs. 

Both the current 1993 AASHTO Design Guide and the forthcoming new design guide from 

NCHRP Project 1-37A are applied to the scenarios in this chapter. Summaries of each of 

these design procedures are provided in Appendices C and D, respectively. Calculations for 

the 1993 AASHTO Guide designs are based on simple spreadsheet evaluation of the flexible 

and rigid pavement design equations. The calculations for the mechanistic-empirical designs 

were performed using Final Report Release version 0.700 (4/7/2004) of the NCHRP 1-37A 

software. This is the final version of the software as submitted to NCHRP at the conclusion 

of Project 1-37A. 

It is important to keep in mind the significant differences between the two design procedures. 

The 1993 AASHTO Design Guide is an empirical methodology in which typically the design 

traffic, environmental conditions, and maximum serviceability (performance) loss are 

specified, and the corresponding required pavement structure—typically described just in 

terms of layer thicknesses—is determined. The NCHRP 1-37A procedure is a mechanisticempirical methodology in which the design traffic, environmental conditions, and pavement 

structure are specified, and the corresponding pavement performance vs. time is predicted. In 

the NCHRP 1-37A procedure, several trial designs generally need to be evaluated in an 

iterative fashion in order to find the one (or more than one) that meets the design 

performance requirements. 

Typically, there are multiple pavement designs that can provide the required performance for 

any scenario. This is true for both the 1993 AASHTO and NCHRP 1-37A design 

methodologies. The final selection of the “best” design should be based upon life-cycle costs, 

constructability, and other issues. Crude economic evaluations can be made in terms of initial 

construction costs, although even this is difficult because of large region-to-region variations 

in unit costs. 

One final note regarding units of measure in this chapter: FHWA policy is to report values in 

SI units in all reports, with the corresponding U.S. Customary equivalent in parentheses. This 

is not done here. All values for the design examples in this chapter are reported in U.S. 

Customary units. There are two important practical reasons that dictate this choice. First, the 
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structural layer coefficients, empirical correlations, and other data in the 1993 AASHTO 

Design Guide were developed and are presented in U.S. Customary units only. Although 

many of these could be converted to SI units, the consequences would be confusing. For 

example, expressing flexible pavement layer thicknesses in millimeters would require 

changing the values of the structural layer coefficients to quantities that would be unfamiliar 

to pavement engineers (e.g., the a1 value for asphalt concrete would change from 0.44 to 

0.017 if asphalt thickness were expressed in millimeters). Second, although an SI version of 

the NCHRP 1-37A software is planned, at the time of this writing, only the U.S. Customary 

version is available. All of the outputs from the software are expressed in U.S. Customary 

units, and, consequently, the inputs described in this report are left in U.S. Customary units 

as well for consistency. The general SI-U.S. Customary conversion table included at the 

beginning of this reference manual can be used, if necessary, for converting units in this 

chapter. 

6.2 BASELINE DESIGNS 

The baseline designs for flexible and rigid pavements are intended to provide very simple 

and ordinary reference cases that can be used as the basis for subsequent exploration of the 

effects of various geotechnical inputs. The design scenario is based on the following 

assumptions: 

• New construction 

• Simple pavement structure 

o Hot mix asphalt concrete (HMA) over crushed stone graded aggregate base 

(GAB) over subgrade (SG) for flexible pavements 

o Jointed plain concrete pavement (JPCP) over graded aggregate base (GAB) 

over subgrade (SG) for rigid pavements 

• Excellent, non-erodable base material (AASHTO A-1-a crushed stone) 

• Nonexpansive subgrade 

• Benign environmental conditions – e.g., no frost heave/thaw or expansive soils 

• Good drainage 

• Simple traffic conditions – e.g., no traffic growth over design period 

Reference pavement designs consistent with these assumptions have been developed for a 

hypothetical new arterial highway outside of College Park, MD. The roadway is assumed to 

have two lanes in each direction and significant truck traffic consistenting primarily of Class 

9 5-axle tractor-trailer units. The subgrade conditions are a non-expansive silty clay subgrade 

(AASHTO A-7-5/USCS MH material), a deep groundwater table, and no shallow bedrock. 

Environmental conditions in the Mid-Atlantic region are mild, so frost heave/thaw is not a 
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design issue, and seasonal variations of the unbound material properties are expected to be 

minor. 

Table 6-1 provides some typical in-place initial construction unit costs for paving materials 

in Maryland. These costs will be used for rough economic evaluations of the designs 

developed in this chapter. 

Table 6-1. Typical in-place unit material costs for use in example design problems 

(MDSHA, 2002). 

Material Reasonable 

Range 

Typical 

Unit Price Typical Unit Price 

Hot mix asphalt concrete 

(12.5 mm PG 64-22) $30-$50/ton $36/ton $14,250/lane-mi-in 

Portland cement concrete (PCC) 

without steel $110-$180/cy $144/cy $28,200/lane-mi-in 

Graded aggregate base $24-$60/cy $42/cy $8,200/lane-mi-in 

6.2.1 1993 AASHTO Design 

Flexible Pavement 

The baseline flexible pavement design is a three-layer system consisting of an asphalt 

concrete (AC) surface layer over a nonstabilized graded aggregate base (GAB) layer over 

subgrade (SG). The input parameters for the baseline design using the 1993 AASHTO 

flexible procedure for new pavements are summarized in Table 6-2. Refer to Chapter 5 for 

detailed explanations of all input parameters and the methods available for their 

determination. 

The methodology by which the input parameters in Table 6-2 are used to determine the final 

structural design in the 1993 AASHTO Guide is described in Appendix C. The calculations 

are sufficiently straightforward that they can be easily performed using a spreadsheet. The 

key output from the 1993 AASHTO design methodology is the required pavement structure, 

which is determined as follows: 

• Required overall structural number SN = 4.61 

• Required structural number for asphalt concrete surface layer SN1 = 2.35 
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• Required minimum thickness of asphalt 1

1

1

5.3 SN D

a = = inches1

• Remaining structural number required for granular base layer SN2 = 1 1 SN D a − = 2.28

• Required thickness of granular base 2

2

2 2

12.7 SN D

m a = = inches1

Since the ratio of the layer coefficients ( ) 1 2 a a/ 0.44 / 0.18 2.44 = = is greater than the ratio of 

the associated in-place unit costs per lane-mile-inch of thickness in Table 6-1 

( ) $14,250 / $8,200 1.74 = , there is an economic benefit from substituting granular base 

thickness with additional asphalt in this design – i.e., replacing 2.4 inches of granular base 

with an additional 1.0 inch of asphalt concrete is both structurally feasible (at least in terms 

of the 1993 AASHTO Guide) and economically beneficial (at least in terms of initial 

construction costs) since it would result in a savings of about $5400 per lane mile at the same 

SN value. However, in order to avoid complicating comparisons between the various design 

scenarios later in this chapter, the baseline flexible pavement structure will be kept at 5.3 

inches of AC over 12.7 inches of GAB (or 5.5 inches of AC over 13 inches of GAB after 

rounding). 

Although the 15-year initial service life specified for this scenario is typical for flexible 

pavements and equal to the values used in the design examples in the 1993 AASHTO Guide, 

current trends are toward longer life or “perpetual” pavement designs. The required 

pavement section for a 30-year initial service life based on the 1993 AASHTO Guide is 6.0 

inches of AC over 13.7 inches of GAB. The “premium” for an additional 15 years of 

pavement life is thus only about three-quarters of an inch of asphalt and one inch of crushed 

stone base. 

 

1

The 1993 AASHTO Design Guide recommends rounding the asphalt layer thickness to the nearest half inch 

and unbound layer thicknesses to the nearest inch. However, all layer thicknesses are rounded to the nearest 0.1 

inch in this chapter to make the comparisons between the various scenarios more meaningful. 
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Table 6-2. Input parameters for 1993 AASHTO flexible pavement baseline design. 

Input Parameter Design Value Notes 

Initial service life 15 years 1 

Traffic (W18) 6.1x106 ESALs 2 

Reliability 90% 3 

Reliability factor (ZR) -1.282 

Overall standard error (So) 0.45 1 

Allowable serviceability deterioration (∆PSI) 1.7 4 

Subgrade resilient modulus (MR) 7,500 psi 5 

Granular base type AASHTO A-1-a 1 

Granular base layer coefficient (a2) 0.18 6 

Granular base drainage coefficient (m2) 1.0 7 

Asphalt concrete layer coefficient (a1) 0.44 1 

 Notes: 

1. Typical value for flexible pavement design. 

2. Consistent with more detailed traffic input in the NCHRP 1-37A design (Section 6.2.2). 

3. Typical value for a principal arterial (AASHTO, 1993). 

4. Typical value for flexible pavements. No serviceability reduction for swelling or frost heave. 

5. Consistent with the Level 3 default input for this soil class in the NCHRP 1-37A design after 

 adjustment for seasonal effects (Section 6.2.2). 

6. Corresponds to an MR value of 40,000 psi, which is consistent with the Level 3 default input for 

 this soil class in the NCHRP 1-37A design (Section 6.2.2). 

7. Representative of good drainage and moderate (5-25%) saturation conditions; matches value 

 typically used by the Maryland State Highway Administration for design. 

 

Rigid Pavement 

The baseline rigid pavement design is a three layer JPCP system consisting of a Portland 

cement concrete (PCC) slab over a nonstabilized graded aggregate base (GAB) layer over 

subgrade (SG). The input parameters for the baseline design using the 1993 AASHTO rigid 

procedure for new pavements are summarized in Table 6-3. Refer to Chapter 5 for detailed 

explanations of all input parameters and the methods available for their determination. The 

rigid pavement design inputs are consistent with those used for the baseline flexible 

pavement design. 

The methodology by which the input parameters in Table 6-3 are used to determine the final 

structural design in the 1993 AASHTO Guide is described in Appendix C. The calculations 

are sufficiently straightforward that they can be easily performed using a spreadsheet. The 

key output from the 1993 AASHTO design methodology is the required pavement structure. 
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This is determined from the input parameters in Table 6-3 and the following additional 

intermediate steps: 

• Assume a 6-inch design thickness for the granular subbase. This value is typical for 

rigid pavements on reasonably competent subgrades. 

• Determine the composite modulus of subgrade reaction k∞ representing the combined 

stiffness of the subgrade and the subgrade layer. For a 6-inch subbase thickness and 

unbound moduli as given in Table 6-3, k∞ = 423 pci. 

• Correct k∞ for loss of support to determine the design modulus of subgrade reaction 

keff. (NOTE: No shallow bedrock correction is required for the assumed subgrade 

conditions.) For k∞ = 423 pci and LS = 2, keff = 38 pci. 

• Determine the required slab thickness D = 10.4 inches from the rigid pavement design 

equation.2

 

The final design selection based upon the design inputs in Table 6-3 is therefore a 10.4 inch 

PCC slab over 6 inches of GAB (or 10.5 inches of PCC over 6 inches of GAB after 

rounding). 

 

2

 The 1993 AASHTO Design Guide recommends rounding the slab thickness to the nearest inch (nearest half 

inch if controlled grade slip form pavers are used). However, all slab thicknesses are rounded to the nearest 0.1 

inch in this chapter to make the comparisons between the various scenarios more meaningful. 
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Table 6-3. Input parameters for 1993 AASHTO rigid pavement baseline design. 

Input Parameter Design Value Notes 

Initial service life 25 years 1 

Traffic (W18) 16.4x106 ESALs 2 

Reliability 90% 3 

Reliability factor (ZR) -1.282 

Overall standard error (So) 0.35 1 

Allowable serviceability deterioration (∆PSI) 1.9 4 

Terminal serviceability level (pt) 2.5 1 

Subgrade resilient modulus (MR) 7,500 psi 5 

Granular subbase type AASHTO A-1-a 1 

Granular subbase resilient modulus (ESB) 40,000 psi 6 

Drainage coefficient (Cd) 1.0 7 

Loss of Support (LS) 2.0 8 

PCC modulus of rupture (Sc') 690 psi 1 

PCC modulus of elasticity (Ec) 4.4x106

 psi 1 

Joint load transfer coefficient (J) 2.8 9 

 Notes: 

1. Typical value for rigid pavement design. 

2. Consistent with more detailed traffic input in the NCHRP 1-37A design (Section 6.2.2). 

3. Typical value for a principal arterial (AASHTO, 1993). 

4. Typical value for rigid pavements. No serviceability reduction for swelling or frost heave. 

5. Consistent with the Level 3 default input for this soil class in the NCHRP 1-37A design after 

 adjustment for seasonal effects (Section 6.2.2). 

6. Consistent with the Level 3 default input for this soil class in the NCHRP 1-37A design 

 (Section 6.2.2). 

7. Representative of good drainage and moderate (5-25%) saturation conditions. 

8. Within AASHTO-recommended range for unbound granular materials. 

9. Typical value for JPCP with tied PCC shoulders and dowelled joints. 
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6.2.2 NCHRP 1-37A Design 

Flexible Pavement 

Consistent with the 1993 AASHTO design, the baseline flexible pavement structure for the 

NCHRP 1-37A design methodology is a three-layer new construction consisting of an asphalt 

concrete (AC) surface layer over a nonstabilized graded aggregate base (GAB) layer over 

subgrade (SG). However, the input parameters required for the NCHRP 1-37A methodology 

are considerably more extensive than those for the 1993 AASHTO Design Guide. The 

NCHRP 1-37A design inputs are summarized in Table 6-4. Refer to Chapter 5 for detailed 

explanations of all input parameters and to Appendix D for a summary of the NCHRP 1-37A 

design procedure. All of the inputs correspond to Level 3 quality, and the default values 

provided within the NCHRP 1-37A software are used wherever appropriate. 

The NCHRP 1-37A procedure requires the evaluation of several trial pavement sections in 

order to find the design that best meets the performance requirements. The baseline pavement 

structure from the 1993 AASHTO design procedure can be conveniently taken as the initial 

trial section. The predicted rutting performance for a trial section corresponding to the 1993 

AASHTO design of 5.3 inches of AC over 12.7 inches of GAB is shown in Figure 6-1; total 

rutting (after adjustment for reliability) at the end of the 15-year initial service life is 0.646 

inches. Fatigue and thermal cracking are negligible for this design scenario, and rutting is the 

controlling distress type. The design limit for predicted total rutting is an explicit input in the 

NCHRP 1-37A procedure that would, in general, be set by individual agency policy. For the 

examples in this chapter, however, the design limit for total rutting is taken as the predicted 

rutting for the 1993 AASHTO design section in order to make the 1993 AASHTO and 

NCHRP 1-37A designs equivalent for the baseline conditions. The design limit for total 

rutting (after adjustment for reliability and rounding) is thus 0.65 inches, which is slightly 

less than the 0.75 inch default value in the NCHRP 1-37A software. 
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Table 6-4. Input parameters for NCHRP 1-37A flexible pavement baseline design. 

Input Parameter Design Value Notes 

General Information

 Design life 15 years 1 

 Base/subbase construction month September 2 

 Pavement construction month September 2 

 Traffic open month October 2 

Site/Project Identification

 Functional class Principal Arterials - Others 

Analysis Parameters

 Initial IRI 63 in./mi 2 

 Terminal IRI 172 in./mi 2 

 Alligator cracking limit 25% 2 

 Total rutting limit 0.65 in See text 

 Reliability 90% 

Traffic

 Initial two-way AADTT 2000 

 Number of lanes in design direction 2 

 Percent of trucks in design direction 50% 2 

 Percent of trucks in design lane 95% 2 

 Operational speed 55 mph 2 

 Monthly adjustment 1.0 throughout 2 

 Vehicle class distribution Level 3 defaults Table 6-5 

 Hourly distribution Level 3 default Table 6-6 

 Traffic growth factor 0% 

 Axle load distribution factors Level 3 defaults Table 6-7 

 Mean wheel location from edge 18 in 2 

 Traffic wander standard deviation 10 in 2 

 Design lane width 12 ft 2 

 Number of axles per truck Level 3 defaults Table 6-8 

 Average axle outside width 8.5 ft 2 

 Dual tire spacing 12 in 2 

 Tire pressure 120 psi 2 

 Tandem axle spacing 51.6 in 2 

 Tridem axle spacing 49.2 in 2 

 Quad axle spacing 49.2 2 

Climate

 Latitude 38.98o 

 Longitude -76.94o 

 Elevation 48 ft 

 Depth of water table 20 ft 
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Input Parameter Design Value Notes 

 College Park, MD climate data Generated 3 

Thermal Cracking

 Average AC tensile strength at 14o

F 366.5 psi 4 

 Creep test duration 100 sec 4 

 Creep compliance Level 3 defaults 4 

 Mixture VMA 14.1% 5 

 Aggregate coefficient of thermal contraction 5x10-6/

o

F 4 

Drainage and Surface Properties

 Surface shortwave absorptivity 0.85 2 

 Infiltration n/a 2 

 Drainage path length n/a 2 

 Pavement cross slope n/a 2 

AC Surface Layer

 Cumulative % retained on 3/4 inch sieve 4 5 

 Cumulative % retained on 3/8 inch sieve 39 5 

 Cumulative % retained on #4 sieve 59 5 

 % passing #200 sieve 3 5 

 Asphalt binder grade PG 64-22 5 

 Reference temperature 70o

F 2 

 Effective binder content 10.1% 5 

 Air voids 4.0% 5 

 Total unit weight 151 pcf 5 

 Poisson's ratio 0.35 2 

 Thermal conductivity 0.67 BTU/hr-ft-o

F 2 

 Heat capacity 0.23 BUT/lb-o

F 2 

Granular Base Layer

 Unbound material type AASHTO A-1-a 

 Analysis type ICM Inputs 

 Poisson's ratio 0.35 2 

 Coefficient of lateral pressure K0 0.5 2 

 Modulus 40,000 psi 2,6 

 Plasticity index 1% 

 % passing #200 sieve 3 

 % passing #4 sieve 20 

D60 8 mm 

 Compaction state Compacted 2 

 Maximum dry unit weight 122.2 pcf 2 

 Specific gravity of solids 2.66 2 

 Saturated hydraulic conductivity 263 ft/hr 2 

 Optimum gravimetric water content 11.1% 2 

 Calculated degree of saturation 82% 2 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 12 May 2006

Input Parameter Design Value Notes 

 SWCC parameter af 11.1 psi 2 

 SWCC parameter bf 1.83 2 

 SWCC parameter cf 0.51 2 

 SWCC parameter hr 361 psi 2 

Compacted Subgrade (top 6 inches)

 Unbound material type AASHTO A-7-5 

 Analysis type ICM Inputs 

 Poisson's ratio 0.35 2 

 Coefficient of lateral pressure K0 0.5 2 

 Modulus 12,000 psi 2,6 

 Plasticity index 30% 2 

 % passing #200 sieve 85 2 

 % passing #4 sieve 99 2 

 D60 0.01 mm 2 

 Compaction state Compacted 

 Maximum dry unit weight 97.1 pcf 2 

 Specific gravity of solids 2.75 2 

 Saturated hydraulic conductivity 3.25x10-5 ft/hr 2 

 Optimum gravimetric water content 24.8% 2 

 Calculated degree of saturation 88.9% 2 

 SWCC parameter af 301 psi 2 

 SWCC parameter bf 0.995 2 

 SWCC parameter cf 0.732 2 

 SWCC parameter hr 1.57x104

 psi 2 

 Natural Subgrade (beneath top 6 inches)

 Unbound material type AASHTO A-7-5 

 Compaction state Uncompacted 

 Maximum dry unit weight 87.4 pcf 2 

 (other properties same as for compacted subgrade) 

Distress Potential

 Block cracking None 2 

 Sealed longitudinal cracks outside wheel 

 path None 2 

Notes: 

1. Typical initial service life for flexible pavement design. 

2. Level 3 default/calculated/derived value from NCHRP 1-37A software. 

3. Based on interpolated climate histories at IAD, DCA, and BWI airports. 

4. Level 3 default/calculated/derived values from NCHRP 1-37A software for baseline AC mixture 

properties. Thermal cracking is not expected for the baseline design. However, these values are 

included here because they will be used in subsequent design scenarios. 

5. Based on a Maryland State Highway Administration 19.0mm Superpave mix design. 

6. Default input value at optimum moisture and density conditions before adjustment for seasonal effects 

(adjustment performed internally within the NCHRP 1-37A software). 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 13 May 2006

Table 6-5. AADTT distribution by truck class 

(Level 3 defaults for Principal Arterials – Others). 

1.3%

8.5%

2.8%

0.3%

7.6%

74.0%

1.2%

3.4%

0.6%

0.3%

Class 4

Class 5

Class 6

Class 7

Class 8

Class 9

Class 10

Class 11

Class 12

Class 13

Table 6-6. Hourly truck traffic distribution 

(Level 3 defaults for Principal Arterials – Others).

Midnight 2.3% Noon 5.9%

1:00 am 2.3% 1:00 pm 5.9%

2:00 am 2.3% 2:00 pm 5.9%

3:00 am 2.3% 3:00 pm 5.9%

4:00 am 2.3% 4:00 pm 4.6%

5:00 am 2.3% 5:00 pm 4.6%

6:00 am 5.0% 6:00 pm 4.6%

7:00 am 5.0% 7:00 pm 4.6%

8:00 am 5.0% 8:00 pm 3.1%

9:00 am 5.0% 9:00 pm 3.1%

10:00 am 5.9% 10:00 pm 3.1%

11:00 am 5.9% 11:00 pm 3.1%

By period beginning:
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 14 May 2006

Table 6-7. Truck axle load distributions: Percentage of axle loads by truck class for 

single axle configurations (Level 3 defaults). 

 Truck Class 

Axle Weight (lbs) 4 5 6 7 8 9 10 11 12 13 

3000 1.80 10.05 2.47 2.14 11.65 1.74 3.64 3.55 6.68 8.88 

4000 0.96 13.21 1.78 0.55 5.37 1.37 1.24 2.91 2.29 2.67 

5000 2.91 16.42 3.45 2.42 7.84 2.84 2.36 5.19 4.87 3.81 

6000 3.99 10.61 3.95 2.70 6.99 3.53 3.38 5.27 5.86 5.23 

7000 6.80 9.22 6.70 3.21 7.99 4.93 5.18 6.32 5.97 6.03 

8000 11.47 8.27 8.45 5.81 9.63 8.43 8.35 6.98 8.86 8.10 

9000 11.30 7.12 11.85 5.26 9.93 13.67 13.85 8.08 9.58 8.35 

10000 10.97 5.85 13.57 7.39 8.51 17.68 17.35 9.68 9.94 10.69 

11000 9.88 4.53 12.13 6.85 6.47 16.71 16.21 8.55 8.59 10.69 

12000 8.54 3.46 9.48 7.42 5.19 11.57 10.27 7.29 7.11 11.11 

13000 7.33 2.56 6.83 8.99 3.99 6.09 6.52 7.16 5.87 7.32 

14000 5.55 1.92 5.05 8.15 3.38 3.52 3.94 5.65 6.61 3.78 

15000 4.23 1.54 3.74 7.77 2.73 1.91 2.33 4.77 4.55 3.10 

16000 3.11 1.19 2.66 6.84 2.19 1.55 1.57 4.35 3.63 2.58 

17000 2.54 0.90 1.92 5.67 1.83 1.10 1.07 3.56 2.56 1.52 

18000 1.98 0.68 1.43 4.63 1.53 0.88 0.71 3.02 2.00 1.32 

19000 1.53 0.52 1.07 3.50 1.16 0.73 0.53 2.06 1.54 1.00 

20000 1.19 0.40 0.82 2.64 0.97 0.53 0.32 1.63 0.98 0.83 

21000 1.16 0.31 0.64 1.90 0.61 0.38 0.29 1.27 0.71 0.64 

22000 0.66 0.31 0.49 1.31 0.55 0.25 0.19 0.76 0.51 0.38 

23000 0.56 0.18 0.38 0.97 0.36 0.17 0.15 0.59 0.29 0.52 

24000 0.37 0.14 0.26 0.67 0.26 0.13 0.17 0.41 0.27 0.22 

25000 0.31 0.15 0.24 0.43 0.19 0.08 0.09 0.25 0.19 0.13 

26000 0.18 0.12 0.13 1.18 0.16 0.06 0.05 0.14 0.15 0.26 

27000 0.18 0.08 0.13 0.26 0.11 0.04 0.03 0.21 0.12 0.28 

28000 0.14 0.05 0.08 0.17 0.08 0.03 0.02 0.07 0.08 0.12 

29000 0.08 0.05 0.08 0.17 0.05 0.02 0.03 0.09 0.09 0.13 

30000 0.05 0.02 0.05 0.08 0.04 0.01 0.02 0.06 0.02 0.05 

31000 0.04 0.02 0.03 0.72 0.04 0.01 0.03 0.03 0.03 0.05 

32000 0.04 0.02 0.03 0.06 0.12 0.01 0.01 0.04 0.01 0.08 

33000 0.04 0.02 0.03 0.03 0.01 0.01 0.02 0.01 0.01 0.06 

34000 0.03 0.02 0.02 0.03 0.02 0.01 0.01 0.00 0.01 0.02 

35000 0.02 0.02 0.01 0.02 0.02 0.00 0.01 0.00 0.00 0.01 

36000 0.02 0.02 0.01 0.02 0.01 0.01 0.00 0.00 0.00 0.01 

37000 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.01 0.01 

38000 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.02 0.01 0.01 

39000 0.01 0.00 0.01 0.01 0.01 0.00 0.01 0.01 0.00 0.01 

40000 0.01 0.00 0.01 0.01 0.00 0.00 0.04 0.02 0.00 0.00 

41000 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

Total % 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 15 May 2006

Table 6-8. Truck axle distribution (Level 3 defaults). 

1.62 0.39 0.00 0.00 

2.00 0.00 0.00 0.00 

1.02 0.99 0.00 0.00 

1.00 0.26 0.83 0.00 

2.38 0.67 0.00 0.00 

1.13 1.93 0.00 0.00 

1.19 1.09 0.89 0.00 

4.29 0.26 0.06 0.00 

3.52 1.14 0.06 0.00 

2.15 2.13 0.35 0.00 

Class 11

Class 12

Class 13

Class 7

Class 8

Class 9

Class 10

Quad 

Axle

Class 4

Class 5

Class 6

Vehicle 

Class

Single 

Axle

Tandem 

Axle

Tridem 

Axle

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 18 36 54 72 90 108 126 144 162 180 198

Pavement Age (month)

Rutting Depth (in)

TotalRutReliability

Total Rutting

SubTotalAC

SubTotalSG

SubTotalBase

Figure 6-1. Predicted rutting performance for NCHRP 1-37A baseline flexible pavement 

design. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 16 May 2006

Rigid Pavement 

Consistent with the 1993 AASHTO design, the baseline rigid pavement structure for the 

NCHRP 1-37A design methodology is a three-layer JPCP construction consisting of a 

Portland cement concrete (PCC) slab over a nonstabilized graded aggregate base (GAB) over 

subgrade (SG). However, the input parameters required for the NCHRP 1-37A methodology 

are considerably more extensive than for the 1993 AASHTO Design Guide. The NCHRP 

1-37A design inputs are summarized in Table 6-9. Refer to Chapter 5 for detailed 

explanations of all input parameters and to Appendix D for a summary of the NCHRP 1-37A 

design methodology. All of the inputs correspond to Level 3 quality, and the default values 

provided within the NCHRP 1-37A software are used wherever appropriate. 

The NCHRP 1-37A procedure requires the evaluation of several trial pavement sections in 

order to find the design that best meets the performance requirements. The baseline pavement 

structure from the 1993 AASHTO design procedure can be conveniently taken as the initial 

trial section. The predicted faulting performance for a trial section corresponding to the 1993 

AASHTO Design section of 10.4 inches of PCC over 6.0 inches of GAB is shown in Figure 

6-2; total faulting (after adjustment for reliability) at the end of the 25-year initial service life 

is 0.117 inches. Transverse fatigue cracking is negligible for this design scenario, and 

faulting is the controlling distress type. The design limit for predicted faulting, which is an 

explicit input in the NCHRP 1-37A procedure, would in general be set by individual agency 

policy. For the examples in this chapter, however, the design faulting limit is taken as the 

predicted faulting for the 1993 AASHTO design section. This is done in order to make the 

1993 AASHTO and NCHRP 1-37A designs equivalent for the baseline conditions. The 

design limit for faulting (after adjustment for reliability and rounding) is thus 0.12 inches, 

which coincidentally equals the default value in the NCHRP 1-37A software. Note that initial 

service life for the baseline rigid pavement is 25.5 years after rounding of the faulting limit. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 17 May 2006

Table 6-9. Input parameters for NCHRP 1-37A rigid pavement baseline design. 

Input Parameter Design Value Notes 

General Information

 Initial service life 25 years 1 

 Pavement construction month September 2 

 Traffic open month October 2 

Site/Project Identification

 Functional class Principal Arterials - Others 

Analysis Parameters

 Initial IRI 63 in./mi 2 

 Terminal IRI 172 in./mi 2 

 Transverse cracking (% slabs cracked) 15% 2 

 Mean joint faulting 0.12 in See text 

 Reliability 90% 

Traffic

 Initial two-way AADTT 2000 

 Number of lanes in design direction 2 

 Percent of trucks in design direction 50% 2 

 Percent of trucks in design lane 95% 2 

 Operational speed 55 mph 2 

 Monthly adjustment 1.0 throughout 2 

 Vehicle class distribution Level 3 defaults Table 6-5 

 Hourly distribution Level 3 defaults Table 6-6 

 Traffic growth factor 0% 

 Axle load distribution factors Level 3 defaults Table 6-7 

 Mean wheel location from edge 18 in 2 

 Traffic wander standard deviation 10 in 2 

 Design lane width 12 ft 2 

 Number of axles per truck Level 3 defaults Table 6-

8Table 6-8 

 Average axle outside width 8.5 ft 2 

 Dual tire spacing 12 in 2 

 Tire pressure 120 psi 2 

 Tandem axle spacing 51.6 in 2 

 Tridem axle spacing 49.2 in 2 

 Quad axle spacing 49.2 2 

 Wheelbase spacing Level 3 defaults Table 6-10 

Climate

 Latitude 38.98o 

 Longitude -76.94o 

 Elevation 48 ft 

 Depth of water table 20 ft 

 College Park, MD climate data Generated 3 

Design Features

 Permanent curl/warp effective -10o

F 2 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 18 May 2006

Input Parameter Design Value Notes 

 temperature difference 

 Joint spacing 15 ft 2 

 Dowel bar diameter 1 in 2 

 Dowel bar spacing 12 in 2 

 Edge support Widened slab 

 Slab width 14 ft 

 Bond at PCC-base interface Unbonded 

 Base erodibility index 4 (Fairly Erodable) 

Drainage and Surface Properties

 Surface shortwave absorptivity 0.85 2 

 Infiltration Minor (10%) 2 

 Drainage path length 12 ft 2 

 Pavement cross slope 2% 2 

PCC Surface Layer

 Unit weight 150 pcf 2 

 Poisson's ratio 0.2 2 

 Coefficient of thermal expansion 5.5x10-6/

o

F 2 

 Thermal conductivity 1.25 BTU/hr-ft-o

F 2 

 Heat capacity 0.28 BTU/lb-o

F 2 

 Cement type Type 1 2 

 Cement content 600 lb/yd3

 2 

 Water/cement ratio 0.42 2 

 Aggregate type Limestone 2 

 PCC zero-stress temperature 120 o

F 4 

 Ultimate shrinkage at 40% relative 

 humidity 632 με 4 

 Reversible shrinkage 50% 2 

 Time to develop 50% of ultimate 

 shrinkage 35 days 2 

 28-day PCC modulus of rupture 690 psi 2 

 28-day PCC elastic modulus 4.4x106

 psi 4 

Granular Base Layer

 Unbound material type AASHTO A-1-a 

 Analysis type ICM Inputs 

 Poisson's ratio 0.35 2 

 Coefficient of lateral pressure K0 0.5 2 

 Modulus 40,000 psi 2,5 

 Plasticity index 1% 

 % passing #200 sieve 3 

 % passing #4 sieve 20 

 D60 8 mm 

 Compaction state Compacted 2 

 Maximum dry unit weight 122.2 pcf 2 

 Specific gravity of solids 2.66 2 

 Saturated hydraulic conductivity 263 ft/hr 2 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 19 May 2006

Input Parameter Design Value Notes 

 Optimum gravimetric water content 11.1% 2 

 Calculated degree of saturation 82% 2 

 SWCC parameter af 11.1 psi 2 

 SWCC parameter bf 1.83 2 

 SWCC parameter cf 0.51 2 

 SWCC parameter hr 361 psi 2 

Compacted Subgrade (top 6 inches)

 Unbound material type AASHTO A-7-5 

 Analysis type ICM Inputs 

 Poisson's ratio 0.35 2 

 Coefficient of lateral pressure K0 0.5 2 

 Modulus 12,000 psi 2,5 

 Plasticity index 30% 2 

 % passing #200 sieve 85 2 

 % passing #4 sieve 99 2 

 D60 0.01 mm 2 

 Compaction state Compacted 

 Maximum dry unit weight 97.1 pcf 2 

 Specific gravity of solids 2.75 2 

 Saturated hydraulic conductivity 3.25x10-5 ft/hr 2 

 Optimum gravimetric water content 24.8% 2 

 Calculated degree of saturation 88.9% 2 

 SWCC parameter af 301 psi 2 

 SWCC parameter bf 0.995 2 

 SWCC parameter cf 0.732 2 

 SWCC parameter hr 1.57x104

 psi 2 

Natural Subgrade (beneath top 6 inches)

 Unbound material type AASHTO A-7-5 

 Compaction state Uncompacted 

 Maximum dry unit weight 87.4 pcf 2 

 (Other properties same as for compacted subgrade) 

Notes: 

1. Typical initial service life for rigid pavement design. 

2. Level 3 default/calculated/derived value from NCHRP 1-37A software. 

3. Based on interpolated climate histories at IAD, DCA, and BWI airports. 

4. Level 3 default/calculated/derived values from NCHRP 1-37A software for baseline PCC mixture 

properties. 

5. Default input value at optimum moisture and density conditions before adjustment for seasonal effects 

(adjustment performed internally within the NCHRP 1-37A software). 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 20 May 2006

Table 6-10. Wheelbase spacing distribution (Level 3 defaults). 

Short Medium Long

12 15 18

33% 33% 34%

Average Axle Spacing (ft)

Percent of trucks

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Pavement age, years

Faulting, in

Faulting

Faulting at specified

reliability

Figure 6-2. Predicted faulting performance for NCHRP 1-37A baseline rigid pavement 

design. 

6.2.3 Summary 

Baseline flexible and rigid pavement designs were developed using the 1993 AASHTO 

Design Guide and the input parameters in Table 6-2 and Table 6-3. The final designs are 5.3 

inches of AC over 12.7 inches of GAB (5.5”/13” after rounding) and 10.4 inches of PCC 

over 6 inches of GAB (10.5”/6” after rounding), respectively. Initial construction costs for 

these designs, based on the unit cost data in Table 6-1, are $180,000 and $342,000 per linemile, respectively ($185,000 and $345,000 per lane-mile using rounded layer thicknesses). 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 21 May 2006

These baseline designs were then analyzed using the NCHRP 1-37A procedures and the 

input parameters in Table 6-4 and Table 6-9 to determine the corresponding distress levels at 

the end of initial service life. Permanent deformations are the controlling distress type for the 

flexible pavement scenario; the predicted total rutting (after adjustment for reliability) for the 

baseline flexible pavement section is 0.65 inches, as compared to the 0.75-inch default value 

in the NCHRP 1-37A software. Joint faulting is the controlling distress type for the rigid 

pavement scenario; the predicted joint faulting (after adjustment for reliability) for the 

baseline rigid pavement design is 0.12 inches, identical to the default value in the NCHRP 

1-37A software. Note that the baseline design scenarios are not greatly different from the 

pavement conditions at the AASHO Road Test (except perhaps for climate), and therefore 

the 1993 AASHTO designs should be in general agreement with those from the more 

sophisticated NCHRP 1-37A methodology. Discrepancies between the design procedures 

should become more pronounced as conditions increasingly deviate from the AASHO Road 

Test conditions. 

6.3 SOFT SUBGRADE 

The design scenario for this case is identical to the baseline conditions in Section 6.2, except 

for a much softer and weaker subgrade. The subgrade is now postulated to be a very soft high 

plasticity clay (AASHTO A-7-6, USCS CH) with MR = 6000 psi at optimum moisture and 

density before adjustment for seasonal effects. Note that this MR value is even lower than the 

NCHRP 1-37A default values for an A-7-6/CH material in order to accentuate the effects of 

low subgrade stiffness. The groundwater depth is left unchanged from the baseline scenario 

in order to focus on the subgrade stiffness effect. Intuitively, more substantial pavement 

sections are expected for this scenario as compared to the baseline conditions to achieve the 

same level of pavement performance. This can be achieved by increasing the thicknesses of 

the AC/PCC/granular base layers, increasing the quality of the AC/PCC/granular base 

materials, stabilizing the granular base layer, treating the soft subgrade soil, or some 

combination of these design modifications. In order to keep the comparisons among 

scenarios simple, only increases in AC or PCC thickness will be considered here. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 22 May 2006

6.3.1 1993 AASHTO Design 

Flexible Pavement 

The only modification to the 1993 AASHTO flexible pavement baseline inputs (Table 6-2) 

required to simulate the soft subgrade condition is a reduction of the seasonally-adjusted 

subgrade resilient modulus MR from 7,500 to 3,800 psi.3

 The W18 traffic capacity for baseline 

flexible pavement section under the soft subgrade conditions is only 1.25x106

 ESALs, 

corresponding to an 80% decrease in initial service life. 

The required pavement structure for the soft subgrade condition is determined from the 1993 

AASHTO design procedure, as follows (assuming changes only in the AC layer thickness): 

• Required overall structural number SN = 5.76 (compared to 4.61 for the baseline 

conditions) 

• Structural number provided by granular base (same thickness D2 as in baseline 

design) 2 22 2 SN m a D == = (1.0)(0.18)(12.7) 2.28 

• Required asphalt structural number 1 2 SN SN SN = −=−= 5.76 2.28 3.48

• Required asphalt layer thickness 1

1

1

7.9 SN D

a = = inches 

The design for the soft subgrade condition is thus 7.9 inches of AC over 12.7 inches of GAB

(before rounding). This represents a 50% increase in AC thickness as compared to the 5.3 

inches in the baseline design, which, in turn, translates to a 20% initial construction cost 

increase of about $37K per lane-mile (using the typical unit cost data in Table 6-1). 

Rigid Pavement 

The only modification to the 1993 AASHTO rigid pavement baseline inputs (Table 6-3) 

required to simulate the soft subgrade condition is a reduction of the seasonally-adjusted 

subgrade resilient modulus MR from 7,500 to 3,800 psi. The W18 traffic capacity for the 

baseline rigid pavement section under the soft subgrade conditions is reduced to 15.5x106

ESALs, corresponding to a 6% decrease in initial service life. 

The reduction in foundation stiffness in this scenario has a direct effect on the design 

modulus of subgrade reaction keff, which decreases from its original value of 38 pci for 

baseline conditions to a value of 27 pci for the soft subgrade case. However, the required slab 

thickness is relatively insensitive to this reduction in foundation stiffness, increasing only 0.1 

inches for a final design of 10.5 inches of PCC over 6 inches of GAB for the soft subgrade 

 

3

 Based on the results from the NCHRP 1-37A analyses for these conditions. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 23 May 2006

condition. Note that after rounding to the nearest half-inch, this design is identical to the rigid 

pavement design for the baseline conditions. 

A common constructability concern under these soft in-situ soil conditions is the requirement 

for a stable working platform. The 6-inch granular subbase is unlikely to provide adequate 

stability. Consequently, a realistic final design would require either a thicker granular 

subbase, a separate granular working platform (not included in the structural design 

calculations), and/or subgrade improvement (see Chapter 7) for constructability. 

6.3.2 NCHRP 1-37A Design 

Flexible Pavement 

Changing the subgrade soil type to A-7-6 changes many of the other Level 3 default inputs 

for the subgrade in the NCHRP 1-37A design methodology. The altered input parameters for 

the soft subgrade condition are summarized in Table 6-11. Figure 6-3 summarizes the 

predicted rutting vs. time for the baseline flexible pavement section (5.3′′ AC over 12.7′′

granular base); the time to the 0.65 inch total rutting design limit is only 93 months (7.75 

years), corresponding to a 48% decrease in initial service life due to the soft subgrade 

conditions. 

The trial designs (assuming only increases in AC thickness) and their corresponding 

predicted performance at end of the initial service life are listed in Table 6-12. Rutting is 

again the critical distress mode controlling the design in all cases; the design limit of 0.65 

inches for total rutting is based on the performance of the baseline pavement section, as 

described previously in Section 6.2.2. Interpolating among the results in Table 6-12, the final 

flexible pavement design section for the soft subgrade conditions consists of 7.9 inches of 

AC over 12.7 inches of GAB. This design section is identical to that obtained from the 1993 

AASHTO Design Guide for this scenario. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 24 May 2006

Table 6-11. Modified input parameters for NCHRP 1-37A 

flexible pavement design: soft subgrade scenario. 

Input Parameter Design Value Notes 

Compacted Subgrade (top 6 inches)

 Unbound material type AASHTO A-7-6 

 Analysis type ICM Inputs 

 Poisson's ratio 0.35 1 

 Coefficient of lateral pressure K0 0.5 1 

 Modulus 6,000 psi 2,3 

 Plasticity index 40% 1 

 % passing #200 sieve 90 1 

 % passing #4 sieve 99 1 

 D60 0.01 mm 1 

 Compaction state Compacted 

 Maximum dry unit weight 91.3 1 

 Specific gravity of solids 2.77 1 

 Saturated hydraulic conductivity 3.25x10-5 ft/hr 1 

 Optimum gravimetric water content 28.8% 1 

 Calculated degree of saturation 89.4% 1 

 SWCC parameter af 750 psi 1 

 SWCC parameter bf 0.911 1 

 SWCC parameter cf 0.772 1 

 SWCC parameter hr 4.75x104

 psi 1 

 Natural Subgrade (beneath top 6 inches)

 Unbound material type AASHTO A-7-6 

 Compaction state Uncompacted 

 Maximum dry unit weight 82.2 pcf 2 

 (Other properties same as for compacted subgrade) 

Notes: 

1. Level 3 default/calculated/derived value from NCHRP 1-37A software. 

2. Set artificially low to simulate a soft subgrade condition. 

3. Input value at optimum moisture and density conditions before adjustment for seasonal effects 

(adjustment performed internally within the NCHRP 1-37A software). 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 25 May 2006

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 18 36 54 72 90 108 126 144 162 180 198

Pavement Age (month)

Total Rutting Design Limit

TotalRutReliability

Total Rutting

SubTotalSG

SubTotalAC

SubTotalBase

Figure 6-3. Predicted rutting performance for soft subgrade scenario. 

Table 6-12. Trial cross sections for NCHRP 1-37A flexible pavement design: 

soft subgrade scenario. 

AC Thickness

(in.) 

Base Thickness

(in.) 

Total Rutting 

(in.) 

5.3 12.7 0.765 

6.0 12.7 0.730 

8.0 12.7 0.643 

10.0 12.7 0.572 

Design Limit: 0.65 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 26 May 2006

Rigid Pavement 

Changing the subgrade soil type to A-7-6 changes many of the other Level 3 default inputs 

for the subgrade in the NCHRP 1-37A design methodology. The altered input parameters for 

the rigid pavement soft subgrade condition are the same as those summarized earlier in Table 

6-11 for the corresponding flexible design condition. Figure 6-4 summarizes the predicted 

faulting vs. time for the baseline rigid pavement section (10.4′′ PCC over 6.0′′ granular base); 

the time to the 0.12 inch faulting design limit is only 22.2 years, corresponding to a 13% 

decrease in initial service life due to the soft subgrade conditions. 

The trial designs (assuming only increases in PCC slab thickness) and their corresponding 

predicted performance at end of design life are listed in Table 6-13. Faulting is again the 

critical distress mode controlling the design in all cases; the design limit of 0.12 inches for 

faulting is based on the performance of the baseline pavement section, as described 

previously in Section 6.2.2. Interpolating among the results in Table 6-13, the final rigid 

pavement design section for the soft subgrade conditions consists of a 10.9 inch PCC slab 

over 6.0 inches of GAB. This slab thickness is 0.5 inch (5%) greater than that obtained from 

the 1993 AASHTO Design Guide for this scenario; this corresponds to an initial construction 

cost increase of $14K (8%) per lane-mile. 

Again, the subgrade soil is so soft and weak in this scenario that some additional design 

features may be required to provide a stable working platform during construction. The 

6 inch granular subbase is unlikely by itself to provide an adequate working platform. 

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Pavement age, years

Faulting Limit

Faulting at specified reliability

Faulting

Figure 6-4. Predicted faulting performance for soft subgrade scenario. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 27 May 2006

Table 6-13. Trial cross sections for NCHRP 1-37A rigid pavement design: 

soft subgrade scenario. 

PCC Thickness

(in.) 

Base Thickness 

(in.) 

Faulting 

(in.) 

10.4 6 0.131 

10.7 6 0.125 

11.0 6 0.118 

Design Limit: 0.12 

6.3.3 Summary 

The design flexible pavement sections for the baseline and soft subgrade scenarios are 

summarized in Figure 6-5. As expected, the soft subgrade condition mandates a thicker 

pavement cross section. For the both the 1993 AASHTO and NCHRP 1-37A designs, the 

required AC thickness increases from 5.3 – 7.9 inches (before rounding and based on the 

simplest assumption of constant GAB thickness). The GAB thickness remains a constant 

12.7 inches for all designs, although as described previously, the granular base thickness 

would probably be increased for constructability purposes in order to provide a stable 

working platform over the soft subgrade. 

As mentioned previously, increased asphalt thickness could economically be used with a 

thinner aggregate base layer for the same structural capacity in the 1993 AASHTO 

procedure, but this adjustment has not been made here. The NCHRP 1-37A methodology 

does not allocate the increased overall section thickness to the individual layers in the same 

way as the 1993 AASHTO procedure, and in practice one should examine multiple thickness 

combinations to find the minimum cost design that meets the specified performance limits. 

Initial construction cost ranges (based on Table 6-1) for the flexible pavement designs are 

summarized in Figure 6-6. The average initial construction costs increase from about $180K 

to $217K (~20%) per lane-mile due to the soft subgrade for both the 1993 AASHTO and 

NCHRP 1-37A designs. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 28 May 2006

1993 AASHTO

SG SG

GAB GAB

AC AC

BASELINE SOFT SG

5.3"

12.7" 12.7"

7.9"

NCHRP 1-37A

SG SG

GAB GAB

AC AC

BASELINE SOFT SG

5.3"

7.9"

12.7" 12.7"

Figure 6-5. Summary of flexible pavement sections: soft subgrade scenario. 

1993 AASHTO

0

100

200

300

400

BASELINE SOFT SG

Construction Cost ($1000/lane-mile)

NCHRP 1-37A

0

100

200

300

400

BASELINE SOFT SG

Construction Cost ($1000/lane-mile)

Figure 6-6. Example construction costs for flexible pavement sections: soft subgrade 

scenario. 

The design rigid pavement sections for the baseline and soft subgrade scenarios are 

summarized in Figure 6-7. The increase in slab thickness required for the soft subgrade 

conditions is slight, from 10.4 to 10.5 inches for the 1993 AASHTO designs and from 10.4 to 

10.9 inches for the NCHRP 1-37A sections (before rounding). The GAB thickness remains a 

constant 6 inches for all designs, although as described previously, this would probably be 

increased for constructability purposes. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 29 May 2006

Initial construction cost ranges (based on Table 6-1) for the rigid pavement designs are 

summarized in Figure 6-8. The average increase in initial construction cost due to the soft 

subgrade are quite small, ranging from about $2K (~1%) for the 1993 AASHTO design to 

about $14K (~8%) for the NCHRP 1-37A sections. Note that these initial construction costs 

for rigid pavements (Figure 6-8) cannot be fairly compared to the initial construction costs 

for flexible pavements (Figure 6-6) because of the different assumptions for the initial 

service life and the different maintenance and repair costs that will be required over their 

design lives. A fair comparison would require evaluation of life-cycle costs, including 

maintenance, repair, rehabilitation, and perhaps user costs, in addition to the initial 

construction expense. 

1993 AASHTO

SG SG

GAB GAB

PCC PCC

BASELINE SOFT SG

10.4"

6" 6"

10.5

NCHRP 1-37A

SG SG

GAB GAB

PCC PCC

BASELINE SOFT SG

10.4" 10.9"

6" 6"

Figure 6-7. Summary of rigid pavement sections: soft subgrade scenario. 

1993 AASHTO

0

100

200

300

400

500

600

BASELINE SOFT SG

Construction Cost ($1000/lane-mile)

NCHRP 1-37A

0

100

200

300

400

500

600

BASELINE SOFT SG

Construction Cost ($1000/lane-mile)

Figure 6-8. Example construction costs for rigid pavement sections: soft subgrade scenario. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 30 May 2006

Given the significant increases in flexible pavement section required to deal with the weak 

subgrade condition, other design approaches should be considered, such as subgrade 

stabilization, geosynthetic reinforcement of the base layer, or switching to a rigid pavement 

system that is more tolerant of poor foundation conditions. The benefits of subgrade 

stabilization for the flexible pavement designs are explored in the next section. 

6.4 SUBGRADE STABILIZATION 

As described in the preceding section, a very soft and weak subgrade requires a substantially 

thicker section for flexible pavements. The effect on required slab thickness for rigid 

pavements is slight; this confirms the conventional wisdom that rigid pavements are 

particularly advantageous for very poor subgrade support conditions. 

Lime stabilization is a common technique for improving soft and weak subgrades beneath 

flexible pavements. A primary benefit of lime stabilization is a greatly increased stiffness 

within the stabilized zone as a function of the lime content. Consequently, the lime content—

or more specifically, the effect of lime content on subgrade properties—and the thickness of 

the stabilized zone are the primary variables for the stabilization design. 

The effect of lime content on the engineering properties of stabilized subgrades will depend 

greatly on the specific subgrade being stabilized. As a simple illustration of the benefits of 

lime stabilization, the sensitivity of predicted rutting to the thickness and resilient modulus of 

the stabilized zone can be examined using the NCHRP 1-37A methodology.4

 For the purpose 

of this illustration, all design inputs are kept the same as for the soft subgrade scenario (Table 

6-4 and Table 6-11) except that the thickness and resilient modulus of the compacted upper 

layer of the subgrade are adjusted to simulate the lime stabilized zone. 

Total rutting (after adjustment for reliability) at the end of the 15-year initial service life as 

predicted by the NCHRP 1-37A design methodology for various thicknesses and stiffnesses 

of the lime stabilized zone are summarized in Figure 6-9. Based on the data in Chapter 7, 

each 1% of lime in the stabilized zone corresponds very roughly to an increase in resilient 

modulus MR of about 10,000 psi. The data in Figure 6-9 suggest that a lime content 

corresponding to an MR value of 60,000 psi over a depth of 18 inches is one of several 

combinations that will meet the 0.65 inch design limit for total rutting. A pavement section 

consisting of 5.3 inches of AC over 12.7 inches of GAB over 18 inches of lime-stabilized 

subgrade (MR=60,000 psi) should provide sufficient performance. This will also be the more 

 

4

 Note: The NCHRP 1-37A design methodology has not been calibrated for lime stabilization because of an 

insufficient numbers of appropriate field sections in the LTPP database. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 31 May 2006

economically feasible design if the cost of the lime stabilization is less than the $37K/lanemile cost of the 2.6 inches of asphalt concrete saved as a consequence of the subgrade 

improvement. 

0.6

0.7

0.8

5 10 15 20

Thickness of Stabilized Zone (inches)

Total Rutting (inches)

Design Limit

Mr=20000 psi

Mr=60000 psi

Mr=100000 psi

Figure 6-9. Effect of lime stabilization on predicted total rutting. 

6.5 LOW QUALITY BASE/SUBBASE 

The design scenario for this case is identical to the baseline conditions in Section 6.2, except 

that a lower quality granular base material is specified. The granular base is now postulated 

to be a clayey sand gravel (AASHTO A-2-6, USCS GC or SC) with MR = 26,000 psi at 

optimum moisture and density. Intuitively, significantly thicker design pavement sections are 

expected for this scenario, as compared to the baseline conditions to achieve the same level 

of pavement performance. 

6.5.1 1993 AASHTO Design 

Flexible Pavement 

Three modifications to the 1993 AASHTO flexible pavement baseline inputs (Table 6-2) are 

made to simulate the low quality base condition: 

1. The seasonally-adjusted resilient modulus MR for the granular base layer is reduced 

from 40,000 to 26,000 psi. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 32 May 2006

2. The structural layer coefficient a2 is reduced from 0.18 to 0.12, consistent with the 

reduction in MR for the granular base (see correlations in Section 5.4.5). 

3. The drainage coefficient m2 is reduced from 1.0 to 0.70 to reflect a reduction in 

drainage quality from good to poor, due to the substantially increased fines content. 

4. This yields an SN value of only 3.4 for the baseline design layer thicknesses. The W18

traffic capacity for this SN value is only 0.88x106

 ESALs, corresponding to an 86% 

decrease in initial service life. 

The required structure is then determined from the design equations, as follows (again 

assuming changes only in the AC layer thickness): 

• Required overall structural number SN = 4.6 (unchanged from baseline conditions) 

• Structural number provided by granular base (same thickness D2 as in baseline 

design) 2 22 2 SN m a D == = (0.7)(0.12)(12.7) 1.07 

• Required asphalt structural number 1 2 SN SN SN = −=−= 4.61 1.07 3.54

• Required asphalt layer thickness 1

1

1

8.0 SN D

a = = inches 

The design for the low quality base condition is thus 8.0 inches of AC over 12.7 inches of 

GAB (before rounding). This represents a significant increase in AC thickness as compared 

to the baseline AC design thickness of 5.3 inches and is even slightly thicker than the 7.9 

inches of AC required for the soft subgrade scenario (Section 6.3.1). This translates to a 21% 

initial construction cost increase of about $38K per lane-mile (using the typical unit cost data 

in Table 6-1 and assuming that the unit cost for the low quality base is the same as for high 

quality crushed stone). 

Rigid Pavement 

Three modifications to the 1993 AASHTO rigid pavement baseline inputs (Table 6-3) are 

made to simulate the low quality subbase condition: 

1. The resilient modulus MR for the granular subbase is reduced from 40,000 to 26,000 

psi. 

2. The Loss of Support LS coefficient is increased from 2 to 3 to reflect the increased 

erosion potential of the fines in the granular subbase. 

3. The drainage coefficient Cd is reduced from 1.0 to 0.85 to reflect a reduction in 

drainage quality from good to poor, due to the substantially increased fines content. 

The W18 traffic capacity for the baseline rigid pavement section under the low quality 

subbase condition is reduced to 8.3x106

 ESALs, corresponding to a 50% decrease in initial 

service life. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 33 May 2006

The first two changes listed above will have a direct effect on the design modulus of 

subgrade reaction keff, which reduces from its baseline value of 38 pci to a value of 17 pci for 

the low quality subbase condition. The 1993 AASHTO design corresponding to this keff and 

Cd of 0.85 consists of an 11.5 inch PCC slab over 6 inches of GAB. Note that this design is 

substantially thicker than the 10.4" PCC over 6" subbase section for the 1993 AASHTO 

baseline scenario; this corresponds to an initial construction cost increase of $31K (9%) per 

lane-mile. The increase in slab thickness is due primarily to the reduction in the drainage 

coefficient Cd; only about 0.2 inches of the slab thickness increase is attributable to the 

increased erodibility of the subbase, as reflected in the larger LS value and the consequently 

reduced value for keff. 

6.5.2 NCHRP 1-37A Design 

Flexible Pavement 

Changing the granular base soil type from A-1-a to A-2-6 changes many of the other Level 3 

default inputs for this layer in the NCHRP 1-37A design methodology. The altered input 

parameters for the low quality base condition are summarized in Table 6-14. Figure 6-10 

summarizes the predicted rutting vs. time for the baseline flexible pavement section (5.3′′ AC 

over 12.7′′ GAB); the time to the 0.65 inch total rutting design limit is only 153 months 

(12.75 years), corresponding to a 15% decrease in initial service life due to the low quality 

base. Although cracking is not the controlling distress for this scenario, reducing the quality 

of the base material does have a significant effect on cracking, more so than on rutting: 

predicted alligator cracking increases by a factor of 3 and longitudinal cracking by a factor of 

over 10 in the low quality base scenario, as compared to baseline conditions. 

The trial designs and their corresponding predicted performance at end of the initial service 

life are listed in Table 6-15. Rutting is again the critical distress mode controlling the design 

in all cases; the design limit of 0.65 inches for total rutting is based on the performance of the 

baseline pavement section, as described previously in Section 6.2.2. Interpolating among the 

results in Table 6-15, the final flexible pavement design section for the low quality base 

condition consists of 5.8 inches of AC over 12.7 inches of GAB. This is significantly thinner 

than the 8 inches of asphalt required by the 1993 AASHTO design for this scenario, but only 

0.5 inches (9%) thicker than in the baseline design, corresponding to a $7.1K/lane-mile (4%) 

increase in initial construction costs. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 34 May 2006

Table 6-14. Modified input parameters for NCHRP 1-37A flexible pavement design: 

low quality base scenario. 

Input Parameter Design Value Notes 

Granular Base Layer 

 Unbound material type AASHTO A-2-6 

 Analysis type ICM Inputs 

 Poisson's ratio 0.35 1 

 Coefficient of lateral pressure K0 0.5 1 

 Modulus 26,000 psi 2 

 Plasticity index 15% 1 

 % passing #200 sieve 20 1 

 % passing #4 sieve 95 1 

 D60 0.1 mm 1 

 Compaction state Compacted 

 Maximum dry unit weight 117.5 pcf 1 

 Specific gravity of solids 2.71 1 

 Saturated hydraulic conductivity 1.73x10-5 ft/hr 1 

 Optimum gravimetric water content 13.9% 1 

 Calculated degree of saturation 85.9% 1 

 SWCC parameter af 23.1 psi 1 

 SWCC parameter bf 1.35 1 

 SWCC parameter cf 0.586 1 

 SWCC parameter hr 794 psi 1 

Notes: 

1. Level 3 default/calculated/derived value from NCHRP 1-37A software. 

2. Level 3 default value for this soil class. Input value is at optimum moisture and density conditions 

before adjustment for seasonal effects (adjustment performed internally within the NCHRP 1-37A 

software). 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 35 May 2006

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 18 36 54 72 90 108 126 144 162 180 198

Pavement Age (month)

Total Rutting Design Limit

TotalRutReliability

Total Rutting

SubTotalAC

SubTotalBase

SubTotalSG

Figure 6-10. Predicted rutting performance for low quality base scenario. 

Table 6-15. Trial cross sections for NCHRP 1-37A flexible pavement design: 

low quality base scenario. 

AC Thickness

(in.) 

Base Thickness

(in.) 

Total Rutting 

(in.) 

5.3 12.7 0.677 

5.5 12.7 0.664 

6.0 12.7 0.635 

Design Limit: 0.65 

Rigid Pavement 

Changing the granular base soil type from A-1-a to A-2-6 changes many of the other Level 3 

default inputs for this layer in the NCHRP 1-37A design methodology. The altered input 

parameters for the base layer in the rigid pavement design are the same as those summarized 

earlier in Table 6-14 for the corresponding flexible design condition. In addition, the 

Erodibility Index for the base layer is changed from “Fairly Erodible (4)” to “Very Erodible 

(5)” (see Table 6-16) to reflect the increased fines content of the A-2-6 base material. In 

reality, the erodibility is probably somewhere in between these categories, but the NCHRP 

1-37A does not permit input of intermediate erodibility conditions. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 36 May 2006

Figure 6-11 summarizes the predicted faulting vs. time for the baseline rigid pavement 

section (10.4′′ PCC over 6.0′′ GAB); the time to the 0.12 inch faulting design limit is only 24 

years, corresponding to a 6% decrease in initial service life due to the low quality base 

condition. 

The trial designs and their corresponding predicted performance at end of design life are 

listed in Table 6-17. Also shown in the table is the design limit for predicted faulting, which 

is the controlling distress for this scenario. A pavement section consisting of a 10.7 inch PCC 

slab over 6 inches of GAB (before rounding) meets the design faulting limit for the low 

quality base scenario. This pavement section is 0.3 inches thicker than the baseline design, 

but significantly thinner than the 11.5′′ slab from the 1993 AASHTO design for this scenario. 

Increased initial construction cost for the additional 0.3 inches of PCC slab is $8.5K (2.5%) 

per lane-mile. 

0.0

0.0

0.0

0.1

0.1

0.1

0.1

0.1

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Pavement age, years

Faulting, in

Faulting Limit

Faulting at specified reliability

Faulting

Figure 6-11. Predicted faulting performance for low quality base scenario. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 37 May 2006

Table 6-16. NCHRP 1-37A recommendations for assessing erosion potential 

of base material (adapted after PIARC, 1987; Christory, 1990). 

Erodibility Class Material Description and Testing 

1 

(1) Lean concrete with previous outstanding past 

performance and a granular subbase layer or a stabilized soil 

layer or a geotextile fabric layer is placed between the 

treated base and subgrade, otherwise class 2. 

(2) Hot mixed asphalt concrete with previous outstanding 

past performance and a granular subbase layer or a stabilized 

soil layer is placed between the treated base and subgrade, 

otherwise class 2. 

(3) Permeable drainage layer (asphalt or cement treated 

aggregate) and a granular or a geotextile separation layer 

between the treated permeable base and subgrade. 

(4) Unbonded PCC Overlays: HMAC separation layer 

(either dense or permeable graded) is specified. 

2 

(1) Cement treated granular material with good past 

performance and a granular subbase layer or a stabilized soil 

or a geotextile fabric layer is placed between the treated base 

and subgrade, otherwise class 3. 

(2) Asphalt treated granular material with good past 

performance and a granular subbase layer or a stabilized soil 

layer or a geotextile soil layer is placed between the treated 

base and subgrade, otherwise class 3. 

3 

(1) Cement-treated granular material that has exhibited some 

erosion and pumping in the past. 

(2) Asphalt treated granular material that has exhibited some 

erosion and pumping in the past. 

(3) Unbonded PCC Overlays: Surface treatment or sand 

asphalt is used.

4 Unbound crushed granular material having dense gradation 

and high quality aggregates.

5 Untreated subgrade soils (compacted).

Table 6-17. Trial cross sections for NCHRP 1-37A rigid pavement design: 

low quality base scenario. 

PCC Thickness

(in.) 

Base Thickness 

(in.) 

Faulting 

(in.) 

10.4 6 0.124 

10.6 6 0.121 

10.7 6 0.119 

11.0 6 0.113 

Design Limit: 0.12 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 38 May 2006

6.5.3 Summary 

The design flexible pavement sections for the baseline and low quality base scenarios are 

summarized in Figure 6-12. As expected, the low quality base condition mandates a thicker 

pavement cross section in both design methodologies. For the 1993 AASHTO design, the 

required asphalt thickness increases from 5.3 to 8.0 inches (assuming constant granular base 

thickness); the NCHRP 1-37A design requires a significantly smaller increase of only an 

additional half-inch of asphalt thickness. Overall, these results suggest that the 1993 

AASHTO Guide assigns more weight—at least with regard to rutting, the controlling distress 

in these scenarios—to the structural contributions from the unbound layers than does the 

NCHRP 1-37A methodology; this trend has been observed in other comparison studies 

between the 1993 AASHTO Guide and the new NCHRP 1-37A procedure. 

Initial construction cost ranges (based on Table 6-1) for the flexible pavement designs are 

summarized in Figure 6-12. Note that these costs are very approximate; in particular, the 

same unit cost has been used for both the high quality (baseline) and low quality granular 

base materials, but in reality these would likely be different. The initial construction costs for 

the low quality base scenario (based on the typical unit costs in Table 6-1) increase by about 

38K or 21% per lane-mile in the 1993 AASHTO designs and only by about $7.1K or 4% in 

the NCHRP 1-37A designs. 

The design rigid pavement sections for the baseline and low quality base scenarios are 

summarized in Figure 6-14. The low quality base condition necessitates a significantly 

thicker pavement cross section in the 1993 AASHTO design, with the slab thickness 

increasing by 1.1 inches. The required slab thickness increased a much smaller 0.3 inches in 

the NCHRP 1-37A design. As for flexible pavements, the 1993 AASHTO Guide appears to 

attach more weight to the structural contributions of the unbound layers than does the 

NCHRP 1-37A procedure. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 39 May 2006

1993 AASHTO

SG SG

GAB GAB

AC AC

BASELINE LOW QUALITY BASE

5.3"

12.7 12.7"

8"

NCHRP 1-37A

SG SG

GAB GAB

AC AC

BASELINE LOW QUALITY BASE

5.3" 5.8"

12.7" 12.7"

Figure 6-12. Summary of flexible pavement sections: low quality base scenario. 

1993 AASHTO

0

100

200

300

400

BASELINE LOW QUALITY

BASE

Construction Cost ($1000/lane-mile)

NCHRP 1-37A

0

100

200

300

400

BASELINE LOW QUALITY

BASE

Construction Cost ($1000/lane-mile)

Figure 6-13. Example construction costs for flexible pavement sections: 

low quality base scenario. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 40 May 2006

1993 AASHTO

SG SG

GAB GAB

PCC PCC

BASELINE LOW QUALITY BASE

10.4"

6" 6"

11.5"

NCHRP 1-37A

SG SG

GAB GAB

PCC PCC

BASELINE LOW QUALITY BASE

10.4" 10.7"

6" 6"

Figure 6-14. Summary of rigid pavement sections: low quality base scenario. 

Initial construction cost ranges (based on Table 6-1) for the rigid pavement designs are 

summarized in Figure 6-15. Note again that these costs are very approximate; in particular, 

the same unit cost has been used for both the high quality (baseline) and low quality granular 

base materials, but in reality these would likely be different. The initial construction costs 

increase by about $31K (9%) per lane-mile for the 1993 AASHTO designs and by $8.4K 

(2.5%) per lane-mile for the NCHRP 1-37A pavement sections. Note again that the overall 

magnitudes of these initial construction costs for rigid pavements (Figure 6-15) cannot be 

fairly compared to the initial construction costs for flexible pavements (Figure 6-13) because 

of the different assumptions regarding initial service life and different maintenance and repair 

expenses over the design lives of these different pavement classes. A fair comparison would 

require evaluation of life-cycle costs, including maintenance, repair, rehabilitation, and 

perhaps user costs, in addition to the initial construction expense. 

The base quality in this scenario is probably not sufficiently low to present a serious problem 

in design. However, if low base quality does become a critical issue and no high quality 

crushed material is available, cement or bituminous stabilization could be employed to 

improve the base quality substantially. Geosynthetics can also be employed for drainage and 

separation. These techniques are described more fully in Chapter 7. 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 41 May 2006

1993 AASHTO

0

100

200

300

400

500

600

BASELINE LOW QUALITY

BASE

Construction Cost ($1000/lane-mile)

NCHRP 1-37A

0

100

200

300

400

500

600

BASELINE LOW QUALITY

BASE

Construction Cost ($1000/lane-mile)

Figure 6-15. Example construction costs for rigid pavement sections: low quality base 

scenario. 

6.6 POOR DRAINAGE 

Good drainage conditions were explicitly assumed in the baseline design scenario. The 

values for the drainage coefficients in the 1993 AASHTO designs corresponded to a “good” 

drainage quality rating, defined as “water removed within 2 hours.” This implies a highpermeability base layer (like the A-1-a material assumed in the baseline design scenario) and 

functioning edge drains. The material properties for the granular base and the drainage length 

specified in the NCHRP 1-37A design procedure were also consistent with a highpermeability base layer and functioning edge drainage. 

In this poor drainage scenario, the material characteristics for the granular base layer remain 

the same as for the baseline conditions. However, it is now assumed that the edge drains are 

clogged (or perhaps nonexistent), and the consequences of the ineffective drainage are 

evaluated. Conceptually, these expected consequences are 

• a decrease in the stiffness (and strength) of the granular base layer because of higher 

average moisture content; 

• a decrease in the stiffness (and strength) of the subgrade because of higher average 

moisture content; 

• an increase in other moisture-related distresses (e.g., freeze/thaw cycles, mixing of 

granular base with subgrade, etc.). 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 42 May 2006

6.6.1 1993 AASHTO Design 

Flexible Pavement 

Three modifications to the 1993 AASHTO flexible pavement baseline inputs (Table 6-2) are 

appropriate for simulating the poor drainage condition: 

1. The seasonally averaged resilient modulus and corresponding structural layer 

coefficient for the granular base material is reduced to reflect the higher average 

moisture content due to the ineffective drainage. From Table 5-39, the granular base 

resilient modulus value for wet conditions may be as low as about 40% of that for dry 

conditions. The EBS = 40,000 psi value for excellent drainage conditions in the 

baseline design scenario is therefore reduced 35% to 26,000 psi for the poor drainage 

condition. This corresponds to a structural layer coefficient a2 = 0.12 using the 

AASHTO correlation in Eq. (5.16). Note that these values coincide with those for the 

low quality granular base scenario in Section 6.5. 

2. The base layer drainage coefficient m2 is reduced from 1.0 to 0.6. This corresponds 

to a reduction in drainage quality rating from “good” to “very poor” under the 

assumption that the pavement is exposed to moisture conditions approaching 

saturation for 5-25% of the time. 

3. The seasonally averaged resilient modulus for the subgrade is reduced to reflect the 

higher average moisture content due to ineffective drainage. From Table 5-39, wet 

resilient modulus values for granular base and subbase materials may be as low as 

about 40% of those for dry conditions. It is likely that the stiffness decrease in 

moisture-sensitive fine-grained subgrade soils will be even greater. Consequently, it 

is assumed here that the MR = 7,500 psi value for good drainage conditions in the 

baseline design scenario is reduced by about 50% to 3,800 psi for the poor drainage 

condition (all MR values are after seasonal adjustment). Note that this value coincides 

with that for the weak subgrade scenario in Section 6.3. 

The W18 traffic capacity for baseline flexible pavement section under these poor drainage 

conditions is only 0.67x106

 ESALs, corresponding to an 89% decrease in initial service life. 

The required structure is then determined from the design equations, as follows (again 

assuming changes only in the AC layer thickness): 

• Required overall structural number SN = 5.76 (unchanged from baseline conditions) 

• Structural number provided by granular base (same thickness D2 as in baseline 

design) 2 22 2 SN m a D == = (0.6)(0.12)(12.7) 0.91

• Required asphalt structural number 1 2 SN SN SN = −=−= 5.67 0.91 4.76

• Required asphalt layer thickness 1

1

1

10.8 SN D

a = = inches 
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The design for the poor drainage condition is thus 10.8 inches of AC over 12.7 inches of 

GAB (before rounding). This represents a significant increase in AC thickness, as compared 

to the baseline AC design thickness of 5.3 inches. The additional asphalt thickness required 

for the poor drainage condition is slightly more than the additional asphalt thicknesses for the 

soft subgrade and low quality base scenarios combined. This increase in asphalt thickness 

translates to a 43% initial construction cost increase of about $78K per lane-mile (using the 

typical unit cost data in Table 6-1). 

Rigid Pavement 

Four modifications to the 1993 AASHTO rigid pavement baseline inputs (Table 6-2) are 

appropriate for simulating the poor drainage condition: 

1. The resilient modulus for the granular subbase material is reduced to reflect the 

higher average moisture content due to the ineffective drainage. Consistent with the 

calculations for the 1993 AASHTO flexible design, the ESB = 40,000 psi value for 

excellent drainage conditions in the baseline design scenario is reduced to 26,000 psi 

for the poor drainage condition. 

2. The Loss of Support LS coefficient is increased from 2 to 3 to reflect the increased 

erosion potential in the granular subbase due to the increased moisture levels. 

3. The resilient modulus for the subgrade is reduced to reflect the higher average 

moisture content due to the ineffective drainage. Consistent with the calculations for 

the 1993 AASHTO flexible design in the preceding subsection, the MR = 7,500 psi 

value for good drainage conditions in the baseline design scenario is reduced to 3,800 

psi for the poor drainage condition (all MR values are after seasonal adjustment). 

4. The drainage coefficient Cd is reduced from 1.0 to 0.75. This corresponds to a 

reduction in drainage quality rating from “good” to “very poor” under the assumption 

that the pavement is exposed to moisture conditions approaching saturation for 5-25% 

of the time. 

The W18 traffic capacity for the baseline rigid pavement section under these postulated poor 

drainage conditions is reduced to 5.2x106

 ESALs, corresponding to a 68% decrease in initial 

service life. 

The first three changes listed above will have a direct effect on the design modulus of 

subgrade reaction keff, which reduces from its baseline value of 38 pci to a value of 12 pci for 

the poor drainage conditions. The 1993 AASHTO design corresponding to this keff and Cd of 

0.75 consists of a 12.3 inch PCC slab over 6 inches of GAB. Note that this design is 

substantially thicker than the 10.4" PCC over 6" subbase section for the 1993 AASHTO 

baseline scenario; this corresponds to an initial construction cost increase of $53K (16%) per 

lane-mile. As was the case for the low quality subbase scenario, the increase in slab thickness 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 44 May 2006

is due primarily to the reduction in the drainage coefficient Cd attributable to the subbase; 

only about 0.25 inches of the slab thickness increase is due to the increased erodibility of the 

subbase as reflected in the larger LS value and the consequently reduced value for keff. 

6.6.2 NCHRP 1-37A Design 

Flexible Pavement 

The current version of the NCHRP 1-37A design software does not yet include the capability 

for directly modeling drainage influences. This in part is due to the paucity of field data 

available for calibrating the empirical distress models for drainage effects. As stated in the 

NCHRP 1-37A final report (NCHRP 1-37A, 2004): 

“The calibration of the flexible pavement distress models assumed that no 

infiltration of moisture occurred throughout the design period. Thus, the flexible 

pavement design procedure does not allow the designer to choose any level of 

infiltration at this time. However, adequate consideration to subdrainage should be 

given when designing flexible pavements.” 

Although it would be possible to run the NCHRP 1-37A analyses using the moduli values 

assumed for the 1993 AASHTO designs (Section 6.6.1), this defeats the purpose of including 

the analysis of seasonal moisture variations on material properties in the mechanisticempirical methodology. It is also probable that this approximate approach would 

underestimate the detrimental effects of excess moisture in the pavement structure. 

Rigid Pavement 

Three modifications to the NCHRP 1-37A rigid pavement baseline inputs (Table 6-9) are 

appropriate for simulating the poor drainage condition: 

1. The surface infiltration condition is changed from “Minor (10%)” to “Extreme 

(100%)”. 

2. The granular base layer is changed from a free-draining A-1-a to a much less 

permeable A-2-6 material. This is the same material as for the low quality base 

scenario in Section 6.5; the modified NCHRP 1-37A design inputs for this material 

are given in Table 6-14. 

3. The Erodibility Index for the base layer is changed from “Fairly Erodible (4)” to 

“Very Erodible (5)” to reflect the increased fines content of the A-2-6 base material 

(see Table 6-16). 

Figure 6-16 summarizes the predicted faulting vs. time for the baseline rigid pavement 

section (10.4′′ PCC over 6.0′′ granular base); the time to the 0.12 inch faulting design limit is 
FHWA NHI-05-037 Chapter 6 – Structural Design & Performance 

Geotechnical Aspects of Pavements 6 - 45 May 2006

only 24 years, corresponding to a 6% decrease in initial service life due to the low poor 

drainage conditions. 

The trial designs and their corresponding predicted performance at end of design life are 

listed in Table 6-18. Also shown in the table is the design limit for predicted faulting, which 

is the controlling distress for this scenario. A pavement section consisting of a 10.7 inch PCC 

slab over 6 inches of GAB meets the design faulting limit for the poor drainage scenario. 

This pavement section is significantly thinner than the 12.3 inch slab required by 1993 

AASHTO design for this scenario, but only 0.3 inches thicker than the design for the baseline 

conditions, corresponding to an $8.5K per lane-mile (2.5%) increase in initial construction 

costs. 

0.0

0.0

0.0

0.1

0.1

0.1

0.1

0.1

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Pavement age, years

Faulting, in

Faulting

Faulting at specified reliability

Faulting Limit

Figure 6-16. Predicted faulting performance for poor drainage scenario. 

Table 6-18. Trial cross sections for NCHRP 1-37A rigid pavement design: 

poor drainage scenario. 

PCC Thickness

(in.) 

Base Thickness 

(in.) 

Faulting 

(in.) 

10.4 6 0.124 

10.7 6 0.119 

11.0 6 0.113 

Design Limit: 0.12 
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6.6.3 Summary 

The design flexible pavement sections for the baseline and poor drainage scenarios are 

summarized in Figure 6-17. Only designs from the 1993 AASHTO procedure are included 

here; as described previously, the NCHRP 1-37A procedure in its present form does not have 

the capability to analyze this scenario. As expected, the poor drainage condition mandates a 

substantially thicker pavement cross section, with the required thickness of the asphalt 

increasing from 5.3 to 10.8 inches (for the simple assumption of constant graded aggregate 

base thickness). 

Initial construction cost ranges (based on Table 6-1) for the flexible pavement designs are 

summarized in Figure 6-18. The initial construction costs based on the typical unit costs in 

Table 6-1 increase by about $78K or 44% per lane-mile as a consequence of the poor 

drainage conditions for the 1993 AASHTO designs. 

1993 AASHTO

SG SG

GAB GAB

AC

AC

BASELINE POOR DRAINAGE

5.3"

12.7" 12.7"

10.8"

Figure 6-17. Summary of flexible pavement sections: poor drainage scenario. 
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Geotechnical Aspects of Pavements 6 - 47 May 2006

1993 AASHTO

0

100

200

300

400

BASELINE POOR DRAINAGE

Construction Cost ($1000/lane-mile)

Figure 6-18. Example construction costs for flexible pavement sections: 

poor drainage scenario. 

The design rigid pavement sections for the baseline and poor drainage scenarios are 

summarized in Figure 6-19. Again, the poor drainage necessitates a thicker pavement cross 

section. For the 1993 AASHTO design, the required thickness for the PCC slab increases 

substantially from 10.4 to 12.3 inches (before rounding). For the NCHRP 1-37A designs, the 

PCC slab thickness increases only from 10.4 to 10.7 inches. These disparities suggest that the 

1993 AASHTO procedure overestimates and/or the NCHRP 1-37A methodology 

underestimates the impact of poor drainage on pavement performance and design 

requirements. Poor drainage impacts the unbound layers most significantly, and so these 

disparities are consistent with the trends observed in the low quality base scenario in Section 

6.5. 

Initial construction cost ranges (based on Table 6-1) for the rigid pavement designs are 

summarized in Figure 6-20. The initial construction costs increase by about $54K (16%) per 

lane-mile due to the poor drainage for the 1993 AASHTO designs and by about $8.5K 

(2.5%) per lane-mile in the NCHRP 1-37A designs. Note again that these construction costs 

for rigid pavements (Figure 6-20) cannot be fairly compared to the construction costs for 

flexible pavements (Figure 6-18) because of the different initial service lives and the 

different maintenance and repair expenses incurred over their design lives. A fair comparison 

would require evaluation of life-cycle costs, including maintenance, repair, rehabilitation, 

and perhaps user costs, in addition to the initial construction expense. 
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Geotechnical Aspects of Pavements 6 - 48 May 2006

These examples clearly show the very substantial effect that ineffective or nonexistent 

drainage can have on the pavement design thickness. Methods for ensuring adequate 

drainage in pavements are described in Chapter 7. 

1993 AASHTO

SG SG

GAB GAB

PCC PCC

BASELINE POOR DRAINAGE

10.4"

6" 6"

12.3"

NCHRP 1-37A

SG SG

GAB GAB

PCC PCC

BASELINE POOR DRAINAGE

10.4" 10.7"

6" 6"

Figure 6-19. Summary of rigid pavement sections: poor drainage scenario. 

1993 AASHTO

0

100

200

300

400

500

600

BASELINE POOR DRAINAGE

Construction Cost ($1000/lane-mile)

NCHRP 1-37A

0

100

200

300

400

500

600

BASELINE POOR

DRAINAGE

Construction Cost ($1000/lane-mile)

Figure 6-20. Example construction costs for rigid pavement sections: poor drainage 

scenario. 
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6.7 SHALLOW BEDROCK 

6.7.1 1993 AASHTO Design 

Flexible Pavement 

Although the presence of shallow bedrock will clearly have a beneficial stiffening effect on 

the pavement foundation, the 1993 AASHTO Guide does not include any provision for 

including this benefit in the flexible design procedure. Consequently, the design pavement 

section for the shallow bedrock scenario will be identical to that for the baseline conditions 

Rigid Pavement 

The presence of shallow bedrock will increase the design modulus of subgrade reaction keff

via the shallow bedrock correction factor (Figure 5.25). Values of keff for several bedrock 

depths are summarized in Table 6-19, along with the corresponding values of required slab 

thickness. All design inputs other than depth to bedrock are equal to the baseline conditions 

(Table 6-3), and the granular subbase thickness is held constant at 6 inches. The results in 

Table 6-19 indicate that, for the design conditions in this example, the depth to bedrock has a 

moderate effect on the design modulus of subgrade reaction keff, but that this has a nearly 

negligible effect on the required slab thickness. The design pavement section for the case of a 

4 foot depth to bedrock is a slightly thinner 10.3 inch PCC slab over 6 inches of GAB. 

Viewed alternatively, bedrock at a 4 foot depth will increase the allowable traffic W18 to 

17.0x106

, increasing the initial service life of the baseline rigid pavement section by 

approximately 1 year (4%). 

Table 6-19. Design pavement sections for 1993 AASHTO rigid pavement design: 

shallow bedrock effects. 

Depth to bedrock (ft) ∞ 8 4 2 

Composite subgrade modulus k∞ (pci) 423 474 577 704 

Design subgrade modulus keff (pci) 38 41 46 52 

Slab thickness D (in.) 10.4 10.4 10.3 10.3 
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6.7.2 NCHRP 1-37A Design 

Flexible Pavement 

Depth to bedrock is an explicit design input in the NCHRP 1-37A procedure. Table 6-20 

summarizes the predicted total rutting (adjusted for reliability) for the baseline flexible 

pavement conditions (Table 6-4) as a function of depth to bedrock, as predicted by the 

NCHRP 1-37A methodology. For the conditions in this example, a 4 foot depth to bedrock 

will reduce the predicted rutting by about 17%, or increase the initial service life by about 18 

years (120%). This translates directly to a thinner required pavement section. Trial designs 

and their corresponding predicted performance at end of design life for the case of a 3 foot 

depth to bedrock are summarized in Table 6-21. A pavement section consisting of 3 inches of 

AC over 12.7 inches of GAB is sufficient for the shallow bedrock scenario. This corresponds 

to an initial construction cost reduction of about $38K (18%). 

Table 6-20. Influence of bedrock depth on predicted total rutting: 

NCHRP 1-37A design methodology. 

Depth to 

Bedrock 

(ft) 

Total Rutting 

(in.) 

∞ 0.646 

10 0.578 

8 0.544 

4 0.533 

Table 6-21. Trial cross sections for NCHRP 1-37A flexible pavement design: 

baseline conditions with shallow bedrock at 4 ft depth. 

AC Thickness 

(in.) 

Base Thickness

(in.) 

Total Rutting 

(in.) 

5.3 12.7 0.533 

4.0 12.7 0.593 

3.0 12.7 0.651 

Design Limit: 0.65 
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Geotechnical Aspects of Pavements 6 - 51 May 2006

Rigid Pavement 

Depth to bedrock is an explicit design input in the NCHRP 1-37A procedure. Table 6-22 

summarizes the predicted joint faulting (adjusted for reliability) for the baseline rigid 

pavement conditions (Table 6-9) as a function of depth to bedrock, as predicted by the 

NCHRP 1-37A methodology. For the conditions in this example, a 4 foot depth to bedrock 

will reduce the predicted faulting by about 75%. This translates directly to a thinner required 

pavement section. Trial designs and their corresponding predicted performance at end of 

design life for the case of a 4 foot depth to bedrock are summarized in Table 6-23. The 

NCHRP 1-37A software cannot model PCC thicknesses less than 7 inches. Even at this 

reduced thickness, however, the predicted faulting for the shallow bedrock conditions is well 

below the design limit. 

Table 6-22. Influence of bedrock depth on predicted joint faulting: 

NCHRP 1-37A design methodology. 

Depth to 

Bedrock 

(ft) 

Faulting 

(in.) 

∞ 0.117 

20 0.093 

10 0.040 

8 0.033 

4 0.028 

Table 6-23. Trial cross sections for NCHRP 1-37A rigid pavement design: 

baseline conditions with shallow bedrock at 3 ft depth. 

PCC Thickness

(in.) 

Base Thickness

(in.) 

Faulting 

(in.) 

10.4 6.0 0.028 

9.0 6.0 0.041 

7.0 6.0 0.079 

Design Limit: 0.12 
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Geotechnical Aspects of Pavements 6 - 52 May 2006

6.8 CONCLUDING COMMENTS 

It is worthwhile to review the primary objectives of the design studies presented in this 

chapter: 

1. to illustrate via examples how the geotechnical properties described in Chapter 5 are 

incorporated in the pavement design calculations in the 1993 AASHTO and NCHRP 

1-37A procedures; and 

2. to highlight the effects of the geotechnical factors and inputs on pavement design 

performance. 

The design scenarios are intentionally highly idealized and simplified. Their point is to 

emphasize in quantitative terms how changes in geotechnical inputs affect the overall 

pavement design and performance. These design scenarios are also good examples of the 

types of sensitivity studies one should perform during design to evaluate the importance of 

the various design inputs, especially with reference to the quality of the information used to 

determine these inputs. As succinctly stated by Hamming (1973) in the frontispiece of his 

pioneering book on engineering computation: “The purpose of computing is insight, not 

numbers.” 

The ultimate measure for comparing the designs for the various scenarios is cost. Life-cycle 

costs are the best measure, but calculation of life-cycle costs is beyond the scope of these 

exercises. As a fallback, the various design scenarios can be compared in terms of their initial 

construction cost ratios using the example unit cost data from Table 6-1 and the baseline 

design conditions as the reference (i.e., cost index=1). These comparisons are presented in 

Figure 6-21 and Figure 6-22 for flexible and rigid pavement designs, respectively. Remember 

that it is inappropriate to compare flexible versus rigid pavements based only on initial 

construction costs because of the different assumptions regarding initial service life and 

different maintenance and repair expenses that would be incurred over the design lives of 

these different pavement classes. 

Some key observations to be drawn from Figure 6-21 and Figure 6-22 include the following: 

• Poor drainage is by far the most detrimental geotechnical factor for flexible 

pavements. For the conditions in these examples, poor drainage drives up the initial 

cost of the flexible pavement design by nearly 50%. 

• For the flexible pavement scenarios considered here, a very soft subgrade is the 

second most detrimental geotechnical factor, driving initial costs up by about 20%. 

However, the soft subgrade condition can be mitigated by lime stabilization or other 

remedial measures, as described in more detail in Chapter 7. 
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Geotechnical Aspects of Pavements 6 - 53 May 2006

• More thickness may be required if lower quality materials are used for the granular 

base layers in flexible pavements, but the thickness increase may be partially or fully 

compensated by lower unit costs for the lower quality materials. This will be very 

sensitive to the specific unit costs of competing materials in the project location. 

• Overall, the rigid pavement designs were much less sensitive to geotechnical factors 

than were the flexible designs. The range and variations in the cost index among the 

various design scenarios were much less for the rigid pavement designs (Figure 6-22) 

than for the flexible pavements (Figure 6-21). 

An alternate way of looking at the impact of the various geotechnical design parameters is in 

terms of their effect on initial service life for the baseline pavement section. Initial service 

lives for the various design scenarios are summarized in Figure 6-23 and Figure 6-24 for the 

flexible and rigid pavement sections, respectively. The observations in terms of service life 

are similar to those from costs. For the flexible pavements, poor drainage, soft subgrade, and 

low quality base had the most impact on service life (especially for the AASHTO designs), 

reducing the initial service life from 15 to as little as 2 years in the most extreme case. For 

the rigid pavements, poor drainage and low quality base again had the most significant 

impact (again, especially for the AASHTO designs), reducing the initial service life from 25 

to as little as 8 years. The soft subgrade conditions had comparatively less effect on the rigid 

pavement service life, as compared to the flexible pavements, confirming the advantages of 

rigid pavements for very poor foundation conditions. 

Of course, all of the specific observations from these results apply only to the particular 

design scenarios considered in these illustrative studies. Pavement design conditions and inplace unit costs will vary considerably across agencies and regions. Nevertheless, the simple 

design scenarios presented here demonstrate quite convincingly the important effects that 

geotechnical factors can have on design pavement sections and costs. 
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0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50

Baseline

Soft Subgrade

Low Quality

Base

Poor Drainage

Shallow

Bedrock (4')

Initial Cost Index1

AASHTO

NCHRP 1-37A

1

Based on typical unit cost data from Table 6-1. For low quality base scenario, unit cost for granular base is assumed to be 

the same as for high quality base; in reality, this unit cost will likely be lower. 

Figure 6-21. Summary of costs for example design scenarios: flexible pavement designs. 

0.90 0.95 1.00 1.05 1.10 1.15 1.20

Baseline

Soft Subgrade

Low Quality

Base

Poor Drainage

Shallow

Bedrock (4')

Initial Cost Index1

AASHTO

NCHRP 1-37A

1

Based on typical unit cost data from Table 6-1. For low quality base scenario, unit cost for granular base is assumed to be 

the same as for high quality base; in reality, this unit cost will likely be lower. 

Figure 6-22. Summary of costs for example design scenarios: rigid pavement designs. 
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Initial Service Life

0 5 10 15 20 25

Baseline

Soft Subgrade

Low Quality Base

Poor Drainage

Shallow Bedrock (4')

Years

AASHTO

NCHRP

Figure 6-23. Initial service lives for example design scenarios: flexible pavement designs. 

Initial Service Life

0 5 10 15 20 25 30

Baseline

Soft Subgrade

Low Quality Base

Poor Drainage

Shallow Bedrock (4')

Years

AASHTO

NCHRP

Figure 6-24. Initial service lives for example design scenarios: rigid pavement designs. 
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6.9 EXERCISES 

Students will divide into groups to develop designs for the Main Highway project. Each 

group will focus on either a flexible or a rigid pavement design. Specific tasks for each group 

are as follows: 

• Develop a pavement design based on the most-likely conditions expected at the site. 

• Identify the short list of critical geotechnical inputs to which the design is expected to 

be most sensitive. 

• Perform some initial sensitivity evaluation for the critical geotechnical inputs. 

• Complete computations using a simple Excel spreadsheet provided by the instructors. 

6.10 REFERENCES 

AASHTO (1993). AASHTO Guide for Design of Pavement Structures, American Association 

of State Highway and Transportation Officials, Washington, D.C. 

Christory, J.P. (1990). “Assessment of PIARC Recommendations on the Combating of 

Pumping in Concrete Pavements,” Proceedings of the Second International Workshop on the 

Design and the Evaluation of Concrete Pavements, Siguenza, Spain. 

Hamming, R.W. (1973). Numerical Methods for Scientists and Engineers, McGraw-Hill, 

New York, NY. 

MDSHA (2002). Pavement Design Guide, Pavement Division, Office of Materials and 

Technology, Maryland State Highway Administration, Baltimore, MD. 

NCHRP 1-37A (2004). Mechanistic-Empirical Design of New and Rehabilitated Pavement 

Structures, Draft Report, Transportation Research Board, National Research Council, 

Washington, D.C. 

PIARC (1987). “Combating Concrete Pavement Slab Pumping,” PIARC Technical 

Committee on Concrete Roads. 
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Geotechnical Aspects of Pavements 7 - 1 May 2006

CHAPTER 7.0 DESIGN DETAILS AND CONSTRUCTION 

CONDITIONS REQUIRING SPECIAL DESIGN ATTENTION 

7.1 INTRODUCTION 

This chapter includes: 

• Design details for key geotechnical components in the pavement system, including 

drainage and base layer requirements. 

• Compaction of subgrades and unbound pavement layers. 

• A general overview of the types of potential subgrade problems. 

• Identification and treatment of select, widely occurring special geotechnical 

challenges, including collapsible or highly compressible soils, expansive or swelling 

soils, subsurface water and saturated soils, and frost-susceptible soils. 

• Detailed guidelines on alternate stabilization methods, which are often used to 

mitigate special problems. 

 

In this chapter, design details for the specific pavement features of base materials and 

drainage systems are provided. Compaction, one of the key geotechnical issues in pavement 

design and construction is also covered. 

Special challenges generally relate to poor subgrade conditions that occur due to the type of 

soil and environmental conditions. In this chapter, the various types of problematic soil 

conditions are reviewed along with widely occurring specific subgrade problems. Although 

these problematic conditions can be detected with a detailed subsurface exploration, 

problematic conditions can potentially go unnoticed if they are located between borings. 

7.2 SUBSURFACE WATER AND DRAINAGE REQUIREMENTS 

The damaging effects of excess moisture on the pavement have long been recognized. 

Moisture from a variety of sources can enter a pavement structure. This moisture, in 

combination with heavy traffic loads and freezing temperatures, can have a profound 

negative effect on both material properties and the overall performance of a pavement 

system. 

As was shown in Figure 3-3, Chapter 3, moisture in the subgrade and pavement structure can 

come from many different sources. Water may seep upward from a high groundwater table, 
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or it may flow laterally from the pavement edges and shoulder ditches. Knowledge of 

groundwater and its movement are critical to the performance of the pavement as well as 

stability of adjacent sideslopes, especially in cut situations. Groundwater can be especially 

troublesome for pavements in low-lying areas. Thus, groundwater control, usually through 

interception and removal before it can enter the pavement section, is an essential part of 

pavement design. 

In some cases, pavements are constructed beneath the permanent or a seasonally high 

watertable. Obviously, drainage systems must perform or very rapid pavement failure will 

occur. In such cases, redundancy in the drainage design is used (e.g., installation of 

underdrains and edgedrains) and, often, some monitoring is used to ensure continual function 

of the drain system. 

 

Capillary action and moisture-vapor movement are also responsible for water accumulating 

beneath a pavement structure (Hindermann, 1968). Capillary effects are the result of surface 

tension and the attraction between water and soil. Moisture vapor movement is associated 

with fluctuating temperatures and other climatic conditions. 

As was previously indicated in Chapter 3, the most significant source of excess water in 

pavements is typically infiltration through the surface. Joints, cracks, shoulder edges, and 

various other defects in the surface provide easy access paths for water. A study by the 

Minnesota Department of Transportation indicates that 40% of rainfall enters the pavement 

structure (Hagen and Cochran 1995). Demonstration Project 87, Drainable Pavement 

Systems, indicates that surface infiltration is the single largest source of moisture-related 

problems in PCC pavements (FHWA 1994). Although AC pavements do not contain joints, 

surface cracks, longitudinal cold joints that crack, and pavement edges provide ample 

pathways for water to infiltrate the pavement structure. 

The problem only worsens with time. As pavements continue to age and deteriorate, cracks 

become wider and more abundant. Meanwhile, joints and edges become more deteriorated 

and develop into channels through which moisture is free to flow. The result is more 

moisture being allowed to enter the pavement structure with increasing pavement age, which 

leads to accelerated development of moisture-related distresses and pavement deterioration. 

7.2.1 Moisture Damage Acceleration 

Excessive moisture within a pavement structure can adversely affect pavement performance. 

A pavement can be stable at a given moisture content, but may become unstable if the 

materials become saturated. High water pressures can develop in saturated soils when 
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subjected to dynamic loading. Subsurface water can freeze, expand, and exert forces of 

considerable magnitude on a given pavement. Water in motion can transport soil particles 

and cause a number of different problems, including clogging of drains, eroding of 

embankments, and pumping of fines. These circumstances must be recognized and accounted 

for in the design of a pavement. 

The detrimental effects of water on the structural support of the pavement system are 

outlined by AASHTO (1993), as follows: 

• Water in the asphalt surface can lead to moisture damage, modulus reduction, and 

loss of tensile strength. Saturation can reduce the dry modulus of the asphalt by as 

much as 30% or more. 

• Added moisture in unbound aggregate base and subbase is anticipated to result in a 

loss of stiffness on the order of 50% or more. 

• Modulus reduction of up to 30% can be expected for asphalt-treated base and increase 

erosion susceptibility of cement or lime treated bases. 

• Saturated fine-grain roadbed soil could experience modulus reductions of more than 

50%. 

As noted in Chapters 3, 4, 5 and 6, modulus is the key pavement design property! 

The influence of saturation on the life of the pavement is illustrated in Figure 7-1. The 

severity factor (shown in the figure) is the anticipated relative damage during wet versus dry 

periods anticipated for the type of road. As an example, Figure 7-1 shows that if the 

pavement system is saturated only 10% of its life (e.g., about one month per year), a 

pavement section with a moderate stability factor will be serviceable only about 50% of its 

fully drained performance period. Specific distresses caused by excessive moisture within 

flexible and rigid pavements are summarized in Table 7-1 and 7-2, respectively. 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 4 May 2006

Figure 7-1. The influence of saturation on the design life of a pavement system (after 

Cedergren, 1987). 

% of Time Structural Section is Saturated

0 10 20 30 40 50

0

20

40

60

80

100

S = Severity Factor

S=5

S=10

S=20
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 5 May 2006

Table 7-1. Moisture-related distresses in flexible (AC) pavements (NHI 13126; adapted after Carpenter et al. 1979). 

Structural Defect Begins in 

Type Distress Manifestation Moisture Problem Climatic Problem Material Problem Load Associated Distress AC Base Subgrade 

Bump or 

Distortion 

Excess 

Moisture Frost Heave Volume Increase No No No Yes 

Corrugation or 

Rippling Slight Moisture and Temperature Unstable Mix Yes Yes Yes No 

Stripping Yes Moisture Loss of Bond No Yes No No 

Rutting 

Excess in 

Granular 

Layers or 

Subgrade 

Moisture 

Plastic 

Deformation, 

Stripping 

Yes Yes Yes Yes 

Depression Excess Moisture Suction & Materials Settlement, Fill Material No No No Yes 

Surface 

Deformation 

Potholes Excess Moisture Moisture, Temperature < Strength > Moisture Yes Yes Yes Yes 

Longitudinal No; Accelerates No Construction No Faulty Construction No No 

Alligator (fatigue) Yes; Accelerates Spring-Thaw Strength loss Thickness Yes Yes, Mix Yes No 

Transverse No; Accelerates Low Temp. Freeze-Thaw Cycles Thermal Properties No Yes, Temp. Susceptible No No Cracking 

Slippage Yes No Loss of Bond Yes Yes, Bond No No 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 6 May 2006

Table 7-2. Moisture-related distresses in rigid (PCC) pavements (NHI 13126; adapted after Carpenter et al. 1979).

Structural Defect Begins in 

Type Distress Manifestation Moisture Problem Climatic Problem Material Problem Load Associated Distress PCC Base Subgrade 

Spalling Possible Freeze/Thaw Cycles Mortar No Yes No No 

Scaling Yes Freeze/Thaw Cycles Chemical Influence No Yes; Finishing No No 

D-Cracking Yes Freeze/Thaw Cycles Aggregate Expansion No Yes No No 

Surface 

Defects 

Crazing No No Rich Mortar No 

Yes; 

Weak 

Surface 

No No 

Blow-up No Temperature Thermal Properties No Yes No No 

Pumping and 

Erosion Yes Moisture Inadequate Strength Yes No Yes Yes 

Faulting Yes Moisture- Suction Erosion- Settlement Yes No Yes Yes Surface Deformation 

Curling/Warping Yes Moisture & Temperature Moisture and Temperature Differentials No Yes No No 

Corner Yes Moisture 

Cracking 

Follows Erosion 

Yes No Yes Yes 

Diagonal 

Transverse 

Longitudinal 

Yes Moisture Follows Erosion Yes No Yes Yes Cracking 

Punchout 

(CRCP) Yes Moisture 

Deformation 

Follows 

Cracking 

Yes No Yes Yes 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 7 May 2006

7.2.2 Approaches to Address Moisture in Pavements 

As was indicated in Chapter 3, to avoid moisture-related problems, a major objective in 

pavement design should be to keep the base, subbase, subgrade, and other susceptible paving 

materials from becoming saturated or even exposed to constant high moisture levels. The 

three approaches described in detail in Chapter 3 for controlling or reducing the problems 

caused by moisture are 

• prevent moisture from entering the pavement system. 

• use materials and design features that are insensitive to the effects of moisture. 

• quickly remove moisture that enters the pavement system 

No single approach can completely negate the effects of moisture on the pavement system 

under heavy traffic loading over many years. For example, it is practically impossible to 

completely seal the pavement, especially from moisture that may enter from the sides or 

beneath the pavement section. While materials can be incorporated into the design that are 

insensitive to moisture, this approach is often costly and in many cases not feasible (e.g., may 

require replacing the subgrade). Drainage systems also add cost to the road. Maintenance is 

required for both drainage systems and sealing systems, for them to effectively perform over 

the life of the system. Thus, it is often necessary to employ all approaches in combination to 

obtain the most effective design. The first two approaches involve the surficial pavement 

materials, which are well covered in the NHI courses on pavement design (e.g., NHI 

131060A “Concrete Pavement Design Details and Construction Practices” and the 

participant’s manual) and will not be covered herein. The geotechnical aspects of these 

approaches include drainage systems for removal of moisture, the requirements of which will 

be reviewed in the following subsections. Durable base material requirements will be 

reviewed in the subsequent section, and followed by subgrade stabilization methods to 

mitigate moisture issues in the subgrade. A method of sealing to reduce moisture intrusion 

into the subgrade will also be reviewed in the subgrade stabilization section. 

7.2.3 Drainage in Pavement Design 

Removal of free water in pavements can be accomplished by draining the free water 

vertically into the subgrade, or laterally though a drainage layer into a system of collector 

pipes. Generally, the actual process will be a combination of the two (ASSHTO, 1993). 

Typically in wet climates, if the subgrade permeability is less than 3 m/day (10 ft/day), some 

form of subsurface drainage or other design features to combat potential moisture problems 

should be considered. Table 7-3 provides additional climatic conditions and traffic 

considerations to assist in the assessment of the need for subsurface drainage. 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 8 May 2006

The quality of drainage is defined in both AASHTO 1993 and NCHRP 1-37A based on the 

principle of time-to-drain. Time-to drain is the time required following any significant 

rainfall event for a pavement system to drain from a saturated state to a specific saturation or 

drainage level (e.g., 50% drainage level in AASHTO 1993). The concept can also be applied 

(at least qualitatively) to other significant moisture events that would saturate the pavement 

(i.e., flood, snow melt, or capillary rise). The definitions of poor to excellent drainage 

provided by AASHTO (1993) are given in Table 7-4. 

Table 7-3. Assessment of need for subsurface drainage in new or reconstructed 

pavements (NCHRP 1-37 A, adapted after NHI 13126). 

Greater than 12 million 20-

yr design lane heavy trucks 

Between 2.5 and 12 million 20-

yr design lane heavy trucks 

Less than 2.5 million 20-yr 

design lane heavy trucks 

ksubgrade (m/day) 

Climatic 

Condition 

< 3 3 to 30 > 30 < 3 3 to 30 > 30 < 3 3 to 30 > 30

WetFreeze 

R R F R R F F NR NR 

WetNo Freeze 

R R F R F F F NR NR 

DryFreeze 

F F NR F F NR NR NR NR 

DryNo Freeze 

F NR NR NR NR NR NR NR NR 

LEGEND: 

 ksubgrade = Subgrade permeability. 

R = Some form of subdrainage or other design features are recommended to combat 

potential moisture problems. 

F = Providing subdrainage is feasible. The following additional factors need to be 

considered in the decision making: 

(1) Past pavement performance and experience in similar conditions, if any. 

(2) Cost differential and anticipated increase in service life through the use of 

various drainage alternatives. 

(3) Anticipated durability and/or erodibility of paving materials. 

 NR = Subsurface drainage is not required in these situations. 

 Wet Climate = Annual precipitation > 508 mm (20 in.) 

 Dry Climate = Annual precipitation < 508 mm (20 in.) 

Freeze = Annual freezing index > 83 oC-days (150 oF-days) 

No-Freeze = Annual freezing index < 83 oC-days (150 oF-days)
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 9 May 2006

Table 7-4. AASHTO definitions for pavement drainage recommended for use in 

both flexible and rigid pavement design (AASHTO, 1993). 

Quality of Drainage Water Removed* Within 

Excellent 2 hours 

Good 1 day 

Fair 1 week 

Poor 1 month 

Very Poor Does not Drain 

* Based on 50% time-to-drain. 

As reviewed in Chapters 3, 5, and 6, drainage effects on pavement performance are 

incorporated into both the AASHTO 1993 and in the NCHRP 1-37A design methods. In 

AASHTO 1993, the effect of drainage is considered by modifying the structural layer 

coefficient (for flexible pavements) and the load transfer coefficient (for rigid pavements) as 

a function of the quality of drainage and the percent of time the pavement structure is near 

saturation. The influence of the drainage coefficient (Cd) for rigid pavement design and a 

drainage modifier (m) for flexible pavement design were demonstrated in the sensitivity 

studies shown in Chapter 6. 

In the NCHRP 1-37A pavement design guide, the impact of moisture on the stiffness 

properties of unbound granular and subgrade materials is considered directly through the 

modeling of the interactions between climatic factors (rainfall and temperatures), 

groundwater fluctuations, and material characteristics of paving layers. Drainage coefficients 

are not used. However, the benefits of incorporating drainage layers are apparent in terms of 

distress predictions, which consider seasonal changes in unbound layers and subgrade 

properties due to moisture and coupled moisture-temperature effects. 

Using either the AASHTO 1993 or NCHRP 1-37A method, the influence on design can be 

significant. For example, in high rainfall areas, the base section of a flexible pavement 

system (with a relatively thick base layer) can be reduced in thickness by as much as a factor 

of 2, or the design life extended by an equivalent amount, if excellent drainage is provided 

versus poor drainage. Likewise, an improvement in drainage leads to a reduction in Portland 

cement concrete (PCC) slab thickness. 

Achieving poor drainage is relatively simple. If the subgrade is not free draining (e.g., not a 

clean sand or gravel), then the pavement section will require drainage features to drain. Even 

with edge drainage (i.e., daylighted base or edgedrains), drainage could still be poor. Many 

designers choose to use dense graded base for its improved construction and presumed 
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Geotechnical Aspects of Pavements 7 - 10 May 2006

structural support over free-draining base. Unfortunately, dense graded base usually does not 

readily drain and, as a result, structural support will most likely decrease over time. 

Due to the low permeability of dense graded base and long drainage path to the edge of the 

road, drainage in dense graded base is, at best, extremely slow. For example, consider that 

the permeability of a dense graded base with a very low percentage of fine-grain soil (less 

than 5% smaller than a 0.075 mm {No. 200 U.S. sieve}) is about 0.3 m/day (1ft/day)(as was 

reviewed in Chapter 5). Also consider that the length of the drainage path for a two-lane road 

(lane width of the road draining from the centerline to the edge) is typically 3.7 m (12 ft). An 

optimistic estimate of the time required to drain a base section that is 300 mm (1 ft) thick and 

has a slope of 0.02 is 2 days. According to AASHTO definitions of drainage, the pavement 

section has “good” to “fair” drainage. If the length of the drainage path is two lanes (i.e., 7.3 

m {24 ft}), it would take up to a week for the pavement to drain; a condition defined as “fair” 

drainage (AASHTO, 1993). Base materials often contain more than 5% fines, in which case 

the permeability and, correspondingly, the drainage can easily be an order of magnitude less 

than the estimated value for the example (AASHTO, 1993)1

. In a recent study a Midwestern 

state found base materials from six different quarries to have 12% to 19% fines and 

corresponding field permeabilities measured at 2 to 0.01 m/day (7 to 0.03 ft/day) (Blanco et 

al., 2003). A month or more will then be estimated for pavement drainage; a condition 

defined as “poor” to “very poor” in AASHTO 1993. In reality, capillary effects and the 

absence of a driving head of water often cause dense graded base to act like a sponge at low 

hydraulic gradients. This results in trapped water in the pavement section and “very poor” 

drainage (e.g., see Dawson and Hill, 1998). 

In order to achieve good to excellent drainage, a more permeable, open-graded base and/or 

subbase will be required, which is tied into a subsurface drainage system. However, this 

approach only works for new or reconstructed pavements. For existing pavements, 

retrofitting drainage along the edges of the pavement is the only option, and the existing base 

material may not drain. However, a significant amount of water can enter the pavement at the 

crack between the shoulder and the pavement, as well as from lateral movement of water 

from outside the shoulder. Specific guidelines do not exist currently for retrofit pavements, as 

only limited data are available. Local experience should be used in selecting pavement 

candidates for retrofitting. Performance of similar retrofitted sections, if available, can be a 

valuable tool in the decision making process. 

 

1

 Based on hydraulic conductivity tests, AASHTO notes a decrease in permeability from 3 m/day (10 ft/day) 

with 0% fines down to 0.02 m/day (0.07 ft/day), with the addition of only 5% non-plastic fines and (0.0003 

m/day (0.001 ft/day) with 10% non-plastic fines. An additional order of magnitude decrease was observed with 

base containing plastic fines.
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 11 May 2006

7.2.4 Types of Subsurface Drainage 

In the past, pavement systems were designed without any subdrainage system. These sections 

are commonly labeled “bathtub” or “trench” sections because infiltrated water is trapped in 

the base and subbase layers of the pavement system. 

Many types of subsurface drainage have been developed over the years to remove moisture 

from the pavement system. These subsurface drainage systems can be classified into several 

groups. One criterion for classifying various subsurface drainage systems is the source of 

moisture that the system is designed to control. For example, a groundwater control system

refers to a subsurface drainage system designed to remove and control the flow of 

groundwater. Similarly, an infiltration control system is designed to remove water that 

seeps into the pavement structural section. A capillary break system is designed to intercept 

and remove rising capillary water and vapor movement. 

Probably the most common way to classify a subsurface drainage system is in terms of its 

location and geometry. Using this classification, subsurface drainage systems are typically 

divided into five distinct types: 

• Longitudinal edgedrains. 

• Transverse and horizontal drains. 

• Permeable bases. 

• Deep drains or underdrains. 

• Interceptor drains. 

Each type may be designed to control several sources of moisture and may perform several 

different functions. In addition, the different types of subsurface drainage system may be 

used in combination to address the specific needs of the pavement being designed. Drains 

constructed primarily to control groundwater general consist of underdrains and/or 

interceptor drains. The interceptor drains are usually placed outside the pavement system to 

intercept the lateral flow of water (e.g., from cut slopes) and remove it before it enters the 

pavement section. Deep underdrains (greater than 1 m {3 ft} deep) are usually installed to 

lower the groundwater level in the vicinity of the pavement. The design and placement of 

these interceptor and underdrains should be addressed as part of the geotechnical 

investigation of the site. 

Edgedrains placed in trenches under the shoulders at shallower depths are used to handle 

water infiltrating the edge of the pavement from above. Edgedrains are combined with 

permeable base and, in some cases, transverse and horizontal drains to form a drainable 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 12 May 2006

pavement system to control surface infiltration water. Drainable pavement systems generally 

consist of the following design features (as shown in Figure 7-2): 

• a full-width permeable base under the AC- or PCC-surfaced travel lanes, 

• a separator layer under the permeable base to prevent contamination from subgrade 

materials, 

• longitudinal edgedrains with closely spaced outlets. An alternative to closely spaced 

outlets is dual drainage systems with parallel collector drains. An alternative to 

edgedrains is daylighting directly into a side ditch. 

Designs not incorporating these combinations of features cannot be expected to function 

properly. Drainage systems for new construction and rehabilitation are described in more 

detail in the following sections. 

Figure 7-2. Design elements of a drainable pavement system (after FHWA, 1992).
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 13 May 2006

7.2.5 Daylighted Base Sections 

Daylighted bases were one of the first attempts to remove surface infiltration water from the 

pavement system. The original daylighted base consists of a dense-graded aggregate base 

that extends to the ditchline or side slope. Daylighted dense-graded bases are expected to 

intercept water that infiltrates through the pavement surface and drain the water through the 

base to the ditch. However, most dense-graded daylighted bases are slow draining and, 

therefore, not very effective in removing infiltrated water. 

This situation led to the development of a new generation of daylighted bases—daylighted 

permeable bases (Fehsenfeld 1988), as illustrated in Figure 7-3. Several studies have reported 

that daylighted permeable bases are as effective in removing infiltrated water and reducing 

moisture-related distresses as permeable bases with edgedrains (Yu et al. 1998b). However, 

they require regular maintenance because the exposed edge of daylighted bases easily 

becomes clogged with fines, soil, vegetation, and other debris. Also, stormwater from ditch 

lines can easily backflow into the pavement structure. Further study into daylighted 

permeable bases is needed to verify long-term performance of this design. 

7.2.6 Longitudinal Edgedrains 

Longitudinal edgedrains consist of a drainage system that runs parallel to the traffic lane. The 

edgedrains collect water that infiltrates the pavement surface and drains water away from the 

pavement through outlets. Four basic types of edgedrains systems have been used: 

• pipe edgedrains in an aggregate filled trench, 

• pipe edgedrains with porous concrete (i.e., cement treated permeable base) filled 

trench, 

• prefabricated geocomposite edgedrains in a sand backfilled trench, and 

• aggregate trench drain (“French” drain). 

The most commonly used edgedrain is a perforated pipe varying in diameter from 100 – 150 

mm (4 – 6 in.). The pipe is generally situated in an aggregate trench to allow water to flow 

toward the pipe. Another type of edgedrain that is often used in rehabilitation projects is a 

geocomposite drain in a sand filled trench with pipe outlets. Typical cross sections of 

edgedrains are illustrated in Figures 7-5 and 7-6. 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 14 May 2006

Figure 7-3. Typical AC pavement with a daylighted base. 

300 m m x 300 m m shou ld er 

d rain w ith filter fabric

#57 stone

100 m m p erforated p ip e

A C sh ou ld er

Trav el lan es Sh ou ld er

AC su rface

Aggregate base

Su bgrad e

Figure 7-4. Typical AC pavement with pipe edgedrains (ERES, 1999).

Sand Backfill

Geocomposite 

drain 

Aggregate base

Subbase/Subgrade

25 mm

100 mm

AC Shoulder PCC pavement

Figure 7-5. Typical PCC pavement with geocomposite edgedrains (ERES, 1999).

Shoulde

r 

Permeable base 

Base layer 

Pavement

Subgrade 

Ditch

Geotextile 

Embankment
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 15 May 2006

Figure 7-6. Typical edgedrains for rehabilitation projects (NCHRP 1-37A). 

The effectiveness of longitudinal edgedrains depends on how they are used. Longitudinal 

edgedrains can be effective if used with other drainage features. Typical application of 

edgedrains include the following: 

• New construction 

o Longitudinal edgedrains (pipe or geocomposite) with nonerodible densegraded bases*. 

o Longitudinal edgedrains (pipe or geocomposite) with permeable bases. 

• Existing pavement 

o Retrofit longitudinal edgedrains (pipe or geocomposite) with nonerodible 

dense-graded bases*. 

• Rehabilitation projects 

o Retrofit longitudinal edgedrains (pipe or geocomposite) with nonerodible 

dense-graded bases*. (On projects using rubblized base or dense graded base 

with erodible fines, the geotextile filter in the trench should not be placed 

between the base and the edgedrain aggregate to avoid clogging the geotextile 

filter - see Figure 7-6.) 

* Retrofit edgedrains are usually not recommended for pavements with dense-graded aggregate 

bases containing more than 15% fines (fraction passing the 0.075 mm (No. 200 sieve)). Excessive 

fines can clog the drains, and the loss of fines through the pipes can lead to significant base erosion. 

As previously indicated, dense graded base with greater than 5% fines is not anticipated to freely 

drain, however edgedrains can be used to effectively remove water entering the longitudinal joint (or 

crack) between the pavement and the shoulder. 

The field performance of edgedrains installed without a permeable base has been mixed. 

Some studies show little or no benefit, but others report significant improvement in pavement 

performance. Considering that only about 30% of all edgedrains in-service are functioning 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 16 May 2006

properly, mostly due to improper construction (Daleiden 1998; Sawyer 1995), the mixed 

report is not surprising. In many cases, the outlet pipes are crushed during construction or 

clogged due to inadequate maintenance. The performance of edgedrains placed in untreated 

dense-graded base sections seems to be dependent to a significant degree on local climatic 

conditions, natural drainage characteristics, subgrade type, pavement design, construction 

and construction inspection, and maintenance. Longitudinal edgedrains with permeable bases 

have been found to be effective in draining pavements and reducing moisture-related 

distresses when well designed, constructed, and maintained. 

The type of geocomposite edgedrains used also affects performance. Older versions did not 

have sufficient hydraulic capacity and had not been recommended for draining permeable 

bases. However, some of the geocomposites available today do provide sufficient hydraulic 

capacity to drain permeable bases. The main disadvantage of geocomposite edgedrains is that 

they are difficult to maintain. 

The use of aggregate trench drains, however, is not recommended because of low hydraulic 

capacity and inability to be maintained. An exception might be permeable cement stabilized 

aggregate placed in a trench. 

The size of the longitudinal perforated pipe in the edgedrain is often based on maintenance 

requirements for cleaning capabilities and reasonable distance between outlets. Maintenance 

personnel should be consulted before finalizing these dimensions. The smallest diameter 

suitable for cleaning is 75 mm (3 in.), however many state highway agencies and the FHWA 

suggest a minimum pipe size of 100 mm (4 in.) based on maintenance considerations 

(FHWA, 1992). FHWA also recommends a maximum outlet spacing of 75 m (250 ft). 

One of the most critical items for edgedrains is the grade of the invert. Construction control 

of very flat grades is usually not possible, leaving ponding areas that result in subgrade 

weakening and premature failures. Although not a popular concept, it may be more 

economical to raise the pavement grade to develop adequate drain slopes for the subsurface 

drainage facilities (e.g., Florida). To achieve a desirable drainage capacity, a minimum slope 

may be required for the edgedrain that is greater than the slope of the road. However, this 

requirement may not be practical, and the pipe will mostly be sloped the same as the 

roadway. It is suggested that rigorous maintenance be anticipated, especially when adequate 

slopes cannot be achieved (FHWA, 1992). 

The ditch or storm drain pipe must be low and large enough to accept the inflow from the 

edgedrain without backup. FHWA recommends the outlet be at least 150 mm (6 in.) above 

the ten-year storm flow line of the ditch or structure. The outlet should also be at a location 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 17 May 2006

and elevation that will allow access for maintenance activities (both cleaning and repair). 

Outlets and shallow pipes should be located well away from areas of expected future surface 

maintenance activities, such as sign replacement and catch basin cleanout or repair. FHWA 

also recommends angled or radius outlet connections to facilitate clean out and video 

inspection. Outlet headwalls, typically precast concrete, are also an essential part of the 

edgedrain system to prevent displacement of the outlet pipe and crushing during roadway and 

ditchline maintenance operations. Locations of guardrail, sign, signal, and light posts must be 

adjusted to prevent damage to the subsurface drainage facilities. 

An offset dual pipe with a large diameter parallel collector drain line is an alternative to 

decrease the number of outlets (see Figure 7-7). The large diameter collector pipe (either 

heavy walled plastic or concrete) runs either adjacent to or below a perforated drainage pipe, 

as shown in Figure 7-7, to facilitate quick removal of subsurface water. The collector pipe 

can outflow into culverts or stormwater systems. Manholes can be installed for cleanout. 

These systems are quite common in Europe and have been used by a few U.S. agencies to 

reduce outlet maintenance issues (e.g., California and, experimentally, in Kentucky). 

7.2.7 Permeable Bases 

A permeable base is designed to rapidly move surface infiltration water from the pavement 

structure to the side ditch through longitudinal edgedrains with outlets or by daylighting 

directly into the side ditch. Permeable bases contain no fines (0% passing the 0.075-mm (No. 

200) sieve) to allow easy flow of water. In order to meet excellent drainage requirements 

(i.e., time-to-drain of less than 2 hours from Table 7-4), permeable bases typically are 

required to have permeability values in excess of 300 m/day (1000 ft/day) and thicknesses of 

100 mm (4 in.) (as recommended by FHWA, 1992). The performance of permeable base 

layers meeting these requirements will be demonstrated later in Section 7.2.12 on design of 

pavement drainage. 

The structural capacity of angular, crushed aggregate permeable base, with a percentage of 

two-face crushing, is usually equivalent to the structural capacity of an equal thickness of 

dense-graded base. However, in order to meet these hydraulic requirements, a coarse uniform 

gravel must be used, which is often difficult to construct. Asphalt or cement treatments are 

often used to stabilize the gravel for construction, as discussed in Section 7.3. While 

stabilizing the base with a cement or asphalt binder will initially offer greater structural 

support than dense-graded base, it should be remembered that the primary purpose of the 

stabilizer is to provide stability of the permeable base during the construction phase. It is 

generally assumed that the binder will either break down or be removed by stripping with 

time. Thus, increase in structural support is generally not assumed for stabilized aggregate. 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 18 May 2006

Figure 7-7. Dual pipe edgedrain systems showing alternate locations of the parallel collector 

pipe, either adjacent to or beneath the drain line (Christopher, 2000).

Typical cross-sections of AC and PCC pavements with permeable bases were illustrated in 

Figures 7-4, 7-5, and 7-6. Note that a geotextile filter should be wrapped around a portion of 

the trench, but not over the interface between the permeable base and drainage aggregate. 

7.2.8 Dense-Graded Stabilized Base with Permeable Shoulders 

This system consists of a nonerodible dense-graded base, typically lean concrete base (LCB) 

or asphalt treated base (ATB), under the traffic lanes and a permeable base under the 

shoulder. Longitudinal edgedrains are placed in the permeable base course to carry the excess 

moisture from the pavement structure. The recommended design for a dense-graded 

stabilized base with permeable shoulders is illustrated in Figure 7-8. This design offers better 

support under the traffic lanes where it is needed most, while still providing a means to 

remove water from the pavement structure. This design is now required for all high-type 

PCC pavements (pavements designed for more than 2.5 million equivalent single axle loads 

{ESALs}) in California (CALTRANS 1995). 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 19 May 2006

Shoulder

Nonerodible base

PCC 

pavement

Aggregate subbase

Geotextile

Slotted pipe

Porous

concrete

Figure 7-8. Recommended design of PCC pavement with a nonerodible dense-graded base 

and permeable shoulders (ERES, 1999). 

7.2.9 Horizontal Geocomposite Drains 

Several states (i.e., Maine, Wisconsin, and Virginia) have experimented with the use of 

horizontal geocomposite drains, with properties sufficient to handle the estimated flow and 

support traffic loads, placed either below or above dense graded base, placed as a drainage 

layer beneath full depth asphalt, or placed between a crack and seat concrete surface and a 

new asphalt layer. When placed below the base aggregate, the geocomposite shortens the 

drainage path and reduces the time-to-drain. When placed directly beneath the pavement 

surface, the geocomposite intercepts and removes infiltration water before it enters the base 

and/or subgrade. The geocomposite is tied into an edgedrain system. Systems using this 

technology have been found to have excellent drainage (i.e., time-to-drain of less than 2 

hours from Table 7-4). Additional information, including preliminary performance 

information, is reported by Christopher et al. (2002). 

7.2.10 Separator Layers 

Separator layers play an essential role in the performance of a pavement with a permeable 

base by preventing fines in the underlying layers and subgrade soils from infiltrating into the 

permeable base, thus maintaining the permeability and effective thickness of the base course. 

Various combinations of materials have been used as separator layers, including the 

following (FHWA 1994a):
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Geotechnical Aspects of Pavements 7 - 20 May 2006

• Dense-graded aggregate (most used by far) 

• Geotextiles 

• Cement-treated granular material 

• Asphalt chip seals 

• Dense-graded asphalt concrete 

These materials have been used with varying degrees of success. Lime- or cement-treated 

subgrades alone are not acceptable as separator layers over fine-grained soils. There have 

been some classic failures of lime-treated soils used as separator layers in which pumping 

into the permeable base caused excessive settlements. 

According to a survey of 33 states, 27 used dense-graded aggregates or asphalt-treated 

mixtures as separator layers on a regular basis. Sixteen states used geotextiles sparingly, and 

11 states used either dense-graded material or geotextiles as separator layers (Yu et al., 

1998). Generally, a dense-graded aggregate or a dense-graded AC material separator layer is 

preferred over a geotextile for competent subgrades because the aggregate layer will provide 

a strong construction platform and distribute traffic loads to the subgrade. However, 

geotextile separator layers have been used directly beneath base layers where the additional 

support of a subbase is not required. For sensitive subgrades that are easily disturbed by 

construction (e.g., silts and saturated cohesive soils), a geotextile separator layer used in 

conjunction with a granular subbase minimizes disturbance and provides a good construction 

platform. Geotextile separators also allow the use of a more open-graded, freer-draining 

subbase, reducing the potential for subbase saturation. Geotextiles can also be used as a 

separator layer in conjunction with compacted or treated subgrades, or granular subbases. If 

appropriately design, geosynthetics can also be used to increase subgrade support, as 

reviewed later in Section 7.6.5. 

 

7.2.11 Performance of Subsurface Drainage 

Many studies have shown the benefits of subsurface drainage in terms of improved 

performance. Cedergren (1988) believes that all important pavements should have internal 

drainage, claiming drainage eliminates damage, increases the life of the pavement, and is 

cost-effective. 

Moisture-related damage to pavements has become more significant as traffic loadings have 

increased over the past 40 years. The annual rate of ESAL applications has virtually doubled 

every 10 years, causing tremendous problems related to moisture accelerated damage. A 

pavement may be adequately drained for one level of traffic, but as traffic increases, 

moisture-related damage may increase greatly. As a result, more and more states have begun 

to employ subsurface drainage systems (Yu et al. 1998b). Many preliminary studies indicate 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 21 May 2006

that drainage systems are indeed beneficial in terms of reducing certain types of pavement 

deterioration. However, due to some instances of poor design, construction, and/or 

maintenance, all have not performed as well as expected. 

One example of unsatisfactory performance is some early cracking observed on a few PCC 

pavements with permeable bases. This occurs for a variety of reasons, including: 

• Inadequate design of permeable bases and separator layers. 

• Inadequate edgedrains. 

• Lack of quality control during construction, such as inadequate joint sawing. 

Sometimes the concrete from the slab enters the permeable base, creating a thicker 

slab than was originally designed. Joints must be sawed deeper to ensure the proper 

depth is obtained to cause cracking through the joint. 

• Lack of maintenance of the drainage system after the highway is open to traffic. 

• Possible settlement of the PCC slab over untreated aggregate permeable bases. 

Permeable bases must be constructed of durable, crushed aggregate to provide good stability 

through aggregate interlock. They must have a separator layer capable of preventing the 

pumping of fines into the permeable base from underlying layers and from preventing any 

intermixing of the permeable base and separator layer. Permeable bases must also have pipe 

edgedrains to drain the infiltrated water with suitable outlets at reasonable outlet spacing or 

must be daylighted directly into the ditch. Finally, to ensure good performance, the drainage 

system must be regularly maintained. 

7.2.12 Design of Pavement Drainage 

Design of pavement drainage consists of determining: 

1. The hydraulic requirements for the permeable layer to achieve the required time-todrain. 

2. The edgedrain pipe size and outlet spacing requirements. 

3. Either the gradation of requirements for a graded aggregate separation layer or the 

opening size, permeability, endurance, and strength requirements for geotextile 

separators. 

4. The opening size, permeability, endurance, and strength requirements for geotextile 

filters, or the gradation of the granular filters (to be used in the edgedrain). 

 The following provides an outline of the design steps and procedures required for the design 

of each of these subsurface drainage components. Complete design details and supporting 

information can be found in NHI 13126 on Pavement Subsurface Drainage Design – 

Reference Manual (ERES, 1999). 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 22 May 2006

7.2.13 Hydraulic Requirements for the Permeable Layer(s) 

Basically there are two approaches to the hydraulic design of a permeable layer: 

1. Time-to-drain 

2. Steady-state flow. 

The time-to-drain approach was previously discussed in Section 7.2.3 and simply means the 

time required for a percentage of the free water (e.g., 50%) to drain, following a moisture 

event where the pavement section becomes saturated. In the steady-state flow approach, 

uniform flow conditions are assumed, and the permeable layer is designed to drain the water 

that infiltrates the pavement surface. The time-to-drain approach will be the basis for design 

in this manual, as it is currently the procedure recommended by the FHWA, AASHTO, and 

NCHRP 1-37A for pavement design. Elements of steady state flow will be used to determine 

outlet spacing. (For additional discussion of steady state flow methods see FHWA, 1992 and 

ERES, 1999.) 

The time-to-drain approach assumes the flow of water into the pavement section until it 

becomes saturated (the drainage layer plus the material above the drainage layer). Excess 

precipitation will not enter the pavement section after it is saturated; this water will simply 

run off the pavement surface. After the rainfall event, the drainage layer will drain to the 

edgedrain system. Engineers must design the permeable layer to drain relatively quickly to 

prevent the pavement from being damaged. 

A time-to-drain of 50% of the drainable water in 1 hour is recommended as a criterion for the 

highest class roads with the greatest amount of traffic (FHWA, 1992). For most other high 

use roadways, a time-to-drain of 50% of the drainable water in 2 hours is recommended. For 

secondary roads, a minimum target value of 1 day is recommended (U.S. Army Corps of 

Engineers, 1992). Remember, in all cases, the goal of drainage is to remove all drainable 

water as quickly as possible. 

 

The time-to-drain is determined by the following equation: 

where, t = time-to-drain in hours 

 T = Time Factor 

 m = “m” factor (see Eq. 7.3) 

 

t =T ×m × 24 Eq. 7.1 
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A simplified design chart for determining a time-to-drain of 50% time factor, T50, is provided 

in Figure 7-9. This chart was developed for one degree (i.e., direction) of drainage and is 

adequate for most designs. For expanded charts to cover additional degrees of drainage and 

desired percent drained see FHWA, 1992 and ERES, 1999. 

The time factor is based on the geometry of the drainage layer (e.g., the permeable base 

layer). The geometry includes the resultant slope (SR) and length (LR); the thickness of the 

drainage layer (H), which is the length the water must travel within a given layer; and, the 

percent drained (U), (i.e., 50%). SR and LR are based on the true length of drainage and are 

determined by finding the resultant of the cross and longitudinal pavement slopes (SX and S, 

respectively) and lengths (Lx and L, respectively). The resultant length is measured from the 

highest point in the pavement cross-section to the point where drainage occurs (i.e.,

edgedrain or daylighted section). First, the slope factor (Sl) must be calculated: 

 where, SR = (S2

 + SX

2

)

1⁄2

 LR = W [1 + (S/SX)

2

]

1⁄2 

 W = width of permeable layer in m (ft) 

 H = thickness of permeable layer in m (ft) 

 1 ft = 0.3 m 

Figure 7-9 is then entered with the Sl, and the resulting T50 to be used in Eq. 7.1 is 

determined. 

The “m” factor in Eq 7.1 is determined by the equation: 

where, No = the effective porosity of the drainage layer 

 k = permeability of drainage layer in m/day (ft/day) 

 H = thickness of drainage layer in m (ft) 

ψ = the transmissivitty of the drainage layer in m2/day (ft2/day) 

 1 ft = 0.3 m 

 Eq. 7.2 

S

L S

H

R R

1 =

m

N L

kH

N L

Eq oR oR = =

2 2

7 3

ψ . .
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Geotechnical Aspects of Pavements 7 - 24 May 2006

Figure 7-9. Time Factor for 50% Drainage (ERES, 1999). 

The intrinsic factors that represent the drainage capabilities of drainage layer base are 

represented by the effective porosity (No) and the coefficient of permeability (k) or, if H is 

known, the transmissivity of the drainage layer. The effective porosity is the ratio of the 

volume of water that can drain under gravity from the material to the total volume of the 

material. It is a measure of the amount of water that can be drained from a material. The 

value can be easily determined by saturating a sample of material and measuring the amount 

of water that drains. Additional information on the determination of these characteristics for 

aggregate drainage layers are covered in detail in FHWA, 1992 and NHI 13126. 

For example, using the recommended 4-inch-thick open-graded base layer with a 

permeability of 300 m/day (1000 ft/day) at a cross slope of 2% in a relatively flat (1% grade) 

road alignment would produce the following time-to-drain for a four lane road draining from 

the center (W = 7.3 m (24 ft)): 

1 

2 

3 

5 

7 

0 

.01 .03 .10 .30 .60

Slope Factor (S1)

Time Factor (T50)
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Geotechnical Aspects of Pavements 7 - 25 May 2006

 SR = (S2

 + SX

2

)

1⁄2 = (0.012

 + 0.022)

1⁄2 = 0.022 

 LR = W((1 + (S/SX)

2

)

1⁄2 = 24 ft [1 + (0.01/0.02)2

]

1⁄2 = 26.8 ft 

 Sl = (LR SR)/H = (26.8 ft x 0.022)/ 0.33 ft = 1.8 

 m = (NeLR

2

) / (kH) = (0.25 x (26.8 ft) 2)/(1000 ft/day x 0.33 ft) = 0.54 days 

From Figure 7-9 with Sl = 1.8, T = 0.16 

Therefore, t = T x m x 24 = 0.16 x 0.54 days x 24 hrs/day = 2.1 hrs 

Since the time-to-drain is close to 2 hrs, the drainage layer would provide excellent drainage, 

as defined in Table 7-4. 

According to a sensitivity analysis on time-to-drain performed in ERES, 1999, time-to-drain 

is most sensitive to changes in the coefficient of permeability and the resultant slope, 

decreasing exponentially with increasing permeability and slope values. Time to drain 

increases linearly with increasing length and effective porosity, while thickness has very little 

effect. 

The DRIP microcomputer program developed by FHWA can be used to rapidly evaluate the 

effectiveness of the drainage system and calculate the design requirements for the permeable 

base design, separator, and edgedrain design, including filtration requirements. The program 

can also be used to determine the drainage path length based on pavement cross and 

longitudinal slopes, lane widths, edgedrain trench widths (if applicable), and cross-section 

geometry crowned or superelevated. The software can be downloaded directly from the 

FHWA WEB page http://www.fhwa.dot.gov/pavement/library.htm and is included with the 

NCHRP 1-37A pavement design software. 

7.2.14 Edgedrain Pipe Size and Outlet Spacing Requirements 

The FHWA recommends a minimum pipe diameter of 100 mm (4 in.) and a maximum outlet 

spacing of 75 m (250 ft) to facilitate cleaning and video inspection. The adequacy of these 

requirements can be confirmed by evaluating the anticipated infiltration rate or, more 

conservatively, from the maximum flow capacity of the drainage layer. 

With the flow capacity method, the estimated discharge rate from drainage layer is 

determined. For example, the conventional 100-mm (4-in.) thick open-graded base layer with 
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Geotechnical Aspects of Pavements 7 - 26 May 2006

a permeability of 300 m/day (1000 ft/day) used in the previous time-to-drain example 

provides excellent drainage for most conditions (FHWA, 1992). This 100-mm (4-in.) thick 

free-draining base layer has a transmissivity (i.e., permeability multiplied by the thickness) of 

about 28 m2

/day (300 ft2/day). For a typical roadway gradient of 0.02 (for a 2% grade), the 

open-graded base layer has a flow capacity of 0.13 ft3/day (6 ft3/day) per ft length of road. 

Thus at an outlet spacing of 75 m (250 ft), the quantity of flow at the discharge (Q) of the 

edgedrain system would be 33 m3

/day (1500 ft3/day). 

The capacity of a circular pipe flowing full can be determined by Manning’s equation: 

 where, Q = Pipe capacity, cu ft/day 

 D = Pipe diameter, in. 

 S = Slope, ft/ft 

 n = Manning’s roughness coefficient 

 = 0.012 for smooth pipe 

 = 0.024 for corrugated pipe 

 1 ft = 0.3 m 

 1 in = 25.4 mm 

Thus, for a 100-mm (4-in.) smooth wall pipe at a 1% grade, the flow capacity is 504 m3

/day 

(17800 ft3/day), which is more than adequate to handle the maximum quantity of flow 

anticipated for the edgedrain system. However, the 100-mm (4-in.) pipe is still recommended 

to facilitate inspection and cleaning. 

 

In the infiltration method, a design rainfall and an infiltration ratio are selected. Pavement 

infiltration is determined by the equation 

 qi = C x R x 1/12 (ft/in) x 24 (hr/day) x 1 ft x 1 ft) Eq. 7.5

which can be simplified to: 

 qi = 2 C R Eq. 7.5a 

where, qi = Pavement infiltration, ft3/day/ft2 of pavement 

 C = Infiltration ratio 

 R = Rainfall rate, in./hr 

Q

n = D S Eq

5301

7 4

8

3

1

2 . . .
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The infiltration ratio C represents the portion of rainfall that enters the pavement through 

joints and cracks. The following design guidance for selecting the infiltration coefficient is 

suggested (FHWA, 1992): 

 Asphalt concrete pavements 0.33 to 0.50 

 Portland cement concrete pavements 0.50 to 0.67 

To simplify the analysis and provide an adequate design, FHWA suggest using a value of 

0.5. The design storm whose frequency and duration will provide an adequate design must be 

selected. A design storm of 2-year frequency, 1-hour duration, is suggested. Figure 7-10 

provides a map of generalized rainfall intensity. 

The analysis is then performed by substituting into the above equation for the specific region 

of the country. The drainage layer discharge rate qd can then be determined by multiplying 

the infiltration rate by the resultant length of the pavement section LR as follows: 

 qd = qi LR Eq. 7.6

This discharge rate can then be compared to the flow capacity of the drainage layer and the 

lower value of the two used to evaluate the outlet spacing and pipe size. 

7.2.15 Separator Layer 

As indicated in the previous section, the separator consists of a layer of aggregate material 

(treated or untreated) or a geotextile layer placed between the permeable base and the 

subgrade or other underlying layers. The separator layer has to maintain separation between 

permeable base and subgrade, and prevent them from intermixing and support construction 

traffic. It may also be desirable to use a low permeable layer that will deflect water from the 

permeable base horizontally toward the pavement edge (NCHRP 1-37A). 

If dense-graded aggregate separator layers are used, the aggregate must be a hard, durable 

material. Based on FHWA guide specifications for materials selection and construction of 

aggregate separation layers, the aggregate should meet the following requirements: 

• The aggregate should have at least two fractured faces, as determined by the material 

retained on the 4.74 mm (No. 4) sieve; preferably, it should consist of 98% crushed 

stone. 

• The L.A. abrasion wear should not exceed 50%, as determined by AASHTO T 96, 

Resistance to Abrasion of Small Size Coarse Aggregate by Use of Los Angeles 

Machine. 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 28 May 2006

Figure 7-10. Rainfall Intensity in in./hr for a 2-year, 1-hour Storm Event (FHWA, 1992). 

• The soundness loss percent should not exceed 12 or 18%, as determined by the 

sodium sulfate or magnesium sulfate tests, respectively. The test shall be in 

accordance with AASHTO T 104, Soundness of Aggregate by the Use of Sodium 

Sulfate or Magnesium Sulfate. 

• The gradation of this layer should be such that it allows a maximum permeability of 

approximately 5 m/day (15 ft/day) with less than 12% of the material passing the 

0.075 mm (No. 200) sieve, by weight. 

• Material passing the 425 mm (No. 40) sieve shall be nonplastic, in accordance with 

AASHTO T 90, “Determining the Plastic Limit and Plasticity Index of Soils.” 

7.2.16 Geotextile Separator and Filter Design 

As a separator, just as with the granular layer, the geotextile must prevent the intermixing of 

the permeable base and the adjacent subgrade or subbase layer. Also as with aggregate 
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Geotechnical Aspects of Pavements 7 - 29 May 2006

separator layers, the geotextile layers will have to satisfy filtration criteria. In order to 

provide support for construction traffic, the geotextile must also satisfy survivability and 

endurance criteria. Additional requirements for subgrade improvement are reviewed in 

Section 7.6.5. Both woven and non-woven geotextiles have been used for the separation 

application. The criteria for filtration and survivability are outlined in the following 

paragraphs and are basically the same as that required for the edgedrain geotextile filters. The 

only notable exception is that the separation layer can have a much lower permeability 

(compatible with the subgrade) than the edgedrain filter (compatible with the permeable 

base). 

As a filter for the edgedrain, the geotextile must be designed to allow unimpeded flow of 

water into edgedrain system over the life of the system. The geotextile must prevent soil from 

washing into the system without clogging over time. The FHWA presents three basic 

principles for geotextile design and selection (Holtz et al., 1998): 

1. If the larger pores in the geotextile filter are smaller than the largest particles of soil, 

these particles will not pass the filter. As with graded granular filters, the larger 

particles of soil form a filter bridge on the geotextile, which, in turn, filters the 

smaller particles of the soil. Thus, the soil is retained and particle movement and 

piping is prevented. 

2. If the smaller openings in the geotextile are sufficiently large so that the smaller 

particles of soil are able to pass through the filter, then the geotextile will not clog. 

3. A large number of openings should be present in the geotextile so that proper flow 

can be maintained even if some of the openings later become clogged. 

The geotextile filtration characteristics must be checked for compatibility with the gradation 

and permeability of the subgrade. The requirements for proper performance can be 

appropriately selected by using the following design steps. 

Step 1. Determine the gradation of the material to be separated/filtered. The filtered material 

is directly above and below the geocomposite drainage layer. Determine D85, D15 and 

percent finer than a 0.075 mm (No. 200) sieve. 

Step 2. Determine the permeability of the base or subbase kbase/subbase, whichever is located 

directly above the geocomposite drainage layer. (For placement directly beneath the 

hot-mix or PCC pavement applications, the default permittivity requirement will be 

used. 

Step 3. Apply design criteria to determine apparent open size (AOS), permeability (k), and 

permittivity (ψ) requirements for the geotextile (after Holtz et al., 1998) 

 AOS ≤ D85 base/subbase (For woven geotextile) 

 AOS ≤ 1.8 D85 subgrade (For nonwoven geotextile)* 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 30 May 2006

 kgeotextile ≥ kbase/subbase 

 ψ ≥ 0.1 sec-1 

 * For noncohesive silts and other highly pumping susceptible soils, a filter bridge 

may not develop, especially considering the potential for dynamic, pulsating flow. A 

conservative (smaller) AOS # D85 subgrade is advised, and laboratory filtration tests are 

recommended. 

Step 4. In order to perform effectively, the geotextile must also survive the installation 

process. AASHTO M288 (1997) provides the criteria for geotextile strength required 

to survive construction of roads, as shown in Table 7-5. Use Class 2 where a 

moderate level of survivability is required (i.e., for subgrade CBR > 3, where at least 

150 mm (6 in.)) of base/subbase and normal weight construction equipment is 

anticipated, and where filters are used in edgedrains). Class 1 geotextiles are 

recommended for CBR < 3 and when heavy construction equipment is anticipated. 

For separation layers, a minimum of 150 mm (6 in.) of base/subbase should be 

maintained between the wheel and geotextile at all times. 

In projects using recycled concrete, rubblizing, or crack-and-seat techniques, geotextiles and 

granular filters are susceptible to clogging by precipitate and should not be indiscriminately 

used to separate the permeable base from the drain or wrapped around pipes. Geotextiles 

should not be placed between the recycled material and the drain, but could be placed 

beneath and on the outside of the drain to prevent infiltration of the subgrade and subbase 

layers (see Figure 7-2.) 

Table 7-5. Geotextile survivability requirements (AASHTO M 288-96). 

Test Test Units Geotextile Class 

 Method Class 1 Class 2 

 < 50%* > 50%* < 50%* > 50%* 

Grab Strength ASTM D 4632 N 1400 900 1100 700 

Seam Strength ASTM D 4632 N 1200 810 990 630 

Tear Strength ASTM D 4533 N 500 350 400 250 

Puncture Strength ASTM D 4833 N 500 350 400 250 

Burst Strength ASTM D 3786 kPa 3500 1700 2700 1300 

*Note: Elongation measured in accordance with ASTM D 4632 with < 50% typical of woven 

geotextiles and > 50% typical of nonwoven geotextiles. (1 N = 0.22 lbs, 1 kPa = 0.145 psi) 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 31 May 2006

7.3 BASE LAYERS: REQUIREMENTS, STABLILIZATION & REINFORCEMENT 

The function of the base course varies according to the type of pavement, as was described in 

Chapter 1. Under rigid pavements, the base course is used to: (1) provide uniform and stable 

support, (2) minimize damaging effects of frost action, (3) provide drainage, (4) prevent 

pumping of fine-grained soils at joints, (5) prevent volume change of the subgrade, (5) 

increase structural capacity of the pavement, and (6) expedite construction. Under flexible 

pavements, the prime function of the base course is to structurally improve the loadsupporting capacity of the pavement by providing added stiffness and resistance to fatigue, as 

well as to provide a relatively thick layer to distribute the load through a finite thickness of 

pavement. The base may also provide drainage and give added protection against frost action 

where necessary. 

To meet these functional requirements, the base course as a minimum should have the 

following characteristics: 

• To prevent pumping, a base course must be either free draining or it must be highly 

resistant to the erosive action of water. Erodibility is covered in more detail in the 

next section. 

• To provide drainage, the base course may or may not be a well-graded material, but it 

should contain little or no materials finer than a 0.075 mm (No. 200) sieve. It may 

sometimes be stabilized with asphalt or cement. 

• A base course design for frost action should be non-frost susceptible and free 

draining. 

• To improve resistance to deformation and improve structural support or reduce the 

thickness, it may be desirable to stabilize the base course with asphalt or cement, as 

reviewed in Section 7.3.2 and 7.3.3, or to reinforce it with geosynthetics, as reviewed 

in Section 7.3.4. 

• A base course need not be free draining to provide structural capacity, but it should be 

well-graded and should resist deformation due to loading. 

The aggregate used for base must be hard, durable material. As a minimum, the aggregate 

should meet the following requirements: 

• The aggregate should have at least two fractured faces; preferably, it should consist of 

98% crushed stone. 

• The L.A. abrasion wear should not exceed 45% as determined by AASHTO T 96, 

Resistance to Abrasion of Small Size Coarse Aggregate by Use of Los Angeles 

Machine. 
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Geotechnical Aspects of Pavements 7 - 32 May 2006

• The soundness loss percent should not exceed 12 or 18%, as determined by the 

sodium sulfate or magnesium sulfate tests, respectively. The test should be 

performed in accordance with AASHTO T 104, Soundness of Aggregate by the Use 

of Sodium Sulfate or Magnesium Sulfate (see Chapter 5). 

• For permeable base, the gradation of this layer should enable free movement of water 

with a minimum permeability value around 300 m/day (1,000 ft/day) (see Section 

7.2) and the material passing the 0.425 mm (No. 40) sieve should be non-plastic in 

accordance with AASHTO T 90, Determining the Plastic Limit and Plasticity Index 

of Soils. 

 

7.3.1 Erodibility of Bases 

Preventing significant erosion of the base and subbase materials is very important for the 

control of moisture-related distresses, such as pumping and faulting in JPCP and punchouts 

in CRCP, as discussed in NCHRP 1-37A. Erodibility is the loss of base material due to 

hydraulic action, most often at the joints in rigid pavements, but also along the edge of both 

rigid and flexible pavements. The condition is related to the durability of the base in relation 

to its potential to break down under dynamic traffic loads, climatic conditions, environmental 

effects, as well as water action. As truck traffic increases, a more erosion resistant base is 

required, along with more adequate joint load transfer design (e.g., use of dowels in joints). 

Traffic level is a very critical factor in the consideration of base/subbase course erosion, 

especially considering that the base/subbase under PCC slabs of reconstructed projects will 

likely receive 10 to 20 times more load repetitions over their design life than in the past. 

While the base course is the layer most often affected by erosion, any layer directly beneath a 

treated base can experience serious erosion. There are many examples of the erosion of fine 

grained soils beneath a stabilized base course causing loss of support and joint faulting. Thus, 

some agencies now place a dense graded granular subbase layer between the base and 

compacted subgrade to reduce this problem. Other agencies stabilize the top layer of a finegrained soil with lime to reduce this problem; however, this approach must produce a 

sufficiently hard material with adequate compressive strength and uniformity along the 

project. Geotextiles are also used as separation layers to hold the subgrade materials in place. 

Another alternative that has been used successfully is to place a layer of recycled crushed 

PCC beneath the dense treated base. 

The NCHRP 1-37A guide provides guidance for assessing the erodibility potential of various 

materials used in new JPCP and CRCP design and in PCC overlays of existing flexible or 

rigid pavements. The effect of erosion is considered empirically in the form of erodibility 

classification assessment for specific design levels. The design procedure provides the 
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Geotechnical Aspects of Pavements 7 - 33 May 2006

framework for which erosion can be considered on a more mechanistic basis in the future 

(such as iterative month-by-month damage accumulation, and inclusion of Level 1 laboratory 

erosion test). Tables 7-6, 7-7, and 7-8 provide the Material Classification requirements for 

Level 1, Level 2, and Level 3 design, respectively. 

7.3.2 Bound Bases 

In order to achieve the highest erodibility levels, stabilized base or subbase materials often 

produced by the addition of a sufficient quantity of stabilizing agent (usually cement or 

asphalt) to produce materials with significant tensile strength (e.g., Erodibility Class 1a in 

Table 7-7). Such materials are considered to be bound bases and have a substantial increase 

in structural capacity over that of unbound and modified (treated) bases. Bound bases or 

subbases are not considered to be geotechnical materials, and are not covered in this manual. 

Users are referred to NHI courses on pavements (e.g., NHI 131033) for additional 

information. 

Table 7-6. Level 1 recommendation for assessing erosion potential of base material 

(NCHRP 1-37A).

Erodibility 

Class Material Description and Testing

Class based on 

the material 

type and test 

results 

Test not fully developed for nationwide uses; thus Level 1 cannot be implemented at 

this time. 

The tests currently being considered to assess the erodibility of paving materials include 

- Rotational shear device for cohesive or stabilized materials (Bhatti et al., 1996). 

- Jetting test (Bhatti et al., 1996). 

- Linear and rotational brush tests (Dempsey, 1982). 

- South African erosion test (DeBeer, 1990). 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

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Table 7-7. Design Level 2 recommendations for assessing erosion potential of base material 

(NCHRP 1-37A adapted after the Permanent International Association of Road Congresses, PIARC, 1987). 

Erodibility 

Class Material Description and Testing

1 

(a) Lean concrete with approximately 8% cement; or with long-term compressive strength 

> 17.2 MPa (2,500 psi) [> 13.8 MPa (2,000 psi) at 28-days] and a granular subbase 

layer or a stabilized soil layer or a geotextile fabric is placed between the bound base 

and subgrade; otherwise Class 2. 

(b) Hot mixed asphalt concrete with 6% asphalt cement that passes appropriate stripping 

tests and aggregate tests and a granular subbase layer or a stabilized soil layer; 

otherwise Class 2. 

(c) (c) Permeable drainage layer (asphalt-treated aggregate or cement-treated aggregate) 

and with an appropriate granular or geotextile separation layer placed between the 

treated permeable base and subgrade. 

2 

(a) Cement-treated granular material with 5% cement manufactured in-plant, or long-term 

compressive strength 13.8 to 17.2 MPa (2,000 to 2,500 psi) [10.3 MPa to 13.8 MPa 

(1,500 to 2,000 psi) at 28-days] and a granular subbase layer or a stabilized soil layer 

or a geotextile fabric is placed between the treated base & subgrade; otherwise Class 3.

(b) (b) Asphalt-treated granular material with 4% asphalt cement that passes appropriate 

stripping test and a granular subbase layer or a treated soil layer or a geotextile is 

placed between the treated base and subgrade; otherwise Class 3. 

3 

(a) Cement-treated granular material with 3.5% cement manufactured in-plant, or with 

long-term compressive strength 6.9 MPa to 13.8 MPa (1,000 to 2,000 psi) [5.2 MPa to 

10.3 MPa (750 to 1,500 psi) at 28-days]. 

(b) Asphalt-treated granular material with 3% asphalt cement that passes appropriate 

stripping test. 

4 Unbound crushed granular material having dense gradation and high quality aggregates. 

5 Untreated soils (PCC slab placed on prepared/compacted subgrade). 
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Table 7-8. Design Level 3 recommendations for assessing erosion potential of base 

material based on material description only (NCHRP 1-37A).

Erodibility 

Class Material Description and Testing

1 

(a) Lean concrete with previous outstanding past performance and a granular subbase 

layer or a stabilized soil layer or a geotextile layer is placed between the treated base 

and subgrade; otherwise Class B. 

(b) Hot mixed asphalt concrete with previous outstanding past performance and a granular 

subbase layer or a stabilized soil layer is placed between the treated base and 

subgrade; otherwise Class B. 

(c) Permeable drainage layer (asphalt- or cement-treated aggregate) and a granular or a 

geotextile separation layer between the treated permeable base and subgrade. 

Unbonded PCC Overlays: HMAC separation layer (either dense or permeable graded) 

is specified. 

2 

(a) Cement-treated granular material with good past performance and a granular subbase 

layer or a stabilized soil or a geotextile layer is placed between the treated base and 

subgrade; otherwise Class C. 

(b) Asphalt-treated granular material with good past performance and a granular subbase 

layer or a stabilized soil layer or a geotextile is placed between the treated base and 

subgrade; otherwise Class C. 

3 

(a) Cement-treated granular material that has exhibited some erosion and pumping in the 

past. 

(b) Asphalt-treated granular material that has exhibited some erosion and pumping in the 

past. Unbonded PCC Overlays: Surface treatment or sand asphalt is used. 

4 Unbound crushed granular material having dense gradation and high quality aggregates. 

5 Untreated subgrade soils (compacted). 

 

7.3.3 Modified (or Treated) Bases 

The addition of cement or asphalt (typically less than 5%) to stabilize unbound base or 

subbase with the primary purpose of improving the stability for construction are considered 

to be modified or treated bases. Modified materials are usually considered to behave 

structurally as unbound granular material. These bases or subbases are considered to be 

geotechnical materials. Stabilization is most often required for open graded (permeable) 

bases (OGB), which tend to rut and weave under construction activities. Tables 7-9 and 7-10 

provide the recommendations for asphalt-treated bases and cement-treated bases, 

respectively. 
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The strength of cement-treated bases will depend in part on adequate curing during 

construction. The mixture must be well compacted at optimum moisture content, and 

adequate density must be obtained throughout the layer. Density control will also be 

important for the uniformity of asphalt-treated base materials. Although stabilization is often 

used to reduce the thickness of the base, it should be recognized that thin bases (less than 150 

mm (6 in.) thickness) are often extremely difficult to construct to the exact depth, creating 

the potential for very thin base layers in localized areas. Construction of thin bases requires a 

very competent subgrade or a good working platform (as reviewed in Section 7.6). 

Construction quality control for cement- and asphalt-treated materials is reviewed in Chapter 8. 

Table 7-9. Recommended asphalt stabilizer properties for asphalt-treated permeable 

base/subbase materials. 

Specification Requirement Test Method

(a) hard, durable material with at least two 

fractured faces; preferably, consisting of 

98% crushed stone. 

Visual Classification 

(b) L.A. abrasion wear should not exceed 45%. AASHTO T 96 

Aggregate 

(c) Soundness loss percent should not exceed 

12 as determined by the sodium sulfate, or 

18% by the magnesium sulfate tests. 

AASHTO T 104, Soundness of 

Aggregate by the Use of Sodium 

Sulfate or Magnesium Sulfate 

AC content AC content must ensure that aggregates are 

well coated. Minimum recommended AC 

content is between 2.5 – 3% by weight. Final 

AC content should be determined according to 

mix gradation and film thickness around the 

coarse aggregates. 

ASTM D 2489, Test Method for 

Degree of Particle Coating of 

Bituminous-Aggregate 

Mixtures.

AC grade A stiff asphalt grade (typically 1 grade stiffer 

than the surface course is recommended). 

Penetration, viscosity, or Superpave 

binder testing can be performed to 

determine AC grade. 

Antistripping 

Anti-stripping test should be performed on all 

AC treated materials. 

AASHTO T283, Resistance of 

Compacted Bituminous Mixture to 

Moisture Induced Damage.

Antistripping 

Agents

Aggregates exhibiting hydrophilic 

characteristics can be counteracted with 

0.5 – 1% lime. 

NCHRP Report 274. 

Permeability Minimum mix permeability: 300 m/day 

(1000 ft/day). 

AASHTO T 3637, Permeability of 

Bituminous Mixtures.
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Table 7-10. Recommended Portland cement stabilizer properties for cement-treated 

permeable base/subbase materials.

Specification Requirement Test Method 

(a) Hard, durable material with at least two 

fractured faces; preferably, consisting of 98 

percent crushed stone. 

Visual Classification 

(b) (a) L.A. abrasion wear should not exceed 

45%. 

AASHTO T 96-94 

Aggregate 

(c) (c) Soundness loss percent should not 

exceed 12 or 18%, as determined by the 

sodium sulfate or magnesium sulfate tests, 

respectively. 

AASHTO T 104-86, “Soundness of 

Aggregate ...Use of Sodium Sulfate 

or Magnesium Sulfate” 

Cement Portland cement content selected must ensure 

that aggregates are well coated. An application 

rate of 130 to 166 kg/m3

 (220 to 285 lb/yd3) is 

recommended. 

Must conform to the specification of 

AASHTO M 85, Portland Cement

Water-tocement ratio 

Recommended water-to-cement ratio to ensure 

strength and workability: 0.3 to 0.5. 

Workability Mix slump should range between 25 – 75 mm 

(1 – 3 in.). 

Cleanness Use only clean aggregates 

Permeability Minimum mix permeability: (300 m/day) 

1,000 ft/day. 

7.3.4 Base Reinforcement 

A more recent form of stabilization is the use of geosynthetics (primarily geogrids) to 

reinforce the base for flexible pavement systems, which has been found under certain 

conditions to provide significant improvement in performance of pavement sections. The 

principal effect of reinforcement in base-reinforced flexible pavements is to provide lateral 

confinement of the aggregate layer. Lateral confinement arises from the development of 

interface shear stresses between the aggregate and the reinforcement, which, in turn, transfers 

load to the reinforcement. The interface shear stress present when a traffic load is removed 

continues to grow with traffic load applications, meaning that the lateral confinement of the 

aggregate increases with increasing load applications. Increases in traffic volume up to a 

factor of 10 to reach the same distress level (25-mm (1 in.) rutting) have been observed for 

reinforced sections, versus unreinforced sections of the same design asphalt and base 

thickness (Berg et al., 2000). Table 7-11 provides a summary of the conditions for which 

various geosynthetic products should be considered for this application. 
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Table 7-11. Qualitative review of reinforcement application potential for paved 

permanent roads (after Berg et al., 2000). 

Roadway Design 

Conditions Geosynthetic Type

Geotextile Geogrid 2 GG-GT Composite

Subgrade

Base / 

Subbase 

Thickness1

(mm)

Nonwoven Woven Extruded

Knitted 

or 

Woven

OpenGraded 

Base3

WellGraded 

Base

Soft 150 – 300 ï ● ● ❏ ● æ

(CBR < 3) 

(MR <30 MPa) > 300 ï ï TM TM TM æ

Firm - Vy. Stiff 150 – 300 ❍ TM ● ❏ ● æ 

(3 ≤ CBR ≤ 8) 

(30 ≤ MR ≤ 80) > 300 ❍ ❍ ❍ ❍ ❍ ❍

KEY: ● — usually applicable TM — applicable for some conditions 

❍ — usually not applicable ❏ — insufficient information at this time æ — see note 

NOTES: 1. Total base or subbase thickness with geosynthetic reinforcement. Reinforcement may be 

placed at bottom of base or subbase, or within base for thicker (usually > 300 mm (12 in.)) 

thicknesses. Thicknesses less than 150 mm (6 in.) not recommended for construction over 

soft subgrade. Placement of less than 150 mm (6 in.) over a geosynthetic not recommended. 

2. For open-graded base or thin bases over wet, fine grained subgrades, a separation geotextile 

should be considered with geogrid reinforcement. 

3. Potential assumes base placed directly on subgrade. A subbase also may provide filtration. 

ï Reinforcement usually applicable, but typically addressed as a subgrade stabilization. 

æ Geotextile component of composite likely is not required for filtration with a well-graded 

base course; therefore, composite reinforcement usually not applicable.

Current design methods for flexible pavements reinforced with a geosynthetic in the unbound 

aggregate base layer are largely empirical methods based on a limited set of design 

conditions over which test sections have been constructed (i.e., AASHTO 4E-SR Standard of 

Practice Guidelines for Base Reinforcement). These design methods have been limited in use 

due to 1) absence of nationally recognized reinforced base design procedure, 2) narrow range 

of test section design conditions from which the method was calibrated, and 3) proprietary 

design methods pertaining to a single geosynthetic product. Recently FHWA sponsored a 

study to develop an interface for including geosynthetic base reinforcement in mechanistic 

empirical design, consistent with the NCHRP 1-37A model. This work is currently in review, 

but shows excellent promise for the incorporation of these methods into pavement design. 
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In the interim, AASHTO 4E includes a design approach that relies upon the assessment of 

reinforcement benefit as defined by a Traffic Benefit Ratio (TBR) or a Base Course reduction 

Ratio (BCR). TBR is defined as the ratio of the number of traffic loads between an otherwise 

identical reinforced and unreinforced pavement that can be applied to reach a particular 

permanent surface deformation of the pavement. BCR defines the percentage reduction in the 

base course thickness of a reinforced pavement such that equivalent life (e.g., surface 

deformation) is obtained between the reinforced and the unreinforced pavement with the 

greater aggregate thickness. The philosophy of this approach is one in which applicability of 

the technology and reinforcement benefits are assessed by empirical considerations. 

Reinforcement benefit defined in this manner is then used to modify an existing unreinforced 

pavement design. 

The proposed design procedure in AASHTO 4E follows the steps listed below: 

Step 1. Initial assessment of applicability of the technology. 

Step 2. Design of the unreinforced pavement. 

Step 3. Definition of the qualitative benefits of reinforcement for the project. 

Step 4. Definition of the quantitative benefits of reinforcement (TBR or BCR). 

Step 5. Design of the reinforced pavement using the benefits defined in Step 4. 

Step 6. Analysis of life-cycle costs. 

Step 7. Development of a project specification. 

Step 8. Development of construction drawings and bid documents. 

Step 9. Construction of the roadway. 

Step 1 involves assessing the project-related variables given in Table 7-11 and making a 

judgment on whether the project conditions are favorable or unfavorable for reinforcement to 

be effective and what types of reinforcement products (as defined in Table 7-11) are 

appropriate for the project. 

Step 2 involves the design of a conventional unreinforced typical pavement design cross 

section or a series of cross sections, if appropriate, for the project. Any acceptable design 

procedure can be used for this step. 

Step 3 involves an assessment of the qualitative benefits that will be derived by the addition 

of the reinforcement. The two main benefits that should be assessed are whether the 

geosynthetic will be used for an extension of the life of the pavement (i.e., the application of 

additional vehicle passes), a reduction of the base aggregate thickness, or a combination of 

the two. Berg et al. (2000) has listed additional secondary benefits that should also be 

considered. 
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Step 4 is the most difficult step in the design process and requires the greatest amount of 

judgment. This step requires the definition of the value, or values, of benefit (TBR and/or 

BCR) that will be used in the design of the reinforced pavement. The definition of these 

benefit values for a range of design conditions is perhaps the most actively debated and most 

currently studied topic within this field. Given the lack of a suitable analytical solution for 

the definition of these terms, Berg et al. (2000) has suggested that these values be determined 

by a careful comparison of project design conditions, as defined in previous steps, to 

conditions present in studies reported in the literature. The majority of these studies have 

been summarized in Berg et al. (2000) in a form that allows direct comparison to known 

project conditions. In the absence of suitable comparison studies, an experimental 

demonstration method involving the construction of reinforced and unreinforced pavement 

test sections has been suggested and described in Berg et al. (2000), and may be used for the 

definition of benefit for the project conditions. The reasonableness of benefit values should 

be carefully evaluated such that the reliability of the pavement is not undermined. 

Step 5 involves the direct application of TBR or BCR to modify the unreinforced pavement 

design defined in Step 2. TBR can be directly used to define an increased number of vehicle 

passes that can be applied to the pavement, while BCR can be used to define a reduced base 

aggregate thickness such that equal life results. Within the context of an AASHTO pavement 

design approach, it is possible to calculate a BCR knowing a TBR and vise versa for the 

specific project design conditions, however this approach has not been experimentally or 

analytically validated. 

With the unreinforced and reinforced pavement designs defined, a life-cycle cost analysis 

should be performed to assess the economic benefit of reinforcement. This step will dictate 

whether it is economically beneficial to use the geosynthetic reinforcement. Remaining steps 

involve the development of project specifications, construction drawings, bid documents, and 

plans for construction monitoring. Berg et al. (2000) has presented a draft specification that 

may be adopted for this application. 

Even though the application of geosynthetic reinforcement of flexible pavements has been 

proposed and examined over the past 20 years, research in this area is quite active, meaning 

that new design methods should be expected in the near future. These new design methods 

will hopefully provide less empirical methods for assessing reinforcement benefit and be 

expressed as a function of the variables that are known to influence benefit. 
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7.4 COMPACTION 

Compaction of the subgrade, unbound base, and subbase materials is a basic design detail 

and is one of the most fundamental geotechnical operations for any pavement project. 

Compaction is used to increase the stiffness and strength, decrease the permeability, and 

increase the erosion resistance of geomaterials. Compaction can also reduce the swelling 

potential for expansive soils. Thus, the intent of compaction is to maximize the soil strength 

(and minimize the potential volume change) by the proper adjustment of moisture and the 

densification at or near the ideal moisture content, as discussed in this section. 

In most instances, once heavy earthwork and fine grading is completed, the uppermost zone 

of subgrade soil (roadbed) is improved. The typical improvement technique is by means of 

water content adjustments and densification by compaction. Higher density requirements are 

routinely established for the top two feet of at-grade roadbeds and for embankments. The soil 

in cut areas may need to be undercut and backfilled to obtain the strength and uniformity 

desired. Heavy proof rolling equipment (270 to 450 kN (30 to 50 tons)) can be used to 

identify areas of non-uniform support in prepared subgrades. Proofrolling and other field 

construction aspects of compaction are covered in Chapter 8. Perhaps the most common 

problem arising from deficient construction is related to moisture-density control, which can 

be avoided or at least minimized with a thorough plan and execution of the plan as it relates 

to QC/QA during construction, as reviewed in Chapter 8. This plan should pay particular 

attention to proper moisture content, proper lift thickness for compaction, and sufficient 

configuration (e.g., weight and width) of the compaction equipment utilized. 

7.4.1 Compaction Theory 

The basic engineering principles of soil compaction date back to work by Proctor in the 

1930s. Compaction can be performed in the laboratory using static, kneading, gyratory, 

vibratory, or impact compactors. Each method has its advantages and disadvantages, but 

impact compaction using a falling hammer is the standard in practice today. Standard 

laboratory compaction tests are described in more detail in Chapter 5. In these tests, soil is 

mixed with water at a range of moisture contents w and compacted using a specified 

compaction energy (e.g., ft-lbs/ft3

 or joules/m3). Figure 7-11 illustrates the effect of 

compaction energy on laboratory compaction curves. As described in Chapter 5, the 

Modified Proctor compaction test (ASTM D1557/ AASHTO T-180) has a compaction 

energy of 2,700 kN-m/m3

 (56,000 ft-lb/ft3), which is nearly 5 times the compaction energy of 

600 kN-m/m3 (12,400 ft-lb/ft3) in the Standard Proctor test (ASTM D698/AASHTO T-99). 

Likewise, increased compaction energy in the field will increase the maximum dry unit 

weight and decrease the associated optimum water content. 
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Figure 7-11. Effect of compaction energy on compaction curves (Coduto, 1999). 

Different soils will generally have differently shaped compaction curves. This fact will aid in 

identifying the corresponding laboratory curve for materials encountered in the field. Figure 

7-12 shows typical compaction curves for several different soils. Coarser, granular soils 

typically have fairly steep compaction curves, with large changes in density for small 

changes in moisture content, while highly plastic clays exhibit fairly flat compaction curves. 

The maximum dry density is higher for coarser soils and the optimum moisture content is 

lower. Some cohesionless soils will also exhibit two peaks in the compaction curve; one at 

very dry conditions, where there are no capillary tensions to resist the compaction effort, and 

the other at the optimum moisture content, where optimum lubrication between particles 

occurs. 

Nearly all compaction specifications are based on achieving a minimum dry unit weight in 

the field. This is usually expressed in terms of the relative compaction CR: 

( )max

R

d 100%

d

C γ

γ

= × Eq. 7-5 

in which γd is the dry unit weight achieved in the field and (γd)max is the maximum dry unit 

weight as determined from a specified laboratory compaction test. 
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Figure 7-12. Laboratory compaction curves for different soils (Rollings and Rollings, 1996). 

The water content at compaction is also sometimes specified because of its effect on soil 

fabric, especially for clays. Clays compacted dry of optimum have a flocculated fabric (see 

Figure 7-13), which generally corresponds to higher permeability, greater strength and 

stiffness, and increased brittleness. Conversely, clays compacted wet of optimum to the same 

equivalent dry density tend to have a more oriented or dispersed fabric, which typically 

corresponds to lower permeability, lower strength and stiffness, but more ductility. 
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Figure 7-13. Effect of compacted water content on soil fabric for clays (Coduto, 1999). 

7.4.2 Effect on Soil Properties 

The principal effects of compaction on soil properties are as follows: 

• Density: As described in the preceding sections, the most direct measurable effect of 

compaction is an increase in soil density. Typical laboratory values of maximum dry 

density values and optimum moisture contents for different soils were summarized in 

Chapter 5, Table 5-18 and 5-19. 

• Strength: Intuitively, one expects strength to increase with compaction energy and to be 

larger at low water contents than at high values. Figure 7-14 summarizes typical strength 

versus water content and compaction energy for a lean clay where strength is quantified 

by CBR (Rollings and Rollings, 1996). The data in the figure generally confirm intuitive 

expectations. The strength dry of the optimum water content is larger for higher 

compaction energies, as expected, and is up to an order of magnitude higher than the 

strength when compacted wet of optimum. Note, however, that higher compaction 

energies can produce slightly lower strength values when a fine-grained soil is compacted 

at water contents higher than the optimum. Also note that the strength in the figure is 

based on unsaturated soils. If material compacted dry of optimum becomes 

saturated, a significant decrease in strength can occur, with strengths even less than 

that of the same soil compacted wet of optimum. Large changes in strength upon 

wetting are associated with fine-grained silts and clays, and are less pronounced or even 

negligible in coarse-grained soils (Rollings and Rollings, 1996). 
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Figure 7-14. Strength as measured by CBR and dry density vs. water content for laboratory 

impact compaction (Rollings and Rollings, 1996). 
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• Stiffness: Figure 7-15 summarizes typical stiffness versus water content and compaction 

energy behavior for clays, where stiffness is defined as the stress required to case 5% and 

25% axial strain in a triaxial compression test (Seed and Chan, 1959). Stiffness increases 

with compaction energy when compacted dry of optimum and is largely independent of 

compaction energy when compacted wet of optimum. The stiffness dry of optimum is 

also substantially larger than when compacted wet of optimum, as would be expected. 

Again however, a significant decrease in stiffness can occur if the material becomes 

saturated to the extent that the stiffness could be less than that of the soil compacted 

wet of optimum.

• Permeability: Permeability at constant compactive effort decreases with increasing water 

content and reaches a minimum at about the optimum moisture content. The permeability 

when compacted dry of optimum is about an order of magnitude higher than the value 

when compacted wet of optimum. 

• Swelling/Shrinkage Potential: Swelling of compacted clays is greater when compacted 

dry of optimum. Dry clays have a greater capacity to absorb water, and thus swell more. 

Soils dry of optimum are in general more sensitive to environmental influences, such as 

changes in water content. The situation is just the opposite for shrinkage (Figure 7-16), 

where samples compacted wet of optimum exhibit the highest shrinkage strains as water 

is removed from the soil. 

7.5 SUBGRADE CONDITIONS REQUIRING SPECIAL DESIGN ATTENTION 

Considering variables such as soil type or mineralogy along a length of roadway, the geology 

(soil genesis and deposition method) and groundwater and flow properties make each project 

unique with respect to subgrade conditions. It is not surprising that certain conditions will 

exist that are not conducive to support, or even construction, of pavement systems. This 

section provides an overview of subgrade conditions that require special design attention. 

These subsurface conditions are often regional in nature and have usually been identified as 

problematic by the agency. Several foundation problems, such as collapsible or highly 

compressible soils, expansive or swelling soils, subsurface water and saturated soils, and 

frost-susceptible soils, occur extensively across the U.S. and are not specific to one region. 

For example, frost heave occurs in over half of the states in the U.S. and damage may be 

most severe in the central states, where many more frost cycles occur than in the mostnorthern states. Identification of these widely variable problematic subgrade conditions are 

also reviewed in this section, along with design and construction alternatives to achieve an 

adequate foundation on which to build the pavement structure. 
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Figure 7-15. Stiffness as a function of compactive effort and water content (after Seed and 

Chan, 1959; from Holtz and Kovacs, 1981). 
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Figure 7-16. Shrinkage as a function of water content and type of compaction (after Seed 

and Chan, 1959; from Holtz and Kovacs, 1981). 
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Most of the subgrade conditions presented in this section can be anticipated through a 

complete exploration program, as described in Chapter 4, and mitigated or at least minimized 

via well-conceived designs. By identifying such subgrade issues in the design stage, or even 

the potential for such problems along an alignment, alternative designs can be established. 

Alternate designs can then be placed in the bid documents with indicators clearly identified 

that show where these alternatives should be considered, and then implemented if and where 

such conditions are encountered. When these special subgrade conditions are not recognized 

in design, they are often identified during construction, usually resulting in claims and 

overruns. However, identifying problems in construction is still somewhat fortunate, 

considering the impact such problems may have on the pavement performance. If the soil 

conditions described in this section go undetected, there typically is decreased serviceability, 

usually resulting in premature localized rehabilitation or, not uncommon, reconstruction of 

the pavement within the first few years of the pavement performance period. 

7.5.1 Problematic Soil Types 

Obviously, a pavement is to be constructed on whatever material and condition is naturally 

occurring. The strength and stability of some soils can present problems during construction 

and certainly can affect the long-term performance of the pavement during its service life. In 

order to properly discuss these potential problems, it is necessary to define some terms as 

they relate to problematic mineralogy (Sowers, 1979). Some of the terms are true geological 

terminology, while some are local or regional terminology. The terms may describe a 

particular material or condition, but all are problematic and care must be taken when 

constructing pavements in regions containing these materials. 

Adobe. Sandy clays of medium plasticity found in the semiarid regions of the southwestern 

U.S. These soils have been used for centuries to make sun-dried brick. The name is also 

applied to some highly plastic clays of the West, which swell significantly when wet. 

Bentonite. Highly plastic clay, usually montmorillonite, resulting from the decomposition of 

volcanic ash. It may be hard when dry, but swells considerably when wet. 

Buckshot clay. Applied to clays of the southern and southwestern United States. Cracks into 

small, hard, relatively uniform sized lumps on drying. Dry lumps will degrade upon wetting 

(e.g., after they have been used as fill). These soils also tend to swell when wet. 
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Caliche. A silt or sand of the semiarid areas of the southwestern United States that is 

cemented with calcium carbonate. The calcium carbonate is deposited by the evaporation of 

water brought to the ground surface by capillary action. The consistency of caliche varies 

from soft rock to firm soil. 

Coquina. A soft, porous limestone made up largely of shells, coral, and fossils cemented 

together. Very friable, and breaks down during construction. 

Gumbo. A fine-grained, highly plastic clay of the Mississippi Valley. It has a sticky, greasy 

feel, highly expansive, and forms large shrinkage cracks on drying. 

Kaolin. A white or pink clay of low plasticity. It is composed largely of minerals of the 

kaolinite family. 

Loam. A surface soil that may be described as a sandy silt of low plasticity or a silty sand 

that is well suited to tilling. It applies to soils within the uppermost horizons and should not 

be used to describe deep deposits of parent material. Loam-type soils are typically sensitive 

to moisture, easily disturbed in construction, and frost susceptible. 

Loess. A deposit of relatively uniform, windblown silt. It has a loose structure, with 

numerous rootholes that produce vertical cleavage and high vertical permeability. It consists 

of angular to subrounded quartz and feldspar particles cemented with calcium carbonate or 

iron oxide. Upon saturation, it becomes soft and compressible because of the loss of 

cementing. Loess altered by weathering in a humid climate often becomes more dense and 

somewhat plastic (loess loam). Loess is also highly frost susceptible. 

Marine clay. Clays deposited in a marine environment, which, if later uplifted, tend to be 

extra sensitive due to salt leaching, dramatically losing strength when disturbed. 

Marl. A water-deposited sand, silt, or clay containing calcium carbonate. Marls are often 

light to dark gray or greenish in color and sometimes contain colloidal organic matter. They 

are often indurated into soft rock. 

Muck or mud. An extremely soft, slimy silt or organic silt found on river and lake bottoms. 

The terms indicate an extremely soft consistency rather than any particular type of soil. Muck 

implies organic matter. 
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Peat. A naturally occuring highly organic substance derived primarily from plant materials 

(ASTM D 5715). Peats are dark brown or black, loose (void ratio may be 5 to 10), and 

extremely compressible. When dried, they will float. Peat bogs often emit quantities of 

inflammable methane gas. These soils will experience significant short-term and long-term 

settlement, even under light loads, and are often moisture sensitive, losing significant 

strength when wet. They are easily disturbed under construction activities. Peat containing a 

high degree of easily identifiable fibers is often called fibrous peat for geotechnical 

applications. Peat containing highly decomposed fibers and a significant highly organic soil 

component is often called amorphous peat. 

Quicksand. Refers to a condition, not a soil type. Gravels, sands, and silts become “quick” 

when an upward flow of groundwater and/or gas takes place to such a degree that the 

particles are lifted. 

Saprolites. Soils developed from in-situ weathering of rocks. Relic joints from the parent 

rock often control the weathered soils’ strength, permeability, and stability. Fragments may 

appear sound, but prove to be weak. Identifying the transition of soil to weathered rock to 

sound rock is difficult, often resulting in claims. 

Shale. Indurated, fine grained, sedimentary rocks, such as mudstones, siltstone, and 

claystone, which are highly variable and troublesome. Some are hard and stable, while others 

are soft and degrade into clay soon after exposure to the atmosphere or during the design life 

of the structure. Clays developed from shale are often highly plastic. 

Sulfate. A mineral compound characterized by the sulfate radical SO4, which may be 

contained in soil. It creates significant expansion problems in lime-stabilized soil and, in 

some cases, distress in concrete. 

Sulfide. A mineral compound characterized by the linkage of sulfur with a metal, such as 

lead or iron, creating galena and pyrite, respectively. 

Till. A mixture of sand, gravel, silt, and clay produced by the plowing action of glaciers. The 

name boulder clay is often given such soils, particularly in Canada and England. The 

characteristics of glacial till vary depending on the sediments and bedrock eroded. The tills in 

New England are typically coarser and less plastic that those from the Midwest. The tills in 

the Northeast tend to be broadly graded and often unstable under water action. The complex 

nature of their deposition creates a highly unpredictable material. 
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Topsoils. Surface soils that support plant life. They usually contain considerable organic 

matter. These soils tend to settle over time as organic matter continues to degrade. They are 

often moisture sensitive, losing significant strength when wet, and are easily disturbed under 

construction activities. 

Tuff. The name applied to deposits of volcanic ash. In humid climates or in areas in which 

ash falls into bodies of water, the tuff becomes cemented into a soft, porous rock. 

Varved clays. Sedimentary deposits consisting of alternate thin layers of silt and clay. 

Ordinarily, each pair of silt and clay layers is from 3 – 13 mm (1/8 – 1/2 in.) thick. They are 

the result of deposition in lakes during periods of alternating high and low water in the 

inflowing streams, and are often formed in glacial lakes. These deposits have a much higher 

horizontal than vertical permeability, with the horizontal seams holding water. They are often 

sensitive, and will lose strength when remolded. 

7.5.2 Compressible Soils 

Effect of Compressible Soils on Pavement Performance 

Highly compressible (very weak) soils are susceptible to large settlements and deformations 

with time that can have a detrimental effect on pavement performance. Highly compressible 

soils are very low density, saturated soils, usually silts, clays, and organic alluvium or wind 

blown deposits and peats. If these compressible soils are not treated properly, large surface 

depressions with random cracking can develop. The surface depressions can allow water to 

pond on the pavement’s surface and more readily infiltrate the pavement structure, 

compounding a severe problem. More importantly, the ponding of water will create a safety 

hazard to the traveling public during wet weather. 

Treatments for Compressible Soils 

The selection of a particular technique depends on the depth of the weak soil, and the 

difference between the in-situ conditions and the minimum compaction or strength 

requirements to limit the amount of anticipated settlement to a permissible value that will not 

adversely affect pavement performance. When constructing roadways in areas with deep 

deposits of highly compressible layers, the specific soil properties must be examined to 

calculate the estimated settlement. Under these conditions, a geotechnical investigation and 

detailed settlement analysis must be completed prior to the pavement design. When existing 

subgrade soils do not meet minimum compaction requirements and are susceptible to large 

settlements over time, consider the following alternatives: 

• Remove and process soil to attain the approximate optimum moisture content, and 

replace and compact. 
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• Remove and replace subgrade soil with suitable borrow or select embankment 

materials. All granular fill materials should be compacted to at least 95% of the 

maximum density, with moisture control, as defined by AASHTO T180. Cohesive fill 

materials should be compacted to no less than 90%, near or slightly greater than 

optimum moisture content (e.g., -1% to +2% of optimum), as defined by AASHTO 

T99. 

• Consider mechanical stabilization using geosynthetics as covered in Section 7.5 to 

reduce the amount of undercut required. 

• If soils are granular (e.g., sands and some silts), consider compaction of the soils from 

the surface to increase the dry density through dynamic compaction techniques. 

Identification of soil characteristics and detailed procedures for the successful 

implementation of this technique covered in FHWA/NHI course 132034 on Ground 

Improvement Techniques (FHWA NHI-04-001). 

• If the soil is extremely wet or saturated, consider dewatering using well points or 

deep horizontal drains. If horizontal drains cannot be daylighted, connection to storm 

drainage pipes or sump pumps may be required. 

• Consolidate deep deposits of very weak saturated soils with large fills prior to 

pavement construction (surcharge). After construction, the fills can either be left inplace or removed, depending on the final elevation. Consider wick drains to 

accelerate consolidation (see FHWA NHI-04-001). 

• Other techniques for deep deposits of compressible soil include piled embankments 

and use of lightweight fill, such as geofoam, as covered in the FHWA Ground 

Improvement Techniques manual (FHWA NHI-04-001). Although more costly than 

most of the previous techniques in terms of construction dollars, these techniques 

offer immediate improvement, thus accelerating construction. On some projects, the 

time savings may be more valuable than the construction cost differential. 

7.5.3 Collapsible Soils 

As with highly compressible soils, collapsible soils can lead to significant localized 

subsidence of the pavement. Collapsible soils are very low density silt type soils, usually 

alluvium or wind blown (loess) deposits, and are susceptible to sudden decreases in volume 

when wetted. Often their unstable structure has been cemented by clay binders or other 

deposits, which will dissolve on saturation, allowing a dramatic decrease in volume (Rollings 

and Rollings, 1996). Native subgrades of collapsible soils should be soaked with water prior 

to construction and rolled with heavy compaction equipment. In some cases, residual soils 

may also be collapsible due to leaching of colloidal and soluble materials. Figure 7-17 

provides a method of identifying the potential for collapsible soils. Other local methods for 

identification may be available. Collapsible soils can also be created in fills when sand type 
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soils are compacted on the dry side of optimum moisture. Meniscus forces between particles 

can create a soil fabric susceptible to collapse. 

If pavement systems are to be constructed over collapsible soils, special remedial measures 

may be required to prevent large-scale cracking and differential settlement. To avoid 

problems, collapse must be induced prior to construction. Methods include 

1. ponding water over the region of collapsible soils. 

2. infiltration wells. 

3. compaction - conventional with heavy vibratory roller for shallow depths (within 0.3 

or 0.6 m (1 or 2 ft)) 

4. compaction - dynamic or vibratory for deeper deposits of more than half a meter (a 

few feet) (could be combined with inundation) 

5. excavated and replaced. 

Figure 7-17. Guide to collapsible soil behavior (Rollings and Rollings, 1996). 
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7.5.4 Swelling Soils 

Effect of Swelling Soils on Pavement Performance 

Swelling or expansive soils are susceptible to volume change (shrink and swell) with 

seasonal fluctuations in moisture content. The magnitude of this volume change is dependent 

on the type of soil (shrink-swell potential) and its change in moisture content. A loss of 

moisture will cause the soil to shrink, while an increase in moisture will cause it to expand or 

swell. This volume change of clay type soils can result in longitudinal cracks near the 

pavement’s edge and significant surface roughness (varying swells and depressions) along 

the pavement’s length. 

Expansive soils are a very significant problem in many parts of the United States (see Figure 

7-18) and are responsible for the application of premature maintenance and rehabilitation 

activities on many miles of roadway each year. Expansive soils are especially a problem 

when deep cuts are made in a dense (over-consolidated) clay soil. 

Identification of Swelling Soils 

Various techniques and procedures exist for identifying potentially expansive soils. 

AASHTO T 258 can be used to identify soils and conditions that are susceptible to swell. 

Two of the more commonly used documents are listed below: 

• An Evaluation of Expedient Methodology for Identification of Potentially Expansive 

Soils, Report No. FHWA-RD-77-94, Federal Highway Administration, Washington, 

D.C., June 1977. 

• Design and Construction of Airport Pavements on Expansive Soils, Report No. 

FAA-RD-76-66, Federal Aviation Administration, U.S. Department of 

Transportation, Washington, D.C., June 1976. 

Clay mineralogy and the availability of water are the key factors in determining the degree to 

which a swelling problem may exist at a given site. Different clay minerals exhibit greater or 

lesser degrees of swell potential based on their specific chemistry. Montmorillonitic clays 

tend to exhibit very high swell potentials due to the particle chemistry, whereas illitic clays 

tend to exhibit very low swell potentials. Identification of clay minerals through chemical or 

microscopic means may be used as a method of identifying the presence of high swell 

potential in soils. The soil fabric will also influence the swell potential, as aggregated 

particles will tend to exhibit higher swell than dispersed particles, and flocculated higher than 

deflocculated. Generally, the finer-grained and more plastic the soil, the higher the swell 

potential the soil will exhibit. 
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The identification of swelling soils in the subgrade is a key component of the geotechnical 

investigation for the roadway. Soils at shallow depths beneath the proposed pavement 

elevation are generally sampled as part of the investigation, and their swell potential may be 

identified in a number of ways. Index testing is a common method for identifying swell 

potential. Laboratory testing to obtain the plastic and liquid limits and/or the shrinkage limit 

will usually be conducted. The soil activity (ASTM D 4318), defined as the ratio of the 

plasticity index to the percentage of the soil by weight finer than 0.002 mm (0.08 mils) is 

also used as an index property for swell potential, since clay minerals of higher activity 

exhibit higher swell. Activity calculation requires measurement of gradation using 

hydrometer methods, which is not typical in geotechnical investigations for pavement design 

in many states. In addition to index testing, agency practice in regions where swelling soils 

are a common problem may include swell testing (e.g., ASTM D 4546), for natural or 

compacted soil samples. Such testing generally includes measurement of the change in height 

(or volume) of a sample exposed to light loading similar to that expected in the field and then 

allowed free access to water. 

Figure 7-18. Estimated location of swelling soils (from Witczak, 1972). 
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Treatment for Swelling Soils 

When expansive soils are encountered along a project in environments and areas where 

significant moisture fluctuations in the subgrade are expected, consideration should be given 

to the following alternatives to minimize future volume change potential of the expansive 

soil: 

• For relatively thin layers of expansive clays near the surface, remove and replace the 

expansive soil with select borrow materials. 

• Extend the width of the subsurface pavement layers to reduce the change (i.e., wetting 

or drying) in subgrade moisture along the pavement’s edge, and increase the roadway 

crown to reduce infiltration moisture. 

• Partial encapsulation along the edge of the pavement or full encapsulation can also be 

used to reduce change in subgrade moisture, as described in greater detail in Section 7.5. 

• Scarify, stabilize, and recompact the upper portion of the expansive clay subgrade. 

Lime or cement stabilization is an accepted method for controlling the swelling of 

soils, as discussed in Section 7.6. (Stabilization, as used for expansive soils, refers to 

the treatment of a soil with such agents as bitumen, Portland cement, slaked or 

hydrated lime, and flyash to limit its volume change characteristics. This can 

substantially increase the strength of the treated material.) 

• In areas with deep cuts in dense, over-consolidated expansive clays, complete the 

excavation of the subsurface soils to the proper elevation, and allow the subsurface 

soils to rebound prior to placing the pavement layers. 

AASHTO 1993 (Appendix C) provides procedures and graphs to predict the direct effect of 

swelling soils on serviceability loss and is treated with respect to the differential effects on 

the longitudinal profile of the road surface. If the swelling is anticipated to be relatively 

uniform, then the procedures do not apply. 

7.5.5 Subsurface Water 

It is important to identify any saturated soil strata, the depth to groundwater, and subsurface 

water flow between soil strata. Subsurface water is especially important to recognize and 

identify in the transition areas between cut and fill segments. If allowed to saturate unbound 

base/subbase materials and subgrade soils, subsurface water can significantly decrease the 

strength and stiffness of these materials. Reductions in strength can result in premature 

surface depressions, rutting, or cracking. Seasonal moisture flow through selected soil strata 

can also significantly magnify the effects of differential volume change in expansive soils. 

Cut areas are particularly critical for subsurface water. 
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Treatments for Subsurface Water 

When saturated soils or subsurface water are encountered, consideration should be given to 

the following alternatives for improving the foundation or supporting subgrade: 

• For saturated soils near the surface, dry or strengthen the wet soils through the use of 

mechanical stabilization techniques to provide a construction platform for the 

pavement structure, as described in Section 7.6. 

• Remove and replace the saturated soils with select borrow materials or soils. (May 

not be an option if excavation is required below the groundwater level). 

• Place and properly compact thick fills or embankments to increase the elevation of 

the subgrade, or in other words, increase the thickness between the saturated soils or 

water table depth and pavement structure. 

• Consideration should also be given to the use of subgrade drains as previously 

detailed in Section 7.2 whenever the following conditions exist: 

o High ground-water levels that may reduce subgrade stability and provide a 

source of water for frost action. 

o Subgrade soils consisting of silts and very fine sands that may become quick 

or spongy when saturated. 

o Water seeps from underlying water-bearing strata or from subgrades in cut 

areas (consider intercepting drains). 

7.5.6 Frost-Susceptible Soils 

Effect of Frost Action on Pavement Performance 

Frost action can cause differential heaving, surface roughness and cracking, blocked 

drainage, and a reduction in bearing capacity during thaw periods. These effects range from 

slight to severe, depending on types and uniformity of subsoil, regional climatic conditions 

(i.e., depth of freeze), and the availability of water. 

One effect of frost action on pavements is frost heaving caused by crystallization of ice 

lenses in voids of soils containing fine particles. As shown in Figure 7-19, three conditions 

must be present to cause frost heaving and associated frost action problems: 

• frost-susceptible soils; 

• subfreezing temperatures in the soil; and, 

• source of water. 

If these conditions occur uniformly, heaving will be uniform; otherwise, differential heaving 

will occur, causing surface irregularities, roughness, and ultimately cracking of the pavement 

surface. 
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Figure 7-19. Elements of frost heave. 

A second effect of frost action is thaw weakening. The bearing capacity may be reduced 

substantially during mid-winter thawing periods, and subsequent frost heaving is usually 

more severe because water is more readily available to the freezing zone. In more-southerly 

areas of the frost zone, several cycles of freeze and thaw may occur during a winter season 

and cause more damage than one longer period of freezing in more-northerly areas. Spring 

thaws normally produce a loss of bearing capacity to well below summer and fall values, 

followed by a gradual recovery over a period of weeks or months. Water is also often trapped 

above frozen soil during the thaw, which occurs from the top down, creating the potential for 

long-term saturated conditions in pavement layers. 

Identification of Frost-Susceptible Soils 

Frost-susceptible soils have been classified into four general groups. Table 7-12 provides a summary 

of the typical soils in each of these four groups based on the amount of fines (material passing the 

0.075 mm (No. 200) sieve. Figure 7-20 graphically displays the expected average rate of frost heave 

for the different soil groups based on portion of soil finer than 0.02 mm (0.8 mils).

Little to no frost action occurs in clean, free draining sands, gravels, crushed rock, and 

similar granular materials, under normal freezing conditions. The large void space permits 

water to freeze in-place without segregation into ice lenses. Conversely, silts are highly frostsusceptible. The condition of relatively small voids, high capillary potential/action, and 

relatively good permeability of these soils accounts for this characteristic. 
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Table 7-12. Frost susceptibility classification of soils (NCHRP 1-37A). 

Frost 

Group 

Degree of 

Frost 

Susceptibility 

Type of Soil 

Percentage Finer 

than 0.075 mm 

(# 200) by wt. 

Typical Soil Classification 

F1 Negligible to low Gravelly soils 3-10 GC, GP, GC-GM, GP-GM 

Gravelly soils 10-20 GM, GC-GM, GP-GM 

F2 Low to medium 

Sands 3-15 SW, SP, SM, SW-SM, SP-SM 

Gravelly Soils Greater than 20 GM-GC 

Sands, except very 

fine silty sands F3 High Greater than 15 SM, SC 

Clays PI>12  CL, CH 

All Silts  ML-MH 

Very Fine Silty 

Sands Greater than 15 SM 

Clays PI<12  CL, CL-ML F4 Very high 

Varied clays and 

other fine grained, 

banded sediments 

 CL, ML, SM, CH 

Figure 7-20. Average rate of heave versus % fines for natural soil gradations (Kaplar, 1974). 
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Clays are cohesive and, although their potential capillary action is high, their capillary rate is 

low. Although frost heaving can occur in clay soils, it is not as severe as for silts, since the 

impervious nature of the clays makes passage of water slow. The supporting capacity of 

clays must be reduced greatly during thaws, even in the absence of significant heave. 

Thawing usually takes place from the top downward, leading to very high moisture contents 

in the upper strata. 

A groundwater level within 1.5 m (5 ft) of the proposed subgrade elevation is an indication 

that sufficient water will exist for ice formation. Homogeneous clay subgrade soils also 

contain sufficient moisture for ice formation, even with depth to groundwater in excess of 

3 m (10 ft). However, the magnitude of influence will be highly dependent on the depth of 

the freezing front (i.e., frost depth penetration). For deep frost penetration, groundwater at 

even a greater depth could have an influence on heave. 

Identification of Frost-Susceptible Conditions 

The most distinguishing factor for identifying a pavement frost hazard condition is water 

supply. For frost susceptible soils within the frost zone, the frost hazard may be rated as high 

or low, according to the following conditions. An unknown rating may be appropriate when 

conditions for both high and low ratings occur and cannot be resolved, or when little or no 

information is available. The inclusion of a frost hazard rating in the site evaluation 

documentation verifies that an evaluation of frost action has been attempted and has not been 

overlooked. When the rating is unknown, a decision to include frost action mitigation 

measures in a design will be based more upon the unacceptable nature of frost damage than 

the probability of occurrence. 

The conditions associated with a high frost hazard potential include 

1. A water table within 3 m (10 ft) of the pavement surface (depth of influence depends 

on the type of soil and frost depth). 

2. Observed frost heaves in the area. 

3. Inorganic soils containing more than 3% (by weight) or more grains finer than 

0.02 mm (0.8 mils) in diameter according to the U.S. Army Corps of Engineers. 

4. A potential for the ponding of surface water and the occurrence of soils between the 

frost zone beneath the pavement and the surface water with permeabilities high 

enough to enable seepage to saturate soils within the frost zone during the term of 

ponding. 

The conditions associated with a low frost hazard potential include 

1. A water table greater than 6 m (20 ft) below the pavement surface (again, could be 

much shallower depending on the type of soil and frost depth). 
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2. Natural moisture content in the frost zone low versus the saturation level. 

3. Seepage barriers between the water supply and the frost zone. 

4. Existing pavements or sidewalks in the vicinity with similar soil and water supply 

conditions and without constructed frost protection measures that have not 

experienced frost damage. 

5. Pavements on embankments with surfaces more than 1 – 2 m (3 – 6 ft) above the 

adjacent grades (provides some insulation and a weighting action to resist heave). 

Treatment for Frost Action 

When frost-susceptible soils are encountered, consideration should be given to the following 

alternatives for improving the foundation or supporting subgrade: 

1. Remove the frost-susceptible soil (generally for groups F3 and F4, Table 7-12) and 

replace with select non-frost susceptible borrow to the expected frost depth 

penetration. 

2. Place and compact select non-frost-susceptible borrow materials to a thickness or 

depth to prevent subgrade freezing for frost susceptible soil groups F2, F3, and F4, 

Table 7-12. 

3. Remove isolated pockets of frost-susceptible soils to eliminate abrupt changes in 

subgrade conditions. 

4. Stabilize the frost-susceptible soil by eliminating the effects of soil fines by three 

processes: a) mechanically removing or immobilizing by means of physical-chemical 

means, such as cementitious bonding, b) effectively reducing the quantity of soil 

moisture available for migration to the freezing plane, as by essentially blocking off 

all migratory passages, or c) altering the freezing point of the soil moisture. 

a. Cementing agents, such as Portland cement, bitumen, lime, and lime-flyash, 

as covered in Section 7.5. These agents effectively remove individual soil 

particles by bonding them together, and also act to partially remove capillary 

passages, thereby reducing the potential for moisture movement. Care must be 

taken when using lime and lime-flyash mixtures with clay soils in seasonal 

frost areas (see Section 7.5 & Appendix F). 

b. Soil moisture available for frost heave can be mitigated through the 

installation of deep drains and/or a capillary barrier such that the water table is 

maintained at a sufficient depth to prevent moisture rise in the freezing zone. 

Capillary barriers can consist of either an open graded gravel layer 

sandwiched between two geotextiles, or a horizontal geocomposite drain. The 

installation of a capillary barrier requires the removal of the frost susceptible 

material to a depth either below frost penetration or sufficiently significant to 

reduce the influence of frost heave on the pavement. The capillary break must 
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be drained. The frost susceptible soil can then be replaced and compacted 

above the capillary barrier to the required subgrade elevation. 

5. Increase the pavement structural layer thickness to account for strength reduction in 

the subgrade during the spring-thaw period for frost-susceptible groups F1, F2, and 

F3. 

Pavement design for frost action often determines the required overall thickness of flexible 

pavements and the need for additional select material beneath both rigid and flexible 

pavements. Three design approaches have been used for pavement in seasonal frost areas: 

• The Complete Protection approach—requires non-frost susceptible materials for the 

entire depth of frost (e.g., treatment methods 1, 2, and 3 above). 

• Limited Subgrade Frost Penetration approach—permits some frost penetration into 

the subgrade, but not enough to allow unacceptable surface roughness to develop. 

• Reduced Subgrade Strength approach—allows more frost penetration into the 

subgrade, but provides adequate strength during thaw weakened periods. 

AASHTO 1993 (Appendix C) provides procedures and graphs to predict the direct effect of 

frost heave on serviceability loss and is treated with respect to the differential effects on the 

longitudinal profile of the road surface. If the frost is anticipated to be relatively uniform, 

then the procedures do not apply. 

For the most part, local frost-resistant design approaches have been developed from 

experience, rather than by application of some rigorous theoretical computational method. A 

more rigorous method is available in the NCHRP 1-37A design procedure to reduce the 

effects of seasonal freezing and thawing to acceptable limits, as discussed in Chapter 6. The 

Enhanced Integrated Climatic Model is used to determine the maximum frost depth for the 

pavement system at a particular location. Various combinations of layer thicknesses and 

material types can be evaluated in terms of their impact on the maximum frost depth and total 

amount of base and select materials necessary to protect the frost susceptible soils from 

freezing. 
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7.5.7 Summary 

Problematic soils can be treated using a variety of methods or a combination thereof. 

Improvement techniques that can be used to improve the strength and reduce the climatic 

variation of the foundation on pavement performance include 

1. Improvement of subsurface drainage (see Section 7.2, and should always be 

considered). 

2. Removal and replacement with better materials (e.g., thick granular layers). 

3. Mechanical stabilization using thick granular layers. 

4. Mechanical stabilization of weak soils with geosynthetics (geotextiles and geogrids) 

in conjunction with granular layers. 

5. Lightweight fill. 

6. Stabilization of weak soils with admixtures (highly plastic or compressible soils). 

7. Soil encapsulation. 

Details for most of these stabilization methods will be reviewed in the next section. 

7.6 SUBGRADE IMPROVEMENT AND STRENGTHENING 

Proper treatment of problem soil conditions and the preparation of the foundation are 

extremely important to ensure a long-lasting pavement structure that does not require 

excessive maintenance. Some agencies have recognized certain materials simply do not 

perform well, and prefer to remove and replace such soils (e.g., a state specification dictating 

that frost susceptible loess cannot be present in the frost penetration zone). However, in many 

cases, this is not the most economical or even desirable treatment (e.g., excavation may 

create disturbance, plus additional problems of removal and disposal). Stabilization provides 

an alternate method to improve the structural support of the foundation for many of the 

subgrade conditions presented in the previous section. In all cases, the provision for a 

uniform soil relative to textural classification, moisture, and density in the upper portion of 

the subgrade cannot be over-emphasized. This uniformity can be achieved through soil subcutting or other stabilization techniques. Stabilization may also be used to improve soil 

workability, provide a weather resistant work platform, reduce swelling of expansive 

materials, and mitigate problems associated with frost heave. In this section, alternate 

stabilization methods will be reviewed, and guidance will be presented for the selection of 

the most appropriate method. 
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7.6.1 Objectives of Soil Stabilization 

Soils that are highly susceptible to volume and strength changes can cause severe roughness 

and accelerate the deterioration of the pavement structure in the form of increased cracking 

and decreased ride quality when combined with truck traffic. Generally, the stiffness (in 

terms of resilient modulus) of some soils is highly dependent on moisture and stress state 

(see Section 5.4). In some cases, the subgrade soil can be treated with various materials to 

improve the strength and stiffness characteristics of the soil. Stabilization of soils is usually 

performed for three reasons: 

1. As a construction platform to dry very wet soils and facilitate compaction of the 

upper layers—for this case, the stabilized soil is usually not considered as a structural 

layer in the pavement design process. 

2. To strengthen a weak soil and restrict the volume change potential of a highly plastic 

or compressible soil—for this case, the modified soil is usually given some structural 

value or credit in the pavement design process. 

3. To reduce moisture susceptibility of fine grain soils. 

A summary of the stabilization methods most commonly used in pavements, the types of 

soils for which they are most appropriate, and their intended effects on soil properties is 

presented in Table 7-13. 

Mechanical stabilization using thick gravel layers or granular layers in conjunction with 

geotextiles or geogrids is an effective technique for improving roadway support over soft, 

wet subgrades. Thick granular layers provide a working platform, but do not provide 

strengthening of the subgrade. In fact, construction of thick granular layers in some cases 

results in disturbance of the subgrade due to required construction activities. Thick granular 

layers are also used to avoid or reduce frost problems by providing a protection to the 

underlying subgrade layers. 
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Table 7-13. Stabilization Methods for Pavements (after Rollings and Rollings, 1996). 

Stabilization Method Soil Type Improvement Remarks 

Mechanical 

- More Gravel Silts and Clays None Reduce dynamic stress 

level 

- Blending Moderately plastic 

Other 

None 

Improve gradation 

Reduce plasticity 

Reduce breakage 

Too difficult to mix 

- Geosynthetics Silts and Clays Strength gain through 

minimum 

disturbance and 

consolidation 

Fast, plus provides longterm separation 

- Lightweight fill Very weak silts, 

clays, peats 

None 

Thermal barrier for frost 

protection 

Fast, and reduces 

dynamic stress 

level 

Admixture 

- Portland cement Plastic 

Coarse 

 Less pronounced 

hydration of cement 

Hydration of cement 

- Lime Plastic Drying 

Strength gain 

Reduce plasticity 

Coarsen texture 

Long-term pozzolanic 

cementing 

Rapid 

Rapid 

Rapid 

Rapid 

Slow 

 Coarse with fines Same as plastic Dependent on quantity 

of plastic fines 

 Nonplastic None No reactive material 

- Lime-flyash Same as lime Same as lime Covers broader range 

- Lime-cement- flyash Same as lime Same as lime Covers broader range 

- Bituminous Coarse Strengthen/bind 

waterproof 

Asphalt cement or 

liquid asphalt 

 Some fines Same as coarse Liquid asphalt 

 Fine None Can’t mix 

- Pozzolanic and slags Silts and coarse Acts as a filler 

Cementing of grains 

Dense and strong 

Slower than cement 

- Chemicals Plastic Strength increase and 

volume stability 

See vendor literature 

Difficult to mix 

Water proofers 

- Asphalt Plastic and 

collapsible 

Reduce change in 

moisture 

Long-term moisture 

migration 

problem 

- Geomembranes Plastic and 

collapsible 

Reduce change in 

moisture 

Long-term moisture 

migration 

problem 
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A common practice in several New England and Northwestern states is to use a meter (3.3 ft) 

or more of gravel beneath the pavement section. The gravel improves drainage of surface 

infiltration water and provides a weighting action that reduces and results in more uniform 

heave. Washington State recently reported the successful use of an 0.4 m (18 in.) layer of cap 

rock beneath the pavement section in severe frost regions (Ulmeyer et al., 2002). 

Blending gravel and, more recently, recycled pavement material with poorer quality soils 

also can provide a working platform. The gravel acts as filler, creating a dryer condition and 

decreasing the influence of plasticity. However, if saturation conditions return, the gravel 

blend can take on the same poorer support characteristics of the subgrade. 

Geotextiles and geogrids used in combination with quality aggregate minimize disturbance 

and allow construction equipment access to sites where the soils are normally too weak to 

support the initial construction work. They also allow compaction of initial lifts on sites 

where the use of ordinary compaction equipment is very difficult or even impossible. 

Geotextiles and geogrids reduce the extent of stress on the subgrade and prevent base 

aggregate from penetrating into the subgrade, thus reducing the thickness of aggregate 

required to stabilize the subgrade. Geotextiles also act as a separator to prevent subgrade 

fines from pumping or otherwise migrating up into the base. Geosynthetics have been found 

to allow for subgrade strength gain over time. However, the primary long-term benefit is 

preventing aggregate-subgrade mixing, thus maintaining the thickness of the base and 

subbase. In turn, rehabilitation of the pavement section should only require maintenance of 

surface pavement layers. 

Stabilization with admixtures, such as lime, cement, and asphalt, have been mixed with 

subgrade soils used for controlling the swelling and frost heave of soils and improving the 

strength characteristics of unsuitable soils. For admixture stabilization or modification of 

cohesive soils, hydrated lime is the most widely used. Lime is applicable in clay soils (CH 

and CL type soils) and in granular soils containing clay binder (GC and SC), while Portland 

cement is more commonly used in non-plastic soils. Lime reduces the Plasticity Index (PI) 

and renders a clay soil less sensitive to moisture changes. The use of lime should be 

considered whenever the PI of the soil is greater than 12. Lime stabilization is used in many 

areas of the U.S. to obtain a good construction platform in wet weather above highly plastic 

clays and other fine-grained soils. It is important to note that changing the physical properties 

of a soil through chemical stabilization can produce a soil that is susceptible to frost heave. 

Following is a brief description of the characteristics of stabilized soils followed by the 

treatment procedures. Additional guidance on soil stabilization with admixtures and 

stabilization with geosynthetics can be obtained from the following resources: 
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• “Lime Stabilization – Reactions, Properties, Design, and Construction,” State of the 

Art Report 5, Transportation Research Board, Washington D.C., 1987. 

• Soil Stabilization for Pavements, Joint Departments of the Army and Air Force, USA, 

TM 5-822-14/AFMAN 32-8010, 1994. 

• Geosynthetics Design and Construction Guidelines, FHWA HI-95-038, 1998. 

• Standard Specifications for Geotextiles - AASHTO M288, 1997. 

7.6.2 Characteristics of Stabilized Soils 

Although mechanical stabilization with thick granular layers or geosythetics and aggregate 

subbase provides the potential for strength improvement of the subgrade over time, this is 

generally not considered in the design of the pavement section, and no increase in structural 

support is attributed to the geosynthetic. However, the increase in gravel thickness (minus an 

allowance for rutting) can contribute to the support of the pavement. Alternatively, the 

aggregate thickness used in conjunction with the geosynthetic is designed to provide an 

equivalent subgrade modulus, which can be considered in the pavement design, discounting 

the additional aggregate thickness of the stabilization layer. Geosynthetics also allow more 

open graded aggregate, thus providing for the potential to drain the subbase into edgedrains 

and improving its support value. 

The improvement of subgrade or unbound aggregate by application of a stabilizing agent is 

intended to cause the improvements outlined above (i.e., construction platform, subgrade 

strengthening, and control of moisture). These improvements arise from several important 

mechanisms that must be considered and understood by the pavement designer. Admixtures 

used as subgrade stabilizing agents may fill or partially fill the voids between the soil 

particles. This reduces the permeability of the soil by increasing the tortuosity of the 

pathways for water to migrate through the soil. Reduction of permeability may be relied upon 

to create a waterproof surface to protect underlying, water sensitive soils from the intrusion 

of surface water. This mechanism must be accompanied by other aspects of the geometric 

design into a comprehensive system. The reduction of void spaces may also tend to change 

the volume change under shear from a contractive to a dilative condition. The admixture type 

stabilizing agent also acts by binding the particles of soil together, adding cohesive shear 

strength and increasing the difficulty with which particles can move into a denser packing 

under load. Particle binding serves to reduce swelling by resisting the tendency of particles to 

move apart. The particles may be bound together by the action of the stabilizing agent itself 

(as in the case of asphalt cement), or may be cemented by chemical reaction between the soil 

and stabilizing agent (as in the case of lime or Portland cement). Additional improvement can 

arise from other chemical-physical reactions that affect the soil fabric (typically by 

flocculation) or the soil chemistry (typically by cation exchange). The down side of 
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admixtures is that they require up front lab testing to confirm their performance and very 

good field control to obtain a uniform, long lasting product, as outlined later in this section. 

There are also issues of dust control and weather dependency, with some methods that should 

be carefully considered in the selection of these methods. 

The zone that may be selected for improvement depends upon a number of factors. Among 

these are the depth of soft soil, anticipated traffic loads, the importance of the transportation 

network, constructability, and the drainage characteristics of the geometric design and the 

underlying soil. When only a thin zone and/or short roadway length is subject to 

improvement, removal and replacement will usually be the preferred alternative by most 

agencies, unless a suitable replacement soil is not economically available. Note that in this 

context, the use of the qualitative term “thin” is intentional, as the thickness of the zone can 

be described as thick or thin, based primarily on the project economics of the earthwork 

requirements and the depth of influence for the vehicle loads. 

7.6.3 Thick Granular Layers 

Many agencies have found that a thick granular layer is an important feature in pavement 

design and performance. Thick granular layers provide several benefits, including increased 

load-bearing capacity, frost protection, and improved drainage. While the composition of this 

layer takes many forms, the underlying strategy of each is to achieve desired pavement 

performance through improved foundation characteristics. The following sections describe 

the benefits of thick granular layers, typical characteristics, and considerations for the design 

and construction of granular embankments. 

Objectives of Thick Granular Layers 

Thick granular layers have been used in design for structural, drainage, and geometric 

reasons. Many times, a granular layer is used to provide uniformity and support as a 

construction platform. In areas with large quantities of readily accessible, good quality 

aggregates, a thick granular layer may be used as an alternative to soil stabilization. 

Whatever the reason, thick granular layers aim to improve the natural soil foundation. By 

doing this, many agencies are recognizing that the proper way to account for weak, poorly 

draining soils is through foundation improvement, as opposed to increasing the pavement 

layer thicknesses. The following is a list of objectives and benefits of thick granular layers: 

• To increase the supporting capacity of weak, fine-grained subgrades. 

• To provide a minimum bearing capacity for the design and construction of 

pavements. 

• To provide uniform subgrade support over sections with highly variable soil 

conditions. 
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• To reduce the seasonal effects of moisture and temperature variations on subgrade 

support. 

• To promote surface runoff through geometric design. 

• To improve subsurface drainage and the removal of moisture from beneath the 

pavement layers. 

• To increase the elevation of pavements in areas with high water tables. 

• To provide frost protection in freezing climatic zones. 

• To reduce subgrade rutting potential of flexible pavements. 

• To reduce pumping and erosion beneath PCC pavements. 

• To meet elevation requirements of geometric design. 

Characteristics of Thick Granular Layers 

Thick granular layers have been incorporated in pavement design in several ways. They can 

be referred to as fills or embankments, an improved or prepared subgrade, and select or 

preferred borrow. Occasionally, a thick granular layer is used as the pavement subbase. The 

two most important characteristics for all of these layers are material properties and 

thickness. While geometric requirements (e.g., vertical profile) and improved surface runoff 

can be achieved by embankments constructed of any soil type, the most beneficial effects are 

produced through utilization of good quality, granular materials. Several methods are used to 

characterize the strength and stiffness of granular materials, including the California Bearing 

Ratio (CBR) and resilient modulus testing. In addition, several types of field plate load tests 

have been used to determine the composite reaction of the embankment and soil 

combination. In general, materials with CBR values of 20% or greater are used, 

corresponding to resilient moduli of approximately 120 MPa (17,500 psi). These are typically 

sand or granular materials, or coarse-grained materials with limited fines, corresponding to 

AASHTO A-1 and A-2 (GW, GP, SW and SP) soils. 

Aggregate gradation and particle shape are other important properties. Typically, 

embankment materials are dense-graded, with a maximum top-size aggregate that varies 

depending on the height of the embankment. Many times, the lowest embankment layer may 

contain cobbles or aggregates of 100 – 200 mm (4 – 8 in.) in diameter. Granular layers 

placed close to the embankment surface have gradations, including maximum size 

aggregates, similar to subbase material specifications. Although dense-graded aggregate 

layers do not provide efficient drainage relative to open-graded materials, a marginal degree 

of subsurface seepage can be achieved by limiting the fines content to less than 10%. The 

type of granular material used is normally a function of material availability and cost. Pit-run 

gravels and crushed stone materials are the most common. The high shear strength of crushed 
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stone is more desirable than rounded, gravelly materials; however, the use of crushed 

materials may not always be economically feasible. 

The thicknesses of granular layers vary, depending upon their intended use. Granular layers 

150 – 300 mm (6 – 12 in.) thick may be used to provide uniformity of support, or act as a 

construction platform for paving of asphalt and concrete layers. To increase the composite 

subgrade design values (i.e., combination of granular layer over natural soil), it is usually 

necessary to place a minimum of 0.5 – 1.5 m (11⁄2 -- 5 ft) of embankment material, depending 

on the strength of the granular material relative to that of the underlying soil. Likewise, 

granular fills placed for frost protection may also range from 0.5 – 1.5 m (11⁄2 – 5 ft). In most 

cases, embankments greater than 2 m (6 1⁄2 ft) thick have diminishing effects in terms of 

strength, frost protection, and drainage. Granular embankments greater than 2 – 3 m (61⁄2 – 

10 ft) thick are usually constructed for purposes of geometric design. 

Considerations for Pavement Structural Design 

The use of a thick granular layer presents an interesting situation for design. The placement 

of a granular layer of substantial thickness over a comparatively weak underlying soil forms, 

essentially, non-homogeneous subgrade in the vertical direction. Pavement design requires a 

single subgrade design value, for example CBR, resilient modulus, or k-value. This is 

generally determined through laboratory or field tests, when the soil mass in the zone of 

influence of vehicle loads is of the same type, or exhibits similar properties. In the case of a 

non-homogeneous subgrade, the composite reaction of the embankment and soil combination 

can vary from that of the natural soil to that of the granular layer. Most commonly, the 

composite reaction is a value somewhere between the two extremes, dependent upon the 

relative difference in moduli between the soil and embankment, and the thicknesses of the 

granular layer. The actual composite subgrade response is not known until the embankment 

layer is placed in the field, and it may be different once the upper pavement layers are placed. 

To account for non-homogenous subgrades in pavement structural design, it is recommended 

to characterize the individual material properties by traditional means, such as resilient 

modulus or CBR testing, and to compare these results to field tests performed over the 

constructed embankment layers, as well as the completed pavement section. Analytical 

models, such as elastic layer programs, can be used to make theoretical predictions of 

composite subgrade response, and these predictions can then be verified by field testing. 

Some agencies use in-situ plate load tests to verify that a minimum composite subgrade 

modulus has been achieved. Deflection devices, including the Falling Weight Deflectometer 

(FWD), can be used for testing over the compacted embankment layer and over the 

constructed pavement surface. 
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It is advisable to use caution when selecting a design subgrade value for a non-homogenous 

subgrade. Experience has shown that a good-quality embankment layer must be of significant 

height, say 1 m (3 ft) or more, before the composite subgrade reaction begins to resemble that 

of the granular layer. This means that, for granular layers up to 1 m (3 ft) in height, the 

composite reaction can be much less than that of the embankment layer itself. If too high a 

subgrade design value is selected, the pavement will be under-designed. Granular layers less 

than 0.5 m (1.6 ft) thick have minimal impact on the composite subgrade reaction, when 

loaded under the completed pavement section. 

7.6.4 Geotextiles and Geogrids 

Geosynthetics are a class of geomaterials that are used to improve soil conditions for a 

number of applications. They consist of manufactured polymeric materials used in contact 

with soil materials or pavements as an integral part of a man-made system (after ASTM 

D4439). The most common applications in general use are in pavement systems for both 

paved and unpaved roadways, for reinforcing embankments and foundation soils, for creating 

barriers to water flow in liners and cutoffs, and for improving drainage. The generic term 

“geosynthetic” is often used to cover a wide range of different materials, including 

geotextiles, geogrids, and geomembranes. Combinations of these materials in layered 

systems are usually called geocomposites. 

Geotextile and geogrid materials are the most commonly used geosynthetics in 

transportation, although certainly others are sometimes used. This generality is more accurate 

when only the pavement itself (not including the adjoining fill or cut slopes, retaining walls, 

abutments, or drainage facilities) is considered. Table 7-14 provides a list of transportation 

applications for specific basic functions of the geosynthetic. Each of these functional classes, 

while potentially related by the specific application being proposed, refers to an individual 

mechanism for the improvement of the soil subgrade. Stabilization, as reviewed in this 

section, is a combination of the separation, filtration, and reinforcement functions. Drainage 

can also play a role. 

The separation function prevents the subgrade and the subbase from intermixing, which 

would most likely occur during construction and in-service due to pumping of the subgrade. 

The filtration function is required because soils requiring stabilization are usually wet and 

saturated. By acting as a filter, the geotextile retains the subgrade without clogging, while 

allowing water from the subgrade to pass up into the subbase, thus allowing destabilizing 

pore pressure to dissipate and promote strength gain due to consolidation. If the subbase is 

dirty (contains high fines), it may be desirable to use a thick, nonwoven geotextile, which 
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will allow for drainage in its plane (i.e., in this case, pore water pressure dissipates through 

the plane of the geotextile). 

Geotextiles and geogrids also provide some level of reinforcement by laterally restraining the 

base or subbase and improving the bearing capacity of the system, thus decreasing shear 

stresses on the subgrade. Soft, weak subgrade soils provide very little lateral restraint, so 

when the aggregate moves or shoves laterally, ruts develop on the aggregate surface and also 

in the subgrade. A geogrid with good interlocking capabilities or a geotextile with good 

frictional capabilities can provide tensile resistance to lateral aggregate movement. The 

geosynthetic also increases the system bearing capacity by forcing the potential bearing 

surface under the wheel load to develop along alternate, longer mobilization paths and, thus, 

higher shear strength surfaces. 

Geotextiles serve best as separators, filters and, in the case of nonwoven geotextiles, drainage 

layers, while geogrids are better at reinforcing. Geogrids, as with geotextiles, prevent the 

subbase from penetrating the subgrade, but they do not prevent the subgrade from pumping 

into the base. When geogrids are used, either the subbase has to be designed as a separator or 

a geotextile must be used in conjunction with the geogrid, either separately or as a 

geocomposite. 

Table 7-14. Transportation uses of geosynthetic materials (after Koerner, 1998). 

General Function Typical Application 

Separation of Dissimilar Materials 

Between subgrade and aggregate base in paved 

and unpaved roads and airfields 

Between subgrade and ballast for railroads 

Between old and new asphalt layers 

Reinforcement of weak materials 

Over soft soils for unpaved roads, paved 

roads, airfield, railroads, construction 

platforms 

Filtration Beneath aggregate base for paved and unpaved 

roads and airfields or railroad ballast 

Drainage Drainage interceptor for horizontal flow 

Drain beneath other geosynthetic systems 
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Table 7-15. Appropriate subgrade conditions for stabilization using geosynthetics 

(after FHWA HI-95-038 ).

Condition Related Measures

Poor soils USCS of SC, CL, CH, ML, MH, OL, OH, PT or 

AASHTO of A-5, A-6, A-7, A-7-6 

Low strength cu<13 psi or CBR<3 or MR < 4500 psi 

High water table Within zone of influence of surface loads 

High sensitivity High undisturbed strength compared to remolded strength 

As defined by AASHTO M288, geotextiles or geogrids in conjunction with an appropriately 

designed thickness of subbase aggregate provide stabilization for soft, wet subgrades with a 

CBR of less than 3 (a resilient modulus less than 30 MPa (4500 psi)). Table 7-15 provides 

subgrade conditions that are considered to be the most appropriate for geosynthetic use. 

These are conditions where the subgrade will not support conventional construction without 

substantial rutting. Engineers have compiled over 20+ years of successful use for this 

application in these types of conditions. Geosynthetics do not provide improvements for 

expansive soils, and use in stabilization for subgrade conditions that are better than those 

defined in Table 7-15 is questionable. However, geosynthetics may still provide a valuable 

function as separators for any subgrade containing large amounts of fines or as base 

reinforcement, even with competent subgrades, as discussed in Section 7.2. 

Separation is a viable function, for soils that are seasonally weak (e.g., from spring thaw) or 

for high fines content soils, which are susceptible to pumping. This is especially the case for 

permeable base applications, as covered in Section 7.2. A greater range of geotextile 

applicability is recognized in the M288 specification (AASHTO, 1997). With a CBR ≥ 3, the 

geotextile application is identified as separation. By simply maintaining the integrity of the 

subbase and base layers over the life of the pavement, the serviceability of the roadway 

section will be extended, and substantial cost benefits can be realized. Research is ongoing to 

quantify the cost-benefit life cycle ratio of using geosynthetics in permanent roadway 

systems. Initial work by Al-Qadi, 1997 indicates that the use a geosynthetic separator may 

increase the number of allowable design vehicles (ESALs) by a factor of two. Considering 

the cost of a geosynthetic is generally $1.25/m2

, while the cost of a modern pavement section 

is on the order of $25/m2

, the life extension of the roadway section will more than make up 

for the cost of the geosynthetic. In addition, as previously indicated, the geosynthetic 

maintains the integrity of the base such that rehabilitation should only require surface 

pavement restoration. The ability of a geosynthetic to prevent premature failure and 

reduce long-term maintenance costs provides extremely low-cost performance 

insurance. 
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The design of the geosynthetic for stabilization is completed using the design-by-function 

approach in conjunction with AASHTO M288, in the steps from FHWA HI-95-038 outlined 

below. A key feature of this method is the assumption that the structural pavement design is 

not modified at all in the procedure. The pavement design proceeds exactly according to 

standard procedures, as if the geosynthetic was not present. The geosynthetic instead replaces 

additional unbound material that might be placed to support construction operations, and 

replaces no part of the pavement section itself. However, this unbound layer will provide 

some additional support. If the soil has a CBR of less than 3, and the aggregate thickness is 

determined based on a low rutting criteria in the following steps, the support for the 

composite system is theoretically equivalent to a CBR = 3 (resilient modulus of 30 Mpa 

(4500 psi)). As with thick aggregate fill used for stabilization, the support value should be 

confirmed though field testing using, for example, a plate load test or FWD test to verify that 

a minimum composite subgrade modulus has been achieved. Note that the FHWA procedure 

is controlled by soil CBR, as measured using ASTM C4429. 

1. Identify properties of the subgrade, including CBR, location of groundwater table, 

AASHTO and/or USCS classification, and sensitivity. 

2. Compare these properties to those in Table 7-15, or with local policies. Determine if a 

geosynthetic will be required. 

3. Design the pavement without consideration of a geosynthetic, using normal pavement 

structural design procedures. 

4. Determine the need for additional imported aggregate to ameliorate mixing at the 

base/subgrade interface. If such aggregate is required, determine its thickness, t1, and 

reduce the thickness by 50%, considering the use of a geosynthetic. 

5. Determine additional aggregate thickness t2 needed for establishment of a 

construction platform. The FHWA procedure requires the use of curves for aggregate 

thickness vs. the expected single tire pressure and the subgrade bearing capacity, as 

shown in Figure 7-21, modified for highway applications. For the purposes of this 

manual, the curves have been correlated with common pavement construction traffic. 

Select Nc based on allowable subgrade ruts, where: 

Nc = 5 for a low rut criteria (< 50 mm (< 2 in.)), 

Nc = 5.5 for moderate rutting (50 – 100 mm (2 – 4 in.)), and 

Nc = 6 for large rutting (> 100 mm (> 4 in.)). 

(For comparison without a geotextile: Nc = 2.8, 3.0, or 3.3 respectively for low to 

large ruts.) 

Alternatively, local policies or charts may be used. 

6. Select the greater of t2 or 50% t1. 

7. Check filtration criteria for the geotextile to be used. For geogrids, check the 

aggregate for filtration compatibility with the subgrade (see Section 7.2), or use a 
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geotextile in combination with the grid meeting the following criteria. The important 

measures include the apparent opening size (AOS), the permeability (k), and 

permittivity (ψ) of the geotextile, and the 95% opening size, defined as the diameter 

of glass beads for which 95% will be retained on the geosynthetic. These values will 

be compared to a minimum standard or to the soil properties as follows 

• AOS ≤ D85 (Wovens) 

• AOS ≤ 1.8 D85 (Nonwovens) 

• kgeotextile ≥ ksoil 

• ψ ≥ 0.1 sec-1 

8. Determine geotextile survival criteria. The design is based on the assumption that the 

geosynthetic cannot function unless it survives the construction process. The 

AASHTO M288-99 standard categorizes the requirements for the geosynthetic based 

on the survival class. The requirements for the standard include the strength (grab, 

seam, tear, puncture, and burst), permittivity, apparent opening size, and resistance to 

UV degradation, based on the survival class. The survival class is determined from 

Table 7-5 (Section 7.2.12). For stabilization of soils, the default is Class 1, and for 

separation, the default is Class 2. These requirements may be reduced based on 

conditions and experience, as detailed in AASHTO M288. For geogrid survivability, 

see AASHTO PP46 and Berg et al. (2000). 

Field installation procedures introduce a number of special concerns; the AASHTO 

M288 standard includes a guide specification for geotextile construction. FHWA HI905-038 (Holtz et al. 1998) recommends that this specification be modified to suit local 

conditions and contractors and provides example specifications. Concerns and criteria 

for field installation include, for example, the seam lap and sewing requirements, and 

construction sequencing and quality control. 

7.6.5 Admixture Stabilization 

As previously indicated in Section 7.6.1, there are a variety of admixtures that can be mixed 

with the subgrade to improve its performance. The various admixture types are shown in 

Table 7-15, along with initial guidance for evaluating the appropriate application of these 

methods. Following is a general overview of each method, followed by a generalized outline 

for determining the optimum admixture content requirements. Design details for each 

specific method are contained in Appendix F. 
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(a) 

(b) 

Figure 7-21. Thickness design curves with geosynthetics for a) single and b) dual wheel oads

(after USFS, 1977, and FHWA NHI-95-038, 1998). 
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Table 7-16. Guide for selection of admixture stabilization method(s) (Austroads, 1998). 

Lime Treatment 

Lime treatment or modification consists of the application of 1 – 3% hydrated lime to aid 

drying of the soil and permit compaction. As such, it is useful in the construction of a 

“working platform” to expedite construction. Lime modification may also be considered to 

condition a soil for follow-on stabilization with cement or asphalt. Lime treatment of 

subgrade soils is intended to expedite construction, and no reduction in the required 

pavement thickness should be made. 

Lime may also be used to treat expansive soils, as discussed in Section 7.3. Expansive soils 

as defined for pavement purposes are those that exhibit swell in excess of 3%. Expansion is 
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characterized by heaving of a pavement or road when water is imbibed in the clay minerals. 

The plasticity characteristics of a soil often are a good indicator of the swell potential, as 

indicated in Table 7-17. If it has been determined that a soil has potential for excessive swell, 

lime treatment may be appropriate. Lime will reduce swell in an expansive soil to greater or 

lesser degrees, depending on the activity of the clay minerals present. The amount of lime to 

be added is the minimum amount that will reduce swell to acceptable limits. Procedures for 

conducting swell tests are indicated in the ASTM D 1883 CBR test and detailed in ASTM D 

4546. 

The depth to which lime should be incorporated into the soil is generally limited by the 

construction equipment used. However, 0.6 – 1 m (2 – 3 ft) generally is the maximum depth 

that can be treated directly without removal of the soil. 

Lime Stabilization 

Lime or pozzolonic stabilization of soils improves the strength characteristics and changes 

the chemical composition of some soils. The strength of fine-grained soils can be 

significantly improved with lime stabilization, while the strength of coarse-grained soils is 

usually moderately improved. Lime has been found most effective in improving workability 

and reducing swelling potential with highly plastic clay soils containing montmorillonite, 

illite, and kaolinite. Lime is also used to reduce the water content of wet soils during field 

compaction. In treating certain soils with lime, some soils are produced that are subject to 

fatigue cracking. 

Lime stabilization has been found to be an effective method to reduce the volume change 

potential of many soils. However, lime treatment of soils can convert the soil that shows 

negligible to moderate frost heave into a soil that is highly susceptible to frost heave, 

acquiring characteristics more typically associated with silts. It has been reported that this 

adverse effect has been caused by an insufficient curing period. Adequate curing is also 

important if the strength characteristics of the soil are to be improved. 

Table 7-17. Swell potential of soils (Joint Departments of the Army & Air Force, 1994).

Liquid Limit Plasticity Index Potential Swell 

> 60 > 35 High 

50 - 60 25 – 35 Marginal 

< 50 < 25 Low 
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The most common varieties of lime for soil stabilization are hydrated lime [Ca(OH)2], 

quicklime [CaO], and the dolomitic variations of these high-calcium limes [Ca(OH)2⋅MgO 

and CaO⋅MgO]. While hydrated lime remains the most commonly used lime stabilization 

admixture in the U.S., use of the more caustic quicklime has grown steadily over the past two 

decades. Lime is usually produced by calcining2

 limestone or dolomite, although some 

lime—typically of more variable and poorer quality—is also produced as a byproduct of 

other chemical processes. 

For lime stabilization of clay (or highly plastic) soils, the lime content should be from 3 – 8% 

of the dry weight of the soil, and the cured mass should have an unconfined compressive 

strength of at least 0.34 MPa (50 psi) within 28 days. The optimum lime content should be 

determined with the use of unconfined compressive strength and the Atterberg limits tests on 

laboratory lime-soil mixtures molded at varying percentages of lime. As discussed later in 

this section, pH can be used to determine the initial, near optimum lime content value. The 

pozzolanic strength gain in clay soils depends on the specific chemistry of the soil – e.g.,

whether it can provide sufficient silica and alumina minerals to support the pozzolanic 

reactions. Plasticity is a rough indicator of reactivity. A plasticity index of about 10 is 

commonly taken as the lower limit for suitability of inorganic clays for lime stabilization. 

The lime-stabilized subgrade layer should be compacted to a minimum density of 95%, as 

defined by AASHTO T99. 

Typical effects of lime stabilization on the engineering properties of a variety of natural soils 

are shown in Table 7-18 and Figure 7-22. These are the result of several chemical processes 

that occur after mixing the lime with the soil. Hydration of the lime absorbs water from the 

soil and causes an immediate drying effect. The addition of lime also introduces calcium 

(Ca+2) and magnesium (Mg+2) cations that exchange with the more active sodium (Na+

) and 

potassium (K+) cations in the natural soil water chemistry; this cation exchange reduces the 

plasticity of the soil, which, in most cases, corresponds to a reduced swell and shrinkage 

potential, diminished susceptibility to strength loss with moisture, and improved workability. 

The changes in the soil-water chemistry also lead to agglomeration of particles and a 

coarsening of the soil gradation; plastic clay soils become more like silt or sand in texture 

after the addition of lime. These drying, plasticity reduction, and texture effects all occur 

very rapidly (usually with 1 hour after addition of lime), provided there is thorough mixing of 

the lime and the soil. 

 

2

 Calcining is the heating of limestone or dolomite to a high temperature below the melting or fusing point that 

decomposes the carbonates into oxides and hydroxides. 
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Table 7-18. Examples of the effects of lime stabilization on various soils 

(Rollings and Rollings, 1996). 
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Figure 7-22. Effect of lime content on 

engineering properties of a CH clay

(from Rollings and Rollings, 1996; 

from data reported by McCallister 

and Petry, 1990).
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When soils are treated properly with lime, it has been observed that the lime-soil mixture 

may be subject to durability problems, the cyclic freezing and thawing of the soil. The 

durability of lime stabilization on swell potential and strength may be adversely affected by 

environmental influences: 

• Water: Although most lime stabilized soils retain 70% to 85% of their long-term 

strength gains when exposed to water, there have been reported cases of poor strength 

retention for stabilized soils exposed to soaking. Therefore, testing of stabilized soils 

in the soaked condition is prudent. 

• Freeze/thaw cycles: Freeze/thaw cycles can lead to strength deterioration, but 

subsequent healing often occurs where the strength loss caused by winter freeze/thaw 

reverses during the following warm season. The most common design approach is to 

specify a sufficiently high initial strength gain to retain sufficient residual strength 

after freeze/thaw damage. 

• Leaching: Leaching of calcium can decrease the cation exchange in lime stabilized 

soil, which, in turn, can reverse the beneficial reduction in plasticity and swell 

potential. The potential for these effects is greater when low lime contents are used. 

• Carbonation: If atmospheric carbon dioxide combines with lime to form calcium 

carbonate, the calcium silicate and calcium aluminate hydrate cements may become 

unstable and revert back to their original silica and alumina forms, reversing the longterm strength increase resulting from the pozzolanic reactions. Although this problem 

has been reported less in the United States than in other countries, its possibility 

should be recognized and its potential minimized by use of ample lime content, 

careful selection, placement, and compaction of the stabilized material to minimize 

carbon dioxide penetration, as well as prompt placement after lime mixing, and good 

curing. 

• Sulfate attack: Sulfates present in the soil or groundwater can combine with the 

calcium from the lime or the alumina from the clay minerals to form ettringite, which 

has a volume that is more than 200% larger than that of its constituents. Massive 

irreversible swelling can therefore occur, and the damage it causes can be quite 

severe. It is difficult to predict the combinations of sulfate content, lime content, clay 

mineralogy and content, and environmental conditions that will trigger sulfate attack. 

Consequently, if there is a suspicion of possible sulfate attack, the lime stabilized soil 

should be tested in the laboratory to see whether it will swell when mixed and 

exposed to moisture.
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Soils classified as CH, CL, MH, ML, SC, and GC with a plasticity index greater than 12 and 

with 10% passing the 0.425 mm (No. 40) sieve are potentially suitable for stabilization with 

lime. Lime-flyash stabilization is applicable to a broader range of soils because the 

cementing action of the material is less dependent on the fines contained within the soil. 

However, long-term durability studies of pavements with lime-flyash stabilization are rather 

limited. 

Hydrated lime, in powder form or mixed with water as a slurry, is used most often for 

stabilization. 

Cement Stabilization 

Portland cement is widely used for stabilizing low-plasticity clays, sandy soils, and granular 

soils to improve the engineering properties of strength and stiffness. Increasing the cement 

content increases the quality of the mixture. At low cement contents, the product is generally 

termed cement-modified soil. A cement-modified soil has improved properties of reduced 

plasticity or expansive characteristics and reduced frost susceptibility. At higher cement 

contents, the end product is termed soil-cement or cement-treated base, subbase, or subgrade. 

For soils to be stabilized with cement, proper mixing requires that the soil have a PI of less 

than 20% and a minimum of 45% passing the 0.425 mm (No. 40) sieve. However, highly 

plastic clays that have been pretreated with lime or flyash are sometimes suitable for 

subsequent treatment with Portland cement. For cement stabilization of granular and/or 

nonplastic soils, the cement content should be 3 – 10% of the dry weight of the soil, and the 

cured material should have an unconfined compressive strength of at least 1 MPa (150 psi) 

within 7 days. The Portland cement should meet the minimum requirements of AASHTO M 

85. The cement-stabilized subgrade should be compacted to a minimum density of 95% as 

defined by AASHTO M 134. 

Several different types of cement have been used successfully for stabilization of soils. Type 

I normal Portland cement and Type IA air-entraining cements were used extensively in the 

past, and produced about the same results. At the present time, Type II cement has largely 

replaced Type I cement as greater sulfate resistance is obtained, while the cost is often the 

same. High early strength cement (Type III) has been found to give a higher strength in some 

soils. Type III cement has a finer particle size and a different compound composition than do 

the other cement types. Chemical and physical property specifications for Portland cement 

can be found in ASTM C 150. 
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The presence of organic matter and/or sulfates may have a deleterious effect on soil cement. 

Tests are available for detection of these materials and should be conducted if their presence 

is suspected. 

(1) Organic matter. A soil may be acid, neutral, or alkaline and still respond well to 

cement treatment. Although certain types of organic matter, such as undecomposed 

vegetation, may not influence stabilization adversely, organic compounds of lower 

molecular weight, such as nucleic acid and dextrose, act as hydration retarders and 

reduce strength. When such organics are present, they inhibit the normal hardening 

process. If the pH of a 10:1 mixture (by weight) of soil and cement 15 minutes after 

mixing is at least 12.0, it is probable that any organics present will not interfere with 

normal hardening. 

(2) Sulfates. Although sulfate attack is known to have an adverse effect on the quality of 

hardened Portland cement concrete, less is known about the sulfate resistance of 

cement stabilized soils. The resistance to sulfate attack differs for cement-treated, 

coarse-grained and fine-grained soils, and is a function of sulfate concentrations. 

Sulfate-clay reactions can cause deterioration of fine-grained soil-cement. On the 

other hand, granular soil-cements do not appear susceptible to sulfate attack. In some 

cases, the presence of small amounts of sulfate in the soil at the time of mixing with 

the cement may even be beneficial. The use of sulfate-resistant cement may not 

improve the resistance of clay-bearing soils, but may be effective in granular soilcements exposed to adjacent soils and/or groundwater containing high sulfate 

concentrations. The use of cement for fine-grained soils containing more than about 

1% sulfate should be avoided. 

Stabilization with Lime-Flyash (LF) and Lime-Cement-Flyash (LCF) 

Stabilization of coarse-grained soils having little or no fines can often be accomplished by 

the use of LF or LCF combinations. Flyash, also termed coal ash, is a mineral residual from 

the combustion of pulverized coal. It contains silicon and aluminum compounds that, when 

mixed with lime and water, forms a hardened cementitious mass capable of obtaining high 

compressive strengths. Lime and flyash in combination can often be used successfully in 

stabilizing granular materials, since the flyash provides an agent with which the lime can 

react. Thus LF or LCF stabilization is often appropriate for base and subbase course 

materials. 

Flyash is classified according to the type of coal from which the ash was derived. Class C 

flyash is derived from the burning of lignite or subbituminous coal and is often referred to as 

“high lime” ash because it contains a high percentage of lime. Class C flyash is self-reactive 

or cementitious in the presence of water, in addition to being pozzolanic. Class F flyash is 

derived from the burning of anthracite or bituminous coal and is sometimes referred to as 
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“low lime” ash. It requires the addition of lime to form a pozzolanic reaction. To be 

acceptable quality, flyash used for stabilization must meet the requirements indicated in 

ASTM C 593.

Design with LF is somewhat different from stabilization with lime or cement. For a given 

combination of materials (aggregate, flyash, and lime), a number of factors can be varied in 

the mix design process, such as percentage of lime-flyash, the moisture content, and the ratio 

of lime to flyash. It is generally recognized that engineering characteristics such as strength 

and durability are directly related to the quality of the matrix material. The matrix material is 

that part consisting of flyash, lime, and minus No. 4 aggregate fines. Basically, higher 

strength and improved durability are achievable when the matrix material is able to “float” 

the coarse aggregate particles. In effect, the fine size particles overfill the void spaces 

between the coarse aggregate particles. For each coarse aggregate material, there is a quantity 

of matrix required to effectively fill the available void spaces and to “float” the coarse 

aggregate particles. The quantity of matrix required for maximum dry density of the total 

mixture is referred to as the optimum fines content. In LF mixtures, it is recommended that 

the quantity of matrix be approximately 2% above the optimum fines content. At the 

recommended fines content, the strength development is also influenced by the ratio of lime 

to flyash. Adjustment of the lime-flyash ratio will yield different values of strength and 

durability properties. 

 

Asphalt Stabilization 

Generally, asphalt-stabilized soils are used for base and subbase construction. Use of asphalt 

as a stabilizing agent produces different effects, depending on the soil, and may be divided 

into three major groups: 1) sand-bitumen, which produces strength in cohesionless soils, such 

as clean sands, or acts as a binder or cementing agent, 2) soil-bitumen, which stabilizes the 

moisture content of cohesive fine-grained soils, and 3) sand-gravel bitumen, which provides 

cohesive strength and waterproofs pit-run gravelly soils with inherent frictional strength. The 

durability of bitumen-stabilized mixtures generally can be assessed by measurement of their 

water absorption characteristics. Treatment of soils containing fines in excess of 20% is not 

recommended. 

Stabilization of soils and aggregates with asphalt differs greatly from cement and lime 

stabilization. The basic mechanism involved in asphalt stabilization of fine-grained soils is a 

waterproofing phenomenon. Soil particles or soil agglomerates are coated with asphalt that 

prevents or slows the penetration of water that could normally result in a decrease in soil 

strength. In addition, asphalt stabilization can improve durability characteristics by making 

the soil resistant to the detrimental effects of water, such as volume. In noncohesive 

materials, such as sands and gravel, crushed gravel, and crushed stone, two basic 
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mechanisms are active: waterproofing and adhesion. The asphalt coating on the cohesionless 

materials provides a membrane that prevents or hinders the penetration of water and thereby 

reduces the tendency of the material to lose strength in the presence of water. The second 

mechanism has been identified as adhesion. The aggregate particles adhere to the asphalt and 

the asphalt acts as a binder or cement. The cementing effect thus increases shear strength by 

increasing cohesion. Criteria for design of bituminous-stabilized soils and aggregates are 

based almost entirely on stability and gradation requirements. Freeze-thaw and wet-dry 

durability tests are not applicable for asphalt-stabilized mixtures. 

There are three basic types of bituminous-stabilized soils, including 

(1) Sand bitumen. A mixture of sand and bitumen in which the sand particles are 

cemented together to provide a material of increased stability. 

(2) Gravel or crushed aggregate bitumen. A mixture of bitumen and a well-graded gravel 

or crushed aggregate that, after compaction, provides a highly stable waterproof mass 

of subbase or base course quality. 

(3) Bitumen lime. A mixture of soil, lime, and bitumen that, after compaction, may 

exhibit the characteristics of any of the bitumen-treated materials indicated above. 

Lime is used with materials that have a high PI, i.e., above 10. 

Bituminous stabilization is generally accomplished using asphalt cement, cutback asphalt, or 

asphalt emulsions. The type of bitumen to be used depends upon the type of soil to be 

stabilized, method of construction, and weather conditions. In frost areas, the use of tar as a 

binder should be avoided because of its high temperature susceptibility. Asphalts are affected 

to a lesser extent by temperature changes, but a grade of asphalt suitable to the prevailing 

climate should be selected. As a general rule, the most satisfactory results are obtained when 

the most viscous liquid asphalt that can be readily mixed into the soil is used. For higher 

quality mixes in which a central plant is used, viscosity-grade asphalt cements should be 

used. Much bituminous stabilization is performed in-place, with the bitumen being applied 

directly on the soil or soil aggregate system, and the mixing and compaction operations being 

conducted immediately thereafter. For this type of construction, liquid asphalts, i.e., cutbacks 

and emulsions, are used. Emulsions are preferred over cutbacks because of energy constraints 

and pollution control efforts. The specific type and grade of bitumen will depend on the 

characteristics of the aggregate, the type of construction equipment, and the climatic 

conditions. Generally, the following types of bituminous materials will be used for the soil 

gradation indicated: 

(1) Open-graded aggregate. 

a. Rapid- and medium-curing liquid asphalts RC-250, RC-800, and MC3000. 

b. Medium-setting asphalt emulsion MS-2 and CMS-2. 
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(2) Well-graded aggregate with little or no material passing the 0.075 mm (No. 200) 

sieve. 

a. Rapid and medium-curing liquid asphalts RC-250, RC-800, MC-250, 

and MC-800. 

b. Slow-curing liquid asphalts SC-250 and SC-800. 

c. Medium-setting and slow-setting asphalt emulsions MS-2, CMS-2, 

SS-1, and CSS-1. 

(3) Aggregate with a considerable percentage of fine aggregates and material passing 

the 0.075 mm (No. 200) sieve. 

a. Medium-curing liquid asphalt MC-250 and MC-800. 

b. Slow-curing liquid asphalts SC-250 and SC-800 

c. Slow-setting asphalt emulsions SS-1, SS-01h, CSS-1, and CSS-lh.

The simplest type of bituminous stabilization is the application of liquid asphalt to the 

surface of an unbound aggregate road. For this type of operation, the slow- and mediumcuring liquid asphalts SC-70, SC-250, MC-70, and MC-250 are used. 

The recommended soil gradations for subgrade materials and base or subbase course 

materials are shown in Tables 7-19 and 7-20, respectively. 

Table 7-19. Recommended gradations for bituminous-stabilized subgrade materials 

(Joint Departments of the Army and Air Force, 1994). 

Sieve Size Percent Passing 

75-mm (3-in.) 

4.75-mm (#4) 

600-μm (#30) 

75-μm (#200) 

100 

50-100 

38-100 

2-30 
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Table 7-20. Recommended gradations for bituminous-stabilized base and subbase 

materials (Joint Departments of the Army and Air Force, 1994).

Sieve Size 

37.5 mm 

(1 1⁄2 in.) 

Maximum 

25 mm 

(1-in.) 

Maximum

19 mm 

(3⁄4-in.) 

Maximum 

12.7 mm 

(1⁄2-in.) 

Maximum 

37.5-mm (11⁄2-in.) 

25-mm (l-in.) 

19-mm (3⁄4-in.) 

M-in 

9.5-mm (3/8-in.) 

0.475-mm (#4) 

2.36-mm (#8) 

1.18-mm (#16) 

600-μm (#30) 

300-μm (#50) 

150-μm (#100) 

75-μm (#200)

100 

8 4 ± 9 

76 ± 9 

66 ± 9 

59 ± 9 

45 ± 9 

35 ± 9 

27 ± 9 

20 ± 9 

14 ± 7 

9 ± 5 

5 ± 2

- 

100 

83 ± 9 

73 ± 9 

64 ± 9 

48 ± 9 

36 ± 9 

28 ± 9 

21 ± 9 

16 ± 7 

11 ± 5 

5 ± 2

- 

- 

100 

82 ± 9 

72 ± 9 

54 ± 9 

41 ± 9 

32 ± 9 

24 ± 9 

17 ± 7 

12 ± 5 

5 ± 2

- 

- 

- 

100 

83 ± 9 

62 ± 9 

47 ± 9 

36 ± 9 

28 ± 9 

20 ± 7 

14 ± 5 

5 ± 2

Stabilization with Lime-Cement and Lime-Bitumen 

The advantage of using combination stabilizers is that one of the stabilizers in the 

combination compensates for the lack of effectiveness of the other in treating a particular 

aspect or characteristic of a given soil. For instance, in clay areas devoid of base material, 

lime has been used jointly with other stabilizers, notably Portland cement or asphalt, to 

provide acceptable base courses. Since Portland cement or asphalt cannot be mixed 

successfully with plastic clays, the lime is added first to reduce the plasticity of the clay. 

While such stabilization practice might be more costly than the conventional single stabilizer 

methods, it may still prove to be economical in areas where base aggregate costs are high. 

Two combination stabilizers are considered in this section: lime-cement and lime-asphalt. 

a) Lime-cement. Lime can be used as an initial additive with Portland cement, or as the 

primary stabilizer. The main purpose of lime is to improve workability 

characteristics, mainly by reducing the plasticity of the soil. The design approach is to 

add enough lime to improve workability and to reduce the plasticity index to 

acceptable levels. The design lime content is the minimum that achieves desired 
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results. The design cement content is determined following procedures for cementstabilized soils presented in Appendix F. 

b) Lime-asphalt. Lime can be used as an initial additive with asphalt, or as the primary 

stabilizer. The main purpose of lime is to improve workability characteristics and to 

act as an anti-stripping agent. In the latter capacity, the lime acts to neutralize acidic 

chemicals in the soil or aggregate that tend to interfere with bonding of the asphalt. 

Generally, about 1 – 2% percent lime is all that is needed for this objective. Since 

asphalt is the primary stabilizer, the procedures for asphalt-stabilized materials, as 

presented Appendix F, should be followed. 

Admixture Design 

Design of admixtures takes on a similar process regardless of the admixture type. The 

following steps are generally followed and are generic to lime, cement, L-FA and L-C-FA, or 

asphalt admixtures. 

Step 1. Classify soil to be stabilized. 

(% < 0.075 mm – No. 200 sieve, % < 0.425 mm – No. 40 Sieve, PI, etc.) 

Step 2. Prepare trial mixes with varying % content. 

 Lime: Select lowest % with pH = 12.4 in 1 hour 

Cement: Use table to estimate cement content requirements 

Asphalt: Use equation & table in Appendix F to estimate the quantity of cutback 

asphalt 

Step 3. Develop moisture-density relationship for initial design. 

Step 4. Prepare triplicate samples and cure specimens at target density. 

 Use optimum water content and % initial admixture, +2% and +4% 

Step 5. Determine index strength. 

Lime and Cement: Determine unconfined compressive strength (ASTM D 5102) 

Asphalt: Determine Marshall stability 

Step 6. Determine resilient modulus for optimum percent admixture. 

 Perform test or estimate using correlations (See Chapter 5) 

Step 7. Conduct freeze-thaw tests (Regional as required). 

(For Cement, CFA, L-C-FA) 
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Step 8. Select % to achieve minimum design strength and F-T durability. 

Step 9. Add 0.5 – 1% to compensate for non-uniform mixing. 

Appendix F provides specific design requirements and design step details for each type of 

admixture reviewed in this section. Additional design and construction information can also 

be obtained from industry publications including 

− Soil-Cement Construction Handbook, Portland Cement Association, Skokie, Il, 1995. 

− Lime-Treated Soil Construction Manual: Lime Stabilization & Lime Modification, 

National Lime Associaiton, Arlington, Virginia, 2004. 

− Flexible Pavement Manual, American Coal Ash Association, Washington, D.C., 

1991. 

− A Basic Emulsion Manual, Asphalt Institute, Manual Series #19. 

− http://www.cement.org/index.asp

− http://www.lime.org/ 

 

7.6.6 Soil Encapsulation 

Soil encapsulation is a foundation improvement technique that has been used to protect 

moisture sensitive soils from large variations in moisture content. The concept of soil 

encapsulation is to keep the fine-grained soils at or slightly below optimum moisture content, 

where the strength of these soils can support heavier trucks and traffic. This technique has 

been used by a number of states (e.g., Texas and Wyoming) on selected projects to improve 

the foundations of higher volume roadways. It is more commonly used as a technique in 

Europe and in foundation or subbase layers for low-volume roadways, where the import of 

higher quality paving materials is restricted from a cost standpoint. More than 100 projects 

have been identified around the world, usually reporting success in controlling expansive 

soils (Steinberg, 1998). 

Fine-grained soils can provide adequate bearing strengths for use as structural layers in 

pavements and embankments, as long as the moisture content remains below the optimum 

moisture content. However, increases in moisture content above the optimum value can cause 

a significant reduction in the stiffness (i.e., resilient modulus) and strength of fine-grained 

materials and soils. Increased moisture content in fine-grained soils below pavements occurs 

over time, especially in areas subject to frost penetration and freeze-thaw cycles. Thus, finegrained soils cannot be used as a base or subbase layer unless the soils are protected from any 

increase in moisture. 
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The soil encapsulation concept, sometimes referred to as membrane encapsulated soil layer 

(MESL), is a method for maintaining the moisture content of the soil at the desired level by 

encapsulating the soil in waterproof membranes. The waterproof membranes prevent water 

from infiltrating the moisture sensitive material. The resilient modulus measured at or below 

optimum conditions remains relatively constant over the design life of the pavement. 

The prepared subgrade is normally sprayed with an asphalt emulsion before the bottom 

membrane of polyethylene is placed. This asphalt emulsion provides added waterproofing 

protection in the event the membrane is punctured during construction operations, and acts as 

an adhesive for the membrane to be placed in windy conditions. The first layer of soil is 

placed in sufficient thickness such that the construction equipment will not displace the 

underlying material. The completed soil embankment is also sprayed with an asphalt 

emulsion before placement of the top membrane. To form a complete encapsulation, the 

bottom membrane is brought up the sides and wrapped around the top, for an excavated 

section, or the top membrane is draped over the sides, for an embankment situation. The top 

of the membrane is sprayed with the same asphalt emulsion and covered with a thin layer of 

clean sand to blot the asphalt and to provide added protection against puncture by the 

construction equipment used to place the upper paving layers. 

The reliability of this method to maintain the resilient modulus and strength of the foundation 

soil over long periods of time is unknown. More importantly, roadway maintenance and the 

installation of utilities in areas over time limit the use of this technique. Thus, this 

improvement technique is not suggested unless there is no other option available. 

If this technique is used, the pavement designer should be cautioned regarding the use of the 

environmental effects model (EICM) to predict changes in moisture over time. Special 

design computations will be needed to restrict the change in moisture content of the MESL 

over time. The resilient modulus used in design for the MESL should be held constant over 

the design life of the pavement. The designer should also remember that any utilities placed 

after pavement construction could make that assumption invalid. 

7.6.7 Lightweight Fill 

When constructing pavements on soft soils, there is always a concern for settlement. For 

deeper deposits where shallow surface stabilization may not be effective, thicker granular 

aggregate as discussed in Section 7.3, may be effective for control deformation under wheel 

load, but would increase the concern for settlement. An alternate to replacement with 

aggregate would be to use lightweight fill. 
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The compacted unit density of most soil deposits consisting of sands, silts, or clays ranges 

from about 1,800 – 2,200 kg/m3

 (112 – 137 lbs/ft3) Lightweight fill materials are available 

from the lower end of this range down to 12 kg/m3

 ( 0.75 lbs/ft3). In many cases, the use of 

lighter weight materials on soft soils will likely result in both reduced settlement and 

increased stability. The worldwide interest and use of lightweight fill materials has led to the 

recent publication by the Permanent International Association of Road Congresses (PIARC) 

of an authoritative reference "Lightweight Filling Materials" in 1997. 

Many types of lightweight fill materials have been used for roadway construction. Some of 

the more common lightweight fills are listed in Table 7-21. There is a wide range in density 

of the lightweight fill materials, but all have a density less than conventional soils. Addtional 

information on the composition and sources of the lightweight fill materials listed in Table 7-

21 can be found in FHWA NHI-04-001 Ground Improvement Methods technical summaries. 

Some lightweight fill materials have been used for decades, while others are relatively recent 

developments. Wood fiber has been used for many years by timber companies for roadways 

crossing peat bogs and low-lying land, as well as for repair of slide zones. 

The steel-making companies have produced slag since the start of the iron and steel making 

industry. Initially, the slag were stockpiled as waste materials, but beginning around 1950, 

the slag were crushed, graded, and sold for fill materials. 

Geofoam is a generic term used to describe any foam material used in a geotechnical 

application. Geofoam includes expanded polystyrene (EPS), extruded polystyrene (XPS), and 

glassfoam (cellular glass). Geofoam was initially developed for insulation material to prevent 

frost from penetrating soils. The initial use for this purpose was in Scandinavia and North 

America in the early 1960s. In 1972, the use of geofoam was extended as a lightweight fill 

for a project in Norway. 

The technique of using pumping equipment to inject foaming agents into concrete was 

developed in the late 1930s. Little is known about the early uses of this product. However, 

the U.S. Army Corps of Engineers used foamed concrete as a tunnel lining and annular fill. 

This product is generally job-produced as a cement/water slurry with preformed foam 

blended for accurate control and immediate placement. 
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Table 7-21. Densities and approximate costs for various lightweight fill materials.

Fill Type Range in Density 

kg/m3

Range in 

Specific Gravity

Approximate Cost1

$/m3

Geofoam (EPS) 12 to 32 0.01 to .03 40.00 to 85.002

Foamed Concrete 320 to 970 0.3 to 0.8 40.00 to 55.00 

Wood Fiber 550 to 960 0.6 to 1.0 12.00 to 20.002

Shredded Tires 600 to 900 0.6 to 0.9 20.00 to 30.002

Expanded Shale 

And Clay 600 to 1040 0.6 to 1.0 40.00 to 55.003

Flyash 1120 to 1440 1.1 to 1.4 15.00 to 21.003

Boiler Slag 1000 to 1750 1.0 to 1.8 3.00 to 4.003

Air-Cooled Slag 1100 to 1500 1.1 to 1.5 7.50 to 9.003

1

 See Chapter 6 for details on cost data 

2

 Price includes transportation and placement cost 

3

 FOB plant

Shredded tires and tire bales are a relatively recent source of lightweight fill materials. The 

availability of this material is increasing each year, and its use as a lightweight fill is further 

promoted by the need to dispose of tires. In most locations, the tires are stockpiled, but they 

are unsightly and present a serious fire and health hazard. Shredded tires have been used for 

lightweight fill in the United States and in other countries since the mid 1980s. More than 85 

fills using shredded tires as a lightweight fill have been constructed in the United States. In 

1995, three tire shred fills with a thickness greater than 8 m (26 ft) experienced an 

unexpected internal heating reaction. As a result, FHWA issued an Interim Guideline to 

minimize internal heating of tire shred fills in 1997, limiting tire shred layers to 3 m (9.8 ft). 

Expanded shale lightweight aggregate has been used for decades to produce aggregate for 

concrete and masonry units. Beginning in about 1980, lightweight aggregates have also been 

used for geotechnical purposes. Completed projects include the Port of Albany, New York 

marine terminal, where lightweight fill was used behind a bulkhead to reduce the lateral 

pressures on the steel sheeting. Other projects include construction of roadways over soft 

ground. The existing high-density soils were partially removed and replaced with lightweight 

aggregate to reduce settlement. Other projects have included improvement of slope stability 

by reduction of the gravitational driving force of the soil in the slope and replacement with a 

lightweight fill. 

Waste products from coal burning include flyash and boiler slag. Both of these materials 

have been used in roadway construction. One of the first documented uses of flyash in an 

engineered highway embankment occurred in England in 1950. Trial embankments led to the 
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acceptance of flyash fills, and other roadway projects were constructed in other European 

countries. In 1965, a flyash roadway embankment was constructed in Illinois. In 1984, a 

project survey found that flyash was used in the construction of 33 embankments and 31 area 

fills. Boiler slag has been used for backfill since the early 1970s. Many state highway 

department specifications allow the use of boiler slag as an acceptable fine or coarse 

aggregate. 

The FHWA NHI-04-001 provides an overview of the more common lightweight fill 

materials that have been used for geotechnical applications in highway construction. Typical 

geotechnical engineering parameters that are important for design are provided. In addition, 

design and construction considerations unique to each of these lightweight fill materials are 

presented. This information can be used for preliminary planning purposes. The technical 

summary also presents guidelines for preparation of specifications along with suggested 

construction control procedures. Four case histories are also presented to demonstrate the 

effectiveness of lightweight fills for specific situations. Approximate costs for the various 

lightweight fill materials are also presented. 

With regard to pavement design, if a minimum of 1 m (3 ft) of good quality gravel type fill is 

placed between the pavement structure and the lightweight materials as a cover, then the 

lightweight material will have little impact on pavement design, even for the more 

compressible tire and geofoam materials. However, if a thinner cover must be used, the 

support value for these materials must be determined. Lab tests can be used, as discussed in 

Chapter 5, especially for the granular type materials. The ideal method is to perform field 

resilient modulus tests on placed material (i.e., on cover soils after placement over the 

lightweight material(s)), especially for the bulkier materials, such as tires and geofoam. 

7.6.8 Deep Foundations and Other Foundation Improvement Methods 

(from Elias et al., 2004) 

In some cases, the extent (area and depth) of poor subgrade conditions are too large for 

surface stabilization or removal. In extreme cases, the soils may be too week to support the 

roadway embankment (even for embankments that only consist of the pavement structure). In 

these cases, other deep ground improvement methods, such as deep foundations, may be 

required. Ground improvement technologies are geotechnical construction methods used to 

alter and improve poor ground conditions so that embankment and structure construction can 

meet project performance requirements where soil replacement is not feasible for 

environmental or technical reasons, or it is too costly. 
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Ground improvement has one or more than one of the following main functions: 

• to increase bearing capacity, shear or frictional strength, 

• to increase density, 

• to control deformations, 

• to accelerate consolidation, 

• to decrease imposed loads, 

• to provide lateral stability, 

• to form seepage cutoffs or fill voids, 

• to increase resistance to liquefaction and, 

• to transfer embankment loads to more competent layers 

There are three strategies available to accomplish the above functions representing different 

approaches. The first method is to increase the shear strength, density, and/or decrease the 

compressibility of the foundation soil. The second method is to utilize a lightweight fill 

embankment to reduce significantly the applied load to the foundation, and the third method 

is to transfer loads to a more competent deeper layer. 

The selection of candidate ground improvement methods for any specific project follows a 

sequential process. The steps in the process include a sequence of evaluations that proceed 

from simple to more detailed, allowing a best method to emerge. The process is described as 

follows: 

1) Identify potential poor ground conditions, their extent, and type of negative impact. 

Poor ground conditions are typically characterized by soft or loose foundation soils, 

which, under load, would cause long-term settlement, or cause construction or postconstruction instability. 

2) Identify or establish performance requirements. Performance requirements generally 

consist of deformation limits (horizontal and vertical), as well as some minimum 

factors of safety for stability. The available time for construction is also a 

performance requirement. 

3) Identify and assess any space or environmental constraints. Space constraints 

typically refer to accessibility for construction equipment to operate safely, and 

environmental constraints may include the disposal of spoil (hazardous or not 

hazardous) and the effect of construction vibrations or noise. 

4) Assessment of subsurface conditions. The type, depth, and extent of the poor soils 

must be considered, as well as the location of the ground-water table. It is further 

valuable to have at least a preliminary assessment of the shear strength and 

compressibility of the identified poor soils. 

5) Preliminary selection. Preliminary selection of potentially applicable method(s) is 

generally made on a qualitative basis, taking into consideration the performance 
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criteria, limitations imposed by subsurface conditions, schedule and environmental 

constraints, and the level of improvement that is required. Table 7-22, which groups 

the available methods in six broad categories, can be used as a guide in this process to 

identify possible methods and eliminate those that by themselves, or in conjunction 

with other methods, cannot produce the desired performance. 

6) Preliminary design. A preliminary design is developed for each method identified 

under “Preliminary selection” and a cost estimate prepared on the basis of data in 

Table 7-23. The guidance in developing preliminary designs is contained within each 

Technical Summary. 

7) Comparison and selection. The selected methods are then compared, and a selection 

made by considering performance, constructability, cost, and other relevant project 

factors. 

State-of-the-art design and construction methods and/or references are provided in each of 

the FHWA NHI-04-001 Ground Improvement Methods technical summaries to form the 

basis of a final design. The success of any ground improvement method is predicated on the 

implementation of a QA/QC program to verify that the desired foundation improvement level 

has been reached. These programs incorporate a combination of construction observations, 

in-situ testing and laboratory testing to evaluate the treated soil in the field. Details are 

provided in each technical summary contained in the FHWA NHI-04-001. 
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Table 7-22. Ground improvement categories, functions, methods and applications 

(Elias et al., 2004). 

Category Function Methods Comment 

Consolidation Accelerate 

consolidation, 

increase shear 

strength 

(1) Wick drains 

(2) Vacuum 

consolidation 

Viable for normally consolidated 

clays. Vacuum consolidation 

viable for very soft clays. Can 

achieve up to 90% consolidation in 

a few months. 

Load Reduction Reduce load on 

foundation, reduce 

settlement 

(1) Geofoam, 

(2) Foamed concrete 

(3) Lightweight granular 

fills, tire chips, etc. 

Density varies from 1 – 12 kN/m3

(6 – 76 lb/ft3

). Granular fills usage 

subject to local availability. 

Densification Increase density, 

bearing capacity 

and frictional 

strength of granular 

soils. Decrease 

settlement and 

increase resistance 

to liquefaction. 

(1) Vibro-compaction 

using vibrators 

(2) Dynamic compaction 

by falling weight impact 

Vibrocompaction viable for clean 

sands with <15% fines. Dynamic 

compaction limited to depths of 

about 10 m (33 ft), but is 

applicable for a wider range of 

soils. Both methods can densify 

granular soils up to 80% Relative 

Density. 

Dynamic compaction generates 

vibrations for a considerable 

lateral distance. 

Reinforcement Internally 

reinforces fills 

and/or cuts. In soft 

foundation soils, 

increases shear 

strength, resistance 

to liquefaction and 

decreases 

compressibility. 

(1) MSE retaining walls 

(2) Soil Nailing walls 

(3) Stone column to 

reinforce foundations 

Soil Nailing may not applicable in 

soft clays or loose fills. Stone 

columns applicable in soft clay 

profiles to increase global shear 

strength and reduce settlement. 

Chemical 

Stabilization by 

Deep Mixing 

Methods 

Physio-chemical 

alteration of 

foundation soils to 

increase their 

tensile, 

compressive and 

shear strength, and 

to decrease 

settlement and/or 

provide lateral 

stability and or 

confinement. 

(1) Wet mixing methods 

using primarily cement 

(2) Dry mixing methods 

using lime-cement 

Applicable in soft to medium stiff 

clays for excavation support where 

the groundwater table must be 

maintained, or for foundation 

support where lateral restraint 

must be provided, or to increase 

global stability and decrease 

settlement. Requires significant 

QA/QC program for verification. 
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Table 7-22. Ground improvement categories, functions, methods and applications (continued).

Category Function Methods Comment 

Chemical 

Stabilization by 

Grouting 

To form seepage 

cutoffs, fill voids, 

increase density, 

increase tensile and 

compressive 

strength 

(1) Permeation grouting 

with particulate or 

chemical grouts 

(2) Compaction grouting 

(3) Jet grouting, and 

(4) Bulk filling 

(1) Permeation grouting to 

increase shear strength or for 

seepage control, (2) compaction 

grouting for densification and (3) 

jet grouting to increase tensile 

and/or compressive strength of 

foundations, and (4) bulk filling of 

any subsurface voids. 

Load Transfer Transfer load to 

deeper bearing 

layer 

Column (Pile) supported 

embankments on 

flexible geosynthetic 

mats 

Applicable for deep soft soil 

profiles or where a tight schedule 

must be maintained. A variety of 

stiff or semi-stiff piles can be used. 

Table 7-23a. Comparative Costs (SI units) (Elias et al., 2004).

Method Unit Cost Cost of Treated Volume $/m3 

Wick Drains $ 1.50 - 4.00/m $ 0.80 - 1.60 

Lightweight Fill 

 Granular 

 Tires-Wood 

 Geofoam 

 Foamed Concrete 

$ 3.00 - 21.00/m3 

$ 12.00 - 30.00/m3 

$ 35.00 - 65.00/m3 

$ 45.00 - 65.00/m3

Vibrocompaction $ 15.00 - 25.00/m $ 1.00 - 4.00 

Dynamic Compaction $ 6.00 - 11.00/m2 $ 1.00 - 2.00 

MSE Walls $ 160.00 - 300.00/m2 

RSS Slopes $ 110.00 - 260.00/m2 

Soil Nail Walls $ 400.00 - 600.00/m2 

Stone Columns $ 40.00 - 60.00/m $ 50 - 75 

Deep Soil Mixing 

 Dry w/lime-cement 

 Wet w/cement 

$30.00/m $ 60 

$ 85 -150 

Grouting 

 Permeation 

 Compaction 

 Jet 

$ 65.00/m + $ 0.70/Liter 

$ 30 - 200 

$ 200 - 275 

Column-Supported Embankments $ 95/m2 + cost of column n/a 
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Table 7-23b. Comparative Costs (U.S. customary units) (Elias et al., 2004).

Method Unit Cost Cost of Treated 

Volume $/yd3 

 

Wick Drains $ 0.46 - 1.22/ft $ 0.60 - 1.20 

Lightweight Fill 

 Granular 

 Tires-Wood 

 Geofoam 

 Foamed Concrete 

$ 2.30 - 16.10/yd3 

$ 9.20 - 23.00/yd3 

$ 26.75 - 50.00/yd3 

$ 34.50 - 50.00/yd3

Vibrocompaction $ 4.60 - 7.60/ft $ 0.75 – 3.00 

Dynamic Compaction $ 5.00 - 9.20/yd2 $ 0.75 - 1.50 

MSE Walls $ 15.00 - 28.00/ft2 

RSS Slopes $ 10.00 - 24.00/ft2 

Soil Nail Walls $ 37.00 - 56.00/ft2 

Stone Columns $ 12.20 - 18.30/ft $ 38 - 57 

Deep Soil Mixing 

 Dry w/lime-cement 

 Wet w/cement 

$9.15/ft $ 46 

$ 65 -115 

Grouting 

 Permeation 

 Compaction 

 Jet 

$ 20/ft + $ 2.65/Gallon 

$ 23 - 153 

$ 150 - 210 

Column Supported Embankments $ 81.50/yd2 + cost of column n/a 

7.7 RECYCLE 

Recycling, in principal, is a very powerful and often political concept. While the benefits of 

recycling including conservation of aggregate and binders and preservation of the 

environment, it requires serious consideration. The long-term performance of recycled 

materials in pavements and, in come cases the environmental impact, must be carefully 

evaluated to avoid costly performance and maintenance issues. In this section, the evaluation 

requirements for recycled materials will be reviewed. There are two forms of recycling in 

pavements: 1) reuse of the pavement materials themselves and 2) the use of recycled waste 

materials for subgrade stabilization or as a substitute for aggregate. 

7.7.1 Pavement Recycling 

The method of recycling the pavement will, in most cases, depend on whether the surface 

pavement has an AC or PCC surface pavement. In either case, the material could be 

rubblized, or, in some cases, processed (e.g., sieving, stockpiling, and reusing the reclaimed 

asphalt pavement (RCP) materials or recycled concrete materials (RCM) plus the aggregate 

base). Both pavement types can also be rubblized in place and compacted. This procedure is 
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Geotechnical Aspects of Pavements 7 - 101 May 2006

known as rubblize and roll for PCC pavements and full-depth reclamation for AC pavements. 

For AC pavement materials, there are also several other methods, including hot mix asphalt 

recycling, hot in-place recycling, and cold in-place recycling, all of which produce a bound 

product, which is beyond the scope of this manual. 

Recycled Asphalt 

The design requirements for RCP aggregates are essentially the same as natural aggregates. 

The strength of the material must be determined using the methods outlined in Chapter 5 and 

Section 7.3, and an assessment must be made of the drainage characteristics, as discussed in 

Section 7.2. With full-depth reclamation, all of the asphalt pavement sections and a 

predetemined amount of underlying materials are treated with recyling agents to produce a 

stabilized base course, and is well covered in FHWA-SA-98-042 (Kandhal, and Mallick, 

1997) . The advantages of this process are establishing high production rate and maintaining 

the geometry of the pavement or shoulder reconstruction. The primary drawbacks are 

aggregate size, depth limitation and depth control, and need for specialized equipment. With 

the sizing, RAP can often only be effectively screened down to a maximum size of 50 mm 

(2 in.). If a significant amount of contaminated base course (i.e., containing significant 

amount of fines) is removed with the asphalt, the hydraulic properties of the aggregate could 

also be poor. 

Recycled Concrete 

Again, the design requirements for RCM aggregates are essentially the same as natural 

aggregates. Recycled concrete has been used by a number of states as base materials since 

the 1980s. However, several states have identified three significant issues, including 

− the formation of tufa (calcium deposits) clogging drains and filter materials; 

− alkaline (high pH) run-off; and, 

− freeze thaw degradation. 

As a result, these states are now primarily using the recycled concrete, mixed with natural 

soils, as embankment fill. 

7.7.2 Recycled Waste Materials 

A number of recylced waste materials have been used in permanent construction, practically 

all of which where covered in Section 7.6.7 since they have a lighter weight than 

conventional aggregate. Other applications not reviewed in Section 7.6.7 include the use of 

recycled materials as a replacement for base materials (e.g., slag and bottom ash) and, in 

some cases (e.g., glass and tire shreds) drainage aggregate. As indicated in Section 7.6.7, the 

materials must be evaluated with respect to the same property requirements as the material 
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Geotechnical Aspects of Pavements 7 - 102 May 2006

they will replace. The pavement support value (e.g., resilient modulus or CBR) should be 

determine based on lab tests reviewed in Chapter 5. Field trails using FWD tests to confirm 

the as constructed properties are also recommended. Durability is a critical issue with many 

of these materials, and, obviously, an assessment of environmental issues must be made. 

7.8 REFERENCES 

AASHTO (1993). AASHTO Guide for Design of Pavement Structures, American Association 

of State Highway and Transportation Officials, Washington, D.C. 

AASHTO (2004). Standard Specifications for Transportation Materials and Methods of 

Sampling and Testing (24rd ed.), American Association of State Highway and Transportation 

Officials, Washington, D.C. 

American Society for Testing & Materials (2000). ASTM Book of Standards, Vol. 4, Section 

01, Construction Materials: Cement; Lime; Gypsum, ASTM, Philadelphia, PA. 

American Society for Testing & Materials (2000). ASTM Book of Standards, Vol. 4, Section 

02, Construction Materials: Concrete and Aggregates, ASTM, Philadelphia, PA. 

American Society for Testing & Materials (2000). ASTM Book of Standards, Vol. 4, Section 

08 and 09, Construction Materials: Soils & Rocks, ASTM, Philadelphia, PA. 

American Society for Testing & Materials (2000). ASTM Book of Standards, Vol. 4, Section 

03, Construction Materials: Road and Paving Materials; Vehicle-Pavement Systems, ASTM, 

Philadelphia, PA. 

Austroads (1998). Guide to Stabilisation in Roadworks, Austroads Publication No. AP-60/98, 

Austroads Inc. 

Berg, R.R, Christopher, B.R. and Perkins, S.W. (2000). Geosynthetic Reinforcement of the 

Aggregate Base/Subbase Courses of Pavement Structures, GMA White Paper II, 

Geosynthetic Materials Association, Roseville, MN, USA, 176 p. 

Bhatti, M.A., Barlow, J.A., and Stoner, J.W. (1996). “Modeling Damage to Rigid Pavements 

Caused by Subgrade Pumping,” ASCE, Journal of Transportation Engineering, Vol. 122, 

No. 1, pp. 12-21. 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 103 May 2006

Blanco, A.M., Bowders, J.J., Donahue, J.P. (2003). “Laboratory and In-Situ Hydraulic 

Conductivity of Pavement Bases in Missouri,” presented at the Subsurface Drainage Session, 

TRB Annual Conference, TRB, Washington, D.C. 

CALTRANS (1995) Design Criteria for Portland Cement Concrete Pavements (PCCP). 

Design Information Bulletin No. 80., California Department of Transportation, Sacramento, 

CA. 

Carpenter, S. H., M. I. Darter, and B. J. Dempsey (1979) “Evaluation of Pavement Systems 

for Moisture-Accelerated Distress.” Transportation Research Record 705, Transportation 

Research Board, Washington, D.C. 

Cedergren, H.R. (1987) Drainage of Highway and Airfield Pavements, Robert E. Krieger 

Publishing Co. Inc. Malabar, FL 

Cedergren, H. R. (1988) “Why All Important Pavements Should be Well Drained.” 

Transportation Research Record 1188, Transportation Research Board, Washington, D.C. 

Christopher, B.R. and V.C. McGuffey (1997). Synthesis of Highway Practice 239: Pavement 

Subsurface Drainage Systems, National Cooperative Highway Research Program, 

Transportation Research Board, National Academy Press, Washington, D.C. 

Christopher, B.R. (2000) Maintenance of Highway Edgedrains, National Cooperative 

Highway Research Program, Synthesis of Highway Practice 285, Transportation Research 

Board, National Academy Press, Washington, D.C., 2000, 62 p. 

Christopher, B.R., Zhao, A., Hayden, S.A. (2002) “Geocomposite for Pavement Subsurface 

Drainage,” Presentation at TRB Committee A2KO6 on Subsurface Drainage. 2002 Annual 

Transportation Research Board Meeting, Washington, DC; also see: Christopher, B.R., 

Hayden, S.A., and Zhao, A., (2000). "Roadway Base and Subgrade Geocomposite Drainage 

Layers," Testing and Performance of Geosynthetics in Subsurface Drainage, ASTM STP 

1390, J.S. Baldwin and L.D. Suits, Eds., American Society for Testing and Materials, West 

Conshohocken, PA. 

Christory, J.P. (1990). “Assessment of PIARC Recommendations on the Combating of 

Pumping in Concrete Pavements,” Proceedings of the Second International Workshop on the 

Design and the Evaluation of Concrete Pavements, Siguenza, Spain. 

Coduto, D.P. (1999) Geotechnical Engineering: Principles and Practices, Prentice Hall, 

Upper Saddle River, New Jersey. 

Daleiden, J. (1998) Video Inspection of Highway Edgedrain Systems, Final Report, Brent 

Rauhut Engineering. 
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Geotechnical Aspects of Pavements 7 - 104 May 2006

Dawson A R and Hill A R. (1998) "Prediction and implication of water régimes in granular 

bases and sub-bases", Proc. Int. Symp. On Subdrainage in Roadway Pavements and 

Subgrades, Granada, Spain, pp 121-128. 

De Beer, M. (1990). “Erodibility of Cementitious Subbase Layers in Flexible and Rigid 

Pavements,” 2nd International Workshop on the Theoretical Design of Concrete Pavements, 

Siguenza, Spain. 

Dempsey, B.J. (1982). “Laboratory and Field Studies of Channeling and Pumping,” 

Transportation Research Board, Transportation Research Record 849, pp. 1-12. 

Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J.G., and Berg, R.R. (2004). Ground 

Improvement Methods, Participant Notebook, NHI Course 132034, FHWA NHI-04-001, 

National Highway Institute, Federal Highway Administration, Washington, D.C., 1022 pp. 

ERES Consultants, Inc. (1999). Pavement Subsurface Drainage Design, Participants 

Reference Manual for NHI Course Number 131026, National Highway Institute, Federal 

Highway Administration. 

Fehsenfeld, F. M. (1988) Performance of Open-Graded “Big Rock Mixes” in Tennessee and 

Indiana, National Asphalt Pavement Association, Riverdale, MD. 

FHWA HI-95-038 (1998 revision), Geosynthetic Design and Construction Guidelines, 

Participant Notebook, NHI Course No. 13213, FHWA Publication No. FHWA HI-95-038 

(revised), authors: Holtz, R.D., Christopher, B.R., and Berg, R.R., Federal Highway 

Administration, Washington, DC. 

FHWA (1992) Drainable Pavement Systems—Participant Notebook. (Demonstration Project 

87). Publication No. FHWA-SA-92-008. Federal Highway Administration, Washington, 

DC. 

FHWA (1994) Drainable Pavement Systems — Instructor’s Guide. (Demonstration Project 

87). Publication No. FHWA-SA-94-062. Office of Technology Applications and Office of 

Engineering, Federal Highway Administration, Washington, DC. 

Hagen, M.G., and G.R. Cochran (1995) "Comparison of Pavement Drainage Systems, Final 

Report,” Minnesota Department of Transportation, Minneapolis, MN. 

Hindermann, W. L. (1968) The Swing to Full-Depth. Information Series No. 146, The 

Asphalt Institute, Lexington, KY. 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 105 May 2006

Holtz, R.D., Christopher, B.R., and Berg, R.R. (1998 revision). Geosynthetic Design and 

Construction Guidelines, Participant Notebook, NHI Course No. 132013, FHWA Publication 

No. FHWA HI-95-038 (revised), Federal Highway Administration, Washington, D.C. 

Holtz, R. D., and Kovacs, W. D. (1981). An Introduction to Geotechnical Engineering, 

Prentice-Hall, Inc., Englewood Cliffs, NJ. 

Kandhal, P.S. and R.B. Mallick (1997) Pavement Recycling Guidelines for State and Local 

Governments. Federal Highway Administration Publication No. FHWA-SA-98-042, 

Washington, D.C. 

Kaplar, C.W. (1974). “A Laboratory Freezing Test to Determine the Relative Frost 

Susceptibility of Soils,” Technical Report TR 250, Cold Regions Research and Engineering 

Laboratory (CRREL), U.S. Army Corps of Engineers, 1974. 

Koerner, R.M. (1998). Designing with Geosynthetics, 4th Edition, Upper Saddle River, NJ, 

Prentice Hall. 

McCallister, L.D. and Petry, T.M. (1990). Property Changes in Lime-Treated Expansive 

Clay Under Continuous Leaching, U.S. Army Corps of Engineers, Washington, D.C., No. 

GL-90-17. 

McKeen, R.G. (1976). Design and Construction of Airport Pavements on Expansive Soils. 

Report No. FAA-RD-76-66. 

NCHRP 1-37A (2004). Mechanistic-Empirical Design of New and Rehabilitated Pavement 

Structures, Final Report, National Cooperative Highway Research Program NCHRP Project 

1-37A, Transportation Research Board, National Research Council, Washington, D.C. 

NHI 131033 Hot Mix Asphalt Pavement Evaluation and Rehabilitation

NHI 131060A Concrete Pavement Design Details and Construction Practices

NHI 13126 (Currently 131026) (1999) Pavement Subsurface Drainage Design, Participants 

Reference Manual, prepared by ERES Consultants, Inc, National Highway Institute, Federal 

Highway Administration, Washington, D.C. 

PIARC Technical Committee on Concrete Roads (1987). “Combating Concrete Pavement 

Slab Pumping.” 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 106 May 2006

Rollings, M.P., and Rollings, R.S. Jr. (1996). Geotechnical Materials in Construction, 

McGraw-Hill, New York, NY. 

Sawyer, S. (1995) Establishment of Underdrain Maintenance Procedure. Report 

FHWA/OK95(04), Federal Highway Administration, Oklahoma City, OK. 

Seed, H.B. and C.K. Chan (1959) "Structure and Strength Characteristics of Compacted 

Clays," ASCE Journal of Soil Mechanics and Foundation Engineering, Vol. 85, No. 5, pp. 

87-128. 

Snethen, D.R., L.D. Johnson, and D.M. Patrick (1977). An Evaluation of Expedient 

Methodology for Identification of Potentially Expansive Soils, Report No. FHWA-RD-77-94, 

Federal Highway Administration, Washington, D.C. 

Sowers, G.F. (1979) Introductory Soil Mechanics and Foundations: Geotechnical 

Engineering (4th edition), MacMillan, New York, NY. 

Steinberg. M.L. (1998). Geomembranes and the Control of Expansive Soils in Construction, 

McGraw Hill, Inc., New York, NY. 

Thompson, M.R. (1970). Soil Stabilization for Pavement Systems – State of the Art, 

Technical Report, Construction Engineering Research Laboratory, Champaign, IL. 

Ulmeyer, J.S., Pierce, L.M., Lovejoy, J.S., Gribner, M.R., Mahoney, J P, and Olson, G.D. 

(2002) “Design and Construction of Rock Cap Roadways: Case Study in Northeast 

Washington State” Prepared for presentation at the 2003 TRB Annual Meeting, 

Transportation Research Board, Washington D.C. Also see Transportation Research Record 

1821. 

U.S. Army Corps of Engineers (1992). Engineering and Design Drainage Layers for 

Pavements, Engineer Technical Letter 1110-3-435, U.S. Army Corps of Engineers, 

Washington, D.C. 

USFS (1977) Guidelines for Use of Fabrics in Construction and Maintenance of LowVolume Roads, authors: Steward, J., Williamson, R. and Mohney, J., USDA, Forest Service, 

Portland, OR. Also reprinted as Report No FHWA-TS-78-205. 
FHWA NHI-05-037 Chapter 7 – Design Details & Construction 

Geotechnical Aspects of Pavements 7 - 107 May 2006

Witczak, M.W. (1972). "Relationships Between Physiographic Units and Highway Design 

Factors," Report 132, National Cooperative Highway Research Program, Highway Research 

Board, Washington, D.C. 

Yu, H. T., K. D. Smith, M. I. Darter, J. Jiang, L. Khazanovich (1998a). Performance of 

Concrete Pavements, Volume III—Improved Concrete Pavement Performance. FHWA-RD95-111, Federal Highway Administration, McLean, VA. 

Yu, H. T., L. Khazanovich, S. P. Rao, M. I. Darter, H. Von Quintus (1998b) Guidelines for 

Subsurface Drainage Based on Performance. NCHRP 1-34 Final Report, National 

Cooperative Highway Research Program, Washington DC. 
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FHWA NHI-05-037 Chapter 8 – Construction & Design Verification 

Geotechnical Aspects of Pavements 8 - 1 May 2006

CHAPTER 8.0 CONSTRUCTION AND DESIGN VERIFICATION 

FOR UNBOUND PAVEMENT MATERIALS 

8.1 INTRODUCTION 

Thus far, this reference manual has described the processes detailing the pre-construction 

phase – site characterization and design. The last phase of work required to complete a 

roadway, the construction phase, is the emphasis of this chapter. As such, construction 

specifications are described, quality control/quality assurance concepts are reviewed, and 

innovative measures of design verification are provided. Issues surrounding the preparation 

of the pavement foundation, focused primarily upon cut and fill soil construction, prepared 

subgrade, chemically stabilized soils, and unbound engineered aggregate layers 

(base/subbase) are detailed. Lastly, a review of monitoring techniques for the finished 

product is provided, an important consideration with the current move toward performance 

specifications and warranties. 

Projects come in all shapes and sizes, each presenting unique challenges. For instance, a new 

roadway alignment could be conceptualized in a flat topography, requiring little earthwork, 

or conversely, in a hilly or mountainous topography that requires large cuts – excavation 

and/or rock blasting – and placement of deep fills. In either case, the engineer typically must 

make do with the local soils and design for the site conditions encountered. Fundamental to 

the pavement construction is the preparation of the pavement foundation (i.e., the subgrade) 

to meet the pavement design support requirements. The designer has made assumptions 

based on the subsurface exploration program as to the support conditions and the field 

requirements anticipated to meet those support conditions (i.e., the anticipated adequacy of 

existing support after grading or modifications required to achieve that support). It is now up 

to construction to achieve these requirements. Proper treatment of the subgrade during 

construction will assure expedient construction of the pavement section, enhance pavement 

performance over its life, and ensure that the pavement design intent is carried through in the 

construction phase (Ohio DOT, 2002). 

8.2 SPECIFICATIONS 

The development of the specifications is usually conducted in the pre-construction phase as 

part of the design. The specification dictates the quality for the pavement section 

construction, with the intent of tying the design to the finished product (design intent ⇔

performance). There is no good practice other than what is specified in the contract. 
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Most agencies have developed as part of the construction process a set of standards and 

specifications. These documents may contain guideline specifications (e.g., AASHTO 

Standard Specifications for Transportation Materials, AASHTO, 2004) modified by the 

local agency to address local conditions, materials available, construction techniques 

commonly employed, and their local experience. 

The specifications can generally be categorized as method, result, or performance

specifications. An example of each type is provided below: 

Method Specification – This form of specification describes in detail the equipment and 

procedures (process) used to achieve a desired result (e.g., a compactive effort of 4 passes 

using a 35-ton sheepsfoot roller – Caterpillar C825 or equivalent – shall be made on each 

8-inch lift of loose soil spread on the grade). 

Result Specification – The result specification is normally shorter and easier to write than the 

method specification. This form of specification states what property must be achieved, 

allowing the contractor some liberty to innovate the process to satisfy the intended result 

(e.g., a dry density of 95% of the maximum dry density – as determined by AASHTO T99, 

standard Proctor – shall be obtained for each lift of soil placed on the grade). 

Performance Specification – A specification for key materials and construction quality 

characteristics that have been demonstrated to correlate significantly with long-term 

performance of the finished work (e.g., the pavement shall support 1 million ESALs without 

developing fatigue cracks or rut depths exceeding 6 mm (0.25 in.)). 

Performance specifications may be presented in one of three forms, including (after 

Chamberlin, 1995) 

• Performance specifications – which directly define the condition of the road, the 

response of the road to load, and/or the condition of the pavement materials at a given 

period of time, 

• Performance-based specifications – which describe desired levels of fundamental 

engineering properties that are predictors of performance; or 

• Performance-related specifications - which describe the desired level of key materials 

and construction quality characteristics that have been demonstrated to correlate 

significantly with long-term performance of the finished work. 
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The main intent of each type of specification with respect to geotechnical factors is to 

confirm the adequacy and/or improve the engineering behavior of the soil or aggregate 

material by modification of moisture content and densification, or compaction, of the soil or 

aggregate. While the result specification is more common, the method specification can be 

utilized where the result is probable based on local experiences, or where the result is 

difficult to measure (i.e., density of coarse rock fill). This form of specification takes 

responsibility away from the contractor and places it on the shoulders of the owner and his 

engineer. The result specification will typically encourage the contractor to utilize the most 

efficient and economical means to achieve the requirements. 

Pavement performance specifications may be appropriate for design/build and warranty 

contracts. However, it is obvious that the above pavement performance specifications cannot 

be used to control the quality of aggregate or subgrade materials used in the construction. 

The pavement is an interdependent layered system consisting of different materials, all of 

which affect performance. During the service life of the pavement, the material properties 

can change from those measured during construction. The performance required by the 

example above is also affected by the thickness of the layers, which is a design element. The 

main challenge with performance specifications is the determination of performance 

measures, as discussed later in Section 8.5. 

No specification type can cover all situations, and each type has relevance depending on the 

circumstance (e.g., Design Build or Design-Build-Let contracting methods). The 

specification, regardless of whether method, result, or performance, should emphasis 

material properties of raw materials (soil classification, limits for maximum particle size, 

grain size distribution, Atterberg limits, and other properties typically used for aggregates in 

base or subbase layers, such as toughness (durability) and soundness, among others). 

Each specification type should contain a provision for corrective action measures to be taken 

when unsuitable conditions (i.e., weak, soft, wet, yielding materials) are encountered. The 

corrective measures should include 

• method of detection (proofroll, QC/QA test, etc.), 

• depth of anticipated treatment, 

• type of treatment (drainage, undercut and replace, installation of geosynthetics, 

chemical modification/stabilization), and 

• quick resolution determining whose responsibility (pay item) it is to implement the 

corrective measure (Owner or Contractor). 

Once established, site preparation, excavation, hauling, placing, compaction, and grading 

objectives can commence. 
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8.3 QUALITY CONTROL AND QUALITY ASSURANCE 

Good quality control/quality assurance (QC/QA) practices are essential to obtain satisfactory 

results in a construction project. QC/QA can be a single plan developed by the Owner to 

review the construction process. A third party or the agency often performs the quality 

control (QC) field observations. Alternatively, the quality control (QC) may refer to a written 

plan submitted by the contractor, which is reviewed and approved by the owner/engineer. 

This document clearly demonstrates how the contractor will control the processes used to 

produce or purchase materials used in construction, as well as control the processes for 

proper installation in order to meet the requirements set forth by the owner/engineer. The QC 

Plan will typically include tests (QC tests) performed on the materials intended for use at a 

prescribed frequency, as summarized in Table 8-1, as well as tests to indicate that the intent 

of the specification is being satisfied (field compaction monitoring and control, again at a 

prescribed frequency). Quality assurance (QA) is documentation that the contractor is 

following the QC Plan, and most likely will consist of some random inspections and testing 

to verify QC observations and results. 

Table 8-1. Typical material properties measured for construction. 

NOTES:

1: different natural (in-situ or borrow) soil or quarry aggregate. 

2: values typically required for quarry aggregate used as base. 

3: frequency intervals identified in the QC Plan.

Test Frequency 

Material Source(s) 1 

Classification 1 per material type 

Atterberg Limits 1 per material type 

Grain Size 1 per material type 

Moisture-Density (Proctor) 1 per material type 

Abrasion 2 1 per material type 

Soundness 2 1 per material type 

Field Installation 

Moisture Content per QC Plan 3

Density per QC Plan 3

Stiffness Assessment (e.g., proof rolling) per QC Plan 3
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8.3.1 QC/QA: Tradition Methods 

The construction specification establishes the framework for QC/QA. With a method 

specification, the quality control (QC) individual would document the equipment utilized and 

continuously monitor its activities during operation. The assurance may be by certification of 

QC tests and reports along with intermittent inspection. With a result specification, the QC 

individual would perform frequent testing at the start of the process, testing for changed 

conditions, and some testing for verification. The assurance testing would typically be a 

prescribed number of tests for a specific quantity of materials at random locations. Statistical 

processing of the test data may be used to determine the amount of payment if pay factors are 

included in the contract. A good practice for quality control is the development and use of a 

checklist for monitoring and inspecting the construction of the pavement system, similar to 

the one shown in Table 8-2. 

Initial observations include confirming that clearing and grubbing operations have been 

adequately accomplished and that the prepared surface is suitable for placement of 

embankment/fill. The “suitability” is often confirmed through proof rolling. 

Proof Rolling

The objective of proof rolling is to point out soft or yielding material. The technique can be 

implemented at any point during construction of the embankment, preparation of the 

subgrade (top 300 mm (12 in.)), and completion of base and/or subbase layers. In fact, as 

described later in Section 8.4, proof rolling observations can be made as material is being 

excavated, hauled, placed, and compacted using the equipment used to perform each of these 

tasks. 

Many agencies have developed vehicle configuration specifications, including weight and 

tire pressures, for performing proof rolling operations, and have established a policy on 

methodology and threshold criteria for acceptable deflections, as well as those requiring 

remediation. For example, Ohio uses proof rolling for all projects types (new, rehabilitation, 

and reconstruction). This practice is good for detecting soft zones that may have passed the 

density requirements of the project, but not necessarily the moisture content, and can detect 

problems that could extend many feet below the tested surface. Once detected, seasoned 

experience can often estimate the depth of probable weakness; however, penetration rods and 

hand augers can be used with more objectivity than the eye guestimate. Once detected and 

properly delineated (aerial extent and depth), remediation actions are typically employed 

(remove/undercut and replace, installation of underdrains, installation of geosynthetics, 

chemical stabilization) that best suit the conditions encountered. The remediation alternative 

selected typically is a result of a cost or schedule constraint. Many agencies have reported 
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historically large change order work dealing with soft subgrade, and have subsequently 

included likely remediation alternatives in the bidding process to establish a competitive rate 

for this work. 

Table 8-2. Field monitoring checklist. 

1. Read the specifications and become familiar with

- site preparation 

- material requirements 

- construction procedures 

- grade/slope requirements 

- drainage requirements 

- tolerances of each of above requirements 

- testing requirements 

- acceptance/rejection criteria 

2. Review the construction plans and become familiar with

- lines, grades, and layer thickness requirements 

- temporary and permanent drainage features, locations, and details 

- details for utility construction, special requirements 

- demolition (if rehabilitation or reconstruction project) 

- corrective action requirements for weak, yielding, unstable materials (undercut/replace, 

chemical stabilization, geosynthetics) 

- construction sequence 

3. Review material requirements, equipment requirements, and approved submittals.

4. Check site conditions. Observe 

- clearing, grubbing requirements, and activities – document the final condition in accordance 

with the specification 

- haul patterns 

- response to load (deflection under heavy equipment traffic) 

- assess need for drainage system 

- perform foundation acceptance – testing as required (e.g., proof rolling, DCP, etc.) 

5. On site monitoring and testing. 

- observe and document hauling, placement, and spreading adequacy (segregation, loose lift 

thickness, moisture content reasonableness) 

- review truck tickets and ensure that material sources are from approved sources (generally for 

engineered products such as aggregate base or flowable fill, bedding material, and off-site 

borrow, etc.) 

- randomly sample for material compliance 

- assess for moisture content reasonableness (not overly wet or dry in conformance with the 

specification) 

- document compaction efforts (size and type of equipment, # of passes) 

- test for compaction (determine conformance to specifications with respect to density and 

moisture – Proctor check point, if necessary) and assess stability 

- non-conformance corrective action – proof roll and assess extent/depth of affected area; 

determine and document responsibility (owner vs. G.C.) and begin corrective action 

- measure and document stabilizing agent type and rate of application, mixing adequacy and 

depth, compactive effort, moisture reasonableness and compaction compliance results (for 

stabilization/modification layers or treatments). Sampling and testing of treated layers and 

determination of strength in the laboratory may also be required. 
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Tests

Test methods used for in-place quality control and acceptance of individual flexible 

pavement layers and of new and rehabilitated flexible pavement systems have changed little 

in past decades. Such quality control and acceptance operations typically rely on nuclear 

density measurements (Figure 8-1), sand cone, balloon or drive tube methods, and the results 

of moisture content determined by a variety of methods, including nuclear gauge, speedy 

moisture, and hot plate or oven drying. These tests are typically performed on embankment 

construction (fill soils), finished subgrade, and unbound base layers, while some are 

applicable to measuring the quality of chemically stabilized materials, as described later in 

this section. 

The old school of thought used compaction testing to calibrate construction methods. After 

the methods were calibrated, observation became as important as testing for quality control. 

Samples were taken at select locations based on observations. Today there is more of an 

emphasis on statistical characterization of constructed materials. Sample locations have 

become more random. Quality assurance specifications often give the contractor the 

responsibility of sampling and testing for process control. Testing by the owner includes 

some verification of the contractor’s test results, and testing for acceptance and payment. 

The amount of payment may be determined by the statistical evaluation of test values 

resulting in pay factors (and no test reports, no pay). 

Figure 8-1. Nuclear densometer (photo courtesy of Troxler). 
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As part of a good QC Plan, process control and measurement of the control can be a valuable 

tool. A test result, or trend of a measured value, may not directly demonstrate compliance or 

non-compliance, but tracking the measured value over time may help explain why another 

process is out of compliance. In the following example, a soil with a standard proctor 

maximum dry density of 15.7 kN/m3

 (100 pcf) at an optimum moisture content of 20% is 

being placed in a single lift along a 300-m (1,000-foot) length of roadway embankment. The 

specification requires that a minimum of 95% of the maximum density be obtained, at or near 

the optimum moisture content. In order to simplify this example, it is assumed that the 

material is uniform in classification, and is being hauled by scrapers from a cut zone nearby. 

QC tests have been recorded and are graphically shown in Figure 8-2. This figure illustrates 

that the density is adequate along the first 120 m (400 feet) of placement, then trends toward 

an ‘out-of-tolerance’ or ‘out-of-control’ situation. The QC Plan may prompt the contractor to 

exert more compactive effort on this ‘out-of-control’ area, or change compactors; however, 

the moisture data suggests that the moisture content may be the ‘out-of-control’ parameter, 

which is, in turn, causing the density to move ‘out-of-control’. By recognizing what part of 

the process is defective, the contractor begins spreading the cut soil in thin lifts, allows some 

drying to occur prior to compacting, and again returns to a product considered satisfactory at 

the 250-m (820-foot) mark. 

90

92

94

96

98

100

100 300 500 700 900

centerline distance (ft.)

% Compaction

10

15

20

25

30

100 300 500 700 900

centerline distance (ft.)

% Moisture

Figure 8-2. Process control, field density and moisture. (3.3 ft = 1 m) 
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This form of process control (density or degree of compaction) is applicable to embankment, 

subgrade, and unbound aggregate base construction. In addition, gradation of aggregate base 

materials is an important process control measurement. Depending on location of sampling 

(on grade, from haul trucks, from stockpiles) segregation and contamination may be detected 

using this measure. It is also an important measure used to ensure that the quarry process 

(crushing and grading) is in control. 

Chemical Stabilization

In addition to density and moisture measurements, several additional controls are required 

and considered good practice when stabilization techniques are employed. An excellent 

source for QC/QA requirements can be found in the Soil Stabilization for Pavements manual 

(Army, Air Force, 1994). Briefly, these elements include 

• Pulverization and scarification – An assessment of the material to be treated is 

required, and generally includes random sampling and testing using a field sieve (25-

mm (1-in.) and 4.75-mm (No. 4) sieves). 

• Stabilizing agent content – The amount of modifier added to the soil should be 

measured, and generally includes sampling the discharge using a canvas of known 

area placed on the soil to be treated, or calculating the area over which a known 

tonnage has been spread. For lime stabilization, pH can provide a good indication that 

the correct dosage has been achieved, as discussed in Appendix E. 

• Uniformity of mixing – Visual observation is made to ensure that uniform mixing has 

been accomplished throughout the full depth of treatment. The use of phenolphthalein 

indicator solution has been used effectively for lime treatments to indicate depth of 

treatment. This solution – a light spray applied to the sides of a hand-excavated hole 

in the treated soil – will react with the lime, turning a brilliant pink color. 

• Compaction and moisture control – Covered in the previous section. 

• Curing – Curing is essential to assure that the modified soil mixture will achieve the 

final properties desired. The use of moist curing (light sprinkling of water) or 

membrane curing (application of a bituminous coating) is common. Regardless of 

method, the entire area must be properly sealed, and documentation of this activity is 

required. 

Again, most QC programs only measure the compaction of the earthwork operation. While 

this methodology is valuable, density does not necessarily translate into performance. This 

type of QC testing is still very important in that it provides for a uniformity in the 

contractor’s work, and can control the moisture of a given soil type at or near its optimum 

moisture content. This moisture control is important in order to minimize volume change 

characteristics, and none of the technologies described above have this capability. 
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8.3.2 QC/QA: Emerging Technologies 

Lost in most projects requiring earthwork operations is the design intent. While the industry 

has accrued decades of experience founded upon successful engineered and constructed 

projects, we still observe the occasional premature failure that could have possibly been 

avoided. Reviewing, site soils are improved for three reasons: 

• there are large quantities available; 

• the natural state is inadequate to support the intended structure; and 

• it is cost-effective. 

For embankment fills, shear strength (slope stability) and consolidation (settlement potential) 

are used in the engineering analyses for design intent. As the elevation gets closer to the 

final grade (below the granular layers and surface layers – or within the anticipated depth or 

zone of repeated loading influence) resistance to deformation – siffness or resilient modulus 

– is used in the engineering analyses for design intent. These parameters are measured in the 

laboratory on soils sampled in the soil exploratory phase of the design. In essence, the design 

parameters are rationalized as an anticipated, educated estimate of what will finally be 

obtained in the field. But rather than measure these design parameters in the field, we 

commonly accept the contractor’s work based on a result specification measurement of 

density. While this may be common and traditional, it does not verify if the design intent has 

been met. 

As an example, the following scenario represents a hypothetical design situation where a 0.4-

km (1⁄4 -mile) roadway, 2 lanes wide is to be constructed along a gently rolling topography. 

Based on soil borings drilled and samples obtained, classified and tested, Table 8-3 was 

compiled for five uniquely different soils (based on visual description and engineering 

classification) ranging from a sandy soil to a highly plastic clay. The five samples are 

identified as Samples A-E. 

In this hypothetical example, it is assumed that each of these materials will be equally 

represented along the length and width of the project, and are semi-infinite in depth. Further, 

the design assumes each material will be field compacted to 95% of the soil maximum dry 

density at or very near its optimum moisture content. Lastly, it is assumed that the soil mass 

will remain at this condition for the performance period. Each of these assumptions is 

idealistic, not realistic. However, it can be demonstrated that the design intent for this 

hypothetical example can be verified in the field during construction. In this example 

(Boudreau, 2003), the design intent is a roadbed stiffness of 20 Mpa (7200 psi) based on a 

stress state of 36 kPa (5.3 psi) vertical and 11 kPa (1.6 psi) horizontal (as estimated by the 

pooled subgrade constitutive model Mr = 9041σv

-0.19526σh 0.19643 with σv, σh in English units). 
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Table 8.3. Summary of design soil properties for example problem (pre-construction). 

Physical Property A B C D E 

Liquid Limit, LL 21 NL 35 64 36 

Plastic Limit, PL 16 NP 14 29 27 

Plasticity Index, PI 5 --- 21 35 9 

P4 (%) 94 100 100 100 100 

P10 (%) 92 100 100 96 96 

P200 (%) 47 20 59 82 48 

Maximum Dry Density,1 γmax (kN/m3

) 18.8 18.2 16.9 14.9 17.8 

(pcf) (119.8) (115.9) (107.8) (94.7) (113.3) 

Optimum Moisture Content, wopt (%)1 12.0 11.8 17.2 25.6 15.0 

AASHTO Classification A-4 A-2-4(0) A-6(9) A-7-6(32) A-4 

Unified Soil Classification SC SM CL CH SC 

Resilient Modulus Parameters2

 K1 10 387 6246 10 274 10 362 7938 

K2 -0.015483 -0.00836 -0.41797 -0.18345 -0.21171 

K3 0.23229 0.30028 0.08425 0.12762 0.23770 

1. Maximum dry density and optimum moisture content, as determined by AASHTO T-99 (standard Proctor). 

2. For modulus equation: Mr = K1SV

K2S3K3 with SV and S3 in English units. Laboratory test specimens prepared 

to 95% of maximum dry density at optimum moisture content (as determined by AASHTO T99).

Based on the information provided, a conventional pavement cross section resulting from the 

subgrade support conditions, determined from the pre-construction laboratory test program 

summarized in the table above (analyses performed per AASHTO 1993 Design Guide using 

estimates for traffic and other inputs per the Boudreau reference cited) is 140 mm (5.5 in.) of 

asphalt concrete on 200 mm (8 in.) of a crushed aggregate base. The question is, if the 

contractor satisfies the result specification for subgrade construction - for this example, the 

contractor must achieve 95 percent compaction at or very near optimum moisture content - 

and the layers above (140 mm (5.5 in.) of asphalt concrete and 200 mm (8 in.) of a crushed 

aggregate base) are constructed with approved materials and constructed to satisfy the result 

specification for these layers, will the pavement perform as designed and expected? The 

answer deserves some examination. 

First, the design intent for the subgrade is stiffness and strength; the measure of acceptance is 

density. Fundamentally, these two measures are uniquely different; one measure does not 

necessarily confirm the other measure. It is possible that the contractor has met the 

compaction specification on the wet side of optimum. The important measure is one of 

stiffness – in this hypothetical example, 50 MPa (7200 psi). 

There are a number of ways to more precisely measure whether the design intent has been 

satisfied. These measures could include field CBR and plate load tests, dynamic cone 
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penetration (DCP) tests, correlation studies, and/or laboratory tests performed on undisturbed 

tube samples obtained at finished grade. More recently, nondestructive testing (NDT) 

methods, including lasers, ground-penetrating radar, falling weight deflectometers (FWD), 

mini or portable lightweight FWD (LWD) cone penetrometers, GeoGauge (providing direct 

stiffness measurements), and infrared and seismic technologies, have been significantly 

improved and have shown potential for use in the quality control and acceptance of flexible 

pavement construction. As mentioned in Section 8-4, another technology in development 

consists of instrumented compaction equipment. This and the others mentioned above require 

field verification studies prior to any endorsement of the technology. The thrust of NCHRP 

Project 10-65 is to explore many of these technologies for this specific application (Von 

Quintus et al., 2004). It is anticipated that some of these techniques will eventually be 

incorporated into performance specifications as the industry gains more knowledge and 

accrues more experience with them. Many of these techniques were briefly reviewed in 

Chapter 4 (see Tables 4-2 through 4-6) and are described in greater detail below. 

With the advent of the much anticipated NCHRP 1-37A Pavement Design Guide and 

extended warranty period of performance, there is an ever increasing need to measure layer 

stiffness properties by owner agencies, an activity that is not presently a typical component in 

the acceptance of a completed project. 

Proof Rolling

A practical approach that many agencies use is the concept of proof rolling, as discussed in 

the Section 8.3.1. Although this approach is observer-dependant, many agencies use the 

technique not to measure design intent (deformations anticipated at stress levels typical under 

repeated load traffic protected by layers of material would result in deformations 

undetectable to the human eye during a proof rolling exercise), but to evaluate gross 

deficiencies including soft, yielding, or pumping subgrade. The objective of this type of 

process is to correct problem areas prior to the placement and compaction of stronger, more 

expensive materials (these soft zones will surely be detected during finishing operations of 

the stronger layer materials in the form of roller cracks). 

With newer more sophisticated technology, including lasers, digital video, and image 

analysis, it is possible to take proof rolling to a new level of direct stiffness measurements. 

Small deformations can now be monitored as the proof roller moves across the site. 

Although this is somewhat of a research topic at this time, the concept is fairly 

straightforward to develop. In fact, Wisconsin DOT has developed a prototype deflection 

measurement system for use with a loaded dump truck, using ultrasonic sensors and a microcontroller, in order to continuously and objectively proof roll subgrade soils. Wisconsin DOT 

concluded that a threshold value of 38 mm (1.5 in.) of deflection indicated “failed” areas that 
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required corrective action, and also found value in analyzing the ratio of the 0-offset and 

0.6 m-offset (24 in.) sensors to determine depth of weakened zones (Wisconsin DOT, 2002). 

Field CBR or Plate Load Tests

These technologies were developed several years ago and were employed as a measure for 

verifying design intent. Each included mobilization of equipment (moderate to heavy plates, 

loading rams, calibrated proving rings or load cells, and dial indicators or electronic 

deflection measurement devices) crew and heavy reaction vehicle (typically readily available 

on an earthwork construction project in the form of a scraper or track-mounted 

excavator/shovel). These tests are often the standard for quality programs in Europe, but have 

not typically been utilized in the U.S., based on their relative cost, time involved to set up 

and perform the test at a specified location, and accuracy issues. The field CBR test could 

measure the in-situ CBR of finished subgrade in order to verify design intent for flexible 

pavements, and the plate load could directly measure the modulus of subgrade reaction, or kvalue, for rigid pavements. The plate load test is the standard practice in Europe for all 

pavement types. 

Each of these measures is time-consuming; thus, only a few locations could be tested per 

day, oftentimes impeding the earthwork contractor’s progress. In cut zones, these tests 

measure soil properties that are not controlled by the contractor, thus it is often difficult to 

expect the contractor to achieve a predetermined CBR or k-value threshold without paying 

for corrective measures. 

It is noted that this type of testing is common and traditional in many European countries, 

using special customized equipment to make this type of testing more automated and more 

productive than that described above. 

Dynamic Cone Penetration (DCP) Tests

The DCP technology consists of a steel shaft with an instrumented penetration head 

conforming to a precise configuration, as was described in Table 4-9. The instrumentation is 

capable of measuring resistance per increment of advancement and used with correlations to 

estimate stiffness of the materials. The benefits of this form of measurement are that the 

device can be quickly and efficiently mobilized to the project site (can be hand-carried or 

mounted inside a vehicle) and can measure to depths beyond surficial soils. The drawbacks 

include discrete point evaluations – leaving zones between points unknown, and the fact that 

the information gathered is used to correlate stiffness and strength. Thus, is only as accurate 

as the correlation models used. However, an added value is that the DCP can readily indicate 

soil support via correlations for construction activities. For example, if the estimated in-situ 

CBR has a value of 6 or less, the soils are expected to rut and deflect under construction 
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operations. If the estimated CBR values are between 6 and 8, the soils are considered 

marginally suitable for construction support (Illinois DOT, 1982). 

Resilient Modulus Testing

The design for the example introduced at the beginning of this section was based upon 

several soil samples characterized in the laboratory. The soils selected for characterization 

were those anticipated in the uppermost zones of the finished subgrade. In order to verify 

design intent, it would seem logical that samples of the earthwork contractor’s finished work 

be sampled and characterized in a similar fashion. This can be accomplished by extending 

short Shelby tubes into the compacted soil and returning to the laboratory for resilient 

modulus testing. The testing can occur on extruded tube samples the same day they are 

obtained from the field. Thus, final reporting can be available the next day. This form of 

sampling and testing has the benefit of comparing actual results with those used for design 

purposes. Additionally, these measures are direct; therefore they are not reliant upon 

correlations. Lastly, because these samples are physical in nature, the density and moisture 

content will be measured and can be compared with the QC test results for accuracy of the 

QC testing program. 

Falling Weight Deflectometer (FWD) Tests

A very mobile device and one that can be utilized to examine the stress dependency of the 

embankment or roadbed soils, a falling weight deflectometer is basically a trailer-mounted 

piece of equipment, which drops a weight transmitted through a hard rubber-type pad to the 

surface (as covered in detail in Table 4-2). The van pulling the trailer is equipped with a 

computer data acquisition system that measures the load and offset surface deflections. For 

field control, there are also portable or lightweight LWD units (as shown in Figure 8-3), 

allowing an individual to carry the unit around in any vehicle. 

This technology, with sophisticated computer models, can directly measure the roadbed 

deflection from which modulus values can be estimated in order to verify design intent. The 

device is relatively quick (less than 4 minutes is required per location to measure the 

properties), thus numerous locations can be measured per hour. There are also several new 

developments with units mounted on sleds or skis such that continuous coverage along the 

length of a project is possible. 
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Figure 8-3. Lightweight deflectometer (photo courtesy of Dynatest). 

GeoGauge

A recent development is the GeoGauge, a lightweight unit capable of measuring stiffness at 

discrete points. The Federal Highway Administration is currently evaluating this technology 

in the form of a Pooled-Fund Study. This device has many perceived benefits, including the 

capability to measure the stiffness of a composite soil mass directly and quickly such that 

numerous discrete points can be evaluated per hour. 

Seismic Methods

The Portable Seismic Pavement Analyzer (PSPA) and a derivative modified for base and 

subgrade measurement, the Dirt Seismic Pavement Analyzer (DSPA), are currently being 

used on a trial basis by the Texas Department of Transportation for QC/QA purposes 

(Nazarian, 2002). The operating principal of the PSPA is based on generating and detecting 

stress waves in a medium. If used appropriately, analyses of the stress waves can be used to 

determine the modulus of the layered material, as well as assess the thickness of the layer 

(aggregate base). These techniques are being utilized with very promising results during 

construction on a few projects in Texas, and are being considered for quality control on 

pavement warranty projects in Texas and New Mexico. This method of measurement and 

analysis are very similar to the principals used in spectral analysis of surface waves (SASW, 

Table 4-6). 
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Automatic Controlled Variable Roller Compaction and Documentation System

(Intelligent Compaction)

While each of the aforementioned techniques have perceived advantages and disadvantages, 

none of the techniques described above has the capability to continuously evaluate design 

intent along the entire length of a project. The use of instrumented compaction equipment 

would appear to have some potential for continuously monitoring conditions along a length 

of a project. 

The real-time automatic controlled variable roller compaction and documentation system 

(a.k.a. intelligent compaction) allows for optimization of compaction rates and real-time 

quality control. The system works by using accelerometers to monitor the speed of the 

dynamic wave through the soil, induced by the vibratory rollers, in order to measure the 

dynamic stiffness of the soil, which generally increases with higher compaction. Efficient 

fill densification is achieved via automatic adjustment of compaction energy and the 

measurement/documentation feedback, eliminating time wasted on compacting areas that are 

already adequately compacted. This energy variability and efficiency is achieved by the use 

of two counter rotating weights in the drum, rather than the conventional single, onedirectional eccentric weight. The weights rotate in opposite directions and only come 

together in a common direction in the downward vertical inclination. This eliminates 

unwanted and wasteful movements in the lateral and upward directions that occur with 

conventional compaction drums. Internally, the entire counterweight assembly is rotated to 

adjust the direction of the point where the two weights act together. If the onboard 

monitoring system determines the soil is compacted to a satisfactory level, it will 

automatically reduce the vertical component of force at the specific time and location. 

In addition, the ability to monitor density improvement during compaction both speeds up 

and improves the aerial extent of quality control. Most importantly, the ability of 

instrumented compaction equipment to provide 100% quality control coverage enables the 

use of performance based approaches to specifications, and the effective implementation of 

warranties and guarantees for both earthwork and pavements. 

This method does require proprietary specialized monitoring equipment, but the equipment 

and process are not patented. The equipment is readily available in the U.S., and requires 

nominal operator training. 

8.3.3 Risk Acceptance 

Warranties for materials and workmanship are common in the construction industry, with 

most performance bonds covering such items for 1 year following completion of a project. 
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However, the new emphasis on warranties for highway construction involves the guarantee 

of the long-term performance of highways. Typically, a long-term warranty is considered to 

cover a period from 2 – 5 years. It is beyond the generally accepted standard warranty period 

of 1 year. This creates a very difficult situation, and one that involves a very high degree of 

uncertainty with our current state of practice and technology in the United States highway 

construction industry (Hancher, 1994 - NCHRP Synthesis 195). 

This shift in philosophy has basically brought about a shift in project risk. Traditionally, the 

owner has assumed nearly all the performance risk by developing the design, specifying 

materials, and either specifying the results (density) or method to achieve the desired result, 

and measuring and accepting the contractor’s work. The contractor is at risk for gross failures 

resulting from noncompliance with contract requirements detected in the first year of 

performance, while the owner assumes responsibility for failures and maintenance following 

the initial year of performance. By extending the initial warranty period from 1 year to a 

period up to 5 years, the owner has shifted some of this inherent risk to the contractor. As a 

result, the contractor has been tasked with becoming a more integral part of the design and 

construction process. 

Extending the period of performance and assigning performance risk to various parties has 

lead to more sophisticated approaches with respect to life-cycle performance monitoring, 

early detection of potential problems, cost analyses, and budget optimization. For a 

contractor to be willing to accept more risk, he or she should have a more active role in 

design. 

8.4 CONSTRUCTION AND CONSTRUCTION MONITORING 

Construction of the pavement involves grading to provide a uniform support layer(s) at the 

appropriate elevation. In modern construction, either major earthwork or reconstruction, 

sophisticated equipment is available to excavate, haul, add water, aerate (decrease water), 

spread, and compact to achieve this purpose. The objective of this type of operation is to 

achieve a structure with specific design intent. Most earthwork projects have the additional 

objective that transforms existing or natural topography to an acceptable and safe vertical and 

horizontal alignment. A common goal is to achieve this new alignment with a balance of site 

materials. Projects that require more soil in fill areas than can be produced from cut zones 

will require additional materials from off-site borrow sources. Projects that generate more 

material from cut zones than can be placed in fill areas will be wasted. 
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The construction process is described in this section, along with the requirements for 

monitoring each phase of the construction activities, as outlined in Section 8.3 and 

summarized in Table 8-2. 

For new construction, subgrade preparation will typically require grubbing and grading 

(either cut or fill) to meet subgrade elevation requirements. In either case, clearing and 

grubbing is very important to remove vegetation, debris, and any organic, soft, or otherwise 

unsuitable materials from the surface of the site, either at subgrade level or before placing 

fill. For reconstruction, the old roadway surface will be removed (possibly recycled), 

possibly along with the base and subbase layers, if they are not suitable for support (i.e.,

intermixed with large amounts of fine-grained soil). Observation of heavy equipment 

operations on the site at this phase provides the first indication of the subgrade adequacy. 

Rutting and deflection during initial earthwork operations indicates an immediate problem. 

This may not be a problem if significant undercut or stabilization was anticipated prior to 

construction. However, if soft materials are encountered at subgrade or initial fill elevation 

and were not anticipated, immediate action should be taken. In either case, the conditions 

must be improved. 

In order to improve the soil conditions at a site, a couple of options are common: 

1. Remove marginal soils and replace with select material of higher quality. 

2. Stabilize in-place soil (using one of the many techniques discussed in Chapter 7). 

3. Improve site drainage. 

Often the removal and replacement alternative is used because it appears to be the easiest. 

This is generally true for small areas or areas where spot locations are identified. However, 

undercutting may not be the most effective or even desirable for large areas. Excess water in 

soil is the principal cause of unstable conditions, and reducing the soil water content either by 

dewatering or stabilization methods may offer a better solution to expedite construction. 

Hauling or loading on a wet subgrade may continually disturb the section. Excavation in wet 

conditions often leads to more excavations. The variation of soil types and saturation levels 

should be evaluated by subsurface investigation to determine the vertical and longitudinal 

extent of the problem before a decision is made on which method to use. Excavation, 

drainage, and confirmation of stabilization will be reviewed in greater detail in the next 

section on construction techniques. 

Proper consideration of existing conditions should be given prior to earthwork construction. 

For example, areas that are to receive fill soils should be cleared and grubbed (i.e.,

vegetation, organic soils, and weak or otherwise unsuitable soils removed). Long-term 

performance issues are often traced to inadequate removal of unsuitable materials. Removal 
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of surface soils containing organic matter is important, not only for settlement, but these soils 

are often moisture sensitive, losing significant strength when wet, and are easily disturbed 

under construction activities. Site QC/QA personnel should monitor and confirm that these 

organic soils have been adequately removed. Documentation should including a visual 

description along with photos of the cleared surface. If additional excavation will not be 

made, the surface should be checked at this point for compliance with specification 

requirements. This will often require proof rolling or other testing such as DCP. Unsuitable 

materials also often find their way back into filling operations. Therefore, the control of 

unsuitable materials should also be documented. Other special earthworks should also be 

observed and documented, including embankments properly sloped to prevent slides, 

keyways (a.k.a. shear keys) constructed to avoid toe slope failures, and erosion protection 

techniques to maintain long-term stability. Large embankments can cause settlement of the 

natural soils, thus analyses examining settlement potential of underlying soils is essential. If 

settlement is anticipated, techniques such as removal-and-replacement or surcharge 

embankments can be utilized to mitigate the problem (See FHWA NHI-00-045, 2000). 

In unusual circumstances, a roadway alignment may be constructed over large voids or 

weakened soil zones, such as caves, faults or collapsed, abandoned mines, and sinkholes. 

Detection of these underlying potential problems prior to construction is ideal. Geophysical 

techniques such as ground penetrating radar (GPR) or spectral analyses of surface waves 

(SASW), again as described in Chapter 4, may be used in conjunction with historical 

information and experience to detect or predict the likelihood of encountering these described 

problems. Mitigation may include excavation and backfill operations, injection grouting, or 

grout columns using tubular fabric forms. For very localized areas and in karstic regions 

where voids are random but anticipated to be small, there has also been some success with 

bridging such areas with thickened reinforced concrete slabs or reinforced base and subbase 

using geogrids or welded wire mesh. However, for these techniques to be successful, the 

areal extent of the subsidence area must be clearly defined. 

8.4.1 Drainage 

If conditions exist during construction that indicate the need for underdrains (e.g., wet, 

saturated conditions) or the cleaning of the existing underdrains outlets, then this work 

should start as soon as possible (after Ohio DOT, 2002). Some examples of these conditions 

are as follows: 

1. existing underdrains with clogged outlets on rehabilitation projects 

2. free water in the subgrade 

3. saturated soils of moderately high permeability, such as sandy silt and silty clay of 

low plasticity 
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4. groundwater seepage through layers of permeable soil 

5. water seeping in the test pits 

6. water seeping from higher elevations in cut locations 

7. water flowing on the top of the rock undercuts 

Significant subgrade stability improvement can be obtained by cleaning out the existing 

underdrain outlets on rehabilitation projects and by adding construction underdrains on new 

construction projects. The FHWA/NHI course manual on Pavement Subsurface Drainage 

Design is a useful reference. Once the underdrain systems are in place and functioning, the 

drainage system can typically reduce subgrade pumping problems within a few days, but may 

take longer depending on the characteristics of the in-situ materials. Soils that are subject to 

densification and are not free draining (percent saturation exceeding 80 – 90%) within 1 m 

(2 – 4 ft) of the surface are not expected to support construction traffic. This order of 

magnitude of saturation is frequently observed to be the limit for compaction stability when 

developing moisture-density curves in the laboratory. Saturated soils with more than 10% 

fines (minus 0.075 mm (No. 200) sieve) are not expected to be drainable with respect to 

supporting construction traffic. Moisture reduction by evaporation (e.g., disking and aeration) 

may be more feasible than gravity drainage for these types of soils. 

For rehabilitation projects, the Contractor should be instructed to unclog the underdrain 

outlets immediately, attempting to perform this work in the timeframe listed above. If the 

project consists of several phases, then the Contractor should perform the outlet cleaning for 

the entire project at the same time. Because of the timeframes involved, construction 

underdrains should not be used for rehabilitation projects. 

For new construction projects, subgrade stability can be achieved by constructing the plan or 

construction underdrains as soon as the water problem is found (see Figure 8-4). New 

construction projects can allow a longer period of time for the underdrain system to work. At 

the beginning of construction, and certainly before winter shut down, are opportune times for 

this work. 

The plan underdrains should be placed only when they will not be contaminated by further 

construction. If contamination is a concern, then sacrificial or temporary construction 

underdrains should be used on the project. 

Construction underdrains are usually placed in the centerline of the roadway. They may also 

be placed in the ditch line, if the water is coming in from a cut section at a higher elevation. 

The porous backfill is extended to the subgrade elevation. The outlets for the construction 

underdrain are the same pipe material and backfill as regular underdrains. The underdrains 
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can be outlet to any convenient location. Some potential outlet locations are catch basins, 

manholes, pipes, or ditches. The project should not be concerned with the contamination in 

the upper portion of construction underdrain backfill. Construction underdrains are sacrificial 

underdrains that will continue to work throughout the life of the contract and afterwards even 

though the upper portion is contaminated. 

For rock or shale cuts, the design underdrains should extend at least 150 mm (6 in.) into the 

existing rock formation. If the underdrains are too high, the water will accumulate at the rock 

and soil interface and cause subgrade instability. 

Figure 8-4. Underdrain installation (photo courtesy of Ohio DOT). 
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8.4.2 Excavation 

Most construction projects will consist of some amount of excavation, or removal of in-situ 

soil to some design elevation or grade line. Excavation is typically accomplished using 

scrapers (also referred to as pans) or shovels, which are among the list of heaviest equipment 

used in modern earthwork. Observations made at this stage of construction are considered the 

first line of QC documentation. Site personnel should observe vertical movements below the 

construction equipment during excavation. Moderate to large deflections are the first 

indication that weak soils exist and some corrective action may be necessary. 

Scrapers have the ability to remove material from grade and spread at another location 

nearby. Some scrapers may need to be pushed by another scraper or by a bulldozer in order 

to advance while cutting into the zone of soil to be removed. Other scrapers are equipped 

with an elevator system (Figure 8-5) that allows the excavated material to be readily loaded 

without the assistance of a push from behind. 

Scrapers are commonly used in cut-fill earthwork operations, where the majority of the soil 

excavated is placed along another portion of the project. Anticipated site conditions that 

consist of wet or saturated soils due to water table elevations may necessitate the use of other 

forms of excavation equipment in order to minimize disturbance to the underlying in-situ 

soils. A common piece of equipment is the track-mounted excavator (shovel) like the one 

illustrated in Figure 8-6. Materials removed or excavated by this means require transfer to a 

secondary piece of equipment for hauling off site, or to another location along the alignment 

where fill soils are required. 

8.4.3 Hauling and Placement 

While scrapers (a.k.a. pans) transport their payload from a cut zone to a fill area, shovels 

require a haul truck to be utilized. There are several types of hauling vehicles, including end 

dump, side dump, bottom dump (or belly dump), and articulated dump trucks, as illustrated 

in Figure 8-7. 
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Figure 8-5. Self-loading scraper (photo courtesy of Caterpillar). 

Figure 8-6. Track-mounted excavator (photo courtesy of Komatsu). 
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Figure 8-7. Articulated dump truck (photo courtesy of Komatsu). 

Some projects will require off-site materials to be hauled in because of an imbalance of site 

materials. These borrow materials will typically be hauled in trucks and dumped near their 

intended final location. Depending on the dumping method, these piles may require 

spreading using a bulldozer or motor-grader (shown in Figures 8-8 and 8-9, respectively). 

Again, observation of this activity can indicate soft or unsuitable areas that will require 

special treatment. When excessive rutting is noted, haul routes should be changed so as to 

minimize the depth of disturbance. An assessment by the engineer should be made as soon as 

practical to determine if underdrains are needed. Often, well-placed and well-timed 

construction underdrains can mitigate the problem, and hauling over the previously unstable 

location may improve the stability by adding compaction to the draining soils. 

Some projects may restrict hauling on existing paved roads in order to eliminate damage to 

existing local roadways. In this case, it is possible to utilize a conveyor system to transport 

the borrow material to the site, like the one shown in Figure 8-10. 

During the hauling operation, material to be used as fill should be sampled and tested by 

QC/QA personnel for compliance with the specification requirements (e.g., soil type, 

gradation, etc.) Laboratory moisture-density tests (a.k.a. Proctors) should also be preformed 

for correlation with field density testing. 
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Figure 8-8. Bulldozer (photo courtesy of Komatsu). Figure 8-9. Motor grader 

(photo courtesy of Caterpillar)

Figure 8-10. Earth-moving conveyor system (Atlanta Airport – 5th Runway Embankment Placement).

8.4.4 Field Compaction 

Compaction can be defined as the densification of soils by the application of mechanical 

energy, oftentimes requiring a modification of water content. The purpose of compaction is 

generally to enhance the strength or load carrying capacity of the material, while minimizing 

long-term settlement potential. By adjusting the moisture content to a value at or near a 

moisture content considered optimum – as described below – reduced volume changes and 

increased strength can be achieved. 
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Significant advances have been made in the science and technology of earth structures in the 

last century. In the early 1900s, soils were placed in embankments by end dumping from 

wagons, with little attempt to compact. Structures that were placed by hand using baskets 

had, at a minimum, foot traffic to unintentionally “compact” the soil. It was observed that 

this foot traffic actually strengthened the soil, thus creating the concept of mechanical 

stabilization. Different types of field compaction equipment are appropriate for different 

types of soils. Steel-wheel rollers, the earliest type of compaction equipment, are suitable for 

cohesionless soils. Vibratory steel rollers have largely replaced static steel-wheel rollers 

because of their higher efficiency. Sheepsfoot rollers, which impart more of a kneading 

compaction effort than smooth steel wheels, are most appropriate for plastic cohesive soils. 

Vibratory versions of sheepsfoot rollers are also available. Pneumatic rubber-tired work well 

for both cohesionless and cohesive soils. A variety of small equipment for hand compaction 

in confined areas is also available. 

Recommended field compaction equipment for various soil types is summarized in Table 8-4. 

The effective depth of compaction of all field equipment is usually limited, so compaction of 

thick fills must be done in a series of lifts, with each lift thickness typically in the range of 

150 – 300 mm (6 – 12 in.) with greater depths possible (up to 0.7 m (2 ft)) through the use of 

specialized high energy equipment and the right type of soil conditions (e.g., free-draining 

granular soils). The soil type, degree of compaction required, field compaction energy (type 

and size of compaction equipment and number of passes), and the contractor’s skill in 

handling the material are key factors determining the maximum lift thickness that can be 

compacted effectively. Control of water content in each lift, either through drying or addition 

of water plus mixing, may be required to achieve required compacted densities and/or to 

meet specifications for compaction water content. 

Proof rolling with heavy rubber-tired rollers is often used to achieve additional compaction 

beyond that from normal compaction and, more important, to identify any remaining soft 

areas. The proof roller must be sized to avoid causing bearing capacity failures in the 

materials that are being proof rolling. Proof rolling is not a replacement for good compaction 

procedures and inspection. QC/QA personnel need to be present on site to watch the 

deflections under the roller in order to identify soft areas. Construction equipment, such as 

loaded scrapers and material delivery trucks, can also be used to help detect soft spots along 

the highway alignment. Details on the determination of suitability using proof rolling 

methods were provided in Section 8.3. 
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Table 8-4. Recommended field compaction equipment for different soils 

(after Rollings and Rollings, 1996). 

Soil First choice Second choice Comment 

Rock fill Vibratory Pneumatic 

Plastic soils, 

CH, MH 

(A-7, A-5) 

Sheepsfoot or 

pad foot Pneumatic Thin lifts usually needed 

Low-plasticity soils, 

CL, ML 

(A-6, A-4) 

Sheepsfoot or 

pad foot 

Pneumatic, vibratory Moisture control 

often critical for 

silty soils

Plastic sands and 

gravels, 

GC, SC 

(A-2-6, A-2-7) 

Vibratory, 

pneumatic 

Pad foot 

Silty sands and gravels, 

SM, GM 

(A-3, A-2-4, A-2-5) 

Vibratory Pneumatic, pad foot Moisture control 

often critical 

Clean sands, 

SW, SP 

(A-1-b) 

Vibratory Impact, pneumatic 

Clean gravels, 

GW, GP (A-1-a) 

Vibratory Pneumatic, impact, 

grid 

Grid useful for 

over-sized particles 

It is very difficult to achieve satisfactory compaction if the lift is not on a firm foundation. 

Figure 8-11 shows a typical stress distribution under a rubber-tired pneumatic roller for two 

different foundations. The first case corresponds to a homogeneous deposit with a constant 

modulus of elasticity equal to 170 MPa (25,000 psi), which is representative of a good 

quality granular material. The second case corresponds to a 150 mm (6-in.) thick lift of 170 

MPa (25,000 psi) granular material over a subgrade soil having a modulus of 35 MPa (5,000 

psi), which is representative of a soft clay having a CBR of around 3 or 4. As is clear from 

the figure, the stresses induced by the roller in the second case are much lower than in the 

first. High levels of compaction will be difficult to achieve in the thin lift over the weak 

subgrade, and the high stresses in the lower soil may produce shear failure and excessive 

rutting, especially if proof rolling is performed. Thus, it is easy to see the importance of 

monitoring cut surfaces through proof rolling and measuring compaction of each lift during 

fill placement. 
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Figure 8-11. Stress distributions under rollers over different foundations (after Rollings and 

Rollings, 1996). (1 ton = 8.9 kPa, 1 psi = 6.9 kPa)

The most common measure of compaction is density. Field moisture and densities can be 

measured using a variety of standard methods. Field density is correlated to moisture-density 

relationships measured in the lab (AASHTO Test Procedures T99 and T180). Moisturedensity relationships for various soils are discussed in Chapter 7, and the lab tests are covered 

in Chapter 5. Optimal engineering properties for a given soil type occur near its compaction 

optimum moisture content (wopt or OMC), as determined by the laboratory test standard. At 

this state, a soil’s void ratio and potential to shrink (if dried) or swell (if inundated with 

water) is minimized. 

In controlling compaction, the appropriate moisture-density laboratory method (e.g., standard 

or modified Proctor) should be matched to the equipment typically used in the local region. 

Higher energy equipment should be controlled with compaction tests based on high energy 

(i.e., modified in lieu of standard Proctor). There is a trend to lower moisture content 

tolerances with consideration for the higher energy equipment; however, this method could 

result in lower compactive efforts. The reason for this move is that the high-energy 
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compaction equipment is causing an apparent pumping to occur when the soil is above its 

optimum moisture. However, this method could ultimately lead to premature failures as the 

subgrade saturates over time (e.g., loss in stiffness/strength, or potential volume change). It is 

considered good practice to compact at the optimum moisture content for the material used. 

If some deviation occurs, it is better to be on the wet side, rather than dry. 

Compaction, or mechanical stabilization (i.e., water content adjustments and densification) is 

the most common and least expensive of all soil improvement techniques. Perhaps the most 

common problem arising from deficient construction is related to mechanical stabilization. 

The intent of mechanical stabilization is to maximize the soil strength (and minimize the 

potential volume change) by the proper adjustment of moisture and the densification at or 

near the ideal moisture content, as discussed in Section 8.4.4. Without proper quality control 

and quality assurance (QC/QA) measures, some deficient work may go unnoticed. This is 

most common in utility trenches and even bridge abutments, where it is difficult to compact 

because of vertical constraints. This type of problem can be avoided or at least minimized 

with a thorough plan and execution of the plan as it relates to QC/QA during construction, as 

reviewed in detail in Section 8.3. This plan should pay particular attention to proper moisture 

content, proper lift thickness for compaction, and sufficient configuration of the compaction 

equipment utilized (weight and width are the most critical). Failure to adequately construct 

and backfill trench lines will most likely result in localized settlement and cracking at the 

pavement surface. 

There are several compaction devices available in modern earthwork, and selection of the 

proper equipment is dependent on the material intended to be densified. Generally, 

compaction can be accomplished using pressure, vibration, and/or kneading action. Heavy 

equipment, as measured by ground pressure, is utilized to accomplish the compaction 

process. Some heavy equipment have low ground pressures, such as tracked vehicles like 

bulldozers, and rubber-tire equipment like front end loaders and motor graders. The low 

ground pressures imparted by these types of vehicles are not effective at compacting soils; 

however, tracked vehicles do provide some limited compaction of granular, cohesionless 

materials by means of vibration. 

A smooth drum roller is perhaps the most common of all compaction devices, capable of 

applying pressure across the width of its drum. Smooth drum rollers can consist of a single 

drum (Figure 8-12) or dual drum. Most drum rollers are equipped with oscillary vibrators to 

increase the energy transmitted to the surface of the layer being compacted. These smooth 

drum rollers are best suited for granular, relatively non-cohesive soils. Some agencies have 

used smooth drum rollers to finish subgrades prior to base construction, and have even 

employed them as a proof rolling instrument. 
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Figure 8-12. Smooth drum roller (photo courtesy of Bomag). 

The sheepsfoot or studded rollers like the one shown in Figure 8-13 are typically used on 

cohesive soils. These rollers are very similar to the smooth drum roller, however, many 

rounded or rectangular protrusions (or feet) are attached to the drum. These protrusions 

provide for a very high contact pressure in a small zone of soil. By spacing these protrusions 

apart, very high vertical stresses, as well as horizontal stresses, are achieved, thus creating a 

kneading action that compacts from the bottom up. During compaction, the roller literally 

“walks out” of the lift once compaction is achieved. This kneading or shearing action has the 

ability to produce a soil structure that maximizes a cohesive soil’s strength at high density 

levels. Some sheepsfoot or studded rollers are also equiped with oscillatory vibrators to 

increase the effectiveness across a broader range of soil. 

Pneumatic or rubber-tire rollers have also been utilized to compact materials. These 

compactors are typically used as an alternate for compacting a variety of soil types (see Table 

8-4). They are particularly effective for non-cohesive silty soils. Some agencies have used 

them successfully in embankment placements and have also employed them as a proof 

rolling instrument. Hauling vehicles (scrapers and loaded dump trucks) have been used for 

compaction purposes. 
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The latest compaction equipment are high-energy impact rollers, which use shaped (e.g.,

triangular ellipsoids or hexagonal), as opposed to round drums, as shown in Figure 8-14. The 

high energy imparted by these systems allows them to achieve compaction at a faster rate and 

to greater depths. A comparison of different types of compaction equipment based on vertical 

settlement with number of passes is shown in Figure 8-15, demonstrating the superior 

effectiveness both in terms of number of passes and influence depth of high-energy 

equipment. 

Most of the research on this equipment has been performed in Europe, and unfortunately the 

availability is limited at this time in the U.S. The Europeans are also experimenting with 

hydraulic and pneumatic impact hammers to achieve compaction at greater depths, especially 

in rubblized fills (Dumas, et al, 2003). This technique uses a 5-tonnes (5.5 ton), 1-m (3.3 ft) 

drop hydraulic pile hammer to drive a large foot into the ground. This technology eliminates 

excavation and allows for compaction of shallow layers (or soils with low moisture content) 

up to 3 m (9 ft) thick. The technique was initially developed by the British and U.S. military 

for rapid airfield repair. 

Figure 8-13. Sheepsfoot roller (photo courtesy of Bomag). 
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Figure 8-14. Impact roller. 

Figure 8-15. Compaction efficiency. (1 in. = 25 mm) 
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Another significant development in compaction equipment is the use of instruments in the 

compaction drums to measure the response of soil (e.g., stiffness). The equipment is 

computerized, allowing for real-time monitoring of foundation response and automated 

feedback controlling vibration amplitude and frequency, and vehicle speed. While intelligent 

compaction equipment was originally developed for contractors to improve their efficiency in 

achieving compaction with a minimum number of passes, it has direct and significant 

application potential for controlling and monitoring compaction effectiveness for pavement 

performance, as discussed in Section 8.3 on QC/QA. 

Other means exist in which to promote deep densification, including dynamic compaction 

and vibroflotation. These processes are discussed in the FHWA/NHI Ground Improvement 

Techniques reference manual (FHWA NHI-04-001) and, given the right conditions, can be 

used to densify soils at depths of over 9 meters (30 ft). Each is limited to successes achieved 

in deep, loose non-cohesive soils, such as sands and gravels. 

As previously discussed, with the advent of newer higher energy compaction equipment, 

agencies should carefully evaluate their current specifications to meet these changing 

demands. 

The final phase of subgrade construction is the confirmation of surficial support prior to 

placement of the base/subbase layers. One or more of the methods outlined in Section 8.3 

should be utilized (e.g., proof rolling, DCP, FWD). 

8.4.5 Stabilization 

In certain instances, when stabilization is a more economical means of constructing a 

pavement section with the desired support characteristics, use of chemical admixtures, such 

as lime, flyash or cement, is common. These mixtures are typically designed in a controlled 

laboratory environment in order to establish volumetric properties, such as admixture design 

content, maximum density, moisture content, and strength, as discussed in Chapter 7. 

Chemical Stabilization/Modification. In the special case where lime (or other pozzolanic 

modifier such as cement or flyash) is to be utilized to enhance the load carrying capability of 

the soil, the additional effort of introducing the modifier to the soil and mixing prior to 

compaction is required. 

The basic construction steps for chemical stabilization of subgrade soils are (1) pozzolan 

delivery and distribution; (2) mixing; (3) compacting; and (4) curing. Pozzolans can be 

applied to a soil either dry or as a slurry. In the case of dry lime, the lime may be either in the 
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form of dry hydrated lime, which is very fine-grained and, thus, may pose dust control 

problems, or dry quicklime, which is granular and much less dusty. 

The pozzolanic material specified for stabilization or modification is distributed along the 

road alignment, either via bags that are spread manually, by pneumatic trucks with spreader 

bars, or by dump trucks with controlled tailgate openings. Lime slurries can be mixed in a 

central mixing plant or in various types of portable mixing systems. A typical lime slurry 

mixture would consist of 0.9 tonne (1 ton) of lime mixed with 1900 liters (500 gal) of water 

to produce 550 tonnes (600 tons) of slurry with 31% lime solids (Transportation Research 

Board, 1987). 

Adequate mixing of the pozzolanic material with the soil is critical; poor mixing is the 

leading cause of unsatisfactory stabilization results. Subgrade soils can be mixed on site with 

the pozzolan by disking, repeated blading, or by traveling rotary or pug-mill mixing 

equipment. 

Figure 8-16. Roadway stabilizer/mixer (photo courtesy of Bomag). 
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Mixing is usually done in thin lifts and often with multiple passes, with the lift thickness and 

number of passes dependent upon the soil type and the mixing equipment being used. A twostage mixing process is sometimes used for highly plastic materials; the reduced plasticity 

and coarser texture that develops during curing for several days after the initial mixing makes 

the soil more workable for final mixing and compaction. 

Compaction of chemically stabilized soil mixtures follows standard procedures. However, 

with respect to lime stabilization, the addition of lime will generally decrease the maximum 

density and increase the optimum water content at a given compaction energy, which may 

cause problems determining the percentage of specified density achieved by the field 

compaction. Compaction curves of the in-situ lime-soil mixture at the time of compaction 

may be required to determine the appropriate density values for field compaction control. 

Curing at temperatures above 4.4o C (40o F) and with adequate moisture is essential for the 

pozzolanic reactions underlying the long-term strength gains in lime-stabilized soils. A cure 

period of 3 – 7 days is typically employed, with adequate moisture maintained either through 

moist curing (e.g., truck sprinklers) or by applying an asphalt seal over the surface. 

Similarly, modified soil (lime, cement and/or flyash) will require special QC/QA 

considerations, as discussed in Section 8.3. Again, a final evaluation of the stabilized 

subgrade surface should be made by one or more of the methods described in Section 8.3 

(i.e., proof rolling, DCP, FWD). 

8.4.6 Base and Subbase Construction 

In the case of aggregate base construction, material is hauled to the site and is typically 

placed directly on grade, spread to a uniform specified thickness, and compacted. Care must 

be exercised to minimize segregation of aggregate blends. Good practices to prevent or 

minimize segregation include eliminate the number of transfer points prior to final grade 

placement (avoidance of intermediate stockpiling), and minimize the amount of spreading 

and movement once on grade. Asphalt pavement spreaders have been successfully utilized to 

distribute aggregate base materials on grade. Use of such equipment allows good control of 

specified thickness and reduces the potential of segregation caused by traditional spreading 

techniques such as motor-grader or dozer operations. This practice can also positively affect 

the overall project profile smoothness objectives. Regardless of the method of placement, 

care should be exercised to avoid the potential to contaminate the aggregate with site soil. 

Contamination is typically introduced when wet soils adhere to construction traffic tires or 

tracks, and are “cleaned off” when traversing over newly placed aggregate layers. Again, 
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QC/QA personnel should document observations and test results, indicating conformance or 

non-conformance with the specification. 

Base and subbase layers are typically aggregate materials containing moderate (dense 

graded) to little (open graded or drainable) fines. Compaction is typically achieved utilizing 

vibratory smooth drum compactors described previously in Section 8.4.4. Failure to achieve 

proper compaction may be a result of several factors, either individually or in combination: 

• Lack of substrate support (Should have been detected and corrected prior to 

placement of layer.) 

• Improper size of compactor 

• Excessive moisture (Perhaps from rainfall. Site surface runoff should be promoted. 

Excess moisture usually can be dried by blading and allowing excessive moisture 

time to evaporate.) 

• Segregation 

Correction in the form of drying and recompacting should work in the majority of cases. If 

problems persist, removal and replacement may be warranted. If the problem is deeper than 

the base or subbase layer, subexcavation and replacement or some form of chemical 

stabilization technique may be required, as discussed in Section 8.4.5. 

Chemical Stabilization. Mixing of lime or cement with coarse aggregates for base and 

subbase layers is often done in a central mixing plant. Although the central mixing plant is 

required primarily for gradation control, it also enables good control of the lime-aggregate or 

cement-aggregate proportions and mixing. Again, testing during construction should closely 

parallel that described for soil stabilization/modification. 

8.4.7 Pavement Drainage Systems 

For construction of pavement drainage systems, design should acquaint agency construction 

personnel with the impact of construction on the design results. Care during construction to 

build the pavement drainage designed section without compromising the effectiveness of 

design is essential to the pavement’s long-term performance. Key performance elements for 

construction personnel to remember include 

• Good pavement starts with a good foundation. A stable platform is required for 

construction of the permeable base. 

• Quality of aggregate and its ability to meet gradation requirements is essential to 

meeting design performance. 

• An awareness that the introduction of fines into the permeable base during 

construction could result in the premature failure of the pavement. 
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• Unstabilized drainable base tends to displace under traffic. 

In addition to these key elements, construction personnel (contractor and inspector) should be 

aware of how each construction activity can impact the performance of the pavement 

drainage system. 

Subgrade Preparation As with all road sections, the foundation surfaces are required to be 

level, somewhat smooth and constructed to required grades. With drainable pavement 

sections, constructing and maintaining the required subsurface grades until pavement 

construction is essential in maintaining positive pavement drainage. Grades that are too flat, 

local depressions resulting from soft areas, and/or depressions from equipment trafficking 

can lead to ponding of water below the pavement structure and subsequent loss of foundation 

support. 

Separation Layers For granular separation layers, the gradation of materials must be 

carefully checked against design requirements. Material that is more open than specification 

requirements may allow migration of fines and contamination of the permeable base. Good 

compaction of the separation layer is essential to the placement of the permeable base. The 

subbase should be observed for rutting during compaction and subsequent trafficking. 

Subbase surface rutting may be an indication of subgrade rutting and requires immediate 

attention (e.g., by reducing equipment loads or increasing the lift thickness). "A separator is 

not a substitute for proper subgrade preparation" (FHWA-SA-92-008).

For construction of geotextile separation layers, material and certification should be checked 

against design specification requirements to make sure the proper materials have been 

received and used. The smooth subgrade surface is desirable. It is recommended that sharp 

rock protrusions or loose rocks (usually greater than 20 mm (3⁄4 in.) in size) be removed to 

avoid damage to the geotextile, unless such conditions have been anticipated and heavyweight (greater than 250 g/m2

 (7.4 oz/yd2)) geotextiles have been specified. 

Edgedrains Proper grading is essential for edgedrains to be effective. Undulating drain lines 

are not acceptable, as water will accumulate in depressed areas. Good practice dictates that 

drains must be properly connected to the permeable base and to outlets. Outlets are required 

to be set at the proper grades and ditch lines graded according to drainage requirements. 

Drain lines are to be carefully marked and care maintained throughout construction to avoid 

crushing the pipe with construction equipment (e.g., concrete trucks and other heavy 

vehicles/equipment are not to be allowed to travel over drain lines). Drains are sometimes 

constructed after pavement construction to avoid this problem. In this case, temporary 

drainage is required for the permeable base to prevent a bathtub effect from water trapped in 

the porous base. 
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As discussed in Section 7-2, the filter (geotextile or aggregate) has to be carefully placed at 

the design location around all sides of the backfill, not in contact with the permeable base. 

The edgedrains are required to be backfilled with material at least as permeable as the 

permeable base. Most states use a graded aggregate, while some states use free-draining 

sand. In either case, the drainage backfill should be placed below the invert of the pipe, and 

compacted to better support the pipe, reduce the risk of crushing the pipe, and to prevent 

subsequent subsidence that could affect the road. As with the trench line, the pipe must be 

placed at the proper grade on a level surface. Drainage backfill is placed to the final elevation 

and protected from fouling until the pavement section is complete. Maintaining an open 

drainage aggregate is critical during the remaining construction period. A shovel full of fines 

could clog the drain. Construction traffic should not be allowed to traverse over the drain 

line. The drain line could be covered with a geotextile to help prevent fouling during 

construction. Also, outlets must properly drain during this phase to provide temporary 

drainage during construction. Ditch lines should be continuously checked and maintained, as 

erosion sediments could back up and foul essential features. Headwalls for outlets should be 

installed and outlets marked so they will not be disturbed by subsequent construction. 

The edgedrain system should be inspected and tested for proper operation toward the end of 

construction, before final acceptance. An acceptance criteria based on performance 

parameters must be established, otherwise signs of poor construction practices will most 

likely not be identified until major structural damage is done and the pavement life has been 

shortened. Inspection techniques can consist of simply pouring water on the drainage layer in 

an upstream section of the drain and measuring the outflow against the anticipated rate. The 

most effective method for post-construction evaluation is video equipment (e.g., Iowa 

borescope and other mini-cameras). Several states do not accept edgedrains until video 

inspection indicates that they have not been damaged during construction. The design of the 

edgedrain system should have included pipe access installed at the "upstream" end of the 

drain line to gain access for camera inspection, effectiveness testing, and subsequent 

maintenance flushing activities. 

Drainable Base Materials Unstabilized permeable base requires close control of the 

material gradation and attention to activities that might cause segregation. An asphalt 

spreader box is usually required to reduce segregation. Unstabilized base tends to weave and 

rut under traffic. 

Asphalt-stabilized permeable base usually contains AASHTO No. 67 or No. 57 crushed 

aggregate plus 2 – 2.5% asphalt by weight. Higher asphalt cement percentages may be 
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required when a less-open gradation is used. Some states prohibit the use of bank run gravel 

aggregate because of the rounded faces. Stabilized aggregate should be placed at 90 – 120o C 

(200 – 250o F) but not rolled until it is below 65o C (150o F). Vibratory rollers are usually not 

allowed, and the number of roller passes is usually between 1 and 3 (FHWA-SA-92-008). 

Cement-stabilized permeable base usually contains 2 to 3 bags for No. 67 and No. 57 crushed 

aggregate. As with asphalt-stabilized base, higher amounts may be required for less-open 

graded aggregate. Cement-stabilized base could be cured similar to pavement. Test strips are 

recommended to determine appropriate curing and compaction methods (FHWA-SA-92-

008).

Care is required to protect the permeable base from fines contamination (e.g., from dirty 

construction equipment, adjacent backfilling operations, erosion sediments, etc.). While the 

drainable base can generally support light construction loads, it should not be used as a haul 

road. Equipment that would cause rutting (e.g., concrete and loaded dump trucks), dirty 

equipment, or equipment transporting fines should not be allowed to traverse over the 

permeable base. Good practice dictates that traffic be restricted to low speeds with minimal 

turning allowed. Traffic should not be allowed until complete drainage of the base and 

subbase has been confirmed. 

Based on a survey of state agencies (Christopher and McGuffy, 1997), good construction of 

subsurface drainage systems appears to depend on a number of factors: 

• The contractor (and inspector) should be knowledgeable in drain installation 

principles and practices. 

• Someone with knowledge of drainable pavements must be on site at startup. 

• Water needs a continuous, unobstructed path to drain, both during and after 

construction. 

• A positive slope is required. 

• Any discontinuity in flow path can destroy the system's effectiveness. 

• The pavement (or shoulder) is supported by the system; therefore, compaction is 

essential. 

• Construction activities for other work in the area can destroy good drainage 

installations. 
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8.5 PERFORMANCE MONITORING 

8.5.1 Pavement Management Systems 

Pavement management systems have been utilized as tools to document and track pavement 

performance. These systems typically rely on the assessment of the pavement wearing 

surface, in the form of distress surveys performed at periodic intervals, in order to not only 

illustrate how the pavement is performing, but to predict how the pavement may perform into 

the future. Through the use of these tools, agencies have been able to detect performance 

problems early, and correct the problems with routine maintenance during the pavement lifecycle. These tools assist agencies to best manage maintenance and capital budgets across 

their broad network of pavements, and can be utilized efficiently at the project level to 

optimize pavement performance for individual construction projects. These tools become 

very important at the project level when considering performance risk, particularly with 

extended performance periods. 

A major disadvantage with the conventional distress survey input for a pavement 

management system, particularly with respect to pavement layers associated with unbound 

materials, is that problems are not detected until failure occurs. Problems caused by moisture 

intrusion into the subgrade and unbound base/subbase layers weaken the pavement system. If 

gone undetected, a pavement’s life can be dramatically shortened. In order to circumvent this 

problem, agencies and particularly design-build teams, have seen the benefit of augmenting a 

solid pavement management system (distress survey) with structural surveys (NDT using one 

of the many geophysical testing techniques previously documented in Chapter 4, and 

described in further detail in the following sections). 

8.5.2 Geophysical 

Geophysical measurements detect differences or anomalies in material properties. However, 

these properties require interpretation as conditions relevant to pavement performance. As 

discussed in Chapter 4, geophysical testing can be used to locate voids beneath pavement 

sections for both construction and long-term performance monitoring. Periodic monitoring 

of a region with known problems such as solution caves or other karstic features can be a 

significant asset in evaluating the effectiveness of grouting programs to solve problems 

during construction and evaluate any long-term developments that could lead to future 

problems. The following two case histories provide a demonstration of effective use of 

geophysics in both short- and long-term monitoring programs. 
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As indicated in Chapter 4, the Finnish government performs resistivity testing on subgrades 

along with other in-situ and geophysical tests to develop a complete map of the subgrade 

system, including moisture and corresponding settlement and frost heave profiles. These 

anticipated profiles are then used to define the performance requirements for roadway 

warranties. The allowable settlement for a 30-year service period and a 5-year warranty 

period is calculated based on this well-documented and detailed site investigation (Tolla, 

2002). 

Widening and realignment of State Route 69 traverses an area of Tertiary-age travertine 

bedrock near Mayer, Arizona. During the design phase of the project, subsurface exploration 

encountered small voids within the right-of-way. A moderate-sized cave structure in the area 

was mapped by local speleologists. Arizona Department of Transportation (ADOT) was 

concerned that cave structures of unknown size might be found within a few feet of the new 

roadway subgrade. As a result, highway construction specifications contained special 

provisions requiring geophysical surveys to identify cave structures that could adversely 

affect the roadway and expose the traveling public to possible subgrade failure hazards. 

ADOT's concern was realized during construction when a D-9 Caterpillar tractor broke 

through a cave roof and dropped about 1.8 m (6 ft) into the void. A geophysical survey 

conducted of the cave-affected alignment identified 130 cave-type anomalies, and 

recommendations were provided to ADOT and the contractor to remediate the cave-affected 

highway section. Survey monuments were established for monitoring roadway performance 

and potential subgrade settlement (Euge et al., 1998). 

8.5.3 Falling Weight Deflectometer 

Much research has been conducted by FHWA in the past decade, particularly as part of the 

Long-Term Pavement Performance (LTPP) study. Although typically utilized as a tool to 

measure structural capacity of a pavement system for the primary purpose of designing 

strengthening and overlay thickness requirements, the FWD can be utilized to monitor the 

subgrade performance, as well as base/subbase performance. This type of program can be 

established by first measuring the deflection profile of a newly completed or rehabilitated 

pavement section at numerous discrete points (baseline data). These measurements 

(particularly deflections away from the loaded plate) can provide useful information about 

the deeper layers in the pavement system. Measurements made at annual or seasonal periods 

and compared with baseline data may indicate when potential problems exist. A loss in 

stiffness in a deep subgrade, or intermediate base or subbase layer, may indicate a poor 

drainage condition exists, one which can be readily corrected prior to premature pavement 

system failures by either constructing underdrains, maintaining existing underdrains, or 

altering the site hydrology in a way that better promotes site drainage. 
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8.5.4 Drainage Inspection (e.g., video logging) 

Performance monitoring of drainage systems is essential for both acceptance of the 

constructed facility and for maintaining a preventive maintenance program (NCHRP 

Synthesis 285). Probably the most significant development in edgedrain inspection has been 

the use of small diameter, optical tube video cameras with closed circuit video systems. 

Video cameras allow the inside of the edgedrain system to be logged, and expose the 

weaknesses in construction and inspection procedures. Iowa was one of the first states to 

effectively use video inspection (Steffes et al., 1991). Random inspection of drains with video 

cameras has exposed many problems including 

• rodent nests in the drain, 

• varied sag from main line to outlet, 

• polyethylene tubing and connector failures, 

• break from stretch or puncture, and 

• geocomposite drain J-buckling. 

As was discussed in Chapter 4, significant effort to evaluate the use of video cameras as an 

inspection tool and demonstrate the technology was undertaken by the Federal Highway 

Administration. In evaluation of 269 outlet pipes that were inspected, 35% of the laterals 

could not be inspected because they were crushed or clogged, and the condition of the 

mainline could not be investigated. Of the mainlines that were evaluated, 17% were blocked 

or clogged. These findings clearly indicated that there were serious inadequacies in the 

edgedrain design, construction, and maintenance practice. The study also showed that the 

video inspection of edgedrains was a viable tool for determining the existing condition of 

edgedrains and had a definite role in providing construction quality assurance. 

The Federal Highway Administration program to promote this technology appears to have 

had a significant impact. Over 17 states reported to have used a video camera. Many agencies 

own their own video camera, with a cost for the system ranging from $13,000 – $40,000. 

Some agencies retain consultants to perform video inspections. Video cameras have proven 

to be a valuable tool for many of the agencies in identifying problems and exposing 

weaknesses in construction and inspection procedures. Many states currently do or will 

shortly require video inspection for construction acceptance. Several agencies have reported 

that they have improved from an edgedrain failure rate of up to 40% to a failure rate of less 

than 5% by improving their QC/QA program, including the use of video cameras. Several 

agencies have incorporated their video camera into their preventive maintenance program, 

with periodic monitoring during routine inspection. 
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8.5.5 Instrumented Geosynthetics 

Geosynthetics provide a convenient delivery system for performance monitoring instruments. 

Instrumentation, including strain gauges for deformation and stress measurements, pore 

pressure transducers to monitor soil suction, dielectric sensors to monitor moisture change, 

and thermistors to measure temperature change, can all be installed in the factory, delivered 

to the site, and hooked directly into a remote data acquisition system with 

telecommunications (no wires). Geosynthetics are currently available in Europe with an array 

of strain gages embedded in the product for monitoring subsidence (e.g., from karst 

conditions and abandoned mines). This allows performance monitoring with practically no 

disruption to construction. Care is still required to avoid damage to the instruments during 

placement of the section and the initial fill over that section. 

8.6 POST-CONSTRUCTION ISSUES AND SPECIAL CASES 

The installation of structural features (e.g., storm water lines and manholes, culverts, 

roadway drainage lines, etc.) adjacent to or beneath pavements can also lead to problems 

during or following construction. Proper installation of such structures is critical and close 

inspection during construction is critical. For example, a precast concrete pipe is installed as 

a storm drain. Each segment of the pipe is grouted, and the pipe is grouted into a junction box 

that also serves as a storm drain surface inlet (surface grate). The pipe is located on a 

100-mm (4-in.) sand bedding at the bottom of a trench excavation. Following installation of 

the pipe, the trench is backfilled adequately, and the pavement is constructed. Imagine 

though, that one of the pipe joints was not adequately grouted, or post-construction 

settlement occurred (e.g., due to inadequate embankment or bedding compaction) causing 

differential movement such that one of the joints separates. Because of the amount of storm 

water carried by the pipe during the most intense rainfall, the turbulent flow of water has 

begun to pipe soil backfill from around the pipe, and has swept it to the junction box and 

further down stream. The progression of this piping and erosion will eventually lead to 

pavement subsidence. 

This type of pavement failure, subsidence of underlying strata, can be manifested by a 

mechanism as described above or by a similar mechanism – water movement through voids, 

piping or eroding fines over time to cause larger voids that eventually collapse. These 

failures, described as sinkholes, generally are catastrophic in nature, and costly to repair 

(construction and delay costs). A key element in the installation of piping systems is proper 

compaction beneath and around the pipe. Granular fill should always be used to form a 

haunch below the pipe for support. Some state agencies are using flowable fill or controlled 
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low-strength material (CLSM) as an alternative to compacted granular fill (NCHRP Project 

24-12). Without this support feature, the weight above the pipe will cause it to deform 

laterally, creating settlement above the pipe and often pipe collapse. Even if a sinkhole does 

not appear, leaks of any water bearing utility will inundate the adjacent pavement layers 

reducing their support capacity. Several agencies have used CLSM around pipes in the 

pavement section. 

Pavement problems also occur when improper fill is used in the embankment beneath the 

pavement system. Placement of tree trunks, large branches and wood pieces in embankment 

fill must not be allowed. Over time, these organic materials decay, causing localized 

settlement and, eventually, voids in the soil. Again, water entering these voids can lead to 

collapse and substantial subsidence of the pavement section. Likewise, placement of large 

stones and boulders in fills creates voids in the mass, either unfilled due to bridging of soil 

over the large particles or filled with finer material that cannot be compacted with 

conventional equipment. Soil above these materials can pipe into the void space creating 

substantial subsidence in the pavement section. These issues can be mitigated with a wellcrafted specification that will not allow these types of materials to be used, and full execution 

of the project QC/QA plan. (e.g., Uniformity Coefficient, Cu = D60/D10 > 15, and Coefficient 

of Curvature, Cc = D30

2

/D10D60 > 5). 

Special cases may require large stone (e.g., blast rock or surge stone) to be used as fill. If 

such material must be used as fill, then select graded granular material and/or geotextiles 

should be placed above and below the large stone to form separator layers. For the use of a 

granular separation layer, the upper layer should be well compacted to choke off the voids in 

the stone. If possible, this layer should be flooded with a hose to confirm its compatibility. 

The gradation of the granular material must be such that it will not move into the void space 

and must also meet the filter criteria for the finer-grained fill material in the embankment. 

These conditions are met if the following gradation criteria are satisfied: 

• D85 graded granular blanket ≥ 0.2 D50 large stone fill 

• D85 graded granular blanket ≤ 5 D85 embankment fill 

• D15 graded granular blanket ≥ 5 D15 embankment fill 

An alternative is to use high-survivability geotextiles (AASHTO M288 Class 1) placed 

immediately above and below the large stone layers to act as separators and prevent soil from 

moving into the void spaces in these materials. 

Transitions between cut zones and fill zones can also create problems, particularly related to 

insufficient removal of weak organic material (clearing and grubbing), as well as neglect of 

subsurface water movements. 
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A specific transition also occurs at bridge approaches. These problems are typically related to 

inadequate compaction, typically a result of improper compaction equipment mobilized to 

the site, lack of supervision and care (e.g., lift placement greater than compaction equipment 

can properly densify). 

Many problems arise only after construction is completed and some amount of service life 

has been consumed. Detection of problems attributed to the geotechnical aspects of the 

pavement can be identified by interpretation of distresses observed. Examples of problems 

associated with flexible and rigid pavements are highlighted in Tables 8-5 and 8-6, 

respectively. 

Pavement design and construction is an ongoing voyage. The industry has encountered 

numerous untold problems, and has found logical and economical solutions to mitigate these 

problems in the design stage, the construction stage, and the performance stage. Local 

agencies have developed their own strategies to solve anticipated problem and deal with 

unanticipated problems. This chapter was intended to summarize and discuss the majority of 

these issues. 

Table 8-5. Geotechnical related post-construction problems in flexible pavements. 

Problem/Distress 

Observed Probable Cause(s) Corrective Action 

Longitudinal crack in wheel 

path (fatigue) 

1. weak subgrade 

2. insufficient pavement 

thickness 

overlay 

Surface rut in wheel path 

1. over-stressed subgrade 

2. post-construction 

densification of asphalt 

layer(s) 

- leveling course in ruts and overlay 

- plane and overlay (i.e., mill & fill) 

Staining in surface cracks – 

color of stains consistent 

with local soil 

drainage problems, wet 

subgrade – fines 

contamination in base, if 

present 

- reconstruction 

- thick overlay (extend life 

somewhat, but mask the 

problem) 

Intermittent depressions, 

subsidence 

1. erratic compaction control 

2. buried organic matter 

3. piping in subsurface voids 

(e.g., around utilities) 

localized demolition and patching 

Edge cracking frost susceptibility and 

drainage issues 

Construct wider shoulder; use 

materials that are not frost 

susceptible above the depth of 

frost penetration. 
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Table 8-6. Geotechnical related post-construction problems in rigid pavements. 

Problem/Distress Observed Probable Cause(s) Corrective Action 

Staining in surface cracks and/or 

joints – color of stains 

consistent with local soil 

drainage problems, wet 

subgrade – fines 

contamination in base, 

if present, pumping 

fines 

- reconstruction 

- thick overlay (extend life 

somewhat, but mask the 

problem) 

Corner break 

pumping of fines resulting 

in void or loss of 

support beneath slab 

localized demolition and patching 

or reconstruction depending on 

extent of problem (mud-jacking 

and under sealing may be a 

good option if voids are 

detected prior to breaks) 

Faulting at joints, subsidence of 

utility patches 

1. erratic compaction 

control 

2. buried organic matter 

3. piping in subsurface 

voids (e.g., around 

utilities) 

4. pumping of fines 

resulting in void or loss 

of support beneath slab 

localized demolition and patching 

or reconstruction depending on 

extent of problem (mud-jacking 

and under sealing may be a 

good option if voids are 

detected prior to faults, 

diamond grinding may be 

required for smoothness) 

Example 8.1. A class exercise will be constructed around problems that are common to the 

agency. Each team will be given a specific pavement subgrade scenario and asked to identify 

the most appropriate soil improvement method(s) and describe the reasons for their selection. 

Other teams will critique the selection(s). This exercise would then be followed by slides 

summarizing the advantages and disadvantages of all of the soil improvement methods. 
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8.7 REFERENCES 

AASHTO (2004). AASHTO Standard Specifications for Transportation Materials and 

Methods of Sampling and Testing (24th Ed.), American Association of State Highway and 

Transportation Officials, Washington, D.C. 

AASHTO (1993). AASHTO Guide for Design of Pavement Structures, American 

Association of State Highway and Transportation Officials, Washington, D.C. 

Boudreau, R.L. (2003). Resilient Modulus – Pavement Subgrade Design Value, ASTM STP 

1437 - Resilient Modulus Testing for Pavement Components, G.N. Durham, A.W. Marr, and 

W.L. De Groff, Eds., American Society for Testing and Materials, West Conshohocken, PA. 

Chamberlin, W.P. (1995). Performance-Related Specifications for Highway Construction 

and Rehabilitation, National Cooperative Highway Research Program Synthesis 212. 

Christopher, B.R. and McGuffey, V.C. (1997). Pavement Subsurface Drainage Systems,

National Cooperative Highway Research Program, Synthesis of Highway Practice 239, 

Transportation Research Board, National Academy Press, Washington, D.C. 

Daleiden, J. (1998) Video Inspection of Highway Edgedrain Systems, Final Report, Brent 

Rauhut Engineering. 

Dumas, C., Mansukhani, S., Porbaha, A., Short, R.D., Cannon, R.R., McLain, K.W., Putcha, 

S., Macnab, A., Lwin, M.M., Pelnik III, T.W., Brown, D.A., Christopher, B.R. (2003) 

Innovative Technology For Accelerated Construction Of Bridge And Embankment 

Foundations In Europe, Prepared by American Trade Initiatives for Federal Highway 

Administration, http://isddc.dot.gov/OLPFiles/FHWA/010940.pdf. 

Euge, K M; Hennon, K; Williams, J C; Priznar, N M (1998). “Ground Penetrating Radar: an 

Effective Tool for Locating Dry Caves along a Portion of State Route 69 Near Mayer, 

Yavapai County, Arizona,” Proceedings and Field Trip Guide of the 49th Highway Geology 

Symposium. Highway Geology Symposium. 

FHWA NHI-04-001 (2004) Ground Improvement Methods, Participant Notebook, NHI 

Course 132034, authors: Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J.G., and Berg, 

R.R., National Highway Institute, Federal Highway Administration, Washington, D.C., 1022 

pp. 
FHWA NHI-05-037 Chapter 8 – Construction & Design Verification 

Geotechnical Aspects of Pavements 8 - 48 May 2006

FHWA NHI-00-045 (2000). Soils and Foundations Workshop, NHI Course No. 132012, 

Federal Highway Administration, Washington, D.C. 

FHWA-SA-92-008 (1992). Demonstration Project 87: Drainable Pavement Systems, 

Participant Notebook, Federal Highway Administration, Washington, D.C. 

Hancher, D.E. (1994). Use of Warranties in Road Construction, National Cooperative 

Highway Research Program Synthesis 195. 

Illinois DOT (1982) Subgrade Stability Manual, Illinois Department of Transportation. 

Joint Departments of the Army and Air Force (1994). Soil Stabilization for Pavements, TM 

5-822-14/AFMAN 32-8010. 

Nazarian, S., Yuan, D., Tandon, V. and Arellano, M. (2002). Quality Management of 

Flexible Pavement Layers with Seismic Methods, Texas DOT Report No. TX-02 1735-3F. 

Ohio Department of Transportation (2002). Subgrade Construction and Stabilization, Office 

of Construction. 

Rollings, M.P., and R.S. Rollings (1996). Geotechnical Materials in Construction, McGrawHill, NY. 

Steffes, R F; Marks, V J; Dirks, K L (1991) “Video Evaluation Of Highway Drainage 

Systems,” Transportation Research Record No. 1329, Transportation Research Board, 

Washington, D.C. 

Tolla, P. (2002). Presentation to the FHWA Scan Team on “Innovative Technology for 

Accelerated Construction of Bridge and Embankment Foundations.” 

Von Quintus, H.L., Rao, C., Stubstad, R.N., Prowell, B., Maghsoodloo, S., Maser, K.R., 

Nazarian, S. and Minchen, R.E. (2004). Nondestructive Testing Technology for Quality 

Control and Acceptance of Flexible Pavement Construction, Draft Interim Report, National 

Cooperative Highway Research Program, NCHRP Project 10-65. 

Wisconsin Department of Transportation (2002). Comprehensive Subgrade Deflection 

Acceptance Criteria, Executive Summary. 
FHWA NHI-05-037 Appendix A – Terminology 

Geotechnical Aspects of Pavements A - 1 May 2006 

APPENDIX A. TERMINOLOGY 

The following provides a definition of the pavement components, along with other terms 

common to the geotechnical aspects of pavements as contained in this manual. (Definitions 

were taken from NCHRP 1-37A, where available). The terms and definitions are organized 

under five general headings: 

• Primary Pavement Components 

• Geotechnical Pavement Components 

• Non-Geotechnical Components 

• Design Terminology 

• Pavement Distress and Failure Terminology 

Primary Pavement Components

subgrade - The top surface of a roadbed upon which the pavement structure and shoulders 

are constructed. 

subbase - The layer or layers of specified or selected materials of designed thickness placed 

on a subgrade to support a base course. Note that the layer directly below the PCC slab is 

now called a base layer, not a subbase layer. 

base - The layer or layers of specified or select material of designed thickness placed on a 

subbase or subgrade to support a surface course. The layer directly beneath the PCC slab is 

called the base layer. 

surface course - One or more layers of a pavement structure designed to accommodate the 

traffic load, the top layer of which resists skidding, traffic abrasion, and the disintegrating 

effects of climate. The top layer of flexible pavements is sometimes called the "wearing" 

course. 

Geotechnical Pavement Components

The geotechnical components of a pavement system as covered in this manual include 

unbound granular base, unbound granular subbase, the subgrade or roadbed, aggregate and 

geosynthetics used in drainage systems, graded granular aggregate and geosynthetic used as 
FHWA NHI-05-037 Appendix A – Terminology 

Geotechnical Aspects of Pavements A - 2 May 2006 

separation and filtration layers, and the roadway embankment foundation. Terms related to 

these components are defined as follows. 

aggregate base (AB) - A base course consisting of compacted mineral aggregates. Also, 

granular base (GB), unbound granular base. 

aggregate subbase (ASB) - A subbase course consisting of compacted mineral aggregates. 

Also, granular subbase, unbound granular subbase. 

asphalt-treated permeable base (ATPB) - A base containing a small percentage of asphalt 

cement to enhance stability. 

asphalt-treated permeable base (ATPB) - A permeable base containing a small percentage 

of asphalt cement to enhance stability. Also, asphalt-treated open-graded base (ATOGB), 

asphalt-treated base-permeable (ATB-Perm). 

cement-treated base (CTB) - A base course consisting of mineral aggregates blended in 

place or through a pugmill with a small percentage of Portland cement to provide 

cementitious properties and strengthening. Also, aggregate cement, cement-stabilized graded 

aggregate (CSGA), cement-stabilized base (CSB). 

cement-treated permeable base (CTPB) - An open-graded aggregate base treated with 

Portland cement to provide enhanced base strength and reduce erosion potential. 

crushed stone base - A base course of designed thickness and constructed of graded and 

mechanically crushed mineral aggregate compacted above a subbase course or subgrade. 

Also, aggregate base (AB), graded aggregate base (GAB), and crushed aggregate (CA). 

crushed stone subbase - A subbase course of designed thickness and constructed of graded 

and mechanically crushed mineral aggregate compacted above a subgrade. 

dense-graded aggregate (DGA) - A mechanically crushed, well graded aggregate having a 

particle size distribution such that when it is compacted, the resulting voids between the 

aggregate particles, expressed as a percentage of the total space occupied by the material, are 

relatively small. 

drainable granular subbase - A subbase constructed of compacted and crushed opengraded aggregate. 
FHWA NHI-05-037 Appendix A – Terminology 

Geotechnical Aspects of Pavements A - 3 May 2006 

geogrid (GG) - a geosynthetic formed by a regular network of tensile elements with 

apertures of sufficient size to interlock with surrounding fill material, used primarily as 

reinforcement of base and subbase layers and in stabilization of soft subgrade layers. Also 

used in overlays for asphalt reinforcement. 

geosynthetic - a planar product manufactured from a polymeric material used with soil, rock, 

earth, or other geotechnical-related material as an integral part of a civil engineering project, 

structure, or system. 

geotextile (GT) - a permeable geosynthetic made of textile materials, used as a separator 

between base, subbase and subgrade layers, used as filters in drainage features, and used in 

stabilization of soft subgrade layers. Also used in asphalt overlays as a membrane absorption 

and/or waterproofing layer. 

gravel - Coarse aggregate resulting from natural disintegration and abrasion of rock or 

processing of weakly bound conglomerate. In geotechnical engineering, the particles of rock 

that range in size from 76.2 mm (3-in. U.S. sieve) to 4.75 mm (No. 4 U.S. sieve). To be 

classified as a gravel in the Unified Classification System (UCS), at least 50% of the material 

must be in this range. (Identification and classification of soils is covered in Chapter 5.) 

gravel base - An unbound base course constructed of compacted gravel. May or may not be 

graded and/or crushed. 

gravel subbase - An unbound subbase course constructed of compacted gravel. May or may 

not be graded and/or crushed. 

gravel subgrade - A subgrade where a natural gravel has been used as the roadbed surface or 

where the native soil has been blended with a gravel additive (a.k.a. gravel-treated subgrade 

for the second case). 

lime-treated subgrade - A prepared and mechanically compacted mixture of hydrated lime, 

water, and soil supporting the pavement system. 

lime-flyash base (LFB or LFA) - A blend of mineral aggregate, lime, flyash, and water, 

combined in proper proportions and producing a dense mass when compacted. 

modified or treated base - The addition of cement or asphalt (typically less than 5%) to 

unbound base with the primary purpose of improving the stability for construction (i.e., no 

improvement anticipated for stiffness or structural support). 
FHWA NHI-05-037 Appendix A – Terminology 

Geotechnical Aspects of Pavements A - 4 May 2006 

open-graded aggregate base (OGAB) - A crushed mineral aggregate base having a particle 

size distribution such that when compacted the interstices will provide enhanced drainage 

properties. Also, granular drainable layer, untreated permeable base (UPB). 

permeable base (PB) - A base course constructed of treated or untreated open-graded 

aggregate. Also, free-draining base. 

prefabricated geocomposite edge drain (PGED) - An edgedrain consisting of an extruded 

plastic drainage core covered with a geotextile filter (also known as panel drains or fin 

drains). 

roadbed - The graded portion of a highway between top and side slopes, prepared as a 

foundation for the pavement structure and shoulder. 

roadbed material - The material below the subgrade in cuts and in embankment 

foundations, extending to such depth as affects the support of the pavement structure. 

soil aggregate - Natural or prepared mixtures consisting predominantly of stone, gravel, or 

sand that contain a significant amount of minus 75-μm (No. 200) silt-clay material. 

soil cement - A mechanically compacted mixture of soil, Portland cement, and water, used as 

a layer in a pavement system to reinforce and protect the subgrade or subbase. Also, cementtreated subgrade (CTS). 

stabilized granular base - A base course with an unspecified stabilizing material, usually 

asphalt cement or Portland cement. 

stabilized permeable base - A permeable base with an unspecified stabilizing material, 

usually asphalt cement or Portland cement. Also, bound drainable base. 

subgrade - the top surface of a roadbed upon which the pavement structure and shoulders are 

constructed with the purpose of providing a platform for construction of the pavement and to 

support the pavement without undue deflection that would impact the pavements 

performance (NCHRP 1-37A). In this manual, the natural and/or prepared soil materials 

beneath the pavement structure that deform under pavement loading or otherwise have an 

influence on the support of the pavement (a.k.a. roadbed, pavement foundation). 
FHWA NHI-05-037 Appendix A – Terminology 

Geotechnical Aspects of Pavements A - 5 May 2006 

Unbound base/subbase - compacted mineral aggregate layer that may be either untreated or 

treated, but has not been modified sufficiently to provide an increase in stiffness or strength 

for design. 

Non-Geotechnical Components

As indicated in Section 1.1, the non-geotechnical components are the surficial pavement 

layers, including asphaltic concrete, Portland cement concrete, and bound aggregate layers. 

Terms related to these components are defined as follows: 

asphalt concrete (AC) - A controlled mixture of asphalt cement and graded aggregate 

compacted to a dense mass. Also, hot-mixed asphalt (HMA), hot-mixed asphalt concrete 

(HMAC), bituminous concrete (BC), plant mix (PM). 

asphalt concrete base (ACB) - Asphalt concrete used as a base course. Also, asphalt base 

course (ABC), asphalt-stabilized base, hot-mixed (ASB-HM), asphalt-treated base (ATB), 

bituminous aggregate base, bituminous concrete base (BCB), bituminous base (BB), hotmixed asphalt base (HMAB). 

asphalt concrete pavement (ACP) - A pavement structure placed above a subgrade or 

improved subgrade and consisting of one or more courses of asphalt concrete or a 

combination of asphalt concrete and stabilized or unstabilized aggregate courses. 

asphalt concrete surface (ACS) - Asphalt concrete used as a surface course. Also, densegraded asphalt concrete (DGAC). 

continuously reinforced concrete pavement (CRCP) - Portland cement concrete pavement 

with no transverse joints and containing longitudinal steel in an amount designed to ensure 

holding shrinkage cracks tightly closed. Joints exist only at construction joints and on-grade 

structures. 

flexible pavement - A pavement structure that maintains intimate contact with and 

distributes loads to the subgrade and depends on aggregate interlock, particle friction, and 

cohesion for stability. 

jointed plain concrete pavement (JPCP) - Jointed Portland cement concrete pavement 

containing no distributed steel to control random cracking; may or may not contain joint load 

transfer devices. 
FHWA NHI-05-037 Appendix A – Terminology 

Geotechnical Aspects of Pavements A - 6 May 2006 

jointed reinforced concrete pavement (JRCP) - Jointed Portland cement concrete paving 

containing distributed steel reinforcement to control random cracking and usually containing 

joint load transfer devices. 

lean concrete base (LCB) - A base course constructed of mineral aggregates plant mixed 

with a sufficient quantity of Portland cement to provide a strong platform for additional 

pavement layers and placed with a paver. 

plain concrete - PCC without reinforcing steel. 

Portland cement concrete (PCC) - A composite material consisting of a Portland or 

hydraulic cement binding medium and embedded particles or fragments of aggregate. 

rigid pavement - A pavement structure that distributes loads to the subgrade, having as one 

course a Portland cement concrete slab of relatively high-bending resistance. 

Design Terminology

In the context of current design practice, pavement designers and geotechnical specialists 

must communicate using design terms with consistent definitions. Terms related to design 

as used in this manual include 

analysis period - (a.k.a. performance period) The time period used for comparing design 

alternatives. An analysis period may contain several maintenance and rehabilitation activities 

during the life cycle of the pavement being evaluated. 

average annual daily traffic (AADT) - The estimate of typical traffic on a road segment for 

all days of the week over the period of a year. 

average annual daily truck traffic (AADTT) - The estimate of typical truck traffic on a 

road segment for all days of the week over the period of a year. 

axle load - The sum of all tire loads on an axle. 

axle load spectrum - The full spectrum (distribution) of single, dual, triple, and other axle 

loads applied to a pavement structure by a given traffic stream. 
FHWA NHI-05-037 Appendix A – Terminology 

Geotechnical Aspects of Pavements A - 7 May 2006 

bound base - The addition of a sufficient amount of cement or asphalt to change the long 

term stiffness and structural characteristics of unbound base to that of lean concrete. 

design life - The length of time for which a pavement structure is being designed, including 

the time from construction until major programmed rehabilitation. 

elastic layer theory - A mathematical process wherein the layers of a pavement structure are 

all assumed to behave elastically. 

equivalent single axle load (ESAL) - A numerical factor that expresses the relationship of a 

given axle load to another axle load in terms of the relative effects of the two loads on the 

serviceability of a pavement structure. Often expressed in terms of 18,000-pound (80 kN) 

single axle loads. 

finite element analysis - The finite element method is one wherein rigorous mathematical 

solution, often employing complex differential equations, of an engineering problem is 

approximated algebraically. The geometry of the problem is described by discrete elements 

of finite dimensions that are analyzed through the application of engineering mechanics 

principles. Results of the finite element analyses are aggregated to approximate the exact 

mathematical solution. 

international roughness index (IRI) - A pavement roughness index computed from a 

longitudinal profile measurement using a quarter-car simulation at a simulation speed of 

50 mph (80 km/h). 

life-cycle cost analysis (LCCA) - An economic assessment of an item, area, system, or 

facility and competing design alternatives considering all significant costs of ownership over 

the economic life (which encompasses several analysis periods), expressed in equivalent 

dollars. 

longitudinal profile - The perpendicular deviations of the pavement surface from an 

established reference parallel to the lane direction, usually measured in the wheel tracks. 

mechanistic-empirical (M-E) - A design philosophy or approach wherein classical 

mechanics of solids is used in conjunction with empirically derived relationships to 

accomplish the design objectives. 
FHWA NHI-05-037 Appendix A – Terminology 

Geotechnical Aspects of Pavements A - 8 May 2006 

pavement performance - Measure of accumulated service provided by a pavement (e.g., the 

adequacy with which it fulfills it purpose). Often referred to as the record of pavement 

condition or serviceability over time or with accumulated traffic. 

performance period - The period of time that an initially constructed or rehabilitated 

pavement structure will last (perform) before reaching its terminal condition when 

rehabilitation is performed. This is also referred to as the design period. 

present serviceability index (PSI) - An index derived by formula for estimating the 

serviceability rating from measurements of physical features of the pavement. 

present serviceability rating (PSR) - A mean rating of the serviceability of a pavement 

(traveled surface) established by a panel rating under controlled conditions. The usual scale 

for highways is 0 to 5, with 5 being excellent. 

reliability - The probability that a given pavement design will last for the anticipated design 

performance period. 

rideability - A subjective judgment of the comparative discomfort induced by traveling over 

a specific section of highway pavement in a vehicle. 

serviceability - The ability at time of observation of a pavement to serve traffic (autos and 

trucks) that uses the facility. 

single axle load - The total load transmitted by all wheels whose centers may be included 

between two parallel transverse vertical planes 1 m (40 in.) apart, extending across the full 

width of the vehicle. 

tandem axle load - The total load transmitted to the pavement by two consecutive axles 

whose centers may be included between parallel vertical planes. 

traffic growth factor - A factor used to describe the annual growth rate of traffic volume on 

a roadway. 

 

transverse profile - The vertical deviations of the pavement surface from a horizontal 

reference perpendicular to the lane direction. 
FHWA NHI-05-037 Appendix A – Terminology 

Geotechnical Aspects of Pavements A - 9 May 2006 

user costs - Those costs realized by the users of a facility. In a life cycle cost analysis, user 

costs could take the form of delay costs or of changes in vehicle operating costs associated 

with various alternatives. 

weigh-in-motion (WIM) - The process of estimating a moving vehicle's gross weight and 

the portion of that weight that is carried by each wheel, axle, or axle group, or combination 

thereof, by measurement and analysis of dynamic forces. 

wheel load - The sum of the tire loads on all tires included in the wheel assembly comprising 

a half axle. 

zero-stress temperature - temperature (after placement and during curing) at which the 

concrete layer exhibits zero thermal stress (at temperatures less than this, concrete exhibits 

tensile stress). 

Pavement Distress and Failure Terminology

Distress refers to conditions that reduce serviceability or indicate structural deterioration. 

Failure is a relative term. In the context of this manual, failure denotes a pavement section 

that experiences excessive rutting or cracking that is greater than anticipated during the 

performance period or that a portion of the pavement is structurally impaired at any time 

during the performance period with incipient failure anticipated from the local distress. There 

are a number of ways that a pavement section can fail. 

alligator cracking - Interconnected or interlaced cracks forming a pattern that resembles an 

alligator's hide. Also, map cracking. 

blowup - An upward eruption of a PCC pavement slab near a crack or joint. 

crack - A fissure or discontinuity in the pavement surface not necessarily extending through 

the entire thickness of the pavement. 

fatigue cracking - Cracking of the pavement surface as a result of repetitive loading; may be 

manifested as longitudinal or alligator cracking in the wheel paths for flexible pavement and 

transverse cracking (and sometimes longitudinal cracking) for jointed concrete pavement. 

faulting - Elevation or depression of a PCC slab in relation to an adjoining slab, usually at 

transverse joints and cracks. 
FHWA NHI-05-037 Appendix A – Terminology 

Geotechnical Aspects of Pavements A - 10 May 2006 

liquefaction - the process of transforming any soil from a solid state to a liquid state, usually 

as a result of increased pore pressure and reduced shearing resistance (ASTM, 2001). 

Spontaneous liquefaction may be caused by a collapse of the structure by shock or other type 

of strain and is associated with a sudden but temporary increase of the prefluid pressure. 

longitudinal cracking - Pavement cracking predominantly parallel to the direction of traffic. 

pumping - The ejection of foundation material, either wet or dry, through joints or cracks, or 

along edges of rigid slabs resulting from vertical movements of the slab under traffic, or from 

cracks in semi-rigid pavements. 

punchouts - A broken area of a CRCP bounded by closely spaced cracks usually spaced less 

than 1 m (3 ft). 

random cracking - Uncontrolled and irregular fracturing of a pavement layer. 

raveling - A pavement distress characterized by the loss of surface material involving the 

dislodgment of aggregate particles and degradation of the binder material. 

reflective cracking - Cracks in asphalt or concrete surfaces of pavements, occurring over 

joints or cracks in the underlying layers. 

rutting - Longitudinal depression or wearing away of the pavement in wheel paths under 

load. 

spalling - The cracking, breaking, or chipping of pavement edges in the vicinity of a joint or 

crack. 

thermal cracking - Cracks in an asphalt pavement surface, usually full width transverse, as a 

result of seasonal or diurnal volume changes of the pavement restrained by friction with an 

underlying layer. 

transverse cracking - Pavement cracking predominantly perpendicular to the direction of 

traffic. 

warping - Deformation of a PCC slab caused by a moisture or temperature differential 

between the upper and lower surfaces. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 1 May 2006

APPENDIX B: MAIN HIGHWAY PROJECT 

B.1 INTRODUCTION 

The Main Highway, the example project for the design exercises in this manual, involves the 

reconstruction and upgrading of an existing county road in the upper northeastern United 

States. This appendix summarizes the geotechnical and other data available for the design of 

this project. 

A topographic map showing the project horizontal alignment is shown in Figure B-1. The 

length of the entire reconstruction project is 1.9 miles. The project subsurface investigation 

consisted of 10 test pits, 10 power auger borings, 5 hand auger borings, and 21 hand rod 

soundings. Seventeen soil samples were collected and tested in the laboratory. 

For the purposes of the design examples in this manual, only a 1500-foot-long subsection of 

the entire project will be considered. This subsection between Sta. 255+00 and 270+00 is 

indicated in Figure B-2. Subsequent sections of this appendix summarize the relevant design 

data for this subsection. 

B.2 SUBSURFACE CONDITIONS 

A soils map for the site is shown in Figure B-2. Detailed plan and profile views of the 

subsection of interest are shown in Figure B-3 through Figure B-6. The locations of the 

various borings, test pits, and soundings are also indicated on these figures. Logs from 

borings within the subsection of interest are summarized in Figure B-7 through Figure B-14. 

Similar observations from the test pits are provided in Figure B-15 through Figure B-17. 

B.2.1 General Conditions 

The dominant soil type along this project is anticipated to consist of clay silt, with some 

remnants of the existing base material from the existing county road. The natural subgrade 

soils are all plastic and have been classified as an AASHTO A-4 or A-6 soil, with a frost 

rating of IV or III. Several undesirable soil conditions may be encountered along this project. 

These include soil support, drainage, and slumping. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 2 May 2006

Soil Support. The group index value is used as a general guide to the load bearing 

characteristics of a soil. It is a function of the liquid limit, plasticity index, and the amount of 

material passing the No. 200 sieve. Zero indicates good subgrade material, whereas a group 

index value of 20 or more indicates a poor subgrade material. Group index values obtained 

on the moist clay silts along this project ranged between 16.3 and 27.3, with an average of 

21.6. CBR tests run on three samples (37791, 90021, 900221) produced soil support values of 

2.6, 2.8, and 2.2, respectively. Based on this, it is anticipated that these soils will have a low 

bearing capacity resulting in poor soil support. It is anticipated that a standard 30 inch section 

will not provide adequate structural support. Due to the poor bearing capacity of the clay 

soils, it has been determined that an 18 inch lift of granular material will be required in 

addition to the structural aggregate base member. This 18 inch granular lift will provide a 

working platform for construction equipment to operate upon. 

Drainage. Drainage of base and subgrade soils is extremely important along this project due 

to the presence of clay silt soils along the subgrade. The strength of the clay soils along this 

project will be dramatically affected by the presence of water. These soils have a high 

volume change between wet and dry states and will shrink and swell with changes in 

moisture content. Clay silt soils have a high dry strength but lose much of this strength upon 

absorbing water. Unfortunately, these soils are poorly drained and may absorb water by 

capillary action, resulting in low bearing capacity. 

Slumping. The clay silt soils along this project have a tendency to slump with slopes greater 

than 2:1. At times, depending on seasonal conditions, these soils may even slump at slopes 

shallower than 2:1. It is anticipated that stone ditch protection and stone protection of some 

backslopes will be required. Vegetation of all exposed soil areas will be very important. 

Additional relevant comments and observations from the geotechnical investigation report 

for this project are as follows: 

• It is recommended that construction activities requiring heavy equipment operation 

on the native subgrade soils not be attempted during early to mid spring due to 

anticipated moist, soft soils. 

• No substantial embankments or cuts are proposed along the project. However, some 

small embankments or cuts (less than about 5 ft) are proposed over/through the clay 

and silty clay native soils. 

 

1

 From locations outside of study section. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 3 May 2006

• Bedrock was not encountered along the project and thus no rock excavation or 

shallow rock conditions are anticipated. 

B.2.2 Detailed Conditions along Study Section (Sta. 255+00 to 270+00) 

This section consists of several cut and fill areas with a maximum cut of 4.0 feet and a 

maximum fill of 3.4 feet along the proposed centerline. Field explorations within this section 

consist of 2 power auger borings, 3 test pits, and 3 hand auger borings. 

Earth excavation is anticipated to consist of existing base material (403) and clay silt (90021, 

3777). The base material is classified as an AASHTO A-1-b soil, with a frost class of I. The 

clay silts are plastic and have been classified as A-6 soils, with a frost class of III. Depending 

on field conditions with respect to moisture, the clay silts along the surface may be moist to 

wet and softer than the underlying clay silts. 

Subgrade soils are anticipated to consist of existing base material (403) and clay silt (90021, 

3777). It is anticipated that most of the clay silts encountered at subgrade within the proposed 

cut sections may be a little firmer than the overlying clay silts. However, this is based upon 

the seasonal conditions at the time of the field explorations, and these conditions could 

change dramatically by the time of construction. 

As previously discussed in other sections, drainage is extremely important with respect to the 

load bearing characteristics of the clay silt subgrade soils. The existing poor pavement 

conditions in this section can be attributed to the lack of adequate drainage and poor 

subgrade soils. It has been recommended that all proposed ditching be deepened to 2 feet 

below proposed subgrade wherever possible. 

B.2.3 Laboratory Test Data 

The sample log for all test specimens along the study subsection (Sta. 255+00 to 270+00) is 

given in Table B-1. The following laboratory test information is available: 

• Gradation curves for several samples, including some from outside the study section 

(Figure B-18 and Figure B-19). 

• Compaction curves and CBR value for the clayey silt subgrade (Figure B-20 and 

Figure B-21). 

• Compaction curves for the granular working pad (Figure B-22). 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 4 May 2006

• Laboratory resilient modulus values for the clayey silt subgrade (Table B-2) and the 

granular base material (Table B-3). Table B-4 summarizes the corresponding stressdependent resilient modulus parameters k1, k2, and k3 for the NCHRP 1-37A level 1 

MR relation: 

2 3

1 1

k k

oct

R a

a a

M kp

p p

   θ τ

= +      

in which 

MR = subgrade resilient modulus (same units as pa) 

θ = bulk stress = σ1+σ2+σ3 (same units as pa) 

τoct = octahedral shear stress = ( )( )( ) 2 22

12 23 13

1

3

σσ σσ σσ − +− +−

 (same units as pa) 

pa = atmospheric pressure (to make equation dimensionless) 

k1,k2,k3 = material properties with constraints k1>0, k2 >0, k3 <0 

B.2.4 Field Test Data 

Several sets of FWD tests were performed at various times during the years prior to 

construction. These tests were used to backcalculate the subgrade resilient modulus and 

pavement effective modulus. These are summarized in Table B-5. The subgrade resilient 

modulus MR is calculated from the FWD results using the standard AASHTO equations: 

2

2 3

0.24

0.7

R

r

e

p

e

R

P M

d r

r a

E

a aD

M

=

≥

   

= +            

in which 

MR = back-calculated subgrade resilient modulus (psi) 

P = applied load (pounds) 

dr = deflection at a distance r from the center of the load (inches) 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 5 May 2006

r = distance from the center of the load (inches) 

ae = radius of the stress bulb at the subgrade-pavement interface (inches) 

a = load plate radius (inches) 

D = total pavement thickness above the subgrade (inches) 

The effective pavement modulus Ep is determined from: 

2

0 2

3

1 1

1

1 1.5

1

p

p

R

R

D

a d pa E

D E

M

a M

  −     +     = +  

        +          

in which 

d0 = deflection measured at the center of the load plate, adjusted to a 

 standard temperature of 68o

F (inches) 

p = load plate pressure (psi) 

and the other terms as previously defined. Subgrade resilient modulus for design purposes is 

usually less than the value back-calculated from FWD data. The AASHTO design guide 

recommends a design subgrade modulus equal to 33% of the FWD value for flexible 

pavements and 25% of the FWD value for rigid pavements. 

B.3 ENVIRONMENTAL CONDITIONS 

The site is located in the northern United States in a cold and wet climate. Freezing Index and 

frost penetration estimates for the project site are summarized in Table B-6. 

B.4 TRAFFIC 

Traffic estimates for use in design are summarized in Table B-7. The average traffic level is 

approximately 750 ESALs per day. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 6 May 2006

Begin Project Sta. 208+30

End Project Sta. 304+00

Study Area: Sta: 255+00 – 270+00

Figure B-1. Project alignment. 

BaC: silt loam, 8-15% slopes

BaD: silt loam, 15-25% slopes

BbD: very stony silt loam,

15-25% slopes

BoB: silt loam, 3-5% slopes

BoC: silt loam, 8-15% slopes

BoD: silt loam, 15-25% slopes

BoE3: silt loam, 25-45% slopes,

severely eroded

DyB: very stony silt loam,

8-15% slopes

MkB: fine sandy loam, 0-8% slopes

MkC: fine sandy loam, 8-15% slopes

Py: fine sandy loam

Sw: silt loam

BaC: silt loam, 8-15% slopes

BaD: silt loam, 15-25% slopes

BbD: very stony silt loam,

15-25% slopes

BoB: silt loam, 3-5% slopes

BoC: silt loam, 8-15% slopes

BoD: silt loam, 15-25% slopes

BoE3: silt loam, 25-45% slopes,

severely eroded

DyB: very stony silt loam,

8-15% slopes

MkB: fine sandy loam, 0-8% slopes

MkC: fine sandy loam, 8-15% slopes

Py: fine sandy loam

Sw: silt loam

Figure B-2. Soils map for project site. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 7 May 2006

Project Start:255+00

Power Auger

Hand Auger

Boring and Sounding

Test Pit

255+50

255+90

255+10

257+50

260+00 262+00

Project Start:255+00

Power Auger

Hand Auger

Boring and Sounding

Test Pit

255+50

255+90

255+10

257+50

260+00 262+00

Figure B-3. Horizontal alignment with subsurface exploration locations: Sta. 255+00 to 263+50. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 8 May 2006

Project End

270+00

Power Auger

Hand Auger

Boring and Sounding

Test Pit

264+07

265+50

268+00

271+00

Project End

270+00

Power Auger

Hand Auger

Boring and Sounding

Test Pit

264+07

265+50

268+00

271+00

Figure B-4. Horizontal alignment with subsurface exploration locations: Sta. 263+50 to 270+00. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 9 May 2006

Project Start:255+00

12 

16

3

Organic

Brown silty sandy gravel

Gray/brown sandy clay silt (soft/moist)

Gray/brown sandy clay silt (firm)

# SPT N value

255+10

255+50

255+90

257+50

260+00

262+00

Figure B-5. Vertical alignment with subsurface exploration findings: Sta. 255+00 to 263+50. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 10 May 2006

Project End

270+00

11 

3

Organic

Brown silty sandy gravel

Gray/brown sandy clay silt (soft/moist)

Gray/brown sandy clay silt (firm)

# SPT N value

264+07

265+50

268+00

270+00

Figure B-6. Vertical alignment with subsurface exploration findings: Sta. 263+50 to 270+00. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 11 May 2006

Figure B-7. Boring log, station 255+10. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 12 May 2006

Figure B-8. Boring log, station 257+50. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 13 May 2006

Figure B-9. Boring log, station 260+00. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 14 May 2006

Figure B-10. Boring log, station 262+00. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 15 May 2006

Figure B-11. Boring log, station 265+50. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 16 May 2006

Figure B-12. Boring log, station 268+00. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 17 May 2006

Figure B-13. Boring log, station 270+00. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 18 May 2006

Figure B-14. Boring log, station 270+10. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 19 May 2006

Figure B-15. Test pit log, station 255+50. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 20 May 2006

Figure B-16. Test pit log, station 255+90. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 21 May 2006

Figure B-17. Test pit log, station 264+07.
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 22 May 2006

Figure B-18. Grain size distribution curves. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 23 May 2006

Figure B-19. Grain size distribution curves.
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 24 May 2006

3777

Figure B-20. Compaction curves for clayey silt subgrade soil (sample 90021).
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 25 May 2006

Figure B-21. Compaction curves for clayey silt subgrade soil (sample 90021). 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 26 May 2006

Figure B-22. Compaction curves for granular working platform. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 27 May 2006

Table B-1. Sample log. 

STATION OFFSET DEPTH SAMPLE 

No.

W.C L.L. P.I. IGN. pH.

AASHTO FROST

257+50 5R 8.0 - 9.2 90021 24 35.2 14.1 6.5 A-6 III

264+07 20L 4.0 - 5.0 3777 24 27.5 9.1 6.5 A-4 IV

265+50 30R 0.3 - 0.8 234 24 26.1 5.6 A-4 IV

265+50 30R 0.8 - 1.8 232 92 65.1 29.4 16.6 A-7-5 III

270+00 35L 1.2 - 2.2 233 23 22 6.4 A-4 IV

CLASSIFICATION

Classification of these soil samples is in accordance with AASHTO Classification System M-145-40. This Classification

is followed by the "Frost Susceptibility Rating"from zero (non-frost susceptible) to Class IV (highly frost susceptible).

The 'Frost Susceptible Rating"is based upon the Corps of Engineers Classification Systems.

Table B-2. Laboratory resilient modulus data for clay silt subgrade (sample 3777). 

Replicate 

Contact 

Stress 

(psi) 

Confining 

Pressure 

(psi) 

Cyclic 

Stress 

(psi) 

θ

(psi) 

τoct 

(psi) 

MR

(psi) 

1 

0.620 

0.743 

0.307 

0.151 

0.636 

0.455 

0.307 

0.184 

8.032 

6.099 

4.107 

2.043 

8.032 

5.969 

3.972 

2.019 

0.946 

0.839 

0.806 

0.757 

1.431 

1.390 

1.316 

1.209 

25.660 

19.880 

13.433 

7.037 

26.164 

19.752 

13.537 

7.450 

0.738 

0.746 

0.525 

0.428 

0.974 

0.870 

0.765 

0.656 

1450 

1417 

1320 

1152 

1094 

1046 

971 

888 

2 

0.603 

0.447 

0.299 

0.167 

0.620 

0.447 

0.307 

0.167 

8.045 

6.186 

4.139 

2.207 

8.001 

6.082 

4.059 

2.010 

0.880 

0.880 

0.831 

0.773 

1.464 

1.431 

1.324 

1.225 

25.618 

19.886 

13.547 

7.560 

26.087 

20.124 

13.808 

7.423 

0.699 

0.626 

0.532 

0.443 

0.982 

0.885 

0.769 

0.656 

1703 

1675 

1519 

1337 

1289 

1240 

1125 

1013 

3 

0.603 

0.463 

0.307 

0.151 

0.603 

0.463 

0.332 

0.200 

8.088 

6.030 

4.036 

2.028 

8.025 

6.083 

4.097 

2.012 

0.806 

0.789 

0.724 

0.674 

1.357 

1.316 

1.250 

1.143 

25.674 

19.342 

13.140 

6.909 

26.036 

20.027 

13.873 

7.381 

0.664 

0.591 

0.486 

0.389 

0.924 

0.839 

0.746 

0.633 

1351 

1341 

1177 

1046 

1025 

970 

898 

808 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 28 May 2006

Table B-3. Laboratory resilient modulus data for granular base material. 

Replicate 

Contact 

Stress 

(psi) 

Confining 

Pressure 

(psi) 

Cyclic 

Stress 

(psi) 

θ

(psi) 

τoct

(psi) 

MR

(psi) 

1 

0.60 

1.17 

2.04 

3.07 

4.02 

0.60 

1.16 

2.04 

3.00 

4.01 

0.61 

1.20 

2.04 

3.03 

4.05 

0.63 

1.19 

2.06 

3.02 

4.01 

0.61 

1.22 

2.08 

3.00 

4.05 

3.07 

5.96 

9.98 

15.08 

20.08 

3.06 

6.00 

9.97 

14.91 

20.02 

3.03 

6.07 

10.03 

14.98 

20.10 

2.97 

6.06 

10.00 

15.04 

19.99 

2.99 

5.95 

10.03 

15.04 

20.03 

0.72 

2.51 

4.89 

8.07 

11.35 

2.17 

5.54 

10.54 

17.48 

23.99 

4.84 

12.30 

23.44 

35.69 

46.70 

7.56 

20.15 

35.35 

51.98 

67.65 

14.79 

35.32 

58.14 

84.55 

108.67 

10.55 

21.55 

36.87 

56.36 

75.60 

11.96 

24.69 

42.48 

65.21 

88.07 

14.55 

31.71 

55.58 

83.66 

111.03 

17.09 

39.52 

67.41 

100.11 

131.64 

24.37 

54.39 

90.30 

132.69 

172.83 

0.62 

1.74 

3.27 

5.25 

7.25 

1.31 

3.16 

5.93 

9.66 

13.20 

2.57 

6.36 

12.01 

18.25 

23.92 

3.86 

10.06 

17.64 

25.93 

33.78 

7.26 

17.23 

28.38 

41.27 

53.14 

17623 

19768 

27007 

34378 

43204 

10960 

16819 

23760 

33003 

42197 

10496 

17847 

27008 

37326 

44990 

10734 

20036 

28956 

37193 

44559 

12506 

21959 

29538 

38762 

49332 

2 

0.61 

1.23 

2.03 

3.00 

4.03 

0.50 

1.21 

2.08 

3.06 

4.08 

0.59 

1.21 

2.00 

3.02 

6.03 

10.04 

15.02 

20.06 

3.06 

6.06 

10.01 

15.04 

19.93 

3.03 

5.85 

10.00 

1.29 

2.63 

4.91 

7.74 

10.84 

2.59 

5.89 

10.13 

16.02 

21.30 

4.90 

11.73 

21.36 

10.97 

21.96 

37.06 

55.82 

75.04 

12.26 

25.28 

42.23 

64.20 

85.17 

14.57 

30.48 

53.37 

0.90 

1.82 

3.27 

5.07 

7.01 

1.46 

3.35 

5.75 

9.00 

11.96 

2.59 

6.10 

11.01 

7386 

11375 

20089 

28547 

38724 

6802 

12349 

20448 

30563 

39487 

7355 

13987 

23784 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 29 May 2006

Replicate 

Contact 

Stress 

(psi) 

Confining 

Pressure 

(psi) 

Cyclic 

Stress 

(psi) 

θ

(psi) 

τoct

(psi) 

MR

(psi) 

3.04 

4.02 

0.60 

1.20 

2.01 

3.03 

4.06 

0.61 

1.21 

2.02 

3.02 

4.09 

15.05 

20.06 

3.02 

6.06 

10.02 

14.93 

20.01 

2.97 

6.05 

9.93 

15.02 

19.97 

32.28 

42.26 

7.81 

18.79 

32.66 

47.85 

62.46 

14.35 

32.38 

53.30 

76.57 

101.06 

80.49 

106.47 

17.46 

38.16 

64.72 

95.66 

126.55 

23.88 

51.73 

85.12 

124.65 

165.06 

16.65 

21.82 

3.97 

9.42 

16.34 

23.98 

31.36 

7.05 

15.84 

26.08 

37.52 

49.57 

34193 

41623 

8253 

16909 

26331 

35554 

43168 

11096 

20945 

30242 

39709 

51042 

Table B-4. Stress-dependent resilient modulus parameters. 

Soil k1 k2 k3

Clay silt subgrade 170 0.450 -16.388 

Granular base 662 1.010 -0.585 

Table B-5. Field backcalculated values for subgrade modulus and effective pavement modulus. 

Section MR (psi) Ep (psi)

255+00 - 261+50 3000 25714

261+50 - 268+00 3857 27142

268+00 - 269+00 2571 27142

Table B-6. Freezing index and frost penetration estimates 

(assuming 32 inches of pavement and base). 

Total Frost Penetration Frost Penetration into Subgrade

Freezing Index Nongranular 

Subgrade 

Granular 

Subgrade 

Nongranular 

Subgrade 

Granular 

Subgrade 

Mean 1200 46 in. 65 in. 14 in. 33 in. 

Design 1700 53 in. 85 in. 21 in. 15 in. 
FHWA NHI-05-037 Appendix B – Main Highway Project 

Geotechnical Aspects of Pavements B - 30 May 2006

Table B-7. Design traffic. 

Vehicle Class 

Est. AADT 

(2 way) 

Est. AADT 

Percentage ESAL Factor Design ESALs 

4 90 1.51% 0.4700 21.15 

5 297 4.99% 0.3000 44.55 

6 361 6.04% 1.3300 240.7 

7 111 1.86% 3.3600 186.48 

8 118 1.96% 0.8600 50.74 

9 127 2.13% 1.0900 69.22 

10 79 1.32% 2.8800 113.76 

11 0 0.00% 1.0000 0.00 

12 0 0.00% 1.0000 0.00 

13 0 0.00% 3.7500 0.00 

Light Vehicles 4727 80.19% 0.0008 1.89 

All Vehicles 5910 100.00% 727.85 
FHWA NHI-05-037 Appendix C – 1993 AASHTO Design Method 

Geotechnical Aspects of Pavements C - 1 May 2006

APPENDIX C: 1993 AASHTO DESIGN METHOD 

C.1 INTRODUCTION 

The AASHTO Guide for Design of Pavement Structures (AASHTO, 1993) is the primary 

document used to design new and rehabilitated highway pavements. Approximately 80% of 

all states use the AASHTO pavement design procedures, with the majority using the 1993 

version. All versions of the AASHTO Design Guide are empirical design methods based on 

field performance data measured at the AASHO Road Test in 1958-60. 

Chapter 3 of this manual describes the evolution of the various versions of the AASHTO 

Design Guide. Geotechnical inputs to the 1993 AASHTO design procedure are detailed in 

Chapter 5. Chapter 6 provides some design examples using the 1993 AASHTO procedures. 

The overall approach of the 1993 AASHTO procedure for both flexible and rigid pavements 

is to design for a specified serviceability loss at the end of the design life of the pavement. 

Serviceability is defined in terms of the Present Serviceability Index, PSI, which varies 

between the limits of 5 (best) and 0 (worst). Serviceability loss at end of design life, ∆PSI, is 

partitioned between traffic and environmental effects, as follows (see also Figure 3.8): 

∆ =∆ +∆ +∆ PSI PSI PSI PSI TR SW FH (C.1) 

in which ∆PSITR, ∆PSISW and ∆PSIFH are the components of serviceability loss attributable to 

traffic, swelling, and frost heave, respectively. The structural design procedures for swelling 

and frost heave are the same for both flexible and rigid pavements; these are detailed in 

Appendix G of the 1993 AASHTO Guide. The structural design procedures for traffic are 

different for flexible and rigid pavement types. These procedures are summarized below in 

Sections C.2 and C.3, respectively. For simplicity, only the design procedures for new 

construction are summarized here. The design procedures for reconstruction are similar, 

except that characterization of recycled materials may be required. See the 1993 AASHTO 

Guide for details of additional procedures (e.g., determination of remaining structural life for 

overlay design) relevant to rehabilitation design. 
FHWA NHI-05-037 Appendix C – 1993 AASHTO Design Method 

Geotechnical Aspects of Pavements C - 2 May 2006

C.2 FLEXIBLE PAVEMENT STRUCTURAL DESIGN 

Design Equation 

The empirical expression relating traffic, pavement structure, and pavement performance for 

flexible pavements is: 

( ) ( )

( )

( )

log 9.36log 1 0.20 10 18 0 10

log10 4.2 1.5 2.32log 8.07 1094 10 0.40 5.19 1

W Z S SN R

PSI

MR

SN

= + +−

  ∆

    − + +−

+

+

 (C.2) 

in which: 

W18 = number of 18 kip equivalent single axle loads (ESALs) 

 ZR = standard normal deviate (function of the design reliability level) 

S0 = overall standard deviation (function of overall design uncertainty) 

∆PSI = allowable serviceability loss at end of design life 

MR = subgrade resilient modulus 

SN = structural number (a measure of required structural capacity) 

The first five parameters typically are the inputs to the design equation, and SN is the output. 

Equation (C.2) must be solved implicitly for the structural number SN as a function of the 

input parameters. The structural number SN is defined as: 

11 2 22 333 SN a D a D m a D m =+ + (C.3) 

in which D1, D2, and D3 are the thicknesses (inches) of the surface, base, and subbase layers, 

respectively, a1, a2, and a3 are corresponding structural layer coefficients, and m2 and m3 are 

drainage coefficients for the base and subbase layers, respectively. Equation (C.3) can be 

generalized for additional bound and/or unbound layers. Note that there may be many 

combinations of layer thicknesses that can provide satisfactory SN values; cost and other 

issues must be considered to determine the optimal final design. 
FHWA NHI-05-037 Appendix C – 1993 AASHTO Design Method 

Geotechnical Aspects of Pavements C - 3 May 2006

Design Inputs 

Analysis Period 

Performance period refers to the time that a pavement design is intended to last before it 

needs rehabilitation. It is equivalent to the time elapsed as a new, reconstructed, or 

rehabilitated pavement structure deteriorates from its initial serviceability to its terminal 

serviceability. The term “analysis period” refers to the overall duration that the design 

strategy must cover. It may be identical to the performance period. However, realistic 

performance limitations may require planned rehabilitation within the desired analysis 

period, in which case, the analysis period may encompass multiple performance periods. 

Analysis period in this context is synonymous with design life in the 1993 AASHTO Guide. 

AASHTO recommendations for analysis periods for different types of roads are summarized 

in Table C-1. 

Table C-1. Guidelines for length of analysis period (AASHTO, 1993). 

Highway conditions Analysis period (years) 

High-volume urban 30 – 50 

High-volume rural 20 – 50 

Low-volume paved 15 – 25 

Low-volume aggregate surface 10 – 20 

Traffic 

Traffic is one of the most important factors in pavement design, and every effort should be 

made to collect accurate data specific to each project. Traffic analysis requires the evaluation 

of initial traffic volume, traffic growth, directional distribution, and traffic type. 

The AASHTO Design Guide is based on cumulative 18 kip (80 KN) equivalent single-axle 

loads (ESALs). Detailed traffic analysis is beyond the scope of this reference manual. 

However, ESALs may be estimated using the following equation: 

0 ( )( )( )( )( )( )(365)( ) ESAL ADT T T G D L Y = f (C.4) 

in which: 

ADT0 = average daily traffic at the start of the design period 

T = percentage of trucks in the ADT 

Tf = truck factor, or the number of 18 kip ESALs per truck 
FHWA NHI-05-037 Appendix C – 1993 AASHTO Design Method 

Geotechnical Aspects of Pavements C - 4 May 2006

G = traffic growth factor 

D = directional distribution factor 

L = lane distribution factor 

Y = design period in years 

AASHTO (1993) and standard pavement engineering textbooks (e.g, Huang, 2004) provide 

details on the determination of all of these parameters and estimation of design ESALs. 

Reliability 

Design reliability is defined as the probability that a pavement section will perform 

satisfactorily over the design period. It must account for uncertainties in traffic loading, 

environmental conditions, and construction materials. The AASHTO design method accounts 

for these uncertainties by incorporating a reliability level R to provide a factor of safety into 

the pavement design and thereby increase the probability that the pavement will perform as 

intended over its design life. The levels of reliability recommended by AASHTO for various 

classes of roads are summarized in Table C-2. 

The reliability level is not included directly in the AASHTO design equations. Rather, it is 

used to determine the standard normal deviate ZR. Values of ZR corresponding to selected 

levels of reliability are summarized in Table C-3. 

The AASHTO design equations also require specification of the overall standard deviation 

S0. For flexible pavements, values for S0 typically range between 0.35 and 0.50, with a value 

of 0.45 commonly used for design. 

Table C-2. Suggested levels of reliability for various functional classifications 

(AASHTO, 1993). 

Recommended level of reliability Functional classification 

Urban Rural 

Interstate and other freeways 85 – 99.9 80 – 99.9 

Principal arterials 80 – 99 75 – 95 

Collectors 80 – 95 75 – 95 

Local 50 – 80 50 – 80 

Note: Results base on a survey of AASHTO Pavement Design Task Force. 
FHWA NHI-05-037 Appendix C – 1993 AASHTO Design Method 

Geotechnical Aspects of Pavements C - 5 May 2006

Table C-3. Standard normal deviates for various levels of reliability. 

Reliability (%) Standard normal 

deviate (ZR) Reliability (%) Standard normal 

deviate (ZR) 

50 0.000 93 -1.476 

60 -0.253 94 -1.555 

70 -0.524 95 -1.645 

75 -0.674 96 -1.751 

80 -0.841 97 -1.881 

85 -1.037 98 -2.054 

90 -1.282 99 -2.327 

91 -1.340 99.9 -3.090 

92 -1.405 99.99 -3.750 

Serviceability 

Serviceability is quantified by the Present Serviceability Index, PSI. Although PSI 

theoretically ranges between 5 and 0, the actual range for real pavements is between about 

4.5 to 1.5. 

The initial serviceability index po corresponds to road conditions immediately after 

construction. A typical value of po for flexible pavements is 4.2. The terminal serviceability 

index pt is defined as the lowest serviceability that will be tolerated before rehabilitation or 

reconstruction becomes necessary. A terminal serviceability index of 2.5 or higher is 

recommended for design of major highways. Thus, a typical allowable serviceability loss due 

to traffic for flexible pavements can be expressed as: 

 4.2 2.5 1.7 ∆ =−= − = PSI p p t o (C.5) 

Subgrade Resilient Modulus 

Pavement subgrade quality is defined in terms of its resilient modulus MR. The resilient 

modulus MR is a basic material property that can be measured directly in the laboratory, 

evaluated in-situ from nondestructive tests, or estimated using various empirical relations as 

detailed in Chapter 5. The 1993 AASHTO Design Guide also incorporates a procedure for 

considering seasonal fluctuations in MR to determine a seasonally averaged value for use in 

design. This procedure is summarized in Section 5.4.3. 
FHWA NHI-05-037 Appendix C – 1993 AASHTO Design Method 

Geotechnical Aspects of Pavements C - 6 May 2006

Layer Properties 

The material properties required for each layer are the structural layer coefficients ai and, for 

unbound materials, the drainage coefficients mi. Methods for evaluating the ai and mi values 

for unbound materials are detailed in Sections 5.4.5 and 5.5.1, respectively. The chart in 

Figure C-1 can be used to estimate the structural layer coefficient for asphalt concrete in 

terms of its elastic modulus at 68o

F. Values of a1 between 0.4 and 0.44 are typically used for 

dense graded asphalt concrete. 

Figure C-1. Chart for estimating structural layer coefficient of dense-graded asphalt concrete 

based on the elastic (resilient) modulus (AASHTO, 1993). 
FHWA NHI-05-037 Appendix C – 1993 AASHTO Design Method 

Geotechnical Aspects of Pavements C - 7 May 2006

Procedure 

The steps in the 1993 AASHTO flexible pavement design procedure are summarized below 

in the context of the example baseline scenario presented in Section 6.2.1: 

1. Determine the analysis period. For the example design scenario, a 30-year design life is 

specified. 

2. Evaluate the design traffic: W18 = 11.6 million ESALs. 

3. Determine the design reliability factors: Reliability = 90% (usually set by agency policy), 

ZR = -1.282, S0 = 0.45. 

4. Determine the allowable serviceability loss due to traffic: ∆PSI = 1.7 (this may be 

reduced if frost heave or swelling soils are an issue). 

5. Evaluate the seasonally averaged subgrade resilient modulus MR using the procedures 

described in Section 5.4.3: MR = 7,500 psi. 

6. Determine the layer properties: 

• Structural layer coefficients ai for all bound layers (see Section 0 for asphalt concrete, 

1993 AASHTO Guide for other stabilized materials) and unbound layers (Section 

5.4.5). Recommendations for appropriate ai values for rehabilitation design are given 

in Table 5-44 in Section 5.4.5. Values for example design: 

a1 = 0.44, a2 = 0.17. 

• Drainage coefficients mi for all unbound layers (Section 5.5.1): m2 = 1.0. 

7. Solve Eq. (C.2) for the required overall structural number: SN = 5.07. 

8. Determine the design layer thicknesses for the pavement section: 

• Using Eq. (C.2) with MR set equal to the granular base resilient modulus EBS = 40,000 

psi (from the correlation in Eq. 5.16), solve for the required structural number for the 

asphalt concrete surface layer: SN1 = 2.62. 

• Convert SN1 to the required thickness of asphalt: 1

1

1

5.95 6 SN D

a = = → inches.1

 

1

After rounding to the nearest half-inch, per the recommendations in the 1993 AASHTO Design Guide. 

Unbound layer thicknesses are rounded to the nearest inch. 
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Geotechnical Aspects of Pavements C - 8 May 2006

• Assign the remaining required structural number to the granular base layer: 

SN2 = 1 1 SN D a − = 2.43. 

• Convert SN2 to the required thickness of granular base: 2

2

2 2

14.3 14 SN D

m a

= =→

inches.1

C.3 RIGID PAVEMENT STRUCTURAL DESIGN 

Design Equation 

The empirical expression relating traffic, pavement structure, and pavement performance for 

rigid pavements is: 

( ) ( )

( )

( ) ( )

( )

10 18 10

0.75 10

7 10

0.75

8.46 0.25

log 7.35log 1 0.06

log 1.132 4.5 1.5 4.22 0.32 log 1.64x10 18.42 1 215.63 1 /

R o

c d

t

c

W ZS D

PSI

SC D

p

J D D E k

= + +−

    ∆     −  −    + +−     +    −  +     

(C.6) 

 in which: 

W18 = number of 18 kip equivalent single axle loads (ESALs) 

 ZR = standard normal deviate (function of the design reliability level) 

S0 = overall standard deviation (function of overall design uncertainty) 

∆PSI = allowable serviceability loss at end of design life 

pt = terminal serviceability 

k = modulus of subgrade reaction (pci) 

Sc = PCC modulus of rupture (psi) 

Ec = PCC modulus of elasticity (psi) 

J = an empirical joint load transfer coefficient 

Cd = an empirical drainage coefficient 

D = required PCC slab thickness (inches) 
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Geotechnical Aspects of Pavements C - 9 May 2006

The first ten parameters typically are the inputs to the design equation, and D is the output. 

Equation (C.6) must be solved implicitly for the slab thickness D as a function of the input 

parameters. 

The design of JRCP and CRCP pavements also requires design of the steel reinforcement. 

Reinforcement design is beyond the scope of this manual; refer to the 1993 AASHTO Guide 

for details on this. 

Design Inputs 

Analysis Period 

Same as for flexible pavements; see Section 0. 

Traffic 

Same as for flexible pavements; see Section 0. Note that the truck factor Tf will not in general 

be the same for rigid and flexible pavements. Refer to the 1993 AASHTO Design Guide or 

standard pavement engineering textbooks like Huang (2004) for determination of the truck 

factor. 

Reliability 

Similar to flexible pavements; see Section 0. For rigid pavements, values for S0 typically 

range between 0.3 and 0.45, with a value of 0.35 commonly used for design. 

Serviceability 

Similar to flexible pavements; see Section 0. A typical value of po for rigid pavements is 4.4. 

As for flexible pavements, a terminal serviceability index of 2.5 or higher is recommended 

for design of major highways. Thus, a typical allowable serviceability loss due to traffic for 

rigid pavements can be expressed as: 

 4.4 2.5 1.9 ∆ =−= − = PSI p p t o (C.7) 

Modulus of Subgrade Reaction 

The design modulus of subgrade reaction k is a computed quantity that is a function of the 

following properties: 

• Subgrade resilient modulus MR

• Thickness of granular subbase DSB

• Resilient modulus of granular subbase ESB

• Depth to bedrock DSG (if shallower than 10 feet) 
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Geotechnical Aspects of Pavements C - 10 May 2006

• Loss of Service LS (an index of the erodibility of the granular subbase) 

See Section 5.4.6 for the procedure for determining the design value for the modulus of 

subgrade reaction k. 

Other Layer Properties 

Other layer properties include the modulus of rupture Sc and elastic modulus Ec for the 

Portland cement concrete slabs, an empirical joint load transfer coefficient J, and the subbase 

drainage coefficient Cd. The PCC parameters Sc and Ec are standard material properties; 

mean values should be used for the pavement design inputs. The joint load transfer 

coefficient J is a function of the shoulder type and the load transfer condition between the 

pavement slab and shoulders; recommended values are summarized in Table C-4. See 

Section 5.5.1 for determination of the drainage coefficient Cd. 

Table C-4. Recommended load transfer coefficients for various pavement types and 

design conditions (AASHTO, 1993). 

 No Shoulders Asphalt Shoulders Tied PCC Shoulders 

With Load 

Transfer 

Devices 

Without 

Load 

Transfer 

Devices 

With Load 

Transfer 

Devices 

Without 

Load 

Transfer 

Devices 

With Load 

Transfer 

Devices 

Without 

Load 

Transfer 

Devices 

JPCP/ 

JRCP 3.2 3.8 – 4.4 3.2 3.8 – 4.4 2.5 – 3.1 3.6 – 4.2 

CRCP 2.9 N.A. 2.9 - 3.2 N.A. 2.3 – 2.9 N.A. 

Procedure 

The steps in the 1993 AASHTO rigid pavement design procedure are summarized below in 

the context of the example baseline scenario presented in Section 6.2.1: 

1. Determine the analysis period. For the example design scenario, a 30-year design life is 

specified. 

2. Evaluate the design traffic: W18 = 18.9 million ESALs 

. 

3. Determine the design reliability factors: Reliability = 90% (usually set by agency policy), 

ZR = -1.282, S0 = 0.45. 
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Geotechnical Aspects of Pavements C - 11 May 2006

4. Determine the terminal serviceability and allowable serviceability loss due to traffic: 

pt = 2.5, ∆PSI = 1.9 (this may be reduced if frost heave or swelling soils are an issue). 

5. Evaluate the effective modulus of subgrade reaction k using the procedures described in 

Section 5.4.6. Specific design inputs to this procedure are the seasonally averaged 

subgrade resilient modulus MR = 7,500 psi, the assumed thickness of the granular subbase 

DSB, the seasonally averaged subbase resilient modulus ESB = 40,000 psi, the depth to 

bedrock DSG (if less than 10 feet—not the case for this example design), and the loss of 

service coefficient LS = 2. 

6. Specify the PCC properties: Sc = 690 psi, Ec = 4.4x106

 psi (these would typically be from 

material specifications; mean values should be used for inputs). 

7. Determine the other input parameters: joint load transfer coefficient J = 3.2, drainage 

coefficient Cd = 1.0. 

8. Solve Eq. (C.6) for the required slab thickness: D = 10.55 ≅ 10.5 inches. 

Note that the thickness assumed for the granular subbase in Step 5 can influence the required 

slab thickness computed in Step 8. If desired, several design alternatives can be evaluated to 

arrive at the optimal design. 

C.4 SOFTWARE 

The empirical design equations for flexible and rigid pavements in Eqs. (C.2) and (C.6) are 

implicit relationships for the required structural number SN and slab thickness D, 

respectively. Consequently, an iterative solution algorithm is required. The 1993 AASHTO 

Design Guide provides nomographs for the graphical evaluation of these equations. They can 

also be evaluated easily using a spreadsheet, e.g., via the Solver tool in Microsoft Excel. 

DARWin, a comprehensive software program tied to the 1993 AASHTO Design Guide 

procedures, is also available through AASHTO. Additional information on DARWin can be 

found at http://darwin.aashtoware.org/index.htm. 
FHWA NHI-05-037 Appendix C – 1993 AASHTO Design Method 

Geotechnical Aspects of Pavements C - 12 May 2006

C.5 REFERENCES 

AASHTO (1993). AASHTO Guide for Design of Pavement Structures, American Association 

of State Highway and Transportation Officials, Washington, D.C. 

Huang, Y.H. (2004). Pavement Analysis and Design (2nd ed.), Prentice-Hall, Englewood 

Cliffs, NJ. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 1 May 2006

APPENDIX D: NCHRP 1-37A DESIGN METHOD

D.1 INTRODUCTION 

The Guide for the Mechanistic-Empirical Design of New and Rehabilitated Pavement 

Structures developed under NCHRP Project 1-37A is the state-of-the-art procedure for the 

design of flexible and rigid pavement structures. The mechanistic-empirical approach at the 

heart of the NCHRP 1-37A methodology represents a fundamental paradigm shift for 

pavement design. In the mechanistic-empirical approach, the response of the pavement – 

defined in terms of stresses, strains, and other parameters – is analyzed using rigorous 

theories of mechanics. Critical response quantities – e.g., tensile strains at the bottom of an 

asphalt or PCC layer – are then related empirically to pavement performance – e.g., fatigue 

cracking. 

Figure D-1 provides a flow chart for the mechanistic-empirical design approach as 

implemented in the NCHRP 1-37A procedures. The major steps are 

1. Define the traffic, environmental, and other general design inputs for the project. In 

the case of rehabilitation designs, this will also include information on existing 

pavement conditions (e.g., distress survey, FWD testing). 

2. Select a trial pavement section for analysis. For rehabilitation designs, this includes 

identification of an appropriate rehabilitation strategy. 

3. Define the properties for the materials in the various pavement layers. 

4. Analyze the pavement response (temperature, moisture, stress, strain) due to traffic 

loading and environmental influences. The pavement response analysis is performed 

on a season-by-season basis in order to include variations in traffic loading, 

environmental conditions, and material behavior over time. 

5. Empirically relate critical pavement responses to damage and distress for the 

pavement distresses of interest. Damage/distress are determined on a season-byseason basis and then accumulated over the design life of the pavement. 

6. Adjust the predicted distresses for the specified design reliability. 

7. Compare the predicted distresses at the end of design life against design limits. If 

necessary, adjust the trial pavement section and repeat Steps 3-7 until all predicted 

distresses are within design limits. 

The corresponding major components required to implement this mechanistic-empirical 

pavement design methodology are 

• Inputs—traffic, climate, materials, others. 

• Pavement response models—to compute critical responses. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 2 May 2006

• Performance models or transfer functions—to predict pavement performance over the 

design life. 

• Design reliability and variability—to add a margin of safety for the design. 

• Performance criteria—to set objective distress limits against which the pavement 

performance will be judged. 

• Software—to implement the mechanistic-empirical models and calculations in a 

usable form. 

Each of these components will be briefly summarized in the following sections. Readers 

should refer to the NCHRP 1-37A final reports (NCHRP 1-37A, 2004) for more thorough 

coverage of each topic. In addition, Chapter 5 provides detailed information on the 

geotechnical inputs to the NCHRP 1-37A procedure and Chapter 6 gives several example 

applications. 

D.2 INPUTS 

D.2.1 Hierarchical Inputs 

As described in Chapter 5, the NCHRP 1-37A design methodology incorporates a 

hierarchical approach for specifying all pavement design inputs. The hierarchical approach is 

based on the philosophy that the level of engineering effort exerted in determining design 

inputs should be commensurate with the relative importance, size, and cost of the design 

project. Three levels are provided for the design inputs in the NCHRP 1-37A procedure: 

Level 1 inputs provide the highest level of accuracy and the lowest level of uncertainty. Level 

1 design inputs would typically be used for heavily trafficked pavements or whenever there 

are serious safety or economic consequences of early failure. Level 1 material inputs require 

field or laboratory evaluation. Subgrade resilient modulus measured from FWD testing in the 

field or triaxial testing in the laboratory is one example of a Level 1 input. 

Level 2 inputs provide an intermediate level of accuracy and are closest to the typical 

procedures used with the AASHTO Design Guides. This level could be used when resources 

or testing equipment are not available for Level 1 characterization. Level 2 inputs would 

typically be derived from a limited testing program or estimated via correlations or 

experience (possibly from an agency database). Subgrade resilient modulus estimated from 

correlations with measured CBR values is one example of a Level 2 input. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 3 May 2006

INPUTS

Traffic Environment Materials

Load spectra Temperature Layers

Subgrade

TRIAL DESIGN

MECHANISTIC RESPONSE MODELS

– Environmental response (T, w)

– Mechanical response (σ, ε)

EMPIRICAL PERFORMANCE MODELS

– Damage (e.g., fatigue cracking)

– Distresses (e.g., rutting)

– Smoothness (IRI)

RELIABILITY

PERFORMANCE CRITERIA

– Cracking (various types)

– Rutting

– Faulting, punchouts, others

– Smoothness

MEETS CRITERIA?

FINAL DESIGN

Yes

No

Design Iterations

INPUTS

Traffic Environment Materials

Load spectra Temperature Layers

Subgrade

TRIAL DESIGN

MECHANISTIC RESPONSE MODELS

– Environmental response (T, w)

– Mechanical response (σ, ε)

EMPIRICAL PERFORMANCE MODELS

– Damage (e.g., fatigue cracking)

– Distresses (e.g., rutting)

– Smoothness (IRI)

RELIABILITY

PERFORMANCE CRITERIA

– Cracking (various types)

– Rutting

– Faulting, punchouts, others

– Smoothness

MEETS CRITERIA?

FINAL DESIGN

Yes

No

Design Iterations

Figure D-1. Flow chart for mechanistic-empirical design methodology. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 4 May 2006

Level 3 inputs provide the lowest level of accuracy. This level might be used for designs in 

which there are minimal consequences of early failure (e.g., low volume roads). Level 3 

material inputs typically are default values that are based on local agency experience. A 

default subgrade resilient modulus based on AASHTO soil class is an example of a Level 3 

input. 

Any given pavement design may incorporate a mix of input data of different levels. For 

example, measured HMA dynamic modulus values used with default resilient modulus 

values for the unbound materials in the pavement structure. However, the algorithms used in 

the design computations are identical for all input levels. In other words, the NCHRP 1-37 

methodology features levels of input data but not levels of design analysis. The composite 

input level determines the overall accuracy and reliability of the pavement performance 

predictions used to judge the acceptability of a trial design. 

D.2.2 Traffic 

Traffic data are key inputs for the analysis and design of pavement structures. Most existing 

design procedures, including all of the AASHTO Design Guides, quantify traffic in terms of 

equivalent single axle loads (ESALs). However, the mechanistic pavement response models 

in the NCHRP 1-37A methodology require the specification of the magnitudes and 

frequencies of the actual wheel loads that the pavement is expected to see over its design life. 

Consequently, traffic must be specified in terms of load spectra rather than ESALs. Load 

spectra are the frequency distributions of axle load magnitudes by axle configuration (single, 

tandem, tridem, quad) and season of year (monthly, typically). 

State highway agencies typically collect two categories of traffic data. Weigh-in-motion 

(WIM) data provide information about the number and configuration of axles observed 

within a set of load groups. Automatic vehicle classification (AVC) data provide information 

about the number and types of vehicles that use a given roadway as counted over some 

period of time. Error! Reference source not found. summarizes the WIM and AVC data 

that are required at each of the hierarchical input levels in the NCHRP 1-37A methodology. 

The traffic data required in the NCHRP 137A methodology are the same for all pavement 

types (flexible or rigid) and construction types (new or rehabilitated). Four categories of 

traffic data are required: 

• Traffic volume—base year information 

o Two-way annual average daily truck traffic (AADTT) 

o Number of lanes in the design direction 

o Percent trucks in design direction 
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o Percent trucks in design lane 

o Vehicle (truck) operational speed

• Traffic volume adjustment factors 

o Monthly adjustment 

o Vehicle class distribution (see Table 6-5 for an example) 

o Hourly truck distribution (see Table 6-6 for an example) 

o Traffic growth factors 

• Axle load distribution factors by season, vehicle class, and axle type (see Table 6-7 

for an example) 

• General traffic inputs 

• Traffic wander data (mean wheel location and standard deviation of lateral wander; 

lane width) 

• Number axles/trucks (see Table 6-8 for an example) 

o Axle configuration (axle width and spacing; tire spacing and pressure) 

o Wheelbase spacing distribution (rigid pavements only; see Table 6-11 for an 

example)

The NCHRP 1-37A design software takes all of these traffic inputs and computes the number 

of applications of each axle load magnitude by axle type (single, tandem, tridem, quad) and 

month. These axle load spectra are a primary input to the mechanistic pavement structural 

response models. 

Table D-1. Traffic data required for each of the three hierarchical input levels 

(NHCRP, 2004).

Input Level Data Sources

1 2 3 

WIM data – site/segment specific X 

WIM data – regional default summaries X 

WIM data – national default summaries X 

AVC data – site/segment specific X 

AVC data – regional default summaries X 

AVC data – national default summaries X 

Vehicle counts – site/segment specific1 X X 

Traffic 

load/volume 

data 

Traffic forecasting and trip generation models2

 X X X 

 1Level depends on whether regional or national default values are used for the WIM or AVC information. 2Level depends on input data and model accuracy/reliability. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 6 May 2006

D.2.3 Environment 

Environmental conditions have a significant effect on the performance of both flexible and 

rigid pavements. External factors such as precipitation, temperature, freeze-thaw cycles, and 

depth to water table play key roles in defining the impact of environment on pavement 

performance. Internal factors such as the susceptibility of the pavement materials to moisture 

and freeze-thaw damage, drainability of the paving layers, and infiltration potential of the 

pavement define the extent to which the pavement will react to the external environmental 

conditions. 

Variations in temperature and moisture profiles within the pavement structure and subgrade 

over the design life of a pavement are simulated in the NCHRP 1-37A design methodology 

via the Enhanced Integrated Climatic Model (EICM—described more fully in Section D.3.1). 

The EICM requires a relatively large number of input parameters. As with all other design 

inputs, EICM input parameters are specified using a hierarchical approach (Levels 1, 2, or 3). 

Since many of the EICM material property inputs are not commonly measured by most 

agency and geotechnical laboratories, Level 3 default values will typically be used for most 

designs. The inputs required by the EICM fall under the following broad categories (see 

Sections 5.5.2 and 5.6.2 for more detail): 

• General information 

o Base/subgrade construction completion date 

o Pavement construction date 

o Traffic opening date 

• Weather-related information (Section 5.6.2) 

o Hourly air temperature 

o Hourly precipitation 

o Hourly wind speed 

o Hourly percentage sunshine (used to determine cloud cover) 

o Hourly relative humidity 

• Groundwater related information (Section 5.6.2) 

o Groundwater table depth 

• Drainage and surface properties (Section 5.5.2) 

o Surface shortwave absorptivity (Section 5.6.2) 

o Infiltration 

o Drainage path length 

o Cross slope 

• Pavement materials 

o Asphalt and Portland cement concrete 

• Thermal conductivity 
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Geotechnical Aspects of Pavements D - 7 May 2006

• Heat capacity 

• Unbound materials (Section 5.5.2) 

• Physical properties (specific gravity, maximum dry unit weight, optimum moisture 

content) 

• Soil water characteristic curve 

• Hydraulic conductivity (permeability) 

• Thermal conductivity 

• Heat capacity 

The weather-related information required by the EICM can be obtained from weather stations 

located near the project site. The software accompanying the NCHRP 1-37A Design Guide 

includes a database from nearly 800 weather stations throughout the United States that can be 

used to generate the weather-related design inputs. 

D.2.4 Material Properties 

The material property inputs required for the environmental effects model in the NCHRP 1-37A 

methodology have already been described in Section D.2.3 (and Sections 5.5.2 and 5.6.2). 

Additional material property inputs are required for the structural response models used to 

calculate the stresses and strains in the pavement. As with all other design inputs, the 

material property inputs can be provided at any of the hierarchical Levels (1, 2, or 3). The 

material property inputs are most conveniently grouped by material type: 

• Asphalt concrete 

o Layer thickness 

o Dynamic modulus (measured value for level 1 or mixture gradation and 

volumetrics for Level 2 and 3 estimation) 

o Asphalt binder properties (dynamic shear stiffness or viscosity for Levels 1 

and 2, binder grade for Level 3) 

o Mixture volumetrics (effective binder content, air voids, unit weight) 

o Poisson’s ratio 

o Thermal cracking properties (low temperature tensile strength, creep 

compliance, thermal expansion coefficient) 

• Portland cement concrete 

o Layer thickness 

o Mixture properties (cement and aggregate type, cement content, water/cement 

ratio, unit weight) 

o Shrinkage characteristics 

o Elastic modulus 
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o Poisson’s ratio 

o Compressive strength 

o Modulus of rupture 

o Thermal expansion coefficient 

• Unbound materials (see Sections 5.3 and 5.4 for more details) 

o Material type 

o Layer thickness 

o Unit weight 

o Coefficient of lateral earth pressure 

o Resilient modulus (see Section 5.4.3 for details on inputs at different 

hierarchical levels) 

o Poisson’s ratio 

D.2.5 Other 

A variety of other input data are required for the NCHRP 1-37A methodology. Some of these 

inputs are dependent upon the particular pavement type (flexible vs. rigid) and construction 

type (new vs. rehabilitation) being considered. A brief summary of these other inputs are as 

follows: 

• General project information 

o Design life 

o Latitude, longitude, and elevation (for accessing weather station database) 

• Rigid pavement design features (all rigid pavement types) 

o Permanent curl/warp effective temperature difference 

o Base erodibility index 

• JPCP design features 

o Joint spacing, sealant type 

o Dowel bar diameter, spacing 

o Edge support (e.g., tied shoulder, widened slab) 

o PCC-base interface bond condition 

• CRCP design features 

o Shoulder type 

o Reinforcement (steel percentage, diameter, depth) 

o Mean crack spacing 

• Flexible pavement distress potential (new construction) 

o Block cracking 

o Longitudinal cracks outside wheel paths 

• Pre-rehabilitation distresses (overlay over AC surface) 

o Rutting 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 9 May 2006

o Fatigue cracking within wheel path 

o Longitudinal cracks outside wheel path 

o Patches 

o Potholes 

• Pre-rehabilitation distresses (overlay over PCC surface) 

o Percent cracked slabs before, after restoration 

o CRCP punchouts 

o Dynamic modulus of subgrade reaction 

Note that no design features are included for jointed reinforced concrete pavements (JRCP). 

The NCHRP 1-37A methodology does not include a design capability for this pavement type. 

D.3 PAVEMENT RESPONSE MODELS 

There are two types of pavement response models in the NCHRP 1-37A methodology: (a) an 

environmental effects model for simulating the time- and depth-dependent temperature and 

moisture conditions in the pavement structure in response to climatic conditions; and (b) 

structural response models for determining the stresses and strains at critical locations in the 

pavement structure in response to traffic loads. The same environmental effects model is 

used for all pavement types. Different structural response models are employed for rigid vs. 

flexible pavements because of the fundamental differences in their mechanical behavior. 

D.3.1 Environmental Effects 

Diurnal and seasonal fluctuations in the moisture and temperature profiles in the pavement 

structure induced by changes in groundwater table, precipitation/infiltration, freeze-thaw 

cycles, and other external factors are incorporated in the NCHRP 1-37A design methodology 

via the Enhanced Integrated Climatic Model (EICM). The EICM is a mechanistic onedimensional coupled heat and moisture flow analysis that simulates changes in the behavior 

and characteristics of pavement and subgrade materials induced by environmental factors. 

The EICM consists of three major components: 

• The Climatic-Materials-Structural Model (CMS Model) originally developed at the 

University of Illinois (Dempsey et al., 1985). 

• The CRREL Frost Heave and Thaw Settlement Model (CRREL Model) originally 

developed at the United States Army Cold Regions Research and Engineering 

Laboratory (CRREL) (Guymon et al., 1986). 

• The Infiltration and Drainage Model (ID Model) originally developed at Texas A&M 

University (Lytton et al., 1990). 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 10 May 2006

Each of these components has been enhanced substantially for use in the NCHRP 1-37A 

design methodology. 

For flexible pavements, the EICM evaluates the following environmental effects: 

• Seasonal changes in moisture content for all subgrade and unbound materials. 

• Changes in resilient modulus, MR, of all subgrade and unbound materials caused by 

changes in soil moisture content. 

• Changes MR due to freezing and subsequent thawing and recovery from frozen 

conditions. 

• Temperature distributions in bound asphalt concrete layers (for determining the 

temperature-dependent asphalt concrete material properties). 

For rigid pavements, the following additional environmental effects are simulated by the 

EICM: 

• Temperature profiles in PCC slabs (for thermal curling prediction). 

• Mean monthly relative humidity values (for estimating moisture warping PCC slabs). 

One of the important outputs from the EICM for both flexible and rigid pavement design is a 

set of adjustment factors for unbound layer materials that account for the effects of 

environmental conditions such as moisture content changes, freezing, thawing, and recovery 

from thawing. This factor, denoted Fenv, varies with position within the pavement structure 

and with time throughout the analysis period. The Fenv factor modifies the resilient modulus 

at optimum moisture and density conditions MRopt to obtain the seasonally adjusted resilient 

modulus MR as a function of depth and time. 

D.3.2 Structural Response 

The mechanistic structural response models determine the stresses, strains, and displacements 

within the pavement system caused by traffic loads and as influenced by environmental 

conditions. Environmental influences may be direct (e.g., strains due to thermal expansion 

and/or contraction) or indirect (e.g., changes in material properties due to temperature and/or 

moisture effects). 

Flexible Pavements 

Two flexible pavement analysis methods have been implemented in the NCHRP 1-37A 

computational procedures. For cases in which all materials in the pavement structure can 

realistically be treated as linearly elastic, multilayer elastic theory (MLET) is used to 

determine the pavement response. MLET provides an excellent combination of analysis 

capabilities, theoretical rigor, and computational speed for linear pavement analyses. In cases 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 11 May 2006

where the consideration of unbound material nonlinearity is desired (i.e., Level 1 resilient 

modulus for new construction), a nonlinear finite element (FE) methodology is employed 

instead for determining the pavement stresses, strains, and displacements. 

A major advantage of MLET solutions is very quick computation times. Solutions for 

multiple wheel loads can be constructed from the fundamental axisymmetric single wheel 

solutions via superposition automatically by the computer program. The principal 

disadvantage of MLET is its restriction to linearly elastic material behavior. Real pavement 

materials, and unbound materials, in particular, often exhibit stress-dependent stiffness. The 

materials may even reach a failure condition in some locations, such as in tension at the 

bottom of the unbound base layer in some pavement structures. These nonlinearities vary 

both vertically through the thickness of the layer and horizontally within the layer. Some 

attempts have been made in the past to incorporate these material nonlinearity effects into 

MLET solutions in an approximate way, but the fundamental axisymmetric formulation of 

MLET makes it impossible to include the spatial variation of stiffness in a realistic manner. 

Some of the limitations of MLET solutions are the strengths of FE analysis. In particular, 

finite element methods can simulate a wide variety of nonlinear material behavior; the 

underlying finite element formulation is not constrained to linear elasticity, as is the case 

with MLET. Stress-dependent stiffness and no-tension conditions for unbound materials can 

all be treated within the finite element framework. However, the FE computational times are 

substantially longer than for MLET analyses. 

The choice of MLET vs. FE structural response model is made automatically by the NCHRP 

1-37A software based on the input data from the user (i.e., whether Level 1 new construction 

inputs are specified for the unbound resilient modulus values). In both cases, the NCHRP 

1-37A software automatically pre-processes all of the input data required for the analysis 

(e.g., automatically generates a finite element mesh), automatically performs the season-byseason analyses over the specified pavement design life, and automatically post-processes all 

of the analysis output data to compute the season-by-season values of the critical pavement 

responses for subsequent use in the empirical performance prediction models. 

Performance prediction requires identification of the locations in the pavement structure 

where the critical pavement responses (stress or strain) attain their most extreme values. For 

multilayer flexible pavement systems, these locations can be difficult to determine. Critical 

responses are evaluated at several depth locations in the NCHRP 1-37A analyses, depending 

upon the distress type: 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 12 May 2006

• Fatigue Depth Locations: 

o Surface of the pavement (z = 0), 

o 0.5 inches from the surface (z = 0.5), 

o Bottom of each bound or stabilized layer.

• Rutting Depth Locations: 

o Mid-depth of each layer/sub-layer, 

o Top of the subgrade, 

o Six inches below the top of the subgrade. 

The horizontal locations for the extreme values of critical responses are more difficult to 

determine. The critical location for the simplest case of a single wheel load can usually be 

determined by inspection – e.g., directly beneath the center of the wheel. The critical location 

under multiple wheels and/or axles will be a function of the wheel load configuration and the 

pavement structure. Mixed traffic conditions (single plus multiple wheel/axle vehicle types) 

further complicate the problem, as the critical location within the pavement structure will not 

generally be the same for all vehicle types. The NCHRP 1-37A calculations address this 

problem by evaluating the pavement responses for a set of potential critical locations. 

Damage/distress magnitudes are calculated from the pavement responses at each location, 

with the final performance prediction based on the location having the maximum 

damage/distress at the end of the analysis period. 

Rigid Pavements 

Finite element analysis has been proven a reliable tool for computing rigid pavement 

structural responses. However, the season-by-season distress/damage calculations 

implemented in the NCHRP 1-37A procedure requires hundreds of thousands of calculations 

to compute incremental damage over a design period of many years. These computations 

would take days to complete using existing rigid pavement finite element programs. To 

reduce computer time to a practical level, neural network models have been developed from 

a large parametric study performed using the ISLAB2000 finite element program 

(Khazanovich et al., 2000). The neural network models, which, in effect, are similar to 

regression models, make it possible to accurately compute critical stresses and deflections 

virtually instantaneously. This in turn makes it possible to perform detailed month-by-month 

incremental analysis within a practical timeframe (i.e., a few minutes). Appendix QQ in the 

NCHRP 1-37A final report (NCHRP, 2004) provides a detailed description of the finite 

element models, parametric study, and neural networks used for the structural analysis of 

rigid pavements. 

A key feature of the rigid pavement structural response model is its treatment of the 

pavement foundation. The ISLAB2000 analysis program and the neural network models 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 13 May 2006

derived from it employ a modified version of the conventional slab-on-Winkler springs 

pavement structural model (also called a “dense liquid” foundation model). As shown in 

Figure D-2, the actual multi-layer pavement structure is replaced by an equivalent 2-layer 

(slab and base) pavement section resting on a Winkler spring foundation having a stiffness 

characterized by k, the modulus of subgrade reaction (see Section 5.4.6). The effective k

value in the equivalent 2-layer pavement is determined by matching the computed surface 

deflections for the actual multi-layer pavement section. The surface deflection profile of the 

actual section is determined using MLET, modeling all layers in the structure. This computed 

deflection profile is then used to backcalculate the effective k value for the equivalent 2-layer 

section. Thus, the effective k value is an internally computed value, not a direct input to the 

design procedure. The exception to this is rehabilitation design, where k determined from 

FWD testing may be input directly. 

The effective k value used in the NCHRP 1-37A methodology is interpreted as a dynamic k

value (e.g., as determined from FWD testing), which should be distinguished from the 

traditional static k values used in previous AASHTO design procedures. 

Concrete Slab

(JPCP, CRCP)

Bedrock

Base Course

(Unbound, Asphalt, Cement)

Subbase Course

(Unbound, Stabilized)

Compacted Subgrade

Natural Subgrade

Ebase

Effective k-value 

obtained through 

backcalculation

E

c

Concrete Slab

(JPCP, CRCP)

Base Course

(Unbound, Asphalt, Cement)

Concrete Slab

(JPCP, CRCP)

Bedrock

Base Course

(Unbound, Asphalt, Cement)

Subbase Course

(Unbound, Stabilized)

Compacted Subgrade

Natural Subgrade

Concrete Slab

(JPCP, CRCP)

Bedrock

Base Course

(Unbound, Asphalt, Cement)

Subbase Course

(Unbound, Stabilized)

Compacted Subgrade

Natural Subgrade

Ebase

Effective k-value 

obtained through 

backcalculation

E

c

Ebase

Effective k-value 

obtained through 

backcalculation

E

c

E Concrete Slab

(JPCP, CRCP)

Base Course

(Unbound, Asphalt, Cement)

Figure D-2. Structural model for rigid pavement structural response computations. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 14 May 2006

D.4 PAVEMENT PERFORMANCE MODELS 

Pavement performance is evaluated in terms of individual distress modes in the NCHRP 1-37A 

methodology. A variety of empirical distress models – also sometimes termed “transfer 

functions” – are incorporated in the NCHRP 1-37A methodology for the major structural 

distresses in flexible and rigid pavements. Empirical models are also provided for estimating 

smoothness as a function of the individual structural distresses and other factors. 

D.4.1 Damage vs. Distress 

Some distresses can be evaluated directly during the season-by-season calculations. For 

example, the empirical model for rutting in the asphalt layers in flexible pavements is of the 

form: 

2 2 3 3

1 1 r r a a

r

r

p a T N β β β

ε

ε = (D.1) 

in which: 

εp = accumulated plastic strain after N repetitions of load at the 

 critical location 

εr = resilient strain at the critical location 

N = number of load repetitions 

T = temperature 

ai = regression coefficients derived from laboratory repeated load 

 permanent deformation tests 

βri = field calibration coefficients (see Section D.4.4) 

 

Each asphalt layer is divided into sublayers, and Eq. (D.1) is evaluated at the midthickness of 

each sublayer. The contribution i ∆Rd to total rutting Rd from sublayer i having thickness hi

can then be expressed as: 

 d p i R h i i ∆ = ε .∆ (D.2) 

The contributions of all of the sublayers l can then be summed to give the total rutting for the 

asphalt concrete layer: 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 15 May 2006

 1 i

l

d d

i

R R == ∑∆ (D.3) 

Other distresses cannot be evaluated directly, but must be quantified in terms of computed 

damage factors. For example, the empirical model for “alligator” fatigue cracking in the 

asphalt layers in flexible pavements is of the form: 

 

22 33

1 1 () () f f k k Nk E fft

β β β ε − − = (D.4) 

in which: 

Nf = number of repetitions to fatigue cracking failure 

εt = tensile strain at the critical location 

E = asphalt concrete stiffness (at appropriate temperature) 

k1, k2, k3 = regression coefficients determined from laboratory fatigue 

 tests 

βf1, βf2, βf3 = field calibration coefficients (see Section D.4.4) 

Computation of fatigue damage is based upon Miner’s Law: 

1

T

i

i f i

n D = N = ∑ (D.5) 

in which: 

D = damage 

T = total number of seasonal periods 

ni = actual traffic for period i 

Nfi = traffic repetitions causing fatigue failure under conditions 

 prevailing during period i 

The damage factor determined using Eq. (D.5) is then related to observed fatigue distress 

quantities (e.g., area of fatigue cracking within the lane) during the field calibration process 

(Section D.4.4). 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 16 May 2006

D.4.2 Distress Models 

Empirical distress prediction models are provided for the following structural distresses in 

the NCHRP 1-37A flexible pavement design methodology: 

• Permanent deformation (rutting) 

o Within asphalt concrete layers 

o Within unbound base and subbase layers 

o Within the subgrade 

• Fatigue cracking 

o Within asphalt concrete layers 

• Bottom-up (classical “alligator” cracking) 

• Top-down (longitudinal fatigue cracking) 

o Within cement stabilized layers 

• Thermal cracking 

The empirical structural distress models for rigid pavements include 

• Transverse joint faulting (JPCP) 

• Transverse fatigue cracking (JPCP) 

• Punchouts (CRCP) 

Note that reflection cracking for asphalt concrete overlays is not included in the current 

version of the NCRHP 1-37A methodology. At the time of the NCHRP 1-37A development, 

it was judged that no suitable empirical reflection cracking models yet existed. It is 

anticipated that a suitable model will be developed and added to the NCHRP 1-37A 

procedure in the future. 

D.4.3 Smoothness 

Pavement smoothness is often used as a composite index of pavement quality. Smoothness 

(or loss thereof) is influenced by nearly all of the distresses of interest in flexible and rigid 

pavement systems. Smoothness data is also regularly and routinely collected and stored as 

part of the pavement management systems at many agencies. Lastly, smoothness is directly 

related to overall ride quality, the factor of most importance to highway users. Because of 

these reasons, empirical smoothness prediction models have been incorporated in the 

NCHRP 1-37A design methodology. 

Pavement smoothness in the NCHRP 1-37A models is characterized in terms of the 

International Roughness Index, or IRI. IRI is predicted as a function of the initial as-
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 17 May 2006

constructed IRI, the subsequent development of distresses over time, and other factors such 

as subgrade type and climatic conditions that may affect smoothness through mechanisms 

such as shrinkage or swelling of subgrade soils and frost heave. The structural distresses 

influencing smoothness are predicted directly by the NCHRP 1-37A mechanistic-empirical 

methodology. However, nonstructural distresses cannot be evaluated using mechanisticempirical principles, so the NCHRP 1-37A procedure provides the option of specifying the 

overall potential for these other distresses. Smoothness loss due to soil 

shrinking/swelling/frost heave and other climatic factors are incorporated into the NCHRP 

1-37A IRI models through the use of a “site factor.” 

The NCHRP 1-37A design method provides IRI prediction models as a function of pavement 

type (flexible vs. rigid), base type (flexible pavements), and construction type (new vs. 

rehabilitation). IRI models are provided for the following cases: 

• AC (new construction) 

o AC over granular base 

o AC over asphalt-treated base 

o AC over cement-stabilized material 

• AC overlay (rehabilitation) 

o AC over flexible pavement 

o AC over rigid pavement 

• JPCP (new construction) 

• JPCP (rehabilitation) 

o JPCP restoration 

o Bonded PCC over JPCP 

o Unbonded PCC over JPCP 

• CRCP (new construction) 

• CRCP (rehabilitation) 

o CRCP restoration 

o Bonded PCC over JPCP 

o Unbonded PCC over CRCP (rehabilitation) 

Appendix OO in the NCHRP 1-37A final documentation (NCHRP, 2004) provides a detailed 

description of the development of these models. 

D.4.4 Field Calibration 

The distress prediction models are key components of the NCHRP 1-37A mechanisticempirical design and analysis procedure. Calibration of these models against field 

performance is an essential part of the model development. Calibration refers to the 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 18 May 2006

mathematical process by which the models are adjusted to minimize the differences between 

predicted and observed values of distress. 

All performance models in the NCHRP 1-37A design method have been calibrated on a 

global level to observed field performance at a representative set of pavement test sites 

around North America. Test sections from the FHWA Long Term Pavement Performance 

(LTPP) program were used extensively in the calibration process because of the consistency 

of the monitored data over time and the diversity of test sections throughout North America. 

However, there were some serious limitations to the NCHRP 1-37A field calibration. Many 

of the material property and site feature inputs required for the NCHRP 1-37A analyses were 

unavailable from the LTPP database. Because of the limited number of pavement test sites 

with complete input data, the minimal material testing available, the use of calculated 

properties from correlations (i.e., Level 3 inputs), and the global scope of the calibration 

effort, the predictions from the calibrated models still have relatively high levels of 

uncertainty and a limited inference space of application. The recently completed NCHRP 

Project 9-30 (Von Quintus et al., 2003) has formulated a plan for developing an enhanced 

database for future recalibration of the NCHRP 1-37A and other similar pavement models. 

The NCHRP 1-37A software also includes a provision for entering local or regional field 

calibration factors instead of the national values derived from the LTPP database. This 

feature permits local agencies to adjust the mechanistic-empirical performance predictions to 

better reflect their local conditions. 

D.5 DESIGN RELIABILITY 

A large amount of uncertainty and variability exists in pavement design and construction, as 

well as in the traffic loads and climatic factors acting over the design life. In the NCHRP 

1-37A mechanistic-empirical design, the key outputs of interest are the individual distress 

quantities. Therefore, variability of the predicted distresses is the focus of design reliability. 

The incorporation of reliability in the NCHRP 1-37A procedure is similar in some respects to 

the way it is treated in the 1993 AASHTO Guide. In the 1993 AASHTO Guide, an overall 

standard deviation or “uncertainty” is specified for the design inputs (the S0 value—see 

Appendix C), a desired reliability level is selected based on agency policy, and the 

combination of the standard deviation and reliability are then used in essence to add a 

“margin of safety” to the design traffic W18. The NCHRP 1-37A methodology differs from 

the 1993 AASHTO procedure in that the standard deviations and reliability levels are set for 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 19 May 2006

each individual distress mode predicted in the mechanistic-empirical computations. The 

default value for the standard deviation of each predicted distress quantity is based on a 

careful analysis of the differences between the predicted versus actual distresses during the 

field calibration of the empirical performance models (Section D.4.4). These estimates of 

error represent the combined effects of input variability, variability in the construction 

process, and model error. 

The desired level of reliability is specified along with the acceptable level of distress at the 

end of design life (Section D.6) to define the performance requirements for a pavement 

design in the NCHRP 1-37A procedure. For example, one criterion might be to limit the 

percent of cracked PCC slabs to 8% at a design reliability of 90%. Then, on average for 100 

projects, 90 would be expected to exhibit fewer than 8% slabs cracked at the end of the 

design life. Different reliability levels may be specified for different distresses in the same 

design. For example, the designer may choose to specify 95% reliability for slab cracking, 

but 90% reliability for faulting and IRI. Of course, increasing design reliability will lead to 

more substantial pavement sections and higher initial costs. The beneficial trade-off is that 

future maintenance costs should be lower for the higher-reliability design. 

D.6 PERFORMANCE CRITERIA 

Performance criteria are definitions of the maximum amounts of individual distress or 

smoothness acceptable to an agency at a given reliability level. Performance criteria are a 

user input in the NCHRP 1-37A methodology and depend on local design and rehabilitation 

policies. Default performance criteria built into the current version of the NCHRP 1-37A 

software are summarized in Table D-2. The designer can select all or some subset of the 

performance criteria to be evaluated during the design. 

D.7 SOFTWARE 

The mechanistic-empirical calculations in the NCHRP 1-37A design methodology cannot be 

performed by hand or simple spreadsheets. A Windows-based program has been developed 

to implement the NCHRP 1-37A methodology by providing: (1) an interface to input all 

design variables, (2) computational engines for analysis and performance prediction, and (3) 

results and outputs from the analyses in formats suitable for use in electronic documents or 

for making hard copies. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 20 May 2006

Table D-2. Default performance criteria in NCHRP 1-37A software. 

Distress Unit Limit1 

Flexible Pavements 

 Top-down (longitudinal) fatigue cracking feet/mile 1000 

 Bottom-up (alligator) fatigue cracking % of wheel path area 25 

 Thermal fracture feet/mile 1000 

 Chemically stabilized layer fatigue cracking % of wheel path area 25 

 Total permanent deformation (rutting) inch 0.75 

 Permanent deformation (rutting) in asphalt layer inch 0.25 

 Terminal IRI2 inches/mile 172 

Rigid Pavements 

 Transverse fatigue cracking (JPCP) % slabs cracked 15 

 Mean joint faulting (JPCP) inch 0.12 

 Punchouts (CRCP) number per mile 10 

 Terminal IRI2 inches/mile 172 1Default value from software version 0.700 (4/7/2004). 

 2

Default initial IRI = 63 inches/mile. 

The software presents a series of information and input screens coordinated through a main 

program layout screen, as illustrated in Figure D-3. On this screen, all access points to the 

information and data input screens are color-coded to guide the designer in providing all data 

needed to run a design analysis. Green tags indicate screens on which the designer has 

already entered/reviewed data, yellow tags indicate screens containing default data that have 

not yet been reviewed/approved by the designer, and red tags indicate screens that have 

missing required data that must still be entered by the designer before the calculations can be 

performed. Clicking on any tag brings up the corresponding data input screen; for example, 

Figure D-4 shows an example data entry screen for subgrade material properties. 

The main program layout screen provides access to the following five groupings of 

information and input screens (screens are denoted by the symbol “•”, subordinate screen 

tabs by the symbol “♦”): 

1. Project Information 

• General Information 

• Site/Project Identification 

• Analysis Parameters 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 21 May 2006

2. Traffic Inputs 

• Traffic Volume Adjustment Factors 

♦ Monthly Adjustment 

♦ Vehicle Class Distribution 

♦ Hourly Distribution 

♦ Traffic Growth Factors 

• Axle Load Distribution Factors 

• General Traffic Inputs 

♦ Number of Axles/Truck 

♦ Axle Configuration 

♦ Wheelbase 

3. Climate Inputs 

• Climate 

4. Structure Inputs 

• Structure 

♦ Drainage and Surface Properties 

♦ Layers 

 Layer Material Properties 

♦ Thermal Cracking 

5. Distress Potential

Note that the Structure Inputs listing above is for the case of a new flexible pavement design. 

The screens will be slightly different for other pavement and construction types, but they all 

conform to the general organization listed above. 

Once all necessary information and input data have been entered into the program, the user 

clicks the Run Analysis button to carry out all the required computations. Separate areas of 

the main program layout screen provide (1) the status (% complete) of the analyses in 

progress and (2) links to summary screens for the inputs to the analyses and their results in 

both tabular and graphical formats. For example, the design analysis of a conventional 

flexible pavement design might provide output plots of HMA modulus, alligator cracking, 

thermal cracking, rutting, and IRI versus pavement age. Figure D-5 is an example of the type 

of output generated by the software. Output can be generated as either Microsoft Excel 

spreadsheets or as HTML documents for easy import into other engineering applications. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 22 May 2006

Figure D-3. Main input screen for NCHRP 1-37A software. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 23 May 2006

Figure D-4. Typical data entry screen for NCHRP 1-37A software. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 24 May 2006

Permanant Deformation: Rutting

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 18 36 54 72 90 108 126 144 162 180 198

Pavement Age (month)

Rutting Depth (in)

SubTotalAC

SubTotalBase

SubTotalSG

Total Rutting

TotalRutReliability

Total Rutting Design Limit

AC Rutting Design Value = 0.25

Total Rutting Design Limit = 0.75

Figure D-5. Typical graphical output from NCHRP 1-37A software. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 25 May 2006

D.8 REFERENCES 

Dempsey, B.J., Herlach, W.A. and Patel, A.J. (1985). The Climatic-Material-Structural 

Pavement Analysis Program, FHWA/RD-84/115, Vol. 3, Final Report, Federal Highway 

Administration, Washington D.C. 

Guymon, G.L., Berg, R.L. and Johnson, T.C. (1986). Mathematical Model of Frost Heave 

and Thaw Settlement in Pavement, Report: U.S. Army Cold Region Research and 

Engineering Laboratory, Hanover, NH. 

Khazanovich, L., Yu, H.T., Rao, S., Galasova, K., Shats, E., and Jones, R. (2000). 

ISLAB2000 - Finite Element Analysis Program for Rigid and Composite Pavements. User’s 

Guide. ERES Consultants Division of Applied Research Associates, Inc., Champaign, IL. 

Lytton, R.L., Pufahl, D.E., Michalak, C.H., Liang, H.S. and Dempsey, B.J. (1990). An 

Integrated Model of the Climatic Effects on Pavement, Texas Transportation Institute, Texas 

A&M University, Report No. FHWA-RD-90-033, Federal Highway Administration, 

McLean, VA. 

NCHRP 1-37A (2004). Mechanistic-Empirical Design of New and Rehabilitated Pavement 

Structures, Draft Report, Transportation Research Board, National Research Council, 

Washington, D.C. 

Von Quintus, H.L., Schwartz, C.W., McCuen, R.H. and Andrei D. (2003). “Experimental 

Plan for Calibration and Validation of Hot Mix Asphalt Performance Models for Mix and 

Structural Design,” Final Report, NCHRP Project 9-30, Transportation Research Board, 

National Research Council, Washington, D.C. 
FHWA NHI-05-037 Appendix D – NCHRP 1-37A Design Method 

Geotechnical Aspects of Pavements D - 26 May 2006
FHWA NHI-05-037 Appendix E – Boring Log Preparation Key 

Geotechnical Aspects of Pavements E - 1 May 2006 

APPENDIX E: 

TYPICAL KEY FOR BORING LOG PREPARATION 

Figure E-1. Example key to boring log. 
FHWA NHI-05-037 Appendix E – Boring Log Preparation Key 

Geotechnical Aspects of Pavements E - 2 May 2006 

Figure E-1. Example key for final boring log (continued). 
FHWA NHI-05-037 Appendix E – Boring Log Preparation Key 

Geotechnical Aspects of Pavements E - 3 May 2006 

Figure E-2. Example key to core boring log. 
FHWA NHI-05-037 Appendix E – Boring Log Preparation Key 

Geotechnical Aspects of Pavements E - 4 May 2006 

Figure E-2. Example key to core boring log (continued). 
FHWA NHI-05-037 Appendix F – Admixture for Subgrade Stabilization 

Geotechnical Aspects of Pavements F - 1 May 2006 

APPENDIX F: 

DETERMINATION OF ADMIXTURE CONTENT FOR SUBGRADE 

STABILIZATION 

(Adopted from Joint Departments of the Army and Air Force, USA, 

TM 5-822-14/AFMAN 32-8010, Soil Stabilization for Pavements, 

25 October 1994.) 

Lime Content for Lime-Stabilized Soils 

 To determine the design lime content for a subgrade soil, the following steps are suggested: 

1. Determine whether the soil has at least 25% passing the 75-μm sieve and has a 

plasticity index (PI) of at least 10. The soil screening criteria also limit soluble 

sulfates to less than 0.3 % by weight in a 10:1 water-to-soil solution. 

2. Determine the initial design lime content by mixing varying amounts of lime with the 

soil in water and measuring the pH levels in 1-hour intervals. Select the lowest lime 

mixture level for which a pH of 12.4 occurs as the initial design lime content. 

3. Using the initial design lime content conduct moisture-density tests to determine the 

maximum dry density and optimum water content of the soil lime mixture defined by 

the user agency, e.g., AASHTO T-99, AASHTO T-180, ASTM D 698, or ASTM D 

1557. The procedures in ASTM D 3551 will be used to prepare the soil-lime mixture. 

4. Prepare specimens at optimum moisture content and specified density requirement 

(e.g., 90% of AASHTO T-180) using the initial design lime content and at about 2% 

and 4% lime above that lime content from Step 1. Cure the test specimens in sealed 

plastic bags for 28 days at 21o

C (73oF). (Alternative – cure for 7 days at 40o C 

(104o

F)). 

5. Determine the unconfined compressive strength for all cured test specimens (e.g.,

ASTM 5102). Select as the construction design lime content the minimum percent 

required to achieve the required compressive strength (e.g., 150 psi). Either prepare a 

sample at the design lime content and perform resilient modulus test (e.g., AASHTO 

T 294-94) or estimate from Unconfined compression strength Qu. A conservative 

estimate for lime-stabilized soils has been reported to be obtained from (Thompson, 

1970): 

MR = 0.124 qu + 9.98 

where, 

 MR = resilient modulus, ksi, 

 qu = unconfined compressive strength, psi, as tested in accordance with 
FHWA NHI-05-037 Appendix F – Admixture for Subgrade Stabilization 

Geotechnical Aspects of Pavements F - 2 May 2006 

 ASTM D 5102, “Standard Test Method for Unconfined Compressive 

 Strength of Compacted Soil-Lime Mixtures” 

6. Add 0.5 – 1% additional lime in the lower percentage ranges to compensate for 

problems associated with non-uniform mixing during construction. 

Laboratory testing should always be performed to check whether the stabilization has the 

desired effect on other engineering properties like plasticity and strength. 

Cement Content for Cement-Modified Soils

(1) Improve plasticity. The amount of cement required to improve the quality of the soil 

through modification is determined by the trial-and-error approach. If it is desired to 

reduce the PI of the soil, successive samples of soil-cement mixtures must be prepared at 

different treatment levels and the PI of each mixture determined. The Referee Test of 

ASTM D 423 and ASTM D 424 procedures will be used to determine the PI of the soilcement mixture. The minimum cement content that yields the desired PI is selected, but 

since it was determined based upon the minus 40 fraction of the material, this value must 

be adjusted to find the design cement content based upon total sample weight expressed 

as: 

 

A = 100BC

where, 

A = design cement content, percent total weight of soil 

B = percent passing No. 40 sieve size, expressed as a decimal 

C = percent cement required to obtain the desired PI of minus 40 material, 

 expressed as a decimal 

(2) Improve gradation. If the objective of modification is to improve the gradation of a 

granular soil through the addition of fines, then particle-size analysis (ASTM D 422) 

should be conducted on samples at various treatment levels to determine the minimum 

acceptable cement content. 

(3) Reduce swell potential. Small amounts of Portland cements may reduce swell potential of 

some swelling soils. However, Portland cement generally is not as effective as lime, and 

may be considered too expensive for this application. The determination of cement 

content to reduce the swell potential of fine-grained plastic soils can be accomplished by 

molding several samples at various cement contents and soaking the specimens along 

with untreated specimens for 4 days. The lowest cement content that eliminates the swell 
FHWA NHI-05-037 Appendix F – Admixture for Subgrade Stabilization 

Geotechnical Aspects of Pavements F - 3 May 2006 

potential or reduces the swell characteristics to the minimum is the design cement 

content. Procedures for measuring swell characteristics of soils are found in ASTM D 

4546 and MIL-STD-621A, Method 101. The cement content determined to accomplish 

soil modification should be checked to see whether it provides an unconfined 

compressive strength great enough to qualify for a reduced thickness design in 

accordance with criteria established for soil stabilization. 

(4) Condition frost areas. Cement-modified soil may also be used in frost areas, but in 

addition to the procedures for mixture design described in (1) and (2) above, cured 

specimens should be subjected to the 12 freeze-thaw cycles prescribed by ASTM D 560 

(but omitting wire-brushing) or other applicable freeze-thaw procedures. This should be 

followed by determination of frost design soil classification by means of standard 

laboratory freezing tests. If cement-modified soil is used as subgrade, its frost 

susceptibility, determined after freeze-thaw cycling, should be used as the basis of the 

pavement thickness design if the reduced subgrade design method is applied. 

Cement Content for Cement-Stabilized Soil 

The following procedure is recommended for determining the design cement content for 

cement-stabilized soils. 

Step 1. Determine the classification and gradation of the untreated soil following 

procedures in ASTM D 422 and D 2487, respectively. 

Step 2. Using the soil classification, select an estimated cement content for moisturedensity tests from Table F-1. 

Table F-1. Cement requirements for various soil types. 

Soil Type Initial Estimated Cement Content 

percent dry weight

GW, SW 

GP, GW-GC, GW-GM, SW-SC, SW-SM 

GC, GM, GP-GC, GP-GM, GM-GC, SC, 

SM, SP-SC, SP-SM, SM-SC, SP 

CL. ML, MH

CH 

5 

6 

7 

9 

11 
FHWA NHI-05-037 Appendix F – Admixture for Subgrade Stabilization 

Geotechnical Aspects of Pavements F - 4 May 2006 

Step 3. Using the estimated cement content, conduct moisture-density tests to determine 

the maximum dry density and optimum water content of the soil-cement mixture. 

The procedure contained in ASTM D 558 will be used to prepare the soil-cement 

mixture and to make the necessary calculations; however, the procedures outlined 

in AASHTO T180 or ASTM D 1557 will be used to conduct the moisture density 

test.

Step 4. Prepare triplicate samples of the soil-cement mixture for unconfined compression 

and durability tests at the cement content selected in Step 2 and at cement 

contents 2% above and 2% below that determined in Step 2. The samples should 

be prepared at the density and water content to be expected in field construction. 

For example, if the design density is 95% of the laboratory maximum density, the 

samples should also be prepared at 95%. The samples should be prepared in 

accordance with ASTM D 1632, except that when more than 35% of the material 

is retained on the 4.75 mm (# 4) sieve, a 100-mm (4-in.) diameter by 200-mmhigh (8-in.) mold should be used to prepare the specimens. Cure the specimens for 

7 days in a humid room before testing. Test three specimens using the unconfined 

compression test in accordance with ASTM D 1633, and subject three specimens 

to durability tests, either wet-dry (ASTM D 559) or freeze-thaw (ASTM D 560) 

tests, as appropriate. The frost susceptibility of the treated material should also be 

determined, as indicated in appropriate pavement design manuals. 

Step 5. Compare the results of the unconfined compressive strength and durability tests 

with the requirements. The lowest cement content that meets the required 

unconfined compressive strength requirement and demonstrates the required 

durability is the design cement content. If the mixture should meet the durability 

requirements, but not the strength requirements, the mixture is considered to be a 

modified soil. If the results of the specimens tested do not meet both the strength 

and durability requirements, then a higher cement content may be selected and 

Steps 1 through 4 above repeated. 

Selection of Lime-Flyash Content for LF and 

the Determination of the Ratio of Lime to Fly LCF Mixtures. 

(1) Step 1. The first step is to determine the optimum fines content that will give the 

maximum density. This is done by conducting a series of moisture-density tests using 

different percentages of flyash and determining the mix level that yields maximum density. 

The initial flyash content should be about 10%, based on dry weight of the mix. It is 

recommended that material larger than 19 mm (3⁄4 in.) be removed and the test conducted on 

the minus 19 mm (3⁄4 in.) fraction. Tests are run at increasing increments of flyash, e.g., 2%, 
FHWA NHI-05-037 Appendix F – Admixture for Subgrade Stabilization 

Geotechnical Aspects of Pavements F - 5 May 2006 

up to a total of about 20%. Moisture density tests should be conducted following procedures 

indicated in AASHTO T99, AASHT T180, and ASTM D 1557. The design flyash content is 

then selected at 2% above that yielding maximum density. An alternate method is to estimate 

optimum water content and conduct single point compaction tests at flyash contents of 10 – 

20%, make a plot of dry density versus flyash content, and determine the flyash content that 

yields maximum density. The design flyash content is 2% above this value. A moisture 

density test is then conducted to determine the optimum water content and maximum dry 

density. 

(2) Step 2. Determine the ratio of lime to flyash that will yield highest strength and 

durability. Using the design flyash content and the optimum water content determined in Step 

1, prepare triplicate specimens at three different lime-flyash ratios, following the selected 

density procedure. Use LF ratios of 1:3, 1:4, and 1:5. If desired, about 1% of Portland cement 

may be added at this time. 

(3) Step 3. Test three specimens using the unconfined compression test. If frost design is a 

consideration, subject three specimens to 12 cycles of freeze-thaw durability tests (ASTM D 

560), except wire brushing is omitted. The frost susceptibility of the treated material shall 

also be determined as indicated in the appropriate design manual. 

(4) Step 4. Compare the results of the unconfined compressive strength and durability tests 

with the requirements. The lowest LF ratio content, i.e., ratio with the lowest lime content 

that meets the required unconfined compressive strength requirement and demonstrates the 

required durability, is the design LF content. The treated material must also meet frost 

susceptibility requirements, as indicated in the appropriate pavement design manuals. If the 

mixture should meet the durability requirements, but not the strength requirements, it is 

considered to be a modified soil. If the results of the specimens tested do not meet both the 

strength and durability requirements, a different LF content may be selected, or additional 

Portland cement used and Steps 2 through 4 repeated. 

Selection of Cement Content for LCF Mixtures. 

Portland cement may also be used in combination with LF for improved strength and 

durability. If it is desired to incorporate cement into the mixture, the same procedures 

indicated for LF design should be followed except that, beginning at Step 2, the cement shall 

be included. Generally, about 1 – 2% cement is used. Cement may be used in place of or in 

addition to lime; however, the total tines content should be maintained. Strength and 

durability tests must be conducted on samples at various LCF ratios to determine the 

combination that gives best results. 
FHWA NHI-05-037 Appendix F – Admixture for Subgrade Stabilization 

Geotechnical Aspects of Pavements F - 6 May 2006 

Selection of Asphalt Content for Bituminous-Stabilized Soil

Guidance for the design of bituminous-stabilized base and subbase courses is contained in 

U.S. Army TM 5-822-8/AFM 88-6, Chap. 9. For subgrade stabilization, the following 

equation may be used for estimating the preliminary quantity of cutback asphalt to be 

selected: 

100

(100 )

0.02( ) 0.07( ) 0.15( ) 0.20( ) x

s

a b c d

p −

+ + =

where 

p = percent cutback asphalt by weight of dry aggregate 

a = percent of mineral aggregate retained on No. 50 sieve 

b = percent of mineral aggregate passing No. 50 sieve and retained on No. 100 sieve 

c = percent of mineral aggregate passing No. 100 sieve and retained on No. 200 sieve 

d = percent of mineral aggregate passing No. 200 sieve 

s = percent solvent 

The preliminary quantity of emulsified asphalt to be used in stabilizing subgrades can be 

determined from Table F-2. The final design content of cutback or emulsified asphalt should 

be selected based upon the results of the Marshal Stability test procedure (AASHTO T 245, 

ASTM D 5581, MIL-STD 620A). The minimum Marshall Stability recommended for 

subgrades is 2.2 kN (500 lb). If a soil does not show increased stability when reasonable 

amounts of bituminous materials are added, the gradation of the soil should be modified, or 

another type of bituminous material should be used. Poorly graded materials may be 

improved by the addition of suitable tines containing considerable material passing the 75 

μm (No. 200) sieve. The amount of bitumen required for a given soil increases with an 

increase in percentage of the liner sizes. 
FHWA NHI-05-037 Appendix F – Admixture for Subgrade Stabilization 

Geotechnical Aspects of Pavements F - 7 May 2006 

Table F-2. Emulsified asphalt requirements. 

Pounds of Emulsified Asphalt per 100 pounds of Dry Aggregate 

at Percent Passing No. 10 Sieve 

Percent Passing 

75-μm (No. 200) 

Sieve <50 60 70 80 90 100 

0 

2 

4 

6 

8 

10 

12 

14 

16 

18 

20 

22 

24 

25 

6.0 

6.3 

6.5 

6.7 

7.0 

7.2 

7.5 

7.2 

7.0 

6.7 

6.5 

6.3 

6.0 

6.2 

6.3 

6.5 

6.7 

7.0 

7.2 

7.5 

7.7 

7.5 

7.2 

7.0 

6.7 

6.5 

6.3 

6.4 

6.5 

6.7 

7.0 

7.2 

7.5 

7.7 

7.9 

7.7 

7.5 

7.2 

7.0 

6.7 

6.5 

6.6 

6.7 

7.0 

7.2 

7.5 

7.7 

7.9 

8.2 

7.9 

7.7 

7.5 

7.2 

7.0 

6.7 

6.9 

7.0 

7.2 

7.5 

7.7 

7.9 

8.2 

8.4 

8.2 

7.9 

7.7 

7.5 

7.2 

7.0 

7.1 

7.2 

7.5 

7.7 

7.9 

8.2 

8.4 

8.6 

8.4 

8.2 

7.9 

7.7 

7.5 

7.2 

7.3 

 1 lb = 0.454 kg 
FHWA NHI-05-037 Appendix F – Admixture for Subgrade Stabilization 

Geotechnical Aspects of Pavements F - 8 May 2006 

Table F-3. Common guidelines for stabilized drainable base mixes 

(after FHWA Demonstration Project 87: Drainable Pavement Systems, FHWA-SA-92-008). 

Stabilization 

Method

Item Requirement

AsphaltStabilized 

Gradation of material 

Amount of asphalt 

Temperature of mix 

AASHTO No. 67 stone, preheat at 135o

 – 

160o C (275o

 – 320o F). 

2 – 2.5% by weight, using a harder asphalt 

like AC 40 or AR 8000. 

Lay at 90o – 120o C (195o – 250o F) and seal 

with one pass of a 7.2 – 10.9 metric ton (8 – 

12 ton) smooth wheel roller. Start compaction 

rolling after the temperature reaches 65o C 

(150o F), but before it drops to 38o C (100o F). 

CementStabilized

Gradation of material 

Amount of cement 

Curing requirements 

AASHTO No. 67 stone. 

Use 110 – 150 kg of cement per cubic meter 

(185 – 250 lbs/yd3). (135 – 150 kg/m3 (230 – 

250 lbs/yd3) for high traffic loads). (A 

minimum compressive strength of 4.1 MPa 

(600 psi) is typically suggested in cold 

regions to resist frost deterioration.) 

Not clearly understood, and may require local 

testing (consider a 150 m-long (500 ft) test 

strip). It is suggested that the mix be covered 

with plastic for five days after laydown, or 

that light misting be done, starting the second 

day after laydown.