U. S. Department of Transportation Publication No. FHWA-NHI-10-024 

Federal Highway Administration	 FHWA GEC 011 – Volume I 

November 2009 

NHI Courses No. 132042 and 132043 

Design and Construction of 

Mechanically Stabilized Earth Walls 

and Reinforced Soil Slopes – Volume I 

Developed following:
 

AASHTO LRFD Bridge Design and AASHTO LRFD Bridge Construction 

Specifications, 4th Edition, 2007, Specifications, 2nd Edition, 2004, with 

with 2008 and 2009 Interims. 2006, 2007, 2008, and 2009 Interims.
 
NOTICE

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

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 object of 

this document. 
Technical Report Documentation Page 

1. REPORT NO. 2. GOVERNMENT 3. RECIPIENT'S CATALOG NO. 

ACCESSION NO. 

FHWA-NHI-10-024 FHWA GEC 011-Vol I 

4. TITLE AND SUBTITLE 5. REPORT DATE 

November 2009 

Design of Mechanically Stabilized Earth Walls 

and Reinforced Soil Slopes – Volume I 6. PERFORMING ORGANIZATION CODE 

7. AUTHOR(S) 8. PERFORMING ORGANIZATION REPORT NO. 

Ryan R. Berg, P.E.; Barry R. Christopher, 

Ph.D., P.E. and Naresh C. Samtani, Ph.D., P.E. 

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

Ryan R. Berg & Associates, Inc. 11. CONTRACT OR GRANT NO. 

2190 Leyland Alcove DTFH61-06-D-00019/T-06-001 

Woodbury, MN 55125 

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

National Highway Institute 

Federal Highway Administration 14. SPONSORING AGENCY CODE 

U.S. Department of Transportation 

Washington, D.C. 

15. SUPPLEMENTARY NOTES 

FHWA Technical Consultants: Rich Barrows, P.E., Silas Nichols, P.E., and Daniel Alzamora, P.E. 

Technical Consultant: James G. Collin, Ph.D., P.E. 

16. ABSTRACT 

 This manual is the reference text used for the FHWA NHI courses No. 132042 and 132043 

on Mechanically Stabilized Earth Walls and Reinforced Soil Slopes and reflects current 

practice for the design, construction and monitoring of these structures. This manual was 

prepared to enable the engineer to identify and evaluate potential applications of MSE walls 

and RSS as an alternative to other construction methods and as a means to solve construction 

problems. The scope is sufficiently broad to be of value for specifications specialists, 

construction and contracting personnel responsible for construction inspection, development 

of material specifications and contracting methods. With the aid of this text, the engineer 

should be able to properly select, design, specify, monitor and contract for the construction of 

MSE walls and RSS embankments. 

 The MSE wall design within this manual is based upon Load and Resistance Factor Design 

(LRFD) procedures. This manual is a revision (to LRFD) and an update to the FHWA NHI00-043 manual (which was based upon allowable stress design (ASD) procedures). 

17. KEY WORDS 18. DISTRIBUTION STATEMENT 

Design, Load and Resistance Factor Design No restrictions. (LRFD), analysis, performance criteria, 

Mechanically Stabilized Earth (MSE) Walls, 

Reinforced Soil Slopes (RSS), soil reinforcement, 

geosynthetics, specifications, contracting methods 

19. SECURITY CLASSIF. 20. SECURITY CLASSIF. 21. NO. OF PAGES 22 

Unclassified Unclassified 306 
 

SI CONVERSION FACTORS 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH 

mm 

m 

m 

km 

millimeters 

meters 

meters 

kilometers 

0.039 

3.28 

1.09 

0.621 

inches 

feet 

yards 

miles 

in 

ft 

yd 

mi 

AREA 

mm2 

m2 

m2 

ha 

km2 

square millimeters 

square meters 

square meters 

hectares 

square kilometers 

0.0016 

10.764 

1.195 

2.47 

0.386 

square inches 

square feet 

square yards 

acres 

square miles 

in2 

ft2 

yd2 

ac 

mi2 

VOLUME 

ml 

l 

m3 

m3 

millimeters 

liters 

cubic meters 

cubic meters 

0.034 

0.264 

35.71 

1.307 

fluid ounces 

gallons 

cubic feet 

cubic yards 

fl oz 

gal 

ft3 

yd3 

MASS 

g 

kg 

grams 

kilograms 

0.035 

2.202 

ounces 

pounds 

oz 

lb 

TEMPERATURE 

°C Celsius 1.8 C + 32 Fahrenheit °F 

kN/m3 

WEIGHT DENSITY 

kilonewton / cubic m 6.36 poundforce / cubic foot pcf 

FORCE and PRESSURE or STRESS 

N 

kN 

kPa 

kPa 

newtons 

kilonewtons 

kilopascals 

kilopascals 

0.225 

225 

0.145 

20.9 

poundforce 

poundforce 

poundforce / sq in. 

poundforce / sq ft 

lbf 

lbf 

psi 

psf 
PREFACE
 

Engineers and specialty material suppliers have been designing reinforced soil structures for 

the past 35 years. Currently, many state DOTs are transitioning their design of substructures 

from Allowable Stress Design (ASD) to Load and Resistance Factor Design (LRFD) 

procedures. 

This manual is based upon LRFD for MSE wall structures. It has been updated from the 

2001 FHWA NHI-00-043 manual. In addition to revision of the wall design to LRFD 

procedures, expanded discussion on wall detailing and general updates throughout the 

manual are provided. The primary purpose of this manual is to support educational programs 

conducted by FHWA for transportation agencies. 

A second purpose of equal importance is to serve as the FHWA standard reference for 

highway projects involving MSE wall and reinforced soil structures. 

This Mechanically Stabilized Earth Walls (MSE) and Reinforced Soil Slopes (RSS), Design 

and Construction Guidelines Manual which is an update of the current FHWA NHI-00-043, 

has evolved from the following AASHTO and FHWA references: 

C AASHTO LRFD Bridge Design Specifications, 4th Edition, 2007, with 2008 and 2009 

Interim Revisions. 

C Earth Retaining Structures, by B.F. Tanyu, P.J. Sabatini, and R.R. Berg, FHWA-NHI-07-

071 (2008). 

C AASHTO LRFD Bridge Construction Specifications, 2nd Edition, 2004, with 2006 

Interim Revisions. 

C Geosynthetic Design and Construction Guidelines, by R.D. Holtz, B.R. Christopher, and 

R.R. Berg, FHWA HI-07-092 (2008). 

C Guidelines for Design, Specification, and Contracting of Geosynthetic Mechanically 

Stabilized Earth Slopes on Firm Foundations, by R.R. Berg, FHWA-SA-93-025, January 

1993. 

C Reinforced Soil Structures - Volume I, Design and Construction Guidelines - Volume II, 

Summary of Research and Systems Information, by B.R. Christopher, S.A. Gill, J.P. 

Giroud, J.K. Mitchell, F. Schlosser, and J. Dunnicliff, FHWA RD 89-043 (1990). 

C Design and Construction Monitoring of Mechanically Stabilized Earth Structures, by 

J.A. DiMaggio, FHWA, (1994). 

C AASHTO Bridge T-15 Technical Committee unpublished working drafts for the update 

of Section 11.0 of the AASHTO LRFD Bridge Design Specifications. 

FHWA NHI-10-024 

MSE Walls and RSS – Vol I iii	 November 2009 
The authors recognize the efforts and contributions of Messrs. Richard Barrows, P.E., Silas 

Nichols, P.E., and Daniel Alzamora P.E. who were the FHWA Technical Consultants for this 

work. 

The authors also recognize the contributions of the other Technical Consultants on this 

project. They are: 

C Tony Allen, P.E. of Washington DOT 

C Christopher Benda, P.E. of Vermont DOT 

C James Brennan, P.E. of Kansas DOT 

C James Collin, Ph.D., P.E. of The Collin Group 

C Jerry DiMaggio, P.E. of the National Academy of Sciences 

C Kenneth L. Fishman, Ph.D., P.E. of Earth Reinforcement Testing, Inc. 

C Kathryn Griswell, P.E. of CALTRANS 

C John Guido, P.E. of Ohio DOT 

C Dan Johnston, P.E. of South Dakota DOT 

C Dov Leshchinsky, Ph.D. of the University of Delaware 

C Michael Simac, P.E. of Earth Improvement Technologies, Inc. 

C James L. Withiam, Ph.D., P.E. of D’Appolonia Engineers 

And the authors acknowledge the contributions of the following industry associations: 

C Association of Metallically Stabilized Earth (AMSE) 

C Geosynthetic Materials Association (GMA) 

C National Concrete Masonry Association (NCMA) 

A special acknowledgement of Mr. Jerry A. DiMaggio, P.E. who was the FHWA Technical 

Consultant for most of the above referenced publications. Mr. DiMaggio's guidance and 

input to this and the previous works has been invaluable. 

Lastly, the authors wish to acknowledge the extensive work of the late Victor Elias, P.E. for 

his vital contributions and significant effort as Lead Author in preparing the earlier two 

(1997, 2001) versions of this manual, and as the author of the earlier companion manuals on 

corrosion/degradation of soil reinforcements. Mr. Elias was instrumental in the introduction 

and implementation of reinforced soil technology in the U.S., as a Vice President for The 

Reinforced Earth Company from 1974 to 1985. He was instrumental in research, refinement 

of design methods, and standards of practice and codes for MSE walls, as a Consultant from 

1985 until 2006. 

FHWA NHI-10-024 

MSE Walls and RSS – Vol I iv November 2009 
 

TABLE OF CONTENTS – Volume I
 

CHAPTER 1 INTRODUCTION ................................................................................................. 1-1 

1.1 OBJECTIVES...................................................................................................... 1-1 

1.1.1 Scope ...................................................................................................... 1-1 

1.1.2 Source Documents ................................................................................... 1-2 

1.1.3 Terminology............................................................................................. 1-3 

1.2 HISTORICAL DEVELOPMENT ....................................................................... 1-4 

1.3 LOAD AND RESISTANCE FACTOR DESIGN (LRFD) ............................... 1-12 

CHAPTER 2 SYSTEMS AND PROJECT EVALUATION ....................................................... 2-1 

2.1 APPLICATIONS ................................................................................................. 2-1 

2.1.1 MSE Walls ............................................................................................... 2-1 

2.1.2 Reinforced Soil Slopes............................................................................. 2-4 

2.2 ADVANTAGES AND POTENTIAL DISADVANTAGES............................... 2-6 

2.2.1 Advantages of Mechanically Stabilized Earth (MSE) Walls................... 2-6 

2.2.2 Advantages of Reinforced Soil Slopes (RSS).......................................... 2-6 

2.2.3 Potential Disadvantages ........................................................................... 2-7 

2.3 RELATIVE COSTS............................................................................................. 2-7 

2.4 DESCRIPTION OF MSE/RSS SYSTEMS....................................................... 2-10 

2.4.1 Systems Differentiation ......................................................................... 2-10 

2.4.2 Types of Systems ................................................................................... 2-11 

2.4.3 Facing Systems ...................................................................................... 2-12 

2.4.4 Reinforcement Types ............................................................................. 2-16 

2.4.5 Reinforced Fill Materials ....................................................................... 2-17 

2.4.6 Appurtenant Materials of Construction ................................................. 2-18 

2.5 CONSTRUCTION SEQUENCE ....................................................................... 2-18 

2.5.1 Construction of MSEW systems with precast facings ........................... 2-18 

2.5.2 Construction of MSE systems with Flexible Facings ............................ 2-24 

2.5.3 RSS Construction................................................................................... 2-28 

2.6 SITE EVALUATION ........................................................................................ 2-28 

2.6.1 Site Exploration ..................................................................................... 2-28 

2.6.2 Field Reconnaissance ............................................................................. 2-30 

2.6.3 Subsurface Exploration .......................................................................... 2-30 

2.6.4 Laboratory Testing ................................................................................. 2-32 

2.6.5 Foundation Soils .................................................................................... 2-33 

2.7 PROJECT EVALUATION................................................................................ 2-34 

2.7.1 Structure Selection Factors .................................................................... 2-34 

2.7.2 Geologic and Topographic Conditions .................................................. 2-35 

2.7.3 Environmental Conditions ..................................................................... 2-36 

2.7.4 Size and Nature of Structure.................................................................. 2-37 

2.7.5 Aesthetics............................................................................................... 2-38 

2.7.6 Questionable Applications ..................................................................... 2-38 

2.8 ESTABLISHMENT OF PROJECT CRITERIA ............................................... 2-38 

2.8.1 Alternates ............................................................................................... 2-39 

2.8.2 Facing Considerations............................................................................ 2-39 

FHWA NHI-10-024 Table of Contents 

MSE Walls and RSS – Vol I v November 2009 
 

2.8.3 Performance Criteria .............................................................................. 2-40 

2.8.4 Design Life............................................................................................. 2-46 

2.9 PROPRIETARY ASPECTS .............................................................................. 2-46 

2.10 CASE HISTORIES -- MSE WALLS ................................................................ 2-47 

2.10.1 Mn/DOT Crosstown Project MSE Walls............................................... 2-47 

2.10.2 Veterans Memorial Overpass True Abutment MSE Walls.................... 2-49 

2.10.3 SEATAC Runway Extension Wall ........................................................ 2-50 

2.10.4 Guanella Pass Roadway Reconstruction................................................ 2-51 

CHAPTER 3 SOIL REINFORCEMENT PRINCIPLES 

AND SYSTEM DESIGN PROPERTIES ........................................................................ 3-1 

3.1 OVERVIEW ........................................................................................................ 3-1 

3.2 ESTABLISHMENT OF ENGINEERING PROPERTIES BASED ON SITE 

EXPLORATION AND TESTING ...................................................................... 3-2 

3.2.1 Reinforced Fill Soil .................................................................................. 3-2 

3.2.2 Retained Backfill and Natural Retained Soil ........................................... 3-8 

3.2.3 Electrochemical Properties ...................................................................... 3-9 

3.3 REINFORCED SOIL CONCEPTS ................................................................... 3-10 

3.3.1 Stress Transfer Mechanisms .................................................................. 3-10 

3.3.2 Mode of Reinforcement Action ............................................................. 3-12 

3.3.3 Geometric Characteristics ...................................................................... 3-12 

3.4 SOIL REINFORCEMENT INTERACTION USING NORMALIZED 

CONCEPTS ....................................................................................................... 3-13 

3.4.1 Evaluation of Pullout Performance ........................................................ 3-13 

3.4.2 Estimate of the Reinforcement Pullout Capacity in MSE Structures .... 3-16 

3.4.3 Interface Shear ....................................................................................... 3-21 

3.5 ESTABLISHMENT OF STRUCTURAL DESIGN PROPERTIES ................. 3-22 

3.5.1 Strength Properties of Steel Reinforcements ......................................... 3-22 

3.5.2 Strength Properties of Geosynthetic Reinforcement.............................. 3-28 

3.5.2.a Ultimate Tensile Strength, Tult ................................................... 3-31 

3.5.2.b Installation Damage Reduction Factor, RFID ............................. 3-31 

3.5.2.c Creep Reduction Factor, RFCR ................................................... 3-32 

3.5.2.d Durability Reduction Factor, RFD.............................................. 3-33 

3.5.2.e Durability Reduction Factor, RFD, at Wall Face Unit ............... 3-38 

3.5.2.f LRFD Geosynthetic Resistance Factor, .................................. 3-38 

3.5.2.g Preliminary Design Reduction Factor, RF ................................. 3-39 

3.5.2.h Serviceability Limit State .......................................................... 3-40 

3.6 FACING MATERIALS ..................................................................................... 3-41 

3.6.1 Precast Concrete Panels ......................................................................... 3-41 

3.6.1.a Segmental Panels ....................................................................... 3-41 

3.6.1.b Full-Height Panels ..................................................................... 3-42 

3.6.2 Modular Block Wall (MBW) Units ....................................................... 3-43 

3.6.3 Welded Wire Mesh Facing .................................................................... 3-46 

3.6.4 Geosynthetic Wrap-Around Facing ....................................................... 3-47 

3.6.5 Other Facings ......................................................................................... 3-47 

3.6.6 Two-Stage Facings................................................................................. 3-50 

FHWA NHI-10-024	 Table of Contents 

MSE Walls and RSS – Vol I vi	 November 2009 
CHAPTER 4 DESIGN OF MSE WALLS .................................................................................. 4-1 

4.1 DESIGN METHODOLOGY AND ANALYSIS METHODS ............................ 4-1 

4.1.1 Load and Resistance Factor Design (LRFD) Platform ............................ 4-1 

4.1.2 Analysis Methods..................................................................................... 4-2 

4.2 LOADS AND LOAD COMBINATIONS ........................................................... 4-3 

4.3 DESIGN OF MSE WALLS USING LRFD METHODOLOGY ........................ 4-6 

4.3.1 Design Steps............................................................................................. 4-7 

4.4 MSE WALLS DESIGN GUIDELINES .............................................................. 4-9 

4.4.1 Step 1 - Establish Project Requirements .................................................. 4-9 

4.4.2 Step 2 - Establish Project Parameters ...................................................... 4-9 

4.4.3 Step 3 - Estimate Wall Embedment Depth & Reinforcement Length ... 4-10 

4.4.4 Step 4 - Define Nominal Loads ............................................................. 4-11 

4.4.5 Step 5 - Summarize Load Combinations, Load Factors, and Resistance 

Factors ................................................................................................... 4-17 

4.4.6 Step 6 - Evaluate External Stability....................................................... 4-18 

4.4.6.a Evaluate Sliding Stability .......................................................... 4-19 

4.4.6.b Evaluate Eccentricity ................................................................. 4-22 

4.4.6.c Evaluate Bearing on Foundation................................................ 4-26 

4.4.6.d Settlement Estimate ................................................................... 4-31 

4.4.7 Step 7 - Evaluate Internal Stability ........................................................ 4-31 

4.4.7.a Select Type of Reinforcement ................................................... 4-32 

4.4.7.b Define Critical Slip Surface ....................................................... 4-33 

4.4.7.c Define Unfactored Loads ........................................................... 4-35 

4.4.7.d Establish Vertical Layout of Soil Reinforcements..................... 4-38 

4.4.7.e Calculate Factored Tensile Forces in Reinforcements............... 4-39 

4.4.7.f Calculate Soil Reinforcement Resistance .................................. 4-47 

4.4.7.g Select Grade of &/or Number of Elements at Each Level ......... 4-49 

4.4.7.h Internal Stability with Respect to Pullout Failure...................... 4-49 

4.4.7.i Check Connection Strength ....................................................... 4-52 

4.4.7.j Lateral Movements .................................................................... 4-57 

4.4.7.k Vertical Movement and Bearing Pads ....................................... 4-57 

4.4.8 Step 8 - Design of Facing Elements....................................................... 4-58 

4.4.8.a Design of Concrete, Steel, and Timber Facings ........................ 4-58 

4.4.8.b Design of Flexible Wall Facings................................................ 4-59 

4.4.9 Step 9 - Assess Overall Stability............................................................ 4-59 

4.4.10 Step 10 - Assess Compound Stability .................................................... 4-61 

4.4.11 Step 11 - Wall Drainage Systems .......................................................... 4-65 

4.11.a Surface Drainage........................................................................ 4-65 

4.11.b Surface Water Runoff ................................................................ 4-65 

4.11.c Scour .......................................................................................... 4-65 

4.5 TEMPORARY WALLS .................................................................................... 4-66 

4.6 DESIGN CHECKLIST ...................................................................................... 4-66 

4.7 COMPUTER-AIDED DESIGN ........................................................................ 4-80 

4.8 VENDOR........................................................................................................... 4-80 

4.9 STANDARD MSEW DESIGNS ....................................................................... 4-80 

FHWA NHI-10-024	 Table of Contents 

MSE Walls and RSS – Vol I vii November 2009 
CHAPTER 5 MSE WALL DESIGN DETAILS ......................................................................... 5-1 

5.1 TOP OF WALL ELEMENTS ............................................................................. 5-1 

5.1.1 Copings .................................................................................................... 5-1 

5.1.2 Traffic Barriers......................................................................................... 5-4 

5.1.3 Parapets.................................................................................................... 5-4 

5.1.4 Post and Beam Barriers............................................................................ 5-4 

5.1.5 Drainage Related to Top of Wall Elements ............................................. 5-6 

5.2 BOTTOM OF WALL ELEMENTS .................................................................... 5-6 

5.3 DRAINAGE......................................................................................................... 5-8 

5.3.1 Internal Drainage Systems ....................................................................... 5-9 

5.3.2 External Drainage .................................................................................. 5-17 

5.3.3 Filtration and Drainage System Component Requirements................... 5-24 

5.3.4 Maintenance of Drainage....................................................................... 5-34 

5.4 INTERNAL DETAILS ...................................................................................... 5-35 

5.4.1 Contact Between Dissimilar Metals....................................................... 5-35 

5.4.2 Vertical Obstructions in Reinforced Soil Mass ..................................... 5-35 

5.4.3 Horizontal Obstructions in Reinforced Soil Mass ................................. 5-42 

5.4.4 Wall Face Penetrations .......................................................................... 5-44 

5.4.5 Slip Joints............................................................................................... 5-46 

5.4.6 Wall Curves ........................................................................................... 5-47 

5.4.7 Wall Corners .......................................................................................... 5-51 

5.4.8 Two-Stage Facing .................................................................................. 5-52 

5.5 WALL INITIATIONS AND TERMINATIONS .............................................. 5-54 

5.6 AESTHETICS.................................................................................................... 5-56 

CHAPTER 6 DESIGN OF MSE WALLS WITH COMPLEX GEOMETRICS ........................ 6-1 

6.1 BRIDGE ABUTMENTS ..................................................................................... 6-3 

6.1.1 MSEW Abutments on Spread Footings ................................................... 6-3 

6.1.2 MSEW Abutments on Stub Footings Supported by Deep Foundations 

through Reinforced Wall Fill ................................................................... 6-6 

6.1.3 Alternative Configuration of MSE Walls at Bridge Abutments ............ 6-10 

6.1.4 Protection of MSE Wall at Abutments .................................................. 6-11 

6.2 SUPERIMPOSED (TIERED) MSE WALLS.................................................... 6-13 

6.2.1 2-Tier Superimposed Wall ..................................................................... 6-13 

6.2.2 Superimposed Walls with More than 2-Tiers ........................................ 6-15 

6.3 WALLS WITH UNEVEN REINFORCEMENT LENGTHS ........................... 6-16 

6.4 BACK-TO-BACK (BBMSE) WALLS ............................................................. 6-17 

6.5 SHORED MSE WALLS FOR STEEP TERRAINS AND LOW VOLUME 

ROADS.............................................................................................................. 6-19 

6.6 STABLE FEATURE MSE (SFMSE) WALLS ................................................. 6-26 

CHAPTER 7 DESIGN OF MSE WALLS FOR EXTREME EVENTS...................................... 7-1 

7.1 SEISMIC EVENTS ............................................................................................. 7-1 

7.1.1 External Stability ..................................................................................... 7-1 

7.1.2 Internal Stability....................................................................................... 7-9 

7.1.2.a Tensile Failure ........................................................................... 7-11 

7.1.2.b Pullout Failure............................................................................ 7-12 

FHWA NHI-10-024	 Table of Contents 

MSE Walls and RSS – Vol I viii November 2009 
7.1.3 Facing Reinforcement Connections ....................................................... 7-13 

7.2 VEHICULAR IMPACT EVENTS .................................................................... 7-14 

7.2.1 Traffic Barriers....................................................................................... 7-15 

7.2.2 Post and Beam Railings ......................................................................... 7-17 

7.3 FLOODS AND SCOUR .................................................................................... 7-18 

FHWA NHI-10-024 Table of Contents 

MSE Walls and RSS – Vol I ix November 2009
 
 

FHWA NHI-10-024 Table of Contents
 

MSE Walls and RSS – Vol I x November 2009
 
 

LIST OF TABLES – Volume I
 

Table 1-1. Summary of Reinforcement and Face Details for MSE Wall Systems. .............. 1-7 

Table 1-2. Representative List of Geogrid and Geotextile Reinforcement Manufacturers 

and Suppliers...................................................................................................... 1-11 

Table 2-1. Typical Minimum Length of Reinforcement ..................................................... 2-43 

Table 2-2. Minimum MSEW Embedment Depths .............................................................. 2-43 

Table 2-3. Relationship Between Joint Width and Limiting Differential 

Settlements for MSE Precast Panels .................................................................. 2-46 

Table 3-1. MSE Wall Select Granular Reinforced Fill Requirements .................................. 3-3 

Table 3-2. RSS Granular Reinforced Fill Requirements....................................................... 3-6 

Table 3-3. Recommended Limits of Electrochemical Properties for Reinforced Fills with 

Steel Reinforcement ............................................................................................. 3-9 

Table 3-4. Recommended Limits of Electrochemical Properties for Reinforced Fills with 

Geosynthetic Reinforcements ............................................................................ 3-10 

Table 3-5. Basic Aspects of Reinforcement Pullout Performance in Granular 

and Cohesive Soils of Low Plasticity ................................................................ 3-15 

Table 3-6. Summary of Pullout Capacity Design Parameters. ............................................ 3-17 

Table 3-7. Minimum Galvanization Thickness by Steel Thickness.................................... 3-26 

Table 3-8. Steel Corrosion Rates for Mildly Corrosive Reinforced Fill ............................. 3-26 

Table 3-9. Installation Damage Reduction Factors. ............................................................ 3-32 

Table 3-10. Anticipated Resistance of Polymers to Specific Environments......................... 3-34 

Table 3-11. Durability (Aging) Reduction Factors for PET. ................................................ 3-36 

Table 3-12. Minimum Requirements for use of Default Durability Reduction Factors 

(RFD) for Primary Geosynthetic Reinforcement................................................ 3-37 

Table 3-13. Minimum Requirements for use of Preliminary Design Reduction Factor (RF) 

for Primary Geosynthetic Reinforcement. ......................................................... 3-40 

Table 3-14. Additional MBW Cold Weather Requirements Recommended by 

 Mn/DOT (2008). ................................................................................................ 3-45

Table 4-1. Typical MSE Wall Load Combinations and Load Factors (after Table 3.4.1-1, 

AASHTO, 2007) ................................................................................................. 4-4 

Table 4-2. Typical MSE Wall Load Factors for Permanent Loads, γp (after Table 3.4.1-2, 

AASHTO, 2007) ................................................................................................. 4-4 

Table 4-3. Basic LRFD Design Steps for MSE Walls .......................................................... 4-8 

Table 4-4. Equivalent Height of Soil, heq, for Traffic Loading on Abutments ........................... 

Perpendicular to Traffic (Table 3.11.6.4-1, AASHTO {2007}) ....................... 4-17 

Table 4-5. External Stability Resistance Factors for MSE Walls (Table 11.5.6-1, 

 AASHTO {2007}) ............................................................................................ 4-19

Table 4-6. Bearing Resistance Factors (Table 10.6.3.1.2a-1, AASHTO {2007}) ............. 4-29 

Table 4-7. Resistance Factors for Tensile and Pullout Resistance for MSE Walls (after 

Table 11.5.6-1, AASHTO (2007........................................................................ 4-48 

Table 4-8. Example MBW Faced MSE Wall Standard Design (Minnesota DOT, 2008) .. 4-82 

FHWA NHI-10-024 Table of Contents 

MSE Walls and RSS – Vol I xi November 2009 
Table 5-1. Values of B for Various Cu Values for Soils with less than 50% Passing the 

No. 200 Sieve ..................................................................................................... 5-27 

Table 5-2. Values of B and AOS for Soils with more than 50% Passing the No. 200 Sieve 

Based on Type of Geotextile ............................................................................... 5-27 

Table 5-3. Geotextile Survivability Requirements (AASHTO M 288, 2006) ................... 5-29 

Table 5-4. Recommended Minimum Properties for General Geomembrane Installation 

Survivability (after Koerner, 1998) ................................................................... 5-33 

FHWA NHI-10-024 Table of Contents 

MSE Walls and RSS – Vol I xii November 2009 
LIST OF FIGURES – Volume I 

Figure 1-1. Generic cross section of an MSE structure. ......................................................... 1-4 

Figure 1-2. Generic cross sections of reinforced soil slope structures, reinforcements used 

to: (a) increase stability of a slope; and (b) provide improved compaction and 

surficial stability at edge of a slopes. ................................................................... 1-5 

Figure 2-1. Representative MSE applications (a) retaining wall; (b) access ramp; (c) 

waterfront structure; and (d) bridge abutment. .................................................... 2-2 

Figure 2-2. MSE walls to support temporary bridge abutment roadway embankment .......... 2-3 

Figure 2-3. MSE wall used to temporarily support a permanent roadway embankment for 

phased construction.............................................................................................. 2-3 

Figure 2-4. Application of reinforced soil slopes. .................................................................. 2-5 

Figure 2-5. Cost evaluation of reinforced soil slopes. .......................................................... 2-10 

Figure 2-6. Example MSE wall surface treatments. ............................................................. 2-13 

Figure 2-7. Examples of commercially available MBW units (NCMA, 1997).................... 2-14 

Figure 2-8. Erection of precast panels. ................................................................................. 2-21 

Figure 2-9. Fill spreading and reinforcement connection..................................................... 2-22 

Figure 2-10. Compaction of reinforced wall fill..................................................................... 2-23 

Figure 2-11. Lift construction sequence for geosynthetic faced MSE walls. ......................... 2-25 

Figure 2-12. Typical geosynthetic face construction detail. ................................................... 2-26 

Figure 2-13. Types of geosynthetic reinforced soil wall facing (after Wu, 1994) ................. 2-27 

Figure 2-14. Reinforced slope construction: (a) geogrid and fill placement; (b) soil filled 

erosion control mat placement; and (c) finished, vegetated 1:1 slope ............... 2-29 

Figure 2-15. Empirical curve for estimating probable anticipated lateral displacement 

during construction for MSE walls (after FHWA 89-043 {Christopher et al., 

1990}) .............................................................................................................. 2-41 

Figure 2-16. MSE wall embedment depth requirements, (a) level toe condition and (b)
 

benched slope toe condition............................................................................... 2-44 

Figure 2-17. MSE wall construction on Mn/DOT Crosstown Project, 2008. ........................ 2-48 

Figure 2-18. Veterans Memorial Overpass............................................................................. 2-49 

Figure 2-19. MSE true bridge abutment ................................................................................. 2-49 

Figure 2-20. Typical cross section, VMO project .................................................................. 2-50 

Figure 2-21. SeaTac Airport runway extension MSE wall..................................................... 2-51 

Figure 2-22. Guanella Pass wire-faced wall........................................................................... 2-52 

Figure 2-23. Guanella Pass architectural concrete faced wall ................................................ 2-52 

Figure 3-1. Examples of reinforced fill zone extension beyond the reinforced zone. ............ 3-8 

Figure 3-2. Stress transfer mechanisms for soil reinforcement. ........................................... 3-11 

Figure 3-3. Coverage ratio.................................................................................................... 3-13 

Figure 3-4. Definition of grid dimensions for calculating pullout capacity ......................... 3-18 

Figure 3-5. Parameters for metal reinforcement strength calculations showing (a) steel 

strips and (b) metallic grids and bar mats .......................................................... 3-24 

Figure 3-6. Long-term geosynthetic reinforcement strength concepts ................................. 3-29 

Figure 3-7. Large, wet-cast concrete face unit with reinforcement placed between units. .. 3-48 

Figure 3-8. Large, wet-cast concrete face unit with embedded reinforcement connectors. . 3-49 

Figure 3-9. Geocell face unit with vegetation. ..................................................................... 3-49 

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MSE Walls and RSS – Vol I xiii November 2009 
Figure 4-1. External stability load factors for simple walls.................................................... 4-5 

Figure 4-2. External analysis: nominal earth pressures; horizontal backslope with traffic 

surcharge (after AASHTO, 2007) ..................................................................... 4-12 

Figure 4-3. External analysis: earth pressure; sloping backfill case (after AASHTO, 

2007). ................................................................................................................ 4-13 

Figure 4-4. External analysis: earth pressure; broken backslope case (after AASHTO, 

2007) ................................................................................................................ 4-14 

Figure 4-5. Notation for Coulomb active earth pressures used with wall batters, , greater 

than 100 (after AASHTO, 2007)..................................................................... 4-16 

Figure 4-6. Potential external failure mechanisms for a MSE wall...................................... 4-18 

Figure 4-7. Calculation of eccentricity and vertical stress for bearing check, for horizontal 

backslope with traffic surcharge condition ........................................................ 4-24 

Figure 4-8. Calculation of eccentricity and vertical stress for bearing check, for sloping
 

backslope condition ........................................................................................... 4-25 

Figure 4-9. Location of potential failure surface for internal stability design of MSE Walls 

(a) inextensible reinforcements and (b) extensible reinforcements ................... 4-34 

Figure 4-10. Variation of the coefficient of lateral stress ratio (K/Ka) with depth in a MSE 

wall (Elias and Christopher, 1997; AASHTO, 2002; after AASHTO, 2007) .. 4-37 

Figure 4-11. Calculation of vertical stress for sloping backfill conditions for internal 

stability............................................................................................................... 4-38 

Figure 4-12. Distribution of stress from concentrated vertical load for internal and external 

stability calculations........................................................................................... 4-44 

Figure 4-13. Distribution of stresses from concentrated horizontal loads.............................. 4-45 

Figure 4-14. Reinforcement load contributory height ............................................................ 4-46 

Figure 4-15. Nominal vertical stress at the reinforcement level in the resistant zone, 

beneath a sloping backfill .................................................................................. 4-50 

Figure 4-16. Bodkin connection detail (looking at cross section of segmental panel face .... 4-53 

Figure 4-17. Determination of hinge height for modular concrete block faced MSE walls 

(NMCA, 1997) .................................................................................................. 4-56 

Figure 4-18. Typical geometries where MSE wall compound stability is of concern: steep 

and tall backslope on top of the wall; tiered walls; slope at the toe of the wall; 

and water at toe of the slope .............................................................................. 4-64 

Figure 4-19. Example of standard MSEW design. (Minnesota DOT, 2008) ....................... 4-83 

Figure 5-1. Example copings for MSE walls.......................................................................... 5-2 

Figure 5-2. Example traffic barrier for MSE walls. (a) Barrier behind coping, (b) barrier 

on top of panels, (c) barrier on top of modular block units ................................. 5-5 

Figure 5-3. Leveling pads, (a) Common size, b) Step detail for precast panel facing units, 

(c) Step detail for modular block facing units ..................................................... 5-7 

Figure 5-4. Potential sources and flowpaths of water........................................................... 5-10 

Figure 5-5. Example layout of filter at joints between segmental precast facing units ........ 5-11 

Figure 5-6. Layout of drainage fabric and drainage fill at the face for modular block units. 

(Collin et al., 2002) ............................................................................................ 5-12 

Figure 5-7. Example layout of geotextile filter near the face for welded wire facing units . 5-13 

Figure 5-8. Example drainage blanket detail behind the retained backfill ........................... 5-14 

Figure 5-9. Example drainage detail using a geocomposite sheet drain............................... 5-15 

 

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Figure 5-10. Example drainage detail using a blanket drain with chimney drain (Collin et 

al., 2002) ............................................................................................................ 5-16 

Figure 5-14. Example detail for wall that may experience inundation................................... 5-17 

Figure 5-12. Example MSE wall overflow sill at top of wall................................................. 5-19 

Figure 5-13. Example drainage swale near top of wall. ((b) Collin et al., 2002) ................... 5-20 

Figure 5-14. (a) Example geomembrane barrier details, (b) Installation of geomembrane 

deicing salt runoff barrier, (c) Geomembrane installation around manhole 

penetration.......................................................................................................... 5-22 

Figure 5-15. Example of undesirable water seepage through pavement due to deficient 

grades (Collin et al., 2002)................................................................................. 5-23 

Figure 5-16. Example of surface flow erosion at the bottom of an MSE wall ....................... 5-24 

Figure 5-17. Examples of avoiding a vertical obstruction without severing soil 

reinforcements.................................................................................................... 5-37 

Figure 5-18. Vertical obstructions in reinforced soil mass with segmental precast facing
 

units ................................................................................................................ 5-39 

Figure 5-19. Example of a structural frame around vertical obstruction (a) with segmental 

precast facing - note that vertically adjacent layers of reinforcement to be 

separated by a minimum of 3-in. (75 mm) of wall fill, (b)-(c) with modular 

block face – note corner detail ........................................................................... 5-40 

Figure 5-20. Example details of reinforcements around vertical obstructions in reinforced 

soil mass with modular block units.................................................................... 5-41 

Figure 5-21. Navigating horizontal obstruction in MSE walls with inextensible 

reinforcements.................................................................................................... 5-43 

Figure 5-22. Navigating horizontal obstruction in MSE walls with extensible 

 reinforcement ..................................................................................................... 5-43

Figure 5-23. Example of backup panels for large horizontal obstructions ............................. 5-44 

Figure 5-24. Example pipe penetrations through segmental precast panel facing units ........ 5-45 

Figure 5-25. Example pipe penetration through modular block facing units ......................... 5-46 

Figure 5-26. Example slip joints for segmental precast panel facings ................................... 5-48 

Figure 5-27. Example slip joint for modular block wall facings ............................................ 5-49 

Figure 5-28. Example layout of geogrid reinforcement for walls with curves....................... 5-50 

Figure 5-29. Example corner details....................................................................................... 5-51 

Figure 5-30. Conceptual connection details for a 2-stage facing system ............................... 5-53 

Figure 5-31. Common joint details between segmental precast panel facing units and CIP 

structures............................................................................................................ 5-55 

Figure 5-32. Common joint between modular block facing units and CIP structures ........... 5-55 

Figure 5-33. Example of MSE wall aesthetics ....................................................................... 5-58 

Figure 5-34. Examples of cast-in-place abutment to MSE wall panel transitions.................. 5-59 

Figure 6-1. Types of complex MSE structures ....................................................................... 6-2 

Figure 6-2. Geometry definition, location of critical failure surface and variation of Kr and 

F* parameters for analysis of a MSEW abutment on spread footing ................. 6-4 

Figure 6-3. Details of a typical pile supported MSE abutment .............................................. 6-7 

Figure 6-4. Geometry definition and typical supplemental lateral pressure distribution 

from deep foundation on MSE wall face ............................................................. 6-9 

Figure 6-5. Example of use of a geosynthetic wrapped face wall behind an integral 

abutment............................................................................................................. 6-12 

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Figure 6-6. Example abutment seat detail ............................................................................ 6-12 

Figure 6-7. Design rules for a 2-tier superimposed MSE wall system ................................. 6-14 

Figure 6-8. Dimensioning of MSE wall with uneven reinforcement lengths ....................... 6-17 

Figure 6-9. Back–to–back MSE walls.................................................................................. 6-18 

Figure 6-10. Generic cross-section of a shored MSE (SMSE) wall system for steep terrains 

(Morrison et al., 2006) ....................................................................................... 6-19 

Figure 6-11. Minimum recommended geometry of a shored MSE wall system in steep 

terrains, (a) with extension of upper two rows of reinforcement, and (b) with 

the upper two rows connected to the shoring wall (Morrison et al., 2006) ....... 6-21 

Figure 6-12. Location of potential failure surface for internal stability design of shored 

MSE walls (a) extensible reinforcements, (b) inextensible reinforcements 

(Morrison et al., 2006) ....................................................................................... 6-23 

Figure 6-13. Distribution of stress from concentrated vertical load for internal and external 

stability calculations (Morrison et al., 2006) ..................................................... 6-23 

Figure 6-14. Computation for TMAX and evaluation of pullout resistance (after Morrison et 

al., 2006) ............................................................................................................ 6-25 

Figure 6-15. Example global stability and compound failure surfaces (Morrison et al., 

2006) ................................................................................................................ 6-26 

Figure 6-16. Minimum recommended geometry of a stable feature MSE (SFMSE) wall 

system ................................................................................................................ 6-28 

Figure 7-1. Definition of heights for seismic analyses ..................................................................7-3 

Figure 7-2. Use of a slope stability approach to compute seismic earth pressure........................7-6 

Figure 7-3. Procedure for determination of ky (Anderson et al., 2008) ........................................7-7 

Figure 7-4. Boundary between WUS and CEUS (Anderson et al. 2008) ...................................7-8 

Figure 7-5. Seismic internal stability of a MSE wall............................................................ 7-10 

Figure 7-6. Comparison of static and dynamic impact force with 1-inch (25 mm) 

maximum displacement (Bligh et al., 2009) ...................................................... 7-15 

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MSE Walls and RSS – Vol I xvi November 2009 
 

TABLE OF CONTENTS – Volume II
 

CHAPTER 8 REINFORCED (STEEPENED) SOIL SLOPES PROJECT EVALUATION ..... 8-1 

8.1 INTRODUCTION ............................................................................................... 8-1 

8.2 REINFORCED SOIL SLOPE SYSTEMS .......................................................... 8-1 

8.2.1 Types of Systems ..................................................................................... 8-1 

8.2.2 Construction Materials ............................................................................. 8-2 

8.3 DESIGN APPROACH ........................................................................................ 8-2 

8.3.1 Use Considerations .................................................................................. 8-2 

8.3.2 Design of Reinforcement for Compaction Aid ........................................ 8-4 

8.3.3 Design of Reinforcement for Steepening Slopes and Slope Repair ........ 8-4 

8.3.4 Computer-Assisted Design ...................................................................... 8-6 

8.3.5 Evaluation of External Stability .............................................................. 8-8 

8.4 CONSTRUCTION SEQUENCE ...................................................................... 8-11 

8.5 TREATMENT OF OUTWARD FACE............................................................. 8-15 

8.5.1 Grass Type Vegetation........................................................................... 8-15 

8.5.2 Soil Bioengineering (Woody Vegetation) ............................................. 8-16 

8.5.3 Armored ................................................................................................. 8-19 

8.6 DESIGN DETAILS ........................................................................................... 8-20 

8.6.1 Guardrail and Traffic Barriers ............................................................... 8-20 

8.6.2 Drainage Considerations ........................................................................ 8-20 

8.6.3 Obstructions ........................................................................................... 8-21 

8.7 CASE HISTORIES ............................................................................................ 8-21 

8.7.1 The Dickey Lake Roadway Grade Improvement Project ...................... 8-21 

8.7.2 Salmon-Lost Trail Roadway Widening Project ..................................... 8-24 

8.7.3 Cannon Creek Alternate Embankment Construction Project ................ 8-26 

8.7.4 Pennsylvania SR 54 Roadway Repair Project ....................................... 8-27 

8.7.5 Massachusetts Turnpike - Use of Soil Bioengineering .......................... 8-29 

8.7.6 242-foot High 1H:1V Reinforced Soil Slope for Airport Runway 

Extension................................................................................................ 8-32 

8.8 STANDARD RSS DESIGNS ............................................................................ 8-34 

CHAPTER 9 DESIGN OF REINFORCED SOIL SLOPES ....................................................... 9-1 

9.1 INTRODUCTION ............................................................................................... 9-1 

9.2 REINFORCED SLOPE DESIGN GUIDELINES............................................... 9-3 

9.2.1 Step 1 - Establish the geometric, loading, and performance 

requirements for design............................................................................ 9-3 

9.2.2 Step 2 - Determine the engineering properties of the in-situ soils........... 9-3 

9.2.3 Step 3 - Determine the properties of reinforced fill and, if different, the 

retained fill............................................................................................... 9-5 

9.2.4 Step 4 - Evaluate design parameters for the reinforcement ..................... 9-5 

9.2.5 Step 5 - Check unreinforced stability....................................................... 9-5 

9.2.6 Step 6 - Design reinforcement to provide a stable slope ......................... 9-7 

9.2.7 Step 7 - Check external stability ............................................................ 9-16 

9.2.8 Step 8 - Seismic stability........................................................................ 9-18 

9.2.9 Step 9 - Evaluate requirements for subsurface and surface water runoff 

control .................................................................................................... 9-19 

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MSE Walls and RSS – Vol I xvii	 November 2009 
9.3 COMPUTER ASSISTED DESIGN .................................................................. 9-23 

9.4 PROJECT COST ESTIMATES ........................................................................ 9-24 

 CHAPTER 10 CONTRACTING METHODS AND SPECIFICATIONS
 

FOR MSE WALLS AND SLOPES............................................................................... 10-1 

10.1 POLICY DEVELOPMENT............................................................................... 10-2 

10.2 SYSTEM OR COMPONENT APPROVALS ................................................... 10-2 

10.3 DESIGN AND PERFORMANCE CRITERIA ................................................. 10-5 

10.4 AGENCY OR SUPPLIER DESIGN ................................................................. 10-5 

10.4.1 Plan and Elevation Sheets...................................................................... 10-6 

10.4.2 Facing/Panel Details ............................................................................ 10-7 

10.4.3 Drainage Facilities/Special Details........................................................ 10-7 

10.4.4 Design Computations............................................................................. 10-7 

10.4.5 Geotechnical Report............................................................................... 10-7 

10.4.6 Construction Specifications ................................................................... 10-8 

10.5 END RESULT DESIGN APPROACH ............................................................. 10-8 

10.5.1 Geometric Requirements ....................................................................... 10-9 

10.5.2 Geotechnical Requirements ................................................................... 10-9 

10.5.3 Structural and Design Requirements ..................................................... 10-9 

10.5.4 Performance Requirements .................................................................. 10-10 

10.6 STANDARD DESIGNS .................................................................................. 10-10 

10.6.1 MSEW Standard Designs .................................................................... 10-10 

10.6.2 RSS Standard Designs ......................................................................... 10-12 

10.7 REVIEW AND APPROVALS........................................................................ 10-13 

10.8 CONSTRUCTION SPECIFICATIONS AND SPECIAL PROVISIONS FOR 

MSEW AND RSS CONSTRUCTION ............................................................ 10-14 

10.9 EXAMPLE SPECIFICATION FOR MSE WALLS ....................................... 10-15 

10.10 CONSTRUCTION SPECIFICATIONS FOR REINFORCED 

SLOPE SYSTEMS ...................................................................................... 10-41 

10.10.1 Specification Guidelines For RSS Construction (Agency Design) 10-42 

10.10.2 Specification for Erosion Control Mat or Blanket......................... 10-46 

10.10.3 Specification for Geosynthetic Drainage Composite..................... 10-48 

10.10.4 Specification Guidelines for Geosynthetic Reinforced Soil 

 Slope Systems ................................................................................ 10-52 

CHAPTER 11 FIELD INSPECTION AND PERFORMANCE MONITORING..................... 11-1 

11.1 PRECONSTRUCTION REVIEWS................................................................... 11-1 

11.1.1 Plans and Specifications ........................................................................ 11-3 

11.1.2 Review of Site Conditions and Foundation Requirements .................... 11-3 

11.2 PREFABRICATED MATERIALS INSPECTION ......................................... 11-11 

11.2.1 Precast Concrete Elements................................................................... 11-11 

11.2.2 Reinforcing Elements........................................................................... 11-13 

11.2.3 Facing Joint Materials.......................................................................... 11-15 

11.2.4 Reinforced Backfill .............................................................................. 11-15 

11.3 CONSTRUCTION CONTROL ....................................................................... 11-16 

11.3.1 Leveling Pad ........................................................................................ 11-16 

11.3.2 Erection of Facing Elements ................................................................ 11-16 

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11.3.3 Reinforced Fill Placement, Compaction .............................................. 11-22 

11.3.4 Placement of Reinforcing Elements..................................................... 11-25 

11.3.5 Placement of Subsequent Facing Courses (Segmental Facings) ......... 11-26 

11.4 PERFORMANCE MONITORING PROGRAMS .......................................... 11-35 

11.4.1 Purpose of Monitoring Program .......................................................... 11-35 

11.4.2 Limited Monitoring Program ............................................................... 11-36 

11.4.3 Comprehensive Monitoring Program .................................................. 11-37 

11.4.4 Program Implementation ..................................................................... 11-37 

11.4.5 Data Interpretation ............................................................................... 11-40 

REFERENCES ............................................................................................................................R-1 

APPENDICES 

APPENDIX A LRFD LOAD COMBINATIONS AND LOAD FACTORS, AND LOAD 

FACTOR FOR PERMANENT LOADS ....................................................................... A-1 

APPENDIX B DETERMINATION OF PULLOUT RESISTANCE FACTORS .................B-1 

B.1 EMPIRICAL PROCEDURES TO DETERMINE F*

 AND α .......................B-1 

B.2 EXPERIMENTAL PROCEDURES TO DETERMINE F* AND α .............B-2 

B.3 CONNECTION RESISTANCE AND STRENGTH OF PARTIAL AND 

FULL FRICTION SEGMENTAL BLOCK/REINFORCEMENT FACING 

CONNECTIONS ...........................................................................................B-5 

B.4 CONNECTION RESISTANCE DEFINED WITH

 SHORT-TERM TESTING .........................................................................B-12 

B.5 REFERENCES ............................................................................................B-15 

APPENDIX C TYPICAL DIMENSIONS OF STEEL REINFORCEMENTS .....................C-1 

APPENDIX D DETERMINATION OF CREEP STRENGTH REDUCTION FACTOR ... D-1 

D.1 BACKGROUND .......................................................................................... D-1 

D.2 STEP-BY-STEP PROCEDURES FOR EXTRAPOLATING STRESS 

RUPTURE DATA -- CONVENTIONAL METHOD .................................. D-5 

D.3 STEP-BY-STEP PROCEDURES FOR EXTRAPOLATING STRESS 

RUPTURE DATA -- STEPPED ISOTHERMAL METHOD (SIM) ......... D-10 

D.4 DETERMINATION OF RFCR .................................................................... D-13 

D.5 USE OF CREEP DATA FROM "SIMILAR" PRODUCTS AND 

EVALUATION OF PRODUCT LINES .................................................... D-14 

D.6 CREEP EXTRAPOLATION EXAMPLES USING STRESS

 RUPTURE DATA .......................................................................................E-16 

D.6.1 Stress Rupture Extrapolation Example ............................................E-16 

D.7 REFERENCES ............................................................................................E-20 

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MSE Walls and RSS – Vol I xix	 November 2009 
 

APPENDIX E EXAMPLE CALCULATIONS 

E.1 MBW Unit Faced MSE Wall with Broken Back Sloping Fill and Live 

Load Surcharge, Reinforced with Geogrid ................................................. E.1-1 

E.2 Bearing Check for Sloping Toe Conditions, with and without Water ........ E.2-1 

E.3 Segmental Precast Panel MSE Wall with Sloping Backfill Surcharge, 

Reinforced with Steel Strips .......................................................................E.3-1 

E.4 Segmental Precast Panel MSE Wall with Level Backfill and Live Load 

Surcharge, Reinforced with Steel Bar Mats................................................E.4-1 

E.5 True Bridge Abutment with Segmental Precast Panel MSE Wall, 

Reinforced with Steel Strips .......................................................................E.5-1 

E.6 Traffic Barrier Impact Loading on Top of an MSE Wall ...........................E.6-1 

E.7 Seismic Loading . . . ., Psuedo-Static Analysis........................................... E.7-1 

E.8 Reinforced Soil Slope Design -- Road Widening .......................................E.8-1 

E.9 Reinforced Soil Slope Design -- High Slope for New Road Construction . E.9-1 

E.10 Reinforced Soil Slope Design -- Facing Stability Calculation .................E.10-1 

APPENDIX F OTHER DESIGN PROCEDURES AND ANALSIS MODELS 

F.1 ASD Procedure and the Simplified Method ..................................................F-2 

F.2 Coherent Gravity Method ..............................................................................F-3 

F.3 National Concrete Masonry Association Procedure ......................................F-5 

F.4 GRS ......................................................................................................F-5 

F.5 FHWA Structural Stiffness Method............................................................... F-7 

F.6 K-Stiffness Method ........................................................................................F-8 

F.7 Deep Patch ...................................................................................................F-11 

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LIST OF TABLES – Volume II 

Table 8-1. RSS Slope Facing Options (after Collin, 1996)................................................. 8-14 

Table 9-1. Estimated Project Costs. .................................................................................... 9-25 

Table 11-1. Outline of MSE/RSS Field Inspection Checklist. .............................................. 11-2 

Table 11-2. Checklist for Drawing Review........................................................................... 11-5 

Table 11-3. Checklist for Specification Compliance. ........................................................... 11-7 

Table 11-4. Out-of-Tolerance Conditions and Possible Causes.......................................... 11-30 

Table 11-5. Checklist for Construction. .............................................................................. 11-32 

Table 11-6. Possible Instruments for Monitoring Reinforced Soil Structures. ................... 11-38 

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FHWA NHI-10-024 Table of Contents
 

MSE Walls and RSS – Vol I xxii November 2009
 
LIST OF FIGURES – Volume II 

Figure 8-1. Failure modes for reinforced soil slopes including internal failure within the 

reinforced soil zone, external failure entirely outside the reinforced soil zone, 

and compound failure starting behind and passing through the reinforced soil 

zone .................................................................................................................. 8-3 

Figure 8-2. Modified limit equilibrium analysis for reinforced slope design......................... 8-5 

Figure 8-3. External failure modes for reinforced soil slopes. ............................................... 8-9 

Figure 8-4. Construction of reinforced soil slopes. .............................................................. 8-12 

Figure 8-5. Welded wire mesh detail for temporary (during construction) or permanent 

face support........................................................................................................ 8-15 

Figure 8-6. Components of a vegetated reinforced slope (VRSS) system ........................... 8-17 

Figure 8-7. Dickey Lake site. ............................................................................................... 8-23 

Figure 8-8. Salmon Lost Trail site. ....................................................................................... 8-25 

Figure 8-9. Cannon Creek project. ....................................................................................... 8-26 

Figure 8-10. Pennsylvania SR54. ........................................................................................... 8-28 

Figure 8-11. Massachusetts Turnpike during construction, immediately after construction 

and after the second growing season. ................................................................ 8-31 

Figure 8-12. Reinforced soil slope for runway extension at Yeager Airport, Charleston, 

West Virginia..................................................................................................... 8-33 

Figure 8-13. Example of standard RSS design. ...................................................................... 8-36 

Figure 9-1. Flow chart of steps for reinforced soil slope design. ........................................... 9-2 

Figure 9-2. Requirements for design of reinforced soil slopes............................................... 9-4 

Figure 9-3. Critical zone defined by rotational and sliding surface that meet the required 

safety factor.......................................................................................................... 9-6 

Figure 9-4. Rotational shear approach to determine required strength of reinforcement....... 9-8 

Figure 9-5. Chart solution for determining the reinforcement strength requirements.......... 9-10 

Figure 9-6. Reinforcement spacing considerations for high slopes...................................... 9-11 

Figure 9-7. Developing reinforcement lengths. .................................................................... 9-14 

Figure 9-8. Failure through the foundation........................................................................... 9-17 

Figure 9-9. Seismic stability analysis. .................................................................................. 9-19 

Figure 9-10. Subsurface drainage considerations. .................................................................. 9-20 

Figure 11-1. Casting yard for precast facing elements. ........................................................ 11-12 

Figure 11-2. Inspect reinforcing elements: top photo shows a variety of reinforcements 

including metallic strips, welded wire mesh, and geosynthetics and bottom 

photo shows reinforcement length painted on geogrid reinforcement............. 11-14 

Figure 11-3. Leveling pads: a) leveling the concrete, b) completed pad, and c) placing the 

facing elements on the leveling pad. ................................................................ 11-17 

Figure 11-4. Checking facing element batter and alignment................................................ 11-18 

Figure 11-5. Full height facing panels require special alignment care. ................................ 11-19 

Figure 11-6. Setting first row of precast facing elements..................................................... 11-21 

Figure 11-7. Placement of reinforced fill. ............................................................................ 11-23 

Figure 11-8. Compaction equipment showing: a) large equipment permitted away from 

face; and b) lightweight equipment within 3 ft (1 m) of the face. ................... 11-24 

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Figure 11-9. Facing connection examples ............................................................................ 11-27 

Figure 11-10. Geotextile joint cover and neoprene pads. ....................................................... 11-29 

FHWA NHI-10-024 Table of Contents 

MSE Walls and RSS – Vol I xxiv November 2009
 
 

CHAPTER 1 

INTRODUCTION
 

1.1 OBJECTIVES 

New methods and technologies of retention and steepened-slope construction continue to be 

developed, often by specialty contractors and suppliers, to solve problems in locations of 

restricted Right-of-Way (ROW), at marginal sites with difficult subsurface conditions and 

other environmental constraints, and to expedite construction. Professionals charged with the 

responsibility for planning, designing, and implementing improvements and additions in such 

locations should understand the application, limitations and costs associated with a host of 

measures and technologies available. 

This manual was prepared to assist design engineers, specification writers, estimators, 

construction inspectors and maintenance personnel with the selection, design, construction 

and maintenance of Mechanically Stabilized Earth Walls (MSEW) and Reinforced Soil 

Slopes (RSS). 

The design, construction and monitoring techniques for these structures have evolved over 

the last three decades as a result of efforts by researchers, material suppliers and government 

agencies to improve some single aspect of the technology or the materials used. This manual 

is a comprehensive document that integrates all design, construction, materials, contracting, 

and monitoring aspects required for successful project implementation. 

This manual has been developed in support of FHWA educational programs on the design, 

construction, and maintenance of MSE wall and RSS structures construction. Its principal 

function is to serve as a reference source to the materials presented. The manual serves as 

FHWA's primary technical guideline on the use of these technologies on transportation 

facilities. 

1.1.1 Scope 

The manual addresses in a comprehensive manner the following areas: 

C Overview of MSE development and the cost, advantages, and disadvantages of using 

MSE structures. 

C Available MSE systems and applications to transportation facilities. 

C Basic soil-reinforcement interaction. 

C Design of routine and complex MSE walls. 

FHWA NHI-10-024 1 – Introduction 

MSE Walls and RSS – Vol I 1 – 1 November 2009
 
 

C Design of MSE walls for extreme events. 

C Design detailing of MSE walls. 

C Design of steepened RSS. 

C Specifications and contracting approaches for both MSE walls and RSS construction. 

C Construction monitoring and inspection. 

C Design examples. 

C A separate companion manual addresses long-term corrosion of metallic reinforcements 

and long-term degradation of polymeric reinforcements. Sections of the 

Corrosion/Degradation manual address the background of full-scale, long-term 

evaluation programs and the procedures required to develop, implement, and evaluate 

them. These procedures have been developed to provide practical information on this 

topic for MSE users for non-corrosion or polymer specialists, who are interested in 

developing long-term monitoring programs for these types of structures. 

As an integral part of this Manual, several example calculations are appended that 

demonstrate individual design aspects. 

1.1.2 Source Documents 

This Mechanically Stabilized Earth Walls (MSE) and Reinforced Soil Slopes (RSS), Design 

and Construction Guidelines Manual is an update of the current FHWA NHI-00-043 (Elias et 

al., 2001), has evolved from the following AASHTO and FHWA references: 

C AASHTO LRFD Bridge Design Specifications, 4th Edition, 2007, with 2008 and 2009 

Interim Revisions 

C Earth Retaining Structures, FHWA-NHI-07-071 (Tanyu et al., 2008) 

C AASHTO LRFD Bridge Construction Specifications, 2nd Edition, 2004, with 2006, 2007, 

2008, and 2009 Interim Revisions 

C Geosynthetic Design and Construction Guidelines, FHWA NHI-07-092 (Holtz et al., 

2008) 

C Guidelines for Design, Specification, and Contracting of Geosynthetic Mechanically 

Stabilized Earth Slopes on Firm Foundations, FHWA-SA-93-025 (Berg, 1993) 

C Reinforced Soil Structures - Volume I, Design and Construction Guidelines - Volume II, 

Summary of Research and Systems Information, FHWA RD 89-043 (Christopher et al. 

1990) 

C Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Design and 

Construction Guidelines, FHWA, (Elias and Christopher, 1997) 

Additional guidance, where not available from other sources, was specifically developed for 

this manual. 

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1.1.3 Terminology 

Certain interchangeable terms will be used throughout this manual. For clarity, they are 

defined as follows: 

Inclusion is a generic term that encompasses all man-made elements incorporated in the soil 

to improve its behavior. Examples of inclusions are steel strips, geotextile sheets, steel or 

polymeric grids, steel nails, and steel tendons between anchorage elements. The term 

reinforcement is used only for those inclusions where soil-inclusion stress transfer occurs 

continuously along the inclusion. 

Mechanically Stabilized Earth Wall (MSE wall or MSEW) is a generic term that includes 

reinforced soil (a term used when multiple layers of inclusions act as reinforcement in soils 

placed as fill). Reinforced Earth® is a trademark for a specific reinforced soil system. 

Reinforced Soil Slopes (RSS) are a form of reinforced soil that incorporate planar 

reinforcing elements in constructed earth-sloped structures with face inclinations of less than 

70 degrees. 

Geosynthetics is a generic term that encompasses flexible polymeric materials used in 

geotechnical engineering such as geotextiles, geomembranes, geonets, and geogrids. 

Facing is a component of the reinforced soil system used to prevent the soil from raveling 

out between the rows of reinforcement. Common facings include precast concrete panels, 

dry cast modular blocks, gabions, welded wire mesh, shotcrete, timber lagging and panels, 

polymeric cellular confinement systems, and wrapped sheets of geosynthetics. The facing 

also plays a minor structural role in the stability of the structure. For RSS structures it 

usually consists of welded wire mesh, geosynthetic wrap-around, and/or some type of erosion 

control material. 

Retained backfill is the fill material located behind the mechanically stabilized soil zone. 

Reinforced fill is the fill material in which the reinforcements are placed. 

Generic cross sections of MSE structures are shown in Figures 1-1 and 1-2. 

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Figure 1-1. Generic cross section of an MSE structure. 

1.2 HISTORICAL DEVELOPMENT 

Retaining structures are essential elements of every highway design. Retaining structures are 

used not only for bridge abutments and wing walls but also for slope stabilization and to 

minimize right-of-way for embankments. For many years, retaining structures were almost 

exclusively made of reinforced concrete and were designed as gravity or cantilever walls 

which are essentially rigid structures and cannot accommodate significant differential 

settlements unless founded on deep foundations. With increasing height of soil to be retained 

and poor subsoil conditions, the cost of reinforced concrete retaining walls increases rapidly. 

Mechanically Stabilized Earth Walls (MSEWs) and Reinforced Soil Slopes (RSSs) are costeffective soil-retaining structures that can tolerate much larger settlements than reinforced 

concrete walls. By placing tensile reinforcing elements (inclusions) in the soil, the strength 

of the soil can be improved significantly. Use of a facing system to prevent soil raveling 

between the reinforcing elements allows very steep slopes and vertical walls to be 

constructed safely. 

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(a) 

(b) 

Figure 1-2. Generic cross sections of reinforced slope structures, reinforcements used to: 

(a) increase stability of a slope; and (b) provide improved compaction and 

surficial stability at edge of slopes (after Berg et al., 1990). 

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Inclusions have been used since prehistoric times to improve soil. The use of straw to 

improve the quality of adobe bricks dates back to earliest human history. Many primitive 

people used sticks and branches to reinforce mud dwellings. During the 17th and 18th 

centuries, French settlers along the Bay of Fundy in Canada used sticks to reinforce mud 

dikes. Some other early examples of man-made soil reinforcement include dikes of earth and 

tree branches, which have been used in China for at least 1,000 years (e.g., western portion of 

the Great Wall) and along the Mississippi River in the 1880s. Other examples include 

wooden pegs used for erosion and landslide control in England, and bamboo or wire mesh, 

used universally for revetment erosion control. Soil reinforcing can also be achieved by 

using live plant roots. 

The modern methods of soil reinforcement for retaining wall construction were pioneered by 

the French architect and engineer Henri Vidal in the early 1960s. His research led to the 

invention and development of Reinforced Earth®, a system in which steel strip reinforcement 

is used. The first wall to use this technology in the United States was built in 1972 on 

California State Highway 39, northeast of Los Angeles. Today, MSE walls are the wall of 

choice in most fill situations, and MSE walls are used extensively in the U.S. and worldwide. 

The highest permanent wall constructed in the United States is on the order of 150 ft (46 m) 

with an exposed height of approximately 135 ft (41 m). 

Since the introduction of Reinforced Earth®, several other proprietary and nonproprietary 

systems have been developed and used. Table 1-1 provides a partial summary of some of the 

current systems by proprietary name, reinforcement type, and facing system. 

There are many available systems, as well as new systems that continue to be introduced into 

the market. Components, engineering details, system quality controls, etc. vary with each 

system. States, therefore, need a process to sort and evaluate MSE wall systems for potential 

pre-approval for use on their projects. The Highway Innovative Technology Evaluation 

Center (HITEC) provides review and evaluation of MSE walls. HITEC was established in 

1994 within the American Society of Civil Engineers (ASCE) organization. HITEC’s 

purpose is to accelerate the introduction of technological advances in products, systems, 

services, materials, and equipment to the highway and bridge markets. The evaluation of 

new and more cost-effective retaining wall systems is performed through HITEC’s 

nationally-focused, earth retaining system (ERS) group evaluation program. The published 

reports provide reviews of design, construction, performance, and quality assurance 

information provided by the wall system suppliers with respect to conformance with the 

state-of-practice criteria as outlined in the HITEC Protocol. Wall system suppliers are 

encouraged to conduct an independent review of newly developed components and/or 

systems related to materials, design, construction, performance, and quality assurance. Some 

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public agencies, especially state DOTs, require HITEC evaluations or independent 

evaluations of wall components or wall systems, and obtaining such reviews has proven 

beneficial to wall system suppliers in securing acceptance of their system. 

Currently, most process patents covering soil-reinforced system construction or components 

have expired, leading to a proliferation of available systems or components that can be 

separately purchased and assembled by the erecting contractor. The combination of 

components needs to be evaluated to assure compatibility with respect to longevity, 

constructability, and connection strength. The remaining patents in force generally cover 

only the method of connection between the reinforcement and the facing. 

In the United States, a segmental precast facing unit 20 to 25 ft2

 (2 to 2.25 m2) generally 

square in shape is the facing unit of choice. More recently, larger precast units of up to 50 ft2 

(4.6 m2

) have been used and are becoming more commonplace. Additionally, smaller drycast concrete masonry units are being used, generally in conjunction with geosynthetic 

reinforcements. 

Table 1-1. Summary of Reinforcement and Face Details for MSE Wall Systems. 

System Name Reinforcement Detail Typical Face Detail1 

Stabilized Earth Wall Galvanized welded steel wire mesh with Precast concrete panels 5 ft x 5 ft x 6 in. 

T&B Structural Systems LLC W7 to W20 bars. Mesh width and spacing thick or 5 ft x 10 ft x 6 in. thick. Different 

6800 Manhattan Blvd, Suite 304 can vary. Epoxy-coated meshes also size panels used at top and bottom to 

Ft. Worth Texas 76120 available. match project requirements. 

Reinforced Earth® Ribbed galvanized steel strips, 0.157 in. Cruciform and square shaped precast 

The Reinforced Earth Company thick, 2 in. wide. Or galvanized steel concrete nominally 5 ft x 5 ft x 5.0 to 5.5 

8614 Westwood Center Drive ladder strips, W10 wire, two longitudinal in. thick. Also rectangular shaped precast 

Suite 1100 

Vienna, VA 22182-2233 

wires and cross bars spaced at 6 in. concrete nominally 5 ft x 10 ft x 5.5 in. 

thick. Variable height panels used at top 

and bottom of wall. 

Retained Earth

The Reinforced Earth Company 

8614 Westwood Center Drive 

Suite 1100 

Vienna, VA 22182-2233 

Rectangular grid of W11, W15 or W20 

galvanized steel wire, 24 x 6 in. grid. 2, 4, 

5 or 6 longitudinal bars. Stainless steel 

mesh used in marine and corrosive 

environments. 

Hexagonal and square precast concrete 5 

ft x 5 ft x 5.5 in. thick. Also rectangular 

shaped precast concrete 5 ft x 10 ft x 5.5 

in. thick. Variable height panels used at 

top and bottom of wall. 

Mechanically Stabilized Embankment 

California Dept. of Transportation 

1801 30th Street 

P.O. Box 168041 

Sacramento, CA 95816 

Rectangular grid of W11, W15, and W20 

galvanized welded wire mats, 6 

longitudinal wires with variable transverse 

spacing. 

Precast concrete; 5 ft square, 6 in. thick. 

ARES HDPE Geogrid Precast concrete panel; rectangular 9 ft 

Tensar International Corporation 

5883 Glenridge Drive, Suite 200 

Atlanta, GA 30328 

wide, 5 ft high, 5.5 in. thick. 

Wire Faced Wall 

T&B Structural Systems LLC 

6800 Manhattan Blvd, Ste 304 

Ft. Worth Texas 76120 

4 ft wide welded steel wire mesh. Mesh is 

8 in. x 12, 18 or 24 in., of W4.5 to W20 

bars. Size and configuration are variable. 

Welded steel wire mesh facing. Several 

veneer facing options available. 

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System Name Reinforcement Detail Typical Face Detail1 

Welded Wire Wall Welded steel wire mesh, Galvanized or Welded steel wire mesh, 2’ tall x 8’ wide 

Hilfiker Retaining Walls Non-Galvanized. Mesh reinforcements typical. Backing mat, Hardware Cloth or 

1902 Hilfiker Lane vary in spacing and gauges to meet project Filter Fabric depending on project. (With 

Eureka, CA 95503 design specifications. geotextile or shotcrete, if desired). 

Reinforced Soil Embankment Welded steel wire mesh, Galvanized or Precast concrete unit 12.5 ft long, 24 in. 

Hilfiker Retaining Walls Non-Galvanized. Mesh reinforcements high. 

1902 Hilfiker Lane vary in spacing and gauges to meet project 

Eureka, CA 95503 design specifications. 

ArtWeld Gabions Welded steel wire mesh, Galvanized or ArtWeld Gabion baskets of various sizes 

Hilfiker Retaining Walls Non-Galvanized. Mesh reinforcements and heights designed per project 

1902 Hilfiker Lane vary in spacing and gauges to meet project requirements. 

Eureka, CA 95503 design specifications. 

Gabion Faced M.S.E. Welded steel wire mesh, Galvanized or ArtWeld Gabions of various sizes and 

Hilfiker Retaining Walls Non-Galvanized. Mesh reinforcements heights connected to reinforcing mesh by 

1902 Hilfiker Lane vary in spacing and gauges to meet project spiral binders. 

Eureka, CA 95503 design specifications. 

Eureka Reinforced Soil Welded steel wire mesh, Galvanized or Precast or cast-in-place concrete facing 

Hilfiker Retaining Walls Non-Galvanized. Mesh reinforcements panels, shotcrete, sculpted shotcrete, or 

1902 Hilfiker Lane vary in spacing and gauges to meet project stacked stone. 

Eureka, CA 95503 design specifications. 

Steepened Slope Welded steel wire mesh, Galvanized or Welded steel wire mesh, 1 to 1 slope 

Hilfiker Retaining Walls Non-Galvanized. Mesh reinforcements typical. Hardware Cloth or Filter Fabric 

1902 Hilfiker Lane vary in spacing and gauges to meet project depending on project. (With geotextile or 

Eureka, CA 95503 design specifications. shotcrete, if desired). 

INTER-LOK 0.63 or 0.75 in. reinforcing steel bars fitted Precast concrete panel; cross-shaped 6 ft 

Atlantic Concrete Industries 

P.O. Box 129 

Tullytown, PA 19007 

with 5 x 10 x 0.4 in. anchor plates and 

connected to a keyplate, and galvanized 

after fabrication. 

wide and 3 ft high, 8 and 10 in. thick. 

ISOGRID Rectangular grid of W11 x W11 Diamond shaped precast concrete units, 5 

Neel Co. 

6520 Deepford Street 

Springfield, VA 22150 

4 bars per grid. ft x 8 ft, 5.5 in. thick. 

T-Block Wall System Rectangular 4 ft wide welded steel wire Dry cast concrete block 8 in high x 16 in 

T&B Structural Systems LLC 

6800 Manhattan Blvd 

Ste 304 

Ft. Worth Texas 76120 

mesh of W7 to W20 steel bars. long x 12 in deep. 

MESA 

Tensar International Corporation 

5883 Glenridge Drive, Suite 200 

Atlanta, GA 30328

 HDPE Geogrid MESA HP (high performance), DOT3 

OR Standard units (8 in. high by 18 in. 

long face, 10.8 in. nominal depth). (dry 

cast concrete) 

PyramidTM 

The Reinforced Earth Company 

8614 Westwood Center Drive 

Suite 1100 

Vienna, VA 22182-2233 

Galvanized welded wire ladders. Size 

varies with design requirements. 

Dry cast concrete units, 8 in. high, 16 in. 

nominal length at face, 10 in. nominal 

depth. 

Omega 

The Reinforced Earth Company 

8614 Westwood Center Drive 

Suite 1100 

Vienna, VA 22182-2233 

Geostraps composed of high tenacity 

polyester with polyethylene sheathing. 

Reinforcement used in marine and 

corrosive environments only. 

Cruciform and square shaped precast 

concrete 5 ft x 5 ft x 5.5 in. thick. Also 

rectangular shaped precast concrete 5 ft x 

10 ft x 5.5 in. thick. Variable height 

panels used at top and bottom of wall. 

GeotrelTM 

The Reinforced Earth Company 

8614 Westwood Center Dr, Ste 1100 

Vienna, VA 22182 

Geostraps composed of high tenacity 

polyester with polyethylene sheathing. 

Only used in temporary walls. 

Welded steel wire mesh with geotextile 

backing. 

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System Name Reinforcement Detail Typical Face Detail1 

TerratrelTM 

The Reinforced Earth Company 

8614 Westwood Center Drive 

Suite 1100 

Vienna, VA 22182-2233 

Ribbed galvanized steel strips, 0.157 in. 

thick, 2 in. wide. Or, galvanized steel 

ladder strips or mesh. Size varies with 

design requirements. 

Welded steel wire mesh with geotextile 

backing or stone fill at wall face. 

Maccaferri Terramesh System 

Maccaferri Gabions, Inc. 

43A Governor Lane Blvd. 

Williamsport, MD 21795 

Continuous sheets of galvanized double 

twisted woven wire mesh with PVC 

coating. 

Rock filled gabion baskets laced to 

reinforcement. 

Strengthened Earth 

Gifford-Hill & Co. 

2515 McKinney Ave. 

Dallas, Texas 75201 

Rectangular grid, W7, W9.5 and W14, 

transverse bars at 9 and 18 in. 

Precast concrete units, rectangular or wing 

shaped, 6 ft x 7 ft x 5.5 in. 

MSE Plus 

SSL 

4740 Scotts Valley Drive 

Scotts Valley, CA 95066 

Rectangular grid with W8 to W24 

longitudinal bars and W8 to W20 

transverse. Mesh may have 2 – 6 

longitudinal bars spaced at 6 or 8 in. 

Rectangular precast concrete panels 5 ft 

high, 5, 6, 10, and 12 ft wide, with a 

thickness of 6 or 7 in. 

KeySystem – Inextensible 

Keystone Retaining Wall Systems 

4444 W. 78th Street 

Minneapolis, MN 55435 

Galvanized welded wire ladder mat of 

W7.5 to W17 bars with crossbars at 6 – 24 

in. 

KeySystem concrete facing unit is 8 in 

high x 18 in. wide x 12 in. deep (dry cast 

concrete). 

KeySystem – Extensible 

Keystone Retaining Wall Systems 

4444 W. 78th Street 

Minneapolis, MN 55435 

Miragrid high-tenacity polyester geogrid 

soil reinforcement by TenCate Mirafi, 

polymer coated. 

Keystone Compac concrete facing units 

are 8 in. high x 18 in. wide x 12 in. deep 

(dry cast concrete). 

Tricon System 

Tricon Precast Ltd. 

15055 Henry Road 

Houston, TX 77060 

 Galvanized welded-wire. Rectangular precast concrete panels with 

a face area of 45 sq. ft. 

Versa-Lok Retaining Wall Systems 

6348 Highway 36 Blvd. 

Oakdale, MN 55128 

PVC coated PET or HDPE geogrids. Versa-Lok concrete unit 6 in. high x 16 in. 

long x 12 in. deep (dry cast concrete) 

Anchor Wall Systems 

5959 Baker Road 

Minnetonka, MN 55345 

PVC coated PET geogrid. Anchor Landmark concrete unit 15 in. 

high x 8 in. long x 12 (small unit) or 12.5 

(large unit) in. deep (dry cast concrete). 

EarthTracTM HA 

EarthTec Inc. 

413 Browning Ct. 

Purcellville, VA 20132 

Ribbed galvanized steel strips, 0.188 in. 

thick by 2.36 in. wide. 

Rectangular 5 ft x 10 ft precast concrete 

panels. 

EarthTracTM Wire 

EarthTec Inc. 

413 Browning Ct. 

Purcellville, VA 20132 

Ribbed steel strips, 0.188 in. thick by 2.36 

in. wide; galvanized for permanent walls. 

Welded wire basket 2.5 ft high by 10 ft 

wide. 

EarthTracTM Synthetic 

EarthTec Inc. 

413 Browning Ct. 

Purcellville, VA 20132 

PVC coated high tenacity polyester 

geostraps. 

Precast concrete panels, rectangular or Tshaped. 

1

Additional facing types are possible with most systems. 

The use of geotextiles in MSE walls and RSS started after the beneficial effect of 

reinforcement with geotextiles was noticed in highway embankments constructed over weak 

subgrades. The first geotextile-reinforced wall was constructed in France in 1971, and the 

first structure of this type in the United States was constructed in 1974. Geogrids for soil 

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reinforcement were developed around 1980. The first use of geogrid in earth reinforcement 

was in 1981. Extensive use of geogrid products in the United States started in about 1983, 

and they now comprise a growing portion of the market. Since the early 1980s, the use of 

geosynthetics in reinforced soil structures has increased significantly. 

The first reported use of reinforced steepened slopes is believed to be the west embankment 

for the Great Wall of China. The introduction and economy of geosynthetic reinforcements 

has made the use of steepened slopes economically attractive. A survey of usage in the mid 

1980s identified several hundred completed projects. At least an order of magnitude more 

RSS structures have been constructed since that study. The highest constructed RSS 

structure in the U.S. to date is 242 ft (74 m) (see Chapter 8). 

A representative list of geosynthetic reinforcement manufacturers and suppliers is shown in 

Table 1-2. 

Current Usage: It is believed that MSEWs have been constructed in every state in the 

United States. Major users include transportation agencies in Georgia, Florida, Texas, 

Pennsylvania, New York, and California, which rank among the largest road building states. 

It is estimated that more than 9,000,000 ft2 (850,000 m2

) of MSE retaining walls with precast 

facing are constructed on average every year in the United States, which may represent more 

than half of all retaining wall usage for transportation applications. 

The majority of the MSEWs for permanent applications either constructed to date or 

presently planned use a segmental precast concrete facing and galvanized steel 

reinforcements. The use of geotextile faced MSEWs in permanent construction has been 

limited to date. They are quite useful for temporary construction, where more extensive use 

has been made. 

Recently, modular block dry cast facing units have gained acceptance due to their lower cost 

and nationwide availability. These small concrete units are generally mated with grid 

reinforcement, and the wall system is referred to as modular block wall (MBW). It is 

estimated that more than 3,000,000 ft2 (280,000 m2

) of MBW walls have been constructed 

yearly in the United States when considering all types of transportation related applications. 

The current yearly usage for transportation-related applications is estimated at about 100 

projects per year. 

The use of RSS structures has expanded dramatically in the last decade, and it is estimated 

that several hundred RSS structures have been constructed in the United States. Currently, 

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100 to 150 RSS projects are being constructed yearly in connection with transportation 

related projects in the United States, with an estimated projected vertical face area of 

2,000,000 ft2

/year (190,000 m2/yr). 

Table 1-2 – Representative List of Geogrid and Geotextile Reinforcement Suppliers. 

ACE Geosynthetics Enterprise Co., Ltd. 

No. 8 Kung 10 Rd. 

Yu-Shih Ind. Park, Tachia 

Taichung 43768 

Taiwan 

www.geoace.com 

Belton Industries Inc. 

5600 Oakbrook Pkwy Ste 150 

Norcross, GA 30093-1843 

www.beltonindustries.com 

Carthage Mills Checkmate Geosynthetics Inc. 

4243 Hunt Rd Unit# 412 44500 South Sumas Rd. 

Cincinnati, OH 45242-6645 Chilliwack, BC V2R 5M3 

www.carthagemills.com Canada 

www.checkmategeogrid.com 

Colbond Inc. Fiberweb PLC 

PO Box 1057 70 Old Hickory Blvd. 

1301 Sand Hill Rd Old Hickory, TN 37138 

Enka, NC 28728-1057 www.fiberweb.com 

www.colbond.com 

Dalco Nonwovens 

PO Box 1479 

2050 Evergreen Dr Ne 

Conover, NC 28613-1479 

www.dalcononwovens.com 

Geo-Synthetics Inc. 

2401 Pewaukee Rd 

Waukesha, WI 53188 

www.geo-synthetics.com 

GSE Lining Technology Inc. 

19103 Gundle Rd 

Houston, TX 77073-3515 

www.gseworld.com 

Highland Industries Inc 

629 Green Valley Rd., Suite 210 

Greensboro, NC 27408 

www.highlandindustries.com 

Huesker Inc. 

PO Box 411529 

Charlotte, NC 28241-1529 

www.hueskerinc.com 

Layfield Plastics Inc. 

11603 180th St SW 

Edmonton, AB T5S 2H6 

Canada 

www.layfieldgroup.com 

Luckenhaus Technical Textiles Inc. 

3130 Bee Tree Ln 

Signal Mountain, TN 37377-1441 

Maccaferri Inc. 

10303 Governor Lane Blvd 

Williamsport, MD 21795-3115 

www.maccaferri-usa.com 

NAUE America Inc. 

3525 Piedmont Rd NE 

7 Piedmont Center Ste 300 

Atlanta, GA 30305-1578 

www.naue.com 

Propex Geosynthetics 

6025 Lee Highway, Ste. 425 

P.O. Box 22788 

Chattanooga, TN 37422 

www.propexinc.com 

Saint-Gobain Technical Fabrics 

1795 Baseline Rd 

Grand Island, NY 14072-2010 

www.glasrid.com 

SKAPS Industries 

335 Athena Dr 

Athens, GA 30601 

www.nevown.com 

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Strata Systems, Inc., Div. Glen Raven, Inc. 

PO Box 791 

Statesville, NC 28687-0791 

www.geogrid.com 

SynTeen Technical Fabrics 

1950 W Meeting St 

Lancaster, SC 29720-8811 

www.synteen.com 

Tenax Corp. 

4800 E Monument St 

Baltimore, MD 21205-3031 

TenCate Geosynthetics 

365 S Holland Dr 

Pendergrass, GA 30567-4625 

www.tencate.com 

Tensar International Corporation 

5883 Glenridge Dr NE Ste 200 

Atlanta, GA 30328-5571 

www.tensar-international.com 

Thrace-LINQ Inc. 

2550 W 5th North St 

Summerville, SC 29483-9665 

TNS Advanced Technologies By Crown Resources 

856 S Pleasantburg Dr 

Greenville, SC 29607-2455 

Vantage Partners, LLC 

1000 Bucks Industrial Park Dr 

Statesville, NC 28625-2575 

www.vp-geos.com 

* List is from the Geosynthetics Materials Association. 

1.3 LOAD AND RESISTANCE FACTOR DESIGN (LRFD) 

The most significant revision/update of this reference manual is the change of design 

procedure for MSE walls from an allowable stress design (ASD) basis to load and resistance 

factor design (LRFD) basis. Transportation superstructures are designed using LRFD 

procedures, and logically the substructures supporting the superstructures should also be 

designed on a LRFD basis to provide design consistency on the overall project. Therefore, 

FHWA and the AASHTO Subcommittee on Bridges and Substructures established an 

October 1, 2010 deadline for implementation of LRFD in wall design. 

Although the implementation of LRFD requires a change in design procedures for engineers 

accustomed to ASD, many advantages do exist. LRFD separately accounts for uncertainty in 

both resistance and load, and when appropriately calibrated, can provide more consistent 

levels of safety in the design of superstructure and substructure components in terms of 

reliability index. Section 11 of the AASHTO LRFD Specification (2007) provides 

information on LRFD for earth retaining structures including mechanically stabilized earth 

(MSE) walls. Section 10.4 of AASHTO (2007) provides detailed information on the 

evaluation of soil and rock properties to be used for design. Section 3 of AASHTO (2007) 

provides detailed information on vertical and lateral loads, and load factors for the design of 

retaining walls. 

For many years, engineers have designed walls for highway and other applications using 

allowable stress design (ASD) methods. (Note that the AASHTO (2002) and FHWA (Elias 

et al., 2001) ASD references will not be updated by AASHTO or FHWA, respectively.) In 

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ASD, all uncertainty in applied loads and material resistance are combined in a factor of 

safety or allowable material stress. Furthermore, the factor of safety is independent of the 

method used to estimate the resistance. In LRFD, uncertainty in load and material resistance 

are accounted for separately. The uncertainty in load is represented by a load factor and the 

uncertainty in material resistance is represented by a resistance factor. More importantly, the 

resistance factor is a function of the method used to estimate the resistance and thus the 

model uncertainty is also included in the design process. 

In the AASHTO-LRFD framework, there are four limit states, which represent distinct 

structural performance criteria: (1) strength limit states; (2) serviceability limit states; (3) 

extreme event limit states; and (4) fatigue limit states. For most earth retaining system 

designs, the strength or service limit states control the design. For walls subject to 

earthquake or vessel/vehicle impact, the extreme limit states may control. 

This manual, and the accompanying training course curriculum materials, have been 

prepared assuming that the user is familiar with LRFD general procedures Agencies can 

receive detailed training and reference materials on LRFD procedures for substructures from 

the FHWA NHI 130082 training course (see www.nhi.fhwa.dot.gov). 

This manual also provides detailed procedures for the design, specification, and construction 

of reinforced soil slopes (RSS). The AASHTO LRFD Bridge Design Specifications (2007) 

do not address RSS structures. Therefore, the design for RSS remains based upon a limit 

equilibrium slope stability basis within this manual. 

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CHAPTER 2 

SYSTEMS AND PROJECT EVALUATION
 

This chapter describes available MSE wall (MSEW) and RSS systems and components, their 

application, advantages, disadvantages and relative costs. Subsequently, it reviews typical 

construction sequence for MSEW and RSS construction, and outlines required site and 

project evaluations leading to the establishment of site-specific project criteria and details. 

2.1 APPLICATIONS 

2.1.1 MSE Walls 

MSEW structures are cost-effective alternatives for most applications where reinforced 

concrete or gravity type walls have traditionally been used to retain soil. These include 

bridge abutments and wing walls, as well as areas where the right-of-way is restricted, such 

that an embankment or excavation with stable side slopes cannot be constructed. They are 

particularly suited to economical construction in steep-sided terrain, in ground subject to 

slope instability, or in areas where foundation soils are poor. 

MSE walls offer significant technical and cost advantages over conventional reinforced 

concrete retaining structures at sites with poor foundation conditions. In such cases, the 

elimination of costs for foundation improvements such as piles and pile caps, that may be 

required for support of conventional structures, have resulted in cost savings of greater than 

50 percent on completed projects. 

Representative uses of MSE walls for various applications are shown in Figure 2-1. 

Temporary MSE wall structures have been especially cost-effective for temporary detours 

necessary for highway reconstruction projects. Temporary MSE walls are used to support 

temporary roadway embankments and temporary bridge abutments, as illustrated in Figure 2­

2. MSE walls are also used as temporary support of permanent roadway embankments for 

phased construction, an example is shown in Figure 2-3. 

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Figure 2-1. Representative MSE wall applications (a) retaining wall; (b) access ramp; (c) 

waterfront structure; and (d) bridge abutment. 

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Figure 2-2. MSE walls to support temporary bridge abutment and roadway embankment. 

Figure 2-3. MSE wall used to temporarily support a permanent roadway embankment for 

phased construction. 

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2.1.2 Reinforced Soil Slopes 

Reinforced soil slopes (RSS) are a form of mechanically stabilized earth that incorporate 

planar reinforcing elements (typically geosynthetics) in constructed earth sloped structures 

with face inclinations of less than 70 degrees. As shown in Figure 2-4, multiple layers of 

reinforcement are placed in the slope during construction or reconstruction to reinforce the 

soil and provide increased slope stability. RSS structures are cost-effective alternatives for 

new construction and reconstruction where the cost of fill, right-of-way, and other 

considerations may make a steeper slope desirable. 

There are two primary purposes for using reinforcement in engineered slopes. 

C To increase the stability of the slope, particularly if a steeper than safe unreinforced slope 

is desirable or after a failure has occurred as shown in Figure 2-4. 

C To provide improved compaction at the edges of a slope, thus decreasing the tendency for 

surface sloughing as shown in Figure 1-2b. 

Reinforcement is used to construct an embankment at an angle steeper than could otherwise 

be safely constructed with the same soil. The increase in stability allows for construction of 

steepened slopes on firm foundations for new highways and as an alternative to flatter 

unreinforced slopes and to retaining walls. Roadways can also be widened over existing 

flatter slopes without encroaching beyond existing right-of-ways. In the case of repairing a 

slope failure, the new slope will be safer, and reusing the slide debris rather than importing 

higher quality backfill may result in substantial cost savings. These applications are 

illustrated in Figure 2-4. 

The second purpose for using reinforcement is at the edges of a compacted fill slope to 

provide lateral resistance during compaction. The increased lateral resistance allows for an 

increase in compacted soil density over that normally achieved and provides increased lateral 

confinement for the soil at the face. Even modest amounts of reinforcement in compacted 

slopes have been found to prevent sloughing and reduce slope erosion. Edge reinforcement 

also allows compaction equipment to more safely operate near the edge of the slope. 

Further compaction improvements have been found in cohesive soils through the use of 

geosynthetics with in-plane drainage capabilities (e.g., nonwoven geotextiles) that allow for 

rapid pore pressure dissipation in the compacted soil. 

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Compaction aids placed as intermediate layers between reinforcement in steepened slopes
 

may also be used to provide improved face stability and to reduce layers of more expensive 

primary reinforcement as shown in Figure 1-2. 

Other applications of reinforced slopes have included: 

C Decreased bridge spans.
 

C Temporary road widening for detours. 

C Prevention of surface sloughing during periods of saturation.
 

C Embankment construction with wet, fine-grained soils. 

C Permanent levees. 

C Temporary flood control structures. 

Figure 2-4. Application of reinforced soil slopes. 

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2.2 ADVANTAGES AND POTENTIAL DISADVANTAGES 

2.2.1 Advantages of Mechanically Stabilized Earth (MSE) Walls 

MSE walls have many advantages compared with conventional reinforced concrete and 

concrete gravity retaining walls. MSE walls: 

C Use simple and rapid construction procedures and do not require as large of construction 

equipment. 

C Do not require special skills for construction. 

C Require less site preparation than other alternatives. 

C Need less space in front of the structure for construction operations. 

C Reduce right-of-way acquisition. 

C Do not need rigid, unyielding foundation support because MSE structures are tolerant to 

deformations. 

C Are cost effective. 

C Are technically feasible to heights in excess of 100 ft (30 m). 

Pre-manufactured materials, rapid construction, and, competition among different proprietary 

systems has resulted in a cost reduction relative to traditional types of retaining walls. MSE 

walls are likely to be more economical than other wall systems for walls higher than about 10 

ft (3 m) or where special foundations would be required for a conventional wall. 

One of the greatest advantages of MSE walls is their flexibility and capability to tolerate 

deformations due to poor subsoil foundation conditions. Also, based on observations in 

seismically active zones, these structures have demonstrated a higher resistance to seismic 

loading than rigid concrete wall structures. 

Precast concrete facing elements for MSE walls can be made with various shapes and 

textures (with little extra cost) for aesthetic considerations. Masonry units, timber, and 

gabions also can be used to blend in the environment. 

2.2.2 Advantages of Reinforced Soil Slopes (RSS) 

The economic advantages of constructing a safe, steeper RSS than would normally be 

possible are the result of material and right-of-way savings. It also may be possible to 

decrease the quality of materials required for construction. For example, in repair of 

landslides it is possible to reuse the slide debris rather than to import higher quality backfill. 

Right-of-way savings can be a substantial benefit, especially for road widening projects in 

urban areas where acquiring new right-of-way is always expensive and, in some cases, 

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unobtainable. RSS also provide an economical alternative to retaining walls. In some cases, 

reinforced slopes can be constructed at about one-half the cost of MSEW structures. 

The use of vegetated-faced reinforced soil slopes that can be landscaped to blend with natural 

environments may also provide an aesthetic advantage over retaining wall structures. 

However, there are some potential maintenance issues that must be addressed such as 

mowing grass-faced steep slopes; however, these can be satisfactorily handled in design. 

In terms of performance, due to inherent conservatism in the design of RSS, they are actually 

safer than flatter, unreinforced slopes designed at the same factor of safety. As a result, there 

is a lower risk of long-term stability problems developing with a reinforced slope. Such 

problems often occur in compacted fill slopes that have been constructed to low factors of 

safety and/or with marginal materials (e.g. deleterious soils such as shale, fine grained low 

cohesive silts, plastic soils, etc.). The reinforcement may also facilitate strength gains in the 

soil over time from soil aging and through improved drainage, further improving long-term 

performance. 

2.2.3 Potential Disadvantages 

The following general potential disadvantages may be associated with all reinforced soil 

structures, and are dependent upon local and project conditions: 

C Require a relatively large space (e.g., excavation if in a cut) behind the wall or slope face 

to install required reinforcement. 

C MSE walls require the use of select granular fill. (At some sites, the cost of importing 

suitable fill material may render the system uneconomical.) Reinforced fill requirements 

for RSS are typically less restrictive. 

C The design of soil-reinforced systems often requires a shared design responsibility 

between material suppliers and owners. 

2.3 RELATIVE COSTS 

Site specific costs of a soil-reinforced structure are a function of many factors, including cutfill requirements, wall/slope size and type, in-situ soil type, available backfill materials, 

facing finish, temporary or permanent application, etc. It has been found that MSE walls 

with precast concrete facings are usually less expensive than reinforced concrete retaining 

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walls for heights greater than about 10 ft (3 m) and average foundation conditions. Modular 

block wall (MBW) unit faced walls are competitive with concrete walls at all heights and 

also for small projects. 

In general, the use of MSE walls results in savings on the order of 25 to 50 percent and 

possibly more with a conventional reinforced concrete retaining structure, especially when 

the latter is supported on a deep foundation system (poor foundation condition). A 

substantial savings is obtained by elimination of the deep foundations, which is usually 

possible because reinforced soil structures can accommodate relatively large total and 

differential settlements. Other cost saving features include ease of construction and speed of 

construction. Typical total costs for permanent transportation MSE walls range from $30 to 

$65 per ft2

 ($320 to $650 per m2) of face, and generally vary as function of height, size of 

project, aesthetic treatment, site accessibility, and cost of select wall fill. However, 

reinforced fill costs vary considerably across the U.S. and regional costs may be much higher 

than the indicated range (not just for MSE walls, but for other wall types as well). Some 

example costs are presented with the case histories in Section 2.10. 

The actual cost of a specific MSEW structure will depend on the cost of each of its principal 

components. For segmental precast concrete faced structures, typical relative costs are: 

C Erection of panels and contractors profit - 20 to 30 percent of total cost. 

C Reinforcing materials - 15 to 30 percent of total cost. 

C Facing system - 20 to 40 percent of total cost. 

C Reinforced wall fill including placement - 30 to 60 percent of total cost, where the fill is a 

select granular fill from an off-site borrow source. 

The additional cost for panel architectural finish treatment ranges from $0.50 to $1.50 per ft2 

($5 to $15 per m2) depending on the complexity of the finish. Traffic barrier costs average 

$170 per linear foot ($550 per linear m). In addition, consideration must be given to the cost 

of excavation, which may be somewhat greater than for other systems due to the required 

width of the reinforcement zone. MBW faced walls at heights less than 15 ft (4.5 m) are 

typically less expensive than segmental panel faced walls by 10 percent or more. 

The economy of using RSS must be assessed on a case-by-case basis, where use is not 

dictated by space constraints. For such cases, an appropriate benefit to cost ratio analysis 

should be conducted to determine whether a steeper slope with the reinforcement is justified 

economically over the alternative flatter slope with its increased right-of-way and materials 

costs, etc. It should be kept in mind that guardrails or traffic barriers are often necessary for 

steeper embankment slopes and additional costs such as erosion control systems for slope 

face protection must be considered. 

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With respect to economy, the factors to consider are as follows: 

C Cut or fill earthwork quantities. 

C Size of slope area. 

C Average height of slope area. 

C Angle of slope. 

C Cost of nonselect versus select backfills. 

C Temporary and permanent erosion protection requirements. 

C Cost and availability of right-of-way needed. 

C Complicated horizontal and vertical alignment changes. 

C Need for temporary excavation support systems. 

C Maintenance of traffic during construction. 

C Aesthetics. 

C Requirements for guardrails and traffic barriers. 

The actual bid cost of a specific RSS structure depends on the cost of each of its principal 

components. Based on limited data, typical relative costs are: 

C Reinforcement - 45 to 65 percent of total cost 

C Reinforced fill - 30 to 50 percent of total cost 

C Face treatment - 5 to 10 percent of total cost 

High RSS structures have relatively higher reinforcement and lower backfill costs. Recent 

bid prices suggest costs ranging from $10/ft2

 to $24/ft2 ($110/m2 to $260/m2) as a function of
 

height. 

For applications in the 30 to 50 ft (10 to 15 m) height range, bid costs of about $16/ft2
 

($170/m2) have been reported. These prices do not include safety features and drainage
 

details. 

A rapid, first-order assessment of cost items for comparing a flatter unreinforced slope with a
 

steeper reinforced slope is presented in Figure 2-5. 

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 V = V 3:1 V3:1 = L 

V2:1 = 2⁄3V V2:1 = 2⁄3L 

V1:1 = 1⁄3V V1:1 = 1⁄3L 

COST: 

3H:1V = VSOIL + LLAND + Guardrail* (?) + Hydroseeding (?) 

2H:1V = 2⁄3VSOIL + 2⁄3LLAND + Guardrail + Erosion Control + High Maintenance 

1H:1V = 1⁄3VSOIL + 1⁄3LLAND + Reinforcement + Guardrail + Erosion Control 

* Include guardrail or traffic barrier cost if required. 

Figure 2-5. Cost evaluation of reinforced soil slopes. 

2.4 DESCRIPTION OF MSE and RSS SYSTEMS 

2.4.1 Systems Differentiation 

Since the expiration of the fundamental process and concrete facing panel patents obtained 

by the Reinforced Earth Company for MSE wall systems and structures, the engineering 

community has adopted a generic term Mechanically Stabilized Earth (MSE) to describe this 

type of retaining wall construction. 

Trademarks, such as Reinforced Earth®, Retained Earth®, Genesis® etc., describe systems 

with some present or past proprietary features or unique components marketed by nationwide 

commercial suppliers. Other trademark names appear yearly to differentiate systems 

marketed by competing commercial entities that may include proprietary or novel 

components or for special applications. 

A system for either MSEW or RSS structures is defined as a complete supplied package that 

includes design, specifications and all prefabricated materials of construction necessary for 

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the complete construction of a reinforced soil structure. Often technical assistance during the 

planning and construction phase is included. Components marketed by commercial entities 

for integration by the owner, or others, into a coherent package are not classified as systems. 

Generic systems created by combining components are also possible; however, the 

components must be tested and evaluated together in the form of the final system. 

Components cannot be substituted without complete evaluation of the impact on the system. 

2.4.2 Types of Systems 

MSE/RSS systems can be described by the reinforcement geometry, stress transfer 

mechanism, reinforcement material, extensibility of the reinforcement material, and the type 

of facing and connections. 

Reinforcement Geometry Three types of reinforcement geometry can be considered: 

C Linear unidirectional. Strips, including smooth or ribbed steel strips, or coated 

geosynthetic strips over a load-carrying fiber. 

C Composite unidirectional. Grids or bar mats characterized by grid spacing greater than 

6 in. (150 mm). 

C Planar bi-directional. Continuous sheets of geosynthetics, welded wire mesh, and 

woven wire mesh. The mesh is characterized by element spacing of less than 6 in. (150 

mm). 

Reinforcement Material Distinction can be made between the characteristics of metallic and
 

nonmetallic reinforcements: 

C Metallic reinforcements. Typically of mild steel. The steel is usually galvanized. 

C Nonmetallic reinforcements. Generally polymeric materials consisting of polyester or 

polyethylene. 

The performance and durability considerations for these two classes of reinforcement vary 

considerably and are detailed in the companion Corrosion/Degradation manual (FHWA NHI09-087; Elias et al., 2009). 

Reinforcement Extensibility There are two classes of extensibility relative to the soil’s 

extensibility: 

C Inextensible. The deformation of the reinforcement at failure is much less than the 

deformability of the soil. Steel strip and bar mat reinforcements are inextensible. 

C Extensible. The deformation of the reinforcement at failure is comparable to or even 

greater than the deformability of the soil. Geogrid, geotextile, and woven steel wire mesh 

reinforcements are extensible. 

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2.4.3 Facing Systems 

The types of facing elements used in the different MSE systems control their aesthetics 

because they are the only visible parts of the completed structure. A wide range of finishes 

and colors can be provided in the facing, as shown in the FHWA Federal Lands Highway 

Division’s Roadway Aesthetic Treatments Photo Album (RATPA) available at 

http://gallery.company39.com/FLH/gallery/. In addition, the facing provides protection 

against backfill sloughing and erosion, and provides, in certain cases, drainage paths. The 

type of facing influences settlement tolerances. Major facing types are: 

C Segmental precast concrete panels. The various shapes and dimensions of segmental 

precast panels are summarized in Table 1-1, and examples are illustrated in Figure 2-6 

(and in Figure 5-33). The precast concrete panels have a minimum thickness of 5-1⁄2 

inches (140 mm) and are of a square, rectangular, cruciform, diamond, or hexagonal 

geometry. Typical nominal panel dimensions are 5-foot (1.5 m) high and 5- or 10-foot 

(1.5 or 3 m) wide. Temperature and tensile reinforcement of the concrete are required 

and should be designed in accordance with Section 5 of AASHTO LRFD Specifications 

for Highway Bridges (2007). 

C Dry cast modular block wall (MBW) units. These are relatively small, squat concrete 

units that have been specifically designed and manufactured for retaining wall 

applications. The weight of these units commonly ranges from 30 to 110 lbs (15 to 50 

kg), with units of 75 to 110 lbs (35 to 50 kg) routinely used for highway projects. Unit 

heights typically range from 4 to 12 in. (100 to 300 mm) for the various manufacturers, 

with 8-in. (200 mm) typical. Exposed face length usually varies from 8 to 18 in. (200 to 

450 mm). Nominal front to back width (dimension perpendicular to the wall face) of 

units typically ranges between 8 and 24 in. (200 and 600 mm). Units may be 

manufactured solid or with cores. Full height cores are filled with aggregate during 

erection. Units are normally dry-stacked (i.e. without mortar or bearing pads) and in a 

running bond configuration. Vertically adjacent units may be connected with shear pins, 

lips, or keys. They are referred to by trademarked names such as Keystone®, Landmark® , 

Mesa®, Versa-Lok®, etc. Several example MBW units are illustrated in Figure 2-7. 

C Welded Wire Mesh (WWM). Wire grid can be bent up at the front of the wall to form 

the wall face. This type of facing is used for example in the Hilfiker, Tensar, and 

Reinforced Earth wire faced retaining wall systems. This type of facing is commonly 

used for RSS with face angles of about 45 degrees and steeper. 

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Figure 2-6. Example MSE wall facing treatments. 

(See Figure 5-33 and http://gallery.company39.com/FLH/gallery/ for 

additional example facings.) 

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Figure 2-7. Examples of commercially available MBW units (NCMA, 1997). 

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C Gabion Facing. Gabions (rock-filled wire baskets) can be used as MSE wall or RSS 

facing with reinforcing elements consisting of welded wire mesh, welded bar-mats, 

geogrids, geotextiles or the double-twisted woven mesh placed between or integrally 

manufactured with the gabion baskets. For example, this facing system is used by 

Maccaferri for their Terramesh® wall system. 

C Geosynthetic Facing. Geosynthetic reinforcements are looped around at the facing to 

form the exposed face of the MSEW or RSS. These faces are susceptible to ultraviolet 

light degradation, vandalism, and damage due to fire. Geogrid used for soil 

reinforcement can be looped around to form the face of the completed retaining structure 

in a similar manner to welded wire mesh and fabric facing. Vegetation can grow through 

the grid structure and can provide both ultraviolet light protection for the geogrid and a 

pleasing appearance. 

C Post-construction Facing. For wrapped faced walls, the facing – whether geotextile, 

geogrid, or wire mesh – can be attached after construction of the wall by shotcreting, 

guniting, cast-in-place concrete or by attaching prefabricated facing panels made of 

concrete, wood, or other materials. This multi-staging facing approach adds cost but is 

advantageous where significant settlement is anticipated. 

Precast elements can be cast in several shapes and provided with facing textures to match 

environmental requirements and blend aesthetically into the environment. Retaining 

structures using precast concrete elements as the facings can have surface finishes similar to 

any reinforced concrete structure. 

Retaining structures with metal facings have the disadvantage of shorter life because of 

corrosion, unless provision is made to compensate for it. Facings using welded wire or 

gabions have the disadvantages of an uneven surface, exposed backfill materials, more 

tendency for erosion of the retained soil, possible shorter life from corrosion of the wires, and 

more susceptibility to vandalism. These disadvantages can, of course, be countered by 

providing shotcrete or by hanging facing panels on the exposed face and compensating for 

possible corrosion with galvanization and thicker wire. The greatest advantages of such 

facings are low cost, ease of installation, design flexibility, good drainage (depending on the 

type of wall fill) that provides increased stability, and possible treatment of the face for 

vegetative and other architectural effects. The facing can easily be adapted and well blended 

with natural country environment. These facings, as well as geosynthetic wrapped facings, 

are especially advantageous for construction of temporary or other structures with a shortterm design life. 

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Dry cast segmental block MBW facings may raise some concerns as to durability in 

aggressive freeze-thaw environments where deicing salts are used. Recent research has 

shown that the MBW mix design must be specifically formulated to produce durable, freezethaw resistant units. Agencies should confirm locally manufactured units resistance with 

laboratory freeze-thaw testing. The current specifications in Chapter 10 have been developed 

to address this issue. Further, because the cement is not completely hydrated during the dry 

cast process, (as is often evidenced by efflorescence on the surface of units), a highly alkaline 

regime may establish itself at or near the face area, and may limit the use of some 

geosynthetic products as reinforcements. 

The slope face of RSS structures is usually vegetated if approximately 1:1 or flatter. The 

vegetation requirements vary by geographic and climatic conditions and are therefore, project 

specific. Details are outlined in Section 10.5. 

2.4.4 Reinforcement Types 

Most, although not all, MSE wall systems with precast concrete panels use steel 

reinforcements that are typically galvanized. The two types of steel reinforcements currently 

in use with segmental panel faced MSE walls are: 

1. Steel strips. The currently commercially available strips are ribbed top and bottom, 2 in. 

(50 mm) wide and 5/32-inch (4 mm) thick. Smooth strips 2- to 43⁄4-in. (60 to 120 mm) 

wide, 1/8 to 5/32-inch (3 to 4 mm) thick have been used. 

2. Steel grids. Welded wire grid using two to six W7.5 to W24 longitudinal wire spaced at 

either 6 or 8 in. (150 or 200 mm). The transverse wire may vary from W11 to W20 and 

are spaced based on design requirements from 9 to 24 in. (230 to 600 mm). Welded steel 

wire mesh spaced at 2 by 2-inch (50 by 50 mm) of thinner wire has been used in 

conjunction with a welded wire facing. Some MBW systems use steel grids with two 

longitudinal wires. 

Most MBW systems use geosynthetic reinforcement, predominantly geogrids. The following 

soil reinforcement types are widely used and available: 

3. High Density Polyethylene (HDPE) geogrid. These are of uniaxial manufacture and are 

available in up to 6 grades of strength. This type of reinforcement is also used with 

segmental panel facing. 

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4. PVC coated polyester (PET) geogrid. Available from a number of manufacturers. They 

are characterized by bundled high tenacity PET fibers in the longitudinal load carrying 

direction. For longevity the PET is supplied as a high molecular weight fiber and is 

further characterized by a low carboxyl end group number. 

Other types of soil reinforcements, and their applications, include: 

5. Geotextiles. High strength geotextiles can be used principally in connection with 

reinforced soil slope (RSS) construction. Both polyester (PET) and polypropylene (PP) 

geotextiles have been used. 

6. Double twisted steel mesh. The Terramesh® system by Maccaferri, Inc. uses a metallic, 

soft-temper, double twisted mesh soil reinforcement that is galvanized and then coated 

with poly vinyl chloride (PVC). This reinforcement is used for RSS and gabion faced 

MSE wall construction. Note that this reinforcement is classified as an extensible type 

of reinforcement due to its manufacturing geometry even though it is metallic. 

7. Geosynthetic strap. Although not (currently) widely used, a geosynthetic strap type 

reinforcement has been used with segmental panel faced MSE walls. The strap consists 

of PET fibers encased in a polyethylene (PE) sheath. 

2.4.5 Reinforced Fill Materials 

MSEW Structures MSE walls require high quality wall fill for durability, good drainage, 

constructability, and good soil reinforcement interaction which can be obtained from well 

graded, granular materials. Many MSE systems depend on friction between the reinforcing 

elements and the soil. In such cases, a material with high friction characteristics is specified 

and required. Some systems rely on passive pressure on reinforcing elements, and, in those 

cases, the quality of reinforced wall fill is still critical. These performance requirements 

generally eliminate soils with high clay contents. 

From a reinforcement capacity point of view, lower quality wall fills could be used for 

MSEW structures; however, a high quality granular wall fill has the advantages of better 

drainage, providing better durability for metallic reinforcement, and requiring less 

reinforcement. There are also significant handling, placement and compaction advantages in 

using granular soils. These include an increased rate of wall erection and improved 

maintenance of wall alignment tolerances. Appropriate use of lower quality reinforced fill 

and design considerations for its use is discussed in Chapter 3. 

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RSS Structures Reinforced Soil Slopes are normally not constructed with rigid facing 

elements. Slopes constructed with a flexible face can thus readily tolerate minor distortions 

that could result from settlement, freezing and thawing, or wetting-drying of the backfill. As 

a result, any soil meeting the requirements for embankment construction could be used in a 

reinforced slope system. However, a higher quality material offers fewer durability concerns 

for the reinforcement, and is easier to handle, place and compact, which speeds up 

construction. 

2.4.6 Appurtenant Materials of Construction 

Walls using precast concrete panels require bearing pads in their horizontal joints that 

provide some compressibility and movement between panels during elastic compression and 

settlement of the reinforced fill and preclude concrete-to-concrete contact. These materials 

are generally EPDM rubber or HDPE. The compressibility and thickness of the horizontal 

joint material should be a function of the wall height. Walls with heights greater than 50 ft 

(15 m) may require thicker or more compressible joints to accommodate the larger vertical 

loads due to the weight of panels in the lower third of the structure. 

All joints of precast concrete panels are covered with a geotextile filer strip to prevent the 

migration of fines from the reinforced wall fill. 

Bearing pads are not routinely used with MBW units. A zone of aggregate fill, usually 1-ft 

wide, is used behind the MBW units and within units with cores. This gravel readily 

compacted and conforms to the MBW unit. A filter is required between the gravel zone and 

wall fill, and can either be a soil filter or a geotextile filter (see Chapter 5). 

2.5 CONSTRUCTION SEQUENCE 

The following is an outline of the principal sequence of construction for MSEW and RSS. 

Specific systems, special appurtenances and specific project requirements may vary from the 

general sequence indicated. 

2.5.1 Construction of MSEW systems with precast panel facings 

The construction of MSEW systems with a precast panel facing is carried out as follows: 

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C Preparation of subgrade. This step involves removal of unsuitable materials from the 

area to be occupied by the retaining structure. All organic matter, vegetation, slide debris 

and other unstable materials should be stripped off and the subgrade compacted. 

In unstable foundation areas, ground improvement methods, such as excavation and 

replacement, or dynamic compaction, stone columns, wick drains, etc. (see FHWA NHI06-019 and NHI-06-020, Elias et al., 2006) would be constructed prior to wall erection. 

C Placement of a leveling pad for the erection of the facing elements. This generally 

unreinforced concrete pad is often only 1 ft (300 mm) wide and 6 in. (150 mm) thick and 

is used for MSEW construction only, where concrete panels are subsequently erected. A 

wider concrete pad is recommended for MBW unit erection. 

The purpose of this pad is to serve as a guide for facing panel erection and is not intended 

as a structural foundation support. 

C Erection of the first row of facing panels on the prepared leveling pad. Facings may 

consist of either precast concrete panels or dry cast MBW units. 

The first row of facing panels may be full, or half-height panels, depending upon the type 

of facing used. Only the first tier of panels must be braced to maintain stability and 

alignment. Subsequent rows of panels are simply wedged and clamped to adjacent 

panels. For construction with MBW units, full sized blocks are used throughout with no 

shoring. 

The erection of facing panels and placement of the soil backfills should proceed 

simultaneously. 

C Placement and compaction of reinforced wall fill on the subgrade to the level of the 

first layer of reinforcement and its compaction. The fill should be compacted to the 

specified density, usually 95 to 100 percent of AASHTO T-99 maximum density and 

within the specified range of optimum moisture content. Compaction moisture contents 

dry of optimum are recommended. 

A key to good performance is consistent placement and compaction. Wall fill lift 

thickness must be controlled based on specification requirements and vertical distribution 

of reinforcement elements. The uniform loose lift thickness of the reinforced backfill 

should not exceed 12 in. (300 mm). Reinforced wall fill should be dumped into or 

parallel to the rear and middle of the reinforcement and bladed toward the front face. 

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Retained backfill placement and compaction behind the reinforced volume should 

proceed simultaneously. 

C Placement of the first layer of reinforcing elements on the wall fill. The 

reinforcements are placed and connected to the facing panels, when the compacted fill 

has been brought up to the level of the connection. The reinforcements are generally 

placed perpendicular to back of the facing panels. More detailed construction control 

procedures associated with each construction step are outlined in Chapter 11. 

C Placement of the wall fill over the reinforcing elements to the level of the next 

reinforcement layer and compaction of the wall fill. The previously outlined steps are 

repeated for each successive layer. 

C Construction of traffic barriers and copings. This final construction sequence is 

undertaken after the final panels have been placed, and the wall fill has been completed to 

its final grade. 

A complete sequence is illustrated in Figures 2-8 through 2-10. 

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Figure 2-8. Erection of precast panels. 

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Figure 2-9. Fill spreading and reinforcement connection. 

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Figure 2-10. Compaction of the reinforced wall fill. 

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2.5.2 Construction of MSE systems with Flexible Facings 

Construction of flexible-faced MSE walls, where the reinforcing material also serves as 

facing material, is similar to that for walls with precast facing elements. For flexible facing 

types such as welded wire mesh, geotextiles, geogrids or gabions, the erection of the first 

level of facing element requires only a level grade. A concrete footing or leveling pad is not 

usually required unless precast elements are to be attached to the system after construction. 

Construction proceeds as outlined for segmental facings with the following exceptions: 

C Placement of first reinforcing layer. Reinforcement with anisotropic strength 

properties (i.e., many geosynthetics) should be placed with the principal strength 

direction perpendicular to face of structure. 

The reinforcement should be secured with retaining pins to prevent movement during 

reinforced fill placement. 

Adjacent sheets should be overlapped a minimum of 6 in. (150 mm) along the edges 

perpendicular to the face. Alternatively, with geogrid or wire mesh reinforcement, the 

edges may be butted and clipped or tied together. 

C Face Construction. Place the geosynthetic layers using face forms as shown in Figure 

2-11. For temporary support of forms at the face, form holders should be placed at the 

base of each layer at approximately 4 ft (1.20 m) horizontal intervals. Details of 

temporary formwork are shown in Figure 2-12. These supports are essential for 

achieving good compaction. When using geogrids or wire mesh, it may be necessary to 

use a geotextile or hardware cloth to retain the wall fill material at the face. 

When compacting wall fill within 3 ft (~1 m) of the wall face, a hand-operated vibratory 

compactor is recommended. 

The return-type method or successive layer tie method as shown in Figure 2-12 can be 

used for facing support. In the return method, the reinforcement is folded at the face over 

the wall fill material, with a minimum return length of 4 ft (1.25 m) to ensure adequate 

pullout resistance. Consistency in face construction and compaction is essential to 

produce a wrapped facing with satisfactory appearance. 

Apply facing treatment (shotcrete, precast facing panels, etc.). Some alternative facing 

systems for flexible faced walls and slopes are shown in Figure 2-13. 

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Figure 2-11. Lift construction sequence for geosynthetic faced MSE walls. 

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Figure 2-12. Typical geosynthetic face construction detail. 

(25 mm = 1 in.) 

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Figure 2-13. Types of geosynthetic reinforced soil wall facing (after Wu, 1994). 

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2.5.3 RSS Construction 

The construction of RSS embankments is considerably simpler and consists of many of the 

elements outlined for MSEW construction. They are summarized as follows: 

C Site preparation. 

C Construct subsurface drainage features. 

C Place reinforcement layer. 

C Place and compact backfill on reinforcement. 

C Construct face. Details of the available methods are outlined in Chapter 8, construction. 

C Place additional reinforcement and reinforced fill. 

C Construct surface drainage features. 

Key stages of construction are illustrated in Figure 2-14, and the complete sequence is fully
 

outlined in Chapter 8. 

2.6 SITE EVALUATION 

2.6.1 Site Exploration 

The feasibility of using an MSEW, RSS or any other type of earth retention system depends 

on the existing topography, subsurface conditions, and soil/rock properties. It is necessary to 

perform a comprehensive subsurface exploration program to evaluate site stability, 

settlement potential, need for drainage, etc., before repairing a slope or designing a new 

retaining wall or bridge abutment. Where the select backfill is to be obtained from on-site 

sources, the extent and quality must be fully explored to minimize contractor claims for 

changed conditions. 

Subsurface investigations are required not only in the area of the construction but also behind 

and in front of the structure to assess overall performance behavior. The subsurface 

exploration program should be oriented not only towards obtaining all the information that 

could influence the design and stability of the final structure, but also to the conditions which 

prevail throughout the construction of the structure, such as the stability of temporary 

construction slopes that may be required. 

The engineer's concerns include the bearing resistance of the foundation materials, the 

allowable deformations, and the stability of the structure. Necessary parameters for these 

analyses must be obtained. 

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Figure 2-14. Reinforced slope construction: (a) geogrid and fill placement; (b) soil filled 

erosion control mat placement; and (c) finished, vegetated 1:1 slope. 

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The cost of a reinforced soil structure is greatly dependent on the availability of the required 

type of reinforced fill and retained backfill materials. Therefore, investigations must be 

conducted to locate and test locally available materials that may be used for reinforced fill 

and retained backfill with the selected system. 

2.6.2 Field Reconnaissance 

Preliminary subsurface investigation or reconnaissance should consist of collecting any 

existing data relating to subsurface conditions and making a field visit to obtain data on: 

C Limits and intervals for topographic cross sections. 

C Access conditions for work forces and equipment. 

C Surface drainage patterns, seepage, and vegetation characteristics. 

C Surface geologic features, including rock outcrops and landforms, and existing cuts or 

excavations that may provide information on subsurface conditions. 

C The extent, nature, and locations of existing or proposed below-grade utilities and 

substructures that may have an impact on the exploration or subsequent construction. 

C Available right-of-way. 

C Areas of potential instability such as deep deposits of weak cohesive and organic soils, 

slide debris, high groundwater table, bedrock outcrops, etc. 

Reconnaissance should be performed by a geotechnical engineer or by an engineering 

geologist. Before the start of field exploration, any data available from previous subsurface 

investigations and those that can be inferred from geologic maps of the area should be 

studied. Topographic maps and aerial photographs, if available, should be studied. Much 

useful information of this type is available from the U.S. Geological Survey, the Natural 

Resources Conservation Service, the U.S. Department of Agriculture, and local planning 

boards or county offices. 

2.6.3 Subsurface Exploration 

The subsurface exploration program generally consists of soil soundings, borings, and test 

pits. The type and extent of the exploration should be decided after review of the preliminary 

data obtained from the field reconnaissance, and in consultation with a geotechnical engineer 

or an engineering geologist. The exploration must be sufficient to evaluate the geologic and 

subsurface profile in the area of construction. For guidance on the extent and type of 

required investigation, the FHWA NHI-01-031 Subsurface Investigations – Geotechnical Site 

Characterization reference manual (Mayne et al., 2002), should be reviewed. 

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The following guidelines are recommended (Christopher et al., 1990) for the subsurface 

exploration for potential MSE applications: 

C Borings: The type (soil boring and/or cone penetration), number, location, and depth of 

investigation points generally are dictated by the project stage (i.e., feasibility study, 

preliminary, or final design), availability of existing geotechnical data, variability of 

subsurface conditions, length of the structure, what the structure supports, and other 

project details. Soil borings should be performed along the front and the back of the 

proposed reinforced soil structure. The width of the MSE wall or slope structure may be 

assumed as 0.8 times the anticipated height. Borings at the following intervals should be 

considered: 

- 100 ft (30 m) along the alignment of the reinforced soil structure; and 

- 150 ft (45 m) along the back of the reinforced soil structure 

C The boring depth should be controlled by the general subsurface conditions. Where 

bedrock is encountered within a reasonable depth, rock cores should be obtained for a 

length of about 10 ft (3 m). This coring will be useful to distinguish between solid rock 

and boulders. Deeper coring may be necessary to better characterize rock slopes behind 

new retaining structures. In areas of soil profile, the borings should extend at least to a 

depth equal to twice the height of the wall/slope. If subsoil conditions within this depth 

are found to be weak and unsuitable for the anticipated pressures from the structure 

height, then the borings must be extended until reasonably strong soils are encountered. 

C In each boring, soil samples should be obtained at 5-foot (1.5 m) depth intervals and at 

changes in strata for visual identification, classification, and laboratory testing. Methods 

of sampling may follow AASHTO T 206 or AASHTO T 207 (Standard Penetration Test 

and Thin-Walled Shelby Tube Sampling, respectively), depending on the type of soil. In 

granular soils, the Standard Penetration Test can be used to obtain disturbed samples. In 

cohesive soils, undisturbed samples should be obtained by thin-walled sampling 

procedures. In each boring, careful observation should be made for the prevailing water 

table, which should be observed not only at the time of sampling but also at later times to 

obtain a good record of prevailing water table conditions. If necessary, piezometers 

should be installed in a few borings to observe long-term water levels. 

C Both the Standard Penetration Test and the Cone Penetration Test, ASTM D3441, 

provide data on the strengths and density of soils. In some situations, it may be desirable 

to perform in-situ tests using a dilatometer, pressuremeter, or similar means to determine 

soil modulus values. 

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C Adequate bulk samples of available soils should be obtained and evaluated as indicted in 

the following testing section to determine the suitability of the soil for use as backfill in 

the MSE structures. Such materials should be obtained from all areas from which 

preliminary reconnaissance indicates that borrow materials will be used. 

C Test-pit explorations should be performed in areas showing instability or to explore 

further availability of the borrow materials for backfill. The locations and number of test 

pits should be decided for each specific site, based on the preliminary reconnaissance. 

2.6.4 Laboratory Testing 

Soil samples should be visually examined and appropriate tests performed for classification 

according to the Unified Soil Classification System (ASTM D2488). These tests permit the 

engineer to decide what further field or laboratory tests will best describe the engineering 

behavior of the soil at a given project site. Index testing includes determination of moisture 

content, Atterberg limits, and gradation. The dry unit weight of representative undisturbed 

samples should also be determined. 

Shear strength determination by unconfined compression tests, direct shear tests, or triaxial 

tests will be needed for external stability analyses of MSE walls and slopes. At sites where 

compressible cohesive soils are encountered below the foundations of the MSE structure, it is 

necessary to perform consolidation tests to obtain parameters for performing service state 

settlement analyses. Both undrained and drained (effective stress) parameters should be 

obtained for cohesive soils, to permit evaluation of both long-term and short-term conditions. 

Of particular significance in the evaluation of any material for possible use as backfill are the 

grain size distribution and plasticity. The effective particle size (D10) can be used to estimate 

the permeability of cohesionless materials. Laboratory permeability tests may also be 

performed on representative samples compacted to the specified density. Additional testing 

should include direct shear tests on a few similarly prepared samples to determine shear 

strength parameters under long and short-term conditions. The compaction behavior of 

potential backfill materials should be investigated by performing laboratory moisture-density 

relationship tests according to AASHTO T 99, or T 180. 

Properties to indicate the potential aggressiveness of the backfill material and the in-situ soils 

behind the reinforced soil zone must be measured. Tests include: 

C pH (AASHTO T 289; ASTM D4972) 

C Electrical resistivity. 

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C Salt content including water soluble sulfate (AASHTO T 290), sulfides (ASTM 

D4327), and chlorides (ASTM D4327). 

The test results will provide necessary information for planning degradation protection 

measures and will help in the selection of reinforcement elements with adequate durability. 

designers. 

2.6.5 Foundation Soils 

The development and implementation of an adequate subsurface investigation program for 

the existing foundation conditions is a key element for ensuring successful project 

implementation. Causes for distress experienced in projects are often traced to inadequate 

subsurface exploration programs that did not disclose local or significant areas of soft soils, 

causing significant local differential settlement and distress to the facing panels. In a few 

documented extreme cases, such foundation weakness caused complete foundation failures 

leading to catastrophic collapses. 

Determination of engineering properties for foundation soils should be focused on 

establishment of bearing resistance, global stability, settlement potential, and position of 

groundwater levels. For bearing capacity determinations, frictional and cohesive parameters 

(, c) as well as unit weights (γT) and groundwater position are normally required in order to 

calculate bearing resistance in accordance with Article 10.6.3.1 for soil and 10.6.3.2 for rock 

in AASHTO (2007). The effects of load inclination and footing shape may be omitted for 

Strength Limit State. 

For foundation settlement determinations, the results of conventional settlement analyses 

with Service Limit State load factors, and using laboratory time-settlement data, coefficients 

of consolidation Cc, in conjunction with approximate value for compression index Cv, 

obtained from correlations to soil index tests (moisture content, Atterberg limits) should be 

used. The results of settlement analyses, especially with respect to differential settlement 

should be used to determine the ability of the facing and connection system to tolerate such 

movements or the necessity for special details or procedures to accommodate the differential 

movement anticipated. 

Major foundation weakness and compressibility may require the consideration of ground 

improvement techniques to achieve adequate bearing capacity, or limiting total or differential 

settlement. Techniques successfully used, include surcharging with or without prefabricated 

vertical drains, stone columns, dynamic compaction, compaction grouting and the use of 

lightweight fill to reduce settlement. Additional information on ground improvement 

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techniques can be found in the FHWA Ground Improvement Manuals, FHWA NHI-06-019 

and FHWA NHI-06-020 (Elias et al., 2006). As an alternate for MSE walls, faces 

constructed of geosynthetic wraps, welded wire mesh or gabion baskets, which will tolerate 

significant differential settlement, could be constructed and permanent facings such as 

concrete panels attached after the settlement has occurred, see Section 3.6.6. Of particular 

concern, are situations where the MSEW structure may terminate adjacent to a rigidly 

supported structure such as a pile supported abutment at the end of a retained approach fill. 

Evaluation of these foundation related issues are typically beyond the scope of services 

provided by wall/slope system suppliers. Evaluations of this type are the responsibility of 

agency engineers or consultant geotechnical and are required before selection of the 

appropriate MSE wall or RSS system. 

2.7 PROJECT EVALUATION 

2.7.1 Structure Selection Factors 

The major factors that influence the selection of an MSE/RSS alternative for any project 

include: 

C Geologic and topographic conditions 

C Environmental conditions 

C Size and nature of the structure 

C Aesthetics 

C Durability considerations 

C Performance criteria 

C Availability of materials 

C Experience with a particular system or application 

C Cost 

Many MSEW systems have proprietary features. Some companies provide services 

including design assistance, preparation of plans and specifications for the structure, supply 

of the manufactured wall components, and construction assistance. 

The various wall systems have different performance histories, and this sometimes creates 

difficulty in adequate technical evaluation. Some systems are more suitable for permanent 

walls, others are more suitable for low walls, and some are applicable for remote areas while 

others are more suited for urban areas. The selection of the most appropriate system will 

thus depend on the specific project requirements. 

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RSS embankments have been constructed with a variety of geosynthetic reinforcements and 

treatments of the slope face. These factors again may create an initial difficulty in adequate 

technical evaluation, but with the use of this manual easily addressed by department 

personnel to prepare generic designs. A number of geosynthetic reinforcement suppliers 

provide design services as well as technical assistance during construction. 

Specific technical issues focused on selection factors are summarized in the following 

sections. 

2.7.2 Geologic and Topographic Conditions 

MSE structures are particularly well suited where a "fill-type" wall must be constructed or 

where side-hill fills are indicated. Under these latter conditions, the volume of excavation 

may be small, and the general economy of this type of construction is not jeopardized. 

Economic advantages diminish with large cut volumes to accommodate the reinforced soil 

structure, but in many instances remain viable. 

The adequacy of the foundation to support the fill weight must be determined as a first-order 

feasibility evaluation. 

Where soft compressible soils are encountered, preliminary stability analyses must be made
 

to determine if sufficient shear strength is available to support the weight of the reinforced
 

fill. As a rough first approximation for vertically faced MSE structures, the available shear 

strength must be equal to at least 2.0 to 2.5 times the weight of the fill structure. For RSS 

embankments the required foundation strength is somewhat less and dependent on the actual 

slope considered. 

Where these conditions are not satisfied, ground improvement techniques (see FHWA NHI06-019 and NHI-06-020, Elias et al., 2006) must be considered to increase the bearing 

capacity at the foundation level. These techniques include but are not limited to: 

C Excavation and removal of soft soils and replacement with a compacted structural fill.
 

C Use of lightweight fill materials. 

C In-situ densification by dynamic compaction or improvement by use of surcharging with 

or without prefabricated vertical drains. 

C Construction of aggregate columns. 

Where marginal to adequate foundation strength is available, preliminary settlement analyses 

should be made to determine the potential for differential settlement, both longitudinally 

along a proposed structure as well as transverse to the face. This second-order feasibility 

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evaluation is useful in determining the appropriate type of facing systems for MSE walls and 

in planning appropriate construction phasing to accommodate the settlement. 

In general, concrete-faced MSE structures using discrete articulating panels can 

accommodate maximum longitudinal differential settlements of about 1/100, without the 

introduction of special sliding joints between panels. Full-height concrete panels are 

considerably less tolerant and generally should not be considered where differential 

settlements are anticipated. MBW unit faced walls can accommodate maximum longitudinal 

differential settlements of about 1/200, with the introduction of special slip joints. 

The performance of reinforced soil slopes generally is not affected by differential 

longitudinal settlements. 

2.7.3 Environmental Conditions 

The primary environmental condition affecting reinforcement type selection and potential 

performance of MSE structures is the aggressiveness of the in-situ ground regime that can 

cause deterioration to the reinforcement. Post construction changes must be considered 

where deicing salts or fertilizers are subsequently used. 

For steel reinforcements, in-situ regimes containing chloride and sulfate salts generally in 

excess of 200 PPM accelerate the corrosive process as do acidic regimes characterized by a 

pH of less than 5 (Elias, 1989). Alkaline regimes characterized by pH > 10 will cause 

accelerated loss of galvanization. 

Certain in-situ regimes have been identified as being potentially aggressive for geosynthetic 

reinforcements. Polyester (PET) degrades in highly alkaline or acidic regimes. Polyolefins 

appear to degrade only under certain highly acidic conditions. 

For additional specific discussions on the potential degradability of reinforcements, refer to 

the companion Corrosion/Degradation reference manual and are summarized in Section 3.5. 

A secondary environmental issue is site accessibility, which may dictate the nature and size 

of the facing for MSE wall construction. Sites with poor accessibility or remote locations 

may lend themselves to lightweight facings such as geotextile or geogrid wrapped facings 

and vegetative covers; metal skins; welded wire mesh, gabions, modular blocks (MBW) 

which could be erected without heavy lifting equipment. 

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RSS construction with an organic vegetative cover must be carefully chosen to be consistent 

with native perennial cover that would establish itself quickly and would thrive with 

available site rainfall. 

2.7.4 Size and Nature of Structure 

Theoretically there is no upper limit to the height of MSE wall that can be constructed. 

Structures up to 135 ft (41 m) have been successfully constructed in the U.S. with steel 

reinforcements, although such heights for transportation-related structures are rare. RSS 

embankments have been constructed to up to a height of 242 ft (74 m) in the U.S. with 

geogrid reinforcements, but again such heights for transportation-related structures are rare. 

Practical limits are often dictated by economy, available ROW, and the tensile strength of 

commercially available soil reinforcing materials. For bridge abutments there is no 

theoretical limit to the span length that can be supported, although the longer the span, the 

greater is the area of footing necessary to support the beams. Since the nominal bearing 

resistance of the reinforced fill for the service limit state is usually limited to 4000 psf (200 

kPa), a large abutment footing further increases the span length, adding cost to the 

superstructure. This additional cost must be balanced by the potential savings of the MSE 

alternate to a conventional abutment wall, which would have a shorter span length. As an 

option in such cases, it might be economical to consider support of the bridge beams on deep 

foundations, placed within (or in front of) the reinforced fill zone. 

The lower limit to height is usually dictated by economy. When used with traffic barriers, 

low walls on good foundations of less than 10 to 14 ft (3 to 4 m) are often uneconomical, as 

the cost of the overturning moment leg of the traffic barrier approaches one-third of the total 

cost of the MSE structure in place. For cantilever walls, the barrier is simply an extension of 

the stem with a smaller impact on overall cost. 

The total size of structure (square feet of face) has little impact on economy compared with 

other retaining wall types. However, the unit cost for small projects of less than 3,000 ft2 

(300 m2) is likely to be 10 to 15 percent higher. 

RSS may be cost effective in rural environments, where ROW restrictions exist or on 

widening projects where long sliver fills are necessary. In urban environments, they should 

be considered where ROW is available, as they are generally more economical than vertically 

faced MSE wall structures. 

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2.7.5 Aesthetics 

Precast concrete facing panels may be cast with an unlimited variety of texture and color for 

an additional premium that seldom exceeds 15 percent of the facing cost, which on average 

would mean a 4 to 6 percent increase on total in-place cost. 

Modular block wall facings are often comparable in cost to precast concrete panels except on 

small projects (less than 4,000 ft2 {400 m2}) where the small size introduces savings in 

erection equipment cost and the need to cast special, made-to-order concrete panels to fit 

what is often irregular geometry. MBW facings may be manufactured in color and with a 

wide variety of surface finishes. 

The outward face treatment of RSS, generally is by vegetation, which is initially more 

economical than the concrete facing used for MSE structures. However, maintenance costs 

may be considerably higher, and the long-term performance of many outward face treatments 

has not been established. 

2.7.6 Questionable Applications 

The current AASHTO LRFD Specifications (2007) states that MSE walls should not be used 

under the following conditions: 

C When utilities other than highway drainage must be constructed within the reinforced 

zone where future access for repair would require the reinforcement layers to be cut. A 

similar limitation should be considered for RSS structures. 

C With galvanized metallic reinforcements exposed to surface or ground water 

contaminated by acid mine drainage or other industrial pollutants as indicted by low pH 

and high chlorides and sulfates. 

C When floodplain erosion may undermine the reinforced fill zone, or where the depth to 

scour cannot be reliably determined. 

2.8 ESTABLISHMENT OF PROJECT CRITERIA 

The engineer should consider each topic area presented in this section at a preliminary design 

stage and determine appropriate elements and performance criteria. 

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The process consists of the following successive steps: 

C Consider all possible alternatives. 

C Choose a system (MSEW or RSS). 

C Consider facing options. 

C Develop performance criteria (loads, design heights, embedment, settlement tolerances, 

foundation capacity, effect on adjoining structures, etc.). 

C Consider effect of site on corrosion/degradation of reinforcements. 

2.8.1 Alternates 

Cantilever, gravity, semi gravity, or counterforted concrete walls or soil embankments are the 

usual alternatives to MSE walls and abutments and RSS. 

In cut situations, in-situ walls such as tieback anchored walls, soil nailed walls or nongravity 

cantilevered walls are often more economical, although where limited ROW is available, a 

combination of a temporary in-situ wall at the back end of the reinforcement and a permanent 

MSE wall is often competitive. 

For waterfront or marine wall applications, sheetpile walls with or without anchorages or 

prefabricated concrete bin walls that can be constructed in the wet are often, if not always, 

both more economical and more practical to construct. 

2.8.2 Facing Considerations 

The development of project-specific aesthetic criteria is principally focused on the type, size, 

and texture of the facing, which is the only visible feature of any MSE structure. 

For permanent applications, considerations should be given to MSE walls with precast 

concrete panels. They are constructed with a (near) vertical face. Currently, the size of 

panels commercially produced varies from 20 to 50 ft2 (1.8 to 4.5 m2 ). Generally, full height 

panels may be considered for walls up to about 14 to 16 ft (4 to 5 m) in height on foundations 

that are not expected to settle. Experienced contractors have successfully constructed taller 

full height panels (e.g., 25 ft {7.5 m}) on competent foundations. The precast concrete 

panels can be manufactured with a variety of surface textures and geometries, as shown in 

Figure 2-6. 

For permanent applications, considerations should be given to MBW facings, which are 

available in a variety of shapes and textures as shown in Figure 2-7. They range in facial 

area from 0.5 to 1 ft2 (0.05 to 0.1 m2

). An integral feature of this type of facing is a front 

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batter ranging from nominal up to 15 degrees. Project geometric constraints, i.e., the bottom 

of wall and top of wall horizontal limits, may limit the amount of permissible batter and, 

thus, the types of MBW units that may be used. Note that the toe of these walls step back as 

the foundation elevation steps up, due to the stacking arrangement and automatic batter. 

Other facing options are gabion, timber faced, or vegetated. 

For temporary walls, significant economy can be achieved with geosynthetic wrapped 

facings. They may be made permanent by applying gunite or cast-in-place concrete in a 

post-construction application. 

For RSS structures, the choice of slope facing may be controlled by climatic and regional 

factors. For structures of less than 33 ft (10 m) height with slopes of approximately 1 

Horizontal:1 Vertical (1H:1V) or flatter, a vegetative "green slope" can be usually 

constructed using an erosion control mat or mesh and local grasses. Where vegetation cannot 

be successfully established and/or significant run-off may occur, armored slopes using 

natural or manufactured materials may be the only choice to reduce future maintenance. For 

additional guidance see Section 8.5. 

2.8.3 Performance Criteria 

Performance criteria for MSE structures with respect to design requirements are governed by 

design practice or codes such as contained in Article 11.10 of 2007 AASHTO LRFD 

Specifications for Highway Bridges. These requirements consider load and resistance factors 

with respect to various failure modes and materials, and for various limit states. No specific 

AASHTO guidance is presently available for RSS structures. 

With respect to lateral wall displacements, no method is presently available to definitively 

predict lateral displacements, most of which occur during construction. The horizontal 

movements depend on compaction effects, reinforcement extensibility, reinforcement length, 

reinforcement-to-panel connection details, and details of the facing system. A rough estimate 

of probable lateral displacements of simple structures that may occur during construction can 

be made based on the reinforcement length to wall-height ratio and reinforcement 

extensibility as shown in Figure 2-15, for the serviceability limit check. 

This figure indicates that increasing the length-to-height ratio of reinforcements from its 

theoretical lower limit of 0.5H to 0.7H, decreases the deformation by 50 percent. 

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For L = 0.7 H
 

Metallic (inextensible) reinforcement . 3⁄4-in. per 10 ft of wall height 

Geogrid (moderately extensible) reinforcement .1 in. per 10 ft of wall height 

Geotextile (extensible) reinforcement .1.5 in. per 10 ft of wall height 

Based on 20 ft high walls, relative displacement increases approximately 25% 

for every 400 psf surcharge. Experience indicates that for higher walls, the 

surcharge effect may be greater. 

NOTE: This figure is only a guide. Actual displacement will depend, in addition to the 

parameters addressed in the figure, on soil characteristics, compaction effort, and contractor
 

Figure 2-15. Empirical curve for estimating lateral displacement during construction for 

MSE walls (after FHWA RD 89-043 {Christopher et al., 1990}). 

 

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Performance criteria are both site and structure-dependent. Structure-dependent criteria 

consist of safety factors or a consistent set of load and resistance factors as well as tolerable 

movement criteria of the specific MSE structure selected. 

Recommended MSE Wall load and resistance factors with respect to the various potential 

failure modes and limit states are presented in Chapter 4. 

A number of site-specific project criteria need to be established at the inception of design: 

C Design limits and wall height. The length and height required to meet project geometric 

requirements must be established to determine the type of structure and external loading 

configurations. 

C Alignment limits. The horizontal (perpendicular to wall face) limits of bottom and top 

of wall alignment must be established as alignments vary with batter of wall system. The 

alignment constraints may limit the type and maximum batter of the wall facing, 

particularly with MBW units. 

C Length of reinforcement. A minimum reinforcement length of 0.7H is recommended 

for MSE walls. Longer lengths are required for structures subject to surcharge loads, or 

where foundation conditions affect lateral sliding and/or global/compound slope stability, 

as listed in Table 2-1. Shorter lengths can be used in special situations (see Chapter 6). 

C External loads. The external loads may be soil surcharges required by the geometry, 

adjoining footing loads, loads as from traffic, and/or traffic impact loads. The magnitude 

of the minimum traffic loads outlined in Article 3.11.6.4 (AASHTO, 2007) is a uniform 

load equivalent to 2 ft (0.6 m) of soil over the traffic lanes. The traffic load is greater for 

some cases (see Tables 4-5 and 4-6). 

C Wall embedment. The minimum embedment depth for walls from adjoining finished 

grade to the top of the leveling pad should be based on bearing, settlement, and slope 

stability considerations. Current practice based on local bearing considerations, 

recommends the minimum embedment depths listed in Table 2-1. 

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Table 2-1. Typical Minimum Length of Reinforcement. 

Case Typical Minimum L/H Ratio 

Static loading with or with traffic surcharge 0.7 

Sloping backfill surcharge 0.8 

Seismic loading 0.8 to 1.1 

Table 2-2. Minimum MSEW Embedment Depths. 

Slope in Front of Wall Minimum Embedment Depth 

to Top of Leveling Pad* 

All Geometries 2 ft minimum 

horizontal (walls) H/20 

horizontal (abutments) H/10 

3H:1V H/10 

2H:1V H/7 

1.5H:1V H/5 

* Minimum depth is the greater of applicable values listed, frost 

depth, or scour depth. 

Larger values may be required, depending on shrinkage and swelling of foundation soils, 

seismic activity, and/or scour. A greater embedment depth may also be required based 

upon bearing, settlement, and/or global stability calculations. As noted, the minimum in 

any case is 2 ft (0.6 m), except for structures founded on rock at the surface, where no 

embedment may be used. Alternately, frost-susceptible soils could be overexcavated and 

replaced with non-frost susceptible fill, hence reducing the embedment depth (and overall 

wall height). 

A minimum horizontal bench 4-ft (1.2 m) wide as measured from the face shall be 

provided in front of walls founded on slopes. The bench may be formed or the slope 

continued above that level (11.10.2.2, AASHTO {2007}), as illustrated in Figure 2-16. 

The horizontal bench is intended to provide resistance against general bearing failure and 

to provide access for maintenance inspections (C11.10.2.2, AASHTO {2007}). 

For walls constructed along rivers and streams where the depth of scour has been reliably 

determined, a minimum embedment of 2 ft (0.6 m) below scour depth is recommended. 

Embedment is not required for RSS unless dictated by stability requirements. 

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(a) 

(b) 

Figure 2-16. MSE wall embedment depth requirements, (a) level toe condition and (b) 

benched slope toe condition (dh = minimum depth for horizontal slope and ds 

= minimum depth for sloping toe, from Table 2-2). 

Seismic Activity. Due to their flexibility, MSE wall and slope structures are quite 

resistant to dynamic forces developed during a seismic event, as confirmed by the 

excellent performance in several recent earthquakes. 

Seismic loading analysis of MSE walls is an Extreme Event limit state. Psuedo-static 

analysis procedures for seismic stability are presented in Chapter 7. Note that for sites 

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C 
 

 

C 

where the anticipated ground acceleration is greater than 0.29 g, significant total lateral 

structure movements may occur, and a deformation analysis for the structure is 

recommended (C11.10.7.1, AASHTO {2007}). 

MSE walls should be designed/checked for seismic stability on all sites where the As 

coefficient is greater than 0.05. For RSS structures, seismic analyses should be included 

regardless of acceleration magnitude. 

Tolerance of precast facing panels to settlement. MSE structures have significant 

deformation tolerance both longitudinally along a wall and perpendicular to the front 

face. Therefore, poor foundation conditions seldom preclude their use. However, where 

significant differential settlements are anticipated (greater than 1/100) sufficient joint 

width and/or slip joints must be provided to preclude panel cracking. This factor may 

influence the type and design of the facing panel selected. 

Square panels generally adapt to larger longitudinal differential settlements better than 

long rectangular panels of the same surface area. A joint width of 3⁄4-inch (20 mm) is 

generally recommended. Guidance on differential settlements that can be tolerated is 

presented in Table 2-3, for panels with a surface of 30 ft2

 (2.8 m2) or less and for panels 

with surface area greater than 30 ft2 (2.8 m2) and less than or equal to 75 ft2 (7 m2). 

Bearing pads used between segmental precast concrete panels should be designed to 

accommodate downdrag forces on it due to elastic settlement of the wall fill. Bearing pad 

design and specification are addressed in Section 3.6.1.a and Section 10.5, respectively. 

MSE walls constructed with full height panels should be limited to differential 

settlements of 1/500. Walls with drycast facing (MBW) should be limited to settlements 

of 1/200. For walls with welded wire facings, the limiting differential settlement should 

be 1/50. 

Where significant differential settlement perpendicular to the wall face is anticipated, the 

reinforcement connection may be overstressed. Where the back of the reinforced soil 

zone will settle more than the face, the reinforcement could be placed on a sloping fill 

surface which is higher at the back end of the reinforcement to compensate for the greater 

vertical settlement. This may be the case where a steep surcharge slope is constructed. 

This latter construction technique, however, requires that surface drainage be carefully 

controlled after each day's construction. Alternatively, where significant differential 

settlements are anticipated, ground improvement techniques may be warranted to limit 

the settlements. 

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Table 2-3. Relationship Between Joint Width and Limiting Differential 

Settlements for MSE Precast Panels (after C11.10.4.1 AASHTO {2007}).
 

Joint Width 

Limiting Differential Settlement 

Area < 30 ft2

 30 ft2 <Area < 75 ft2 

3⁄4-in. (20 mm) 1/100 1/200 

2.8.4 Design Life 

MSE walls should be designed for a service life based on consideration of the potential longterm effects of material deterioration, seepage, stray currents and other potentially deleterious 

environmental factors on each of the material components comprising the wall. For most 

applications, permanent retaining walls should be designed for a minimum service life of 75 

years. Retaining walls for temporary applications are typically designed for a service life of 

36 months or less. 

A greater level of safety and/or longer service life (i.e., 100 years) may be appropriate for 

walls that support true bridge abutments, buildings, critical utilities, or other facilities for 

which the consequences of poor performance or failure would be severe. 

The quality of in-service performance is an important consideration in the design of 

permanent retaining walls. Permanent walls shall be designed to retain an aesthetically 

pleasing appearance, and not require significant maintenance throughout their design service 

life. 

For RSS structures, similar minimum design life ranges should be adopted. 

2.9 PROPRIETARY ASPECTS 

The distinguishing characteristics of MSE trademarked systems from generic systems are 

patented features or materials of construction. 

At present the following significant components are known to be covered by unexpired 

patents: 

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C Connection details between grid reinforcement and precast panel covered by a number of 

patents issued to various suppliers. In general, these patents cover a specific design for 

the concrete-embedded portion of connecting member only. 

C Most MBW facing units are covered by recent design patents. 

A number of patents may be in force for specific MSE construction methods under water, 

specific types of traffic barriers constructed over MSE walls, and facing attachments to 

temporary facings. 

2.10 CASE HISTORIES – MSE WALLS 

2.10.1 Mn/DOT Crosstown Project MSE Walls 

MSE walls were used extensively on the Crosstown Project, located in Minneapolis, 

Minnesota. The walls are used to widen existing roadways, construct new ramps, and 

construct new bridge approaches. The project enlarges and streamlines the I35W and Mn 

Hwy 62 interchange. This is a heavily traveled roadway in a congested urban area. Several 

bridges were widened and several new bridges were constructed as part of this project. The 

detailed wall designs and the wall components were supplied by a Minnesota Department of 

Transportation (Mn/DOT) pre-approved MSE wall vendor. Design followed the ASD 

method (AASHTO 17th Edition, 2002). Most of the walls were constructed during the 2008 

and 2009 construction seasons. 

Approximately 300,000 ft2

 (28,000 m2) of MSE walls were constructed in 24 separate walls. 

Typical wall heights are approximately 25 to 30 ft (7.5 to 9 m), and the maximum wall height 

is 45 ft (14 m). The MSE walls are faced with architectural segmental precast panels and 

reinforced with steel bar mats. Facing panels are painted after wall construction. The 

architectural relief included false columns, on the long and tall walls, as shown in Figure 2­

17. The reinforced wall fill is an angular, well-graded sand. The walls are designed for a 

100-year life. Many walls have traffic barriers on top of the reinforced zoned. The barriers 

were designed by the Mn/DOT project design consultant. A geomembrane was specified and 

installed across the top of the reinforced zones to prevent, or minimize, infiltration of deicing salt runoff into the reinforced fill. 

Temporary welded wire mesh (WWM) faced walls were also used on this project for 

temporary bridge abutments (see Figure 2-2), bridge approach embankments, and 

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construction staging. These walls were also designed and supplied by the same Mn/DOT 

pre-approved vendor. 

The use of MSE walls on this project provided a relatively rapid means of wall construction, 

and produced structures designed for a 100-year life. Cast-in-place, concrete cantilever walls 

were also extensively used on this project. Approximately 500,000 ft2

 (49,000 m2) of C.I.P. 

walls were constructed. 

The cost of these MSE walls was $30.50 per ft2

 ($330 per m2) of face area, plus $17.00 per 

yd3 ($20 per m3) for the reinforced wall fill and $375.00 per yd3 (($450 per m3) of concrete 

for the traffic barrier moment slab. MSE wall cost with the select granular wall fill and 

moment slab was approximately $54.50 per ft2

 ($585 per m2) of face area. The MSE wall 

costs do not include the traffic barrier and noise wall. The cost of the cast-in-place walls on 

the project was $67.20 per ft2 ($723 per m2) of face area plus cost of backfill, for an 

approximate total cost of $76 per ft2 ($820 per m2) of face area. 

Figure 2-17. MSE wall construction on Mn/DOT Crosstown Project, 2008. 

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2.10.2 Veterans Memorial Overpass True Abutment MSE Walls 

The Pima County Department of Transportation’s Veterans Memorial Overpass (VMO) 

project is located in Tucson, Arizona. The project included a 5-span, 348-ft long bridge 

(shown in Figure 2-18) that takes Palo Verde Road over Aviation Parkway, Union Pacific 

railroad assembly area tracks, and 36th Street. The North and South abutments consist of 

spread footings on top of 25-ft and 35-ft high MSE walls, respectively. MSE walls were 

proposed as part of value engineering proposal to replace cast-in-place walls. The walls were 

designed using the ASD method (AASHTO 17th Edition, 2002). The walls were constructed 

in 2004 and 2005. 

An abutment is shown in Figure 2-19, and abutment cross section is illustrated in Figure 2­

20. Both abutments are 150 ft long. The bridge consists of simply supported AASHTO 

Type III girders on elastomeric bearing pads resting on an abutment footing. The bridge 

footing at each abutment is 10.75 ft wide and 10.2 ft high. Clearance between the back of the 

coping and the toe of the footing is 6 in. The length of reinforcements was equal to the 

height of the abutment. Reinforced fill was a select granular fill, in accordance with 

AASHTO/FHWA requirements. Ribbed steel reinforcing strips were used for soil 

reinforcements, with 5-ft tall x 10-ft wide precast concrete segmental panels. 

The upper 9 ft of foundation soils were over-excavated and replaced with engineered fill 

because they were loose and potentially collapsible. Underlying soils were dense to very 

dense clayey sands with refusal N-values. Groundwater depth is greater than 150 ft. 

This project was monitored with over 500 survey points. Settlements of less than 1 in., 

primarily occurring during construction, were measured. No noticeable post-construction 

settlement has been observed. 

 

Figure 2-18. Veterans Memorial Overpass. Figure 2-19. MSE true bridge abutment. 

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Figure 2-20. Typical cross section, VMO project. 

2.10.3 SeaTac Airport Runway Extension MSE Wall 

The tallest MSE wall in the U.S. to date has an exposed height of 138 ft (42 m) at its tallest 

section and was constructed to limit encroachment on adjacent creeks and wetlands and 

increase the land use area for the Third Runway project at SeaTac Airport (Figure 2-21). The 

West MSE wall was one of several walls constructed for the runway extension and is 

approximately 1430 ft (436 m) long, has four tiers formed by 8 ft (2.4 m) setbacks, and had a 

constructed height of 150 ft (148 m). The wall supports 20 ft (6.1 m) high, unreinforced 

2H:1V slope. The MSE wall used steel reinforcing strips with concrete facing panels. In the 

lower tier, up to 25, 2 in. (50 mm) wide by 0.24 in. (6 mm) thick strips with a length of 116 ft 

(35.4 m) were connected to the 5 by 5-ft (1.5 by 1.5-m) panels. A full discussion of project 

background, design aspects, and instrumentation of these MSE walls are provided by Sankey 

et al. (2007) and Stuedlein et al. (2007). 

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Figure 2-21. SeaTac Airport runway extension MSE wall. 

2.10.4 Guanella Pass Roadway Reconstruction 

The Guanella Pass project is located in the Front Range of the Colorado Rocky Mountains 

within the Pike and Arapaho National Forests approximately 50 miles west of Denver. The 

overall goal of the Guanella Pass Road improvement project is to balance transportation 

requirements and roadway maintenance within a sensitive human and natural environment. 

The 24-mile long route connects two principal east-west corridors, US 285 and Interstate 

Highway 70. The original roadway supported two-way traffic with various widths and many 

sharp switchbacks in very steep mountainous terrain. 

Twenty-one wired-faced MSE walls, extending a total length of almost 12,000 feet, were 

constructed along the project to gain adequate roadway width (typical wall is shown in 

Figure 2-22). Alternately, very long and steep down-slope embankments could have been 

constructed, but would have significantly impacted the forest. The MSE walls were able to 

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limit the width of disturbance to a little more than the roadway width. Some of the walls, 

which were visible from the road, were faced with an architectural concrete cast-in-place 

facing (as shown in Figure 2-23). The facing form liner was specifically designed by the 

Forest Service to simulate a more natural rockery type of wall, which was used extensively 

on the cut side of the road. 

Due to the site geometry, these walls were constructed on very steep slopes ranging from 

1.3H:1V to 1.5H:1V. MSE retaining walls for this project were evaluated for global stability 

using limit equilibrium methods using a minimum factor of safety for global stability of 1.3. 

In order to achieve this factor of safety under these geometric and loading conditions, the 

designers needed to work with each site individually. The reinforcement lengths were longer 

than typically used on MSE walls; they ranged from 70% to 120% of the wall height. In 

addition, the designers were able to vary the wall embedment below finished grade. These 

two parameters were used to provide a stable structure to support the new roadway. 

In addition, the project was able to utilize the on-site soils for the reinforced backfill. The 

reinforced backfill met most of the AASHTO and FHWA requirements with the exception of 

the No. 200 sieve. In order to be able to use most of the soils excavated during construction, 

the project specifications allowed the use of up to 20% passing the No. 200 sieve instead of 

15%. This was a significant savings to the project since it would have been difficult to waste 

the excavated material within the construction limits and it would have been very costly to 

import material for the walls, since the project was so remote. 

MSE walls were selected for this project primarily due to their ease of construction and 

flexibility in difficult terrain and remote sites. 

Figure 2-22. Guanella Pass wire-faced wall. 

 

Figure 2-23. Guanella Pass architectural 

 concrete faced wall. 

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CHAPTER 3 

SOIL REINFORCEMENT PRINCIPLES 

AND SYSTEM DESIGN PROPERTIES
 

This chapter outlines the fundamental soil reinforcement principles that governs structure 

behavior, and develops system design parameters which are used for specific MSE wall and
 

RSS design, detailed in Chapters 4, 6, 7, and 9. 

The objectives of this chapter are to develop: 

C An understanding of soil-reinforcement interaction. 

C Introduce normalized pullout capacity concepts. 

C Develop design soil parameters for select reinforced fill, retained backfill and foundation 

bearing capacity. 

C Establish structural design properties. 

3.1 OVERVIEW 

As discussed in Chapter 2, mechanically stabilized earth systems (MSEW and RSS) have 

three major components: reinforcing elements, facing system, and reinforced fill. 

Reinforcing elements may be classified by stress/strain behavior and geometry. In terms of 

stress/strain behavior, reinforcing elements may be considered inextensible (metallic) or 

extensible (polymeric). This division is not strictly correct because some newer glass-fiber 

reinforced composites and ultra-high-modulus polymers have moduli that approach that of 

mild steel. Likewise, certain metallic woven wire mesh reinforcements, such as hexagon 

gabion material, have a structure that will deform more than the soil at failure and are thus 

considered extensible. Based on their geometric shapes, reinforcements can be categorized 

as strips, grids or sheets. Facing elements, when employed, can be precast concrete panels or 

modular blocks, gabions, welded wire mesh, cast-in-place concrete, timber, shotcrete, 

vegetation, or geosynthetic material. Reinforced fill refers to the soil material placed within 

the zone of reinforcement. The retained soil refers to the material, placed or in-situ, directly 

adjacent to the reinforced fill zone. The retained soil is the source of earth pressures that the 

reinforced zone must resist. A drainage system below and behind the reinforced fill is also 

an important component, especially when using poorly draining backfill. 

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3.2 ESTABLISHMENT OF REINFORCED AND RETAINED FILL 

ENGINEERING PROPERTIES 

3.2.1 Reinforced Fill Soil 

The selection criteria of reinforced fill should consider long-term performance of the 

completed structure, construction phase stability and the degradation environment created for 

the reinforcements. Much of engineering communities’ knowledge and experience with 

MSE wall structures to date has been with select, cohesionless backfill. Hence, knowledge 

about internal stress distribution, pullout resistance, and failure surface shape is constrained 

and influenced by the unique engineering properties of these soil types. Granular soils are 

ideally suited to MSE wall and RSS structures. Many agencies have adopted conservative 

reinforced fill requirements for both walls and slopes. These conservative properties are 

suitable for inclusion in standard specifications or special provisions when project specific 

testing is not feasible and when the quality of construction control and inspection may be in 

question. It should be recognized, however, that using conservative reinforced fill 

property criteria cannot completely replace a reasonable degree of construction control 

and inspection. 

In general, these select reinforced fill materials will be more expensive than lower quality 

materials. The specification criteria for each application (walls and slopes) differ somewhat 

primarily based on performance requirements of the completed structure (allowable 

deformations) and the design approach. Material suppliers of proprietary MSE systems each 

have their own criteria for reinforced fills. Detailed project reinforced fill specifications, 

which uniformly apply to all MSE wall and RSS systems, should be provided by the 

contracting agency. The following requirements are consistent with current practice: 

Select Granular Fill Material for the Reinforced Zone of Walls. All fill material used in 

the structure volume for MSE wall structures should be reasonably free from organic or other 

deleterious materials and should conform to the gradation limits, PI and soundness criteria 

listed in Table 3-1. Note that Table 3-1 presents a broad gradation range that is applicable 

across the United States. Individual DOTs may adjust this range based upon locally 

available and economical select granular fill. The reinforced fill should be well-graded in 

accordance with the Unified Soil Classification System (USCS) in ASTM D2487. Unstable 

broadly graded soils (i.e., Cu > 20 with concave upward grain size distributions) and gapgraded soils should be avoided (see Kenney and Lau, 1985, 1986 for a method to identify 

unstable soils). These soils tend to pipe and erode internally, creating problems with both 

loss of materials and clogging of drainage systems. 

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Table 3-1. MSE Wall Select Granular Reinforced Fill Requirements. 

Gradation: 

(AASHTO T-27) 

U.S. Sieve Size Percent Passing(a)

 4 in. (102 mm)(a,b) 100 

 No. 40 (0.425 mm) 0-60 

No. 200 (0.075 mm) 0-15 

Plasticity Index, PI 

(AASHTO T-90) PI < 6 

Soundness: 

(AASHTO T-104) 

The materials shall be substantially free of shale or other soft, 

poor durability particles. The material shall have a magnesium 

sulfate soundness loss of less than 30 percent after four cycles 

(or a sodium sulfate value less than 15 percent after five cycles). 

Notes: 

(a) To apply default F* values, Cu, should be greater than or equal to 4. 

(b) As a result of recent research on construction survivability of geosynthetics and epoxy coated 

reinforcements, it is recommended that the maximum particle size for these materials be 

reduced to 3⁄4-in. (19 mm) for geosynthetics, and epoxy and PVC coated steel reinforcements 

unless construction damage assessment tests are or have been performed on the reinforcement 

combination with the specific or similarly graded large size granular fill. Prequalification tests 

on reinforcements using standard agency fill materials should be considered. 

The fill material must be free of organic matter and other deleterious substances, as these 

materials generally result in poor performance of the structure and enhance degradation for 

reinforcements. Other materials such as soils containing mica, gypsum, smectite, 

montomorrilonite or other soft durability particles should be carefully evaluated as large 

strains are typically required to reach peak strength and pullout capacity, resulting in larger 

lateral and vertical deformation than with higher quality granular fills. Use of salvaged 

materials such as asphaltic concrete millings or Portland Cement Concrete rubble is not 

recommended. Recycled asphalt is prone to creep resulting in both wall deformation and 

reinforcement pullout. Recycled concrete has a potential to produce tufa precipitate from 

unhydrated cement, which can clog drains and exude a white pasty substance onto the wall 

face creating aesthetic problems. The recycled concrete typically does not meet 

electrochemical properties and its corrosion potential has also not been fully evaluated, 

especially if residual wire and rebar are present that could create problems with dissimilar 

metals. 

The compaction specifications should include a specified lift thickness and allowable range 

of moisture content with reference to optimum. Compaction moisture control should be ±2% 

of optimum moisture, wopt. 

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The compaction requirements of reinforced fill are different in close proximity to the wall 

facing (within 3 ft {1 m}). Lighter compaction equipment (e.g., walk-behind vibratory plate 

or roller) and thinner lifts are used near the wall face to prevent buildup of high lateral 

pressures from the compaction and to prevent facing panel movement. Because of the use of 

this lighter equipment, a reinforced fill material of good quality in terms of both 

friction and drainage, such as crushed stone may be used close to the face of the wall to 

provide adequate strength and minimize settlement in this zone. If an open graded fill is 

used adjacent to the face, filtration requirements with the reinforced wall fill must be 

addressed, see Section 5.3.3. It should be noted that granular fill containing even a few 

percent fines (which can even develop or increase due to breakdown during compaction) may 

not be free draining and drainage requirements should always be carefully evaluated. 

Marginal Reinforced Fill for MSE walls. MSE wall reinforced fill materials outside of 

these gradation and plasticity index requirements (Table 3-1) have been used successfully; 

however, problems including significant distortion and structural failure have been observed 

with finer grained and/or more plastic soils. A recent NCHRP research study (NCHRP 24­

22) on Selecting Reinforced Fill Materials for MSE Retaining Walls has confirmed that that 

reinforced fill with up to 35% passing a No. 200 (0.75 mm) sieve could be safely allowed in 

the reinforced fill, provided the properties of the materials are well defined and controls are 

established to address the design issues. Design issues include drainage, corrosion, 

deformations, reinforcement pullout, constructability, and performance expectations. While 

there may be a significant savings in using lower quality reinforced fill, the affect on 

performance must be carefully evaluated. 

For MSE walls constructed with reinforced fill containing more than 15% passing a No. 200 

(0.075 mm) sieve and/or a PI exceeding 6, both total and effective shear strength parameters 

should be evaluated in order to obtain an accurate assessment of horizontal stresses, sliding, 

compound failure (behind and through the reinforced zone) and the influence of drainage on 

the analysis. Both long-term and short-term reinforcement pullout tests as well as 

soil/reinforcement interface friction tests should be performed. Settlement characteristics 

must be carefully evaluated, especially in relation to downdrag stresses imposed on 

connections at the face and settlement of supported structures. Drainage requirements at the 

back, face, and beneath the reinforced zone must be carefully evaluated (e.g., use flow nets to 

evaluate influence of seepage forces and hydrostatic pressure). If marginal fill is used the 

surface of the wall should be positively sloped such that water drains away from the wall 

(which is a good practice for all MSE walls as discussed in Chapter 5, but most important if 

marginal fills are used). In addition, a geomembrane is recommended above the wall to 

preclude infiltration of seepage water into the fill (see Chapter 5, Section 5.3 for drainage 

design details). Again, these drainage features are good practice for all MSE walls. The 

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length of the upper 2 layers of reinforcement should be extended at least 3 to 5 ft beyond the 

lower reinforcement layers to reduce the potential for tension cracks to develop directly 

behind the reinforced zone. If the soil reinforcement is steel, the extended layers must be 

contained within select granular fill to avoid differential corrosion conditions. 

Electrochemical tests should be performed on the reinforced fill to obtain data for evaluating 

degradation of reinforcements and facing connections (see Section 3.2.3). Moisture and 

density control during construction must be carefully controlled in order to obtain strength 

and interaction values. Deformation during construction also must be carefully monitored 

and maintained within defined design limits. Performance monitoring is also recommended 

for reinforced fill soils that fall outside of the requirements listed above, as detailed in 

Chapter 11. 

Reinforced Rock Fill for Wall or RSS Structures. Material that is composed primarily of 

rock fragments (material having less than 25 percent passing a 3⁄4 in (20 mm) sieve) should be 

considered to be a rock backfill. The maximum particle size should not exceed the limits 

listed in Table 3-1. Such material should meet all the other non-gradation requirements such 

as soundness and electrochemical properties in Tables 3-1 to 3-4. When such material is 

used, a very high survivability geotextile filter (e.g., Type 1 geotextile in accordance with 

AASHTO M 288), designed for filtration performance following the guidelines in FHWA 

NHI-07-092 (Holtz et al., 2008), should encapsulate the rock backfill to within 3 ft (1 m) 

below the wall coping. Adjoining sections of separation fabric should be overlapped by a 

minimum of 12 in. (0.30 m). Additionally, the upper 3 ft (1 m) of fill should contain no 

stones greater than 3 in. (75 mm) in their greatest dimension, and should be composed of 

material not considered to be rock backfill, as defined herein. Where density testing is not 

possible, trial fill sections should be constructed with agency supervisory personnel and 

geotechnical specialist present to determine appropriate watering, in situ modification 

requirements (e.g., grading), lift thickness, and number of passes to achieve adequate 

compaction. Compaction can be determined by measuring the settlement of the trial section 

at a number of points after each pass (e.g., a minimum of 5 points measured at the center of a 

1 ft square plate is typically required). Several lifts should be constructed to determine the 

appropriate number of passes, which will maximize compaction without excessively crushing 

the rock at the surface. The number of passes to achieve at least 80 percent of the maximum 

settlement should be required. 

Select Reinforced Fill for RSS Structures. Less select reinforced fill can be used for RSS 

since facings are typically flexible and can tolerate some distortion during construction. 

Even so, a high quality embankment fill meeting the following gradation requirements to 

facilitate compaction and minimize reinforcement requirements is recommended. The 

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guidelines listed in Table 3-2 are provided as recommended reinforced fill requirements for 

RSS construction. 

RSS reinforced fill compaction should be based on 95% of AASHTO T-99, and ±2% of 

optimum moisture, wopt. 

RSS fill materials outside of these gradation and plasticity index requirements have been 

used successfully as well as unsuccessfully. For fill materials outside of these limits, default 

values for strength and pullout are no longer applicable and laboratory tests must be 

performed. Issues with drainage problems, excessive distortion and settlement (as discussed 

above for marginal fill in MSE walls) must be carefully evaluated with finer grained and/or 

more plastic soils. Performance monitoring is also recommended for reinforced fill soils that 

fall outside of the requirements listed above, as detailed in Chapter 11. 

Table 3-2. RSS Granular Reinforced Fill Requirements. 

Gradation: 

(AASHTO T-27) 

U.S. Sieve Size Percent Passing 

4 in. (102 mm)(a,b) 3⁄4-inch (20 

mm)(a) 

100 

No. 4 (4.76 mm) 100 – 20 

 No. 40 (0.425 mm) 0-60 

No. 200 (0.075 mm) 0 – 50 

Plasticity Index, PI 

(AASHTO T-90) PI < 20 

Soundness: 

(AASHTO T-104) 

Magnesium sulfate soundness loss less than 30% after 4 

cycles, based on AASHTO T-104 or equivalent sodium 

sulfate soundness of less than 15 percent after 5 cycles. 

Note: 

(a) To apply default F* values, Cu, should be greater than or equal to 4. 

(b) As a result of recent research on construction survivability of geosynthetics and epoxy 

coated reinforcements, it is recommended that the maximum particle size for these 

materials be reduced to 3⁄4-in. (19 mm) for geosynthetics, and epoxy and PVC coated steel 

reinforcements unless construction damage assessment tests are or have been performed 

on the reinforcement combination with the specific or similarly graded large size granular 

fill. Prequalification tests on reinforcements using standard agency fill materials should 

be considered. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 6 November 2009
 
 

Design Strength of Select Granular Reinforced Fill. The MSE wall and RSS reinforced 

fill criteria outlined previously represent materials that have been successfully used 

throughout the United States and resulted in excellent performance of MSEW and RSS 

structures. Peak shear strength parameters are used in the wall and slope analyses. For MSE 

walls using well fill meeting the gradation requirements in Table 3-1, a maximum effective 

friction angle  of 34 degrees is usually assumed (in accordance with Article 11.10.6.2, 

AASHTO, 2007), unless project-specific fill is tested by triaxial (per AASHTO T-296) or 

direct shear (per AASHTO T-236), per Article 11.10.6.2 (AASHTO, 2007). However, some 

nearly uniform fine sands meeting the specifications limits may exhibit friction angles of 30 

to 32 degrees. When contractor furnished sources are used, the specification may also require 

testing of the source material to verify that its friction angle meets specification requirements 

(e.g., 34 degrees). Higher values may be used if substantiated by laboratory direct shear or 

triaxial test results for the site specific material used or proposed. If the measured friction 

angle is greater than 40 degrees, the angle of friction used for design should not exceed 40 

degrees (Article 11.10.6.2, AASHTO {2007}). In all cases, the cohesion of the reinforced 

fill is assumed to be zero. 

For RSS structures, where a considerably greater percentage of fines (minus No. 200 sieve) is 

permitted, lower bound values of frictional strength equal to 28 to 30 degrees would be 

typical for the reinforced fill requirements listed. A significant economy could again be 

achieved if laboratory direct shear or triaxial test results on the proposed fill are performed, 

justifying a higher value. Likewise, soils outside the gradation range listed should be 

carefully evaluated and monitored. 

Limits of Reinforced Fill. For MSE walls, except back-to-back walls, and RSS, many 

agencies extend the reinforced fill beyond the free end of the reinforcement. Some agencies 

extend the reinforced fill 1 ft (0.3 m) beyond the reinforcement length, and some others 

extend the fill in a wedge behind the reinforced zone, as illustrated in Figure 3-1. For backto-back walls wherein the free ends of the reinforcement of the two walls are spaced apart 

less than or equal to one-half the design height of the taller wall, reinforced backfill should 

be used for the space between the free ends of the reinforcements as well. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 7 November 2009
 
 

Figure 3-1. Examples of reinforced fill zone extension beyond the reinforced zone. 

3.2.2 Retained Backfill and Natural Retained Soil 

The key engineering properties required for the retained backfill are the strength and unit 

weight based on evaluation and testing of subsurface or borrow pit data. Friction angles () 

may be determined from either by consolidated drained triaxial tests with pore pressure 

measurements or drained direct shear tests. As with reinforced fill, a cohesion value of zero 

is conservatively recommended for the long-term, effective strength of the retained fill. For 

backcut construction, if undisturbed samples cannot be obtained, friction angles may be 

obtained from in-situ tests or by correlations with index properties. The strength properties 

are required for the determination of the coefficients of earth pressure used in design as well 

as for overall stability analysis. In addition, the position of groundwater levels above the 

proposed base of construction must be determined in order to evaluate hydrostatic stresses in 

the retained fill and plan an appropriate drainage scheme to control ground water conditions. 

For most retained backfills lower bound frictional strength values of 28 to 30 degrees are 

reasonable for granular and low plasticity cohesive soils. For highly plastic retained fills and 

natural soils (PI > 20), even lower values would be indicated and should be evaluated for 

both drained and undrained conditions. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 8 November 2009
 
 

 

 

Backfill and natural soil behind the limits of the reinforced fill should be considered to be in 

the retained zone for a distance equal to 50 percent of the design height of the MSE wall. For 

the reasons discussed previously for reinforced fill, use of soils containing shale, mica, 

gypsum, smectite, montmorillonite or other soft particles of poor durability is discourage and 

soundness limits should meet the criteria in Table 3-1. 

The following are good practice to preclude potential problems with retained backfill soils. 

The percent fines, i.e., the fraction passing No. 200 sieve (0.075 mm), should be less than 50 

and the Liquid Limit and Plasticity Index (PI) should be less than 40 and 20 percent, 

respectively, as determined in accordance with AASHTO T-90. The potential differential 

settlement/performance between the reinforced fill and retained backfill should be assessed. 

The agency should consider transition detailing between the reinforced zone and retained 

backfill by lengthening the upper two layers of soil reinforcement or extending the reinforced 

zone beyond the reinforcement length, as previously discussed. The maximum particle size 

in the retained backfill should limited to the maximum particle size in the reinforced wall fill, 

at least within this transition zone. Material that is composed primarily of rock fragments 

(material having less than 25 percent passing a 3⁄4-inch sieve), should be considered to be a 

rock backfill (see Section 3.2.1). 

3.2.3 Electrochemical Properties 

The design of buried steel elements of MSE structures is predicated on reinforced fills 

exhibiting minimum or maximum electrochemical index properties and then designing the 

structure for maximum corrosion rates associated with these properties. These recommended 

index properties and their corresponding limits are shown in Table 3-3. Reinforced fill soils 

must meet the indicated criteria to be qualified for use in MSE construction using steel 

reinforcements. 

Table 3-3. Recommended Limits of Electrochemical Properties for Reinforced Fills 

with Steel Reinforcement. 

Property Criteria Test Method 

Resistivity > 3000 ohm-cm AASHTO T-288 

pH > 5 and < 10 AASHTO T-289 

Chlorides < 100 PPM ASMT D4327 

Sulfates < 200 PPM ASTM D4327 

Organic Content 1% max. AASHTO T-267 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 9 November 2009
 
 

Table 3-4. Recommended Limits of Electrochemical Properties for Reinforced Fills 

with Geosynthetic Reinforcements (FHWA NHI-09-087, Elias et al., 2009). 

Base Polymer Property Criteria Test Method 

Polyester (PET) pH 3 < pH < 9 AASHTO T-289 

Polyolefin (PP & HDPE) pH pH > 3 AASHTO T-289 

Where geosynthetic reinforcements are planned, the limits for electrochemical criteria will 

vary depending on the polymer. Limits, based on current research, are shown in Table 3-4. 

3.3 REINFORCED SOIL CONCEPTS 

A reinforced soil mass is somewhat analogous to reinforced concrete in that the mechanical 

properties of the mass are improved by reinforcement placed parallel to the principal strain 

direction to compensate for soil's lack of tensile resistance. The improved tensile properties 

are a result of the interaction between the reinforcement and the soil. The composite material 

has the following characteristics: 

C Stress transfer between the soil and reinforcement takes place continuously along the 

reinforcement. 

C Reinforcements are distributed throughout the soil zone with a degree of regularity. 

localized. 

3.3.1 Stress Transfer Mechanisms 

Stresses are transferred between soil and reinforcement by friction (Figure 3-2a) and/or 

passive resistance (Figure 3-2b) depending on the reinforcement geometry. 

Friction develops at locations where there is a relative shear displacement and corresponding 

shear stress between soil and the reinforcement surface. Reinforcing elements dependent on 

friction should be aligned with the direction of soil reinforcement relative movement. 

Examples of such reinforcing elements are steel strips, longitudinal bars in grids, geotextile, 

geosynthetic straps, and some geogrid layers. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 10 November 2009
 
 

Figure 3-2.Stress transfer mechanisms for soil reinforcement. 

Passive resistance occurs through the development of bearing type stresses on "transverse" 

reinforcement surfaces normal to the direction of soil reinforcement relative movement. 

Passive resistance is generally considered to be the primary interaction for bar mat, wire 

mesh reinforcements, and geogrids with relatively stiff cross machine direction ribs, The 

transverse ridges on "ribbed" strip reinforcement also provide some passive resistance. 

The contribution of each transfer mechanism for a particular reinforcement will depend on 

the roughness of the surface (skin friction), normal effective stress, grid opening dimensions, 

thickness of the transverse members, and elongation characteristics of the reinforcement. 

Equally important for interaction development are the soil characteristics, including grain 

size, grain size distribution, particle shape, density, water content, cohesion, and stiffness. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 11 November 2009
 
 

 

 

3.3.2 Mode of Reinforcement Action 

The primary function of reinforcements is to restrain soil deformations. In so doing, stresses 

are transferred from the soil to the reinforcement. These stresses are resisted by the 

reinforcement tension and/or shear and bending. 

 Tension is the most common mode of action of tensile reinforcements. All 

"longitudinal" reinforcing elements (i.e., reinforcing elements aligned in the direction of 

soil extension) are generally subjected to high tensile stresses. Tensile stresses are also 

developed in flexible reinforcements that cross shear planes. 

 Shear and Bending. "Transverse" reinforcing elements that have some rigidity, can 

withstand shear stress and bending moments.

 3.3.3 Geometric Characteristics 

Two types can be considered: 

C Strips, bars, and steel grids. A layer of steel strips, bars, or grids is characterized by the 

cross-sectional area, the thickness and perimeter of the reinforcement element, and the 

center-to-center horizontal distance between elements (for steel grids, an element is 

considered to be a longitudinal member of the grid that extends into the wall). 

C Geotextiles and geogrids. A layer of geosynthetic strips is characterized by the width of 

the strips and the center-to-center horizontal distance between them. The cross-sectional 

area is not needed, since the strength of a geosynthetic strip is expressed by a tensile 

force per unit width, rather than by stress. Difficulties in measuring the thickness of 

these thin and relatively compressible materials preclude reliable estimates of stress. 

The coverage ratio Rc is used to relate the force per unit width of discrete reinforcement to 

the force per unit width required across the entire structure. See Figure 3-3 (and later Figure 

3-5) for an illustration of these terms. 

 R b c  (3-1) S h 

where: 

b = the gross width of the strip, sheet, or grid. For grids, b is measured from the center 

to center of the outside longitudinal bars as shown in Figure 3-3. 

Sh = center-to-center horizontal spacing between strips, sheets, or grids 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 12 November 2009
 
 

Note, Rc = 1 in the case of continuous reinforcement, i.e., each reinforcement layer covers 

the entire horizontal surface of the reinforced soil zone. Alternatively, for discrete 

reinforcements and segmental precast concrete facing, force per width may be more 

conveniently calculated per panel width, as defined later in Equation 4-25c, for layout and 

detailing. 

Figure 3-3. Coverage ratio. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 13 November 2009
 
 

 

3.4 SOIL REINFORCEMENT INTERACTION USING NORMALIZED CONCEPTS 

Soil-interaction (pullout capacity) coefficients have been developed by laboratory and field 

studies, using a number of different approaches, methods, and evaluation criteria. A unified 

normalized approach developed in a FHWA research project is detailed below. 

3.4.1 Evaluation of Pullout Performance 

The design of the soil reinforcement system requires an evaluation of the long-term pullout 

performance with respect: 

C Pullout capacity, i.e., the pullout resistance of each reinforcement should be adequate to 

resist the factored tensile force in the reinforcement with a specified resistance factor (or 

factor of safety in the case of RSS). 

C Allowable displacement, i.e., the relative soil-to-reinforcement displacement required to 

mobilize the design tensile force should be smaller than the allowable displacement. 

C Long-term displacement, i.e., the pullout load should be smaller than the critical creep 

load. 

The pullout resistance of the reinforcement is mobilized through one or a combination of the 

two basic soil-reinforcement interaction mechanisms, interface friction and passive soil 

resistance against transverse elements of reinforcements such as bar mats, wire meshes, or 

geogrids. The load transfer mechanisms mobilized by a specific reinforcement depends 

primarily upon its structural geometry (i.e., composite reinforcement such as grids, versus 

linear or planar elements, thickness of transverse elements, and aperture dimension). The 

soil-to-reinforcement relative movement required to mobilize the design tensile force 

depends mainly upon the load transfer mechanism, the extensibility of the reinforcement 

material, the soil type, and the confining pressure. 

The long-term pullout performance (i.e., displacement under constant design load) is 

predominantly controlled by the creep characteristics of the soil and the reinforcement 

material. Soil reinforcement systems will generally not be used with cohesive soils 

susceptible to creep. Therefore, creep is primarily controlled by the type of reinforcement. 

Pullout performance in terms of the main load transfer mechanism, relative soil-toreinforcement displacement required to fully mobilize the pullout resistance, and creep 

potential of the reinforcement in granular (and low plasticity cohesive) soils for generic 

reinforcement types is provided in Table 3-5. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 14 November 2009
 
 

 

 

 

 

Table 3-5. Reinforcement Pullout Performance in Granular and Cohesive 

Soils of Low Plasticity.
 

Generic 

Reinforcement 

Type 

Major Load 

Transfer 

Mechanism 

Range of 

Displacement 

at Specimen Front 

Long Term 

Deformation 

Inextensible strips 

Smooth Frictional 0.05 in. 

(1.2 mm) 

Noncreeping 

Ribbed Frictional + passive 0.5 in. 

(12 mm) 

Noncreeping 

Extensible composite 

plastic strips 

Frictional Dependent on 

reinforcement 

extensibility 

Dependent on 

reinforcement structure 

and polymer creep 

Extensible sheets 

Geotextiles Frictional Dependent on 

reinforcement 

extensibility 

(1 to 4 in.) 

{25 to 100 mm} 

Dependent on 

reinforcement structure 

and polymer creep 

characteristics 

Inextensible grids 

bar mats Passive + frictional 0.5 to 2 in. 

(12 to 50 mm) 

Noncreeping 

welded wire 

meshes 

Frictional + passive 0.5 to 2 in. 

(12 to 50 mm) 

Noncreeping 

Extensible grids 

Geogrids Frictional + passive Dependent on 

extensibility 

(1 to 2 in.) 

{25 to 50 mm} 

Dependent on 

reinforcement structure 

and polymer creep 

characteristics 

woven wire meshes Frictional + passive 1 to 2 in. 

(25 to 50 mm) 

Noncreeping 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 15 November 2009
 
 

 

 

 

 

 

3.4.2 Estimate of the Reinforcement Pullout Capacity in MSE Structures 

The pullout resistance of the reinforcement is defined by the ultimate tensile load required to 

generate outward sliding of the reinforcement through the reinforced soil zone. Several 

approaches and design equations have been developed and are currently used to estimate the 

pullout resistance by considering frictional resistance, passive resistance, or a combination of 

both. The design equations use different interaction parameters, so it is difficult to compare 

the pullout performance of different reinforcements for a specific application. 

For design and comparison purposes, a normalized definition of pullout resistance will be 

used throughout the manual. The pullout resistance, Pr, at each of the reinforcement levels 

per unit width of reinforcement is given by: 

 P r  F * α σ' 

v Le C (3-2) 

where: 

Le C = the total surface area per unit width of the reinforcement in the resistive zone 

behind the failure surface 

Le = the embedment or adherence length in the resisting zone behind the failure 

surface 

C = the reinforcement effective unit perimeter; e.g., C = 2 for and sheets, and 

because the edges are neglected C = 2 for strips and grids 

F* = the pullout resistance (or friction-bearing-interaction) factor 

α = a scale effect correction factor to account for a non linear stress reduction over 

the embedded length of highly extensible reinforcements, based on laboratory 

data (generally 1.0 for metallic reinforcements and 0.6 to 1.0 for geosynthetic 

reinforcements, see Table 3-6). 

σv = the effective vertical stress at the soil-reinforcement interfaces. 

The correction factor α depends, therefore, primarily upon the strain softening of the 

compacted granular backfill material, and the extensibility and the length of the 

reinforcement. For inextensible reinforcement, α is approximately 1, but it can be 

substantially smaller than 1 for extensible reinforcements. The α factor (a scale correction 

factor) can be obtained from pullout tests on reinforcements with different lengths as 

presented in Appendix B, or derived using analytical or numerical load transfer models 

which have been "calibrated" through numerical test simulations. In the absence of test data, 

α = 0.8 for geogrids and α = 0.6 for geotextiles (extensible sheets) is recommended (see 

Table 3-6). 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 16 November 2009
 
 

 

Table 3-6. Summary of Pullout Capacity Design Parameters. 

Reinforcement Type Sopt 

Grid 

Spacing Tan ρ Fq αβ

α

Default 

Value 

Inextensible strips NA Obtain Tan ρ

from tests, or 

use default 

values 

NA NA 1.0 

Inextensible grids (bar 

mats and welded wire) 2 tan

t Fq  SS optt Obtain Tan ρ

from tests 

NA NA 1.0* 

2 tan

t Fq  SS optt NA Obtain Fq 

from tests, or 

use default 

values 

t 2S 

t 1.0* 

Extensible grids with 

1 

d50 

min. grid opening 

 2 tan

t Fq  SS optt Obtain Tan ρ

from tests 

NA NA 0.8 

2 tan

t Fq  SS optt NA Obtain Fq 

from tests, or 

use default 

values 

t 

b 

2S 

tf 0.8 

Extensible grids with 

1 

d50 

min. grid opening 



 NA Obtain Tan ρ

from tests 

NA NA 0.8 

Extensible sheets NA Obtain Tan ρ

from tests 

NA NA 0.6 

NOTES: 

(i) It is acceptable to use the empirical values provided in or referenced by this table to determine F* 

in the absence of product and backfill specific test data, provided granular reinforced fill as specified in 

Table 3-1 for MSE walls is used and Cu ≥ 4. For fill outside these limits, tests must be run. 

(ii) Pullout testing to determine α is recommended if α shown in table is less than 1.0. These values 

of α represent highly extensible geosynthetics. 

(iii) For grids where Tan ρ is applicable, apply Tan ρ to the entire surface area of the reinforcement 

sheet (i.e., soil and grid), not just the surface area of the grid elements. 

(iv) NA means "not applicable."  is the soil friction angle. ρ is the interface friction angle mobilized 

along the reinforcement. Sopt is the optimum transverse grid element spacing to mobilize maximum 

pullout resistance as obtained from pullout tests (typically 6 in. (150 mm) or greater). St is the spacing 

of the transverse grid elements. t is the thickness of the transverse elements. Fq is the embedment (or 

surcharge) bearing capacity factor. α is a structural geometric factor for passive resistance. fb is the 

fraction of the transverse member on which bearing can be fully developed (typically ranging from 0.6 

to 1.0) as obtained from an evaluation of the bearing surface shape. D50 is the backfill grain size at 

50% passing by weight. α is the scale effect correction factor. 

(v) Definitions of the geometric variables are illustrated in Figure 3-4. 

* For longitudinal bars/wires spacing greater than 6 inches,  may be less than 1.0 and pullout tests 

are required. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 17 November 2009
 
 

 

t = Thickness of transverse bar 

D = Nominal diameter of longitudinal 

Notes: 

1. Transverse bar thickness does not need to be reduced for corrosion. 

2. This is applicable up to a maximum transverse bar spacing of 24 in. (610 mm). 

Figure 3-4. Definition of grid dimensions for calculating pullout capacity. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 18 November 2009
 
 

 

 

The Pullout Resistance Factor F* can be obtained most accurately from laboratory or field 

pullout tests performed in the specific backfill to be used on the project. Test procedures for 

determining pullout parameters are presented in Appendix B. Alternatively, F* can be 

derived from empirical or theoretical relationships developed for each soil-reinforcement 

interaction mechanism and provided by the reinforcement supplier. For any reinforcement, 

F* can be estimated using the general equation: 

F* = Passive Resistance + Frictional Resistance 

or, F* = Fq · αβ + tan ρ (3-3) 

where: 

Fq = the embedment (or surcharge) bearing capacity factor 

αβ = a bearing factor for passive resistance which is based on the thickness per unit 

width of the bearing member. 

ρ = the soil-reinforcement interaction friction angle. 

The pullout capacity parameters for Equation 3-3 are summarized in Table 3-6 and Figure 3­

4 for the soil reinforcement systems considered in this manual. 

A significant number of laboratory pullout tests have been performed for many commonly 

used reinforcement backfill combinations and correlated to representative field pullout tests. 

Therefore, the need for additional laboratory and/or field pullout tests, is generally limited to 

reinforcement/reinforced fill combinations where this data is sparse or nonexistent (e.g., 

uniform and marginal reinforced fill discussed in Section 3.1). Where applicable, laboratory 

pullout tests should be made in accordance with the device and procedures in ASTM D6706 

and Appendix B of this manual. Note that this test procedure provides a short-term pullout 

capacity and does not account for soil or reinforcement creep deformations, which may be 

significant in MSE wall and RSS structures utilizing fine grained soil fills. 

When using laboratory pullout tests to determine design parameters, vertical stress variations 

and reinforcement element configurations for the actual project should be used. Tests should 

be performed on samples with a minimum embedded length of 24 in. (600 mm). The pullout 

resistance is the greater of the peak pullout resistance value prior to, or the value achieved at, 

a maximum deformation of 3⁄4-in. (20 mm) as measured at the front of the embedded section 

for inextensible reinforcements and 5/8-in. (15 mm) as measured at the end of the embedded 

sample for extensible reinforcements. This allowable deflection criterion is based on a need 

to limit the structure deformations, which are necessary to develop sufficient pullout 

capacity. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 19 November 2009
 
 

Long-term pullout tests to assess soil/reinforcement creep behavior should be conducted 

when silt or clay reinforced fill is being used. Soil properties and reinforcement type will 

determine if the allowable pullout resistance is governed by creep deformations. The 

placement and compaction procedures for both short-term and long-term pullout tests should 

simulate field conditions. The allowable deformation criteria in the previous paragraph 

should be applied. 

A summary of the procedures for evaluating laboratory tests to obtain pullout design 

parameters is outlined in Appendix B of this manual. 

Most specialty system suppliers have developed recommended pullout parameters for their 

products when used in conjunction with the select backfill detailed in this chapter for MSEW 

and RSS structures. The semi-empirical relationships summarized below are consistent with 

results obtained from laboratory and field pullout testing at a 95 percent confidence limit, and 

generally consistent with suppliers developed data. Some additional economy can be 

obtained from site/product specific testing, where the source of the backfill in the reinforced 

volume has been identified during design. 

In the absence of site-specific pullout testing data, it is reasonable to use the semi-empirical 

relationships described in the following paragraphs in conjunction with the standard 

specifications for reinforced fill to provide a conservative evaluation of pullout resistance. 

For steel ribbed reinforcement, the Pullout Resistance Factor F* is commonly taken as: 

 

F* = tan ρ = 1.2 + log Cu at the top of the structure = 2.0 maximum (3-4) 

F* = tan  at a depth of 20 ft (6 m) and below (3-5) 

where Cu is the uniformity coefficient of the backfill (D60/D10). If the specific Cu for the 

wall backfill is unknown at design time, a Cu = 4 should be assumed (i.e., F* = 1.8 at the 

top of the wall) for reinforced fills meeting the requirements of Section 3.1 of this 

chapter. 

For steel grid reinforcements with transverse spacing of St > 6 inches (150 mm) (see Figure 

3-4), F* is a function of a bearing or embedment factor, Fq, applied over the contributing 

bearing αβ, as follows: 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 20 November 2009
 
 

 

 

F*= Fq αβ = 40 αβ = 40 (t/2St) = 20 (t/St) at the top of the structure (3-6) 

F*= Fq αβ = 20 αβ = 20 (t/2St) = 10 (t/St) at a depth of 20 ft (6 m) and below (3-7) 

Where, t is the thickness of the transverse bar. St must be uniform throughout the length of 

the reinforcement rather than having transverse grid members concentrated only in the 

resistant zone. The maximum St distance is 24 in. (610 mm). For sloping backfills see 

Figure 4-15. 

For geosynthetic (i.e., geogrid and geotextile) sheet reinforcement, the pullout resistance is 

based on a reduction in the available soil friction with the reduction factor often referred to as 

an Interaction Factor, Ci. In the absence of test data, the F* value for geosynthetic 

reinforcement should conservatively be taken as: 

F *  2 

3 tan (3-8) 

Where used in the above relationships,  is the peak friction angle of the soil which for MSE 

walls using select granular backfill, is taken as a maximum of 34 degrees unless project 

specific test data substantiates higher values. For RSS structures, the  angle of the 

reinforced backfill is normally established by test, as a reasonably wide range of backfills can 

be used. A lower bound value of 28 degrees is often used. 

3.4.3 Interface Shear 

The interface shear between sheet type geosynthetics (geotextiles, geogrids and 

geocomposite drains) and the soil is often lower than the friction angle of the soil itself and 

can form a slip plane. Therefore the interface friction coefficient tan ρ must be determined in 

order to evaluate sliding along the geosynthetic interface with the reinforced fill and, if 

appropriate, the foundation or retained backfill soil. The interface friction angle ρ is 

determined from soil-geosynthetic direct shear tests in accordance with ASTM D 5321. In 

the absence of test results, the interface friction coefficient can be conservatively taken as: 

ρ  2 

3 tan (3-9) 

for geotextiles, geogrids and geonet type drainage composites. Other geosynthetics such as 

geomembranes and some geocomposite drain cores may have much lower interface values 

and tests should accordingly be performed. 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 21 November 2009
 
 

 

 3.5 ESTABLISHMENT OF STRUCTURAL DESIGN PROPERTIES 

The structural design properties of reinforcement materials are a function of geometric 

characteristics, strength and stiffness, durability, and material type. The two most commonly 

used reinforcement materials, steel and geosynthetics, must be considered separately as 

follows: 

3.5.1 Strength Properties of Steel Reinforcements 

For steel reinforcements, the design life is achieved by reducing the cross-sectional area of 

the reinforcement used in design calculations by the anticipated corrosion losses over the 

design life period as follows: 

Ec  En  ER (3-10) 

where Ec is the thickness of the reinforcement at the end of the design life, En the nominal 

thickness at construction, and ER the sacrificial thickness of metal expected to be lost by 

uniform corrosion during the service life of the structure. 

The nominal long-term tensile strength of the reinforcement, Tal, is obtained for steel strips 

and grids as shown in the following equations. Tal in units force per unit width is used to 

provide a unified strength approach, which can be applied to any reinforcement. Tensile 

strength of a known steel or grid reinforcement can also be expressed in terms of the tensile 

load carried by the reinforcement, Ptal. The desired designation of reinforcement tensile 

strength (Tal or Ptal) varies depending on whether one is designing with a known system, 

designing with an undefined reinforcement, checking a design layout, performing connection 

design, or performing reinforcement pullout calculations. Thus, nominal tensile strength may 

be calculated and expressed in the following terms: 

 

F A

Tal  y 

b 

c (in strength per unit reinforcement width {kips/ft}) (3-11a) 

Ptal  Fy Ac (in strength per reinforcement element {kips}) (3-11b) 

where: 

b = the gross width of the strip, sheet or grid (see Figure 3-5) 

Fy = yield stress of steel 

FHWA NHI-10-024 3 – Soil Reinforcement Principles 

MSE Walls and RSS – Vol I 3 – 22 November 2009
 

 

 

 Ac = design cross section area of the steel, defined as the original cross section area 

minus corrosion losses anticipated to occur during the design life of the wall. 

The LRFD resistance factors for steel reinforcements in MSE walls are listed in Table 4-8. 

The resistance factor for strip reinforcements under static conditions is 0.75. The resistance 

factors for steel grid MSE wall reinforcements, for static loading, is 0.65 when reinforcement 

is connected to a rigid facing element and is 0.75 when connected to a flexible facing. The 

lower resistance factor for grid reinforcing members connected to a rigid facing element 

(e.g., a concrete panel or block) is used to account for the greater potential for local 

overstress due to load nonuniformities for steel grids than for steel strips or bars. Transverse 

and longitudinal grid members are sized in accordance with ASTM A185. 

The quantities needed to determine Ac for steel strips and grids are shown in Figure 3-5. 

Typical dimensions for common steel reinforcements are provided in Appendix C. The use 

of hardened and otherwise low strain (very high strength) steels may increase the potential 

for catastrophic failure; therefore, a lower resistance factor may be warranted with such 

materials. 

For metallic reinforcement, the life of the structure will depend on the corrosion resistance of 

the reinforcement. Practically all the metallic reinforcements used in construction of 

embankments and walls, whether they are strips, bar mats, or wire mesh, are made of 

galvanized mild steel. Woven meshes with PVC coatings provide some corrosion protection, 

provided the coating is not significantly damaged during construction. Epoxy coatings can 

be used for corrosion protection, but are susceptible to construction damage, which can 

significantly reduce the coatings effectiveness. When PVC or epoxy coatings are used, the 

maximum particle size of the backfill should be restricted to 3⁄4-inch (19 mm) or less to 

reduce the potential for construction damage. For a more detailed discussion of 

requirements, refer to the Corrosion/Degradation manual, FHWA NHI-09-087 (Elias et al., 

2009). 

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MSE Walls and RSS – Vol I 3 – 23 November 2009
 

 

(a) 

(b) 

 D*

2  A  No. of longitudinal bars 



  c	 4   

D* = Diameter of bar or wire corrected for corrosion loss 

b = Unit width of reinforcement (if reinforcement is continuous count the number of 

bars for reinforcement width of 1 unit) 

 R F A c y c T max   R Tc al 	  b 

where Tmax = Maximum factored load applied to reinforcement (load/unit wall width) 

Tal = Nominal long-term tensile strength of the reinforcement 

(strength/unit reinforcement width) 

 = 0.75 for steel strip 

 = 0.65 for steel grid and rigid face 

Fy = Yield strength of steel 

b 

Rc = Reinforcement coverage ratio  

Sh 

Use Rc	 = 1 for continuous reinforcement (i.e., Sh = b = 1 unit width) 

Figure 3-5.	 Parameters for metal reinforcement nominal strength calculations showing (a) 

steel strips and (b) metallic grids and bar mats. 

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MSE Walls and RSS – Vol I 3 – 24 November 2009
 
 

 

Several DOTs have used resin-bonded epoxy coated steel reinforcing elements. The 

effectiveness of these coatings in MSE wall structures has not been well documented. If 

used, the minimum coating thickness should be on the order of 18 mils (45 m), and applied 

in accordance with ASTM A884 for grid reinforcement and AASHTO M284 for strip 

reinforcement. The in-ground design life of the coating should be considered as equal to that 

of a galvanized reinforcement with a coating thickness of 3.4 mils (85 m), unless durability 

exposure testing has been performed on the specific coating that identifies a longer effective 

life as discussed in FHWA NHI-09-087 (Elias et al., 2009). Where other metals, such as 

aluminum alloys or stainless steel have been used, corrosion, unexpectedly, has been a severe 

problem, and their use has been discontinued. 

The in-ground degradation resistance of PVC coated mesh has not been sufficiently 

demonstrated. Anecdotal evidence of satisfactory performance in excess of 25 years does not 

exist. 

Extensive studies have been made to determine the rate of corrosion of galvanized mild steel 

bars or strips buried in different types of soils commonly used in reinforced soil. Based on 

these studies, deterioration of steel strips, mesh, bars and mats can be estimated and 

accounted for by using increased metal thickness. 

The majority of MSE walls constructed to date have used galvanized steel and backfill 

materials with low corrosive potential. A minimum galvanization coating of 2.0 oz/ft2

 (605 

g/m2) or 3.4 mils (85 μm) thickness is required per Article 11.10.6.4.2a (AASHTO, 2007). 

Galvanization shall be applied in accordance with AASHTO M 111 (ASTM A 123) for strip 

type, bar mat, or grid type reinforcements and ASTM A 153 for accessory parts such as bolts 

and tie strips. Galvanization shall be applied after fabrication in accordance with ASTM 

A123. The zinc coating provides a sacrificial anode that corrodes while protecting the base 

metal. Galvanization also assists in preventing the formation of pits in the base metal during 

the first years of aggressive corrosion (which can occur in non-galvanized or “black” steel). 

After the zinc is oxidized (consumed), corrosion of the base metal starts. 

The ASTM and AASHTO standards for galvanization provide different required minimum 

galvanization coating thickness as a function of the bar or wire thickness. However, as noted 

previously AASHTO (2007) requires a minimum thickness of 3.4 mils (85 μm) for MSE 

walls. Galvanization requirements using this minimum and AASHTO M 111 are 

summarized in Table 3-7. 

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Table 3-7. Minimum Galvanization Thickness by Steel Thickness 

(after AASHTO M 111 and ASTM A123). 

Category Steel 

Thickness 

Minimum Galvanization 

Thickness 

Strip < 1⁄4 in. (6.4 mm) 3.4 mils (85 m) 

> 1⁄4 in. (6.4 mm) 3.9 mils (100 m) 

Wire* All diameters 3.4 mils (85 m) 

* For bar mats fabricated from uncoated steel wire. 

The corrosion rates presented in Table 3-8 are suitable for conservative design. These rates 

assume a moderately corrosive backfill material having the controlled electrochemical 

property limits that are discussed under electrochemical properties in this chapter. 

Table 3-8 Steel Corrosion Rates for Moderately Corrosive Reinforced Fill. 

For zinc/side: 0.58 mils/yr (15 μm/year) (first 2 years) 

0.16 mils/yr (4 μm/year) (thereafter) 

For residual carbon steel/side: 0.47 mils/yr (12 μm/year) (thereafter) 

Based on these rates, complete corrosion of galvanization with the minimum required 

thickness of 3.4 mils (85 μm) (AASHTO, 2007) is estimated to occur during the first 16 

years and a carbon steel thickness or diameter loss of 0.055 in. to 0.08 in. (1.42 mm to 2.02 

mm) would be anticipated over the remaining years of a 75 to 100 year design life, 

respectively. Galvanization can also be damaged during handling and construction by 

abrasion, scratching, notching, and cracking. Care must be taken during handling and 

construction to avoid damage. Construction equipment should not travel directly on 

reinforcing elements and elements should not be dragged, excessively bent, or field cut. 

Galvanized reinforcement should be well supported during lifting and handling to prevent 

excessive bending. Any damaged section should be field repaired by coating the damaged 

area with a field grade zinc-rich paint. 

The look of galvanized WWM face may not be desired on some projects due to aesthetic 

requirements. As previously noted, black (ungalvanized) steel is not allowed on permanent 

structures. Staining of galvanized WWM has been used to achieve desired aesthetics on 

some projects. 

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MSE Walls and RSS – Vol I 3 – 26 November 2009
 
 

 

The designer of an MSE structure should also consider the potential for changes in the 

reinforced fill environment during the structure's service life. In certain parts of the United 

States, it can be expected that deicing salts, coastal storm surges, or contaminated runoff or 

groundwater might cause such an environment change. For this problem, the depth of 

chloride infiltration and concentration are of concern such that additional protective measures 

may be required. 

For permanent structures directly supporting roadways exposed to deicing salts, limited 

data indicate that the upper 8 ft (2.5 m) of the reinforced backfill (as measured from the 

roadway surface) or greater depths, depending on the gradation and compaction of the fill, 

are affected by higher corrosion rates not presently defined. Under these conditions, it is 

recommended that a 30 mil (minimum) geomembrane be placed below the road base and tied 

into a drainage system to mitigate the penetration of the deicing salts in lieu of higher 

corrosion rates as shown in the Design Details section in Chapter 5. Alternatively free 

draining reinforced fill (e.g., AASHTO No. 57 stone) has also been found to allow salts to 

“flush out” and limit corrosion as discussed in FHWA NHI-09-087 (Elias et al., 2009). Note 

that value of “higher” corrosion rate for deicing salt exposure is not defined. 

The following project situations lie outside the scope of the previously presented values: 

C Structures exposed to a marine or other chloride-rich environment. (Excluding locations 

where de-icing salts are used.) For marine saltwater structures, carbon steel losses on the 

order of 3.2 mils (80 μm) per side or radius should be anticipated in the first few years, 

reducing to 0.67 to 0.7 mils (17 to 20 μm) thereafter. Zinc losses are likely to be quite 

rapid as compared to losses in reinforced fills meeting the MSE electrochemical criteria. 

Total loss of zinc (3.4 mils {85 m}) should be anticipated in the first year. 

C Structures exposed to stray currents, such as from nearby underground power lines, and 

structures supporting or located adjacent to electrical railways. 

C Structures exposed to acidic water emanating from mine waste, abandoned coal mines, or 

pyrite-rich soil and rock strata. 

Each of these situations creates a special set of conditions that should be specifically 

analyzed by a corrosion specialist. 

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MSE Walls and RSS – Vol I 3 – 27 November 2009
 
 

 

3.5.2 Strength Properties of Geosynthetic Reinforcement 

Selection of long-term nominal tensile strength, Tal, for geosynthetic reinforcement is 

determined by thorough consideration of all possible strength time dependent strength losses 

over the design life period. The tensile properties of geosynthetics are affected by factors 

such as creep, installation damage, aging, temperature, and confining stress. Furthermore, 

characteristics of geosynthetic products manufactured with the same base polymer can vary 

widely requiring a Tal determination for each individual product with consideration of all 

these factors. 

Polymeric reinforcement, although not susceptible to corrosion, may degrade due to 

physicochemical activity in the soil such as hydrolysis, oxidation, and environmental stress 

cracking depending on polymer type. In addition, these materials are susceptible to 

installation damage and the effects of high temperature at the facing and connections. 

Temperature acts to accelerate creep and aging processes and temperature effects are 

accounted for through their determination. While the normal range of in-ground temperature 

vary from 55o

 F (12o C) in cold and temperate climates to 85o F (30o C) in arid desert 

climates, temperatures at the facing and reinforcement connections can be as high as 120o F 

(50o C). Confining stress is not directly taken into account other than indirectly when 

installation damage is evaluated. For creep and durability, confining stress generally will 

tend to improve the long-term strength of the reinforcement. 

The available long-term strength, Tal, is calculated as follows: 

Tult 	 	 Tult Tal  	 (in strength per unit reinforcement width) (3-12) RF RFID CR D 	  RF  RF

where, 

Tult = Ultimate Tensile Strength (strength per unit width). The tensile strength of 

the reinforcement is determined from wide strip tests per ASTM D4595 

(geotextiles) or D6637 (geogrids) based on the minimum average roll value 

(MARV) for the product. 

RF = Reduction Factor. The product of all applicable reduction factors. 

RFID = Installation Damage Reduction Factor. A reduction factor that accounts for the 

damaging effects of placement and compaction of soil or aggregate over the 

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MSE Walls and RSS – Vol I 3 – 28 November 2009
 
 

geosynthetic during installation. A minimum reduction factor of 1.1 should 

be used to account for testing uncertainties. 

RFCR = Creep Reduction Factor. A reduction factor that accounts for the effect of 

creep resulting from long-term sustained tensile load applied to the 

geosynthetic. 

RFD = Durability Reduction Factor. A reduction factor that accounts for the strength 

loss caused by chemical degradation (aging) of the polymer used in the 

geosynthetic reinforcement (e.g., oxidation of polyolefins, hydrolysis of 

polyesters, etc.). 

RFID, RFCR, and RFD reflect actual long-term strength losses, analogous to loss of steel 

strength due to corrosion. This long-term geosynthetic reinforcement strength loss concept is 

illustrated in Figure 3-6. As shown in the figure, some strength losses occur immediately 

upon installation, and others occur throughout the design life of the reinforcement. Much of 

the long-term strength loss does not begin to occur until near the end of the reinforcement 

design life. 

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MSE Walls and RSS – Vol I 3 – 29 November 2009 

Tult 

Tult/RFID 

0 Time Design Life

Strength Retained

Tult/(RFIDRFCRRFD)

}Immediate loss due to 

installation stresses and abrasion

Long-term loss due to } Creep and chemical 

Degradation (assumes

constant load near

creep limit applied)

0 Time Design Life

Strength Retained

Tult/(RFIDRFCRRFD) 

} Immediate loss due to 

installation stresses and abrasion 

Long-term loss due to } Creep and chemical 

Degradation (assumes 

constant load near 

creep limit applied) 

Figure 3-6. Long-term geosynthetic reinforcement strength concepts. 
 

 

 

Because of varying polymer types, quality, additives and product geometry, each 

geosynthetic is different in its resistance to aging and attack by different chemical agents. 

Therefore, each product must be investigated individually, or in the context of product line 

where the same polymer source and additives are used, and the manufacturing process is the 

same for all products in the product line. This product line approach makes it possible to 

interpolate reduction factors for products in the product line not specifically tested using the 

reduction factors determined for the products in the product line that are specifically tested 

for each degradation mechanism. 

The AASHTO LRFD Bridge Design Specifications provide minimum requirements for the 

assessment of Tal for use in the design of geosynthetic reinforced soil structures. Protocols 

for evaluating Tal are included in Appendix D with supporting information on testing 

procedures provided in the companion Corrosion/Degradation document (Elias et al., 2009). 

It is recommended that Tal values for specific products be determined from in-house, agency 

evaluation or third-party evaluation of independent test results such as the Highway 

Innovative Technology Evaluation Center (HITEC) or AASHTO National Transportation 

Product Evaluation Program (NTPEP). Agencies can approve reduction factors and 

allowable strength values based these reports or require that vendor designs use reduction 

factors substantiated by these or equivalent third party reports. Alternatively, Tal could be 

obtained directly from the manufacturer based on independent test results, though third party 

testing is the preferred approach. If manufacturer data is used, it should meet the same 

standard of quality and completeness that can be obtained from the third party testing 

programs such as NTPEP, and the designer should check to make sure that the manufacturer 

data are representative of the products likely to be received at the project site (i.e., the 

product test data should be current, and the product manufacturing process, polymer source, 

etc., should not have changed since the testing was conducted). In all cases, the geosynthetic 

product line must be reevaluated on a periodic to assess any changes that may affect the 

product and corresponding reduction values (e.g., NTPEP requires that the geosynthetic 

reinforcement product/product line be retested every 3 years). 

In lieu of third party testing or manufacturer generated data, in-house agency testing to 

establish Tal with regard to the full suite of tests is generally not practical. However, agencies 

are encouraged to at least perform some of the index testing themselves, both for product 

qualification purposes (i.e., development of a qualified product or approved products list) as 

well as project specific product acceptance purposes. Agencies should also consider site 

specific installation damage testing, especially if relatively coarse, uniformly graded crushed 

or otherwise angular aggregate is used as backfill, or if other relatively severe installation 

conditions are anticipated. 

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The determination of reduction factors for each geosynthetic product and product line 

requires extensive field and/or laboratory testing which can take a year or more to complete. 

Background regarding the determination of each long-term strength reduction factor is 

briefly summarized as follows: 

3.5.2.a Ultimate Tensile Strength, Tult 

The value selected for Tult, for design purposes, is the minimum average roll value (MARV) 

for the product. This minimum average roll value, accounts for statistical variance in the 

material strength. Other sources of uncertainty and variability in the long-term strength 

result from installation damage, creep extrapolation, and the chemical degradation process. 

It is assumed that the observed variability in the creep rupture envelope is 100% correlated 

with the short-term tensile strength, as the creep strength is typically directly proportional to 

the short-term tensile strength within a product line. Therefore, the MARV of Tult adequately 

takes into account variability in the creep strength. Note that the MARV of Tult is the 

minimum certifiable wide width tensile strength provided by the product manufacturer. 

3.5.2.b Installation Damage Reduction Factor, RFID 

Damage during handling and construction, such as from abrasion and wear, punching and 

tear or scratching, notching, and cracking may occur in geosynthetics. These types of 

damage can only be avoided by using care during handling and construction. Construction 

equipment should not travel directly on geosynthetic materials. 

Damage during reinforced fill placement and compaction operations is a function of the 

severity of loading imposed on the geosynthetic during construction operations and the size 

and angularity of the reinforced fill. For MSE walls and RSS construction, lightweight, low 

strength geotextiles and geogrids should be avoided to minimize damage with ensuing loss of 

strength. 

Protocols for field testing for this reduction factor are detailed in the companion 

Corrosion/Degradation document (Elias et al., 2009) and in ASTM D-5818 (see also 

WSDOT T925). These protocols require that the geosynthetic material be subjected to a 

reinforced fill placement and compaction cycle, consistent with field practice. The ratio of 

the initial strength, to the strength of retrieved samples defines this reduction factor. For 

reinforcement applications, a minimum weight of 8.0 oz/yd2 (270 g/m2) for geotextiles is 

recommended to minimize installation damage. This roughly corresponds to a Class 1 

geotextile as specified in AASHTO M 288. In general, the combination of geosynthetic 

reinforcement, and backfill placement and gradation characteristics, should not result in a 

value of RFID greater than 1.7. If testing indicates that RFID will be greater than 1.7 

(approximately a 40 percent strength loss), that combination of geosynthetic and backfill 

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conditions should not be used, as this or greater levels of damage will cause the remaining 

strength to be highly variable and therefore not adequately reliable for design. 

Table 3-9 provides a summary of typical RFID values for a range of soil gradations and 

geosynthetic types. 

In general, RFID is strongly dependent on the backfill soil gradation characteristics and its 

angularity, especially for lighter weight geosynthetics. Provided a minimum of 6 inches of 

backfill material is placed between the reinforcement surface and the compaction and 

spreading equipment wheels/tracks, the backfill placement and compaction technique will 

have a lesser effect on RFID. Regarding geosynthetic characteristics, the geosynthetic 

weight/thickness or tensile strength may have a significant effect on RFID. However, for 

coated polyester geogrids, the coating thickness may overwhelm the effect of the product unit 

weight or thickness on RFID. 

Table 3-9. Installation Damage Reduction Factors. 

Reduction Factor, RFID 

Geosynthetic 

Type 1 Backfill 

Max. Size 4 in. (100mm) 

D50 about 11⁄4-in. (30 mm) 

Type 2 Backfill 

Max. Size 3⁄4 -in. (20mm) 

D50 about #30 (0.7 mm) 

HDPE uniaxial geogrid 1.20 - 1.45 1.10 - 1.20 

PP biaxial geogrid 1.20 - 1.45 1.10 - 1.20 

PVC coated PET geogrid 1.30 - 1.85 1.10 - 1.30 

Acrylic coated PET geogrid 1.30 - 2.05 1.20 - 1.40 

Woven geotextiles (PP&PET)a 1.40 - 2.20 1.10 - 1.40 

Non woven geotextiles (PP&PET) a 1.40 - 2.50 1.10 - 1.40 

Slit film woven PP geotextile a 1.60 - 3.00 1.10 - 2.00 

a. Minimum weight 8.0 oz/yd2 (270 g/m2). 

3.5.2.c Creep Reduction Factor, RFCR 

The creep reduction factor is required to limit the load in the reinforcement to a level known 

as the creep limit, that will preclude excessive elongation and creep rupture over the life of 

the structure. The creep limit strength is thus analogous to yield strength in steel. Creep is 

essentially a long-term deformation process. As load is applied, molecular chains move 

relative to each other through straightening out of folded or curved/kinked chains or through 

breaking of inter-molecular bonds, resulting in no strength loss, but increased elongation. 

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Eventually, if the load levels are sufficiently high i.e., constant load near the creep limit), the 

molecular chains can straighten/elongate no more without breaking the molecular chains. 

Significant strength loss occurs only when the straightening/slipping process is exhausted. If 

the load is high enough, molecular chains break, and both elongation and strength loss occur 

at an accelerating rate, eventually resulting in rupture. Generally this strength loss occurs 

only near the end of the design life of the geosynthetic under a given load level. 

The creep reduction factor is obtained from long term laboratory creep testing as detailed in 

Appendix D. Creep testing is essentially a constant load test on multiple product samples, 

loaded to various percentages of the ultimate product load, for periods of up to 10,000 hours. 

For creep testing one of two approaches may be used: 1) “conventional” creep testing per 

ASTM D5262, or 2) a combination of Stepped Isothermal Method (SIM) per ASTM D6992, 

which is an accelerated method using stepped increases in temperature to allow tests to be 

performed in a matter of days, and “conventional” creep testing. The creep reduction factor 

is the ratio of the ultimate load to the extrapolated maximum sustainable load (i.e., creep 

rupture limit) within the design life of the structure (e.g., several years for temporary 

structures, 75 to 100 years for permanent structures). 

Typical ranges of RFCR as a function of polymer type are: 

 

 

 

Polymer Type Creep Reduction Factors

Polyester (PET) 2.5 to 1.6 

Polypropylene (PP) 5 to 4.0 

High Density Polyethylene (HDPE) 5 to 2.6 

3.5.2.d Durability Reduction Factor, RFD 

This reduction factor is dependent on the susceptibility of the geosynthetic to attack by 

chemicals, thermal oxidation, hydrolysis, environmental stress cracking, and 

microorganisms, and can vary typically from 1.1 to 2.0. 

Typically, polyester products (PET) are susceptible to aging strength reductions due to 

hydrolysis (water must be available). Hydrolysis and the resulting fiber dissolution are 

accelerated in alkaline regimes, percent of water saturation in the surrounding soil, and 

temperature. Polyolefin products (PP and HDPE) are susceptible to aging strength losses due 

to oxidation (contact with oxygen). The level of oxygen in reinforced fills is a function of 

soil porosity, groundwater location and other factors, and has been found to be slightly less 

than oxygen levels in the atmosphere (21 percent). Therefore, oxidation of geosynthetics inground may proceed at a rate equal those used above ground. Oxidation is accelerated by the 

presence of transition metals (Fe, Cu, Mn, Co, Cr) in the reinforced fill as found in acid 

sulphate soils (e.g., pyrite), slag and cinder fills, other industrial wastes or mine tailings 

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containing transition metals, and elevated temperature. It should be noted that the resistance 

of polyolefin geosynthetics to oxidation is primarily a function of the proprietary antioxidant 

package added to the base resin, which differs for each product brand, even when formulated 

with the same base resin. 

The relative resistance of polymers to these identified regimes is shown in Table 3-10 and a 

choice can be made, therefore, consistent with the in-ground regimes indicated. 

Table 3-10. Anticipated Resistance of Polymers to Specific Environments. 

Soil Environment 

Polymer 

PET PE PP 

Acid Sulphate Soils 

Organic Soils 

Saline Soils pH < 9 

Ferruiginous 

Calcareous Soils 

Modified Soils/Lime, Cement 

Sodic Soils, pH > 9 

Soils with Transition Metals 

NE 

NE 

NE 

NE 

ETR 

ETR 

ETR 

NE 

ETR 

NE 

NE 

ETR 

NE 

NE 

NE 

ETR 

ETR 

NE 

NE 

ETR 

NE 

NE 

NE 

ETR 

NE =No Effect 

ETR =Exposure Tests Required 

Most geosynthetic reinforcement is buried, and therefore ultraviolet (UV) stability is only of 

concern during construction and when the geosynthetic is used to wrap the wall or slope face. 

If used in exposed locations, the geosynthetic should be protected with coatings or facing 

units to prevent deterioration. UV tests (ASTM D4355) extended beyond the normal 500 

hour test duration should be performed on materials that will be directly exposed for long 

periods of time (more than several months) in order to evaluate the materials anticipated 

design life. Vegetative covers can also be considered in the case of open weave geotextiles 

or geogrids. Thick geosynthetics with ultraviolet stabilizers can be left exposed for several 

years or more without protection; however, long-term maintenance should be anticipated 

because of both UV deterioration and possible vandalism. 

Protocols for testing to obtain this reduction factor have been proposed and are detailed in 

FHWA RD-97-144 (Elias et al. 1999). In general, for polyolefins, they consist of oven aging 

polyolefins (PP and HDPE) samples to accelerate oxidation and measure their strength 

reduction, as a function of time, temperature and oxygen concentration. This high 

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temperature data must then be extrapolated to a temperature consistent with field conditions. 

For polyesters (PET) the aging is conducted in an aqueous media at varying pHs and 

relatively high temperature to accelerate hydrolysis, with data extrapolated to a temperature 

consistent with field conditions. For more detailed explanations, see the companion 

Corrosion/Degradation manual, FHWA NHI-09-087 (Elias et al., 2009). 

Due to the long-term nature of these durability evaluation protocols (2 to 3 years could be
 

required to complete such tests), it is generally not practical to conduct such tests for typical 

geosynthetic reinforcement design, but are generally more suited for research activities. 

However, short-term index type tests can be conducted as indicators of good long-term
 

durability performance, based on correlation to the long-term research results obtained and 

reported by Elias et al. (1999). Such index test results, combined with a criteria applied to 

the test results that can be considered to indicate good long-term performance, can be used to 

justify the use of a default value for RFD that can be used for the determination of Tal. 

The following recommendations are stated in this companion document in regards to
 

defining a RFD factor. With respect to aging degradation, current research results suggest the 

following. 

Polyester Geosynthetics
 

PET geosynthetics are recommended for use only in environments characterized by 3 < pH < 

9. The reduction factors for PET aging (RFD) listed in Table 3-11 are developed for a 100­

year design life in the absence of long-term product specific testing. Based on these research 

results, for polyester reinforcement, the AASHTO LRFD specifications recommend a 

minimum number average molecular weight of 25,000 and a maximum carboxyl end group 

content (CEG) of 30 to allow the use of a default reduction factor for durability. 

Polyolefin Geosynthetics 

To mitigate thermal and oxidative degradative processes, polyolefin (i.e., PP and HDPE) 

products are stabilized by the addition of antioxidants for both processing stability and longterm functional stability. These antioxidant packages are proprietary to each manufacturer 

and their type, quantity, and effectiveness varies. Without residual antioxidant protection 

(after processing), PP products are vulnerable to oxidation and significant strength loss 

within a projected 75 to 100 year design life at 20oC. Current data suggests that unstabilized 

PP has a half-life of less than 50 years. 

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Table 3-11. Durability (Aging) Reduction Factors for PET. 

Producta 

Durability Reduction Factor, RFD 

5 ≤ pH ≤ 8 3b < pH ≤ 5 

8 ≤ pH < 9 

Geotextiles 

Mn < 20,000, 40 < CEG < 50 

1.6 2.0 

Coated geogrids, Geotextiles 

Mn > 25,000, CEG < 30 

1.15 1.3 

Mn = number average molecular weight 

CEG = carboxyl end group 

Notes: 

a. Use of materials outside the indicated molecular property range requires specific 

product testing. Use of products outside of 3 < pH < 9 range is not recommended. 

b. Lower limit of pH for permanent applications is 4.5 and lower limit for temporary 

applications is 3, per Article 11.10.6.4.2b (AASHTO, 2007). 

Therefore the anticipated functional life of a PP geosynthetic is to a great extent a function of 

the type and post-production antioxidant levels, and the rate of subsequent antioxidant 

consumption. Antioxidant consumption is related to the in-ground oxygen content, which in 

fills is only slightly less than atmospheric. 

A detailed discussion of the effectiveness of oven aging and other protocols to allow 

estimation of long-term strength loss due to the combination of heat aging and oxidative 

degradation of various polyolefins is provided in Elias et al. (1999) and Elias et al. (2009). 

At present, index tests and associated test result criteria that can be considered indicative of 

sufficient long-term durability consist of shorter-term relatively high temperature oven aging 

tests (ENV ISO 13438:1999 and UV degradation tests (i.e., ASTM D4355). The current 

AASHTO LRFD specifications currently only specify a requirement for the UV test as an 

indirect indicator of the presence of long-term residual antioxidant protection, requiring 

polyolefins to have a minimum of 70 percent strength retained after 500 hours in a 

weatherometer per ASTM D4355. In addition, in Europe and in the NTPEP testing program, 

oven aging test are also required to justify the use of a default value for RFD for polyolefins. 

For both polyester and polyolefins, if these index test criteria are met, a default value for RFD 

of 1.3 could be used to determine Tal for design purposes. These index criteria are 

summarized in Table 3-12. If the effective in-soil site temperature is anticipated to be 

approximately 85o F (30o C) plus or minus a few degrees, a higher default reduction factor 

for RFD should be considered. 

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Environmental stress cracking is an aging phenomenon that is really as much related to creep 

as it is to durability. In certain environments, such as when surfactants are present, the creep 

rupture process, through making it easier for the tie molecules to pull out of the crystalline 

structure, can be accelerated, allowing cracks in the polymer to form, and premature rupture. 

Additional information on this phenomenon is provided in Elias et al. (2009). For most in 

ground conditions, the chemicals necessary to cause this to happen are generally not present, 

and the results from laboratory creep testing are sufficient to address strength loss under 

constant load. 

Table 3-12. Minimum Requirements for Use of Default Durability
 

Reduction Factors (RFD) for Primary Geosynthetic Reinforcement. 

Type Property Test Method Criteria to Allow Use 

of Default RF 

Polypropylene 

and 

Polyethylene 

UV 

Oxidation 

Resistance 

ASTM D4355 Min. 70% strength 

retained after 500 hrs. 

in weatherometer 

Polyester UV 

Oxidation 

Resistance 

ASTM D4355 Min. 50% strength 

retained after 500 hrs. 

in weatherometer if 

geosynthetic will be 

buried within one 

week, 70% if left 

exposed for more than 

one week 

Polypropylene 

and 

Polyethylene 

ThermoOxidation 

Resistance 

ENV ISO 13438:1999, Method A 

(Polypropylene) or B 

(Polyethylene) 

Min. 50% strength 

retained after 28 days 

(PP) or 56 days 

(HDPE) 

Polyester Hydrolysis 

Resistance 

Inherent Viscosity Method 

(ASTM D4603 and GRI Test 

Method GG8), or Determine 

Directly Using Gel Permeation 

Chromatography 

Min. Number (Mn) 

Molecular Weight of 

25,000 

Polyester Hydrolysis 

Resistance 

GRI GG7 Max. Carboxyl End 

Group Number of 30 

All Polymers Survivability 1

Weight per Unit Area, 

ASTM D5261 

Min. 8 oz/yd2 

(270 g/m2) 

All Polymers % Post Consumer 

Recycled Material 

by Weight 

Certification of Material used Maximum 0% 

1

Alternatively, a default RFD = 1.3 may be used if product specific installation damage testing is 

performed and it is determined that RFID is 1.7 or less, and if the other requirements in Table 3-12 

are met. 

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Note that biological degradation due to micro-organisms is rarely a concern, as most 

geosynthetic reinforcement products only contain high molecular weight polymers, and the 

biological agents have great difficulty in finding the molecular chain endings that would 

allow them to begin consuming the polymer. Therefore, biological degradation is usually not 

considered in the determination of RFD. 

3.5.2.e Durability Reduction Factor, RFD, at Wall Face Unit 

As noted in Section 4.4.7.i Connection Strength, the long-term environmental aging factor 

(RFD) may be significantly different than that used in computing the in-soil nominal longterm reinforcement strength Tal. For these applications, it is recommended that the use of 

polyesters be limited to a pH range of > 3 and < 9, as noted in Table 3-11. 

Of particular concern is the use of polyester geogrid and geotextile reinforcements with 

concrete facings because of the potential high pH environment. PET geogrids and 

geotextiles should not be cast into concrete for connections, due to the potential for chemical 

degradation. 

Use of PET reinforcements connected to dry-cast MBW units by laying the reinforcement 

between units may be subject to additional strength reductions. An FHWA sponsored field 

monitoring study to examine pH conditions within and adjacent to MBW units was 

performed (Koerner et al., 2000), which provided a large database of pH measurements of 25 

MSE wall structures in the United States. The results indicated that the pH regime within the 

blocks in the connection zone is only occasionally above 9 and then for only the first few 

years. The pH subsequently decreases to the pH of the ambient backfill (Koerner et al., 

2000). It therefore appears that for coated PET geogrids no further reduction is warranted. 

For geotextiles a small further reduction should be considered to account for a few years at a 

pH in excess of 9. 

Caution is advised in situations where the MBW units will be saturated for extended periods 

of time such as structures in lakes or streams. For such cases, long-term pH tests should be 

performed on saturated block. If the pH exceeds 9, polyester reinforcements should not be 

used in the section of the structure. 

3.5.2.f LRFD Geosynthetic Reinforcement Resistance Factor,  

The resistance factor for geosynthetic reinforcement accounts for potential of local overstress 

due to load nonuniformity and uncertainties in long-term reinforcement strength. For 

Strength I limit state conditions, a resistance factor () equal to 0.90 is used for geosynthetic 

reinforcements (see Table 4-7). This is higher than the resistance factors for steel 

reinforcements due to the ductile nature of geosynthetic systems at failure. 

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The recommended resistance factor of  of 0.90 can be further justified by considering the 

following: 

C For geosynthetic reinforcements, the reinforced fill soil controls the amount of strain in 

the reinforcement which for granular fills is limited to considerably less than the rupture 

strain of the reinforcement. Therefore even at a limit state, overstress of the geosynthetic 

reinforcement would cause visible, time-dependent strain in the wall system rather than 

sudden collapse. 

C The long-term properties of geosynthetics, based on limited data, are significantly 

improved when confined in soil. Confinement is presently not considered in developing 

nominal long-term strength. 

C Measurements of stress in MSE walls reinforced with geosynthetics have consistently 

indicated lower stress levels than used for design as developed in Chapter 4. 

Note that Tal is used for RSS structures design with limit equilibrium analysis and 

computation of a factor of safety against instability. 

3.5.2.g Preliminary Design Reduction Factor, RF For preliminary design of permanent 

structures or for applications defined by the user as not having severe consequences should 

poor performance or failure occur, the nominal long-term tensile strength Tal, may be 

evaluated without product specific data, as: 

 T T ult ult T   (3-13) al RF 7 

Further, RF = 7 should be limited (i.e., do not use Eq. 3-13 where following requirements are
 

not met) to projects where the project environment meets the following requirements: 

C Granular soils (sands, gravels) used in the reinforced volume. 

C 4.5 < pH < 9 

C Site temperature < 85o

 F (30o C) 

C Maximum backfill particle size of 3⁄4-inch (19 mm) 

C Maximum MSEW height is 35 ft (10 m) and 

C Maximum RSS height is 50 ft (15 m)
 

Site temperature is defined as the temperature which is halfway between the average yearly 

air temperature and normal daily air temperature for the highest month at the site. 

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RF = 7 has been established by multiplying lower bound partial reduction factors obtained 

from currently available test data, for products which meet the minimum requirements in 

Table 3-13. It should be noted that the total Reduction Factor may be reduced 

significantly with appropriate test data. It is not uncommon for products with creep, 

installation damage and aging data, to develop total Reduction Factors in the range of 3 to 

6 or even less with the development of new materials. 

For temporary applications not having severe consequences should poor performance or 

failure occur, a default value for RF of 3.5 rather than 7 may be considered. 

Table 3-13. Minimum Requirements for use of Preliminary Design 

Reduction Factor for Primary Geosynthetic Reinforcement. 

Type Property Test Method Criteria to Allow Use 

of Default RF 

Polypropylene UV 

Oxidation 

Resistance 

ASTM D4355 Min. 70% strength 

retained after 500 hrs. 

in weatherometer 

Polyethylene UV 

Oxidation 

Resistance 

ASTM D4355 Min. 70% strength 

retained after 500 hrs. 

in weatherometer 

Polyester Hydrolysis 

Resistance 

Inherent Viscosity Method 

(ASTM D4603) with Correlation 

or Determine Directly Using Gel 

Permeation Chromatography 

Min. Number (Mn) 

Molecular Weight of 

25,000 

Polyester Hydrolysis 

Resistance 

GRI GG7 Max. Carboxyl End 

Group Number of 30 

All Polymers Survivability Weight per Unit Area, 

ASTM D5261 

Min. 8 oz/yd2 

(270 g/m2) 

All Polymers % Post Consumer 

Recycled Material 

by Weight 

Certification of Material used Maximum 0% 

3.5.2.h Serviceability Limit State 

Serviceability limit state deflection requirements for geosynthetic reinforcements are met 

through the use of low stress levels resulting from reduction factors combined with the 

inherent constraining effects of granular soils. With regard to strain limits on the 

reinforcement, methods for estimating of strain vary widely with no present consensus on an 

appropriate analytical method capable of modeling strains in the structure. Measurements 

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from instrumented field structures have consistently measured much lower strain levels in the 

reinforcement (typically less than 1 percent) than predicted by most current analytical 

methods. Therefore, until an appropriate method of determination is agreed upon, it is 

recommended that strain limit requirements not be imposed on the reinforcement. 

3.6 FACING MATERIALS 

The material aspects of the various facings used with MSE walls structures are discussed 

below, by facing type. Typical dimensions, manufacturing process and controls, details, 

durability, and associated materials are discussed. Aesthetics were discussed in Chapter 2. 

Tolerances of precast panels to settlement were presented in Section 2.8.3. Design aspects of 

the more commonly used facings are addressed in Section 4.4.8. Specifications are 

addressed in Chapter 10. 

3.6.1 Precast Concrete Panels 

3.6.1.a Segmental Panels 

Segmental, precast concrete panels are commonly square or rectangular in shape with typical 

dimensions of 5 to 8-in. (125 to 200 mm) thick and 5-foot (1.5-m) high and a front face width 

of 5 or 10-ft (1.5 or 3-m). Panels with cruciform, diamond, and hexagonal face geometry are 

also used. The panels are typically cast with the exposed face down, so they may have a 

smooth or a form-liner finish. Panels may also be prepared with an exposed aggregate finish. 

The edges of adjacent panels are cast with a butt, shiplap, or tongue-and-groove joint. 

Agencies should check the raw materials, mix design, and precasting operation as they do for 

other precast, structural items. Generally, agencies have reviewed and approved these items 

for a particular precaster. Panels are usually produced by a local precaster for, and with 

forms provided by, the wall vendor. Form dimensions, concrete steel reinforcement 

placement, and connection hardware placement should be examined for conformance to the 

vendor’s quality control and tolerances. The units must be fully supported until the concrete 

reaches a minimum compressive strength of 1,000 psi (6.9 MPa). The units may be shipped 

after reaching a minimum compressive strength of 3,400 psi (23.4 MPa). At the option of the 

contractor, the units may be installed after the concrete reaches a minimum compressive 

strength of 3,400 psi (23.4 MPa). The concrete must have a minimum 4,000 psi (27.6 MPa) 

compressive strength at 28 days. Temperature and tensile steel reinforcement should be 

designed in accordance with Section 5 of AASHTO LRFD Specifications for Highway 

Bridges (2007). 

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Metal connection hardware that is cast into the panel and extends out the back face of the 

panel for attachment to the soil reinforcement should not be placed in direct contact with the 

concrete steel reinforcement. This type of placement could accelerate corrosion of metal soil 

reinforcement. Direct contact is permissible if both have the same protection (e.g., 

galvanized). 

Bearing pads are placed on all horizontal (and diagonal, if applicable) joints of adjacent 

segmental precast panels as they are erected. Two pads are usually used on 5-foot (1.5-m) 

wide panels and at least three bearing pads with 10-foot (3-m) wide panels. A minimum of 

two bearing pads are used per horizontal panel joint. The bearing pads are used to prevent or 

minimize point loadings or stress concentrations between adjacent panels, and to 

accommodate small vertical deformation of the panels as the wall height increases and the 

reinforced wall fill compresses. 

Bearing pads shall meet or exceed the following material requirements: 

C Preformed EPDM (Ethylene Propylene Diene Monomer) rubber pads conforming to 

ASTM D2000 Grade 2, Type A, Class A with a Durometer Hardness of 60 + 5. 

C Preformed HDPE (High Density Polyethylene) pads with a minimum density of 0.946 

grams per cubic centimeter in accordance with ASTM D 1505. 

The stiffness (axial and lateral), size, and number of bearing pads must be determined such 

that the final joint opening is not less than the required joint width after compression (e.g., 1⁄2 

in.) unless otherwise shown on the plans. The MSE wall designer must submit substantiating 

calculations verifying the stiffness (axial and lateral), size, and number of bearing pads 

assuming, as a minimum, a vertical loading at a given joint equal to 2 times the weight of 

facing panels directly above that level. As part of the substantiating calculations, the MSE 

wall designer must submit results of certified laboratory tests in the form of vertical loadvertical strain and vertical load-lateral strain curves for the specific bearing pads proposed by 

the MSE wall designer. The vertical load-vertical strain curve should extend beyond the first 

yield point of the proposed bearing pad. 

3.6.1.b Full-Height Panels 

Typical dimensions of full-height panels are 6 to 8-in. (150 to 200-mm) thick and 8 or 10-ft 

(2.4 to 3-m) wide. Single, full-height panel walls have been constructed to a height of 

approximately 32 ft (10 m). Full-height panels are externally braced until the reinforced soil 

reaches 2/3 to full height of the wall. 

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Full-height panels do not provide the same ability to adjust face panel alignment and rotation 

during construction, as do segmental panels. Nor are bearing pads used to accommodate 

elastic settlement of the reinforced fill so the connection detailing and strength must 

accommodate this deformation. Therefore, if the full-height panels will be used, Agencies 

should specify experience requirements for the wall vendor, wall designer (if different than 

the wall vendor), and the wall contractor. Additionally, the maximum height should be 

limited to about 32 feet (10 m), or less. 

Agency controls are the same as for segmental, with the exception that taller, full-height 

panels have multiple heights of pick-up point hardware cast into the panel. Handling of the 

panels for shipping and erection should be monitored to ensure panels are not cracked by 

these operations. 

No bearing pads are used with full-height panels. Therefore, high quality reinforced fill 

should always be used with full-height panel walls. Individual wall systems should address 

how the reinforcement connection is designed to tolerate elastic fill settlement. 

3.6.2 Modular Block Wall Units 

Modular block wall (MBW) MSE face units have typical dimensions of 4 to 15-in. (100 to 

375-mm) high and 8 to 18-in. (200 to 450-mm) in exposed face length, and 8 to 24-in. (200 

to 600-mm) in depth (perpendicular to wall face). MBW units are produced in a masonry 

manufacturing process. Therefore, the concrete is dry-cast, and unlike wet-cast panels 

cannot be air entrained or reinforced with steel. These units are also known as “segmental 

retaining wall” units. 

There are a wide variety of commercially available MBW units, as noted in Section 2.4.3. 

These units are normally produced near the project site by a licensed manufacturer. Quality 

control requirements and quality assurance vary by licensor and licensee. Therefore, 

Agencies should control the raw materials, mix design, and casting operation as they do for 

wet-cast concrete, structural items. Form or cast units should be examined for dimensional 

tolerances. Many of these units have the face sheared off after casting to create a roughened, 

rock-like texture for aesthetic reasons. 

Dry-cast concrete MBW units are susceptible to freeze-thaw degradation with exposure to 

deicing salts and cold temperatures. This is a concern in northern tier states that use deicing 

salts. Some vendors have developed mix designs, with additive(s), and manufacturing 

processes that result in units that are very durable and resistant to freeze-thaw degradation. 

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The current specifications in Chapter 10 have been developed to address this issue and clarify 

requirements depending on the susceptibility to freeze-thaw conditions and salt exposure. 

Based on good performance experience by several agencies, ASTM C1372, Standard 

Specification for Segmental Retaining Wall Units should be used as a model, except that the 

compressive strength for units should be increased to 4,000 psi (28 MPa) to increase 

durability, maximum water absorption be limited to 5 percent, requirements for freeze-thaw 

testing modified, and tolerance limits expanded. 

Note that more stringent durability requirements are being used by the Minnesota 

Department of Transportation (Mn/DOT) based upon their experience, research, climatic 

conditions and de-icing salt usage. The Mn/DOT criteria (2008) state that wall and cap units 

shall conform to ASTM C1372, except for the items in Table 3-14. 

Several research projects investigating the freeze-thaw durability and degradation of MBW 

units have been performed. Reports are available from FHWA (Chan et al., 2007) and the 

University of Minnesota (Embacher et al., 2001a,b). 

Freeze-thaw resistance of MBW units is tested following ASTM C1262. These tests 

generally take more than 3 months to perform. Therefore, the testing is not suited for 

approval of materials on an individual project basis. The testing is better suited to an agency 

evaluating and placing MBW units on an approved products list. 

MBW units are erected using a running bond configuration. Full-height cores are filled with 

aggregate during erection units are normally dry-stacked (i.e. without mortar), and erected 

using a running bond configuration. Vertically adjacent units may be connected with shear 

pins, lips, or keys. 

Geogrid soil reinforcement is typically used with MBW units, though some systems use 

geotextile and some use steel mat soil reinforcement. The soil reinforcement is connected to 

the MBW units via a frictional, mechanical, or combination mechanical and frictional-type 

connection. Bearing pads between vertically adjacent units are not used with MBW units. 

Therefore, the connection detailing and strength, and the soil placement and compaction must 

accommodate deformation caused by elastic compression of the reinforced fill. On certain 

systems, geosynthetic soil reinforcement sandwiched between vertically adjacent units 

provides some cushioning to distribute bearing loads between blocks. 

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Table 3-14. Additional MBW Cold Weather Requirements 

Recommended by Mn/DOT (2008). 

Item Test Standard Requirement 

Compressive ASTM C140, except that Section 6.2.4 5,500 psi (38 MPa) min. 

strength shall be deleted and replaced with: “The 

specimens shall be coupons cut from a 

finished side or back shell of each unit and 

sawn to remove any face shell projections. 

The coupon size shall have a height to 

thickness ratio of 2 to 1 before capping and 

a length to thickness ratio of 4 to 1. The 

coupon shall be cut from the unit such that 

the coupon height dimension is in the same 

direction as the unit height dimension. 

Compressive testing of full size units will 

not be permitted. The compressive strength 

of the coupon shall be assumed to represent 

the net area compressive strength of the 

whole unit.” 

5,800 psi (40 MPa) min. Average for 3 

units 

Freeze-thaw (1) the mean weight loss of five test 

durability of 

wall units The freeze/thaw durability of wall units 

tested in accordance with ASTM C1262 in 

a 3% saline solution shall be the minimum 

of the following: 

specimens at the conclusion of 90 cycles 

shall not exceed 1% of its initial weight; 

(2) the mean weight loss of the 4 lowest 

out of 5 test specimens at the conclusion 

of 100 cycles shall not exceed 1.5% of 

its initial weight. Test results shall be 

recorded and reported in 10 cycle 

intervals showing the weight of all 

specimens and not just the mean value. 

Freeze-thaw The freeze/thaw durability of cap units (1) the mean weight loss of five test 

durability of tested in accordance with ASTM C1262 in specimens at the conclusion of 40 cycles 

cap units a 3% saline solution shall be the minimum shall not exceed 1% of its initial weight; 

of the following: (2) the mean weight loss of the 4 lowest 

out of 5 test specimens at the conclusion 

of 50 cycles shall not exceed 1.5% of its 

initial weight. Test results shall be 

recorded and reported in 10 cycle 

intervals showing the weight of all 

specimens and not just the mean value. 

Cap unit — Top surface sloped at 1 inch fall per 10 

inch run (1 mm fall per 10 mm run) 

front to back or crowned at the center. 

Surface sealer Contact Mn/DOT Concrete Engineering 

Unit, or 

www.mrr.dot.state.mn.us/pavement/ 

concrete products.asp, for requirements. 

All segmental masonry retaining walls 

shall have their surfaces sealed. Apply 

surface sealer to the top, exposed front 

face, and backside of the upper three 

courses of all walls. 

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Therefore, it is recommended that agencies specify wall height experience requirements for 

the wall vendor, wall designer (if different than the wall vendor), and the wall contractor 

when MBW unit faced walls are to be used. Additionally, it is recommended that the 

maximum height typically be limited to about 32 feet (10 m), or less, unless setbacks are 

used to separate wall facing loads. Taller walls without setbacks require that bearing 

between units and possible stress concentrations due to geometric variations along the length 

of the wall be specifically addressed in the design and detailing. Typically, this can be 

accomplished with horizontal bearing pads or other compression members in the lower 

portion of the wall and/or vertical joints to separate geometric variations. 

The use of polyester geogrid or geotextile soil reinforcements connected to the dry-cast 

MBW concrete units are discussed in Section 3.5.2.e. Recommendations for design as 

addressed in Section 3.5.3.e3, Durability Reduction Factor, RFD, at the Wall Face Unit. 

3.6.3 Welded Wire Mesh Facing 

Welded wire mesh (WWM) is a popular facing for temporary walls and slopes, and is used in 

permanent walls and slopes. In permanent walls and slopes, the WWM may be the primary 

face soil retention element. For these cases, galvanized steel is used. The reinforcements in 

temporary structures should be galvanized if contact between reinforcements of the 

temporary structure and of a permanent (galvanized) structure is possible. In some 

permanent, geosynthetic-reinforced slopes and walls, the WWM is used as a forming device 

that is left in–place. The geosynthetic is the primary face soil retention element, and for 

these cases, plain (a.k.a, black) steel is typically used. A temporary WWM wall with a 

geotextile for retention at the face is shown in Figure 2-3. 

Steel facings should be galvanized consistent with the use of galvanized reinforcements. Hot 

dip galvanizing of at least 2 oz/ft2 should protect the steel in atmospheric conditions for a 

period between 20 and 50 years (AGA, 2004). Forty to 50 years are expected in rural and 

suburban environments, 25 to 30 years in coastal areas, and approximately 20 years if located 

in proximity to industrial areas where the atmosphere may be acidic. A typical corrosion rate 

for temporary, non-galvanized steel facing is 1.0 mil/yr (25 m/yr). Substantially higher 

rates should be used if the wall face will be vegetated, where road salts are used, if 

atmospheric conditions are corrosive such as marine environments or when air quality may 

be compromised buy nearby industrial activity. Corrosion potential can be reduced by using 

open graded stone in the facing. Note that a corrosion rate of 28 m/yr should be applied to 

plain steel soil reinforcements, if the reinforced fill is not corrosive or only mildly aggressive, 

for temporary walls. 

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Hardware cloth that is sometimes used with welded wire facings to contain fill material may 

be vulnerable to corrosion (if steel) or degradation from UV radiation (if geosynthetic). 

Designers should assume that the hardware cloth will degrade over time, in permanent walls, 

and that the WWM will have to retain the wall fill adjacent to the face or maintenance (i.e., 

repair, replace) of the hardware cloth 

For permanent walls, vertical and horizontal spacing of metallic reinforcements for flexible 

face (welded wire or similar) wall systems should not exceed 18 inches. The stiffness of the 

facing and spacing of reinforcements must be such that the maximum local horizontal 

deformation between soil reinforcement layers is limited to less than 1 to 2 in. as specified by 

the agency. The maximum local horizontal deformation between soil reinforcement layers 

should also be limited to less than 1 to 2 in. for temporary walls, i.e., walls with up to 36 

months service life. This recommendation is particularly important if the temporary wall will 

be incorporated into a permanent feature, e.g., buried within an embankment fill. 

The look of galvanized WWM face may not be desired on some projects due to aesthetic 

requirements. On some projects, staining of galvanized WWM has been used to achieve 

desired aesthetics. 

3.6.4 Geosynthetic Wrap-Around Facing 

Geosynthetic facing elements should not be left exposed to sunlight (specifically ultraviolet 

radiation) for permanent walls. If geosynthetic facing elements must be left exposed to 

sunlight, for permanent or temporary structures, the geosynthetic must be stabilized to be 

resistant to ultraviolet radiation. Furthermore, product specific test data should be provided 

which can be extrapolated to the intended design life and which proves that the product will 

be capable of performing as intended in an exposed environment. Vegetative covers provide 

some protection from UV and in many cases, a healthy vegetative cover can prevent 

exposure altogether. Alternately, a protective facing must be constructed in addition (e.g., 

concrete, shotcrete, etc.). A temporary wrap-around wall is shown in Figure 2-3. 

3.6.5 Other Facings 

Other facings being used on permanent walls, and sometimes on slopes, include: large, up to 

3-ft (0.9-m) high and 3 to 10-ft (0.9 to 3-m) in width, wet cast concrete units, gabions, and 

geocells. 

The large wet cast units are typically stacked, similar to MBW units. Generally, geosynthetic 

soil reinforcements are used with these units. The reinforcement is usually connected to the 

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facing by friction, i.e., sandwiched between vertically adjacent units, as shown in Figure 3-7. 

Attachments may also be cast into the units and mechanical connection used, as shown in 

Figure 3-8. 

Rock filled gabions are another large face unit used with MSE walls. One system uses 

woven-wire soil reinforcement that is integral with the gabion face, so no connection is 

required. Other systems connect reinforcement to the facing by friction by sandwiching the 

reinforcement between vertically adjacent units. Connecting the reinforcement by 

mechanically clipping it to the back of a gabion should be avoided. Most gabions are 3 ft by 

3 ft (0.9 by 0.9 m), thus vertical reinforcement exceeds the 32 in. (0.8 m) recommended 

maximum spacing. This greater spacing may be offset by the size/mass of the facing. 

Although 36-in. reinforcement spacing has been used successfully on many projects, it is not 

in agreement with the 32-in. limit to ensure a coherent MSE mass. The Owner should 

exercise caution in the evaluation of the maximum reinforcement spacing when specific 

loading conditions, unusual geometries, or soft foundation exist. The Owner and/or wall 

designer should consider use of secondary reinforcement layers placed in at the center of the 

unit heights to reduce reinforcement vertical spacing. 

Geocells are used to face reinforced soil walls and slopes. Eight-inch (200-mm) high 

geocells and nominally about 3 ft (0.9 m) wide are typically used. Connection to the soil 

reinforcement is by friction, i.e., sandwiched between vertically adjacent mats of geocells. 

The lifts of geocells may be offset and the outer cells filled with topsoil and vegetated, as 

shown in Figure 3-9. 

Figure 3-7. Large, wet-cast concrete 

face unit with reinforcement placed 

between units. 

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Figure 3-8. Large, wet-cast concrete face unit with embedded reinforcement connectors. 

Figure 3-9. Geocell face unit with vegetation. 

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3.6.6 Two-Stage Facings 

Two-stage MSE wall construction is used to construct walls on foundations that will undergo 

significant settlement. The first stage is construction of an MSE wall with a flexible facing 

(i.e., WWM or geosynthetic wrap). Connectors or form anchors are embedded in the first 

stage construction. The foundation soils are allowed to settle under the load of the first stage, 

with or without an additional surcharge load. The second stage consists of facing the first 

stage with cast-in-place or precast concrete panels. Either full height or segmental precast 

panels are used and are mechanically connected to the first-stage reinforced soil mass. 

Connection mechanisms and details may be proprietary to the wall vendor. For cast in place 

facings, the design of the connection mechanism must consider fluid pressure that develops 

during pouring of the concrete, which may require staging to avoid connection overstressing. 

Precast material control is discussed in Section 3.6.1. Design issues include; 1) estimation of 

settlement and establishing tolerance limits for the first-stage wall construction, 2) estimating 

additional long term settlement after construction of the second stage including additional 

loading from the facing system, and 3) evaluating the long-term durability of the connection 

hardware between the concrete and MSE mass with consideration for long term differential 

settlement. Corrosion needs to be addressed for steel connectors and durability for any 

geosynthetic connectors. 

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CHAPTER 4 

DESIGN OF MSE WALLS 

This chapter details design guidelines common to all MSE wall structures. It is limited to 

MSE walls having a near-vertical face, and uniform length of soil reinforcements. MSE wall 

design details are addressed in Chapter 5. Design guidelines for complex structures, or 

structures with unusual features are covered in Chapter 6. Detailed example calculations for 

both routine and complex structures are presented in Appendix E of this reference manual. 

This chapter is organized sequentially as follows: 

 Overview of design methods. 

o LRFD 

o Other methods 

 Loads and load combinations. 

o LRFD design of MSE walls 

 MSE wall design guidelines (step-by-step) 

o Sizing for external stability 

o Sizing for internal stability 

 Temporary walls 

 Design checklist 

 Computer aided design 

 Standard MSE wall designs 

4.1 DESIGN METHODOLOGY AND ANALSIS METHODS 

4.1.1 Load and Resistance Factor Design (LRFD) Platform 

Traditionally, the MSE wall design has been performed using the Allowable Stress Design 

(ASD) methodology. The LRFD methodology is the latest advancement in transportation 

structures design practice. The LRFD method in various forms is now being applied 

throughout the world. For example, EuroCode uses the limit state design (LSD) 

methodology, which is very similar to the LRFD methodology. Regardless of the design 

methodology, the core analytical methods for MSE walls such as external and internal 

stability evaluation remain unchanged. The assumption of a coherent gravity mass for 

external stability, the shape of the internal failure planes, and treatment of reinforcements as 

discrete elements remains unchanged. The primary change is in the way the loads and 

resistances are compared and how uncertainty is incorporated into the design process. 

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Specific to the topic of MSE walls the following points regarding LRFD methodology should 

be noted to prevent any confusion in application of the various theories and equations 

presented in this chapter:
 

C The symbol  (phi) is used for both the soil friction angle and the LRFD resistance factor. 

C The symbol γ (gamma) is used for both soil unit weight and the LRFD load factor. 

C Load and resistance factors for MSE walls are currently calibrated by fitting to ASD 

results. Therefore, designs using LRFD procedure should not significantly vary from 

past, expected ASD designs. 

C For most MSE wall system designs, strength limit states generally control the member 

sizes. Service limit states may control aspects such as joint width openings and 

construction sequence based on the anticipated deformations. Extreme event limit states 

may affect both the member sizes as well as deformations. 

4.1.2 Analysis Methods 

As noted earlier, the core analysis methods for MSE walls are unchanged relative to ASD 

practice. AASHTO (2002), which is based on the ASD method, recommended the use of the 

Simplified Method (a.k.a., Simplified Coherent Gravity Method) provided in the previous 

version of this manual. {Note: The AASHTO (2002) and FHWA (Elias et al., 2001) ASD 

references will not be updated by AASHTO or FHWA, respectively.} 

It is acknowledged that other analytical methods are also available in the literature as 

follows: 

C Allowable Stress Design (ASD) Procedure and the Simplified Method (AASHTO, 2002 

and FHWA NHI-00-043 {Elias et al., 2001}) 

C Coherent Gravity Method Analysis Model 

C National Concrete Masonry Association (NCMA) Procedure (NCMA, 2009) 

C Geosynthetic Reinforced Soil (GRS) Method (Wu et al., 2006) 

C K-Stiffness Method (Allen and Bathurst, 2003; Allen et al., 2003; Allen et al., 2004; 

WSDOT, 2006; and Bathurst et al., 2008a) 

The LRFD methodology permits consideration of any of the above methods as long as 

appropriate calibrations are performed for resistance factors using acceptable quality 

statistical data. This chapter concentrates on application of the Simplified Method which is 

recommended due to its applicability to a variety of soil reinforcement types (in contrast to 

the limited applicability of the alternative methods to specific type of reinforcements, e.g., 

GRS method is strictly applicable to geosynthetic reinforcements), and it is a methodology 

that has been successfully used in practice for many years. Brief descriptions of these other 

analytical methods are included in Appendix F. 

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4.2 LOADS AND LOAD COMBINATIONS 

A complete list of various loads, load factors and load combinations that need to be 

considered in design of bridge structures and associated transportation structures such as 

retaining walls and culverts is presented in Section 3 of AASHTO (2007). Many load types 

are commonplace to design of bridge structures and not applicable to retaining walls as noted 

in Section 11 of AASHTO (2007). With respect to MSE wall structures, only a few of the 

loads and load combinations are applicable on a routine basis. The applicable loads for most 

MSE wall applications are summarized below followed by a summary of applicable load 

combinations in Tables 4-1 and 4-2. Complete load combination and load factor tables (per 

AASHTO, 2007) are contained in Appendix A. 

Applicable Loads

 Permanent Loads 

 EH = Horizontal earth loads 

 ES = Earth surcharge load 

 EV = Vertical pressure from dead load of earth fill 

 

 Transient Loads 

 CT = Vehicular collision force 

 EQ = Earthquake load 

 LL = Vehicular live load 

 LS = Live load surcharge 

An example of an ES load on an MSE wall is the pressure from a spread footing above the 

reinforced mass. An example EV load is a sloping fill above the top of an MSE wall. 

Further distinction is made under the external and the internal design steps that follow. 

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Table 4-1. Typical MSE Wall Load Combinations and Load Factors 

(after Table 3.4.1-1, AASHTO {2007}). 

Load 

Combination 

Limit State 

EH 

ES 

EV 

LL 

LS 

Use One of These at a Time 

EQ CT 

STRENGTH I γp 1.75 – – 

EXTREME EVENT I γp γEQ 1.00 – 

EXTREME EVENT II γp 0.50 – 1.00 

SERVICE I 1.00 1.00 – – 

Notes: 

γp = load factor for permanent loading. May subscript as P-EV, P-EH, etc. 

γEQ = load factor for live load applied simultaneously with seismic loads 

Table 4-2. Typical MSE Wall Load Factors for Permanent Loads, γp 

(after Table 3.4.1-2, AASHTO {2007}). 

Type of Load Load Factor 

Maximum Minimum 

DC: Component and Attachments 1.25 0.90 

EH: Horizontal Earth Pressure 

 Active 1.50 0.90 

EV: Vertical Earth Pressure 

 Overall Stability 

 Retaining Walls and Abutments 

1.00 

1.35 

N/A 

1.00 

ES: Earth Surcharge 1.50 0.75 

Note: May subscript as EV-MIN, EV-MAX, EH-MIN, EH-MAX, etc. 

Maximum and Minimum Load Factors 

Two load factors, a maximum and a minimum, are listed in Table 4-2. It is important to 

understand the application of these load factors in context of MSE walls. Article 3.4.1 

AASHTO (2007) states that: “The factors shall be selected to produce the total extreme 

factored force effect. For each load combination, both positive and negative extremes shall 

be investigated. In load combinations where one force effect decreases another effect, the 

minimum value shall be applied to the load reducing the force effect. For permanent force 

effects, the load factor that produces the more critical combination shall be selected. . . . 

Where the permanent load increases the stability or load-carrying capacity of a component 

or bridge, the minimum value of the load factor for that permanent load shall also be 

investigated.” 

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In general, AASHTO’s guidance can be applied by using minimum load factors if permanent 

loads increase stability and use maximum load factors if permanent loads reduce stability. 

For simple walls, e.g., level backfill with or without surcharges due to traffic, or sloping 

backfill, the load factor (minimum or maximum) to use for a particular stability check may 

be readily identifiable. The load factors to use for such simple walls for external stability 

calculations are illustrated in Figure 4-1. The maximum EV load factor should be used for 

internal stability calculations. 

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a. Typical load factors for sliding stability and eccentricity check. 

b. Typical load factors for bearing calculations. 

Figure 4-1. External stability load factors for simple walls. 
 

 

The basic concept of load combinations using maximum and minimum load factors is 

applicable to more complex MSE wall configurations, such as those that may be experienced 

at bridge abutments or walls with complex geometries (see Chapter 6). Therefore, different 

combinations of load factors will need to be investigated to determine the total extreme 

factored (critical) force effect for each applicable limit state. 

While the positive and negative extremes are the two bounds, an intermediate combination of 

maximum and minimum load factors can create the critical force effect for design purposes. 

This is particularly applicable to retaining walls, wherein various components within a wall 

system may separately experience maximum or minimum loads. For example, in MSE walls, 

while the reinforced soil mass may be constructed such that it results in a maximum load, it is 

conceivable that the construction of retained fill may be at a minimum load level. Therefore, 

a critical combination of loads needs to be evaluated based on applicable maximum and 

minimum load factors. The detailed design examples complex MSE wall configurations in 

Appendix E use the concept of using minimum and maximum load factors. 

4.3 DESIGN OF MSE WALLS USING LRFD METHODOLOGY 

The procedure for design of MSE walls using LRFD methodology is very similar to that 

using ASD methodology. In LRFD, the external and internal stability of the MSE wall is 

evaluated at all appropriate strength limit states and overall stability and lateral/vertical wall 

movement are evaluated at the service limit state. Extreme event load combinations are used 

to design and analyze for conditions such as vehicle impact and seismic loading (see Chapter 

7 for extreme event design). The specific checks for the strength and service limit states 

required for MSE wall design are listed below. 

Strength Limit States for MSE walls
 

 External Stability 

o Limiting Eccentricity 

o Sliding 

o Bearing Resistance 

 Internal Stability 

o Tensile Resistance of Reinforcement 

o Pullout Resistance of Reinforcement 

o Structural Resistance of Face Elements 

o Structural Resistance of Face Element Connections 

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Service Limit States for MSE walls 

 External Stability 

o Vertical Wall Movements 

o Lateral Wall Movements 

Global Stability of MSE walls
 

 Overall Stability 

 Compound Stability 

The external stability of an MSE wall is evaluated assuming that the reinforced soil zone acts 

as a rigid body. This is because, when properly designed, the wall facing and the reinforced 

soil act as a coherent block with lateral earth pressures acting on the back side of that block. 

The internal stability of the reinforced soil zone is dependent on three fundamental 

characteristics: 

C the soil-reinforcement interaction (resistance to pullout and to sliding, for sheet-type 

reinforcements); 

C the tensile resistance of the reinforcement; and 

C the durability of the reinforcing material. 

Therefore, the internal stability analyses of an MSE wall in LRFD is evaluated by (a) 

determining the maximum factored load in each reinforcement and (b) comparing this 

maximum factored load to the factored pullout resistance and to the factored tensile
 

resistance of the reinforcement for all applicable strength, service, and extreme event limit 

states. 

Capacity to Demand Ratio (CDR) 
 

With LRFD, the goal is to have the factored resistance greater than the factored load. The 

term capacity to demand ratio, CDR, is used to quantify the ratio of the factored resistance to
 

the factored load. This term is useful in identifying critical and controlling limit states. 

4.3.1 Design Steps 

There are eleven basic design steps for an MSE wall, as listed in Table 4-3. Some of these 

steps have several sub-steps in the design process. These steps are for walls with simple 

geometries, as discussed in this chapter. Steps can vary somewhat depending on on type of 

reinforcement and/or whether or not type of reinforcement is initially defined. Additional 

steps are required for more complex cases such as true bridge abutments, as discussed in 

Chapter 6. 

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Table 4-3. Basic LRFD Design Steps for MSE Walls. 

Step 1. Establish Project Requirements 

– including all geometry, loading conditions (permanent, transient, seismic, 

etc.), performance criteria, and construction constraints. 

Step 2. Establish Project Parameters 

– evaluate existing topography, site subsurface conditions, reinforced wall fill 

properties, and retained backfill properties. 

Step 3. Estimate Wall Embedment Depth, Design Height(s), and Reinforcement Length 

Step 4 Define nominal loads 

Step 5 Summarize Load Combinations, Load Factors, and Resistance Factors 

Step 6. Evaluate External Stability 

a. Evaluate sliding 

b. Evaluate eccentricity 

c. Evaluate bearing on foundation soil 

d. Settlement analysis (at service limit state) 

Step 7. Evaluate Internal Stability 

a. Select type of soil reinforcement 

b. Define critical failure surface (for selected soil reinforcement type) 

c. Define unfactored loads 

d. Establish vertical layout of soil reinforcements 

e. Calculate factored horizontal stress and maximum tension at each 

reinforcement level. 

f. Calculate nominal and factored long-term tensile resistance of soil 

reinforcements 

g. Select grade (strength) of soil reinforcement and/or number of soil 

reinforcement elements at each level. 

h. Calculate nominal and factored pullout resistance of soil reinforcements, 

and check established layout 

i. Check connection resistance requirements at facing 

j. Estimate lateral wall movements (at service limit state) 

k. Check vertical movement and compression pads 

Step 8. Design of Facing Elements 

Step 9. Assess Overall Global Stability 

Step 10. Assess Compound Stability 

Step 11. Design Wall Drainage Systems. 

a. Subsurface drainage 

b. Surface drainage 

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4.4 MSE WALL DESIGN GUIDELINES 

4.4.1 Step 1 – Establish Project Requirements 

Prior to proceeding with the design, the following parameters must be defined: 

C Geometry 

o Wall heights 

o Wall batter 

o Backslope 

o Toe slope 

C Loading Conditions 

o Soil surcharges 

o Live (transient) load surcharges 

o Dead (permanent) load surcharges 

o Loads from adjacent structures that may influence the internal or external stability 

of MSE wall system, e.g., spread footings, deep foundations, etc. 

o Seismic 

o Traffic barrier impact 

C Performance Criteria 

o Design code (e.g., AASHTO LRFD) 

o Maximum tolerable differential settlement 

o Maximum tolerable horizontal displacement 

o Design life 

o Construction Constraints 

The chosen performance criteria should reflect site conditions and agency or AASHTO code 

requirements, which are discussed in detail in Chapters 2 and 3 of this manual. 

4.4.2 Step 2 – Establish Project Parameters 

The following must be defined by the agency (Owner) and/or its designer: 

Existing and proposed topography 

C Subsurface conditions across the site 

o Engineering properties of foundation soils (γf, c'f, ' f, cu) 

o Groundwater conditions 

C Reinforced wall fill – engineering properties of the reinforced soil volume (γr, ' r) 

C Retained backfill – engineering properties of the retained fill (γb, c'b, ' b), addressing all 

possible fills (e.g., in-situ, imported, on-site, etc.). Cohesion in the retained backfill is 

usually assumed to be equal to zero. See FHWA Earth Retaining Structures reference 

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manual (Tanyu et al., 2008) for guidance on value of cohesion and calculation of the 

lateral pressure if a cohesion value is used in design. 

Note that AASHTO uses the subscript f for both the foundation and retained backfill soils. In 

the text of this reference manual, the subscript f is used for foundation soil and subscript b is 

used for the retained backfill. 

The reinforced wall fill should be a select granular material, as detailed in Chapter 3 of this 

manual and in Article 7.3.6.3 AASHTO LRFD Bridge Construction Specifications (2004). 

Per Article 11.10.6.2 (AASHTO, 2007) the maximum friction angle of the select granular 

reinforced fill should be assumed to be 34°, unless the project specific fill is tested for 

frictional strength by triaxial or direct shear testing methods. A design friction angle greater 

than 40° should not used, even if the measured friction angle is greater than 40°. Note, that 

while 34° is a maximum value in absence of testing, some soils such as semi-rounded to 

round, uniform sands, that meet the specified gradation have a friction angle lower than 34°. 

In geologic areas where such soils are found (e.g., Florida, Wisconsin, Minnesota, etc.), it is 

recommended that project specific fill shear strength tests be performed. Similarly, where 

soils are micaceous, project specific shear strength tests should be performed. Also note, it is 

assumed that the select granular reinforced fill is noncohesive, i.e., cohesion is assumed 

equal to zero. 

For the foundation soil, Article 11.10.5.3 (AASHTO, 2007) notes that in absence of specific 

data, a maximum friction angle, ' fof 30° may be used. The use of an assumed, non-specific 

parameter is recommended only for preliminary sizing. As discussed in Chapter 2, a project 

specific site evaluation, that defines subsurface conditions and properties, is required for 

design of MSE wall structures. 

An assumed friction angle, ' bof 30° is often used for the retained (i.e., behind the 

reinforced zone) backfill. The use of an assumed, non-specific parameter is recommended 

only for preliminary sizing. As discussed in Chapter 2, a project specific site evaluation, that 

defines subsurface conditions and properties, is required for the design of MSE wall 

structures; or the use of a backfill specification that assures that the minimum friction angle 

is obtained. Most agencies have defined allowable property ranges for the retained fill (may 

be classified as an embankment fill material) and have appropriate friction angle(s) 

established for design. 

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4.4.3 Step 3 – Estimate Wall Embedment Depth and Reinforcement Length 

The process of sizing the structure begins by determining the required embedment, 

established under Project Criteria (Section 2.8.3, see Table 2-2), and the final exposed wall 

height, the combination of which is the full design height, H, for each section or station to be 

investigated. Use of the full height condition is required for design as this condition usually 

prevails in bottom-up constructed structures, at least to the end of construction. 

A preliminary length of reinforcement is chosen to initiate design. The length should be the 

greater of 0.7H or 8 ft (2.5 m), where H is the design height of the structure. Structures with 

sloping surcharge fills or other concentrated loads, such as abutments, generally require 

longer reinforcements for stability, often on the order of 0.8H to 1.1H (see Table 2-1). This 

preliminary reinforcement length is checked in the external and the internal stability 

calculations. 

Generally, the reinforcement length should be uniform throughout the entire height of the 

wall. One exception is special structures with shorter reinforcement lengths at the base of the 

wall; these are addressed in Chapter 6. Another exception is the use of longer layers of 

reinforcement at the top of a wall. It is recommended that the upper two layers of soil 

reinforcement be extended 3 ft (0.9 m) beyond the other layers where post-construction 

movements at the reinforced zone and retained backfill have been observed on previous, 

similar projects or if a seismic loading could lead to tension cracks in the backfill soil 

immediately behind the reinforcement. The design can be completed assuming uniform 

lengths, and the extra length added to the top two layers when detailing and specifying. 

The 8 ft (2.5 m) minimum is used to accommodate the typical size of fill spreading and 

compaction equipment used on transportation works. As noted in Commentary C.11.10.2.1 

AASHTO (2007), a minimum soil reinforcement length, on the order of 6.0 ft (1.8 m) can be 

considered for short walls if smaller compaction equipment is used and other wall design 

requirements are met. But, the minimum of 0.7H should be maintained. This shorter 

minimum length of 6 ft (1.8 m) is generally used only for landscape features (e.g., walls not 

supporting traffic). 

4.4.4 Step 4 – Define Nominal Loads 

The primary sources of external loading on an MSE wall are the earth pressure from the 

retained backfill behind the reinforced zone and any surcharge loadings above the reinforced 

zone. Thus, the loads for MSE walls may include loads due to horizontal earth pressure 

(EH), vertical earth pressure (EV), live load surcharge (LS), and earth surcharge (ES). Water 

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(WA) and seismic (EQ) should also be evaluated if applicable. Stability computations for 

walls with a near vertical face are made by assuming that the MSE wall acts as a rigid body 

with earth pressures developed on a vertical pressure plane at the back end of the 

reinforcements, as shown in Figures 4-2, 4-3, and 4-4. Estimation of earth pressures on MSE 

walls for three different conditions (i.e., horizontal backslope with traffic surcharge, sloping 

backslope, and broken backslope) follows. 

Figure 4-2. External analysis: nominal earth pressures; horizontal backslope with traffic 

surcharge (after AASHTO, 2007). 

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Figure 4-3. External analysis: earth pressure; sloping backfill case (after AASHTO, 

2007). 

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Figure 4-4. External analysis: earth pressure; broken backslope case (after AASHTO, 

2007). 

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Vertical Wall and Horizontal Backslope: The active coefficient of earth pressure is 

calculated for near vertical walls (defined as walls with a face batter of less than 10 degrees 

from vertical) and a horizontal backslope from: 

 

' 

2 	  b  Kab  tan 

 45  	 (4-1) 2   

where: ’ b = friction angle of retained backfill. 

Vertical Wall and a Surcharge Slope: The active coefficient of earth pressure is calculated 

for near vertical walls (defined as walls with a face batter of less than 10 degrees from 

vertical) and a sloping backfill from: 

 

 sin2 ( + b 

' ) Kab = 2	 (4-2)  sin  sin ( -  ) 

where: 

 

  ' ' sin ( b +  ) sin ( b -  )  

2 

 = 1 + 	 (4-3) 	 sin ( -  ) sin ( +  )    

β = Nominal slope of backfill behind wall (deg) 

δ = Angle of friction between retained backfill and reinforced soil, set equal to 

(deg) 

b = effective friction angle of retained backfill (deg) 

θ = 90o for vertical, or near (< 10◦) vertical, wall (deg) 

Note that the earth pressure force, (FT) in Figure 4-3, is oriented at the same angle as the 

backslope, , as it is assumed that  . 

 

Vertical Wall with Broken Backslope: The active earth pressure coefficient (Ka) for this 

condition is computed using Equations 4-2 and 4-3, with the design β angle and the interface 

angle  both set equal to I, as defined in Figure 4-4. 

Battered Wall with or without Backslope: For an inclined front face and reinforced zone 

(i.e., batter) equal or greater than 10 degrees from vertical, the coefficient of earth pressure 

can be calculated using Equations 4-2 and 4-3 where θ is the face inclination from horizontal, 

and β the surcharge slope angle as shown in Figure 4-5. The wall friction angle δ is assumed 

to be equal to β. 

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Figure 4-5. Notation for Coulomb active earth pressures used with wall batters, , greater 

than 100 (after AASHTO, 2007). 

Traffic Loads 

Traffic loads should be treated as uniform surcharge live load of not less than 2.0 ft (0.6 m) 

of earth (Article 11.10.10.2, AASHTO {2007}). For external stability, traffic load for walls 

parallel to traffic will have an equivalent height of soil, heq equal to 2.0 ft. For internal 

stability, traffic load for walls parallel to traffic will have a heq, equal to 2.0 ft unless traffic is 

allowed within 1.0 ft of the back of the wall facing. Commonly the wheel path is more then 

1-ft behind the wall backface due to the presence of a traffic barrier and, therefore, a heq 

value of 2 ft is applicable. 

Equivalent heights of soil, heq, for uniform surcharge loadings on retaining wall abutments 

with traffic running perpendicular to the wall may be taken from Table 4-4. Linear 

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interpolation is used for intermediate wall heights. Typically, the abutment heq will be acting 

on the stub abutment that sits on top of the reinforced soil zone (see Figure 4-13). If a 

structural approach slab is used and is supported on the backwall of the abutment (and not by 

the soil), the load is directly transmitted to the abutment; in this case heq = 0 is used unless 

otherwise mandated by an owner. 

If the surcharge is for other than highway vehicular loading, the owner should specify or 

approve different surcharge load. 

Table 4-4. Equivalent Height of Soil, heq, for Traffic Loading on Abutments 

Perpendicular to Traffic (Table 3.11.6.4-1, AASHTO {2007}). 

Abutment Height (ft) heq (ft) 

5.0 4.0 

10.0 3.0 

> 20.0 2.0 

Soil Compaction-Induced Earth Pressures 

Compaction stresses are already included in the design model and specified compaction 

procedures for MSE walls (Article C3.11.2, AASHTO {2007}). Therefore, no additional 

design considerations are required. 

4.4.5 Step 5 – Summarize Load Combinations, Load Factors, and Resistance Factors 

Load combinations were discussed in Section 4.2, and typically may include Strength I, 

Extreme I and/or II, and Service I limits. Note however, that in certain states, the Strength II 

limit state is more critical than the Strength I limit state because owner prescribed legal loads 

are greater than those provided in the AASHTO specifications (2007). Maximum permanent 

loads, minimum permanent loads, and total extremes should be checked for a particular load 

combination for walls with complex geometry and/or loadings to identify the critical loading. 

Examination of only the critical loading combination, as described in Section 4.2, is 

sufficient for simple walls. Load factors typically used for MSE walls are listed in Tables 4­

1 and 4-2. Refer to the information in Appendix E or Section 3 of AASHTO (2007) for load 

factors to use with complex MSE wall configurations and loadings. 

Live loads are not used on specific design steps since they contribute to stability. These are 

identified in subsequent design steps. 

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Sliding Sliding Sliding Limiting Eccentricity Limiting Eccentricit Limiting ELimiting ELimiting ELimiting ELimiting ELimiting Eccentricitccentricitccentricitccentricitccentricitccentricity

(Overturning) (Overturning) (Overturning)

Bearing Bearing
 

Figure 4-6. Potential external failure mechanisms for a MSE wall. 

 

 

Resistance factors for external stability and for internal stability are presented in respective 

design step discussions that follow. Internal stability resistance factors are listed later in 

Table 4-7. 

4.4.6 Step 6 – Evaluate External Stability 

As with classical gravity and semigravity retaining structures, four potential external failure 

mechanisms are usually considered in sizing MSE walls, as shown in Figure 4-6. They 

include: 

C Sliding on the base 

C Limiting eccentricity (formerly known as overturning) 

C Bearing resistance 

C Overall/global stability (see Step 8) 

The resistance factor for external stability analyses of MSE walls are listed in Table 4-5. 

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Table 4-5. External Stability Resistance Factors for MSE Walls 

(Table 11.5.6-1, AASHTO {2007}). 

Stability Mode Condition Resistance 

Factor 

Bearing Resistance 0.65 

Sliding 1.0

Overall (global) 

Stability 

Where geotechnical parameters are well 

defined, and the slope does not support or 

contain a structural element 

0.75 

Where geotechnical parameters are based on 

limited information, or the slope contains or 0.65 

supports a structural element 

4.4.6.a Evaluate Sliding Stability 

Check the preliminary sizing with respect to sliding of the reinforced zone where the 

resisting force is the lesser of the shear resistance along the base of the wall or of a weak 

layer near the base of the MSE wall, and the sliding force is the horizontal component of the 

thrust on the vertical plane at the back of the wall (see Figures 4-2 through 4-4). The live 

load surcharge is not considered as a stabilizing force when checking sliding, i.e., the sliding 

stability check only applies the live load above the retained backfill, as shown in Figure 4-2. 

The driving forces generally include factored horizontal loads due to earth, water, seismic, 

and surcharges. 

Sliding resistance along the base of the wall is evaluated using the same procedures as for 

spread footings on soil as per Article 10.6.3.4 (AASHTO, 2007). The factored resistance 

against failure by sliding (RR) can be estimated by: 

 RR =  R (4-4) 

where: 

τ = resistance factor for shear resistance between soil and foundation 

 (equal to 1.0 for sliding of soil-on-soil, see Table 4-5) 

Rτ = nominal sliding resistance between reinforced fill and foundation soil 

Note that any soil passive resistance at the toe due to embedment is ignored due to the 

potential for the soil to be removed through natural or manmade processes during its service 

life (e.g. erosion, utility installation, etc.). Also, passive resistance is usually not available 

during construction. The shear strength of the facing system is also conservatively neglected. 
 

 

 

Calculation steps and equations to compute sliding for two typical cases follow. These 

equations should be extended to include other loads and geometries, for other cases, such as 

additional live and dead load surcharge loads. 

1) Calculate nominal thrust, per unit width, acting on the back of the reinforced zone. 

Wall with Horizontal Backslope: (see Figure 4-2) 

The retained backfill resultant, F1, is: 

F 1  1

2 Kab γ b H2	 (4-5) 

For a uniform surcharge, the resultant is: 

F 2	  Kab q H (4-6) 

 

 

 

 

 

where: 

Kab = active earth pressure coefficient for the retained backfill 

b = moist unit weight of the retained backfill soil 

H = height of the retaining wall 

q = uniform live load surcharge = (r) (heq) 

Wall with Sloping Backfill: (see Figure 4-3) 

Calculate nominal retained backfill force resultant per unit width, FT
 

2 FT  1

2 Kab γ b h	 (4-7) 

 

 

 

where: 

Kab = active earth pressure coefficient for the sloping backfill, see Eq. 4-2 

h = total height of wall, H, and slope at the back of the reinforced zone 

= H + L tan β

For a broken backslope (see Figure 4-4), h - H should not exceed the height of the upper 

crest. If the broken backslope height is defined as “S”, then (H + L tan) < (H + S); use 

(H + S) if (L tan) > S. 

2) Calculate the nominal and the factored horizontal driving forces. For a horizontal 

backslope and uniform live load surcharge: 

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F  F 1  F2	 (4-8) 

P d	  γEH F1  γLS F2 (4-9) 

For a sloping backfill condition: 

F H	  FT cosβ (4-10) 

P d	  γEH FH  γEH FT cosβ (4-11) 

Use the maximum EH load factor (= 1.50) in these equations because it creates the 

maximum driving force effect for the sliding limit state. 

3) Determine the most critical frictional properties at the base. Choose the minimum soil 

friction angle,  for three possibilities: 

i) Sliding along the foundation soil, if its shear strength (based on c'f + tan ' f and/or 

cu for cohesive soils) is smaller than that of the reinforced fill material shear 

strength (tan ' r). 

ii) Sliding along the reinforced fill (' r). 

iii) For sheet type reinforcement, sliding along the weaker of the upper and lower 

soil-reinforcement interfaces. The soil-reinforcement friction angle ρ, should 

preferably be measured by means of interface direct shear tests. In absence of 

testing, it may be taken as 2⁄3 tan ' r. 

4) Calculate the nominal components of resisting force and the factored resisting force per 

unit length of wall. For a horizontal backslope and uniform live load surcharge, the live 

load is excluded since it increases sliding stability: 

R	  γ V  μ (4-12) r EV 1 

For a sloping backfill condition: 

R	  [ γ (V  V )  γ (F sinβ )]  μ (4-13) r EV 1 2 EH T 

where 

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 μ = minimum soil friction angle  [tan ' f, tan ' r, or (for continuous 

reinforcement) tan ρ] 

External loads that increase sliding resistance should only be included if those loads are 

permanent. 

Use the minimum EV load factor (= 1.00) in these equations because it results in 

minimum resistance for the sliding limit state. 

5) Compare factored sliding resistance, Rr, to the factored driving force, Pd, to check that 

resistance is greater. 

6) Check the capacity demand ratio (CDR) for sliding, CDR = Rr/Pd. If the CDR < 1.0, 

increase the reinforcement length, L, and repeat the calculations. 

4.4.6.b Eccentricity Limit Check 

The system of forces for checking the eccentricity at the base of the wall is shown on Figure 

4-7. It should be noted that the weight and width of the wall facing is typically neglected in 

the calculations. Limiting eccentricity is a strength limit state check. The eccentricity limit 

check only applies the live load above the retained backfill, as shown in Figure 4-2. 

The eccentricity, e, is the distance between the resultant foundation load and the center of the 

reinforced zone (i.e., L/2), as illustrated in Figure 4-7. The quantity e is calculated by 

summing the overturning and the resisting moments about the bottom, center of the base 

length, and dividing by the vertical load. 

 M  M 

e  D R	 (4-14) V 

Equations to compute eccentricity for two typical cases follow. These equations should be 

extended to include other loads and geometries, for other cases. 

Wall with Horizontal Backslope: Calculation steps for the determination of the eccentricity 

beneath a wall with a horizontal backslope and a uniform live load surcharge are as follows, 

with respect to Figure 4-7. 

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Calculate nominal retained backfill and surcharge force resultants per unit width. See 

Equations 4-5 and 4-6 for walls with a horizontal backslope and uniform live load surcharge. 

See Equation 4-7 for walls with sloping backfill. 

For a vertical wall, with horizontal backslope and uniform live load surcharge, calculate the 

eccentricity e as follows: 

γ F1 H 

3  γ F  H 2 EH-MAX LS q LS 

e  (4-15)

γEV-MIN V1 

Wall with Sloping Backfill: The eccentricity beneath a wall with a sloping backfill, and no 

surcharges, is calculated as follows, with respect to Figure 4-8. 

Calculate e with factored loads. For a wall with a sloping backfill the eccentricity is equal to: 

γ F cosβh   γ F sinβL   γ V L  EH-MAX T EH-MAX T EV-MIN 2 3 2 6 e  (4-16)

γEV-MIN 1 EV-MIN 2 EH-MAX T V  γ V  γ F sinβ

Eccentricity Check Criteria: The eccentricity, e, is considered acceptable if the calculated 

location of the resultant vertical force (based on factored loads) is within the middle one-half 

of the base width for soil foundations (i.e., emax = L / 4) and middle three-fourths of the base 

width for rock foundations (i.e., emax = 3/8 L). Therefore, for each strength limit load group, 

e must be less than emax. If e is greater, than a longer length of reinforcement is required. 

Examination of only the critical loading combination, as describe in Section 4.2, (i.e., use the 

minimum EV and maximum EH load factors) is sufficient for simple walls. Maximum 

permanent loads, minimum permanent loads, and total extremes should be checked for 

complex (geometry and/or loadings) walls to identify the critical loading. 

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Figure 4-7. Calculation of eccentricity and vertical stress for bearing check, for horizontal 

backslope with traffic surcharge condition. 

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Figure 4-8.	 Calculation of eccentricity and vertical stress for bearing check, for sloping 

backslope condition. 

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4.4.6.c Evaluate Bearing on Foundation 

Two modes of bearing capacity failure exist, general shear failure and local shear failure. 

Local shear is characterized by a punching or squeezing of the foundation soil when soft or 

loose soils exist below the wall. 

Bearing calculations require both a strength limit state and a service limit state calculation. 

Strength limit calculations check that the factored bearing pressure is less than the factored 

bearing resistance. Service limit calculations are used to compute nominal bearing pressure 

for use in settlement calculations. It should be noted that the weight and width of the wall 

facing is typically neglected in the calculations. The bearing check applies live load above 

both the reinforced zone and the retained backfill, as shown in Figure 4-2. 

General Shear. To prevent bearing failure on a uniform foundation soil, it is required that the 

factored vertical pressure at the base of the wall, as calculated with the uniform Meyerhoftype distribution, does not exceed the factored bearing resistance of the foundation soil: 

q R  quniform (4-17) 

The uniform vertical pressure is calculated as: 

σ  (4-18) V L  2eB 

V

where: 

 V = summation of vertical forces 

L = width of foundation, equal to reinforcement length L 

eB = eccentricity for bearing calculation (not equal to eccentricity check e) 

This step, 6.c, requires a different computation of the eccentricity value computed in Step 6.a 

because different, i.e., maximum in lieu of minimum, load factor(s) are used. Also note that 

the bearing check applies the live load above both the reinforced zone and the retained 

backfill, as shown in Figure 4-2. In addition to walls founded on soil, a uniform vertical 

pressure is also used for walls founded on rock due to the flexibility of MSE walls and their 

limited ability to transmit moment (Article C11.10.5.4 {AASHTO, 2007}). 

 

Calculation steps for MSE walls with either a horizontal backslope and uniform live load 

surcharge and for sloping backfills follow. Again, note that these equations should be 

extended to include other loads and geometries, for other cases. 

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1) Calculate the eccentricity, eB, of the resulting force at the base of the wall. The e value 

from the eccentricity check, Step 6.a, cannot be used, Calculate e with factored loads. 

For a wall with horizontal backslope and uniform live load surcharge centered about the 

reinforced zone, the eccentricity is equal to: 

γ F H	 H

1  3	 2  F  EH-MAX 	 γLS qLS  eB	  (4-19)

γEV-MAX 1 LS V  γ q L

where terms were previously defined. The maximum load factors for EH and EV are 

used to be consistent with the computation for v (below) where maximum load factors 

results in the maximum vertical stress. 

For walls with sloping backfill see Equation 4-16. Again, note that these equations should 

be extended to include other loads and geometries, for other cases. 

Note that when checking the various load factors, and load combinations, the value of 

eccentricity, eB, will vary. Also note that when the calculated value of eccentricity, eB, is 

negative, a value of 0 should be carried forward in the design stress equation, i.e., set L’ = 

L, per AASHTO C11.10.5.4 (2007). 

2) Calculate the factored vertical stress V-F at the base assuming Meyerhof-type 

distribution. For a horizontal backslope and uniform live load surcharge the factored 

bearing pressure is: 

γEV-MAX 1 LS V  γ q L  V-F 	 (4-20) L  2eB 

This approach, proposed originally by Meyerhof, assumes that a stress distribution due to 

eccentric loading can be approximated by a uniform stress distribution over a reduced 

area at the base of the wall. This area is defined by a width equal to the wall width minus 

twice the eccentricity as shown in Figures 4-7 and 4-8. The effect of eccentricity and 

load inclination is addressed with use of the effective width, L – 2eB, in lieu of the full 

width, L. 

For wall with sloping backfill the factored bearing stress is: 

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γEV-MAX 1 EV-MAX 2 EH-MAX T V  γ V  γ F sinβ q  (4-21) V-F L  2eB 

 

 

Note that (L – 2eB) is set equal to L when the value of eccentricity is negative. A 

negative value of eccentricity may be found for some extreme geometries, e.g. a wall 

section with very long reinforcements and a steep, infinite backslope. Note that when 

checking the various load factors and load combinations the value of eccentricity, eB, 

will vary and a critical value must be determined by comparisons of applicable load 

combinations. 

Where applicable, in the computation of bearing stress, V-F, include the influence of 

factored surcharge and factored concentrated loads. Maintain consistency with loads and 

load factors used in the eccentricity calculation and corresponding bearing stress 

calculation. 

3) Determine the nominal bearing resistance, qn, Equation 10.6.3.1.2a-1 of AASHTO 

(2007). For a level grade in front of a MSE wall and no groundwater influence: 

q n	  cf Nc  0.5L'γf Nγ (4-22) 

where: cf = the cohesion of the foundation soil 

γf = the unit weight of the foundation soil 

Nc and Nγ = dimensionless bearing capacity coefficients 

L’ = effective foundation width, equal to L – 2eB; set L’ equal to L if eB is a 

negative value 

The dimensionless bearing capacity factors can be obtained from Table 10.6.3.1.2a-1 of 

AASHTO (2007) and, for convenience, are shown in Table 4-6. Modifications to qn 

(Equation 4-22) for a ground surface slope and for high groundwater level are provided in 

10.6.3.1.2 AASHTO (2007). The beneficial effect of wall embedment is neglected. 

(Note: for excessive embedment (i.e., embedment greater that the minimum 

requirements, see Table 2-2), partial embedment may be considered in the determination 

of qn provided that the fill in front of the wall is placed and compacted as the reinforced 

fill is placed and all possible failure modes are examined. Bearing capacity is addressed 

in detail in the following two NHI courses: 132037 Shallow Foundations, and reference 

manual FHWA NHI-01-023 (Munfakh et al., 2001); and 132012 Soils & Foundations, 

and reference manual Volume I, FHWA NHI-06-089 (Samtani and Nowatzki, 2006). 

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Table 4-6. Bearing Resistance Factors 

(Table 10.6.3.1.2a-1, AASHTO {2007}). 

 Nc Nq Nγ  Nc Nq Nγ

0 5.14 1.0 0.0 23 18.1 8.7 8.2 

1 5.4 1.1 0.1 24 19.3 9.6 9.4 

2 5.6 1.2 0.2 25 20.7 10.7 10.9 

3 5.9 1.3 0.2 26 22.3 11.9 12.5 

4 6.2 1.4 0.3 27 23.9 13.2 14.5 

5 6.5 1.6 0.5 28 25.8 14.7 16.7 

6 6.8 1.7 0.6 29 27.9 16.4 19.3 

7 7.2 1.9 0.7 30 30.1 18.4 22.4 

8 7.5 2.1 0.9 31 32.7 20.6 25.9 

9 7.9 2.3 1.0 32 35.5 23.2 30.2 

10 8.4 2.5 1.2 33 38.6 26.1 35.2 

11 8.8 2.7 1.4 34 42.2 29.4 41.1 

12 9.3 3.0 1.7 35 46.1 33.3 48.0 

13 9.8 3.3 2.0 36 50.6 37.8 56.3 

14 10.4 3.6 2.3 37 55.6 42.9 66.2 

15 11.0 3.9 2.7 38 61.4 48.9 78.0 

16 11.6 4.3 3.1 39 37.9 56.0 92.3 

17 12.3 4.8 3.5 40 75.3 64.2 109.4 

18 13.1 5.3 4.1 41 83.9 73.9 130.2 

19 13.9 5.8 4.7 42 93.7 85.4 155.6 

20 14.8 6.4 5.4 43 105.1 99.0 186.5 

21 15.8 7.1 6.2 44 118.4 115.3 224.6 

22 16.9 7.8 7.1 45 133.9 134.9 271.8 

Note: 

Nc (Prandtl, 1921), Nq (Reisnner, 1924), and N (Vesic, 1975). 

Nq is embedment term, which is typically not used in MSE wall design. 

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4) Check that factored bearing resistance is greater than the factored bearing stress, i.e., qR > 

qV-F. The factored bearing resistance (qR) is given as: 

 qR   qn (4-23) 

where: 

 = resistance factor, for MSE walls this factor is 0.65 (Table 11.5.6-1, AASHTO 

{2007}) 

5) As indicated in step 2) and step 3), qV-F can be decreased and qR increased by lengthening 

the reinforcements, though only marginally. The nominal bearing resistance often may 

be increased by additional subsurface investigation and better definition of the foundation 

soil properties. If adequate support conditions cannot be achieved or lengthening 

reinforcements significantly increases costs, improvement of the foundation soil may be 

considered (dynamic compaction, soil replacement, stone columns, precompression, etc.) 

– see FHWA Ground Improvement Methods reference manuals NHI-06-019 and NHI06-020 (Elias et al., 2006). 

Local Shear, Punching Shear And Lateral Squeeze. Local shear is a transition between 

general shear and punching shear, which can occur in loose or compressible soils, in weak 

soils under slow (drained) loading. If local shear or punching shear failure is possible, 

Section 10.6.3.1.2b of AASHTO (2007) requires the use of reduced shear strength 

parameters for calculating the nominal bearing resistance. The reduced effective stress 

cohesion, c* is set equal to 0.67c'. The reduced effective stress soil friction angle, * is set 

equal to tan-1(0.67 tan ' f). 

Lateral squeeze is a special case of local shear that can occur when bearing on a weak 

cohesive soil layer overlying a firm soil layer. Lateral squeeze failure results in significant 

horizontal movement of the soil under the structure. 

To prevent local shear of structures bearing on weak cohesive soils it is required that: 

 γr H ≤ 3 cu (4-24) 

where γr is the nominal unit weight of the reinforced fill, H is the height of the wall and cu is 

the nominal total stress cohesion of the foundation soil. 

If adequate support conditions cannot be achieved, either the soft soils should be removed or 

ground improvement of the foundation soils is required. Local shear, as well as bearing on 

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two layered soil systems in undrained and drained loading, are addressed in Section 

10.6.3.1.2 of AASHTO (2007). Local shear and lateral squeeze is addressed in detail in NHI 

course 132012 Soils & Foundations, and reference manual Volume II, FHWA NHI-06-088 

(Samtani and Nowatzki, 2006). 

4.4.6.d Settlement Estimate 

Conventional settlement analyses should be carried out to ensure that immediate, 

consolidation, and secondary settlement of the wall are less than the performance 

requirements of the project (see FHWA NHI-06-088 and NHI-06-089, Soils and Foundations 

Reference manuals {Samtani and Nowatzki, 2006}). Settlement is evaluated under bearing 

pressure computed at a Service I limit state. 

Significant estimated post-construction foundation settlements indicate that the planned top 

of wall elevations need to be adjusted. This can be accomplished by increasing the top of 

wall elevations during wall design, or by providing height adjustment within the top of wall 

coping, and/or by delaying the casting of the top row of panels to the end of erection. The 

required height of the top row, would then be determined with possible further allowance for 

continuing settlements. Significant differential settlements (greater than 1/100), indicate the 

need of slip joints, which allow for independent vertical movement of adjacent precast 

panels. Where the anticipated settlements and their duration, cannot be accommodated by 

these measures, consideration must be given to ground improvement techniques such as wick 

drains, stone columns, dynamic compaction, the use of lightweight fill or the implementation 

of two-phased construction in which the first phase facing is typically a wire facing. 

4.4.7 Step 7 –EVALUATE INTERNAL STABILITY 

Internal failure of a MSE wall can occur in two different ways: 

C The tensile forces (and, in the case of rigid reinforcements, the shear forces) in the
 

inclusions become so large that the inclusions elongate excessively or break, leading to 

large movements and/or possible collapse of the structure. This mode of failure is called 

failure by elongation or breakage of the reinforcements. 

C The tensile forces in the reinforcements become larger than the pullout resistance, leading 

to large movements and/or possible collapse of the structure. This mode of failure is 

called failure by pullout. 

The process of sizing and designing to preclude internal failure, therefore, consists of 

determining the maximum developed tension forces, their location along a locus of critical 

slip surfaces and the resistance provided by the reinforcements both in pullout capacity and 

tensile strength. Internal stability also includes an evaluation of serviceability requirements 

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such as tolerable lateral movement of supported structures and control of downdrag stress on 

reinforcement connections. 

4.4.7.a Select Type of Soil Reinforcement 

Soil reinforcements are either inextensible (i.e., mostly metallic) or extensible (i.e., mostly 

polymeric materials), as discussed in Chapter 2. The internal wall design model varies by 

material type due to their extensibility relative to soil at failure. Therefore, the choice of 

material type should be made at this step of the design. The variations are: whether life 

prediction is based on metal corrosion or polymer degradation; critical failure plane geometry 

assumed for design; and lateral stress used for design. Distinction can be made between the 

characteristics of inextensible and extensible reinforcements, as follows. 

Design Methods, Inextensible (e.g., Metallic) Reinforcements 

The current method of limit equilibrium analysis uses a coherent gravity structure approach 

to determine external stability of the reinforced mass, similar to the analysis for any 

conventional or traditional gravity structure. For internal stability evaluations, it considers a 

bi-linear failure surface that divides the reinforced zone in active and resistant zones and 

requires that an equilibrium state be achieved for successful design. 

The lateral earth pressure distribution for external stability, is assumed to be based on 

Coulomb’s method with a wall friction angle δ assumed to be zero. For internal stability 

lateral pressure varying from a multiple of Ka to an active earth pressure state, Ka is used for 

design. Previous research (FHWA RD 89-043) has focused on developing the state of stress 

for internal stability, as a function of Ka, type of reinforcement used (geotextile, geogrid, 

metal strip or metal grid), and depth. The results from these and more recent (Allen et al., 

2001) efforts have been synthesized in a simplified method, which will be used throughout 

this manual. 

Design Methods, Extensible (e.g., Geosynthetic) Reinforcements 

For external stability calculations, the current method assumes an earth pressure 

distribution, consistent with the method used for inextensible reinforcements. 

For internal stability computations using the simplified method, the internal coefficient of 

earth pressure is again a function of the type of reinforcement, where the minimum 

coefficient (Ka) is used for walls constructed with continuous sheets of geotextiles and 

geogrids. For internal stability, a Rankine failure surface is considered, because the 

extensible reinforcements can elongate more than the soil, before failure, and do not 

significantly modify the shape of the soil failure surface. 

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4.4.7.b Define Critical Slip Surface 

The critical slip surface in a simple reinforced soil wall is assumed to coincide with the locus 

of the maximum tensile force, TMAX, in each reinforcement layer. The shape and location of 

the critical failure surface is based upon instrumented structures and theoretical studies. 

This critical failure surface has been assumed to be approximately bilinear in the case of 

inextensible reinforcements (Figure 4-9), approximately linear in the case of extensible 

reinforcements (Figure 4-9), and passes through the toe of the wall in both cases. 

When failure develops, the reinforcement may elongate and be deformed at its intersection 

with the failure surface. As a result, the tensile force in the reinforcement would increase and 

rotate. Consequently, the component in the direction of the failure surface would increase 

and the normal component may increase or decrease. Elongation and rotation of the 

reinforcements may be negligible for stiff inextensible reinforcements such as steel strips but 

may be significant with geosynthetics. Any reinforcement rotation is ignored for internal 

wall stability calculations with the simplified method. However, reinforcement rotation may 

be considered in compound slope stability analysis (see Chapters 8 and 9). 

For extensible reinforcements, the Coulomb earth pressure relationship shown on Figure 4-5 

should be used to define the failure surface, per AASHTO Figure 11.10.6.3.1-1 (2007), 

where the wall front batter from vertical is greater than 10 degrees. 

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Figure 4-9. Location of potential failure surface for internal stability design of MSE Walls 

(a) inextensible reinforcements and (b) extensible reinforcements. 

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4.4.7.c Define Unfactored Loads 

The primary sources of internal loading of an MSE wall is the earth pressure from the 

reinforced fill and any surcharge loadings on top of the reinforced zone. The unfactored 

loads for MSE walls may include loads due to, vertical earth pressure (EV), live load 

surcharge (LS), and earth surcharge (ES). Water, seismic, and vehicle impact loads should 

also be evaluated, as appropriate. 

Research studies (Collin, 1986; Christopher et al., 1990; Allen et al., 2001) have indicated 

that the maximum tensile force is primarily related to the type of reinforcement in the MSE 

wall, which, in turn, is a function of the modulus, extensibility and density of reinforcement. 

Based on this research, a relationship between the type of the reinforcement and the 

overburden stress has been developed, and shown in Figure 4-10. The Kr/Ka ratio for 

metallic (inextensible) reinforcements decreases from the top of the reinforced wall fill to a 

constant value 20 ft (6 m) below this elevation. In contrast to inextensible reinforcements, 

the Kr/Ka for extensible (e.g., geosynthetic) reinforcement is a constant. Note that the 

resulting Kr/Ka ratio is referenced to the top of the wall at the face, excluding any copings 

and appurtenances (i.e., the top of the reinforced soil zone at the face) for both walls with 

level and with sloping backfills. The Kr/Ka starting elevation for an MSE wall supporting a 

spread footing bridge abutment is the top of the backfill, see Chapter 6 and appended design 

example. 

The simplified approach used herein was developed in order to avoid iterative design 

procedures required by some of the complex refinements of the available methods i.e., the 

coherent gravity method (AASHTO, 1994 Interims) and the structural stiffness method 

(FHWA RD 89-043, Christopher et al., 1990). The simplified method (a.k.a. simplified 

coherent gravity method) (Elias and Christopher, 1997; Allen et al., 2001) is based on the 

same empirical data used to develop these two methods. 

Figure 4-10 was prepared by back analysis of the lateral stress ratio Kr from available field 

data where stresses in the reinforcements were measured and normalized as a function of the 

Rankine active earth pressure coefficient, Ka. The Rankine active earth pressure theory 

assumes lateral pressure is independent of backfill slope and interface friction. The ratios 

shown on Figure 4-10 correspond to values representative of the specific reinforcement 

systems that are known to give satisfactory results assuming that the vertical stress is equal to 

the weight of the overburden (γH). This provides a simplified evaluation method for all 

cohesionless reinforced fill walls. Future data may lead to modifications in Figure 4-10, 

including relationships for newly developed reinforcement types, effect of full height panels, 

etc. These relationships can be developed by instrumenting structures and using numerical 

models to verify the Kr/Ka ratio for routine and complex walls. 

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The lateral earth pressure coefficient Kr is determined by applying a multiplier to the active 

earth pressure coefficient. The active earth pressure coefficient is determined using a 

Coulomb earth pressure relationship, assuming no wall friction and a β angle equal to zero 

(i.e., equivalent to the Rankine earth pressure coefficient). For a vertical wall the earth 

pressure therefore reduces to the Rankine equation: 

 ' 

2   r  K a  tan  45   (4-25) 2   

For wall face batters equal to or greater than 10° from the vertical, the following simplified 

form of the Coulomb equation can be used: 

 

2 ' sin  r  Ka  (4-26) 

3  sinr 

'  sin  1    sin  

where θ is the inclination of the back of the facing as measured from the horizontal starting in 

front of the wall, as shown in Figure 4-5. Commentary C11.10.6.2.1 AASHTO (2007) states 

that above equation can be used for battered walls. The 10◦ value recommendation is 

consistent with the equation to determine the failure surface location for walls with 10◦ or 

greater batter (C11.10.6.3.1, AASHTO {2007}). 

The stress, 2, due to a sloping backfill on top of an MSE wall can be determined as shown 

in Figure 4-11. An equivalent soil height, S, is computed based upon the slope geometry. 

The value of Seq should not exceed the slope height for broken back sloping fills. A 

reinforcement length of 0.7H is used to compute the sloping backfill stress, 2, on the soil 

reinforcement, as a greater length would only have minimal effect on the reinforcement. The 

vertical stress is equal to the product equivalent soil height and the reinforced fill unit weight, 

and is uniformly applied across the top of the MSE zone. 

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Figure 4-10. Variation of the coefficient of lateral stress ratio (Kr/Ka) with depth in a MSE 

wall (Elias and Christopher, 1997; AASHTO; 2002; & after AASHTO, 2007). 

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Figure 4-11. Calculation of vertical stress for sloping backfill conditions for internal 

stability. 

4.4.7.d Establish Vertical Layout of Soil Reinforcements 

Use of a constant reinforcement section and spacing for the full height of the wall usually 

gives more reinforcement in the upper portion of the wall than is required for stability. 

Therefore, a more economical design may be possible by varying the reinforcement density 

with depth. However, to provide a coherent reinforced soil zone, vertical spacing of 

reinforcement should not exceed 32 in. (800 mm). 

There are generally two practical ways to accomplish this for MSE walls: 

For reinforcements consisting of strips, grids, or mats used with segmental precast 

concrete facings, the vertical spacing is maintained constant and the reinforcement 

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C 
 

 

density is increased with depth by increasing the number and/or the size of the 

reinforcements. For instance, the typical horizontal spacing of 2-in. (50 mm) x 5/32-in. 

(4 mm) strips is 30 in. (0.75 m), but this can be decreased by adding horizontal 

reinforcement locations. 

C For continuous sheet reinforcements, made of geotextiles or geogrids, a common way of 

varying the reinforcement density Tal/Sv is to change the vertical spacing Sv, especially if 

wrapped facing is used, because it easily accommodates spacing variations. The range of 

acceptable spacing is governed by consideration of placement and compaction of the 

backfill (e.g., Sv taken as 1, 2 or 3 times the compacted lift thickness). The reinforcement 

density Tal/Sv can also be varied by changing the strength (Tal) especially if wrapped 

facing techniques requiring a constant wrap height are used. 

Low-to medium-height walls (e.g., < 16 ft {5 m}) are usually constructed with one strength 

geosynthetic. Taller walls use multiple strength geosynthetics. For example the 41 ft (12.6 

m) high Seattle preload wall used four strengths of geotextiles (Allen et al., 1992). A 

maximum spacing of 16 in. (400 mm) is typical for wrapped faced geosynthetic walls, 

although a smaller spacing may be desirable to minimize bulging. 

For walls constructed with modular blocks, the maximum vertical spacing of reinforcement 

should be limited to two times the block depth (front face to back face) or 32 in. (810 mm), 

which ever is less, to assure construction and long-term stability. The top row of 

reinforcement should be limited to 1.5 the block depth (e.g. one unit plus a cap unit). 

(AASHTO 11.10.2.3.1 {AASHTO, 2007}). 

For large face units, such as 3 ft by 3 ft (0.9 m by 0.9 m) gabions, a vertical spacing equal to 

the face height (i.e., 3 ft {0.9 m}) is typically used. This spacing slightly exceeds the limit 

noted above, but this may be offset by the contributions of the large facing unit to internal 

(i.e., bulging) stability. 

4.4.7.e Calculate Factored Tensile Forces in the Reinforcement Layers 

e.1 Calculate Horizontal Stress
 

For internal stability analysis, the distribution of horizontal stress, σH, is first established.
 

The horizontal stress at any given depth within the reinforced soil zone is expressed as 

follows: 

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σH = Kr [σv]+ σH (4-27) 
 

 

where Kr is the coefficient of lateral earth pressure in the reinforced soil zone and is obtained 

from Figure 4-10, σv is the factored vertical pressure at the depth of interest, and σh is the 

supplemental factored horizontal stress due to external surcharges. 

For internal stability analysis, the following assumptions are made in the computation of 

factored vertical pressure, σv: 

1. Vertical pressure due to the weight of the reinforced soil zone is assigned a load type 

“EV” with a corresponding (maximum) load factor, γP-EV = 1.35. The maximum load 

factor of 1.35, and not the minimum load factor of 1.00, is always used to find the critical 

stress. 

2. Any vertical surcharge above the reinforced soil zone that is due to soil or considered as 

an equivalent soil surcharge is assigned a load type “EV.” In this scenario, a live load 

traffic surcharge that is represented by an equivalent uniform soil surcharge of height heq 

is assumed as load type “EV.” This is in contrast to the external stability analysis where 

the live load traffic surcharge is assumed as load type “LS” because in external stability 

analysis the MSE wall is assumed to be a rigid block. For internal stability analysis, the 

assumption of load type “EV” is used so that the amount of soil reinforcement within the 

reinforced soil zone is approximately the same as obtained using past working stress 

design approach (i.e., calibration by fitting). 

3. The unit weight of the equivalent soil surcharge is assumed to be the same as the unit 

weight of the reinforced soil zone, r, which is generally greater than or equal to the unit 

weight of the retained backfill. 

4. Any vertical surcharge that is due to non-soil source is assigned a load type “ES.” 

Example of such a load is the bearing pressure under a spread footing on top of 

reinforced soil zone. However, the application of the load factor of γP-ES = 1.50 that is 

assigned to load type “ES” is a function of how the vertical pressures are computed as 

follows: 

	 If the vertical pressures are based on nominal (i.e., unfactored) loads, then use γP-ES 

=1.50. 

	 If the vertical pressures were based on factored loads, then use γP-ES = 1.00. This is 

because once the loads are factored they should not be factored again. 

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It is recommended that the factored vertical pressure be evaluated using both the above 

approaches and the larger value chosen for analysis. 

The supplemental factored horizontal pressure, σh, could be from a variety of sources. Two 

examples of supplemental horizontal pressures are as follows: 

1. Horizontal pressures due to the horizontal (shear) stresses at the bottom of a spread 

footing on top of reinforced soil zone. 

2. Horizontal pressures from deep foundation elements extending through the reinforced 

soil zone. 

Supplemental horizontal pressures are assigned a load type “ES” since they represent 

surcharges on or within the reinforced soil zone. However, similar to the vertical pressures 

due to non-soil loads, the application of the maximum load factor of γP-ES = 1.50 that is 

assigned to load type “ES” is a function of how the horizontal pressures are computed as 

follows: 

	 If the horizontal pressures are based on nominal (i.e., unfactored) loads, then use γESMAX = 1.50. 

	 If the horizontal pressures were based on factored loads, then use γP-ES = 1.00. This is 

because once the loads are factored they should not be factored again. 

As with vertical pressure, it is recommended that the factored horizontal pressure be 

evaluated using both the above approaches and the larger value chosen for analysis. 

The application of the above guidance is illustrated below for four MSE wall configurations 

ranging from simple to complex geometries. The logic used in development of these 

equations can be extended to any other MSE wall configuration with complex system of 

surcharges. 

Example 1: MSE wall with level backfill and no surcharge. This represents the simplest 

MSE wall configuration for which the horizontal stress at any given depth Z below the 

top of the reinforced soil zone is given as follows: 

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σH 	 = Kr [(r Z) γEV-MAX] (4-28) 
 

 

 

where, r is the unit weight of soil in the reinforced soil zone, and γEV-MAX is the 

maximum load factor (=1.35) for load type “EV.” The value of Kr is obtained by 

assuming that: (i) the variation of Kr/Ka ratio shown in Figure 4-10 starts from the top of 

the reinforced soil zone, and (ii) Ka is computed using the Rankine formula (Eq. 4-25). 

Example 2: MSE wall with sloping backfill. This configuration is commonly used for 

side-hill retaining wall applications. Example of this configuration is shown in Figure 4­

10. As shown in Figure 4-11, the sloping surcharge is approximated by an equivalent 

uniform soil surcharge of height, Seq. For this case, the horizontal stress at any depth Z 

below the top of the reinforced soil zone can be written as follows: 

σH = Kr [r (Z + Seq) γEV-MAX] (4-29) 

The value of Kr is obtained by assuming that: (i) the variation of Kr/Ka ratio shown in 

Figure 4-10 starts from the top of the reinforced soil zone, and (ii) Ka is computed using 

the Rankine formula (Eq. 4-25) assuming that the backfill is level. Use of Equation 4-29 

is demonstrated in Example Problem E-3 in Appendix E. 

Example 3: MSE wall with level backfill and live load surcharge. This configuration is 

commonly used for grade-separated roadways. Assuming that the live load is expressed 

as an equivalent uniform soil surcharge of height, heq, (equal to 2 ft) the horizontal stress 

at any depth Z below the top of the reinforced soil zone can be written as follows: 

σH = Kr [r (Z + heq) γEV-MAX] (4-30) 

The value of Kr is obtained by assuming that: (i) the variation of Kr/Ka ratio shown in 

Figure 4-10 starts from the top of the reinforced soil zone, and (ii) Ka is computed using 

the Rankine formula (Eq. 4-25). Use of Equation 4-30 is demonstrated in Example 

Problem E-4 in Appendix E. 

Example 4: Bridge abutment with a spread footing on top of MSE wall. In this 

configuration the bridge superstructure rests on a spread footing on top of a MSE wall. 

This configuration is discussed in detail in Chapter 6. It is included here as an example 

of a complex system of surcharges that can be used to explain the computation of 

horizontal stress for such cases. For development of the equation of horizontal stress, 

refer to Figures 4-12 and 4-13. Assumptions are that the live load is expressed as an 

equivalent uniform soil surcharge of height, heq, as per Table 4-4, the height of the 

roadway fill above the reinforced soil zone is h, and σv and σH increase TMAX. Then, 

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the horizontal stress at any depth Z below the top of the reinforced soil zone can be 

written as follows: 

σH = Kr [r (Z + h + heq) γEV-MAX + (Δσv-footing) γP-ES]+ (σH) γP-ES (4-31) 

where σv and σH are the vertical (normal) and horizontal (shear) pressures at the 

bottom of the spread footing. As noted earlier, the value of γP-ES is 1.50 if nominal (i.e., 

unfactored) pressures are used, and is 1.00 if factored pressures are used with the final 

value being chosen based on larger values of (σv-footing)γP-ES and (σH)γP-ES. 

The value of Kr is obtained by assuming that: (i) the variation of Kr/Ka ratio shown in 

Figure 4-10 starts from the finished pavement grade behind the spread footing, and (ii) Ka 

is computed using the Rankine formula (Eq. 4-25). Use of Equation 4-31 is demonstrated 

in Example Problem E-5 in Appendix E. 

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feet 

Figure 4-12. Distribution of stress from concentrated vertical load for internal and external 

stability calculations. 

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Figure 4-13. Distribution of stresses from concentrated horizontal loads. 

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Figure 4-14. Reinforcement load contributory height. 

e.2 Calculate Maximum Tension, TMAX 

Calculate the maximum factored tension TMAX in each reinforcement layer per unit width of 

wall based on the vertical spacing Sv from: 

T MAX H v  σ S (in force per unit reinforcement width {kips/ft}) (4-32a)

The term Sv is equal to the vertical reinforcement spacing for a layer where vertically 

adjacent reinforcements are equally spaced from the layer under consideration. In this case, 

H, calculated at the level of the reinforcement, is at the center of the contributory height. 

The contributory height is defined as the midpoint between vertically adjacent reinforcement 

elevations, except for the top and bottom layers reinforcement. 

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For the top and bottom layers of reinforcement, Sv is the distance from top or bottom of wall, 

respectively, to the midpoint between the first and second layer (from top or bottom of wall, 

respectively) of reinforcement. Sv distances are illustrated in Figure 4-14. 

The maximum reinforcement tension, TMAX, for the top and bottom layers of reinforcement, 

and for intermediate layers that do not have equally spaced adjacent layers, is calculated as 

the product of the contributory height and the average factored horizontal stress acting upon 

that contributory height. The average stress can be calculated based upon the tributary 

trapezoidal area (i.e. average of the stress at top and at the bottom of the contributory height) 

or at the midpoint of the contributory height, as illustrated in Figure 4-14. 

Alternatively, for discrete reinforcements (metal strips, bar mats, geogrids, etc.) TMAX (force 

per unit width) may be calculated at each level as PTMAX-UWR in terms of force per unit width 

of reinforcement, as: 

 σ H Sv P  (4-32b) TMAX-UWR Rc 

where: 

Rc = ratio of gross width of strip, sheet, or grid to the center-to-center 

horizontal spacing between the strips, sheets, or grids (see Eq. 3-9 and 

Figure 3-3); e.g., Rc = 1 for full coverage reinforcement. 

For discrete reinforcements of known spacing and segmental precast concrete facing of 

known panel dimensions, TMAX (force per unit width) can alternatively be calculated per 

discrete reinforcement, PTMAX-D, per panel width, defined as. 

 σH Sv WP P  (4-32c) TMAXD NP 

where: 

PTMAX-D = maximum factored load in discrete reinforcement element 

WP = width of panel 

NP = number of discrete reinforcements per panel width (e.g., 2, 3, etc.) 

4.4.7.f Calculate Soil Reinforcement Resistance 

The procedure and discussion on definition of nominal long-term reinforcement design 

strength (Tal), for both steel and geosynthetic reinforcements, are presented in Section 3.5 of 

this manual. The factored soil resistance is the product of the nominal long-term strength, 

coverage ratio, and applicable resistance factor, . The resistance factors for tensile rupture 

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of MSE wall soil reinforcements are summarized in Table 4-7. The factored tensile 

resistance, Tr, is equal to: 

Tr =  Tal (4-33) 

Tal (as noted in Section 3.5) and Tr may be expressed in terms of strength per unit width of 

wall, per reinforcement element, or per unit reinforcement width. 

Table 4-7. Resistance Factors, , for Tensile and Pullout Resistance for MSE Walls 

(after Table 11.5.6-1, AASHTO {2007}). 

Reinforcement Type and Loading Condition Resistance 

Factor 

Metallic reinforcement and 

connectors 

Strip reinforcements (A) 

Static loading 0.75 

Combined static/earthquake loading 1.00 

Combined static/traffic barrier impact (B) 1.00 

Grid reinforcements (A, C) 

Static loading 0.65 

Combined static/earthquake loading 0.85 

Combined static/traffic barrier impact (B) 0.85 

Geosynthetic reinforcement 

and connectors 

Static loading 0.90 

Combined static/earthquake loading 1.20 

Combined static/traffic barrier impact (B) 1.20 

Pullout resistance of tensile 

reinforcement (metallic and 

geosynthetic) 

Static loading 0.90 

Combined static/earthquake loading 1.20 

Combined static/traffic barrier impact (B) 1.00 

Notes: 

A. 

B. 

C. 

Apply to gross cross-section less sacrificial area. For sections with holes, reduce gross 

area in accordance with AASHTO (2007) Article 6.8.3 and apply to net section less 

sacrificial area. 

Combined static/traffic barrier impact resistance factors are not presented in AASHTO. 

Applies to grid reinforcements connected to rigid facing element, e.g., a concrete panel or 

block. For grid reinforcements connected to a flexible facing mat or which are continuous 

with the facing mat, use the resistance factor for strip reinforcements. 

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4.4.7.g Select Grade of and/or Number of Soil Reinforcement Elements at Each Level 

The soil reinforcement vertical layout, the factored tensile force at each reinforcement level, 

and the factored soil reinforcement resistance were defined in the previous three steps. With 

this information, select suitable grades (strength) of reinforcement, or number of discrete 

(e.g., strip) reinforcements, for the defined vertical reinforcement layout. Then with this 

layout check pullout and, as applicable, extreme event loadings. Adjust layout if/as 

necessary. 

Stability with respect to breakage of the reinforcements requires that: 

TMAX  T r (4-34) 

Where TMAX is the maximum factored load in a reinforcement (Eqs. 4-32) and Tr is the 

factored reinforcement tensile resistance (Eq. 4-33). 

4.4.7.h Internal Stability with Respect to Pullout Failure 

Stability with respect to pullout of the reinforcements requires that the factored effective 

pullout length is greater than or equal to the factored tensile load in the reinforcement, TMAX. 

Each layer of reinforcement should be checked, as pullout resistance and/or tensile loads may 

vary with reinforcement layer. Therefore, the following criteria should be satisfied: 

 TMAX  L  (4-35) e F *ασv CRc 

 

 

 

 

 

 

 

where: 

Le = The length of embedment in the resisting zone. Note that the boundary 

between the resisting and active zones may be modified by concentrated 

loadings. 

TMAX = Maximum reinforcement tension 

 = Resistance factor for soil reinforcement pullout. See Table 4-7. 

F* = Pullout resistance factor (see Chapter 3) with variation in depth starting at 

the same elevation as that for Kr/Ka variation. 

α = Scale correction factor (see Chapter 3) 

σv = Nominal (i.e., unfactored) vertical stress at the reinforcement level in the 

resistant zone, including distributed dead load surcharges, neglecting 

traffic loads. See Figure 4-15 for computing v for sloping backfills. 

C = 2 for strip, grid, and sheet type reinforcement 

Rc = Coverage ratio 

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Figure 4-15. Nominal vertical stress at the reinforcement level in the resistant zone, 

beneath a sloping backfill. 

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Therefore, the required embedment length in the resistance zone (i.e., beyond the potential 

failure surface) can be determined from: 

TMAX L   3 ft 1 m (4-36) e  F *  v CRc 

If a traffic or other live load is present, it is recommended that TMAX be computed with the 

live loads and that the pullout resistance be computed excluding the live loads. This 

addresses the possibility of the live loads being present near the front of the wall but not 

above the reinforcement embedment length. The pullout resistance and the TMAX can be 

calculated with the live load excluded (AASHTO {2009 Interims} specifications) if it can be 

shown that the live load will be on the active and resistant zones at the same time or on the 

resistant zone alone. An agency should note their pullout calculation requirement, if it varies 

from AASHTO, in their specifications. 

Commentary C11.10.6.2.1 (AASHTO, 2009 Interims) notes that traffic loads and other live 

loads are not included for pullout calculations. Therefore, if TMAX calculation for checking 

the reinforcement and connection strengths included a live load surcharge the value must be 

recomputed, without the surcharge load, for Equation 4-35 or 4-36. 

If the criterion is not satisfied for all reinforcement layers, the reinforcement length has to be 

increased and/or reinforcement with a greater pullout resistance per unit width must be used, 

or the reinforcement vertical spacing may be reduced which would reduce TMAX. 

The total length of reinforcement, L, required for internal stability is then determined from: 

 L = La + Le (4-37) 

where La is obtained from Figure 4-9 for simple structures not supporting concentrated 

external loads such as bridge abutments. Based on this figure the following relationships can 

be obtained for La: 

For MSE walls with extensible reinforcement, vertical face and horizontal backfill: 

La = (H - Z) tan (45 - ’/2) (4-38) 

where Z is the depth to the reinforcement level. 

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For walls with inextensible reinforcement, vertical face and horizontal backfill, from the 

base up to H/2: 

La = 0.6 (H-Z) (4-39) 

For the upper half of a wall with inextensible reinforcements, vertical face, and horizontal 

backfill: 

La = 0.3H (4-40) 

For construction ease, a final uniform length is commonly chosen, based on the maximum 

length required. However, if internal stability controls the length, it could be varied from the 

base, increasing with the height of the wall to the maximum length requirement based on a 

combination of internal and maximum external stability requirements. See Chapter 6, section 

6.3 for additional guidance. 

4.4.7.i Check Connection Strength 

The connection of the reinforcements with the facing, should be designed for TMAX for all 

limit states. The resistance factors () for the connectors are the same as for the 

reinforcement strength, and are listed in Table 4-7 (Article 11.10.6.2.2 (AASHTO, 2007). 

Connections to Concrete Panels 

The metallic reinforcements for MSE systems constructed with segmental precast panels are 

structurally connected to the facing by either bolting the reinforcement to a tie strip cast in 

the panel or connected with a bar connector to suitable anchorage devices in the panels. The 

capacity of the embedded connector as an anchorage must be checked by tests as required by 

Article 5.11.3 AASHTO (2007) for each geometry used. Connections between metallic 

reinforcements and facing units should be designed in accordance with AASHTO Article 

6.13.3, and consider corrosion losses in accordance with AASHTO Article 11.10.6.4.2a. The 

design load at the connection is equal to the maximum load on the reinforcement. 

Polyethylene geogrid reinforcements may be structurally connected to segmental precast 

panels by casting a tab of the geogrid into the panel and connecting to the full length of 

geogrid with a bodkin joint, as illustrated in Figure 4-16. The capacity of the embedded 

connector as an anchorage must be checked by tests as required by Article 5.11.3 AASHTO 

(2007) for each geometry used. A slat of polyethylene is used for the bodkin. Care should 

be exercised during construction to eliminate slack from this connection. 

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Polyester geogrids and geotextiles should not be cast into concrete for connections, due to 

potential chemical degradation. Other types of geotextiles also are not cast into concrete for 

connections due to fabrication and field connection requirements. 

Figure 4-16. Bodkin connection detail (looking at cross section of segmental panel face). 

Connections to MBW Units 

MSE walls constructed with MBW units are connected either by (i) a structural connection 

subject to verification under AASHTO Article 5.11.3, (ii) friction between the units and the 

reinforcement, including the friction developed from the aggregate contained within the core 

of the units, or, (iii) a combination of friction and shear from connection devices. This 

strength will vary with each unit depending on its geometry, unit batter, normal pressure, 

depth of unit, and unit infill gravel (if applicable). The connection strength is therefore 

specific to each unit/reinforcement combination and must be developed uniquely by test for 

each combination. 

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The nominal long-term connection strength, Talc developed by frictional and/or structural 

means is determined as follows: 

T CR T ult cr

alc  (4-41) RFD 

 

where: 

Talc = nominal long-term reinforcement/facing connection strength per unit 

reinforcement width at a specified confining pressure 

Tult = ultimate tensile strength of the geosynthetic soil reinforcement, 

defined as the minimum average roll value (MARV) 

 RFD = reduction factor to account for chemical and biological degradation 

 CRcr = long-term connection strength reduction factor to account for reduced 

ultimate strength resulting from the connection 

CRcr may be obtained from long-term or short-term tests, as described below. 

 CRcr Defined with Long-Term Testing 

A series of connection creep tests are performed over extended periods of time to 

evaluate creep rupture at the connection. The long-term connection creep rupture 

data is extrapolated to the specified design life (e.g., 75 years, 100 years) to define the 

creep reduced connection strength, Tcrc, at the specified design life. Details for longterm testing and interpretation of results are presented in Appendix B. With this 

long-term testing, CRcr is defined as follows: 

 

 Tcrc CRcr  (4-42) Tlot 

Tlot is the ultimate wide width tensile strength of the reinforcement material roll/lot 

used for the connection strength testing. The Tlot strength, for example, might be 

103% to 115% of the minimum average roll value (MARV) ultimate strength, Tult (or 

noted Tult-MARV). 

CRcr Defined with Short-Term Testing 

Short-term (i.e., quick) ultimate strength tests, per ASTM D6638, are used to define 

an ultimate connection strength, Tultconn, at a specified confining pressure. Tests 

should be performed in accordance with ASTM D6638, Determining Connection 

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Strength Between Geosynthetic Reinforcement and Segmental Concrete Units 

(Modular Concrete Blocks). With short-term testing, CRcr, is defined as follows: 

Tultconn CR  (4-43) cr RF T cr lot 

RFcr is the geosynthetic creep reduction factor (see Chapter 3), and Tlot is the 

ultimate wide width tensile strength of the reinforcement material roll/lot used for 

the connection strength testing. 

Raw data from short-term connection strength laboratory testing should not be 

used for design. The wall designer should evaluate the data and define the 

nominal long-term connection strength, Talc. Steps for this data reduction are 

summarized and discussed in Appendix B. An example of reduction of shortterm connection strength data is presented in Appendix B. 

Note that the environment between and directly behind the modular blocks at the connection 

may not be the same as the environment within the reinforced soil zone. Therefore, the longterm environmental aging factor (RFD) may be significantly different than that used in 

computing the nominal long-term reinforcement strength Tal. 

The connection strength as developed above is a function of normal pressure, which is 

developed by the weight of the units. Thus, it will vary from a minimum in the upper portion 

of the structure to a maximum near the bottom of the structure for walls with no batter. 

Further, since many MBW walls are constructed with a front batter, the column weight above 

the base of the wall or above any other interface may not correspond to the weight of the 

facing units above the reference elevation. The concept is shown in Figure 4-17, and is 

termed a hinge height (Simac et al., 1993). Hence, for walls with a nominal batter of more 

than 8 degrees, the normal stress is limited to the lesser of the hinge height or the height of 

the wall above the interface. This vertical pressure range should be used in developing CRcr. 

This recommendation is based on research findings that indicated that the hinge height 

concept is overly conservative for walls with small batters (Bathurst et al., 2000). 

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Figure 4-17.	 Determination of hinge height for modular concrete block faced MSE walls 

(NMCA, 1997). 

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4.4.7.j Lateral Movements 

The evaluation of lateral wall movements in LRFD is the same as in ASD as the 

deformations are evaluated at the Service I limit state. In general, most internal lateral 

deformations of an MSE wall face usually occur during construction. Post construction 

movements, however, may take place due to post construction surcharge loads, settlement of 

wall fill, or long-term settlement of the foundation soils. 

The magnitude of lateral displacement depends on fill placement techniques, compaction 

effects, reinforcement extensibility, reinforcement length, reinforcement-to-facing 

connection details, and details of the wall facing. The rough estimate of probable lateral 

displacements of simple MSE walls that may occur during construction can be estimated 

based on empirical correlations (see Figure 2-15). In general, increasing the length-to-height 

ratio of reinforcement, from its theoretical lower limit of 0.5H to the AASHTO specified 

0.7H, decreases the deformation by about 50 percent. For critical structures requiring precise 

tolerances, such as bridge abutments, more accurate calculations using numerical modeling 

may be warranted. 

A deformation response analysis allows for an evaluation of the anticipated performance of 

the structure with respect to horizontal (and vertical) displacement. Horizontal deformation 

analyses are the most difficult and least certain of the performed analyses. In many cases, 

they are done only approximately. The results may impact the choice of facing, facing 

connections, or backfilling sequences. 

4.4.7.k Vertical Movement and Bearing Pads 

Bearing pads are placed in horizontal joints of segmental precast concrete panels in order to 

allow the panel and the reinforcement to move down with the reinforced fill as it is placed 

and settles, mitigate downdrag stress, and provide flexibility for differential foundation 

settlements. Internal settlement within the reinforced fill is practically immediate with some 

minor movement occurring after construction due to elastic compression in granular 

materials. The amount of total movement is the combination of the internal movement and 

external differential movement. The bearing/compression pad thickness and compressibility 

could be adjusted according to the anticipated movement. Otherwise concrete panel cracking 

and/or downdrag on connections resulting in bending of connections and/or out of plane 

panel movement can occur. Calculation of the external settlement was reviewed in Section 

4.4.6.d. Normally the internal movement is negligible for well graded, granular fill and 

external movement will usually control the compression pad requirements as listed in Table 

2-3. However, when using sand type fill and/or marginal fill containing an appreciable 

amount of fines, the internal movement can be significant and should be calculated to 

evaluate additional thickness requirements of the bearing pad. Immediate settlement of 

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granular fill can be calculated using the Schmertmann method, as described in the FHWA 

NHI-06-088 and NHI-06-089, Soils and Foundation Reference manuals (Samtani and 

Nowatzki, 2006). 

The stiffness (axial and lateral), size, and number of bearing pads should be sized such that 

the final joint opening will be at least 3⁄4 +1/8-inch, unless otherwise shown on the plans. As 

noted in Chapter 2, a minimum initial joint width of 3⁄4-inch is recommended. The stiffness 

(axial and lateral), size, and number of bearing pads are should be checked assuming a 

vertical loading at a given joint equal to 2 to 3 times the weight of facing panels directly 

above that level. Laboratory tests in the form of vertical load-vertical strain and vertical 

load-lateral strain curves of the bearing pads are required for this check. 

4.4.8 Step 8 – Design of Facing Elements 

4.4.8.a Design of Concrete, Steel and Timber Facings 

Facing elements are designed to resist the horizontal forces developed in Section 4.3. 

Reinforcement is provided to resist the maximum loading conditions at each depth in 

accordance with structural design requirements in Section 5, 6 and 8 of AASHTO (2007) for 

concrete, steel and timber facings, respectively. The embedment of the soil reinforcement 

to panel connector must be developed by test, to ensure that it can resist the TMAX loads. 

As a minimum, temperature and shrinkage steel must be provided for segmental precast 

panel facing. Epoxy protection of panel reinforcement or a minimum of 3 in. (75 mm) of 

concrete cover is recommended where salt spray is anticipated. 

For modular concrete facing blocks (MBW), sufficient inter-unit shear capacity must be 

available, and the maximum spacing between reinforcement layers should be limited to twice 

the front to back width, Wu, as defined in Figure 4-17, of the modular concrete facing unit or 

2.7 ft (32 in., 800 mm) whichever is less. The maximum depth of facing below the bottom 

reinforcement layer should typically be limited to the width, Wu (see Figure 4-17), of the 

modular concrete facing unit used. The top row of reinforcement should be limited to 1.5 the 

block depth (e.g. one unit plus a cap unit) (AASHTO 11.10.2.3.1 {2007}). 

The factored inter-unit shear capacity as obtained by testing (ASTM D6916) at the 

appropriate normal load should exceed the factored horizontal earth pressure at the facing. 

For seismic performance Zones 3 or 4, facing connections in modular block faced walls 

(MBW) should use shear resisting devices between the MBW units and soil reinforcement, 

and should not be fully dependent on frictional resistance between the soil reinforcement and 

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facing blocks. Shear resisting devices between the facing blocks and soil reinforcement such 

as shear keys, pins, etc. should be used. For connections partially or fully dependent on 

friction between the facing blocks and the soil reinforcement, the nominal long-term 

connection strength Tac, should be reduced to 80 percent of its static value. Further, the 

blocks above the uppermost layer of soil reinforcement must be secured against toppling 

under all seismic events. 

4.4.8.b Design of Flexible Wall Facings 

Welded wire or similar facing panels should be designed in a manner which prevents the 

occurrence of excessive bulging as backfill behind the facing elements compresses due to 

compaction stresses, self weight of the backfill or lack of section modulus. Bulging at the 

face between soil reinforcement elements in both the horizontal and vertical direction 

generally should be limited to 1 to 2 in. (25 to 50 mm) as measured from the theoretical wall 

line. Specification requirements and design detailing to help achieve this tolerance might 

include limiting the face panel height to 18 in. (460 mm) or less, the placement of a nominal 

2 ft (0.6 m) wide zone of rockfill or cobbles directly behind the facing, decreasing the 

vertical and horizontal spacing between reinforcements, increasing the section modulus of 

the facing material, and/or by providing sufficient overlap between adjacent facing panels. 

Furthermore, the top of the flexible facing panel at the top of the wall should be attached to a 

soil reinforcement layer to provide stability to the top of the facing panel. 

Geosynthetic facing elements generally should not be left exposed to sunlight (specifically 

ultraviolet radiation) for permanent walls. If geosynthetic facing elements must be left 

exposed permanently to sunlight, the geosynthetic should be stabilized to be resistant to 

ultraviolet radiation. Furthermore, product specific test data should be provided which can 

be extrapolated to the intended design life and which proves that the product will be capable 

of performing as intended in an exposed environment. Alternately a protective facing should 

be constructed in addition (e.g., concrete, shotcrete, etc.). 

4.4.9 Step 9 – Assess Overall/Global Stability 

This design step is performed to check the overall, or global, stability of the wall. Overall 

stability is determined using rotational or wedge analyses, as appropriate, to examine 

potential failure planes passing behind and under the reinforced zone. Analyses can be 

performed using a classical slope stability analysis method with standard slope stability 

computer programs. In this step, the reinforced soil wall is considered analogous to a rigid 

body and only failure surfaces completely outside a reinforced zone (e.g., global failure 

planes) are considered. Computer programs that directly incorporate reinforcement elements 

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(e.g., ReSSA) can be used for analyses that investigate both global and compound failure 

planes. See Section 4.4.10 for failure planes that pass partially through the reinforced zone. 

Per Article 11.6.2.3 AASHTO (2007), the evaluation of overall stability of MSE walls should 

be investigated at the Service I Load Combination, and using an appropriate resistance factor. 

Commonly used slope stability programs can be used to conduct this evaluation. The load 

factor at Service I limit state is 1.0 for permanent loads. In lieu of better information, the soil 

shear resistance factor () is defined in Article 11.6.2.3 (AASHTO, 2007) as: 

 = 0.75; where the geotechnical parameters are well defined, and the slope does not 

support or contain a structural element; and

 = 0.65; where the geotechnical parameters are based on limited information, or the 

slope contains or supports a structural element 

The intent of the term “structural element” is that a resistance factor of 0.65 should be used 

for slope stability analysis if the slope/wall supports a bridge foundation, a building, or 

similar structure foundation that cannot tolerate significant movement or if the consequences 

of the failure of the supported structure are severe. A resistance factor of 0.75 may be more 

appropriate for slopes/walls that support structures such as a sign foundation where 

movements may not be detrimental or the consequences of the failure are not significant. 

The Agency/Owner should define whether the MSE wall structure itself is a classified as a 

significant “structural element” (i.e., consequences of failure are severe) and a resistance 

factor of 0.65 is applicable, or if it is a minor structure and a resistance factor of 0.75 is 

applicable. (Also note that a slope supporting a structural element should have well defined 

geotechnical parameters.) 

The codification of LRFD load and resistance factors by probabilistic calibrations for the 

design of slopes are currently being research and developed. Commercial slope stability 

analysis programs fully compatible with AASHTO LRFD procedures are not readily 

available. Therefore, designs today might be performed by traditional (non-LRFD) methods 

and with existing slope stability programs, and a comparison of computed safety factor to 

target resistance factor. 

 

The AASHTO (2007) stated resistance factors of 0.75 and 0.65 are (generally) approximately 

equivalent to the safety factors of 1.3 and 1.5, respectively , that is: 

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  0.75  1  1.3  FS and   0.65  1  1.5  FS 0.75 0.65 
 

 

Note that AASHTO resistance factors are stated to the nearest 0.05, so as to not overstate the 

level of accuracy of a resistance value. Therefore, if assessing global stability with limit 

equilibrium slope stability methods, the target safety factors are: 

FS = 1.30 where the geotechnical parameters are well defined; 

FS = 1.50 where the geotechnical parameters are based on limited information; and 

FS = 1.50 where the wall/slope contains or supports a structural element. 

 This is consistent with past practice, per FHWA NHI-00-043 (Elias et al., 2001). 

The evaluation of overall stability should be performed with reasonable estimates of shortand long-term water pressures (a geotechnical parameter) acting on the wall. If the 

evaluation of overall stability does not indicate a satisfactory result then the reinforcement 

length may have to be increased or the foundation soil may have to be improved. The design 

must be revised according to these changes. 

Most agencies typically perform global stability assessments for MSE walls. Global stability 

generally is assessed by the agency during feasibility design, which might result in ground 

improvement or other wall options, and again after the wall is designed. The MSE wall 

vendors/suppliers typically exclude overall stability check and responsibility in their package 

unless contract documents require such an evaluation by the wall vendor/supplier. 

4.4.10 Step 10 – Assess Compound Stability 

Additional slope stability analyses should be performed for MSE walls to investigate 

potential compound failure surfaces, i.e., failure planes that pass behind or under and through 

a portion of the reinforced soil zone as illustrated in Figure 4-18. For simple structures with 

rectangular geometry, relatively uniform reinforcement spacing, and a near vertical face, 

compound failures passing both through the unreinforced and reinforced zones will not 

generally be critical. However, if complex conditions exist such as changes in reinforced 

soil types or reinforcement lengths, high surcharge loads, seismic loading, sloping faced 

structures, significant slopes at the toe or above the wall, or stacked (tiered) structures, 

compound failures must be considered. 

This design step is performed to check potential compound failure planes passing through the 

reinforced soil zone. Compound stability is determined using rotational or wedge analyses, 

as appropriate, performed with computer programs that directly incorporate reinforcement 

elements (e.g., ReSSA) in the analyses. The reinforced soil wall is not considered a rigid 

body and is modeled with appropriate soil properties and the soil reinforcement layers as 

discrete elements. The long-term strength of each reinforcement layer intersected by the 

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failure surface should be considered as resisting forces in the limit equilibrium slope stability 

analysis. The facing system should be modeled with separate, but appropriate strength 

properties. 

Proposed Procedure: Current AASHTO (2007) states that compound stability should be 

investigated. However, procedures (i.e., load and resistance factors) are not specifically 

defined. The recommended procedure is to follow global stability procedures and include 

reinforcement strength. 

If assessing compound stability with limit equilibrium slope stability methods (e.g., modified 

Bishop, Spencer, etc.) a load factor of 1.0 should be used. Compound analyses should use 

the same AASHTO (2007) stated global stability resistance factors () of 0.75 and 0.65. 

These resistance factors are approximately equivalent to safety factors of 1.3 and 1.5, 

respectively, as previously noted. 

Therefore, if assessing compound stability with limit equilibrium slope stability methods, the 

target safety factors with limit equilibrium analysis are: 

FS = 1.30 where the geotechnical parameters are well defined; 

FS = 1.50 where the geotechnical parameters are based on limited information; and 

FS = 1.50 where the wall/slope contains or supports a structural element 

 This is consistent with past practice, per FHWA NHI-00-043 (Elias et al., 2001). 

Note, however, that the method of incorporating the soil reinforcement strength into the 

stability calculations does affect the magnitude of factor of safety computed. See Section 9.3 

for recommendations on how reinforcement strength should be incorporated. 

The evaluation of compound stability should be performed with reasonable estimates of 

short- and long-term water pressures. If the evaluation of compound stability does not 

indicate a satisfactory result then the reinforcement length, reinforcement strength, 

reinforcement vertical spacing, and/or depth of wall may have to be increased, or the 

foundation soil may have to be improved. The design must be revised according to these 

changes, and compound stability rechecked as appropriate. 

Compound stability analyses require detailed information on both the subsurface conditions 

(typically defined by the agency) and the soil reinforcement layout (typically vendor 

defined). Unlike global stability analyses, the responsibility for this analysis is not clearly 

defined. Agencies should perform an initial assessment of a proposed MSE wall structure 

with an assumed reinforcement layout to determine if compound stability is a concern and 

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must be addressed in final design. Typical geometries where compound stability is of 

concern are illustrated in Figure 4-18. Generally, MSE wall vendors/suppliers exclude 

compound stability check and responsibility in their package, unless specifically required by 

the Owner. 

Compound stability can be addressed by selecting one of the following three options for 

specifying and bidding the MSE wall (Schwanz et al., 1997): 

1. Agency Design. Agency prepares complete design for the MSE wall including external, 

internal, global, and compound stability analyses. This requires material specifications 

for all wall components. 

2. Vendor Design. Agency prepares line and grade plans, and allows approved vendors to 

supply the complete design and wall components. Agency is responsible for and must 

provide detailed subsurface profile(s), soil shear strength, soil unit weight, and 

groundwater information for the vendor to use in external, global, and compound stability 

analyses. Agency should perform a feasibility analysis to ensure global stability can be 

achieved with the line and grade provided to the vendors. 

3. Combined Design. Agency prepares line and grade plans, assesses global and compound 

stability requirements, and specifies/detail reinforcement requirements for adequate 

stability resistance. For example, the agency might specify two layers of reinforcement 

within a range of elevations (at bottom of wall) with minimum strength and minimum 

lengths required. Wall vendor completes wall design with incorporation of reinforcement 

required for adequate compound stability resistance. 

Applications of the above three options by the St. Paul District of the U.S. Army Corps of 

Engineers are summarized by Schwanz et al. (1997). Advantages and disadvantages with 

each option are discussed in the cited reference. 

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Figure 4-18.	 Typical geometries where MSE wall compound stability is of concern: steep 

and tall backslope on top of the wall; tiered walls; slope at the toe of the wall; 

and water at toe of the slope. 

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4.4.11 Step 11 – Wall Drainage Systems 

Drainage is a very important aspect in the design and specifying of MSE walls. The Agency 

should detail and specify drainage requirements for vendor designed walls. Furthermore, the 

Agency should coordinate the drainage design and detailing (e.g., outlets) within its own 

designers and with the vendor. The Agency is also responsible for long-term maintenance of 

drainage features, as discussed in Section 5.3.4. 

4.4.11.a Subsurface Drainage 

Subsurface drainage must be addressed in design. The primary component of an MSE wall is 

soil. Water has a profound effect on this primary component of soil, as it can both decrease 

the soil shear strength (i.e., resistance) and increase destabilizing forces (i.e., load). Thus, 

FHWA recommends drainage features be required in all walls unless the engineer determines 

such feature is, or features are, not required for a specific project or structure. 

Drainage design and detailing are addressed in Section 5.3. Note that MSE walls using free 

draining reinforced fill do not typically need a full drainage system, but do need a method for 

discharging water collected within the reinforced wall fill. Also note that MSE walls can be 

designed for water loads, if needed. Basic soil mechanics principles should be used to 

determine the effect of phreatic surface on wall loads. See discussion in Chapter 7 for design 

of MSE walls for flood and scour events. 

4.4.11.b Surface Water Runoff 

Surface drainage is an important aspect of ensuring wall performance and must be addressed 

during design and during construction. Appropriate drainage measures to prevent surface 

water from infiltrating into the wall fill should be included in the design of a MSE wall 

structure. Surface drainage design and detailing are addressed in Section 5.3. 

4.4.11.c Scour 

There are additional detailing considerations for walls that are exposed to potential scour. 

The wall embedment depth must be below the Agency predicted scour depth. Wall initiation 

and termination detailing should consider and be design to protect from scour. Riprap may 

be used to protect the base and ends of a wall. A coarse stone wall fill may desired to drain 

rapidly. The reinforced wall fill at the bottom of the structure may be wrapped with a 

geotextile filter to minimize loss of fill should scour exceed design predictions. These items 

are discussed in detail in Chapter 5. 

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4.5 TEMPORARY WALLS 

Temporary walls are normally considered wall structures with a 36 month or less service life 

(Article 11.5.1 {AASHTO, 2007}). The design method remains the same as for permanent 

walls, except for the calculation of the soil reinforcement long-term nominal strength, Tal. 

Metallic soil reinforcements are not normally galvanized for temporary walls. An exception 

might be when aggressive wall fill materials are being used and galvanization is specified to 

provide corrosion resistance. 

The long-term nominal strength for black steel (i.e., non-galvanized) in non-aggressive 

reinforced fill soil may be calculated with the whole steel cross-section for temporary walls. 

The long-term nominal strength for black steel (i.e., non-galvanized) and non-aggressive wall 

fill soil may be calculated with a corrosion rate of 1.1 mils/yr (28 m/yr) (FHWA NHI-09­

087 {Elias et al., 2009}). Higher corrosion rates need to be considered for reinforced fills 

that are moderately aggressive or corrosive, and a corrosion specialist should be consulted to 

assess the sacrificial steel requirements or other possible corrosion protection measures. 

Steel reinforcement should be galvanized if a service life greater than 36 months is required 

for a temporary structure. 

For geosynthetic soil reinforcements, the long-term nominal strength may be calculated with 

a minimum durability reduction factor of 1.0 in lieu of 1.1 minimum used for permanent 

walls. This is for temporary walls and for geosynthetics that meet the minimum 

requirements listed in Table 3-12. 

4.6 DESIGN CHECKLIST 

Agencies should have an established, or should establish a, protocol for checking designs. 

This is particularly important for vendor supplied designs, but should also be used with inhouse designs. The protocol should assign responsibilities for the review and list items that 

should be checked. Thus, the protocol can be in the form of a checklist. 

Based upon work by the Arizona Department of Transportation (ADOT), an example design 

checklist follows. This example may be used by agencies to develop their own checklist with 

their defined responsibilities and references to the agency’s standard specifications, standard 

provisions, etc. Some of the items on the following checklist are project specific, and others 

are project and wall structure specific. 

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MSE WALLS - DESIGN REVIEW 

EXAMPLE CHECKLIST 

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TO BE FILLED BY RESIDENT ENGINEER 

Project (Name, Contract No., etc.) 

Resident Engineer (RE) 

Date MSE submittal received 

Is this a re-submittal? If yes, attach previous checklist 

Name of Engineer of Record (ER) 

Date submittal transmitted to ER 

Date comments due back to RE 

 REVIEWED BY

Materials Date Date TO BE FILLED BY ER Group Due Name Organization Received Reviewed Date** 

Professional Engineer 

of Record*(ER) 

Date completed checklist sent to RE 

* Contact designated agency Design Engineer immediately upon receipt of the 

submittal(s) from RE. 

** Due date for submittal to Design Engineer. 

This checklist has been completed 

under the supervision of the 

Professional Engineer of Record 

whose seal and signature appears 

hereon. 
 

 

 

 

LEGEND FOR ABBREVIATIONS / ACRONYMS 

AASHTO American Association of State Highway and Transportation Officials 

DE Agency Design Engineer from assigned to the project. 

APL Approved Products List (For latest APL visit______________). 

ER Engineer of Record (Registered Professional Engineer in the state) 

FHWA Federal Highway Administration 

MBW Modular Block Wall 

MSE Mechanically Stabilized Earth 

MSEW 3.0 Version 3.0 of proprietary software, MSEW, by ADAMA Engineering (visit 

www.geoprograms.com) 

NA Not Applicable 

NHI National Highway Institute 

PE Professional Engineer of Record (shall be a Registered Professional 

Engineer) 

PGR Project Geotechnical Report 

Project 

Drawings 

Complete final plan set for the project 

RE Resident Engineer 

Section #, 

Figure # or 

Table # 

This refers to an appropriate section, figure or table in the following manual 

by FHWA/NHI: 

“Design and Construction of Mechanically Stabilized Earth Walls and 

Reinforced Soil Slopes,” Publication No. FHWA NHI-10-024 Vol I and 

NHI-10-025 Vol II, (Authors: Ryan R. Berg, Barry R. Christopher and 

Naresh C. Samtani) 

Spec Project specification including standard specification and special provisions 

_______ ______ Department of Transportation 

Vendor 

Drawings 

Working drawings provided by MSE wall vendor 

All symbols used within the questions are consistent with those used in the documents 

in the “Reference” column 

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NOTES FOR CHECKLIST 

1. The following information/material should be collected before starting the checklist: 

a. Contractor submittals (transmittal letter, design drawings, design calculations) 

b. Project documents (final plan set, standard specifications, special provisions, 

Project Geotechnical Report) 

c. FHWA/NHI manual (“Mechanically Stabilized Earth Walls and Reinforced Soil 

Slopes,” Publication Nos. FHWA NHI-10-024 Vol I and NHI-10-025 Vol II, 

December 2009; Authors: Ryan R. Berg, Barry R. Christopher and Naresh C. 

Samtani) 

d. Latest version of AASHTO LRFD Bridge Design Specifications, including 

interims 

e. All due dates for checklist 

f. Name of the structural engineer 

g. Name of the roadway engineer 

h. Name of “prime” designer 

2. Each question must have a “Yes”, “No” or “NA” box checked. Any comment or 

action required should be entered in the “Comments/Action Required” column. If the 

“No” or “NA” box is checked then an appropriate comment or action required must 

be entered. Use separate sheets if comments cannot be fitted in the space within the 

checklist. 

3. The documents listed under the “Reference” column in the checklist are not intended 

to be a complete list of documents. Rather, the most common documents are listed 

where guidance/information related to the question in the checklist may be found. 

More stringent criteria may exist in other project documents (e.g., drainage, signage, 

utilities, etc.) that may be relevant to a given question. In such an event, the 

governing document should be noted in the “Comments/Action Required” column of 

the checklist. 

4. Add any pertinent project specific questions to the checklist as necessary. Two empty 

rows are provided at end of each section for this purpose. Use additional sheets as 

necessary if more space is required. 

5. This checklist is intended to be completed, signed, and sealed by the Engineer of 

Record who is Registered Professional Engineer in the state. 

6. The Engineer of Record should contact the project structural or roadway engineer in 

case of discrepancies between the contractor submittals and reference documents. 

7. Wall details that were reviewed and approved as part of the “Approved Products List” 

will be available on a website; contact the _______ for further information. 

8. After completing the checklist the ER should include an attachment that identifies 

specific questions that the MSE wall vendor (or in-house designer) has to address. 

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Reference 

(See Note 3) 

Yes No NA Comments/ 

Action 

Required 

I. GENERAL INFORMATION 

1. Is the wall vendor pre-approved? 

(visit _____for a list of pre-approved 

wall systems) 

APL    

2. Is the wall within the limitations of the 

pre-approved product? (e.g., wall height, 

external loading, environmental 

constraints, seismic loading and other 

project specific constraints; visit 

_________________ for limitations) 

APL    

3. Has the Contractor used the correct 

design survey data (e.g., existing ground 

elevations and horizontal offsets) for wall 

design? 

Project/vendor 

drawings 

   

4. Has the Contractor correctly reflected the 

location of utilities in the area of the 

wall(s)? 

Project/vendor 

Drawings 

   

5. Is the wall profile (top and bottom 

elevations) including start and end 

stations correct? 

Project/vendor 

Drawings 

   

6. Is the wall design life specified? Spec/ 

Section 2.8 

   

7. Have the following items been specified 

by the vendor and are they in 

conformance with the project 

requirements? 

a. Material requirements 

i. Soil Properties (strength, gradation, 

 PI, soundness, electrochemical) 

Spec   

 ii. Soil Reinforcement (ultimate and 

yield tensile strengths, reduction 

 factors for geosynthetics) 

Spec   

 iii. Concrete (strength and other 

 properties) 

Spec/Project 

Drawings 

  

 iv. Concrete reinforcement (type, 

 number and strength) 

Spec/Project 

Drawings 

  

 v. Leveling Pad (strength) Spec/Project 

Drawings 

  

 vi. Steel facing elements for wire mesh

 systems (ultimate and yield tensile 

strengths) 

Spec    

b. Construction procedures including 

sequence 

APL    

c. Soil compaction procedures and 

restrictions for reinforced fill, retained 

fill and foundation preparation 

APL/spec/ 

PGR 

   

d. Facing alignment tolerances Spec    

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Reference 

(See Note 3) 

Yes No NA Comments/ 

Action 

Required 

e. Acceptance/rejection criteria 

(tolerances, facing finish, etc.) 

Spec    

f. Corrosion protection systems for soil 

reinforcement 

Spec    

g. Handling and storage of 

reinforcements 

Spec/APL/ 

PGR 

   

8. Is the initial wall batter during 

construction specified? 

APL    

9. Are the structural (select) backfill 

dimensions shown? 

Spec    

10. Are the wall quantities (area of wall, 

volume of structural fill, etc.) listed in 

accordance with the pay quantity 

schedule in the project specifications? 

Spec    

11. Wall installation guide    

a. Has the proprietary vendor submitted 

a wall installation guide? 

APL    

b. Does the submitted wall installation 

guide address site-specific conditions? 

PGR/Spec    

12. Is the Contractor’s transmittal letter 

acceptable? (e.g., does it contain 

acceptable statements consistent with the 

submittal?) 

   

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

II. TOP OF WALL 

1. Do the top of wall elevations match the 

roadway design elevations? 

Project 

drawings 

   

2. Are top of wall elevations such that they 

can allow for proper interfacing with 

barriers, copings, surface ditches, bridge 

abutments, etc. as shown on the plans? 

Project 

drawings 

   

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

III. LEVELING PAD (Note: Only lean 

concrete leveling pads are allowed) 

1. Are the leveling pad dimensions shown? Spec    

2. Does the leveling pad profile satisfy the 

minimum depth of embedment criteria? 

Section 2.8/ 

PGR/ Project 

Drawings 

   

3. Are the leveling pad elevations such that 

they allow for transverse and longitudinal 

Project 

drawings 

   

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Reference 

(See Note 3) 

Yes No NA Comments/ 

Action 

Required 

drainage structures shown on the plans? 

4. Are leveling pad steps such that they can 

accommodate the bottom row facing unit 

type and size without cutting and/or 

splicing of the facing units? 

APL/vendor 

drawings 

   

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

IV. FACING UNITS AND JOINTS 

1. Are the facing units from the preapproved list? APL    

2. Do facing units meet the project aesthetic 

criteria? 

Spec/Project 

Drawings 

   

3. Have the material properties of the facing 

units been specified? (Examples: 

density, strength, freeze-thaw, etc.) 

Section 

4.4.8/Spec 

   

4. Are the materials properties of the facing 

units in conformance with the project 

criteria? (Examples: density, strength, 

freeze-thaw, etc.) 

Section 

4.4.8/Spec 

5. Are the facing units structurally adequate 

as per the project facing unit structural 

criteria and/or per AASHTO? 

(deformation of facing elements 

including local bending should be within 

allowable limits) 

Section 

4.4.8/Spec 

   

6. Is the horizontal joint width between 

facing units in conformance with project 

criteria? 

Section 2.8, 

Table 2-1 

   

7. Does the joint bearing pad material 

conform to project specifications? 

Spec    

8. Is the joint bearing pad material of proper 

compressive strength such that facing 

unit to facing unit crushing and /or high 

stress concentrations on any facing units 

are prevented? 

Spec/APL    

9. For Modular Block Wall (MBW) units 

with geosynthetic soil reinforcement has 

the hinge height concept been used for 

establishing connection details? 

Section 4.4.7    

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

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Reference 

(See Note 3) 

Yes No NA Comments/ 

Action 

Required 

V. DRAINAGE 

1. Are all vertical and horizontal joints 

covered with geotextile fabric on the 

backside of the wall facing units? 

Spec    

2. Is the geotextile fabric covering the joints 

of sufficient width and continuous across 

the joints? 

Spec    

3. Do the geotextile fabric properties 

(survivability, filtration and permittivity) 

covering the joints meet project 

specifications? 

Spec    

4. Has drainage along the backcut been 

included as per project criteria? 

PGR/Spec    

5. If geocomposite is used for drainage, 

then is it pre-approved and do its 

properties (flow capacity, filtration and 

permeability) meet project requirements? 

PGR/APL/ 

Spec 

   

6. Is the water from subsurface drainage 

adequately led out of the wall system? 

e.g., collector and drain system with 

weepholes, grades towards wall ends, etc. 

PGR/Spec    

7. Is surface drainage in accordance with 

project criteria? 

Project 

Drawings 

   

8. If Modular Block Wall (MBW) units are 

used for facing then has adequate drain 

fill been provided? 

Section 

5.35/Figure 

5.6/Spec 

   

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

VI. SPECIAL WALL DETAILS 

1. Are wall interface details with other 

walls that will be constructed before, 

during, or after this contract shown? 

Spec/ 

Section5.5 

   

2. Are following special wall details shown 

and are they adequate?: 

a. special facing element if interfacing 

with other wall systems 

Spec/APL/ 

Section 5.5    

b. slip joint(s) (e.g., at wing walls, 

differential settlement concerns, etc.) Spec/APL    

c. wall end(s) Spec/APL    

d. connection to appurtenances (e.g., box 

inlets and large obstructions) 

Spec/APL/ 

Section 5.5    

e. acute angles Spec/APL/ 

PGR    

f. coping Spec/APL/ 

Section 4.5    

FHWA NHI-10-024 4 – Design of MSE Walls
 

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Reference 

(See Note 3) 

Yes No NA Comments/ 

Action 

Required 

g. railing, guard rails or traffic barriers Spec/APL/ 

Section 5.1    

h. miscellaneous obstructions (e.g., 

utilities) below ground elevation 

Spec/APL/ 

Section 4.55.4    

i. measures to prevent migration of deicing salts in the reinforced fill Spec/APL/ 

Section 5.3    

j. measures to protect against rapid 

drawdown conditions and hydrostatic 

pressures 

Spec/APL/ 

Section 4.55.3    

3. Are structural frames (“yokes”) provided 

to navigate the bar mat soil 

reinforcements around vertical 

obstructions within the MSE backfill? 

(examples of vertical obstructions 

include piles, shafts, inlet structures, etc.) 

Spec/APL/ 

Section 4.55.4 

   

4. Are the structural frames designed 

properly so that moments and torques are 

not introduced in the bar mat soil 

reinforcements and/or the 

reinforcement/facing unit connection? 

APL/ 

Bridge 

Group 

   

5. Is the splay of strip reinforcements 

limited to less than 15 degrees? 

Spec    

6. If strip reinforcements are splayed, then 

is the length increased to compensate for 

reduction in effective length? 

PGR/Spec    

7. Is the maximum vertical bend (maximum 

15 degrees) in metallic soil 

reinforcements within acceptable limits? 

Spec/ 

Section 5.4 

   

8. Are geosynthetic reinforcement details 

around vertical obstructions acceptable? 

APL    

9. Are overlapping reinforcements 

separated vertically by at least 3-in. of 

soil? 

Spec    

10. If walls are tiered, then are they in 

accordance with project criteria?, e.g., 

bench widths, aesthetics within benches, 

etc. 

Spec/ 

Section 6.2 

   

11. If instrumentation is required per project 

specs, then is it provided? (List the 

instrumentation in the comments column) 

PGR/ 

Spec 

   

12. Are corrosion/durability protection 

details acceptable? 

Spec/ 

Section 3.5 

   

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

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Reference 

(See Note 3) 

Yes No NA Comments/ 

Action 

Required 

VII. SOIL REINFORCEMENT 

1. Is the soil reinforcement type (extensible 

or inextensible) and configuration (strip, 

grid or sheet) in conformance with preapproved list? 

APL    

2. Are the following soil reinforcement 

dimensions in conformance with those 

approved by the Agency during the preapproval process? 

APL 

a. strip thickness or bar diameter APL    

b. strip width or bar mat width APL    

c. center to center spacing of the 

longitudinal bars in bar mats 

APL    

d. center to center spacing of the 

transverse bars in bar mats 

APL    

e. Geosynthetic grid (uniaxial/biaxial) 

openings and junction sizes 

APL    

3. Is the connection of the soil 

reinforcement to the facing units as per 

the pre-approved connection detail? 

APL    

4. Is the soil reinforcement specified to 

have the correct type and thickness of the 

corrosion protection as per the project 

specifications? 

Spec/ 

Section 3.5 

   

5. Is all soil reinforcement, except at acute 

angle corners, perpendicular to the face 

of the wall facing units? If no, please 

comment. 

Spec/ 

PGR 

   

6. Is all soil reinforcement connected to 

facing units? 

Spec    

7. If metallic soil reinforcements are cut 

and/or spliced then have the corrosion 

protection measures at cuts/connections 

been provided and are they acceptable? 

(Note: cutting transverse bars of bar mats 

is not allowed) 

Spec/ 

APL 

   

8. Are means and methods for splicing of 

geosynthetic reinforcement (overlap, 

mechanical connections, edge seams, 

etc.) in accordance with that approved by 

the Agency during the pre-approval 

process? 

APL    

9. Are placement procedures for 

reinforcement acceptable? 

APL    

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

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Reference 

(See Note 3) 

Yes No NA Comments/ 

Action 

Required 

VIII. EXTERNAL STABILITY 

1. Have all assumed soil parameters 

(Cohesion, Angle of Internal Friction, 

Soil Unit Weight, and Sliding Friction 

Coefficient) for retained, reinforced and 

foundation soils been listed? 

PGR/Spec/ 

Section 3.3, 

4.4.6 

   

2. Are soil parameters consistent with those 

recommended in the geotechnical report / 

project specifications? 

PGR/Spec    

3. Have the maximum bearing pressures 

been listed along the length of the wall? 

Vendor 

drawings 

   

4. Have all the loads been incorporated into 

the wall analysis and design? (e.g., traffic 

loads, seismic loads, sloping surcharge, 

broken-back surcharges, etc.) 

PGR/ 

Section 4.4.4 

   

5. Have all the critical sections along all 

walls been analyzed? (e.g., highest wall 

sections, sections where slopes above and 

below the walls are steepest, etc.) 

Project 

Drawings/ 

PGR 

   

6. Are the static and seismic analyses 

adequate (as per performance 

requirements) for the following failure 

modes? 

Spec/Section 

4.4.6, 7.1.1 

  

 a. Sliding Spec/Section 

4.4.6.a 

  

 b. Eccentricity (overturning) Spec/Section 

4.4.6.c 

  

 c. Bearing Spec/Section 

4.4.6.c 

i. General bearing capacity Spec/Section 

4.4.6.c 

  

 ii. Local bearing capacity / lateral 

squeeze 

Spec/Section 

4.4.6.c 

   

iii. Is the bearing resistance greater 

 than the maximum bearing pressure 

 at all locations along the wall? 

PGR    

7. Is the wall embedment equal to or greater 

than the project requirements? 

PGR    

8. Has total settlement analysis been 

performed? 

PGR    

9. Has differential settlement analysis been 

performed? 

PGR    

10. Have slip joints been provided to prevent 

stresses due to large anticipated 

differential settlements? 

PGR/APL/ 

Section 5.4.5 

   

11. Is an undercut needed due to soft or poor 

soils? If so, is the depth of treatment and 

the replacement material specified? 

PGR/ 

Spec 

   

FHWA NHI-10-024 4 – Design of MSE Walls
 

MSE Walls and RSS – Vol I 4 – 76 November 2009
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Reference 

(See Note 3) 

Yes No NA Comments/ 

Action 

Required 

12. Will deep foundations be needed for very 

deep layers of soft/loose soils? 

PGR/ 

Spec 

   

13. Will waiting period(s) and stage 

construction be needed if the design wall 

pressure exceeds the maximum allowable 

bearing pressure? 

PGR/ 

Spec 

   

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

IX. INTERNAL STABILITY 

1. Have calculations for internal stability of 

the wall been performed? 

PGR/ 

Spec 

   

2. Has the static and seismic internal 

stability evaluation been performed by 

the “Simplified Method”? 

PGR/Spec/ 

Section 7.1 

   

3. Have all the critical sections along all 

walls been analyzed? (e.g., highest wall 

sections, sections where slopes above and 

below the walls are steepest, etc.) 

Project 

Drawings/ 

PGR 

   

4. Is pullout resistance adequate at each 

level of the reinforcement? 

PGR/Spec/ 

Section .4.7.h 

   

5. Is the correct value of nominal strength 

of steel used? (e.g., 0.55 Fy for strips and 

0.48Fy for bar mats) 

PGR/Spec/ 

Section 3.5 

   

6. Are corrosion loss rates in conformance 

with project criteria? 

PGR/Spec/ 

Section 3.5 

   

7. Has the cross-sectional area for the soil 

reinforcement been corrected for 

corrosion losses over the design life of 

the structure? 

PGR/Spec/ 

Section 3.5 

   

8. Is resistance against tensile failure 

adequate at each level of reinforcement? 

PGR/Spec/ 

ection 4.4.7.f 

   

9. Are the connections designed for 

maximum tension in soil reinforcements? 

Spec/Section 

4.4.7.i 

   

10. Have the proper values of F* (including 

Cu, Fq, , tan and variation with depth) 

been used? 

Section 3.4, 

4.4.7.h 

   

11. Is the correct value for the scale 

correction factor, , been used? 

Section 3.4, 

4.4.7.h 

   

12. Is the correct value of unit perimeter, C, 

used? 

Section 3.4, 

4.4.7.h 

   

13. For geosynthetic reinforcement have the 

reduction factors for creep (RFCR), 

durability (RFD) and installation damage 

(RFID) been specified and are they 

acceptable? 

Section 3.5/ 

Spec 

   

FHWA NHI-10-024 4 – Design of MSE Walls
 

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Reference 

(See Note 3) 

Yes No NA Comments/ 

Action 

Required 

14. For geosynthetic reinforcement is the 

computation of long-term allowable 

strength acceptable? 

Section 3.5/ 

Spec 

   

15. Have the correct stress ratio (Kr/Ka) and 

lateral pressure coefficient (Ka) been 

used for computing internal loads? 

Section 

4.4.7.c, 

Figure 4-10 

   

16. Has the correct internal failure surface 

been used for static and seismic cases? 

Section 

4.4.7.b 

   

17. Has the vertical stress been computed as 

per the requirements of the Simplified 

Method? 

Section 

4.4.7.e 

18. Are the definitions of the reinforcement 

configuration (grid openings, ratios of the 

bar diameters to spacing of bars in bar 

mats, etc.) consistent with pre-approved 

product list? 

APL/Section 

3.4 

   

19. Have all the external loads been 

incorporated into the wall analysis and 

design? (e.g., traffic impact loads, 

seismic loads, sloping surcharge, brokenback surcharges, etc.) 

Section 

4.4.5, 7.1.1 

   

20. Have all the internal loads been 

incorporated into the wall analysis and 

design? (e.g., lateral loads from piles at 

abutments or overhead mast structures) 

PGR/Spec/ 

Section 

4.4.7, 6.1 

   

21. Has the internal stability evaluation 

accounted for complex geometries such 

as tiered structures, acute corners, backto-back walls, and obstructions? 

PGR/Spec/ 

Section 

6.1 – 6.6 

   

22. Is the vendor’s analysis acceptable to the 

Geotechnical Engineer of Record based 

on an independent verification using 

“Simplified Method” and MSEW 3.0 or 

hand calculations? Please attach a copy 

of the verification calculations using the 

Simplified Method. 

GER/ 

PGR 

   

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

X. GLOBAL / COMPOUND 

STABILITY 

1. Has the owner’s geotechnical engineer of 

record checked global stability? 

PGR    

2. Has the vendor checked compound 

stability? 

PGR/Spec/ 

Section 4.4.10 

   

3. Has the vendor checked the global 

stability? 

PGR/Spec    

FHWA NHI-10-024 4 – Design of MSE Walls
 

MSE Walls and RSS – Vol I 4 – 78 November 2009
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Reference 

(See Note 3) 

Yes No NA Comments/ 

Action 

Required 

4. Is the safety factor against global stability 

failure adequate? 

PGR/Spec    

5. Is the safety factor against compound 

stability failure adequate? 

PGR/Spec    

6. Are the geotechnical parameters for 

global and compound stability analyses 

appropriate and consistent with those 

used for other failure modes? 

PGR/Spec    

7. Is ground improvement needed based on 

global stability analysis? 

PGR    

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

XI. FILE INFORMATION 

1. Has the Geotechnical Engineer of Record 

completed this checklist? If not, who? 

   

2. Has a representative from agency’s ____ 

Group ensured that this checklist has 

been completed and outstanding issues 

identified? 

   

{add as appropriate, may be agency or 

project specific} 

   

{add as appropriate, may be agency or 

project specific} 

   

LIST OF ATTACHMENTS BY ENGINEER OF RECORD 

No. Attachment Comments/Action Required 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

As a minimum the ER should include an attachment that identifies the specific issues that 

need to be addressed by the MSE wall designer (vendor). 

FHWA NHI-10-024 4 – Design of MSE Walls 

MSE Walls and RSS – Vol I 4 – 79 November 2009 
 

 

4.7 COMPUTER-AIDED DESIGN 

The repetitive nature of the computations required at each level of reinforcement lends itself 

to computer-assisted design. The computer program MSEW (ADAMA, 2000) developed 

under FHWA sponsorship analyzes and/or designs MSE walls using any type of metallic or 

geosynthetic reinforcement in conjunction with any type of facing (precast concrete, MBW, 

etc.). Version 1.0 has been designated exclusively for use by U.S. State Highway Agencies 

and by U.S. Federal agencies and performs computations in compliance with the ASD design 

methods in FHWA (Elias et al., 2001) and AASHTO (2002). Version 3.0 is available for 

purchase through ADAMA Engineering (www.MSEW.com) and includes LRFD-based 

computations. Alternatively, spreadsheet based solutions can be developed. The example 

problems in Appendix E, provide comprehensive step-by-step solutions that can be easily 

programmed into a spreadsheet. 

Other MSE wall analysis and design programs are available. Many wall vendors have their 

own programs that are tailored to their system, and may have additional features for 

estimating quantities and costs. Agency personnel should understand the features and finer 

points of the computer program and spreadsheets that they use to design or check vendor 

designs. Likewise, wall vendors and design consultants should understand the features and 

finer points of computer programs and spreadsheets they use. This is particularly important 

with the recent change to an LRFD design platform. 

4.8 VENDOR DESIGNS 

As previously discussed, it is recommended that Agencies use a pre-approved proprietary 

wall system list (an approved products list) for specifying MSE walls with a performance or 

end-result approach. Specific wall systems and respective vendors, along with any 

application restrictions (e.g., height limit), are provided on the list. Detailed evaluations are 

typically required for placement on an approved products list. The design program and 

spreadsheets used by the vendor should be reviewed by the Agency as part of this evaluation. 

4.9 STANDARD MSEW DESIGNS 

MSEW structures are customarily designed on a project-specific basis. Most agencies use a 

line-and-grade contracting approach, with the contractor selected MSEW vendor providing 

the detailed design after contract bid and award. This approach works well for segmental and 

full-height panel faced walls, and can be used for MBW unit faced walls. However, standard 

FHWA NHI-10-024 4 – Design of MSE Walls
 

MSE Walls and RSS – Vol I 4 – 80 November 2009
 
 

 

designs can be developed and implemented by an agency for MSEW structures, somewhat 

similar to standard concrete cantilever wall designs used by many agencies. 

Use of standard designs for MSEW structures could offer the following advantages over a 

line-and-grade approach: 

 Agency is more responsible for design details and integrating wall design with other 

components. 

 Pre evaluation and approval of materials and material combinations, as opposed to 

evaluating contractor submittal post bid. 

 Economy of agency design versus vendor design/stamping of small walls. 

 Agency makes design decisions versus vendors making design decisions. 

 More equitable bid environment as agency is responsible for design details, and vendors 

are not making varying assumptions. 

 Reduces the possibility of substandard work, systems and designs with associated 

approved product lists. 

The Minnesota Department of Transportation (Mn/DOT), with support of the FHWA (via 

Demo 82 project) developed and implemented standardized MSEW designs (Berg, 2000) for 

MBW unit faced and geosynthetic reinforced MSEW structures. The use of these standard 

designs are limited by geometric, subsurface and economic constraints. Structures outside of 

these constraints should be designed on a project-specific basis. The general approach used 

in developing these standards could be followed by other agencies to develop their own, 

agency-specific standard designs. 

Standardized designs require generic designs and generic materials. Generic designs require 

definition of wall geometry and surcharge loads, soil reinforcement strength, structure height 

limit, and MBW unit properties of width and batter. As an example, the Mn/DOT standard 

designs address four geometric and surcharge loading cases, and could be used for walls up 

to 23 ft (7 m) in height. Since original development the number of cases has been reduced to 

three and the maximum height has been reduced to 12 ft (3.6 m) due to MBW durability 

concerns (see Section 3.6.2 for discussion on MBW freeze-thaw durability). 

Definition of generic material properties for the standard designs requires the development of 

approved product lists for MBW units, soil reinforcement and MBW unit-soil reinforcement 

combinations. The combinations require a separate approved product list as the connection 

strength is specific to each unique combination of MBW unit and reinforcement, and often 

controls the reinforcement design strength. An additional requirement for MBW units is an 

approved manufacturing quality control plan on file with the agency. This requirement is a 

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result of the stringent durability (to freeze thaw and deicing salt conditions) specifications for 

the units and the long duration testing used to demonstrate durability. 

An example design cross section and reinforcement layout table from the Mn/DOT standard 

designs are presented in Figure 4-19 and Table 4-8. A list of approved combinations 

(Mn/DOT, 2009) of MBW units and soil reinforcements with classification as MBW-700, 

MBW-1050, or MBW-1400, is used in conjunction with the table and figure. Note that the 

Mn/DOT standard designs are not directly applicable to, nor should they be used by, other 

agencies. 

Another example of an agency standard design is Washington DOT’s geosynthetic walls. 

Standard designs for two stage construction of walls up to 35 ft (11 m) are provided. The 

geosynthetic wrap-around walls are constructed in the first stage. The walls can be faced 

with shotcrete or cast-in-place concrete in the second stage. 

Table 4-8. Example MBW Faced MSE Wall Standard Design (Minnesota DOT, 2008). 

MODULAR BLOCK WALL REINFORCEMENT LAYOUT 

— CASE 4 - 1:3 FILL SLOPE — 

MBW 

Reinforcement 

Class 

Strength of Soil 

Reinf. (plf) Minimum 

Reinforcement 

Length, L 

(ft) 

Maximum 

Wall 

Height 

(ft) 

Nominal 

Block 

Width 

(in.) 

Wall Batter 

Range 

(degrees) 

Maximum 

Unreinforced 

Wall Height 

(in.) 

Zone 1 Zone 2 Zone 3 

L. Term 

(Tal) 

Design 

(Ta) > < H1 

(ft) 

S1max 

(in.) 

H2 

(ft) 

S1max 

(in.) 

H3 

(ft) 

S1max 

(in.) 

MBW-700 1050 700 0.7 H 

12.0 12 

0 3 24 8.5 24 3.5 16 

3 7 24 9.2 24 2.8 16 

7 10 24 11.2 24 0.8 16 

10 15 24 12.0 24 

21 

0 3 42 4.6 32 3.9 24 3.5 16 

3 7 42 5.2 32 3.9 24 2.9 16 

7 10 42 5.2 32 5.9 24 0.9 16 

10 15 42 5.9 32 6.1 24 

MBW-1050 1575 1050 0.7 H 12 

12.0 

0 3 24 12.0 24 

3 7 24 12.0 24 

7 10 24 12.0 24 

10 15 24 12.0 24 

21 

0 3 42 5.6 42 3.3 32 3.1 24 

3 7 42 8.2 42 2.6 32 1.2 24 

7 10 42 8.5 42 3.5 32 

10 15 42 9.8 42 2.2 32 

MBW-1400 2100 1400 0.7 H 12 

12.0 

0 3 24 12.0 24 

3 7 24 12.0 24 

7 10 24 12.0 24 

10 15 24 12.0 24 

21 

0 3 42 8.9 42 3.1 32 

3 7 42 10.8 42 1.2 32 

7 10 42 12.0 42 

10 15 42 12.0 42 

FHWA NHI-10-024 4 – Design of MSE Walls
 

MSE Walls and RSS – Vol I 4 – 82 November 2009
 
 

 

Figure 4-19. Example of standard MSEW design. (Minnesota DOT, 2008) 

FHWA NHI-10-024 4 – Design of MSE Walls 

MSE Walls and RSS – Vol I 4 – 83 November 2009 
 

 

FHWA NHI-10-024 4 – Design of MSE Walls
 

MSE Walls and RSS – Vol I 4 – 84 November 2009
 
 

CHAPTER 5 

MSE WALL DETAILS
 

Proper attention to details of various components is critical to the successful implementation 

of MSE wall projects. This Chapter discusses various details related to the following 

elements of MSE walls: 

 Top of wall elements such as copings, traffic barriers and geomembrane caps 

 Bottom of wall elements such as leveling pads 

 Drainage features such as filters, drains and pipes 

 Internal elements such as obstructions in reinforced soil mass and slip joints 

 Wall initiations and terminations 

 Aesthetics 

The example details shown in this chapter have been used successfully in actual 

projects. However, these details may need modifications to fit the requirements of 

specific projects. Therefore, the user should treat the details in this chapter as initial 

guidance and modify them as appropriate before actual implementation on a given 

project and for a given product. 

5.1 TOP OF WALL ELEMENTS 

The top of wall is important from both aesthetic as well as technical aspects. Aesthetically, 

the top of a MSE wall should provide a smooth profile. Technically, the top of wall needs to 

integrate roadway elements such as pavements, traffic barriers, and drainage features. 

5.1.1 Copings 

The purpose of a coping is to create a smooth and aesthetically pleasing clean line at the top 

of an MSE wall. Copings can be cast-in-place or precast. Precast coping can generally be 

installed more rapidly than cast-in-place coping. Figures 5-1a to 5-1c provide common 

details for cast-in-place and precast copings for segmental precast concrete facing units. 

Figure 5-1d shows a common detail for a precast cap unit on top of modular block wall. 

FHWA NHI-10-024 5 – MSE Wall Design Details
 

MSE Walls and RSS – Vol I 5 – 1 November 2009
 
 

 

(a) 

(b) (c)

(d) 

Figure 5-1. Example copings for MSE walls. 

FHWA NHI-10-024 

MSE Walls and RSS – Vol I 5 – 2 

5 – MSE Wall Design Details 

November 2009 
 

Regardless of whether the coping is cast-in-place or precast, the coping should have fulldepth open joints, i.e., reinforcement should not be continuous across the joints. General 

guidance for copings is as follows: 

	 For segmental precast concrete facing units, the joints for a cast-in-place coping should 

align with the vertical joints in the MSE wall face at a frequency not exceeding 10 ft (3 

m) on centers with a preferable frequency of every panel width. The spacing of the joints 

may be increased to 20 ft (6 m) if differential settlement is not a concern. 

	 For modular block facing units, the joints in a cast-in-place coping should line up with 

the vertical joints in the face at a frequency not exceeding 5 ft (1.5 m) or less as required 

to line up with block joints. The spacing of the joints may be increased to 10 ft (3 m) if 

differential settlement is not a concern. Precast cap units should have a width equal to 

the width of a modular block unit and should be attached to the top modular facing unit 

using a mortar and pin connection. Adhesives should not be used for permanent 

structures unless the agency is prepared to perform continuous maintenance to check and 

reattach the cap blocks as necessary. Warranties for separation and displacement of 

glued cap blocks are, at best, usually on the order of ten years and that assumes that 

blocks meet the required installation conditions of the adhesive manufacturer, e.g., the 

blocks are clean, dry and bonded at the required curing temperature. 

The cast-in-place coping can provide a smooth finish and be adjusted to meet final top of 

wall elevations after settlements have occurred. Cast-in-place copings are also recommended 

in situations where a wall follows a horizontal or vertical curve with less than a 100 ft (30 m) 

curve. Since precast coping sections are cast with square ends, joints between coping 

sections as seen from the front of the wall may become too tight or too wide depending on 

whether the radius point is in front or behind the wall face, respectively. Custom fitted castin-place coping should be used at kink points and corners in the wall and at slip joints so that 

the in-plane movement on each side of the slip joint can be tolerated without compromising 

the purpose of the slip joint. 

Before installing precast coping, the top of the wall must be smooth and free of steps or 

irregularities. To accomplish this, level-up concrete is cast on top of the facing units. The 

smooth finished grade of this concrete fill should follow a line approximately 9 in. (225 mm) 

below the top of coping elevation. Top facing units that are to receive precast coping may 

have protruding dowels that tie in to the level-up concrete. The dowels are field trimmed 1 to 

2 in. (25 to 50 mm) below the top of level-up concrete before pouring the level-up concrete. 

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5.1.2 Traffic Barriers 

Figure 5-2 presents a variety of traffic barrier configurations. Typically, the base (or 

moment) slab length is a minimum of 20 ft (6 m) and jointed to adjacent slabs with shear 

dowels. The width typically varies from 4 ft (1.2m) to 6 ft (1.8m). The actual designs of 

traffic barriers should be in accordance with AASHTO (2007). In all cases, the base slab 

must be sized to prevent overturning and sliding of the barrier system during impact. When 

the base slab extends over the tops of the facing units to form a coping, a recess into which 

the facing units fit must be designed in the underside of the slab and a positive bond breaker 

must be provided to ensure isolation of the barrier from the facing units. Both vertical and 

horizontal bond breaks are required to avoid direct impact loads on the facing unit and to 

prevent prying loads on the top panels during traffic loading. If a precast coping or precast 

traffic barrier is used, the top of the wall must be smooth and free of steps or irregularities. 

To accomplish this, level-up concrete fill is cast on top of the facing units (similar to that for 

coping). 

5.1.3 Parapets 

Where only pedestrian or bicycle loads are anticipated, the safety railing may be in the form 

of a concrete pedestrian parapet. A parapet is a cast-in-place or precast concrete rail located 

directly or nearly on top of the facing units. Although not designed for vehicular impact 

loading parapets do use a moment slab for stability. The moment slab may also serve as a 

sidewalk. The moment slab should be strong enough to resist the nominal (ultimate) strength 

of the pedestrian parapet. Where there is a possibility of vehicular load, the parapet should 

be protected by a non-mountable curb at the edge of the traveled roadway or the parapet be 

designed for impact load. 

5.1.4 Post and Beam Barriers 

Where post and beam barriers such as guardrail systems are used, the posts are driven 

directly into the reinforced soil mass or installed in concrete-filled forms placed during 

backfill placement and compaction. The posts should be placed at a minimum distance of 3 

ft (1 m) from the wall face, driven 5 ft (1.5 m) below grade, and reinforced spaced to miss the 

posts where possible. If the reinforcements cannot be missed, the wall should be designed to 

account for the presence of an obstruction as discussed later in this Chapter. 

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min 

3⁄4" 

(a)	 (b) 

MODULAR 

BLOCK 

COMPRESSIBLE 

JOINT MATERIAL 

(c) 

Figure 5-2. Example traffic barrier for MSE walls. (a) Barrier behind coping, (b) barrier 

on top of panels, (c) barrier on top of modular block units. 

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MSE Walls and RSS – Vol I 5 – 5 November 2009
 
 

5.1.5 Drainage Related Top of Wall Elements 

Whenever possible, the top surface of wall should be graded such that water drains away 

from the wall. A grassed swale or concrete ditch can be used behind the facing to collect and 

remove water. However, when this is not possible, depending on the configuration of the 

backslope with respect to the top of the wall and local hydrogeological considerations, 

several different details may need to be implemented from a drainage perspective. These 

elements are discussed in Section 5.3. 

5.2 BOTTOM OF WALL ELEMENTS 

The primary bottom of wall element is a leveling pad. Figure 5-3 shows common details of a 

leveling pad. Following are some considerations for the leveling pad: 

	 For structural walls, the leveling pad should be constructed from lean (e.g., 2,500 psi 

{17.2 MPa}) unreinforced concrete. The strength and thickness should be such that it 

allows cracking of the leveling pad during differential settlement as/if needed to relieve 

stress concentrations that can occur. Gravel pads may be allowed only for non-structural 

walls such as those for landscaping purposes. 

	 The common thickness of a leveling pad is 6 in. (150 mm). The width of the leveling pad 

should be such that it extends at least 3 in. (75 mm) beyond the thickness of the facing 

unit. Thus, for example, if the segmental precast concrete facing unit is 6 in. (150 mm) 

thick, then the width of the leveling pad shall be at least 12 in. (300 mm). At sharp 

curves the width of the leveling pad may be increased for segmental precast concrete 

facing units which are typically 5 ft (1.5 m) or 10 ft (3 m) wide so that the entire panel is 

resting on the leveling pad and at least 3 in. (75 mm) overhang of the leveling pad on 

each side of the facing unit. 

	 For ease of construction and to prevent misalignment of joints, the top of the leveling pad 

within any given step should be such that it does not vary by more than 1/8 in. (3 mm) 

over any 10 ft (3 m) run. 

	 Any openings between leveling pad steps should be completely filled after erection of the 

first row of panels. Where openings are more than 3 in. (75 mm) wide, filling with lean 

unreinforced cast-in-place concrete is preferred. For smaller openings, a geotextile filter 

with sufficient overlap of the panels and foundation soil could be used to fill openings. 

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MSE Walls and RSS – Vol I 5 – 6 November 2009
 
 

When there is a roadway immediately adjacent to the bottom of the wall, it is advisable to 

provide a crash protection barrier to protect the wall against vehicular impact. The size and 

configuration of this barrier is a function of the vehicle sizes and speed limits on the roadway 

in front of the wall. An alternative to a crash barrier is a non-mountable, i.e., a high, curb at 

the edge of the traveled roadway adjacent to the wall. 

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MSE Walls and RSS – Vol I 5 – 7 November 2009
 

(a) 

(b) 

(c) 

Figure 5-3. Leveling pads, (a) Common size, b) Step detail for precast panel facing units, 

(c) Step detail for modular block facing units. 
 

5.3 DRAINAGE 

Good drainage is essential to the proper performance of an MSE wall. There are two types of 

drainage considerations for an MSE wall, internal and external. Internal drainage
 

considerations are related to control of surface or subgrade water that may infiltrate the 

reinforced soil mass. The internal drainage of an MSE wall depends on the characteristics of 

the backfill used in the reinforced soil mass. External drainage considerations deal with 

water that may flow externally over and/or around the wall surface taxing the internal 

drainage and/or creating external erosion issues. The external drainage depends on the
 

location of the MSE wall with respect to local hydrogeological factors and generally deals 

with diverting water flow away from the reinforced soil structure. 

Regardless of the source of the water, i.e., internal or external, the cardinal rule in the design
 

of MSE walls, as with any other wall type, is to allow unimpeded flow of water through the 

wall and/or collect and remove water before it enters the zone of influence of the wall to
 

prevent the following: 

 build-up of hydrostatic forces that increase lateral pressures, 

 piping, i.e., erosion of one soil into another, which creates paths for additional water flow 

or clogging of drainage aggregate, and 

 external soil erosion from the toe, around the edges or at the top of the wall. 

It is recommended that adequate drainage features be required for all walls unless the 

engineer determines that such features are not needed for a specific project. During a 

determination of the need for drainage features, the engineer must include consideration for 

both subsurface (e.g., ground water, perched water, flooding and tidal action) and surface 

infiltration water (e.g., rain, runoff, and snow melt). 

Effect of Fines on Drainage 

Soil particles with sizes smaller than the U.S. No. 200 (0.075 mm) sieve are referred to as 

“fines.” The permeability of an overall soil mass is affected significantly by the amount of 

fines. In general, soils with less than 3 to 5% non-plastic fines by weight are considered to 

be free draining and water can readily flow through the soil mass even under low hydraulic 

gradients. In the case of MSE walls, a reinforced soil mass with less than 3 to 5% non-plastic 

fines will allow unimpeded flow provided the permeability of the reinforced fill is greater 

than the permeability of the retained fill and the wall is not exposed to significant water 

events such as flooding, tidal action or significant snow melt. 

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When the amount of the fines is more than 3 to 5%, the permeability is significantly reduced 

and drainage requirements must be carefully evaluated as groundwater and/or infiltration of 

surface water can result in build-up of seepage/hydrostatic forces within the reinforced soil 

mass. Surface water that infiltrates into the reinforced soil mass will tend to move toward the 

permeable face of an MSE wall and can have a destabilizing effect due to a potential increase 

in seepage forces (Terzaghi et al., 1996; Cedergren, 1989). Such a condition can occur 

during severe rainstorms, if the permeability of the fill is equal to or less than about 0.002 

cm/sec (Terzaghi et al., 1996; Cedergren, 1989). Therefore, good drainage features should 

be incorporated into the design if low permeability reinforced fill is used, i.e., if the 

reinforced fill has more than 3 to 5% non plastic fines. Special precaution is also advised for 

hillside construction due to the potential for seepage to occur through retained soil and rock 

seams, faults and joints during rain events that may not be apparent during subsurface 

exploration and construction. 

Internal and external drainage details, which represent good drainage, are presented in the 

following Sections 5.3.1 and 5.3.2, respectively. Good design of drainage features requires 

proper consideration of the filtration properties of various geomaterials within and external to 

the MSE wall as well as drains that are adequately sized to effectively remove any seepage 

water. The drainage components including filtration criteria and drain component 

requirements are presented in Section 5.3.3. 

5.3.1 Internal Drainage Systems 

There are two specific forms of internal drainage as shown in Figure 5-4, (a) drainage near 

wall face due to infiltration of surface water near the wall face, and (b) drainage behind and 

under reinforced soil mass from groundwater. Groundwater may be present at an elevation 

above the bottom of the wall and would flow to the MSE walls from an excavation backcut; 

or it may be present beneath the bottom of the MSE wall. A groundwater surface beneath a 

MSE wall may rise into the reinforced soil mass, depending on the hydrogeology of the site. 

Surface water may infiltrate into the reinforced soil mass from above or from the front face 

of the wall, for the case of flowing water in front of the structure. 

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Surface Water Infiltration 

Reinforced fill 

Retained fill 

Groundwater 

Foundation Soil 

Wall 

face 

Figure 5-4. Potential sources and flowpaths of water. 

Internal Drainage Near Wall Face 

A filter is provided at all vertical and horizontal joints in the wall face to prevent the 

migration of fines from the reinforced soil mass through the joints. The location and 

configuration of the filter is a function of the type of wall facing units as follows: 

	 For segmental precast wall facing units, the filter is commonly in the form of geotextile 

fabric that is placed across all horizontal and vertical joints as shown in Figure 5-5. The 

geotextile should extend a minimum of 4 in. (100 mm) on either side of the joint and up 

into the coping to prevent soil from moving around the geotextile. The geotextile filter 

characteristics should be such that it is compatible with the backfill in the reinforced soil 

mass as discussed in Section 5.3.3. 

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Figure 5-5. Example layout of filter at joints between segmental precast facing units. 

	 Modular block wall (MBW) facing units are typically constructed with a zone of free 

drainage aggregate adjacent to the back face of the units. The minimum width of this 

aggregate zone is typically 1 ft (300 mm). In addition to serving as a back face drain, this 

aggregate is required for stiffness of the wall face and constructability, i.e., placement 

and compaction of wall fill may be difficult based on the configuration of the MBW 

units. This column of aggregate is often a high permeability well graded gravel as 

discussed in Section 5.3.3. The gradation of the aggregate should be used to determine 

the maximum allowable vertical joint opening between MBW units, using slot criterion 

given by Equation 5-8 in Section 5.3.3. The configuration of the gravel filter is a 

function of whether the modular block unit is solid or with a hollow-core. For solid 

modular block units, the well graded gravel should be at least 1 ft (300 mm) wide as 

shown in Figure 5-6a. For hollow-core modular block units, the well graded gravel 

should be at least 1 ft (300 mm) wide with a minimum volume of 1 ft3

 per ft2 (0.3 m3/m2) 

of wall face as illustrated in Figure 5-6b. The gradation of the gravel should be sized to 

be compatible with the reinforced wall fill gradation in the reinforced soil mass, i.e. meet 

soil filter criteria as discussed in Section 5.3.3. Alternatively, a geotextile may be used 

between the gravel and reinforced wall fill to meet filtration requirements, as illustrated 

in Figure 5-6b. Finally, the construction sequence should be specified to ensure a 

workable drain system. 

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MSE Walls and RSS – Vol I 5 – 11 November 2009
 
 

(a) 

(b) 

1-ft min 

1-ft 

min 

Figure 5-6. Layout of drainage fabric and drainage fill at the face for modular block units. 

(Collin et al., 2002). 

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MSE Walls and RSS – Vol I 5 – 12 November 2009
 
 

 Figure 5-7 provides a common detail for face drainage in the case of wire-faced walls. In 

this case, the geotextile filter is placed between the facing stones and the reinforced soil 

mass. 

Internal Drainage Under and Behind the MSE Wall 

For walls in locations where groundwater can result in build-up of seepage/hydrostatic forces 

within the height of the reinforced soil mass and/or surface water infiltration is anticipated, a 

base drain that provides drainage beneath the MSE wall and a back or chimney drain that 

provides drainage behind the reinforced soil mass is strongly recommended to ensure proper 

long-term functionality of the MSE wall. This is because, as noted earlier, a reinforced fill 

with more than 3 to 5% non plastic fines is not “free draining.” 

The base drain and back drain should be designed to collect and remove groundwater before 

it enters the reinforced mass and allows infiltration water to preferentially flow downward 

and toward the back of the wall, away from the face. An example of such a drainage system 

is illustrated in Figure 5-8 for segmental precast facing unit structure. Figure 5-6a shows a 

common detail for modular block unit faced structures. Figures 5-9 and 5-10 show 

alternative drainage systems that include geocomposite drains and blanket drains in lieu of 

open graded gravel drains with a geotextile or well-graded soil filter. Information on the 

various drains to relieve hydrostatic pressures is provided below. Design of the base drain 

and backdrain and the drainage system components is covered in Section 5.3.3. 

Figure 5-7. Example layout of geotextile filter near the face for welded wire facing units. 

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Note: Number 

preceding foot and 

inch dimensions is 

in millimeters. 

Figure 5-8. Example drainage blanket detail behind the retained backfill. 

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Figure 5-9. Example drainage detail using a geocomposite sheet drain. 

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MSE Walls and RSS – Vol I 5 – 15 November 2009 
 

TOP SOIL 

LOW PERMEABILITY FILL 

CURB 

ASPHALT 

DRAINAGE AGGREGATE 

DRAINAGE COMPOSITE 

(CHIMNEY DRAIN) 

GEOSYNTHETIC REINFORCEMENT 

GEOTEXTILE FILTER (TYP.) 

BLANKET DRAIN 

COLLECTOR PIPE 

LEVELING PAD (WELL GRADED SAND AND GRAVEL) 

Figure 5-10.	 Example drainage detail using a blanket drain with chimney drain (Collin et 

al., 2002). 

Walls with Possibility of Inundation 

For walls potentially subject to inundation, such as those located adjacent to rivers, canals, 

detention basins or retention basins, a minimum hydrostatic pressure equal to 3 ft (1 m) 

should be applied at the high-water level for the design flood event. Effective unit weights 

should be used in the calculations for internal and external stability beginning at levels just 

below the equivalent surface of the pressure head line. Where the wall is influenced by water 

fluctuations, the wall should be designed for rapid drawdown conditions which could result 

in differential hydrostatic pressure greater than 3 ft (1 m). As an alternative to designing for 

rapid drawdown conditions, No. 57 coarse aggregate, as specified in AASHTO M 43, could 

be provided as reinforced backfill for the full reinforced zone of the wall and to the 

maximum height of submergence of the wall. A geotextile filter should be provided at the 

interface of the No. 57 coarse aggregate and reinforced backfill above it, at the interface of 

the retained backfill behind it, and at the interface of the coarse gravel and subgrade beneath 

it, unless the coarse aggregate meets the soil filtration criteria for the adjacent soils (see 

Section 5.3.3). The geotextile should meet the filtration and survivability criteria in Section 

5.3.3. Adjoining sections of geotextile filter/separator shall be overlapped by a minimum of 

1 ft (0.3 m). An example detail is shown in Figure 5-11. 

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MSE Walls and RSS – Vol I 5 – 16 November 2009
 
 

Note: Number 

preceding foot and 

inch dimensions is in 

millimeters. 

SHORING IF 

NEEDED 

300 (1′-0") 

ABOVE 

Q500 LEVEL 

Figure 5-11. Example detail for wall that may experience inundation. 

5.3.2 External Drainage 

Surface drainage is an important aspect of ensuring MSE wall performance and must be 

addressed during design. Appropriate measures to prevent surface water from infiltrating 

into the wall backfill should be included in the design of all MSE walls. This typically 

requires coordination with designers of other project elements. 

During construction of an MSE wall, the Contractor should grade the wall fill surface away 

from the wall face at the end of each day of construction to prevent water from ponding 

behind the wall and saturating the soil. In addition to softening the subgrade, surface water 

running onto a partially completed wall fill can carry fine-grained soils into the backfill work 

area and locally contaminate a free-draining granular backfill with fines. If finer grained 

backfill is being utilized for the reinforced wall fill, saturation can cause movements of the 

partially constructed wall. 

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When possible, finished grading at the top of a wall structure should provide positive 

drainage away from the wall to prevent or minimize infiltration of surface water into the 

reinforced wall fill. If the area above the wall is paved, a curb and gutter is typically used to 

direct the flow away from the wall. Drainage swales lined with concrete or asphalt can be used 

to collect and discharge surface water. Vegetation lined swales may be used where a 

vegetated finished grade slopes to the wall. Water runoff over the top of a wall where the 

backfill slopes towards it can lead to erosion behind the top of the wall and underming of the 

wall. Such runoff can also cause staining of the wall face as soil is carried with the water. 

Construction of a collection swale close to the wall will help to prevent runoff from going 

over the top of the wall. Runoff flow will concentrate at grading low points behind the face 

and cause ponding which leads to undesirable infiltration of water into the backfill and 

increased compressibility due to softening of the backfill. 

Collection and conveyance swales should prevent overtopping of the wall for the design 

storm event. Extreme events such as heavy rainfalls of short duration have been known to 

cause substantial damage to earth retaining structures due to erosion and undermining, 

flooding, and/or increased hydrostatic pressures both during and after construction. This is 

particularly true for sites where surface drainage flows toward the wall structure and where 

finer-grained backfills are used. 

If the surface grading is such that there is likelihood of surface water flowing towards an 

MSE structure, then the water should be collected in a gutter or other collection feature that 

is part of the site drainage features. Such site drainage features are designed for an assumed 

or prescribed design storm event. For MSE walls, the design storm event should be based on 

a minimum 100 year event. However, extreme events can occur that result in short duration 

flows, e.g., 1 to 3 hours, which significantly exceed the design capacity of the stormwater 

management system. When such events occur, site flooding can cause overtopping of the 

wall, erosion and undermining, and an increase in hydrostatic forces within and behind the 

reinforced soil mass. Therefore, the site layout and wall structure should include features for 

handling flows greater than the design event as is typically done in the design of an overflow 

spillway for a dam. The project civil engineer should address potential excess flows and 

coordinate work with the wall designer. Consideration should be given to incorporating 

details of overflow features, such as a spillway, into the wall design for sites where surface 

water flows towards the wall structure. An example of an overflow feature is shown in 

Figure 5-12. Maintenance issues included in Section 5.3.4 should be addressed to ensure that 

all site drainage features are performing adequately. 

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Figure 5-12. Example MSE wall overflow sill at top of wall. 

Drainage Swale at Top of Wall 

A drainage swale is a man-made depression in the ground surface used to intercept surface 

water and direct it in a controlled manner to an outlet. Drainage swale can also be used to 

reduce the potential for surface water from overtopping the wall. Figure 5-13 shows typical 

drainage swale details for segmental precast concrete facing and modular block wall facing 

units. When a drainage swale is used, the project civil engineer and the wall designer should 

address and detail the outlet(s) for the swale. For example, the swale can be detailed to 

discharge water at the end of the wall structure or to low overflow points along the wall 

length. Overflow points should be detailed on the construction drawings. The designer 

should anticipate and address in design and detailing the possibility of water runoff from 

extreme events which will overtop the drainage swale and run down the wall face, unless the 

swales are specifically sized for such events. For sloping backfills, the wall designer should 

also address collection and diversion of water at the top of the slope. Site water runoff from 

above the backslope should not be directed toward the MSE wall backslope. 

Vegetated swales as shown in Figure 5-13b can provide an aesthetically pleasing appearance. 

However, the effectiveness of the low permeability soil in preventing water from migrating into 

the reinforced soil mass and drainage aggregate should be evaluated. Shrinkage cracks in the 

low permeability soil during periods of extended dry weather may increase the permeability of 

the layer to the extent that it is no longer an effective barrier layer. Therefore, a geomembrane 

should be used beneath any vegetated swale. 

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(a) 

3'-0" 

TOPSOIL 8" MIN. 

8" LOW PERMEABILITY 

SOIL 

IMPERMEABLE 4" CONCRETE 

OR ASPHALT LINING 

COMPACTED INFILL 

DRAINAGE GEOTEXTILE 

AGGREGATE 

SEGMENTAL 

UNIT 

3'-0" 

TOPSOIL 8" MIN. 

8" LOW PERMEABILITY 

SOIL 

SWALE LINED WITH VEGETATION 

GEOMEMBRANE (OPTIONAL) 

COMPACTED INFILL 

DRAINAGE GEOTEXTILE 

AGGREGATE 

SEGMENTAL 

UNIT 

(b) 

Figure 5-13. Example drainage swale near top of wall. ((b) Collin et al., 2002). 

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Geomembrane Barriers 

A geomembrane barrier can be used to prevent surface water infiltration and associated 

seepage forces that can occur when using poorly draining reinforced fill. In addition steel 

soil reinforcements in the upper portion of MSE walls exposed to runoff containing deicing 

salts are affected by higher corrosion rates than defined by current corrosion rate models. 

Therefore, a geomembrane barrier should be placed below the road base and just above the 

first layer of soil reinforcement. The geomembrane should be tied into a drainage system to 

collect and discharge the runoff. As per Article 11.10.8 of AASHTO (2007), a roughened 

surface PVC, HDPE or LLDPE geomembrane with a minimum thickness of 30 mils (0.75 

mm) should be used. All seams in the membrane should be glued or welded to prevent 

leakage. 

An example detail for use of geomembrane barrier to prevent infiltration of runoff into the 

reinforced soil mass is illustrated in Figure 5-14a. As shown in Figure 5-14a, the 

geomembrane should be sloped to drain away from the facing to an intercepting longitudinal 

drain outletted beyond the reinforced mass. Installation of a geomembrane infiltration barrier 

is shown in Figure 5-14b and 5-14c. Design requirements for the geomembrane are covered 

in Section 5.3.3. 

Pavement Permeability and Runoff 

Pavements are porous structures. Surface water flows through asphalt pavement cracks and 

concrete joints and cracks into the pavement base material(s). The flow into the base 

aggregates can be significant, with up to 50% of the water falling on the pavement finding its 

way to the base course, and much more if there are cracks in the pavement, e.g., upwards of 

97% will flow though a 1/8 in. (3 mm) crack according to AASHTO (1986). This water then 

saturates the subgrade because the relatively high permeability base aggregate ponds the 

water above the MSE wall. The situation is compounded if the site and pavement grades 

toward a low spot as shown in Figure 5-15. The MSE wall designer should interact with the 

project civil engineer to ensure that such a condition is mitigated and positive drainage 

measures are provided to capture the pavement drainage in the form of proper grading away 

from the wall and edge drains. Consideration should also be given to using the 

geomembrane detail shown in Figure 5-14, to intercept and discharge the water seeping 

through cracks in the pavement. 

Surface runoff on the pavements that overtops the wall can cause undermining of the wall. 

Sloping the roadway towards a ditch is a common way to guard against wall overtopping. 

This is also sometimes referred to as roadway "in sloping." 

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(HDPE)

> 5%

5 ft

Front Face 

of MSE wall 

30 mil (min) 

(a) 

6” Perforated Pipe

Cover in Geotextile

(Drain to end of wall)

Drainage Layer

#57 Aggregagate

Impervious

Membrane 

Wearing 

Surface

Base

Course

Specified 

Backfill

1 ft

6” Perforated Pipe 

Cover in Geotextile 

(Drain to end of wall) 

Drainage Layer 

#57 Aggregagate 

Impervious 

Membrane 

Wearing 

Surface 

Base 

Course 

Specified 

Backfill 

1 ft 

> 5% 

5 ft 

(b) (c) 

Figure 5-14. (a) Example geomembrane barrier details, (b) Installation of geomembrane 

deicing salt runoff barrier, (c) Geomembrane installation around manhole 

penetration. 

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Figure 5-15. Example of undesirable water seepage through pavement due to deficient 

grades (Collin et al., 2002) 

Grade at Toe and Ends of the Wall 

The final grade at the toe and ends of the wall, both as designed and as constructed, is an 

important consideration for water flow conditions. Surface water flow along the toe of a 

MSE wall may occur around the ends or along the face of the structure and has the potential 

to erode the soil. An example of water damage is shown in Figure 5-16. Erosion of soil at 

the toe of a wall eventually may undermine the MSE wall facing units. Thus, design and 

construction details normally should direct flow away from the toe of wall structures. This 

can be accomplished with site grading and with a soil berm or slope at the toe of the wall. 

Erosion control details are required where water will flow adjacent to the wall toe. 

Geotextile lined riprap stone or other means should be used to prevent scour. The designer 

also may elect to embed the wall deeper (i.e., lower foundation level) where the potential for 

erosion of the wall toe exists. Consideration should be given to turning the wall 90 degrees 

inward from the face. 

The ends of the wall that terminate in or intercept embankment slopes should also be 

protected from erosion. Walls that terminate in slopes should be adequately keyed into the 

slope and a swale used to divert water away from the ends of the wall to mitigate erosion. 

Wing walls for approach fills should also be design such that water does not flow down the 

slope along the back of the wall face. Again a swale can be used to divert water and the 

surface of the slope should be graded to promote water flows away from the wall. 

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Figure 5-16. Example of surface flow er osion at the bottom of an MSE wall. 

5.3.3 Filtration and Drainage System Component Requirements 

Construction of an MSE wall may involve several types of soils. Groundwater flow from 

one soil type to another, and then to a drain and outlet feature, should be unimpeded. Soil 

filtration and permeability requirements must be met between adjacent zones of different 

soils to prevent impeded flow or piping. Adjacent soils of interest in an MSE wall system are 

as follows: 

 the reinforced fill and any drainage layers, 

 the reinforced fill, facing elements such as joints and/or face drainage aggregate and 

geotextile covering the joints, 

 the reinforced fill and retained fill, 

 the reinforced fill and foundation soil, and 

 the reinforced fill and embankment fill above the wall and low permeable surface fill that 

may be used to reduce infiltration. 

Filters may be in the form of a graded granular soil or a geotextile. Design of both soil filters 

and geotextile filters are discussed below. Design of geocomposite drains, drainage inflow 

and outflow, drain collection and outlet pipes and geomembrane barriers are also discussed. 

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Soil Filters 

As water flows from one soil zone to another, the downstream soil must meet filter criteria to 

prevent piping of the upstream soil. Furthermore, the downstream soil must have adequate 

permeability relative to the adjacent, upstream soil. Therefore, the downstream soil must 

have the correct gradation range to function properly as a filter. The gradation requirements 

of the filter are also a function of the upstream soil gradation because the design flow capacity 

of the filter cannot be realized if the upstream soil pipes into the downstream soil. The pore 

sizes in the filter soil must be small enough to retain the larger size particles of the soil, which in 

turn retain the smaller sizes of the retained soil. The filter pore size is mathematically a function 

of its controlling particle size. 

Design criteria for soil filters are summarized below and are based upon gradations of two 

adjacent soils. The particle sizes used in design are the D15, D50, and D85 sizes (subscript 

denotes the percentage of material, by weight, which has a smaller diameter). These criteria are 

applicable to adjacent soils with gradation curves that are approximately parallel. The 

equations are not applicable to gap-graded soils, soil-rock mixtures, non steady-state flow 

and soils with gradation curves that are not approximately parallel. When criteria are not 

applicable, filter design should be based upon laboratory filtration tests. The reader is 

referred to Cedergren (1989) for a comprehensive discussion on soil filtration. 

The soil filtration criterion to prevent piping (i.e., retention) of the upstream soil into the filter is: 

D15(filter) 

D 

 5 (5-1)

85(soil) 

To ensure sufficient permeability of the filter material, the ratio of the filter D15 to the 

upstream soil D15 should be as shown in Equation 5-2. 

D15(filter) 4   20 (5-2) D15(soil) 

An additional criterion to prevent movement of soil particles into or through filters is presented 

in Equation 5-3. For CL and CH soils without sand or silt particles, the D15 size of the filter in 

Equation 5-2 may be as great as 0.016 in, and Equation 5-3 may be disregarded. However, if 

the upstream soil, i.e., retained fill or backcut soils, contains particles of uniform non-plastic fine 

sand and silt sizes, the filter must be designed to meet these criteria. 

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D50(filter)  25 (5-3) D50(soil) 

 

Geotextile Filters 

A geotextile is often used as a filter between a finer-grained and a more permeable soil. The 

geotextile must retain the finer-grained soil, while allowing water to readily pass into the more 

permeable soil, and function throughout the life of the earth retaining structure. Thus, geotextile 

design must address retention, permeability, and clogging. The geotextile must also survive the 

installation process. 

The following design steps are from the FHWA Geosynthetic Design and Construction 

Guidelines Manual (Holtz et al. 2008). 

Step 1. Determine the gradation of the material to be separated/filtered. The filtered material 

is directly upstream or downstream of the geotextile filter for the drainage layer. 

Determine D85, D15, Cu = D60/D10 and the percent passing a No. 200 (0.075 mm) 

sieve. When the soil contains particles 1 in. (25 mm) and larger, use only the 

gradation of soil passing the No.4 (4.75 mm) sieve in selecting the geotextile (i.e., 

scalp off the + No.4 (+4.75 mm) material). 

Step 2. Determine the permeability of the upstream or downstream material to be filtered. 

These include the reinforced fill, foundation soil, retained fill and the natural soil in 

cut slope. 

Step 3. Apply design criteria for retention, permeability and clogging resistance to determine 

apparent open size (AOS), permeability (k), and permittivity (ψ) requirements for the 

geotextile (after Holtz et al., 2008). AOS, k and ψ of the candidate geotextile are 

determined from standard ASTM tests and is typically the value published by the 

geotextile manufacturers/suppliers. Use only needlepunched nonwoven or 

monofilament woven geotextiles (i.e., slit film woven geotextiles shall not be used). 

A. Retention Criteria – Steady State Flow 

Using the D85 and Cu values from Step 1, determine the largest allowable pore size as 

follows: 

AOS < B D85 (5-4)

 where:
 

AOS = apparent opening size of the geotextile
 

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B = dimensionless coefficient 

D85 = soil particle size for which 85% are smaller 

The AOS value of the candidate geotextile is determined from the results of the ASTM 

D4751 test method, and is typically the value published by the geotextile 

manufacturers/suppliers. The B coefficient ranges from 0.5 to 2 and is a function of the 

upstream finer-grained soil, type of geotextile, and/or the flow conditions. For sands, 

gravelly sands, silty sands and clayey sands (i.e., sands with less than 50% passing the 

No. 200 sieve), B is a function of the uniformity coefficient, Cu (Cu= D60/D10), of the 

upstream soil. Table 5-1 presents values of B for various values of Cu. 

If the upstream soil contains any fines, only the portion passing the No. 200 sieve should 

be used for selecting the geotextile. For silts and clays (more than 50% passing the No. 

200 sieve), B is a function of the type of geotextile as given in Table 5-2. 

These retention criteria are for internally stable soils and steady-state seepage conditions. 

Laboratory performance tests should be conducted for unstable soils. For soils with a Cu 

> 20, unsteady seepage may occur. For dynamic and cyclic flow condition use AOS < 

0.5D85. See Holtz et al. (2008) for further information on dynamic flow conditions such 

as wave action. 

Table 5-1. Values of B for Various Cu Values 

for Soils with Less than 50% Passing the No. 200 Sieve. 

Cu B 

Cu < 2 1 

2 < Cu < 4 0.5 Cu 

4 < Cu < 8 8 / Cu 

Cu > 8 1 

Table 5-2. Values of B and AOS for Soils with
 

More than 50% Passing the No. 200 Sieve Based on Type of Geotextile. 

Type of Geotextile B AOS 

Woven monofilament B = 1 AOS < D85 

Nonwoven B = 1.8 AOS < 1.8D85 

Both woven and nonwoven - AOS < 0.012 in. (0.3 mm) 

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B. Permeability/Permittivity Criteria 

For steady-state flow, low hydraulic gradient and well graded or uniform upstream soil, 

the permeability and permittivity criteria are: 

 

 

	 For kgeotextile > ksoil (Less Critical / Less Severe) (5-5a) 

permeability kgeotextile > 10 ksoil (Critical / Severe) 

 For  > 0.5 sec-1 for < 15% passing No. 200 sieve (5-5b) 

permittivity  > 0.2 sec-1 for 15% to 50% passing No. 200 sieve (5-5c) 

 > 0.1 sec-1 for > 50% passing No. 200 sieve (5-5d) 

where: 

k = coefficient of permeability (or hydraulic conductivity) and 

 = geotextile permittivity, which is equal to kgeotextile/tgeotextile.
 

Critical or severe applications are described in Holtz et al. (2008) and, as indicated in 

Equation 5-5a, a geotextile permeability of 10 times the soil permeability should be 

used. The geotextile permittivity is determined from the results of the ASTM D4491 

test method. 

C.	 Clogging Criteria 

a. For steady state flow, low hydraulic gradient and well graded or uniform upstream 

soil, the clogging criterion is: 

AOS > 3 D15(upstream soil) (5-6)

This equation applies to soils with Cu > 3. For soils with Cu < 3, a geotextile with the 

maximum AOS value from the retention criteria should be used. 

b. Other qualifiers 

Nonwoven geotextiles: Porosity (geotextile) > 50%
 

Woven geotextiles: Percent open area > 4% 

c. Alternative: Run filtration tests, especially for critical and severe applications 

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Step 4. In order to perform effectively, the geotextile must also survive the installation 

process. AASHTO M288 (2006) provides the criteria for geotextile strength required 

to survive construction of roads, as shown in Table 5-3. Use geotextile Class 2 where 

a moderate level of survivability is required (e.g., for geotextile filters at the wall face 

and on back drains). Class 1 geotextiles are recommended when heavy construction 

equipment is used and/or angular fill will be placed directly above or below the 

geotextile (e.g., geotextile filters for base drains). A minimum of 6 in. (150 mm) of 

base/subbase should be maintained between the wheel and geotextile at all times. 

Table 5-3. Geotextile Survivability Requirements (AASHTO M 288, 2006). 

Test Test 

Method 

Units 

Geotextile Class 

Class 1 Class 2 

< 50%* > 50%* < 50%* > 50%* 

Grab Strength ASTM 

D4632 

N 

(lb) 

1400 

(315) 

900 

(200) 

1100 

(250) 

700 

(157) 

Seam Strength ASTM 

D4632 

N 

(lb) 

1200 

(260) 

810 

(180) 

990 

(220) 

630 

(140) 

Tear Strength ASTM 

D4533 

N 

(lb) 

500 

(110) 

350 

(80) 

400 

(90) 

250 

(56) 

Puncture Strength ASTM 

D6241 

N 

(lb) 

2750 

(620) 

1925 

(433) 

2200 

(495) 

1375 

(309) 

Ultraviolet Stability 

(Retained Strength) 

ASTM 

D4355 % At face joints - 70% after 500 hours of exposure 

Buried in wall - 50% after 500 hours of exposure 

*Note: Elongation measured in accordance with ASTM D4632 with < 50% typical of woven 

geotextiles and > 50% typical of nonwoven geotextiles. (1 N = 0.22 lbs, 1 kPa = 0.145 psi) 

Step 5. Collect samples of geotextile, reinforced fill and retained fill at time of construction 

to confirm acceptance. 

Step 6. Monitor installation during construction. 

Step 7. Observe effectiveness of drainage system during and after storm events. 

For a more thorough treatment of geotextile drains see Holtz et al. (2008). 

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Geocomposite Drain 

A geocomposite, or prefabricated, drain consists of a geotextile filter and a water collection 

and conveyance core. The cores convey the water and are generally made of plastic waffles, 

three-dimensional meshes or mats, extruded and fluted plastic sheets, or nets. A wide variety 

of geocomposites are readily available. For MSE wall design, only geocomposites that allow 

two-sided flow (i.e., flow into the drains from both sides) should be used. However, the 

filtration and flow properties, detailing requirements, and installation recommendations vary 

and may be poorly defined for some products. The geotextile of the geocomposite should be 

designed to meet filter and permeability requirements discussed previously in this section. The 

flow capacity of geocomposite drains can be determined by using the procedures described in 

ASTM D4716. Long-term compressive stresses and eccentric loadings on the geocomposite 

core should be considered during design and selection. 

MSE walls can place a significant stress on the geocomposite. Hence, the design pressure on a 

geocomposite core should be limited to either of the following: 

 the maximum pressure sustained on the core in a test of 10,000 hr minimum duration; or 

 the crushing pressure of a core, as defined with a quick loading test, divided by a safety 

factor of five. 

Finally, as with in drain system, consideration should be given to system performance factors 

such as distance between drain outlets, hydraulic gradient of the drains, potential for 

blockage due to small animals, freezing, etc. Other design aspects of geocomposite drains 

are addressed in Holtz et al. (2008). 

Installation details, such as joining adjacent sections of the geocomposite and connections to 

outlets, are usually product-specific. Product-specific variances should be considered and 

addressed in the design, specification, detailing and construction phases of a project. General 

construction specification requirements will be review in Chapter 10. Post installation 

examination of the drainage core/path with a camera scope should be considered for critical 

applications. 

Drainage Inflow and Outflow Design Requirements 

For proper design of the drains at the back or base of the reinforced soil mass, the flow into 

the system and the flow in the drain must be evaluated. These flow conditions are discussed 

below and apply to either gravel or geocomposite drains. Cedergren (1989) and Huntington 

(1957) present a more thorough treatment of pressures induced by the influence of ground 

water and seepage acting on retaining walls as well as drainage design. 

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Flow into the System. Anticipated flow into the drain system may be estimated using 

Darcy’s Law. Flow is equal to: 

q = k i A (5-7)

 where: 

q = infiltration rate 

k = effective permeability of retained backfill soil 

i = average hydraulic gradient in retained backfill soil 

A = area of soil normal to the direction of flow 

Conventional flow net analysis can be used to calculate the hydraulic gradient. 

Some drains consist of drainage aggregate surrounding a perforated pipe with a filter (usually 

a geotextile) surrounding the drainage aggregate. Flow into the drainage aggregate may be 

calculated with Equation 5-7. Flow from the drainage aggregate into the pipe is through the 

circular or slot perforations. Perforated, corrugated HDPE pipe is manufactured with 

minimum inlet openings of approximately 1 square inch per 1 foot length (20 cm2

 per meter 

length) for standard pipe (AASHTO M252, 2006). Standard pipe is generally adequate for 

most subsurface drainage applications. Hole diameter or slot width must be checked relative 

to the size of the surrounding drainage aggregate, to ensure soil retention. For slots, Equation 

5-8 may be used to check retention. 

D85(drain fill)  1.2 to1.4 (5-8) Slot Width 

For circular perforations, Equation 5-9 may be used to check retention. 

D85(drain fill)  1.0 (5-9) Hole Diameter 

 

Flow Capacity of the Drain. Flow capacity within aggregate drains can be estimated with 

Equation 5-7, using k and i for the soil drain material. Flow capacity within geocomposite 

drains is expressed in term of unit width using the following form of Darcy’s Law. 

 

q =  i B (5-10) 

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where: 

q = flow rate 

 = transmissivity of geocomposite drain 

i = hydraulic gradient in drain 

B = width of geocomposite drain 

The geocomposite transmissivity should be evaluated with an appropriate laboratory model 

test. Product long-term transmissivity should be quantified at anticipated (or greater) design 

pressure and over time to evaluate potential decrease of flow capacity due to creep (i.e., creep 

of geotextile into flow channel). 

Flow Capacity of the Drain Pipe. Flow capacity within drain pipes, flowing full, can be 

computed with the Manning’s equation. Flow is equal to: 

0.463 8/ 3 1/ 2 q  d s (5-11)

n 

where: 

q = flow rate (cfs) 

n = roughness coefficient, or Manning’s value 

d = diameter of pipe (feet) 

s = slope of energy grade line (ft per ft) 

Drain Collection and Outlet Pipes 

Collection and outlet pipes are often used with the drain directly behind the facing units and 

with the drain at the back of the reinforced soil mass. Examples of such drains are shown in 

Figures 5-8 and 5-10. Pipes are generally laid at required slopes, with a minimum of 2% for 

constructability and to ensure positive flow. Outlets are generally spaced based on the flow 

capacity of the pipes or alternatively at 20 ft (6 m) to 50 ft (15 m) maximum lateral spacing, 

and protected as noted in a later discussion on maintenance. The outlet pipes should be solid 

and gravity flow (e.g., 2% minimum grade) to daylight or the storm drain system. 

Geomembrane Barriers 

Design and specification of a geomembrane as a deicing salt barrier must address installation 

requirements. A geomembrane must be capable of withstanding the rigors of installation to 

ensure the integrity of the barrier. The subgrade material, subgrade preparation, 

geomembrane placement method, overlying soil fill type, and placement and compaction of 

overlying fill soil all affect the geosynthetic barrier's survivability. Recommended properties 

of geomembrane barriers (Koerner, 1998) are presented in Table 5-4. A minimum thickness 

of 30 mils (0.75 mm) is recommended for geomembranes above MSE walls. 

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Three areas of construction which are critical to a successful installation are: 

 subgrade preparation; 

 handling/installation including field seaming; and 

 sealing around penetrations and adjacent structures. 

The subgrade must provide support to the geosynthetic barrier and minimal point loadings. 

The subgrade must be well-compacted and devoid of large stones, sharp stones, grade stakes, 

etc., that could puncture the geosynthetic barrier. In general, no objects greater than 1⁄2 in. 

(12 mm) should be protruding above the prepared subgrade (Daniel and Koerner, 1993). 

Handling and installation specifications for geomembrane and other geosynthetic barriers 

should, as a minimum, conform to the manufacturer's recommendations. All seams in the 

membrane should be glued or welded to prevent leakage. Special project requirements for 

geomembranes should be noted in the construction specifications and plans. 

Table 5-4. Recommended Minimum Properties for General Geomembrane Installation 

Survivability (after Koerner, 1998). 

Property and test method 

Required degree of survivability 

Medium1 Very high2 

Thickness, mils (mm) – ASTM D5199 or 

ASTM D5994 for Textured 30 (0.75) 40 (1.00) 

Tear (Die C), lbf (N) - ASTM D1004 10 (45) 20 (90) 

Puncture, lbf (N) - ASTM D4833 32 (140) 45 (200) 

NOTES: 

1. Medium refers to placement on machine-graded subgrade with medium loads. Soil 

fill should have a maximum size of 3⁄4-inch. 

2. Very high refers to placement on machine-graded subgrade of very poor texture. 

Soil fill with maximum size greater than 3⁄4-inch. 

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Geomembrane selection should also consider installation details of attachment to the MSE 

wall facing and details around penetrations. Construction details around penetrations and 

adjacent structures depend upon the chosen geosynthetic material and the project design. As 

such, they must be individually designed and detailed. For example, batten strips and 

mechanical fasteners were used with the 30 mil (0.75 mm) thick HDPE geomembrane shown 

in Figure 5-14c. Geosynthetic manufacturers and waste containment manuals can provide 

design guidance. 

Another design consideration may be the frictional resistance of the geomembrane. As per 

Article 11.10.8 of AASHTO (2007), typically, a roughened surface PVC, HDPE or LLDPE 

geomembrane with a minimum thickness of 30 mils (0.75 mm) should be used. Such 

roughened geomembranes are readily available in the marketplace. 

5.3.4 Maintenance of Drainage 

Features that minimize water flow into an MSE wall and features that preserve MSE wall 

drainage should be maintained over the life of the structure. For example, cracks in 

pavement above MSE walls should be sealed. Differential settlements and pavement cracks 

around catch basins should be corrected to minimize potential inflow into the reinforced soil 

or retained soil mass. These maintenance items are for non-wall features and the wall 

designer may have little influence on these items. However, in interacting with designers of 

other project features, the need to maintain items that potentially could affect the wall should 

be discussed. 

One of the maintenance items that the wall designer has control over is the drain outlet(s). 

Screens should be installed and maintained on drainpipe outlets. Screening is used to prevent 

small animals from nesting in and clogging the pipe. Outlet screens and cleanouts to provide 

access to clogged drainage should be detailed on the retaining wall construction drawings. 

Additional items should be detailed when outlets are located in a soil embankment beneath 

the MSE walls. Drains are not effective unless the outlets are maintained, i.e., not clogged. 

Outlets in soil embankments should drain onto a concrete (usually precast) apron and should 

be marked with a permanent metal fence post. The apron and post minimize the chance of 

the outlet being run over and crushed by mowers or covered in subsequent construction 

activities. The apron and post should be detailed on the wall construction drawings. 

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5.4 INTERNAL DETAILS 

There are a number of internal details that must be properly designed and implemented to 

ensure that the MSE wall system performs in an acceptable manner over its design life. This 

section presents common internal details 

5.4.1 Contact between Dissimilar Metals 

Often, several different types of metallic elements such as steel piles and drain pipes are placed 

in the reinforced soil mass. The metals used in such elements are different than those used in 

steel reinforcements. Corrosion can occur when dissimilar metals come in contact with each 

other due to galvanic action. Therefore, all steel soil reinforcements should be separated from 

other metallic elements by at least 3 in. (75 mm). 

5.4.2 Vertical Obstructions in Reinforced Soil Mass 

Vertical obstructions are structures that are embedded in or extend vertically through the 

reinforced soil mass. Examples of vertical obstructions are a catch basin, grate inlet, sign 

foundation, bridge foundation, light poles, guardrail post, or culvert. Under no 

circumstances, should any reinforcement be left unconnected to the wall face or 

arbitrarily cut/bent in the field to avoid the obstruction. A review of any modification to 

the design to avoid an obstruction must be made and approved by the wall designer of record. 

Additional consideration must be given to obstructions that exert a load on the wall (e.g., 

deep foundations, overturning of signs and light poles). Such applications may require 

additional reinforcement and facing support to resist the local increase in lateral stress. 

Likewise, the wall may exert lateral earth pressure or vertical downdrag stress on the 

obstruction due to movement of the wall, the consequences of which to the obstruction 

design and performance must also be evaluated. 

The best design is to adjust the location of the obstruction and/or the soil reinforcement so 

that there is no interference. In some cases, where interference between the vertical 

obstruction and the soil reinforcement is unavoidable, the design of the wall near the 

obstruction may be modified using one of the following alternatives. 

Alternative 1 - Fit the soil reinforcement around the obstruction without cutting the soil 

reinforcement as shown in Figure 5-17. In this alternative, the facing units near the 

obstruction are fitted with extra facing connections such that soil reinforcing can be 

connected at locations away from the vertical obstruction. For example, as shown in Figure 

5-17a, a 5 ft (1.5 m) panel that needs a 4-wire bar mat may be fitted with 8 clevis loop 

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connections and in the field the bar mat can be attached to any 4 consecutive clevis loops 

depending on the location of the vertical obstruction. Similarly, as shown in Figure 5-17b, a 

5 ft (1.5 m) panel with the obstruction blocking 2 strip reinforcements, a strong back 

consisting of a galvanized steel angle can bridge across two panels. The strong back then 

allows the two displaced reinforcements to be attached to each side of the obstruction. 

Where soil reinforcements are not centered on the panel, eccentric load of the facing panel 

must be evaluated with respect to the structural capacity of the face to resist increased 

bending moments and the potential for face rotation. 

In case of strip reinforcements with a nut and bolt connection, it may be possible to splay the 

reinforcements around the obstruction as shown in Figure 5-17c. In such cases, the splay 

angle should be less than 15-degrees and the tensile capacity of the splayed reinforcement 

shall be reduced by the cosine of the splay angle. The splay angle is defined as the angle 

measured from a line perpendicular to the wall face in a horizontal plane. Due to the clevis 

loop or similar connection for bar mats splaying at more than 3- to 5-degrees is not possible 

without introducing moments at the connection and uneven loading of the clevis loop 

connectors. Under no circumstances should a bar mat be cut to force longitudinal wires 

around the vertical obstruction because it creates damaging moments on the cross bar welds 

as well as at the facing connection and cracking of galvanization. Bar mats should not be 

splayed if the connection mechanism does not accommodate such splay without cutting of 

cross bars. 

If the soil reinforcements are navigated around the vertical obstruction, then care must be 

taken to balance forces in the wall face to assure that the wall panels do not rotate outward. 

Facing units with a joint in front of the vertical obstruction may be structurally connected 

across the joint as shown in Figure 5-17, or a longer panel may be considered, e.g., 10 ft (3 

m) instead of 5 ft (1.5 m) wide panel. The structural connection should not extend across 

more than one joint, i.e., 2 panels. If such a condition occurs, then Alternatives 2 and 3 

should be implemented as appropriate. 

Alternative 2 - Assuming that reconfiguration of soil reinforcements as per Alternative 1 is 

not possible and the reinforcement layers must be partially or fully severed in the location of 

the obstruction, the surrounding reinforcement layers should be designed to carry the 

additional load which would have been carried by the severed reinforcements. 

In this alternative, the portion of the wall facing in front of the obstruction should be made 

stable against a toppling (overturning) or sliding failure. 

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(a) 

(b) 

SPLAY REINFORCING STRIPS TO AVOID OBSTRUCTION 

(c) 

Figure 5-17. Examples of avoiding a vertical obstruction without severing soil 

reinforcements. 

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Alternative 3 - A structural frame can be placed around the obstruction which is capable of 

carrying the load from the facing in front of the obstruction to reinforcement connected to the 

structural frame behind the obstruction. This is illustrated in Figure 5-18a and 5-18b and in 

Figure 5-19. The structural frame and connections should be designed in accordance with 

Section 6 (“Steel Structures”) of AASHTO (2007) for the maximum tension at any level of 

reinforcements within the reinforced soil mass. The structural frame should be designed such 

that moments in the soil reinforcement or connection at the wall face are not generated. 

Note that as shown in Figure 5-18c it may be feasible to connect the soil reinforcement 

directly to the obstruction depending on the reinforcement type and the nature of the 

obstruction. Figure 5-20 shows example details for MSE walls with modular block units 

with limited height vertical obstructions such as catch basin or fence post foundations near 

the wall face. 

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Wall face 

(a) 

Wall face 

(b) 

(c) 

Figure 5-18. Vertical obstructions in reinforced soil mass with segmental precast facing units. 

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(a) 

(b) (c)

Figure 5-19. Example of a structural frame around vertical obstruction (a) with segmental 

precast facing - note that vertically adjacent layers of reinforcement to be 

separated by a minimum of 3-in. (75 mm) of wall fill, (b)-(c) with modular 

block face – note corner detail. 

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(a) 

CAP UNIT 

PRIMARY 

GEOGRID 

SECONDARY 

GEOGRID 

FACING UNIT 

10 FT GEOGRID EMBEDMENT LENGTH 

(b) 

Figure 5-20. Example details of reinforcements around vertical obstructions in reinforced 

soil mass with modular block units. 

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5.4.3 Horizontal Obstructions in Reinforced Soil Mass 

Horizontal obstructions are structures which are embedded in or extend horizontally through 

the reinforced soil mass for a substantial length along the wall. The horizontal obstructions 

are commonly due to utilities such as storm drain pipes. Horizontal obstructions in 

reinforced soil mass should be avoided because not only do they create construction 

problems but obstructions such as utility pipes can be very expensive to repair and 

maintain as it may require dismantling the wall system. If horizontal obstructions cannot 

be avoided, then some considerations for design are provided below: 

	 For inextensible reinforcements, the horizontal obstruction may be avoided if it is 

possible to deflect the reinforcement in a smooth manner up to 15 degrees of vertical 

skew as shown in Figure 5-21. Deflections greater than 15 degrees tend to break the 

galvanization and may reduce the tensile and pullout resistance of the inextensible soil 

reinforcements. 

	 Guidance for extensible reinforcements such as geogrids is presented in Figure 5-22. 

	 In cases where it is not possible to orient the reinforcements as shown in Figures 5-21 and 

5-22, use of back-up panels may be considered as shown in Figure 5-23. 

	 It is not recommended to tie the reinforcements to pipes. Special details must be 

developed to accommodate the obstruction without attaching to it. 

	 Utility pipes in the reinforced mass are likely to settle differentially as the fill settles 

during construction. Downdrag stress should be anticipated where pipes intersect the 

wall face or a vertical structure such as a drop inlet. Significant leakage of water into 

MSE walls has been known to create wall problems including failures. Therefore, 

utilities should only be placed in double wall design systems such as locating utilities 

inside box culverts with inspection galleries or using double wall pipe with 

instrumentation to indicate leakage. Only leak proof joints should be used on drainage 

pipes. Where differential movement and downdrag stresses are anticipated, flexible 

connections should be used and designed to tolerate the estimated movement and stress. 

	 Pressurized water mains should not be constructed within an MSE structure. 

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3” (min) 

soil cover 

(a) 

Figure 5-21. Navigating horizontal obstruction in MSE walls with inextensible 

reinforcements. 

Additional depth (d) required, in. Required minimum distance (X) 

to achieve smooth bend, in. 

3 27

6 39

9 48

12 60

15 72

Figure 5-22. Navigating horizontal obstruction in MSE walls with extensible reinforcement. 

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Figure 5-23. Example of backup panels for large horizontal obstructions. 

5.4.4 Wall Face Penetrations 

In some cases, pipes must penetrate the MSE wall or pass through the retained wall fill. 

Penetrations through the reinforced soil and/or wall facing units maybe at skew or 

perpendicular angles from the wall face. 

If a pipe must penetrate through the face of the wall, the wall facing elements should be 

designed to fit around the pipe such that the facing elements are stable and the wall backfill 

an not spill through the wall face where it joins the obstruction. Differential movement 

between the facing and reinforced fill should be anticipated and associated downdrag stress 

must be consider in the design. Therefore, dry packing around the pipe should be done after 

the wall is substantially complete. Common details for penetrations through segmental 

precast concrete and modular block facing units are shown in Figures 5-24 and 5-25, 

respectively. Not noted on these details are the bedding and backfill for the pipe. Granular 

bedding may be significantly more permeable than the reinforced fill and/or the retained 

backfill. In these cases, the pipe bedding is a potential conduit for bringing water to the MSE 

wall structure. Therefore, a headwall is required to at the end of the pipe to prevent water 

from entering the bedding. Potential flow should be addressed in the wall details. A clear 

flow path, with filtration criteria addressed, from the pipe bedding and backfill to the 

drainage aggregate should be detailed. Weep holes through the concrete face collar may be 

needed to drain the pipe bedding and backfill. 

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Figure 5-24. Example pipe penetrations through segmental precast panel facing units. 

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Collar 

REFER TO ELEVATION VIEW FOR 

LENGTH, TYPE AND LOCATION OF 

SOIL REINFORCEMENT 

Figure 5-25. Example pipe penetration through modular block facing units. 

Catch basins and manholes may penetrate vertically through the reinforced fill or retained 

backfill. The backfill around these manholes may be a granular soil. If the manhole backfill 

soils are more permeable than the wall fill soils, the manhole backfill is a potential conduit 

for water flow and collection. The wall designer should address this potential, as provided 

drainage if the surrounding wall fill soils are less permeable. 

For critical wall structures, the wall designer may want to consider the possibility of leaking 

pipes saturating the surrounding soil. If this is a concern, a high permeability soil (relative to 

the wall fill) around the pipe leading to a drain or outlet may be used to provide a safety flow 

path. 

5.4.5 Slip Joints 

Where subsurface conditions and/or wall profile change abruptly, significant differential 

settlement may occur at the wall face with associated problems such as joint openings and 

facing unit to unit contact. In such cases, consideration may be given to use of slip joints 

which are continuous vertical joints. A slip joint is different than a regular vertical joint 

between panels in that there is a vertical separation between adjacent facing units that 

extends the full height of the wall. Due to this configuration of the joint, the wall on each 

side behaves independently. 

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Following conditions merit consideration of slip joints: 

 Where abrupt differential settlement of more than 1% (or 0.01) is expected. 

 Where there is an abrupt change in wall height of 5-ft or more. 

 Where the wall is underlain by a relatively rigid feature such as an abutment footing or 

rock outcrop. 

 Where a light weight rigid structure such as a box culvert intersects the face of a MSE 

wall. 

 Where the wall terminates into a cast-in-place structure (see Section 5.5 for additional 

information) 

 Where tight horizontal curves occur. 

Figure 5-26 shows common slip joint details for segmental precast concrete facing units. As 

shown in the figure, the slip joint design uses either an exposed slip joint panel having its 

own soil reinforcement element or a hidden “backup” panel in the backfill behind the facing 

panel. In either case the normal connection between two panels is broken and independent 

movement on each side of the slip joint is possible. Figure 5-27 shows a slip joint detail for 

modular block facing walls. 

5.4.6 Wall Curves 

Curves in walls are approximated by chords that are equal to the nominal width of the facing 

units. Therefore, smaller wall facing units such as the modular block units are able to 

navigate sharp curves better than larger precast concrete facing units. Similarly, 5-ft (1.5 m) 

wide precast concrete facing units can navigate sharper curves than 10 ft (3 m) wide facing 

units. For precast concrete facing units, curves with radius as small as 50 ft (15 m) can be 

achieved for 5 ft (1.5 m) wide facing units with a 3⁄4-in. (19 mm) joint opening. For curved 

walls, regardless of the type of wall facing, it is critical to provide details for wall layout. 

The relationship of wall alignment to roadway alignment should be clearly provided. Clear 

dimensions need to be provided on project drawings for offsets from reference alignments 

and whether these offsets are relative to top of wall or bottom of wall, especially in the event 

of stepped foundations. 

Figure 5-28 shows a typical detail for layout of geogrid reinforcement for walls with modular 

block facing units. Geogrid reinforcements typically require 100% area coverage whereas 

steel reinforcements are generally discrete and can be placed perpendicular to the wall face 

curves. In the case of geosynthetic reinforcements excessive overlap can result in reduced 

pullout resistance since contact between geosynthetics is smoother than contact between soil 

and geosynthetic. Therefore, a minimum soil layer of 3 in. (75 mm) between geosynthetics 

in the overlap zone is recommended as shown in Figure 5-28. 

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Figure 5-26. Example slip joints for segmental precast panel facings. 

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FIELD CUT FACING UNIT AT 

SLIP JOINT LOCATION 

Figure 5-27. Example slip joint for modular block wall facings. 

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Figure 5-28. Example layout of geogrid reinforcement for walls with curves. 

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5.4.7 Wall Corners 

When two MSE wall segments intersect to form an “external” (e.g., 90 degree) or an 

“internal” (e.g., 270 degree) corner, both wall segments will tend to move laterally such that 

corners tend to open up. Corner elements should be provided as shown in Figures 5-26a and 

5-26b to accommodate differential movements, prevent fill from moving through the crack, 

and provide aesthetic treatment. 

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Geotextile 

 Wall facing 

panel 

Soil reinforcement 

Corner element 

(a) External corner 

Wall facing 

Geotextile panel 

Corner element 

Soil reinforcement 

(b) Internal corner 

Figure 5-29. Example corner details. 
 

Acute Angle Corners 

External wall corners with an angle of less than 70 degrees, i.e., acute angle, should be 

avoided because of construction difficulties, e.g., compaction in corners and placement of 

reinforcements. However, if such a situation cannot be avoided, then the wall corner should 

based on following considerations: 

	 The acute angle corner should be designed as a bin wall for the extent of the wall where 

the full length of the reinforcement cannot be installed without encountering the 

opposite wall face. In the bin wall section, the reinforcing elements are either 

structurally connected to both wall faces forming the acute angle corner or overlapped if 

there is adequate space to develop the required pullout resistance. 

	 Full-height vertical slip joints should be provided at the interface of acute corner and 

after the last column of panels where full length reinforcements can be placed. 

	 The soil reinforcement attached to the slip joints should be oriented perpendicular to the 

slip joint panels and shall be the full design length. 

	 Light weight concrete should be considered as an alternate to placing and compacting 

fill. 

 	 Deformation compatibility between the bin wall section and the rest of the MSE 

structure should be carefully evaluated. 

5.4.7 Two-Stage Facing 

MSE walls with 2-stage facing systems can be used where significant (e.g., > 1/100) 

differential settlements are anticipated and use of slip joints, larger joint openings and/or 

ground improvement are not feasible to minimize the adverse effects of differential 

settlements. In an MSE wall with 2-stage facing, the primary MSE wall is constructed with a 

flexible face such as wire face or geosynthetic. After the primary flexible face wall has been 

constructed, it is left in place for a pre-determined amount of time to induce the settlements. 

Once the settlements are within acceptable limits, the facing units are installed in the second 

(final) stage. Figure 5-30 shows conceptual details of a 2-stage system that has been 

implemented in the industry; other similar details can be developed. Following are some 

general considerations for a 2-stage MSE wall system: 

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 The 2nd facing while usually consists of concrete panels should have a 

leveling/foundation pad with alignment/restraining mechanisms such as pins or dowels to 

receive and align the facing units as well as provide bearing resistance. 

 In addition to the usual connections between facing units, e.g., tongue-and-grove joints, 

additional connection elements such as dowels may be needed based on the facing unit 

type. 

 Turn-buckle type of connectors are used between the 1st stage wire mesh facing and the 

2nd stage concrete facing units. The size and type of the turn-buckles and the number of 

connectors is a function of the facing panel size, distance between the two facing units, 

the type of infill used as well as the amount of relative settlement anticipated between the 

two facing systems after the 2nd stage facing is constructed. Detailed structural analysis 

and design of the connections should be performed. 

 The sequence of construction should be clearly noted on the plans. 

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Flexible facing 

MSE wall 

system 

Connectors (turn-buckle 

type); use appropriate 

connector for top most 

facing unit near top of wall 

Dowel 

Joint between two 

facing units 

2nd stage facing 

units 

Space between 1st and 

2nd stage facings with 

appropriate infill 

Alignment/restraining 

pin/dowel mechanisms 

Leveling/foundation pad 

Figure 5-30. Conceptual connection details for a 2-stage facing system. 
 

 

5.5 WALL INITIATIONS AND TERMINATIONS 

The initiation and/or termination of an MSE wall may abut into another structure feature, 

slope or existing ground. The junctures of MSE walls with other structures are critical 

locations that are often observed to have distress such as misaligned facing units, leakage of 

backfill and erosion. Therefore, proper detailing is required at these locations. Following are 

some recommendations for wall initiations and terminations: 

	 The juncture of MSE walls and cast-in-place structures must be designed to prevent loss 

of fines and must allow for differential settlement between the two types of construction. 

Typical configurations for segmental precast panel facing units are shown in Figure 5-31. 

Either bituminous joint filler as shown in Figure 5-5a or a backer rod system and sealant 

as shown in Figure 5-31 is used. A common detail for a MBW facing unit is shown in 

Figure 5-32. 

	 A cast-in-place structure may have a lip to mask the joint as shown in Figure 5-31c. 

Sufficient distance between the facing and lip should be provided to allow for outward 

movement of the wall during construction. A geotextile should be used behind the joint 

to contain the soil. Joint filler such as expanded polystyrene may be used between the 

edge of the facing panel and the cast-in-place structure. 

	 Abrupt changes in wall heights should be avoided near wall initiation and termination 

points. This results in differential settlements and undesirable rotation of the facing units 

due to reduced confining pressures at such locations. Consideration may be given to not 

stepping the leveling pad within 10 ft (3 m) of the start or end of the wall. 

	 When starting or terminating into slopes and existing ground, the wall should be 

protected against erosion by vegetation and adequate embedment. In cases where the 

wall is adjacent to a steep slope or stream, riprap underlain by a filtration aggregate or 

geotextile may be needed. Swales should be used to divert water away from the end of 

the wall as discussed in section 5.3.3. 

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(a) (b)

(c) (d)

Figure 5-31.	 Common joint details between segmental precast panel facing units and CIP 

structures. 

GEOTEXTILE FIELD CUT MODULAR BLOCK 

UNIT – MINIMUM SIZE = 1⁄4 UNIT 

4" WIDE ASPHALT IMPREGNATED EXPANSION 

JOINT GLUED TO END/EDGE OF BLOCK 

Figure 5-32. Common joint between modular block facing units and CIP structures. 

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5.6 AESTHESTICS 

One of the attractive features of MSE walls is that aesthetics can be readily incorporated into the 

precast facing units. Several examples of wall aesthetics are shown in Figure 5-33. The choice 

of aesthetic treatments is virtually unlimited, however, it must be recognized that any aesthetic 

treatment should be compatible with the precasting processes and the construction tolerances. 

Following are some general guidelines that should be considered while developing aesthetic 

treatments: 

	 Cost of treatments that require special form-liners must be considered in the project cost 

estimate because form-liners require special fabrication and their number of uses is limited. 

	 Generally, the relief of the protruding artwork should be less than 1.5 in. (38 mm). 

	 Do not hang heavy aesthetic treatments from the facing units unless the facing units and the 

internal soil reinforcements are designed to withstand the forces from the artwork and 

environmental forces due to wind, snow, etc. 

	 Consider facing construction tolerances in formliner fabrication processes. 

	 Consider compatibility of the wall construction tolerances with the tolerances in details of 

the adjacent aesthetic treatments. Relief patterns are difficult to maintain between a cast-inplace structure and an adjacent MSE walls as illustrated in Figure 5-33(a), and is not 

recommended. Relief patterns between structures should be interrupted by a false column 

or other feature, as illustrated in Figure 5-33(b). 

	 Horizontal patterns parallel to the horizontal panel joints may not align after construction 

due to differential settlement. 

	 Consider using irregular patterns such as Ashlar stone that tend to hide inevitable 

imperfections in lines across joints between facing units. 

	 Consider the angle of sunlight expected at the location of the wall. At various times of the 

day, sunlight tends to accentuate the effect of the aesthetic features. The effect of 

imperfections resulting from regular construction tolerances on the artwork may be 

exaggerated leading to a false sense of problems and/or poor artwork. 

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	 Consider use of colors that are compatible with the wall facing material and weathering of 

the color scheme. 

	 Aesthetic treatments may use obstructions, acute corners, and face penetrations for effect, 

which require careful design review along with increased inspection. 

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(a) (b)

(c) (d)

(e) (f)

Figure 5-33. Example of MSE wall aesthetics. 

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(a) 

(b) 

Figure 5-34. Examples of cast-in-place abutment to MSE wall panel transitions, (a) no 

transition between C.I.P. and precast panels and difficult to match lines, (b) a false column 

between C.I.P. and precast panels masks lines that may not match. 

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CHAPTER 6 

DESIGN OF MSE WALLS WITH COMPLEX GEOMETRICS 

The basic design methods outlined in Chapter 4 consider MSE structures with simple geometries 

with reinforcement layers of the same length supporting either a horizontal backfill or a 

surcharge slope. Although most MSE structures fall into this category, structures with more 

complex geometries or significant external loads are feasible and require consideration during 

the selection process. They include: 

 Bridge abutments with MSE walls 

 Superimposed (tiered) MSE walls 

 MSE Walls with uneven length reinforcements (trapezoidal walls) 

 Back-to-back MSE (BBMSE) walls 

 Shored MSE (SMSE) walls for steep terrains and low volume roads 

 Stable feature MSE (SFMSE) walls 

Schematics of these complex cases are illustrated in Figure 6-1. 

The shape and location of the maximum tensile forces line are generally altered by both the 

geometry and the loads applied on the complex MSE wall structure. It is possible to assume an 

approximate maximum tensile force line for each. However, supporting experience and analysis 

are more limited than for rectangular reinforced soil walls. 

For complex or compound structures, it is always difficult to separate internal stability from 

external stability because the most critical slip-failure surface may pass through both reinforced 

and unreinforced sections of the structure. For this reason, both global and compound stability 

analyses are required for these types of complex structures. The current method for performing 

these analyses is to use an ASD reinforced soil slope stability computer method, as detailed in 

Chapter 9. An alternative method is to adapt the simple modification to the global and 

compound stability analyses for the LRFD procedure as discussed in Chapter 4. 

The following sections give guidelines for each complex case identified in Figure 6-1. 

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H1 

Lt 

H2 

Lb 

H′

L 

H
 

(a) Bridge abutment (b) Superimposed (Tiered) walls 

Lt 

Lb 

H 

(c) Trapezoidal walls (d) Back-to-back walls 

Lt Lt 

Stable Feature 

(rock face) 

H H

Lb 

Shoring Wall 

MSE Reinforcement Lb 

MSE Reinforcement 

(e) Shored walls (f) Stable feature walls 

Figure 6-1. Types of complex MSE structures. 

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6.1 BRIDGE ABUTMENTS WITH MSE WALLS 

Bridge abutments have been designed to support the bridge superstructure on a spread 

foundation constructed directly on the reinforced soil zone, or on a deep foundations constructed 

through the reinforced soil zone. 

The configuration wherein bridge superstructure is supported on a spread footing on top of the 

reinforced soil zone may be more economical compared to abutments supported by deep 

foundation through the reinforced soil zone, and should be considered when the projected 

settlement of the foundation and reinforced volume is rapid/small or essentially complete, prior 

to the erection of the bridge beams. Based on field studies of actual structures, AASHTO (2007) 

suggests, that tolerable angular distortions (i.e., limiting differential settlements) between 

abutments or between piers and abutments be limited to the following angular distortions (in 

radians): 

 0.008 for simple spans, and 

 0.004 for continuous spans 

This criteria, suggests that for a 100 ft (30 m) span for instance, differential settlements of 4.8 in. 

(120 mm) for a continuous span or 9.6 in. (240 mm) for a simple span, would be acceptable, with 

no ensuing overstress and damage to superstructure elements. On an individual project basis 

differential settlements of smaller amounts may be required from functional or performance 

criteria. Settlements well within the tolerable range can often be achieved with MSEW 

abutments abutments on spread footings. 

6.1.1 MSEW Abutments on Spread Footings 

Where fully supporting the bridge loads, MSEW bridge abutments are designed by considering 

them as rectangular walls with surcharge loads at the top. The base width of the bridge support 

spread footing, bf, and the location of the toe of the footing with respect to the back face of the 

walls panels, cf, is commonly such that bf + cf is greater than H/3. In this case, the shape of the 

maximum tensile force line, i.e., the critical failure surface, has to be modified to extend to the 

back edge of the spread footing. The variation of Kr/Ka and F* also need to be modified. Figure 

6-2 shows definitions of various parameters including measurements of heights and depths. 

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Roadway bf cf Surface Kr/Ka F* z=0 z=0 

h 

z 

z=20 ft z=20 ft

H' H Z

z=H' z=H'

L 

d z z 

Resistant 

Zone 

Active 

Zone 

0.3 H' 

1 

0.6 

z' 

Notes: 

 d is the depth of embedment 

 Z is measured below bottom of footing; z is measured from top of spread footing 

 H is measured from top of leveling pad to bottom of bridge support spread footing 

 h is height of the wall as measured from bottom of bridge support spread footing to 

finished roadway surface 

 H' is height of wall as measured from top of leveling pad to finished roadway surface 

 z = Z + h; z' = H – (cf + bf)/0.6 

 Within height z' the length of the reinforcement in the active zone is La= cf + bf 

Figure 6-2. Geometry definition, location of critical failure surface and variation of Kr and 

F* parameters for analysis of a MSEW abutment on spread footing. 

Although MSEW abutments on spread footings have historically almost always used 

inextensible, steel reinforcements, they can also be used with extensible reinforcements. 

However, similar shifts in the maximum tension line to the back of the footing have been 

observed for extensible reinforcement. Therefore, the maximum tensile force line should also be 

modified for extensible reinforcement if the back edge of the footing extends beyond a distance 

of H*tan(45°-°/2) from the wall face. These maximum tensile force lines should be compared 

with the critical failure surface from compound stability analysis and the more conservative 

profile of the failure surface should be selected. 

Successful experience with construction of MSEW abutments on spread footings has suggested 

that the following additional details be implemented: 

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	 Require a minimum offset from the front of the facing to the centerline of the bridge bearing 

of 3.5 ft (1 m). 

	 Require a minimum clear distance, cf, of 6 in. (150 mm) between the back face of the facing 

panels and the front edge of the footing. 

	 In areas that are susceptible for frost, the frost effect can develop from both the top of the 

wall as well as the front of the wall. Where significant frost penetration is anticipated, place 

the abutment footing on a bed of non-frost susceptible compacted coarse aggregate (e.g., No. 

57 as specified in AASHTO M 43). The thickness of the aggregate bed should be minimum 

3 ft (1 m) or 1 ft (0.3 m) below deepest anticipated frost penetration depth, whichever is 

greater. Separation geotextile should be provided at the interface of No. 57 coarse aggregate 

and the surrounding fills (reinforced, retained and above the footing base). Adjoining 

sections of the separation geotextile should be overlapped by a minimum of 1 ft (0.3 m). 

	 For the analysis of the spread footing on top of the reinforced soil zone, use the following 

values of bearing resistance of the reinforced soil zone 

o For service limit state, bearing resistance = 4 ksf (200 kPa) to limit the vertical 

movement to less than approximately 0.5 in. (12.5 mm) 

o For strength limit state, factored bearing resistance = 7 ksf (335 kPa) 

(Note: AASHTO does not provide a value of factored bearing resistance at strength 

limit state and the recommended value is based on the authors’ experience.) 

	 Use the maximum horizontal force at top reinforcement level below the abutment for the 

design of connections of the panels at all reinforcement levels. 

	 Extend the density, length and cross-section of reinforcements of the abutments to wingwalls, 

for a horizontal distance which is greater of the following: 

o 50 percent of the maximum height, H, of the abutment wall face. 

o cf + bf + 3 ft (1 m) where cf and bf are as shown in Figure 6-2 

	 There will be 2-way soil reinforcement within the length of reinforcement perpendicular to 

the abutment face. It is preferable that reinforcement is not placed on top of each other in the 

zone of 2-way reinforcement. The overlapping reinforcement should be separated by 3 to 6 

in. (75 to 150 mm) of soil or some multiple of compacted fill height. This may be achieved 

by appropriately adjusting the steps of the leveling pad between the abutment face wall and 

the wing walls. This practice is especially recommended where a corrosion monitoring 

program is implemented in the abutment area (Elias et al., 2009). 

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	 To prevent adverse stress concentrations at the reinforcement connections, the minimum 

vertical clearance between the bottom of the bridge support spread footing and the top level 

of reinforcement should be 1 ft (0.3 m). 

	 Due to the relatively high bearing pressures near the panel connections, the adequacy and 

nominal capacity of panel connections should be determined by conducting pullout and 

flexural tests on full-sized panels. 

	 The seismic design forces should also include seismic forces transferred from the bridge 

though bearing supports which do not slide freely (e.g., elastomeric bearings). 

In the LRFD context, the design of a MSEW abutment on spread footing requires careful 

separation of various load types. This results in a complex set of inter-related equations which 

are best illustrated by a worked example. Example E4 presents a comprehensive step-by-step 

illustration of both external and internal stability of a MSEW abutment on spread footing. The 

reader should become familiar with Example E5 because the principles and computations used in 

the example problem can also be applied to different complex geometries. 

6.1.2 MSEW Abutments on Stub Footings Supported by Deep Foundations through 

Reinforced Wall Fill 

For cases where MSEW abutments on spread footings may not be viable based on considerations 

of unacceptable post-construction settlements or other reasons, the bridge superstructure is 

placed on stub footings supported by deep foundations such as driven piles or drilled shafts. In 

this configuration, vertical loads are not considered in analysis since they are transmitted to an 

appropriate bearing stratum by deep foundations. However, the horizontal bridge and abutment 

backwall forces must be resisted by methods dependent on the type of abutment support, namely: 

	 For conventional abutments, the horizontal forces must be resisted by extending soil 

reinforcement from the back edge of the abutment footing (cap). The resistance is provided 

by the interaction between the soil and reinforcement over the full length of the 

reinforcement. A typical detail is shown in Figure 6-3. Alternatively, the horizontal forces 

may be resisted by the lateral resistance of the deep foundation or by other means. 

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Note: All dimensions are in mm [1 in. = 25.4 mm] 

Figure 6-3. Details of a typical pile supported MSE abutment. 

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	 For integral abutments, the horizontal forces and its distribution with depth may be 

developed using a lateral load (p)-lateral deflection (y), i.e., p-y methods. These horizontal 

forces are added as a supplementary force to be resisted by the reinforcements. These forces 

will vary depending on the following: 

o magnitude of the horizontal loads and moments, 

o diameter and spacing of deep foundations, and 

o clear distance between the back face of wall panels and front of the deep foundation 

elements. 

Several agencies have constructed integral abutments in front of MSE walls as discussed in 

Section 6.1.3 in order to avoid applying a lateral stress to soil behind the abutment. 

Figure 6-4 shows a typical supplemental lateral pressure that must be considered in the internal 

stability analysis. This lateral pressure is addressed in a fashion similar to the inverted triangular 

lateral pressure distribution shown in Figure E5-2 of Example Problem E5. The effect of the 

roadway fill and the live load surcharge above the MSE wall is also addressed in a fashion 

similar to that for the same features in Example E5. The balance of the computations remains 

identical to those in Chapter 4. 

Based on successful experience of the authors with abutment construction of MSE walls with 

deep foundations through the reinforced fill, following are suggested additional details, as 

applicable: 

	 Where significant settlement of the embankment is anticipated, provide casings (e.g., 

sonotubes or corrugated metal pipes) in the reinforced soil zone to permit construction of 

deep foundations after the MSE wall is constructed and settlement has occurred. In the case 

of driven piles it may be possible to isolate the piles from the casings by filling the annulus 

with loose sand just prior to construction of the footing at top of the piles. In the case of 

drilled shafts it may not be possible to isolate the shaft from the casing in an economical 

manner unless another internal casing is used. 

	 In the case where deep foundations are constructed prior to MSE wall construction, and 

negative skin friction, i.e., downdrag force, is anticipated, provide a casing around the deep 

foundation element, through the reinforced fill. The casing is filled with sand just prior to the 

construction of the footing at top of the deep foundation element. Alternatively, a bond 

breaker can be used on the deep foundation element when negative skin friction, i.e., 

downdrag force, is anticipated. 

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Roadway Traffic Surcharge 

qT 

Simplified supplemental lateral 

pressure distribution on back of 

wall facing (may have nonlinear 

distribution with depth based on 

actual geometries and loads) 

qB 

Deep foundation extending more 

than 1 time the height of the MSE 

wall below the wall or socketed 

into sound rock 

d 

H Reinforced fill 

Surface 

Bridge Superstructure Roadway fill above MSE wall 

Retained 

fill 

Foundation soil 

z 

h 

Z 

Notes: 

 d is the depth of embedment 

 H is measured from top of leveling pad to top of coping 

 z is measured below finished grade behind footing 

 The Kr/Ka and F* values are assumed to start at z = 0, i.e., below the finished grade 

behind the footing. 

Figure 6-4. Geometry definition and typical supplemental lateral pressure distribution from

 deep foundation on MSE wall face. 

	 In the case where deep foundations are constructed prior to MSE wall construction and/or the 

deep foundation element is not isolated from the casing as noted above, the horizontal 

stresses as shown in Figure 6-4 must be included in the analysis of MSE wall. If the deep 

foundations are constructed through casings and isolated from the casings, the horizontal 

stresses may be neglected in the design of the MSE wall. However, it must be realized that 

this configuration leads to a longer unsupported length of the deep foundation that may result 

in undesirable movements at the bridge seat level in addition to increased size of the deep 

foundation element. 

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	 For driven piles extending though the reinforced soil zone, require a minimum offset from 

the back face of the wall panels and the front of the driven pile elements as 1.5 ft (0.5 m). 

	 For drilled shaft extending through the reinforced soil zone, require a minimum offset from 

the back face of the wall panels and the front of drilled shaft elements as 3 ft (1 m). This 

criterion provides the necessary clear space to achieve proper compaction of the soil in this 

area. Thus, for example, if drilled shafts with a maximum dimension of 2 ft (0.61 m) are used 

then the minimum clearance is 3 ft (1 m). For walls where reinforcements will be splayed 

(e.g., steel strip reinforcements), require a minimum offset from the back face of the wall 

panels and the front of deep foundation elements as the greater of 3 ft (1 m) or 1 times the 

diameter of the deep foundation. Thus, for example, if a drilled shaft of 4 ft (1.2 m) diameter 

is used then the minimum clearance is 4 ft (1.2 m). These criteria provides the necessary 

clear space to achieve proper compaction of the soil in this area and adequate distance for 

splaying of reinforcements within the acceptable limits noted in Chapter 5. 

	 Provide soil reinforcements in the soil behind the abutment footing (cap) as shown in Figure 

6-3. 

Interference between Soil Reinforcements and Deep Foundations 

Design of MSE walls with deep foundations needs careful consideration of the interference 

between the soil reinforcements and the deep foundation element(s). Where deep foundation 

elements interfere with the reinforcements, specific methods for field installation must be 

developed and presented on the plans. Simple cutting and then bending of the reinforcements 

during construction should not be allowed. Guidance for navigating soil reinforcement 

around vertical obstructions is presented in Chapter 5. Soil cover as recommended in Section 

5.4.1 of Chapter 5 between dissimilar metals should be implemented as appropriate. 

6.1.3 Alternative Configuration of MSE Walls at Bridge Abutments 

An alternative to construction of MSEW abutments that use deep foundations through reinforced 

backfill is to construct the MSE walls behind abutment foundations that are constructed. In this 

configuration, the foundations are not constructed within or on top of reinforced fills. Rather the 

MSE walls supports only the approach fills while the abutments are constructed in configuration 

of piers. Special details (e.g., bridge approach slabs) are required for this configuration to 

integrate the MSE walls with the bridge abutment. The major advantage of this type of abutment 

configuration is that the construction of the foundations for the abutments can be performed 

independently of the MSE wall construction and that better construction control can be exercised 

for MSE walls since there are no obstructions through the reinforced backfill. 

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Based on successful experience of the authors with construction of an abutment configuration 

with MSE wall behind the abutment foundations, following are suggested additional details, as 

applicable: 

	 Require that the bridge superstructure be placed after the construction of the MSE walls so 

that most of the possible foundation deformations have occurred. 

	 Construct the foundations prior to construction of the MSE wall, but construct the abutment 

columns after the construction of the MSE wall. In this construction sequence, the 

foundation deformation due to the construction of the adjacent MSE wall can be 

compensated for by adjusting the connection of the abutment structure rather than running 

the risk of abutment structure deforming to the extent that it does not fit with the bridge 

superstructure at the beam seat level. 

	 Consider construction of a false wall in front of the abutment substructure, i.e., the element 

between the foundation and the superstructure. This false panel serves to protect the 

abutment elements against vehicular impact as well as protecting vehicle occupants. The 

false wall may be structurally connected to the abutment substructure or an independent wall 

with a separation of 3 to 6 in. (75 mm to 150 mm) with the decision based on the design of 

the abutment substructure and its ability to absorb vehicular impacts. 

	 For integral abutments with a wall supported on deep foundations, a wrapped or wire faced 

MSE wall can be constructed behind the abutment wall, using the abutment as and offset 

form with a spacer to maintain the distance between the MSEW and the abutment wall as 

shown in Figure 6-5. 

6.1.4 Protection of MSE Wall at Abutments 

At abutment locations, the permeation or water through expansion joints into the MSE wall 

results in a number of seepage problems as discussed in Chapter 5 including the potential for 

salt-laden runoff, which could result in a chloride rich, corrosive environment near the face panel 

connection for a significant percentage of the wall height. To minimize this problem, seepage 

should be controlled as shown on Figure 6-6. 

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Figure 6-5. Example of use of a geosynthetic wrapped face wall behind an integral abutment. 

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Note: All dimensions are in mm [1 in. = 25.4 mm] 

Figure 6-6. Example abutment seat detail. 
 

6.2 SUPERIMPOSED (TIERED) MSE WALLS 

For tall walls consideration should be given to superimposed (tiered) walls from the viewpoint of 

constructability. Reconfiguring a tall wall in superimposed walls with smaller heights permits a 

fresh start with a new leveling pad, reduces vertical stress on facing elements, and permits better 

control of vertical alignment of the wall face. Analytically, depending on the offsets between the 

superimposed walls, another beneficial effect might be an overall (equivalent) sloped wall face 

that results in lesser lateral force on the whole wall system. 

6.2.1 2-Tier Superimposed Walls 

Figure 6-7 shows a configuration of a 2-tier superimposed MSE wall system. The design of 

superimposed MSE walls requires two analyses as follows: 

(1) A design using simplified design rules for calculating external stability and locating the 

internal failure plane for internal stability as shown in Figure 6-7. 

(2) A slope stability analysis, including both compound and global stability using a reinforced 

soil global stability computer program outlined in Chapter 4. This is an essential 

computation. 

The definition of wall heights, H1 and H2, and offset D between walls for a 2-tier superimposed 

wall configuration is shown in Figure 6-7. Using the definitions in Figure 6-7, for preliminary 

design, the following minimum values for reinforcement length, of L1 and L2, should be used for 

offsets (D) greater than [1/20 (H1 + H2)]: 

Upper wall: L'1 ≥ 0.7 H1 

Lower wall: L'2 ≥ 0.6 H where H = H1 + H2 

Based on the definitions in Figure 6-7, following are basic design guidelines: 

	 Where the offset distance (D) is greater than H2 tan (90-r), walls are not considered 

superimposed and are independently designed from an internal stability viewpoint, 

	 For a small upper wall offset; D ≤ [1/20 (H1 + H2)], it is assumed that the failure surface does 

not fundamentally change and it is simply adjusted laterally by the offset distance D. The 

walls should be designed as a single wall with a height H. 

In both of the above cases, compound and global stability should be checked. 

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Figure 6-7. Design rules for a 2-tier superimposed MSE wall system. 

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The stability analysis for a 2-tier superimposed MSE wall system is performed as follows: 

	 External stability calculations for the upper wall are conventionally performed as outlined in 

Chapter 4. For the lower wall, consider the upper wall as a surcharge (Load type “ES”) in 

computing bearing pressures. In lieu of a conventional external sliding stability computation, 

perform a wedge type slope stability analysis with failure surfaces along and exiting at the 

base as well as below the base. The overall stability should be investigated at the Service I 

load combination and a sliding resistance factor of 0.65. 

	 For calculating the internal stability, the maximum tensile force lines are as indicated in 

figure 6-7a. These relationships are somewhat empirical and geometrically derived. 

	 For intermediate offset distances, see Figure 6-7a for the location of the failure surface and 

consider the vertical pressures in Figure 6-7b for internal stress calculations. 

	 For large setback distances, [D ≥ H2 tan (90-r)], the maximum tensile force lines are 

considered independently, without regard to the geometry of the two superimposed walls. 

For internal stability computations, the upper wall is neglected. 

	 The balance of the computations remains identical as in Chapter 4. 

6.2.2 Superimposed walls with more than 2-tiers 

The criteria for 2-tier wall presented in Figure 6-7 can be extended to walls with more than two 

tiers. For such configurations, the global and compound stability analysis becomes even more 

critical. Methods outlined in Chapter 4 may be used for evaluating the global and compound 

stability. For internal stability analysis, Wright (2005) and Leschinsky and Han (2004) found 

that the criteria for additional vertical stress in Figure 6-7b may be used for walls with more than 

2-tiers provided that only the immediately overlying tier is considered to contribute to the 

increase in vertical stress on the lower tier. As an alternative, Wright (2005) presents an elastic 

solution based on an assumption of “rigid” walls for estimating additional vertical stresses in a 

given tier of a multi-tier wall due to the effect of all overlying wall tiers. Regardless of the 

approach used for estimating the increase in vertical stresses for evaluation of internal stability, 

the analysis of tiered walls should proceed from the top wall to the bottom wall so that the 

stresses are properly accumulated and accounted for in the design of the bottom-most wall. For 

preliminary design, the length of the reinforcement of the bottom-most tier can be assumed to be 

0.6 times the total height of the wall system. 

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6.3 MSE WALLS WITH UNEVEN REINFORCEMENT LENGTHS (TRAPEZOIDAL 

WALLS) 

Use of this type of reinforcement geometry should be considered only if the base of the MSE 

structure is in rock or competent foundation material, i.e., foundation materials which will 

exhibit minimal post construction settlements. Examples of competent foundation materials 

include materials with SPT N60 value greater than 50 and sound rock. 

The design of these walls requires two analyses as follows: 

1. A design using simplified design rules for determining internal and external stability. 

2. A slope stability analysis performed using a reinforced soil stability program checking both 

global (i.e., circular and wedge type analysis) and compound failure planes. 

Simplified design rules for these structures are as follows: 

	 As shown in Figure 6-8, the wall is represented by a rectangular block (Lo, H) having the 

same total height and the same cross-sectional area as the stepped section for external 

stability calculations. 

	 The maximum tensile force line is the same as in rectangular walls (bilinear or linear 

according to the extensibility of the reinforcements). 

	 Minimum base length (L3) of 0.4H or 8 ft (2.5 m) whichever is greater, with the difference in 

length in each zones being less than 0.15 H. 

For internal stability calculations, the wall is divided in rectangular sections and for each section 

the appropriate L (L1, L2, L3), is used for pullout calculations, using methods developed in 

Chapter 4. 

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L1 

L H 2 

L3 

L0 (0.7 H min) 

Figure 6-8. Dimensioning of MSE wall with uneven reinforcement lengths. 

 

6.4 BACK–TO–BACK MSE (BBMSE) WALLS 

Back-to-back walls are often used for highway ramps. For walls which are built back-to-back as 

shown in Figure 6-9, a modified value of lateral pressure influences the external stability 

calculations. As indicated in Figure 6-9, two cases can be considered and are discussed below. 

 Case I 

For Case I, the overall base width is large enough so that each wall behaves and can be designed 

independently. In particular, there is no overlapping of the reinforcements. Theoretically, if the 

distance, D, between the two walls is shorter than D = H1 tan (45° - °/2) where H1 is the taller 

of the parallel walls, then the active wedges at the back of each wall cannot fully spread out and 

the active thrust is reduced. However, for design it is assumed that for values of D > H1 tan (45o 

- °/2) ≈ 0.5H1 then full active thrust is mobilized. 

 Case II 

For Case II, there is an overlapping of the reinforcements such that the two walls interact. When 

the overlap, LR, is greater than 0.3H2, where H2 is the shorter of the parallel walls, no active 

earth thrust from the backfill needs to be considered for external stability calculations. 

For intermediate geometries between Case I and Case II, the active earth thrust may be linearly 

interpolated from the full active case to zero. 

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LR 45°+/2 45°+/2

H2 H H2 L1 1 H1 

L2 

L2 D L1 Wb 

Case I Case II 

Figure 6-9. Back–to–back MSE walls. 

For Case II geometries with overlaps (LR) greater than 0.3H2, the following guidelines should be 

used: 

 L1/H1 ≥ 0.6 where L1 and H1 is the length of the reinforcement and height, respectively, of 

the taller wall. 

 L2/H2 ≥ 0.6 where L2 and H2 is the length of the reinforcement and height, respectively of the 

shorter wall. 

 Wb/H1 ≥ 1.1 where Wb is the base width as shown in Figure 6-9 and H1 is the height of the 

taller wall. 

The above guidelines are valid for static load conditions or in areas where the seismic horizontal 

accelerations at the foundation level are less than 0.05g. Back-to-back walls in seismically 

active areas should be designed based on a more detailed analysis that includes effects of 

potential non-uniform distribution of seismic and inertial forces within the wall. 

For back-to-back walls designers might be tempted to use single layers of reinforcements that are 

connected to both wall facings. This alternative creates an unyielding structure creating an at 

rest stress state (Ko) from the top to the bottom of the wall, resulting in much higher 

reinforcement tensions than previously used in the design method in this manual. The design 

must include the increases in lateral stress in the determination of the tension in reinforcement 

and connection and in the design of facing elements. Additionally compaction may induce 

higher stress at the connection, which must be accounted for in the lateral earth pressure 

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calculations. Furthermore, difficulties in maintaining wall alignment could be encountered 

during construction, especially when the walls are not in a tangent section. The exception is the 

use of geosynthetic wrapped faced walls, where alignment with connections is not an issue. 

However, while there is a potential for stress relief due to extensible reinforcements, very few 

instrumented structures have been constructed and therefore even in the case of geosynthetic 

reinforcements Ko should conservatively be used to calculate the tension in the reinforcements 

unless numerical modeling is performed to evaluate the anticipated stress state and 

instrumentation is used to confirm the actual stress conditions. 

6.5 SHORED MSE WALLS FOR STEEP TERRAINS AND LOW VOLUME ROADS 

In steep terrains MSE wall construction necessitate excavation to establish a flat bench to 

accommodate the soil reinforcements with a minimum length of greater than 8 ft (2.5 m) or 70% 

of the height of the wall. Additionally, the required depths of embedment are proportional to the 

steepness of the slope below the wall toe. In some cases, the excavation required for 

construction of a MSE wall becomes substantial, and unshored excavation for the MSE wall is 

not practical, particularly if traffic must be maintained during construction of the MSE wall. 

Shoring, most often in the form of soil nail walls, has been employed to stabilize the backslope 

(or back-cut), with a MSE walls being designed and constructed in front of it. Figure 6-10 shows 

a generic cross-section of this configuration. In this configuration, if the shoring wall is designed 

as a permanent wall it can significantly reduce the long-term lateral pressures on the MSE wall. 

Such MSE wall configuration is known as a shored MSE or SMSE wall. Details of SMSE walls 

systems are presented in FHWA-CFL/TD-06-001 (Morrison et al., 2006). 

Figure 6-10. Generic cross-section of a shored MSE (SMSE) wall system for steep terrains 

(Morrison et al., 2006). 

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For successful implementation of the SMSE walls, the following guidelines should be 

implemented. These guidelines are valid for static load conditions or in areas where the seismic 

horizontal accelerations at the foundation level are less than 0.05g. SMSE walls in seismically 

active areas should be designed based on a more detailed analysis that includes effects of 

potential non-uniform distribution of seismic and inertial forces within the wall system (both the 

MSE and the shoring components). Finally, it should be recognized that these walls were 

developed for low volume roads in mountains and are not recommended in urban areas for 

roadway widening applications because of the relatively high risk for tension cracks under 

dynamic effects of traffic at the interface between an existing wall and new MSE wall. 

	 The shoring wall should be designed as a permanent wall whose design life is equal to or 

greater than that for the MSE wall. For design of shoring systems using ground anchors and 

soil nail walls, see Sabatini et al. (1999) and Lazarte et al., (2003), respectively. 

	 Ensure that the drainage features of the MSE wall system and the permanent shoring wall 

behind it are integrated so that there are no lateral pressures due to hydrostatic conditions in 

either wall. Note, as discussed in Chapter 5, thin vertical drains behind the face of the soil 

nail wall do not necessarily fully relieve hydrostatic stress. Thus, some level of hydrostatic 

stress based on flow net analysis should be included in the design or horizontal drains and 

should be considered in the design of SMSE walls. 

	 Figure 6-11 presents the minimum recommended geometry of a SMSE system. The 

minimum length of the reinforcement is 0.3H or 5 ft (1.5 m) whichever is greater. Where 

adequate construction space is available (or can be made temporarily available with 

permanent underground easement), it is recommended that the upper two layers of 

reinforcement are extended to a minimum length of 0.6H or a minimum of 5 ft (1.5 m) 

beyond the shoring wall interface, whichever is greater, as illustrated in Figure 6-11a. This 

feature limits the potential for tension cracks to develop at the shoring/MSE wall interface, 

and resists lateral loading effects. Extension of the upper two layers is intended to result in a 

wall cross-section as depicted in Figure 6-11a, where the height of the shoring wall is at least 

2/3 of the MSE wall height, H. These guidelines should only be applied to wall designs that 

meet this constraint over the majority of their length. Walls with short shoring walls, i.e., 

heights less than 2/3H over most of their length are outside the scope of these guidelines. It 

should be noted that near the ends of the retaining wall the height usually tapers, and the 

shoring wall height may be less than 2/3 of the MSE height for a short distance. However, 

application of these guidelines will result in MSE reinforcements not less than 10 ft (3 m) 

long at the top of the MSE wall (5 ft (1.5 m) minimum plus 5 ft (1.5 m) minimum). 

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(a) Lb = Greater of 0.3H or 5 ft 1 m = 3.28 ft 

(b) Lb = Greater of 0.3H or 5 ft 

Figure 6-11. Minimum recommended geometry of a shored MSE wall system in steep terrains, 

(a) with extension of upper two rows of reinforcement, and (b) with the upper two 

rows connected to the shoring wall (Morrison et al., 2006). 

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 Where the shoring wall is less than 2/3 of the height of the MSE wall, as may occur as the 

wall ends taper, the engineer should check to assure that reinforcement lengths in the upper 

part of the MSE mass is greater than the conventional 0.7H as discussed in Chapter 4. 

Generally, this will be satisfied, as long as the total retaining wall height in such sections is 

less than about 14 ft (4 m). 

 If extension of the upper reinforcements is not feasible, a positive mechanical connection 

between the upper two or more reinforcements and the shoring wall is recommended as 

shown in Figure 6-11b. Incorporation of interface connections may limit differential 

movement between the shoring wall and MSE wall components, as a result limiting 

development of a tension crack, especially if the slack in the MSE reinforcements can be 

effectively removed. This could potentially be accomplished through the fastening 

mechanism or by surcharge loading. Extension of the upper MSE reinforcements is 

considered superior to mechanical connection of the reinforcements and is 

recommended by the authors. 

 The critical failure surfaces for SMSE walls with extensible and inextensible reinforcements 

are presented in Figure 6-12. The critical failure surface is approximated using Rankine’s 

active earth pressure theory within the reinforced soil mass, assuming that the remaining 

portion lies along the shoring/MSE interface. The critical failure surfaces are consistent with 

those presented in Chapter 4 (except pullout calculations). Design for internal stability 

conservatively neglects the additional retaining benefits provided by longer upper 

reinforcement layers shown in Figure 6-10a or the resistance from connections shown in 

Figure 6-11b. 

 For SMSE walls, lateral pressures are essentially the result of reaction of reinforced soil mass 

against the shoring wall, and are thus internal to the MSE mass. At each reinforcement level, 

the horizontal stress, h, along the potential failure line is computed using exactly the 

procedures in Chapter 4. If superimposed concentrated vertical loads are present then the 

increment of vertical stress (v) maybe computed by a modified version of the 2:1 method 

as shown in Figure 6-13. 

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(a) (b)

Figure 6-12. Location of potential failure surface for internal stability design of shored MSE walls 

(a) extensible reinforcements, (b) inextensible reinforcements (Morrison et al., 2006). 

Notes: 

1.	 The measurement of x may be from either the face of the MSE wall or the shoring wall, depending on 

the location of the load footing and the slopes of the various walls. 

2.	 The figure is applicable to MSE walls constructed without a batter, and where the load footing does 

not straddle the shoring wall. When wall batters are employed, as is generally recommended, the 

vertical stresses can be estimated by geometrically calculating D1 at each reinforcement depth. In the 

case where the footing straddles the shoring wall, D1 is always greater than z2, as defined in the figure. 

Figure 6-13. Distribution of stress from concentrated vertical load for internal and external 

stability calculations (Morrison et al., 2006). 

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	 Internal design differs from design of a conventional MSE wall with regard to pullout of the 

reinforcements. Conventional MSE design requires that each layer of reinforcement resist 

pullout by extending beyond the estimated failure surface as indicated in Chapter 4. In the 

case of a SMSE wall system, only the lower reinforcement layers (i.e., those that extend into 

the resistant zone) are designed to resist pullout for the entire “active” MSE mass. The 

relevant equations of TMAX and pullout resistance are given in Figure 6-14. The tensile 

resistance of the reinforcement as well as the connection strength is evaluated in accordance 

with the procedures in Chapter 4. 

	 External stability design of the MSE component of a SMSE wall should address bearing 

capacity and settlement of foundation materials based strength limit state and service limit 

state considerations. Limiting eccentricity (i.e., overturning) and sliding are not included as 

failure mechanisms due to stabilization provided by the shoring wall. Hydrostatic forces are 

eliminated by incorporating internal drainage into the design. Procedures for evaluating 

bearing capacity and settlement analysis are the same as those in Chapter 4. 

	 As part of the design of the individual MSE wall and shoring components, stability internal 

to these individual components will have been achieved. However, a global stability 

evaluation of the SMSE wall system as a compound structure must also be evaluated. 

Various failure modes are shown in Figure 6-15. Although, all failure five failure modes 

shown in Figure 6-15 must be evaluated, the most critical failure mechanisms are along the 

shoring/MSE interface (Mode 4 in Figure 6-15) and global stability external to the SMSE 

wall system must be evaluated (Mode 1 in Figure 6-15). Morrison et al. (2006) present 

suggestions for global stability analyses and measures to improve stability. Stability analyses 

for the SMSE wall system should use conventional (i.e., ASD) limit equilibrium analysis 

methods. As with any earth stability evaluation, selection of appropriate material parameters 

is of utmost importance in obtaining a realistic evaluation. In addition, the compound nature 

of the SMSE wall system requires defining other factors such as drainage issues which affect 

its behavior. 

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Case 1: For LW < H tan

  Lw   Lw 

H    q  FV 2 tan   	   T max	 	  FH tan

Case 2: For LW = 0.3H 

  3H   3H 

H    q  FV 20 tan   	   T max	 	  FH 10 tan

Case 3: For LW ≥ H tan

H tan H  2q 2FV T max	 	  FH 2 tan

Notes: 

1. The loads FV, FH and W should be multiplied by the appropriate load factors when evaluating 

the strength and service limit state load combinations. 

2. The pullout resistance of the MSE wall component of a SMSE wall system is considered 

adequate if TMAX ≤ pFpo where Fpo is the summation of the pullout resistances from all 

layers of reinforcement based on the length of the reinforcement beyond the active zone and 

p is the resistance factor as follows: 

a. p = 0.90 for L/H > 0.4 

b. p = 0.65 for L/H ≤ 0.4 

Figure 6-14. Computation for TMAX and evaluation of pullout resistance (after Morrison et al., 

2006). 

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Figure 6-15. Example global stability and compound failure surfaces (Morrison et al., 2006). 

6.6 STABLE FEATURE MSE (SFMSE) WALLS 

MSE walls can be considered in front of apparently stable features such as a rock face as shown 

in Figure 6-16. Depending on the space between the MSE wall face and the stable feature, the 

behavior of the SFMSE wall may be similar to that of a SMSE wall. Following are some 

guidelines for such cases: 

	 Establish that the feature behind the proposed SFMSE wall line is stable and will be stable 

during the design life of the SFMSE wall. The feature should be stabilized to the extent 

necessary to be compatible with the design life of the SFMSE wall that is being proposed at 

that particular location. 

	 Evaluate the deformation and strength behavior of the feature (rock face or existing wall) 

under additional stresses behind it. Hydrostatic pressure and or other lateral pressures may 

contribute to the instability of a rock cut in front of which a SFMSE wall is being proposed. 

The stability analysis should include an evaluation of potential lateral movements under 

anticipated additional loadings on the existing feature. 

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	 Perform a deformation analysis of the foundation under the SFMSE wall and evaluate the 

effect of the estimated deformations on the facilities above the top of the wall and in 

particular at and immediately above the interface between the existing feature and the 

SFMSE wall. 

	 Evaluate the effect of the increased stresses at the base of the MSE wall on the settlement of 

the existing feature. If the existing feature is a retaining wall then it might experience 

detrimental settlement in the immediate and long-term as well as downdrag forces at the 

interface between the MSE wall and the existing feature. 

	 Ensure that the drainage features of the SFMSE wall system and the stable feature behind it 

are integrated so that there are no lateral pressures due to hydrostatic conditions. 

	 For SFMSE wall systems, the configuration in Figure 6-16 is recommended wherein at least 

the top two reinforcements are extended over the top of the stable feature rather than being 

mechanically connected to the stable feature. For roadway widening projects where the 

stable feature may be an existing wall, it is recommended that the top of the wall be trimmed 

as necessary to accommodate the top layers of reinforcements and mitigate long-term 

maintenance issues. 

	 Extend all soil reinforcement layers above the top of the stable feature a distance back of Lt, 

per Figure 6-15, with a minimum of two layers as previously noted. 

	 Establish the reinforcement layout based on the TMAX values obtained using the guidance 

provided in Figure 6-14 and other guidance provided in Section 6.5. The minimum clearance 

between the top of the stable feature and the reinforcement layer above it should be 6 in. (150 

mm) to prevent adverse stress concentrations in this area and contact between dissimilar 

materials. 

	 Global stability analysis should be performed as the MSE wall will increase driving forces. 

Global analysis is especially needed where structures are constructed with a slope at the toe 

or on soft ground. All failure modes similar to those shown in Figure 6-15 should be 

evaluated. 

The above guidelines are valid for static load conditions or in areas where the seismic horizontal 

accelerations at the foundation level are less than 0.05g. SFMSE walls in seismically active 

areas should be designed based on a more detailed analysis that includes effects of potential nonuniform distribution of seismic and inertial forces within the wall system (both the MSE and the 

stable feature components). Finally, these types of walls are not recommended in urban areas for 

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Lt 

3 ft min 

H 

6.5 ft min 

Stable Feature 

(rock face) 

MSE Reinforcement Lb 

1 m = 3.28 ft Lt = 0.8 H min; Lb = Greater of 0.3H or 5 ft 

Figure 6-16. Minimum recommended geometry of a stable feature MSE (SFMSE) wall system. 

cases such as roadway widening because of the relatively high risk for tension cracks under 

dynamic effects of traffic at the interface between the existing feature such as a wall and new 

MSE wall. 

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CHAPTER 7 

DESIGN OF MSE WALLS FOR EXTREME EVENTS
 

As per AASHTO (2007) an extreme event is one whose recurrence interval can be thought to 

exceed design life. AASHTO (2007) has two limit states to deal with such events. These limit 

states are labeled Extreme Event I and Extreme Event II. In the context of MSE walls, the 

extreme events with the applicable limit state shown in parentheses that require consideration in 

the design process are as follows: 

 Seismic events (Extreme Event I 

 Vehicular impact events (Extreme Event II) 

 Superflood events and scour (Extreme Event II) 

This chapter addresses each of the above extreme events along with a review of the applicable 

limit state, i.e., Extreme Event I or Extreme Event II. 

7.1 SEISMIC EVENTS 

Seismic events are analyzed under Extreme Event I limit state as per AASHTO (20071). Seismic 

events tend to affect both external and internal stability of MSE walls. Guidance for seismic 

analysis presented in this section is based on Anderson et al. (2008) and Kavazanjian (2009) and 

represents updated procedures to those in AASHTO (2007). 

7.1.1 External Stability 

The external stability uses a displacement based approach. The recommended design methodology 

is presented in the following steps. 

Step 1 Establish an initial wall design based on static loading using information in Chapters 4, 5 

and 6. 

Step 2 Establish the seismic hazard using Article 3.10.2 of AASHTO (2007). Using the 1,000-

yr return period seismic hazard maps in AASHTO (2007), estimate the following sitespecific values: 

 The site peak ground acceleration (PGA), and 

 Spectral acceleration at 1-second, S1 

 

1

 AASHTO 4th Edition 2007 including 2008 and 2009 Interims. 2008 Interims contain significant seismic revisions. 

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Step 3 For the project under consideration, establish the Site Effects in accordance with Article 

3.10.3 of AASHTO (2007). This includes the determination of Site Class as per Article 

3.10.3.1 of AASHTO (2007) and Site Factors, Fpga and Fv from Tables 3.10.3.2-1 and 

3.10.3.2-3, respectively, of AASHTO (2007). The procedure described herein is 

applicable to Site Classes A, B, C, D and E. For all sites in Site Class F, site-specific 

geotechnical investigations and dynamic site response analysis should be performed. 

Step 4 Determine the maximum accelerations, kmax, and peak ground velocity (PGV) as follows: 

kmax = Fpga (PGA) (7-1) 

PGV (in/sec) = 38FvS1 (7-2)

where Fpga and Fv are site factors determined in Step 3 and PGA and S1 are site peak 

ground acceleration and spectral acceleration at the 1-second period, respectively, as 

obtained in Step 2. 

Step 5 Using a wall height dependent reduction factor, , obtain an average peak ground 

acceleration, kav, within the reinforced soil zone as follows: 

kav = kmax	 (7-3) 

where the value of  is based on the Site Class of the foundation soils as follows: 

	 For Site Class C, D and E (i.e., soils) 

	  Fv S1   α  1  0.01H0.5

   1 (7-4) k 	  max   

where H is the wall height in feet at the wall face as shown in Figure 7-1. 

	 For Site Class A and B foundation conditions (i.e., hard and soft rock), the values of 

 determined by Equation 7-4 should be increased by 20 percent. 

For practical purposes, walls less than approximately 20 ft in height and on very firm 

ground conditions (i.e., Site Class B or C), kav ≈ kmax. For wall heights greater than 100 

ft, site-specific geotechnical investigations and dynamic site response analysis should be 

performed. 

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Ground Surface 

Surface Loads 

H 

h 

h/2 

tan I (0.5H) h  H  ;where I is the backfillslopeangle 10.5tan I

h/2 is measured from back of wall facing 

Figure 7-1. Definition of heights for seismic analyses. 

Step 6 Determine the total (static + dynamic) thrust PAE using one of the following two 

methods: 

Method 1: Mononobe-Okabe (M-O) formulation 

2 P AE  0.5K γ h	 (7-5) 	 AE b 

where h is the wall height along the vertical plane within the reinforced soil mass as 

shown in Figure 7-1, b is the unit weight of the retained fill and KAE is obtained as 

follows: 

K  cos2 (' 

b 90)

AE
 2 

2  sin(' 

b )sin(' b I) 

 (7-6) 

coscos	 (90 )cos(  90)1  cos( 90)cosI 90  

where, 

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1  k   tan  h    with k

1 h = horizontal seismic coefficient and kv = vertical seismic  kv 

coefficient 

 = angle of wall friction = lesser of the angle of friction for the reinforced soil mass 

(' r) and the retained backfill (' b) 

I = the backfill slope angle = β (see Figure 4-3) {Note: use GLE for broken back 

slopes, see Comment 2 below} 

' b = angle of internal friction for retained backfill 

 = the slope angle of the face (see Figure 4-5 in Chapter 4) 

 

To use the Mononobe-Okabe formulation, two seismic coefficient, kh and kv, must be 

defined. It is assumed that these coefficients are applied simultaneously and uniformly 

to all parts of the structure, i.e., to the reinforced and retained fill. Typically, the vertical 

seismic coefficient, kv, is assumed to be zero. The horizontal seismic coefficient, kh is 

taken to be equal to kmax determined in Step 2. 

The total thrust, PAE, calculated as per Equation 7-5 is assumed to act at h/2, i.e., midheight of the vertical plane of height h shown in Figure 7-1. Therefore, the stress due to 

thrust PAE is assumed to be distributed uniformly over the height h. 

Comments on use of M-O formulation: 

1. For backfills that are sloped at 3H:1V or steeper, it may not be possible to obtain a 

solution for a certain combination of variables in the M-O formulation. This is 

because the term sin( –  – I) in Equation 7-6 may become negative and represents 

a limiting condition since at I =  –  an unstable slope condition occurs (i.e., FS=1 

wherein the failure surface coincides with the slope surface. As the limiting 

condition is approached the earth pressures based on M-O formulation become 

unrealistically large. 

2.	 M-O formulation is strictly applicable to homogeneous cohesionless soils and may 

not yield realistic solutions for more complex cases involving (a) soils which derive 

their shear strength from both cohesion and friction, i.e., c- soils, (b) non-uniform 

backslope profiles, and (c) complex surface loadings. 

For the cases where M-O formulation leads to unrealistic results, it is recommended that 

numerical procedures using the same principles of M-O formulation may be used, such 

as the well-known graphical Culmann method or Coulomb’s trial wedge method. 

However, the more versatile approach for such cases is to utilize the conventional slope 

stability programs as described in Method 2. 

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Method 2: Generalized Limit Equilibrium (GLE) slope stability 

a. Define the wall geometry, nominal surface loadings (i.e., loadings with load factor = 

1.0), groundwater profile, and design soil properties. The plane where the earth 

pressure needs to be calculated should be modeled as a free boundary. This 

boundary is a vertical plane located at a distance of h/2 from the back of the wall 

facing as shown in Figure 7-2. 

b. Choose an appropriate slope stability analysis method. Spencer’s method generally 

yields good results because it satisfies the equilibrium of forces and moments. 

c.	 Choose an appropriate sliding surface search scheme, e.g., circular, linear, bi-linear, 

block, etc. 

d.	 For seismic analysis, use kh = kav and kv = 0. 

 

e. Apply the earth pressure as a boundary force, PAE, on the face of vertical plane of 

height h as shown in Figure 7-2. The angle of the applied force with respect to 

horizontal depends on assumed friction angle between the wall and soil which is 

lesser of the angle of friction for the reinforced soil mass (' r) and the retained 

backfill (' b). Different application points between h/3 and 2h/3 from the base need 

to be examined to determine the maximum value of PAE. Change the magnitude of 

the applied load until a capacity:demand ratio (CDR) of 1.0 is obtained i.e., the load 

and the resistance are balanced. Thus, the force corresponding to a CDR of 1.0 is 

equal to the total thrust on the retaining structure. 

f. Verify design assumptions and material properties by examining the loads on 

individual slices in the output. 

g.	 Once the maximum value of total thrust, PAE, is determined, apply the force at mid 

height (h/2) as shown in Figure 7-2 for analysis in following steps. 

Step 7. Determine the horizontal inertial force, PIR, of the total reinforced wall mass as follows: 

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PIR = 0.5(kav)(W) (7-7)

where W is the weight of the full reinforced soil mass and any overlying permanent 

slopes and/or permanent surcharges within the limits of the reinforced soil mass. The 

inertial force is assumed to act at the centroid of the mass used to determine the weight 

W. 
 

Ground Surface 

Typical 

Slice 

PAE 

Appropriate 

potential slide 

surface (circular, 

non-circular, etc.) 

Plane for calculating 

earth pressure 

H 

Surface Loads 

h 

h/2 

h/2 

Figure 7-2. Use of a slope stability approach to compute seismic earth pressure. 

 

 

 

Step 8 Check the sliding stability using a resistance factor, , equal to 1.0 and the full, nominal 

weight of the reinforced zone and any overlying permament sucharges. If the sliding 

stability is met, the design is satisfactory and go to Step 11. If not, go to Step 9. 

Compute the total horizontal force, THF, is as follows: 

 For M-O method: 

THF = Horizontal component of PAE(cos  + PIR + EQ(qLS)KAEH + other horizontal 

nominal forces due to surcharges (with load factor =1.0) 

where, EQ is the load factor for live load in Extreme Event I limit state and qLL is the 

intensity of the live load surcharge. 

 For GLE method: 

THF = Horizontal component of PAE (since all surcharges are included in the slope 

stability analysis) + PIR 

Compute the sliding resistance, R, as follows: 

R = V (

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PAE 

k 

Horizontal Force 

k 

Driving 

Force Resisting 

Force 

ky 

Figure 7-3. Procedure for determination of ky (Anderson et al., 2008). 

where  is the minimum of tan' r, tan' f or (for continuous reinforcement) tan as 

discussed in Section 4.5.6.a and V is the summation of the vertical forces as follows: 

V = W + PAEsin + permanent nominal surcharge loads within the limits of the 

reinforced soil mass 

The sliding stability capacity to demand ratio (CDR) is calculated as follows: 

CDRsliding = R / THF 

If CDRsliding > 1, the design is satisfactory and go to Step 11 otherwise go to Step 9. 

Step 9 Determine the wall yield seismic coefficient, ky, where wall sliding is initiated. This 

coefficient is obtained by iterative analysis as follows: 

a. Determine values of PAE as a function of the seismic coefficient k (< kmax) as shown 

in Figure 7-3a. 

b.	 Determine horizontal driving and resisting forces as a function of k (using 

spreadsheet calculations) and plot as a function of k as shown in Figure 7-3b. The 

value of ky corresponds to the point where the two forces are equal, i.e., the CDR 

against sliding equals 1.0. 

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Step 10 Determine the wall sliding displacement, d, in inches based on the following 

relationships between d, ky/kmax, kmax, and PGV based on whether the site is located in 

Western United States (WUS) or Central and Eastern United States (CEUS) as per 

Figure 7-4: 

 

	 For WUS soil and rock sites and CEUS soil sites
 

log(d) = -1.51 – 0.74log(ky/kmax) + 3.27log(1- ky/kmax) – (7-8) 0.80log(kmax) + 1.59log(PGV) 

 

	 For CEUS rock sites
 

log(d) = -1.31 – 0.93log(ky/kmax) + 4.52log(1- ky/kmax) – (7-9) 0.46 log(kmax) + 1.12log(PGV) 

Figure 7-4. Boundary between WUS and CEUS (Anderson et al. 2008). 

Step 11 Evaluate the limiting eccentricity and bearing resistance using the same principles 

discussed in Chapter 4. Include all applicable loads for Extreme Event I. If M-O 

method is used then add other applicable forces to PAE. If GLE method is used then no 

additional forces need to be added to PAE since the slope stability analysis includes all 

applicable forces. Check the limit states using the following criteria: 

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1. For limiting eccentricity, for foundations on soil and rock, the location of the 

resultant of the applicable forces should be within the middle two-thirds of the wall 

base for EQ = 0.0 and within the middle eight-tenths of the wall base for EQ = 1.0. 

Interpolate linearly between these values as appropriate. 

2. For bearing resistance compare the effective uniform bearing pressure to the 

nominal bearing resistance that is based on the full width of the reinforced zone. A 

resistance factor of 1.0 is used per Article 10.5.5.3.3 (AASHTO, 2007). 

Step 12 If Step 11 criteria are not met, adjust the wall geometry and repeat Steps 6 to 11 as 

needed. 

Step 13 If Step 11 criteria are met, assess acceptability of sliding displacement, d. The amount of 

displacement which is tolerable will depend on the nature of the wall and what it 

supports, as well as what is in front of the wall. Typical practice is to limit the lateral 

displacement in the range of 2.0 in. (50 mm) to 4.0 in (100 mm) assuming that structures 

on top or at toe of the wall can tolerate such displacements. 

7.1.2 Internal Stability 

For internal stability, the active wedge is assumed to develop an internal dynamic force, Pi, that is 

equal to the product of the mass in the active zone and the wall height dependent average seismic 

coefficient, kav. Thus, Pi is expressed as follows: 

Pi = kav Wa (7-10)

where Wa is the soil weight of the active zone as shown by shaded area in Figure 7-5 and kav given 

by Equation 7-3. The force Pi is assumed to act as shown in Figure 7-5. If the weight of the facing 

is significant then include it in Wa computation. 

The supplementary inertial force, Pi, will lead to dynamic increases in the maximum tensile forces 

in the reinforcements. Reinforcements should be designed to withstand horizontal forces generated 

by the internal inertia force, Pi, in addition to the static forces. During the internal stability 

evaluation, it is assumed that the location and the maximum tensile force lines do not change during 

seismic loading. 

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Figure 7-5. Seismic internal stability of a MSE wall. 

The inertial force is distributed to the reinforcements equally as follows: 

P T i

md  (7-11) 

n 

where: 

Tmd = factored incremental dynamic intertia force at layer i 

Pi = internal inertia force due to the weight of backfill within the active zone, i.e., the 

 shaded area in Figure 7-5 

n = number of soil reinforcement layers within the reinforced soil zone, 

The load factor for seismic forces is equal to 1.0. Therefore, the total factored load applied to the 

reinforcement on a load per unit of wall width basis is determined as follows: 

T total max md  T  T (7-12) 

where Tmax is the factored static load applied to the reinforcements determined using the appropriate 

equations in Chapters 4 and 6. The reinforcement must be designed to resist the dynamic 

component of the load at any time during its design life. This includes consideration of both tensile 

and pullout failures as discussed next. 

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7.1.2.a Tensile Failure 

Design for static loads requires the strength of the reinforcement at the end of the design life to be 

reduced to account for corrosion for metallic reinforcement, and for creep and other degradation 

mechanisms for geosynthetic reinforcements. The adjustment for metallic corrosion losses are 

exactly the same described in Chapter 4 for static analysis. For metallic reinforcements, use the 

following resistance factors while evaluating tensile failure under combined static and earthquake 

loading (per Table 11.5.6-1 of AASHTO {2007}): 

 Strip reinforcements: 1.00 

 Grid reinforcements: 0.85 

In contrast, the procedures for geosynthetic do not require a creep reduction for the short duration 

seismic loading condition and only reductions for geosynthetic degradation losses are required. 

Strength loss in geosynthetics due to creep requires long-term, sustained loading. The dynamic 

component of load for seismic design is a transient load and does not cause strength loss due to 

creep. Therefore, the resistance of the reinforcement to the static component of load, Tmax, must be 

handled separately from the dynamic component of load, Tmd. The strength required to resist Tmax 

must include the effects of creep, but the strength required to resist Tmd should not include the 

effects of creep. Thus, for geosynthetic reinforcement rupture, the reinforcement is designed to 

resist the static and dynamic components of the load determined as follows: 

For the static component: 

Tmax RF S rs  (7-13) R c 

For the dynamic component: 

T RF RF md ID D Srt  (7-14) R c 

 

 

where: 

 = resistance factor for combined static/earthquake loading = 1.20 from Table 11.5.6-1 of 

 AASHTO (2007) 

Srs = ultimate reinforcement tensile resistance required to resist static load component 

Srt = ultimate reinforcement tensile resistance required to resist dynamic load component 

Rc = reinforcement coverage ratio 

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RF = combined strength reduction factor to account for potential long-term degradation 

due to installation damage, creep, and chemical aging, equal to RFCR x RFID x RFD 

(see Chapter 3) 

RFID = strength reduction factor to account for installation damage to reinforcement 

RFD = strength reduction factor to prevent rupture of reinforcement due to chemical and 

 biological degradation 

Using the above equations, the required ultimate tensile resistance of the geosynthetic reinforcement 

is determined as follows: 

Tult = Srs + Srt (7-15)

7.1.2.b Pullout Failure 

For pullout of steel or geosynthetic reinforcement, the following equation is used: 

total Le 

T 

(7-16) (0.8F *ασ CR ) v c 

 

where: 

Le = length of reinforcement in resisting zone 

Ttotal = maximum factored reinforcement tension from Equation 7-12 

 = resistance factor for reinforcement pullout = 1.20 from Table 11.5.6-1 of AASHTO 

(2007) 

F* = pullout friction factor 

α = scale effect correction factor 

σv = unfactored vertical stress at the reinforcement level in the resistant zone 

C = overall reinforcement surface area geometry factor 

Rc = reinforcement coverage ratio 

For seismic loading conditions, the value of F*, the pullout resistance factor, is reduced to 80 

percent of the value used for static design, unless dynamic pullout tests are performed to directly 

determine the F* value. 

7.1.3 Facing Reinforcement Connections 

Facing elements are designed to resist the total (static + seismic) factored load, i.e., Ttotal. Facing 

elements should be designed in accordance with applicable provisions of Sections, 5, 6, and 8 of 

AASHTO (2007) for reinforced concrete, steel, and timber, respectively. 

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For segmental concrete block faced walls, the blocks located above the uppermost reinforcement 

layer should be designed to resist toppling failure during seismic loading. 

For geosynthetic connections subjected to seismic loading, the factored long-term connection 

strength, Tac, must be greater than Ttotal (i.e., Tmax + Tmd). If the connection strength is partially or 

fully dependent on friction between the facing blocks and the reinforcement (e.g., MBW facing), the 

connection strength to resist seismic loads should be reduced to 80 percent of its static value as 

follows: 

For the static component of the load: 

max D Srs 

T RF 

(7-17) 0.8  (CR ) R cr c 

 

 

 

 

 

 

For the dynamic component of the load: 

Srt 

Tmd RFD (7-18) 0.8 (CR u ) R c 

where: 

Srs = ultimate reinforcement tensile resistance required to resist static load component 

Tmax = applied load to reinforcement 

RFD = reduction factor to prevent rupture of reinforcement due to chemical and biological

 degradation from Chapter 3 

 = resistance factor = 1.20 applied to both the static and the dynamic components, from 

Table 11.5.6.4-1 of AASHTO (2007) 

CRcr = long-term connection strength reduction factor to account for reduced ultimate 

 strength resulting from connection 

Rc = reinforcement coverage ratio 

Srt = ultimate reinforcement tensile resistance required to resist dynamic load component 

Tmd = factored incremental dynamic inertia force 

CRu = short-term reduction factor to account for reduced ultimate strength resulting from 

 connection. 

For mechanical connections that do not rely on a frictional component, the 0.8 multiplier is removed 

from Equations 7-17 and 7-18. 

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The required ultimate tensile resistance of the geosynthetic reinforcement at the connection is: 

Tult-conn = Srs + Srt (7-19) 

The connection capacity of a facing/reinforcement connection system that is fully dependent on the 

shear resisting devices for the connection capacity will not be significantly influenced by the normal 

stress between facing blocks. The percentage of connection load carried by the shear resisting 

devices relative to the frictional resistance to meet the specification requirements should be 

determined based on past successful performance of the connection system. 

For cases where seismic analysis is required as per Section 4 of AASHTO, facing connections in 

MBW unit faced walls should use shear resisting devices between the facing blocks and soil 

reinforcement such as shear keys and structural pins (i.e., pins manufactured from material meeting 

the design life of the structure, e.g., steel and HDPE) and should not be fully dependent on frictional 

resistance between the soil reinforcement and facing blocks. 

For steel reinforcement connections, AASHTO (2007) recommends that the resistance factors for 

combined static and seismic loads as follows: 

 Strip reinforcements: 1.00 

 Grid reinforcements: 0.85 

7.2 VEHICULAR IMPACT EVENTS 

Traffic railing impact loads are analyzed under Extreme Event II limit state as per Article A13.2 

(AASHTO, 2007). Traffic railing impact events tend to affect only the internal stability of MSE 

walls. Guidance for traffic barrier analysis presented in this section is based on NCHRP 22-20 

(Bligh et al., 2009), which is an extension of the previous FHWA (Elias et al., 2001) method based 

on laboratory and full-scale field tests. Guidance for post and beam railings is based upon 

AASHTO (2007). 

7.2.1 Traffic Barriers 

The impact traffic load on barriers constructed over the front face of MSE walls, must be 

designed to resist the overturning moment by their own mass per Article 11.10.10.2 (AASHTO, 

2007). 

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Static Impact Load 

The recommended static impact force is 10,000 lb (45 kN) applied on a barrier with a minimum 

height of 32 in. (810 mm) above the roadway. Bligh et al. (2009) found that a 10,000 lbs (45 

kN) static impact load is equivalent to a dynamic TL-4 railing test level of 54,000 lb (240 kN), as 

illustrated in Figure 7-6. 

The wall design should ensure that the reinforcement does not rupture or pullout during the 

impact event. Where the impact barrier moment slab is cast integrally with a concrete pavement, 

the additional force may be neglected. The recommended static impact forces for rupture and for 

pullout are based upon the recent NCHRP 22-20 project (Bligh et al., 2009) and past practice. 

Load Combination and Load Factors 

The load factors and load combination for an Extreme Event II are summarized in Table 4-1. A 

load factor, P-EV = 1.35 is used for the static soil load. The traffic surcharge, modeled as an 

equivalent soil height of 2 ft, also uses the load factor P-EV = 1.35 (and not  = 0.50), for internal 

stability analysis. The static equivalent impact loads are multiplied by a load factor, CT =1.00. 

Figure 7-6. Comparison of static and dynamic impact force with 1-inch (25 mm) maximum 

displacement (Bligh et al., 2009). (1 kip = 4.44 kN; 1 ft = 0.3 m) 

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Reinforcement Rupture 

The static impact force, adds an additional horizontal force to the upper 2 layers of soil 

reinforcement. It is recommended that the upper layer of soil reinforcement be designed for a 

rupture impact load equivalent to a static load of 2,300 lb/lft (33.5 kN/m) of wall; and the second 

layer be designed with a rupture impact load equivalent to a static load of 600 lb/lft (8.8 kN/m). 

A distribution of stresses, as discussed in Article 11.101.10.2 and illustrated in Figure 3.11.6.3-2 

(AASHTO, 2007), is not recommended. 

The load factor for impact is equal to 1.0. Therefore, the total factored load applied to the 

reinforcement on a load per unit of wall width basis is determined as follows: 

T total  T max  T I (7-20a)

where: 

TI = factored impact load at layer 1 or 2, respectively 

TMAX = reinforcement tension from static earth and traffic loads 

With terms defined, this equation is: 

Ttotal  S V K r γr [(Z  heq ) γ EV -MAX ]  ti (γ CT ) (7-20b) 

where: 

ti = equivalent static load for impact load at layer i, (t1 = 2,300 lb/lft and t2 = 600 lb/lft) 

 and other terms as previously defined (Chapter 4 and/or 7). 

An example calculation is presented in Appendix E.6. Note that for geosynthetic reinforcements, 

the nominal strength used to structurally size the reinforcements to resist the impact load is not 

increased by eliminating the reduction factor for creep, as was done for internal seismic design in 

Section 7.2.1. This is recommended because full-scale traffic barrier impact testing with 

geosynthetic soil reinforcement has not been performed to date. 

Reinforcement Pullout 

The pullout resistance of the soil reinforcement to the impact load is resisted over the full-length 

of the reinforcements (i.e., L). The traffic surcharge, modeled as an equivalent soil height of 2 ft, 

is included in the nominal vertical stress, v, for pullout resistance calculation. Pullout is resisted 

over a greater length of wall than the reinforcement rupture loads. Therefore, for pullout, it is 

recommended that the upper layer of soil reinforcement be designed for a pullout impact load 

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equivalent to a static load of 1,300 lb/lft (19.0 kN/m) of wall; and the second layer be designed 

with a pullout impact load equivalent to a static load of 600 lb/lft (8.8 kN/m). 

Resistance Factors for Tensile and Pullout Resistance 

The resistance factors presented in Table 4-7 for “Combined static/traffic barrier impact” are 

recommended for Extreme Event II impact loading. (Note that AASHTO does not specifically 

address tensile resistance factors for impact loading.) The tensile and connection rupture 

resistance factors are a function of the type of reinforcement. 

A pullout resistance factor of 1.00 is recommended for metallic and geosynthetic reinforcements. 

(Note that AASHTO does not specifically address pullout resistance factors for impact loading.) 

Barrier, Coping, and Moment Slab Design 

Example traffic barriers are illustrated in Figure 5-2. Typically, the base slab length is 20 ft (6 

m) and jointed to adjacent slabs with shear dowels. Parapet reinforcement shall be designed in 

accordance with AASHTO Section 13 Railings. See NCHRP 22-20 report (Bligh et al., 2009) 

for barrier, coping, and moment loading recommendations. The anchoring slab shall be strong 

enough to resist the ultimate strength of the standard parapet, and sized to provide adequate 

resistance to sliding and overturning. 

MSE Facing Panel Design 

The upper facing panel must be separated from the barrier slab with 1 to 2 in. (25 to 50 mm) of 

expanded polystyrene (see Figure 5-2(b)). The distance should be adequate to allow the barrier 

and slab to resist the impact load in sliding and overturning without loading the facing panel. 

Separation between the precast facing and cast-in-place resistance slab is required to prevent 

stressing on the facing panels due to slab curing and shrinking. 

7.2.2 Post and Beam Railings 

Flexible post and beam barriers, when used, shall be placed at a minimum distance of 3.0 ft (0.9 

m) from the wall face, driven 5.0 ft (1.5 m) below grade, and spaced to miss the reinforcements 

where possible. If the reinforcements cannot be missed, the wall shall be designed accounting 

for the presence of an obstruction. Each of the upper two rows of reinforcement shall be 

designed for an additional horizontal load of 150 lb/lft (2.2 kN/m) of wall, for a total additional 

load of 300 lb/ft (4.4 kN/m). 

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7.3 SUPERFLOOD EVENTS AND SCOUR 

The stability of walls and abutments in areas of turbulent flow must be addressed in design. Wall 

design should be based on the total scour depths estimated per Article 2.6.4.4.2 (AASHTO, 2007). 

Scour should be investigated for two flood conditions: 

C Design Flood 

C Check Flood 

The design flood (storm surge, tide, or mixed population flood) is the more severe of the 100-year 

event or an overtopping flood of lesser recurrence interval. Stability design of the wall should be 

assessed assuming that the streambed material above the total scour line has been removed. This 

should be analyzed as a strength limit state. 

The check flood (storm surge, tide, or mixed population flood) is the more severe of the 500-year 

event or an overtopping flood of lesser recurrence interval. Stability design of the wall should be 

assessed assuming that the streambed material above the total scour line has been removed. This is 

an extreme event, and the extreme event limit state applies. Resistance factors for this extreme limit 

state may be taken at 1.0, per Articles 10.6.4 and 10.5.5.3.3 (AASHTO, 2007). 

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