U.S. Department of Transportation Publication No. FHWA NHI-10-039 

Federal Highway Administration February 2011 

 

NHI Courses No. 132083 ___________________ 

Implementation of LRFD Geotechnical 

Design for Bridge Foundations 

Reference Manual 

State DOT LRFD 

Design Guidance for 

Bridge Foundation

 National Highway Institute
 1 

NOTICE

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

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

policy of the Department of Transportation. This manual does not constitute a standard, 

specification, or regulation. The United States Government does not endorse products or 

manufacturers. Trade or manufacturer's names appear herein only because they are 

considered essential to the objective of this document
 2 

Technical Report Documentation Page

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

FHWA NHI-10-039

4. TITLE AND SUBTITLE 5. REPORT DATE October 2010.

IMPLEMENTATION OF LRFD GEOTECHNICAL DESIGN Updated Draft, February 2011 

FOR BRIDGE FOUNDATIONS

6. PERFORMING ORGANIZATION CODE

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

Naser Abu-Hejleh, Ph.D., P.E; Jerry A. DiMaggio, 

P.E.;William M. Kramer, P.E.; 

Scott Anderson, Ph.D., P.E; Silas Nichols, P.E. 

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

FHWA Resource Center 11. CONTRACT OR GRANT NO.

12. SPONSORING AGENCY NAME AND ADDRESS

National Highway Institute 13. TYPE OF REPORT & PERIOD COVERED

Federal Highway Administration

U.S. Department of Transportation 14. SPONSORING AGENCY CODE

Washington, D.C.

15. SUPPLEMENTARY NOTES

FHWA COTR – Louisa Ward 

16. ABSTRACT. This document is the reference manual for NHI training course No. 132083, both 

developed to assist State Departments of Transportation (DOTs) in the successful development 

of LRFD Design Guidance for bridge foundations based on the 2010 AASHTO LRFD Bridge 

Design Specifications and their local experiences. Initially, an LRFD Implementation Plan of 

six consecutive steps is discussed and the remainder of the manual presents recommendations to 

assist DOTS with implementation of these steps. The principal changes in the AASHTO design 

specifications from ASD to LRFD are presented. The DOTs have three options for selection of

LRFD geotechnical design methods: adoption of AASHTO’s LRFD methods or development of 

local LRFD methods by fitting to ASD or through reliability analysis of information collected at 

load test sites. The calibration methods and conditions for these three options are discussed to 

assist DOTs in implementation of these options. Procedures are furnished to evaluate and 

address the project site variability. Then, the three implementation options are evaluated and 

compared to assist DOTs with selection of the most appropriate option. The advantages of 

development of local reliability-based LRFD design methods over other implementation options 

are demonstrated. The implementation of AASHTO LRFD for geotechnical design for bridge 

foundations will lead to savings or to equivalent foundation costs compared with ASD methods. 

At the end, a roadmap for development of LRFD design guidance that consists of LRFD design 

specifications and delivery processes for bridge foundations is discussed.

17. KEY WORDS: AASHTO, LRFD, ASD, limit 18. DISTRIBUTION STATEMENT states, foundations, safety factor, resistance 

factor, calibration, conditions, reliability, load No restrictions.

tests, design, construction, NCHRP Report 507 

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

Unclassified Unclassified 97 
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iv

FORWARD

State departments of transportation (DOTs) are at various stages of implementing the AASHTO

Load and Resistance Factor Design (LRFD) specifications (AASHTO LRFD, 2007) for the 

design of bridge foundations. Some DOTs still use the allowable stress design (ASD) method 

(AASHTO Standard Specifications, 2002) for the geotechnical design of foundations. Other 

DOTs have developed LRFD design manuals that heavily refer to AASHTO LRFD without fully 

understanding the impact of these specifications on their design practices or the conditions they 

should adhere to when using them. Engineers from several DOTs have expressed an interest in 

having some form of guidance on how to implement the AASHTO LRFD design specifications 

with consideration of their local experiences. Consequently, the Federal Highway Administration 

(FHWA) has developed a new web-based National Highway Institute (NHI) training course 

called “Implementation of LRFD Geotechnical Design for Bridge Foundations.” The goal of this 

training course is to assist DOTs in the successful development of LRFD Design Guidance for 

bridge foundations based on the 2010 AASHTO LRFD Bridge Design Specifications and with 

the consideration of their local experience. This manual is the reference manual for this course 

and includes seven chapters, with each chapter serving as a reference to a lesson in the training 

course. Note that this course supplements NHI Course 130082 “LRFD for Highway Bridge 

Substructures and Earth Retaining Structures” as it covers additional and more specific guidance 

to assist DOTs with implementation of LRFD. 

It is hoped that that this course will help DOTs with their successful implementation of 

AASHTO LRFD and address their specific local LRFD implementation issues. Improvements to 

the AASHTO LRFD platform will continue in the future based on applied research studies, the 

results of additional load tests, and the experience of the highway community with 

implementation of LRFD. The LRFD reliability calibration process is a dynamic process: it 

allows for continued refinement of the resistance factors with more data. DOTs will need to keep 

updating their own LRFD design specifications to keep pace with future refinements and updates 

of AASHTO LRFD. 

This course will continue to be updated in the future to address any errata, include new 

improvements, and to reflect new revisions in the AASHTO LRFD Geotechnical design 

specifications for bridge foundations.

.
 v 

TABLE OF CONTENTS

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

 

CHAPTER 2: LRFD IMPLEMENTATION PLAN...............................................2-1 

 

CHAPTER 3: CHANGES IN THE AASHTO DESIGN SPECIFICATION FROM 

ASD TO LRFD............................................................................3-1 

 3.1 INCORPORATION OF LIMIT STATE DESIGN....................................3-1 

3.1.1 Overview of the AASHTO LRFD Limit State Design for Bridge 

Foundations..........................................................................3-1 

3.1.2 Addressing All Applicable Structural and Geotechnical Limit States...... 3-3 

3.2 LOAD AND RESISTANCE FACTORS TO ACCOUNT FOR DESIGN

UNCERTAINTIES ........................................................................3-5

3.2.1 Load Combinations, Load Factors, and Resistance Factors at Various 

 Limit States...........................................................................3-6 

3.2.2 Resistance Factors at the Strength Limit........................................3-8 

3.2.3 Load Combinations and Factors at the Strength Limit........................3-8 

3.3 NEW AND IMPROVED METHODS TO DETERMINE LOADS, 

RESISTANCES, AND DISPLACEMENTS.........................................3-10 

3.3.1 Examples of Improved AASHTO Methods to Determine Loads.........3-10

3.3.2 Improved AASHTO Methods to Determine Foundation Displacements and 

Resistances.........................................................................3-11 

CHAPTER 4: CALIBRATION METHODS FOR GEOTECHNICAL RESISTANCE 

FACTORS..............................................................................4-1 

4.1 CALIBRATION BY FITTING TO ASD METHODS..............................4-1 

4.2 RELIABILITY ANALYSIS OF DATA COLLECTED AT LOAD TEST 

SITES.........................................................................................4-2 

4.2.1 Compilation of Information at Load Test Sites.................................4-3 

4.2.2 Statistical Analysis to Account for Design/Construction Uncertainties.....4-4 

4.2.3 Reliability Analysis to Determine Resistance Factors........................4-7 

4.2.4 Evaluation of Economics and Improvement of Accuracy of the Calibrated 

Design Methods.....................................................................4-8 

4.3 AASHTO’S LRFD CALIBRATION METHODS.................................4-10 

4.3.1 AASHTO’s Resistance Factors for Driven Piles and Drilled Shafts.......4-11

CHAPTER 5: CALIBRATION CONDITIONS AND ASSESSMENT OF SITE 

VARIABILITY..........................................................................5-1 

5.1 AASHTO’S LRFD CONDITIONS......................................................5-1 

5.1.1 Design Conditions..................................................................5-2 

 
 vi 

5.1.2 Construction and Load Testing Conditions.....................................5-4 

5.1.3 Calibration Analysis Methods and Parameters.................................5-5 

5.2 SELECTION OF CONDITIONS FOR LOCAL CALIBRATION...............5-6 

5.2.1 Conditions from AASHTO’s Reliability Conditions..............................5-7 

5.2.2 Statistical and Reliability Analyses.......................................... ...5-7 

5.2.3 Local Design and Construction Conditions.....................................5-8 

5.2.4 Conditions at Load Test Sites......................................................5-9 

5.3 ASSESSMENT OF SITE VARIABILITY..............................................5-11

5.3.1 Evaluation of Site Variability....................................................5-11 

5.3.2 Addressing Site Variability......................................................5-14 

CHAPTER 6: SELECTION OF LRFD GEOTECHNICAL DESIGN METHODS

6.1 COMAPRSION OF 2010 AASHTO LRFD AND AASHTO STANDARDS...6-1 

 6.1.1 Drilled Shafts.......................................................................6-1 

 6.1.2 Driven Piles........................................................................6-1 

6.2 COMPARISON OF AASHTO LRFD AND LOCAL ASD METHODS.........6-3 

6.2.1 Selection of Local ASD Design Method to Develop the LRFD Design 

Method...............................................................................6-5 

6.3 ADVANTAGES OF LOCAL RELIABILITY CALIBRATION..................6-6 

6.3.1 Advantages of Reliability Calibration over Calibration by Fitting.........6-6 

6.2.2 Advantages of Local over AASHTO Reliability Calibrations...............6-7 

6.4 SUMMARY AND CONCLUSIONS..................................................6-7 

7. DEVELOPMENT OF LRFD DESIGN GUIDANCE FOR BRIDGE 

FOUNDATIONS 

7.1 DEVELOPMENT OF LRFD DESIGN SPECIFICATIONS.....................7-1 

7.1.1 Materials Needed for Development.............................................7-1 

7.1.2 Roles and Responsibilities of Various Groups in the DOT..................7-2 

7.1.3 Contents of the LRFD Design Specifications...................................7-3 

7.2 DEVELOPMENT OF LRFD DESIGN DELIVERY PROCESSES.............7-7 

7.2.1 Overview............................................................................7-7 

7.2.2 Role of the Construction Group.................................................7-7 

7.2.3 Roles of the Geotechnical and Hydraulic Groups.............................7-8 

7.2.4 Roles of Structural and Geotechnical Groups.................................7-8 

 

REFERENCES..............................................................................................8-1 
 vii 

LIST OF FIGURES

Figure 4.1. Example of Histogram and Frequency Distribution of Bias Resistance Data...4-5 

Figure 4.2. Resistance Factor as a Function of Resistance Mean Bias, λ, and COV..........4-6 

Figure 4.3. Pile Nominal Bearing Resistance, Rn, from Various Pile Design Methods......4-10 

Figure 5.1. Redundancy of a Group of Deep Foundation Elements.............................5-6 

Figure 7.1. Example of the Information the Geotechnical Engineer Needs to 

Develop in the Preliminary Design................................................... 7-10 
viii

LIST OF TABLES

Table 3.1. AASHTO LRFD Load Combinations and Load Factors at Various 

Limit States..................................................................................3-7 

Table 3.2. AASHTO LRFD Maximum and Minimum Load Factors for Permanent Loads...3-7 

Table 3.3. Description of the Strength Limit Load Combinations.................................3-9 

Table 4.1. Demonstration for Determination of Resistances in the Field with Pile Dynamic 

Analysis Methods............................................................................4-3 

Table 4.2. Demonstration Example: Statistical Analysis of Bias Resistance Data...............4-4 

Table 4.3. Resistance Factors for Driven Piles Design Methods from NCHRP report 507......4-8 

Table 5.1. Values of Coefficient of Variation for Geotechnical Properties......................5-12 

Table 5.2. Determination of Site Variability, Step 1.............................................5-13 

Table 5.3. Determination of Site Variability, Step 2................................................5-14 

Table 6.1. Economical Comparison of ASD and LRFD Design Methods Conducted 

by FHWA for Illinois Department of Transportation......................................6-5 
ix

LIST OF SYMBOLS

AASHTO American Association of State Highway and Transportation Officials

ASD Allowable stress design 

As Shaft base area 

As Steel cross-sectional area 

BOR Beginning of redrive conditions

Bpf Number of blows to drive 1 foot in the Standard Penetration Test 

bpi Number of hammer blows to drive the pile 1 inch

DD Downdrag load

DOT

Dscour

State Department of Transportation

Scour depth due to combined effect of degradation scour, contraction 

scour, and local scour

COV Coefficient of variation of bias resistance data and bias loads data in the 

reliability analysis; and coefficient of variation of the measured soil and 

rock properties across the site in the assessment of site variability. 

COVinhernt COV of measured soil and rock properties due to inherent (natural) 

variability in subsurface materials and testing methods

CPT Cone penetration test

DSC Differing site conditions

Ed Developed hammer energy 

EN dynamic formula Engineering News dynamic formula

EOD End of pile driving conditions 

FHWA Federal Highway Administration

FORM method First Order Reliability method 

FOSM method First Order Second Moment method

FS Factor of safety

fy Steel yield strength

GL Geotechnical resistance losses 

GWL Groundwater level

IGM Intermediate geomaterial

L, Lmax Pile penetration length and the maximum length a pile can be safely to 

without damage

LL Live load

Pf Probability of failure

LFD Load factor design

LRFD Load resistance factor design 

Nb Number of hammer blows for 1 inch of pile permanent set

n Number of data

N Uncorrected SPT blow count 

N60 SPT blow count value corrected for hammer efficiency 

N160 SPT blow count value corrected for both overburden and hammer 

efficiency effects

NHI National Highway Institute

NCHRP National Cooperative of Highway Research Program 
 x 

Pn Nominal structural resistance 

QC and QA Quality control and Quality Assurance 

Q Force effect on foundation or a foundation design (service) load

Qf and Qfmax Factored load applied to a footing (excluding xowndrag loads) and the 

maximum factored load a footing can support 

Qs and Qsmax Service load applied to a footing (excluding xowndrag loads) and the 

maximum service load a footing can support 

qu Unconfined compressive strength of rock

RQD Rock quality designation

RMR Rock mass rating

Rm Measured geotechnical resistance on a test foundation from load test

Rn Nominal geotechnical resistance of a footing; predicted geotechnical 

resistance for a load test foundation 

TRB Transportation Research Board 

SPT Standard penetration test

Su Undrained shear strength of soils

s Pile permanent set in inches per 1 hammer blow

WEAP Wave equation analysis program

β Reliability index 

λ Mean of bias resistance or load data 

γ, γp , and γave Load factor, load factor for downdrag load, and average load factor

φ Resistance factor 

φsta Resistance factor for the piles static analysis methods

φdyn Resistance factor for the piles for the dynamic analysis and static load 

test methods

η Load modifier 

 
 1-1 

CHAPTER 1

INTRODUCTION

In 2000, the American Association of State Highway and Transportation Officials (AASHTO) 

recommended, and the Federal Highway Administration (FHWA) concurred, that all State 

departments of transportation (DOTs) should follow Load and Resistance Factor Design (LRFD) 

principles in the design of all new highway bridges by October 2007. To implement AASHTO 

LRFD design and construction specifications for bridges, each DOT should develop: 

• An LRFD Design Manual based on the most updated version of the AASHTO LRFD Bridge 

Design Specifications and on the DOT’s own design practices and specifications. 

• An LRFD Construction Manual consistent with the DOT’s LRFD Design Manual and based 

on the most updated version of the AASHTO LRFD Bridge Construction Specifications and 

on the DOT’s construction practices and specifications. 

• LRFD-based design and construction delivery processes consistent with the DOT’s LRFD 

Design and Construction Specifications. These processes are often presented in the DOTs 

Design and Construction Manuals. 

To assist DOTs in implementing the LRFD design platform, FHWA has developed several 

National Highway Institute (NHI) training courses and LRFD-based technical manuals, and has 

provided direct technical support to DOTs. Most DOTs have widely and quickly accepted and 

implemented the AASHTO LRFD Bridge Design Specifications (AASHTO, 2010a) for the 

design of bridge superstructures. This is not the case for the design of bridge foundations. 

DOTs are at various stages of implementing the AASHTO LRFD Bridge Design Specifications

for the design of bridge foundations. Some DOTs still use the allowable stress design (ASD) 

method for the geotechnical design of foundations (AASHTO Standards, 2002). Other DOTs 

have developed LRFD design manuals that heavily refer to AASHTO LRFD (AASHTO 2010a) 

without fully understanding the impact of these specifications on their design practices or the 

conditions they should adhere to when using them. Engineers from several DOTs have expressed 

an interest in having some form of guidance on how to implement LRFD. Consequently, the 

FHWA has developed a new web-based National Highway Institute (NHI) training course called 

“Implementation of LRFD Geotechnical Design for Bridge Foundation.” The goal of this 

training course is to assist DOTs in the successful development of LRFD Design Guidance for 

bridge foundations based on the 2010 AASHTO LRFD Bridge Design Specifications and with 

consideration of their local experience. It is important to realize that this course supplements 
 1-2 

NHI Course 130082 “LRFD for Highway Bridge Substructures and Earth Retaining Structures” 

(NHI, 2005) as it covers additional and more specific guidance to assist DOTs with 

implementation of LRFD. 

This document is the reference manual for the training course described above and includes 

seven chapters, with each chapter serving as a reference to a lesson in the training course. 

• Chapter 2: LRFD Implementation Plan. This chapter presents the steps needed for the 

development of LRFD Design Guidance for bridge foundations and briefly describes how 

these steps are addressed in Chapters 3 to 7 of this manual.

• Chapter 3: Changes in the AASHTO Design Specifications from ASD to LRFD. This 

chapter provides an overview of the content of AASHTO LRFD Section 10 and other 

sections in AASHTO LRFD that are referenced within Section 10, with emphasis on the 

following three principal changes between AASHTO ASD and LRFD design platforms: i) 

Incorporation of Limit State Designs; ii) Load and Resistance Factors to Account for 

Uncertainties; and iii) “New and improved Methods to Determine Foundations Loads, 

Displacements, and Resistances.” 

• Chapter 4: Calibration Methods for Geotechnical Resistance Factors. This chapter 

describes the two calibration methods—by fitting to ASD, and through reliability analysis—

to the extent needed by DOTs for the local calibration of their resistance factors, and then 

describes the methods employed to develop all the geotechnical resistance factors in the 

AASHTO LRFD Design Specifications. 

• Chapter 5: Calibration Conditions and Assessment of Site Variability. This chapter 

describes conditions that the DOTs need to adhere to when they adopt AASHTO’s LRFD 

design methods and provides recommendations to the DOTs to consider when they develop 

local LRFD design methods. It concludes with recommendations to evaluate and address the 

project site variability.

• Chapter 6: Selection of LRFD Geotechnical Design Methods. This chapter evaluates and 

compares the options for selection of LRFD geotechnical design methods in order to assist 

DOTs in selecting and finalizing the most appropriate option. This chapter demonstrates the 

advantages of development of local reliability-based LRFD design methods over other 

implementation options. 
 1-3 

• Chapter 7: Development of the LRFD Design Guidance. The manual concludes with 

general recommendations for development of LRFD Design Guidance that consists of LRFD 

design specifications and delivery processes for bridge foundations. 
2-1

CHAPTER 2

LRFD IMPLEMENTATION PLAN

This chapter presents the sequence of steps recommended for the development of LRFD design 

guidance for bridge foundations and briefly describes how these steps are addressed in Chapters 

3 to 7 of this manual. These steps constitute the LRFD implementation plan.

2.1 STEP 1: FORM LRFD IMPLEMENTATION COMMITTEE 

Formation of this committee needs sponsorship from within the DOT. It should include members 

from all DOT offices involved with the design and construction of foundations (structural, 

geotechnical, and hydraulic), and possibly from the FHWA and academia, with defined roles and 

responsibilities and a schedule with commitment. This Committee will be responsible for: 

• Development of state-specific LRFD Design Guidance for Bridge Foundations. To achieve 

this goal, the Committee needs to develop LRFD implementation plan based on the steps

furnished in this chapter, and execute this plan based on the recommendations presented in 

Chapters 3 to 7 of this manual. 

• Monitoring the implementation of LRFD. It is very important to capture the impact of LRFD 

on DOT practices, in terms of time, cost, and design procedures and practices. Lessons 

learned during implementation of the LRFD platform need to be documented and used in 

future refinements and improvements to the LRFD platform. 

• Sponsoring and providing LRFD training and technical support. LRFD training with real 

LRFD examples should be offered to all personnel involved with the use of LRFD. Require 

that consultants and local agencies attend LRFD training before they perform LRFD design. 

• Sponsoring needed LRFD research studies, and tracking results of relevant LRFD research 

studies conducted by others. Research needs can be identified during implementation of 

LRFD and by monitoring the implementation.

2.2 STEP 2: REVIEW KEY LRFD DESIGN REFERENCES

At a minimum, the following LRFD references should be reviewed to provide adequate 

background and direction for the implementation committee. 

• AASHTO LRFD Section 10 and related sections referenced within Section 10 (e.g., 

AASHTO LRFD Sections 2 to 8 covering hydraulic issues, loads, and structural design). It is 

important to review the most updated and recent version of AASHTO LRFD (AASHTO 

2007 with 2008, 2009, and 2010 Interims). The 2006 Interim Revisions to the AASHTO 
2-2

LRFD Bridge Design Specifications contained the most significant changes to foundation 

design as compared with the AASHTO Standard Specifications and previous LRFD editions. 

A new article on micropiles (Article 10.9) was added in the 2008 AASHTO Interim 

Revisions. In the 2009 AASHTO LRFD interims, a new Article 10.5.4.2 “Liquefaction 

Design Requirements” was added with valuable information in the corresponding 

commentary article. In 2009, AASHTO released guide specifications for LRFD seismic 

bridge design (AASHTO 2009a) and guide specification and commentary for vessel collision 

design of highway bridges (AASHTO 2009b). The 2010 Interim Revisions to the AASHTO 

LRFD Bridge Design Specifications included many changes to AASHTO LRFD Section 10 

that will be discussed more in subsequent chapters. 

• References for development of AASHTO’s resistance factors (Allen, 2005; Paikowsky et al., 

2004) as will be discussed in subsequent chapters. 

• Reference Manual for NHI Course 130082 (NHI, 2005), “LRFD for Highway Bridge 

Substructures and Earth Retaining Structures.” Note that this course is currently based on the 

2006 AASHTO LRFD interim revisions. 

• FHWA’s LRFD Design Examples (see http://www.fhwa.dot.gov/bridge/lrfd/examples.htm). 

Note that these examples are based on the 2004 AASHTO LRFD platform. 

• The FHWA LRFD drilled shaft foundation manual (Brown et. al, 2010). 

• The FHWA Soils and Foundations Manual (Samtani and Nowatzki, 2006). 

• FHWA is the final stages of preparing two NHI courses on the seismic design of bridges and 

other structures. The reference manuals for these should be reviewed if available.

• The experiences of other DOTs that have developed LRFD Design Manuals (such as Florida, 

Arizona and Washington States). The experiences and efforts of some DOTs in 

implementing LRFD for foundation design are detailed in the 2009 Transportation Research 

Board (TRB) publication “Implementation Status of Geotechnical Load and Resistance 

Factor Design in State Departments of Transportation,” which can be accessed online at

http://onlinepubs.trb.org/onlinepubs/circulars/ec136.pdf. 

2.3 STEP 3: IDENTIFY CHANGES NEEDED TO TRANSITION TO LRFD 

The DOTs should review their current ASD design specifications against AASHTO Section 10 

LRFD design specifications, and then identify changes needed to transition to LRFD. It is 

possible to begin by identifying the common foundation types, the typical geomaterials (sand, 

clays, soft and hard rocks) that support each foundation type, and the applicable extreme event 

limit states (seismic event, vessel impact, and check flood). Then continue with review of current

ASD foundation design methods for the applicable limit states and compare them to those

furnished in Section 10 of AASHTO LRFD design specifications. This review will result in the 

changes needed to transition to LRFD, which can be either: 
2-3

• In full accordance with AASHTO LRFD Section 10. These changes will be either adopted 

from or be based on AASHTO LRFD specifications. This would be the case if the DOT 

decides to adopt AASHTO’s LRFD design methods. Chapter 3 covers the changes in the

AASHTO design specifications from ASD to LRFD. The AASHTO LRFD geotechnical 

resistance factors were developed based on reliability analysis of data collected at load test 

sites, calibration by fitting to ASD, and engineering judgment.

• Exceptions (deviations) from AASHTO Section 10 (deletions, additions or modifications to 

the contents of AASHTO Section 10). This would be the case if the DOT decides to develop 

LRFD design methods with locally calibrated resistance factors. Exception from AASHTO 

(e.g., Local calibration of resistance factors) is recognized by AASHTO. For example,

Article 10.5.5.2.1 of AASHTO LRFD (2010a) allows the use of higher φ values developed 

locally if they are based on substantial successful experience. AASHTO LRFD Article 

10.5.5.2.1 allows the use of higher φ developed locally if they are based on substantial 

successful experience OR statistical data with calibration (Load tests). The DOTs need to 

provide justifications for these exceptions from AASHTO LRFD based on their own longterm successful experience and engineering judgment, research results, and, or to address 

local issues not addressed by AASHTO. 

A change in design platform is an excellent opportunity for moving away from ‘business as 

usual’ and making other improvements to practice. The DOTs should consider improvements of 

their geotechnical design practices at the same time as they’re transitioning to LRFD. 

2.4 STEP 4: SELECT GEOTECHNICAL LRFD DESIGN METHODS 

Based on the previous step, DOTs have three options for the selection of LRFD geotechnical 

design methods: adopt AASHTO’s LRFD methods; develop local LRFD methods by fitting to 

local ASD methods that have track records of long-term success, or to develop local LRFD 

methods through reliability analysis of information collected at load test sites. 

Chapters 4, 5, and 6 are developed to assist the DOTs with development, evaluation, and 

comparison of these options. This would help the DOTs to select and implement the most 

appropriate LRFD geotechnical design methods. What is most appropriate may change through 

time. For example, if few load tests are initially available, a local reliability analysis is not 

appropriate. However, if more tests are obtained through time, local reliability analysis could 

become most appropriate and lead to the most efficient designs. 

2.5 STEPS 5 AND 6: DEVELOP LRFD DESIGN SPECIFICATIONS AND DELIVERY 

PROCESSES 
2-4

Based on the results of Steps 1 through 4, the DOTs will be ready for development of LRFD 

Guidance consisting of specifications and delivery processes for each type of their bridge 

foundations. The development of specifications should be based on Section 10 of the 2010 

AASHTO LRFD Bridge Specifications and with consideration of the DOT’s ASD specifications. 

The delivery processes should be consistent with the developed LRFD specifications and with 

consideration of the DOT’s ASD delivery processes. Chapter 7 presents a roadmap to develop 

the contents of this guidance and describe roles and responsibilities of various groups in the DOT 

in the development. 
3-1

CHAPTER 3

CHANGES IN THE AASHTO DESIGN SPECIFICATIONS FROM ASD TO LRFD

This chapter is developed to assist DOTs with identification of the required changes to move 

their practice from ASD to LRFD for the design of bridge foundations. It provides an overview 

of the content of AASHTO LRFD Section 10 and other sections in AASHTO LRFD that are 

referenced within Section 10, with emphasis on the following three principal changes between 

AASHTO ASD and LRFD design platforms: 

• Incorporation of limit states designs

• Load and resistance factors to account for design uncertainties and assure desired safety

• New and improved methods to determine foundations loads, displacements, and resistances. 

3.1 INCORPORATION OF LIMIT STATE DESIGNS

In contrast to the ASD platform, the concept of “limit state design” is an explicit and integral 

component of the AASHTO LRFD platform. 

3.1.1 Overview of the AASHTO LRFD Limit State Design for Bridge Foundations 

All possible structural and geotechnical failure modes for foundations present during the design 

life of the bridge are grouped into three distinct structural and geotechnical limit states. Note that 

these failures do not necessary correspond to “true failure” but rather to when a certain criterion 

is met or exceeded (per Eqs. 1 and 2 discussed later). The various limit states for bridge 

foundations are discussed in AASHTO (2010a) Sections 1 and 3, and in Articles 10.5.1 to 10.5.4: 

• Service limit states. Failure modes are related to the function and performance of the bridge 

due to foundation under regular operating conditions. For example, bridge settlement caused 

by foundation settlement that exceeds the bridge tolerable settlement. In this case, failure 

means generated displacements exceeding the tolerable displacements. In the LRFD design,

the bridge/foundation displacements under regular service conditions must be kept below the 

tolerable values.

• Strength limit states. Failure modes are the collapse or damage of the bridge or its 

foundation under loads applied continuously or frequently during its design life. In the LRFD 

design, the foundations must have adequate structural and geotechnical resistances to resist 

the loads the bridge is expected to experience during its life with an adequate margin of 

safety against damage or collapse. 

• Extreme event limit states. Failure modes are the collapse of the bridge or its foundation 
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due to events that have a return period greater than the design life; for example, a major 

earthquake or flood, or vessel or vehicle collision. In the LRFD design, the foundations must 

have adequate structural and geotechnical resistances to withstand the extreme events the 

bridge may experience during its life without causing collapse of the bridge. The concern 

here is survival of the bridge and protection of life safety (some damage to the structure is 

allowable). 

LRFD design for foundations requires that the summation of factored force effects is kept equal 

to or below the summation of factored geotechnical resistances for all applicable geotechnical 

limit states, and summation of factored force effects is kept equal or below the summation of 

factored structural resistances for all applicable structural limit states, as illustrated in the 

following equations: 

Σ ηi γi Qi ≤ ∑φi Rni for all applicable geotechnical limit states (3.1)

and

Σ ηi γi Qi ≤ ∑φi Pni for all applicable structural limit states (3.2) 

Where: 

• ∑: summation for a failure mode (e.g., axial compression failure, excessive settlement, 

overturning) identified in the limit state

• Qi : is the force effect on the foundation (e.g., axial compression load) from a load 

applied on the bridge (e.g., dead load) and γi is the load factor for that load (e.g., dead 

load). 

• Rni = geotechnical resistance available to resist the force effect (e.g., shaft side or 

resistances) and φi is its resistance factor

• Pni = structural resistance available to resist the force effect (e.g., moment resistance) and 

φi is its resistance factor

• γi and φi will account for the uncertainties in the computation of each load component 

and resistance component. 

• η is a load modifier relating to ductility, redundancy, and operational importance. In 

AASHTO LRFD, η values for superstructure design are furnished, but no specific 

guidance on the application of this factor to foundations is provided. For deep 

foundations, redundancy is addressed by reducing the geotechnical resistance factor, as 

will be discussed later. 

AASHTO (2010a) Section 10 provides the design specifications for design of bridge foundations 

at all structural and geotechnical limit states: 

• The structural and geotechnical resistances (or failure modes) that should be evaluated 

(e.g., bearing and sliding resistances for spread foundations at the strength limit) and their 

design requirements are discussed. 

• Methods to compute foundation nominal geotechnical resistances (Rni) (or displacements) 

and their resistance factors (φi) are furnished, which are needed to evaluate the foundation 
3-3

factored geotechnical resistance (∑φi Rni ). 

• Earth loads (e.g., downdrag, uplift) are discussed. Other loads are referred to Section 3. 

• The load combinations are referred to Section 3, and the computation of the effect of these 

load combinations on foundations (Qi in equations 3.1 and 3.2) is referred to Section 4. 

Note here that the effect of load, Qi, needs to be computed in the design and then factored 

NOT to compute the effect of factored load.

• Factored structural resistances are briefly discussed, and the reader is referred (in most 

cases) to AASHTO (2010a) Section 5 for concrete foundation elements, Section 6 for steel 

foundation elements, and Section 8 for wood foundation elements. 

• The influence of scour on foundation design is discussed (addressed more in Section 

3.3.2.8 of this report). Scour at foundations should be investigated for two types of floods, 

as discussed in AASHTO LRFD Article 2.6.4.4.2: 

o Design flood: the flood of a 100-year event or an overtopping flood of a lesser 

recurrence interval. This type of flood needs to be considered under the service and 

strength limit states.

o Check flood: a flood not to exceed a 500-year event or an overtopping flood of a lesser 

recurrence interval. This type of flood needs to be considered under the extreme event 

limit state. 

3.1.2 Addressing All Applicable Structural and Geotechnical Limit States 

Article 1.3.2.1 of AASHTO LRFD (2010a) reads: “All limit states shall be considered of equal 

importance.” This means that the service, strength, and the applicable extreme event limit states 

must always be checked during foundation design. Some of the implications of this requirement 

on DOTs practices are discussed next. 

The DOTs should ensure close interaction and communication among the entire design and 

construction team (e.g., structural, geotechnical, hydraulic, and construction). The construction 

personnel should ensure that the design requirements for construction of foundations are met 

during construction. The roles of the structural and geotechnical engineers in the design phase 

should complement each other, as both structural and geotechnical resistances should be 

evaluated in the LRFD design of foundations, and the structural engineer is responsible for the 

development of foundation factored loads at various limit states needed in the evaluation of 

various geotechnical resistances. While the foundation factored loads and tolerable 

displacements are finalized by the structural engineer, the geotechnical engineer often computes 

the foundation displacements. Hence, close communication between structural and geotechnical 

engineers is needed to address this limit state and avoid costly overdesigns. There are several 

other common design issues that should be jointly addressed by the structural and geotechnical 
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engineers, such as earth loads, use of the same design methods to evaluate both the structural and 

geotechnical limit states under lateral loading, and the service limit state. 

The DOTs should explicitly address the service limit state in the foundation design to ensure that

the generated displacements are less than the tolerable displacements. As discussed by Samtani 

(2008), overstress allowances (larger loads than estimated) provided in the AASHTO Standard 

Specifications (2002) were used by the structural engineers as a safety umbrella for not 

performing deformation-based analysis. The 2002 AASHTO Standard Specifications also 

allowed for some arbitrary reduction factors for the computed base resistance of drilled shafts to 

avoid conducting a settlement analysis. Geotechnical engineers often employed conservative 

ASD strength limit design methods with large safety factors to avoid conducting foundation 

settlement analysis. However, there are no overstress allowances or reduction factors in the 

AASHTO LRFD specifications (2010a) since the service limit is addressed explicitly.

The DOTs should explicitly address the drivability limit state for driven piles to ensure that the 

pile can be driven safely to the required depth or resistance without damage. Inclusion of a 

drivability analysis using wave equation analysis and dynamic testing is now prescribed in the 

LRFD specifications (Article 10.7.8). Failure to evaluate pile drivability is one of the most 

common deficiencies in driven pile design practice. Drivability analysis is not specifically 

addressed in ASD because the maximum allowable pile design loads, Qsmax, that a pile can 

support, were developed based only on pile structural capacity and by applying a large safety 

factor (FS) to maintain a low Qsmax. For example, the AASHTO Standard Specifications’ 

recommendation for H-piles is Qsmax = 0.25 fyAs , where fy is the steel yield strength, As is the 

cross sectional area of the steel, and the safety factor is 4. The traditional conservative ASD 

approach evolved many years ago when tools were unavailable to estimate and measure pile 

driving stresses. The LRFD approach recognizes the now mainstream application of tools such as 

wave equation analysis and dynamic measurements for drivability analysis. In contrary to 

AASHTO Standard Specifications that recommends Qsmax values for different pile types, the 

AASHTO LRFD does not provide specific values for the maximum axial factored load a pile can 

support, Qfmax. The Qfmax in the LRFD can be determined in the design by meeting all axial 

compression strength limit states: geotechnical, structural under static loading, and drivability. 

The DOTs should explicitly address the AASHTO’s LRFD extreme limit applicable to them. 

When compared to AASHTO Standards, there have been significant changes in the AASHTO 

LRFD design specifications for the extreme event design methods that would impact the design 

of foundations. In 2007 and 2008, AASHTO approved significant changes to the LRFD seismic 

design specifications. In the 2009 AASHTO LRFD interims, a new Article 10.5.4.2 

“Liquefaction Design Requirements” was added with valuable information in the corresponding 

commentary article. The return period was increased from 500 years to 1,000 years, which led to 
3-5

changes in the seismic zones. Current AASHTO LRFD seismic design specifications are 

considered to be “force based,” where the bridge is designed to have adequate strength (capacity) 

to resist earthquake forces. In 2009, AASHTO released the AASHTO Guide Specifications for 

LRFD Seismic Bridge Design (2009a) which are displacement-based specifications developed to 

address the seismic deflection of the bridge rather than forces, and a guide specification and 

commentary for vessel collision design of highway bridges (AASHTO 2009b). 

This change to explicitly address all limit states has the potential to lead to more accurate and 

economical design when compared to ASD practices. A few examples are discussed next. 

• AASHTO LRFD (2010a) does not promote or endorse using conservative strength limit 

analysis methods to compensate for not addressing the service limit. As a result, greater 

geotechnical capacity for foundations can be used. For structures where the service limit does 

not control the design, more economical foundations are the result. 

• LRFD allows piles to accept greater axial loads than ASD if drivability is investigated in the 

design phase. The upper limit for Qfmax is the pile’s factored axial structural resistance, for 

example for H-piles and assuming severe driving conditions, Qfmax is 0.5fyAs. With this 

Qfmax, H-piles support larger design loads when structural capacity controls, as it might the 

case for H-piles seated on hard rocks or with friction piles when static load tests are 

considered. 

• AASHTO LRFD (2010a) allows for consideration of both the side and tip resistances of 

shafts installed in rocks as long as the requirements for construction methods and quality 

control assumed in the design are met. It has been a common practice for DOTs to ignore or 

use a smaller portion of the end-bearing resistance in the design of drilled shafts to help 

ensure that settlement does not exceed tolerable values. For high quality shafts, this results in 

underutilization of their capacity, and less economical foundations. 

3.2 LOAD AND RESISTANCE FACTORS TO ACCOUNT FOR DESIGN 

UNCERTAINTIES

In the ASD design equation, the uncertainties or in the calculations of loads and resistances are 

accounted for through a single factor of safety. In the LRFD, the load factor (γ) accounts for 

uncertainties associated with the calculated design load, and the resistance factor (φ) accounts for 

uncertainties associated with the calculated nominal geotechnical resistance. The sources of 

uncertainties are separated to make it simpler and more rational to evaluate load and resistance 

factors based on scientific methods. In reliability-based calibrations, the load and resistance 

factors are tied together through a target reliability index, β, which is a quantification of the 

probability of failure, Pf, which is equivalent to not meeting the performance intended by the 

limit state as defined in equations 3.1 and 3.2. 
3-6

Thus, the safety factor in ASD is replaced in LRFD with the reliability index, β, and load and 

resistance factors; design loads in the ASD are replaced with factored loads; and allowable 

capacities are replaced in the LRFD with factored resistance. Some DOTs employed both 

factored loads in the load factor design (LFD) structural design of foundations and unfactored 

loads in the ASD geotechnical design of foundations. With LRFD, factored loads should be 

employed in both the structural and geotechnical design of foundations. 

3.2.1 Load Combinations, Load Factors, and Resistance Factors at Various Limit States 

Section 3 of AASHTO (2010a) presents 12 load combinations that could act on the bridge during 

its design life and describe various types of loads. For each load combination a unique 

combination of loads is specified for each limit state and a load factor is assigned to each load 

type (see Tables 3.1 and 3.2). These combinations of loads and their load factors are needed to 

compute the maximum effects of factored loads (ΣγiQi in Equations 3.1 and 3.2) on all 

components of the bridge structure. The load combinations relevant to bridge foundations are: 

• Strength Limit State. The Strength I, II, III, IV, and V load combinations need to be 

considered. 

• Service Limit State. Only the Service I load combination is needed to evaluate this limit 

for foundations. 

• Extreme Event Limit States. Extreme events are considered one at a time. The 

following events may or may not be relevant: 

o Extreme Event I. Load combination including earthquake (EQ)

o Extreme Event II. Load combinations including ice load (IC), collision with vehicle

(CV), or collision by vessel (CT), Or

 Check flood. Consider the strength limit load combination in the design, but with a 

load factor of 1 assigned to all loads. 

Overall stability should theoretically be addressed under the strength limit state because it is the 

shear strength that is being evaluated and the consequence of failure is global instability. 

However, it is investigated under the service limit state (Article 11.6.2.3) because soil weight 

appears on both the load and resistance sides of the equation and the analytical consequence is 

complex. As a result, current slope stability design methods and programs do not allow the loads 

to be factored. Therefore, for overall stability, φ is computed through calibration by fitting as

1/FS since the load factor is 1 for the service limit state. 
3-7

Table 3.1. AASHTO LRFD Load Combinations and Load Factors at Various Limit States

(after 2010 AASHTO LRFD, Table 3.4.1-1; see AASHTO LRFD Section 3 for definitions of all 

abbreviations)

 

Table 3.2. AASHTO LRFD Maximum/Minimum Load Factors for Permanent Loads 

(after AASHTO 2007, Table 3.4.1-2)
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For the service and extreme event limit states, the changes to design procedures are minor since 

FS = 1 in the ASD design equation, and φ = γ = 1 in the LRFD design equation for most 

resistances and loads considered with these two limit states. This means that the design 

procedures for these limit states under ASD and LRFD would be similar. The most significant 

changes introduced with the LRFD platform are for the strength limit as discussed next. 

3.2.2 Resistance Factors at the Strength Limit 

The AASHTO LRFD geotechnical resistance factors at the strength limit (2010a) were 

developed based on reliability analysis of data collected at load test sites, calibration by fitting to 

ASD, and engineering judgment. Reliability calibration of AASHTO’s resistance factors is

considered mainly for the axial compression resistance determination methods of a driven pile 

and a drilled shaft at the strength limit so these methods could be impacted by the transition from 

ASD to LRFD and Chapters 4 to 6 is focused on. 

3.2.3 Load Combinations and Factors at the Strength Limit 

For the strength limit, five load combinations with maximum and minimum load factors (Tables 

3.1 and 3.2) need to be considered in the design. Hence, the procedure in which the loads are 

combined in the LRFD equation for the strength limit and compared to resistances is 

significantly different from the ASD design procedure as illustrated next. 

Table 3.3 describes the conditions for which the five strength limit load combinations should be 

considered in LRFD design. The Strength I load combination is the most common and will be 

applied for most routine short-span bridges, while the Strength IV load combination should be 

considered for long-span bridges (span length exceeding 200 feet). Strength I or IV load 

combinations must be always considered in the design. Additionally, if the bridge may be 

exposed to wind up to 55 mph during bridge design life, then Strength V load combination 

should be considered in the design, and if wind may exceed 55 mph during bridge design life, 

Strength III load combination need to be also considered in the design. 

 

The load factors for the same load type vary among the different load combinations. For 

example, the load factor for live loads (LL) varies from zero in the Strength III and Strength IV 

load combinations to 1.35 in the Strength II and V load combinations to 1.75 in the Strength I 

load combination. As will be discussed in chapter 4, Strength I load factors are considered in the 

calibration of the geotechnical resistance factors, where the dominant loads are dead load (DL, 

load factor 1.25) and live load (LL, load factor 1.75), with an average load factor of around 1.4 

(see Table 3.3). These resistance factors should be employed in the design with all the other 

strength limit load combinations. As discussed in Table 3.3, the load factors of other load 
3-9

combinations (Strength II, III, IV, and V) are selected to reflect the certainty, importance, and 

weight of these loads and to generate overall reliability with loads close to those considered 

under the Strength I load combination. This may explain why the average load factor for some of 

these load combinations is close to 1.4, similar to that used with the Strength I load combination 

(see Table 3.3). Consequently, resistance factors calibrated using the Strength I load combination 

are believed to be reasonable for use with other strength limit load combinations, but this may be 

changed in the future. 

Table 3.3. Descriptions of the Strength Limit Load Combinations 

Strength Limit Description Load factors

I Normal use of short span 

bridge without wind. The basic 

and most common group. 

Load factor, γ= 1.25 for Dead load (DL) and 

1.75 for Live loads (LL), with an average 

load factor, γave, of 1.4 for DL/LL ratio of 2 

to 3.

II For use with specified design 

vehicles- No wind. 

As with Load Combination I but with γ= 

1.35 for LL because of the more certainty of 

this load than with Load Combination I.

III When Bridge is exposed to 

wind> 55 mph. 

γ=0 for LL (high winds prevent the presence 

of live loads on the bridge), and 1.4 for the 

wind load (the most dominant).

IV For long spans (>250 ft) with 

DD/LL ratio > 7.

γ = 0 for LL and 1.5 for DL, leading to γave

= 1.35 for a DL/LL ratio of 9.

V For normal use (as Strength I) 

but with wind speed up to 55 

mph. 

Smaller γ for LL (1.35) than in Strength I 

Load Combination because wind load is 

considered in this load combination 

For permanent loads, AASHTO LRFD provides maximum and minimum load factors (see Table 

3.2). For live loads, maximum load factors are provided (see Table 3.1), but a minimum load 

factor of zero can be considered for these loads. Maximum and minimum load factors are

employed in the strength limit to maximize the force effect for the failure mode investigated, 

either by increasing the loads that contribute to this failure (e.g., consider maximum load factors 

with the vertical loads in the evaluation of bearing failure for shallow foundations), or 

decreasing the loads that will increase the resistances to this failure (e.g., consider minimum load 

factors with the vertical loads in the evaluation of sliding failure for shallow foundations). 

To maximize the force effect for any strength limit’s geotechnical resistance, first determine the 

applicable load combinations that should be evaluated in the design (based on Table 3.3). Then, 

for each load in the load combination, consider either maximum or minimum load factors. If you 

are unsure of which to use, try both the maximum and minimum load factors in the design. The 
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consideration of all applicable load combinations with maximum and minimum load factors 

would lead to the identification of the critical load combination with the maximum force effects. 

The Reference Manual for NHI Course 130082 (NHI, 2005) provides good examples of how to 

maximize the force effect in the evaluation of various geotechnical and structural resistances for 

foundations. 

The need to consider various combinations of loads with maximum and minimum load factors in 

the evaluation of strength limit resistances represents a major difference from ASD design, in 

which only one load combination (Dead loads+ Live Loads) is considered in the geotechnical 

design of foundations. This concept is introduced in the LRFD platform to account for all 

possible combinations of loads that may act on the structure during its design life.

 

3.3 NEW AND IMPROVED METHODS TO DETERMINE LOADS, RESISTANCES, 

AND DISPLACEMENTS 

The governing LRFD and ASD design equations at each limit state are: 

LRFD: Σγi Qi ≤ ∑φi Rni (3.3) 

ASD: ΣQi ≤ ∑Rni/FSi (3.4)

Although foundation design load (Qi) and nominal resistance (Rni) are used in both the ASD and 

LRFD platforms, methods to compute them continue to be improved and updated in the LRFD

platform. By contrast, the last technical update to the AASHTO Standard Specifications was in 

1998, with no plans for future updates. Improvements to the methods to compute foundation 

design load (Qi) and geotechnical nominal resistance (Rni) would be applicable to the ASD 

platform if updates to the Standard Specifications were to be continued. 

3.3.1 Examples of Improved AASHTO Methods to Determine Loads

• The truck loading model (HL-93) was adopted due to the increase in live loads carried by 

trucks. The increased live load from HS-20 (ASD) to HL-93 (LRFD) is a pure loading issue 

that would be applicable to ASD if updates to the Standard Specifications were to be 

continued. 

• In AASHTO LRFD, there are new methods to estimate earthquake and vessel collision loads 

(AASHTO 2009a and 2009b) that would be applicable to ASD if updates to the Standard 

Specifications were to be continued. 

• According to AASHTO LRFD Articles 3.11.8 and 10.7.3.7 (AASHTO LRFD, 2010a), 

downdrag (DD) is an axial compressive load that should be considered in the design of 
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drilled shafts and driven piles at all limit states using a load factor of γp. DD load is computed 

as the nominal side geotechnical resistances of the soil layers located in and above the lowest 

layer contributing to downdrag (see AASHTO Article 10.7.1.6.2). The total factored axial 

compressive load per pile/shaft is calculated as ΣγiQi + γp DD; where ΣγiQi is the factored 

axial load applied to the top of the pile from the superstructure and γp DD is the factored 

downdrag load. In the ASD platform at the strength limit, DD effect is treated by considering 

it as a negative resistance, while in the LRFD, DD is treated as an additional load and lost 

resistance (zero resistance as will discussed later). If the ASD specifications were to continue 

to be updated, DD will be treated as an additional load (total axial compressive load per 

pile/shaft would be calculated as ∑Qi + DD) and lost resistance. 

3.3.2 Improved Methods to Determine Foundation Displacements and Resistances 

AASHTO LRFD covers popular methods to calculate the foundation nominal geotechnical 

resistances, Rn, and displacements that have track records of long-term success. For example, the 

shaft nominal axial unit tip resistance in rocks can be computed as Rn = 2.5Abqu, where Ab is the 

shaft base area and qu is the unconfined compressive strength of the rock mass. The three types 

of resistance and displacement determination methods discussed in AASHTO are:

Static analysis methods. These methods have two components: a) soil/rock strength design 

properties collected in the design phase from the subsurface exploration program (e.g., qu), as 

discussed in AASHTO LRFD Article 10.4; and b) analytical expression used to calculate Rn

(e.g., Rn = 2.5Abqu) or displacement. These methods are discussed in AASHTO LRFD Articles 

10.6 to 10.9 for different foundation types. 

Field dynamic analysis methods. These methods are used with driven piles to determine their 

axial static compression geotechnical resistance, Rn, in the field using a) information collected 

during driving the pile (e.g., hammer developed energy and number of blows to drive 1 inch), 

and b) analytical model. These methods are discussed in AASHTO LRFD Article 10.7, and 

include dynamic testing with signal matching (referred to here as dynamic load test), wave 

equation analysis methods, and the FHWA modified Gates and Engineering News (EN) dynamic 

formulas. 

Static load testing. With this design method, foundation geotechnical resistance and 

displacement are measured directly in the field. This method is described in AASHTO LRFD 

Articles 10.6 to 10.9 for different foundation types. 
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3.3.2.1. AASHTO LRFD Article 10.4: Soil and Rock Properties 

This Article was improved and expanded in AASHTO LRFD (2010a), when compared to 

AASHTO Standards, to emphasize the importance of geotechnical investigation and testing work 

in geotechnical design. It describes the determination and selection of soil/rock properties needed 

for the design and construction of foundations. Recommended AASHTO guidance for 

subsurface investigation programs includes the location, number, and depth of subsurface 

borings, and DOTs need to adhere to this guidance. The level of subsurface exploration in 

AASHTO LRFD Table 10.4.2-1 should be considered a minimum and should be increased 

based on past experience, the degree of site variability, and the importance of the structure. The 

laboratory and in situ tests for the determination of the design geotechnical properties for various 

geomaterials are discussed. Geophysical testing methods for soils and rocks are discussed. 

Article 10.4.6 discusses the selection of deformation and strength properties for soil and rock 

mass. SPT-N values (from the Standard Penetration Test) should always be corrected for 

hammer efficiency (N60) and sometimes for overburden N160. Rock classification is based on 

rock mass rating (RMR), rather than solely on rock quality designation (RQD) as in older ASD 

methods. Erodability (scourability) of rock mass is also discussed in Article 10.4.6. DOTs need 

to know that AASHTO LRFD allows for consideration of local experience and specific geology 

in the selection of soil and rock properties. 

3.3.2.2 AASHTO LRFD Article 10.5.5: Tolerable Movement and Movement Criteria for 

Foundations

Vertical and lateral displacements and rotation of the foundation lead to movements of the bridge 

superstructure at critical locations (e.g., at the girder seat). 

In contrast to AASHTO standards (2002), specific tolerable movements are not provided in the 

AASHTO LRFD (2010a). Both AASHTO LRFD (Article 10.5.5) and the FHWA Soils and 

Foundations Workshop Manual (2006) suggest the development of project-specific movement 

criteria for foundations as a function of structural tolerance of the bridge, the tolerance of the 

structures around the bridge (the approach slab), and rideability, economy (cost of future 

maintenance and repair), safety (clearance), and aesthetics. The DOTs should establish final 

guidelines for movement criteria that are appropriate to their bridges, field conditions, and design 

requirements. 

AASHTO LRFD (2010a) indicates that transient live loads may be omitted in the timedependent settlement analysis of foundations bearing on cohesive soils. Note also that the goal of 

the settlement analysis should be to determine the settlement of the bridge that would impact its 

performance, not the settlement of the foundation. Therefore, only certain loads should be 
3-13

considered in the estimation of the bridge settlement, not all loads that lead to settlement of the 

foundation. 

For the service limit, the force effect can be defined as the foundation displacement generated by 

the foundation design load (OR the foundation design load), and the nominal resistance as the 

tolerable displacement (OR the foundation design load that would generate the tolerable 

displacement). In both cases, tolerable movement values, as discussed in AASHTO LRFD 

Article 10.5.2, and estimation of the foundation displacement, as discussed in AASHTO LRFD 

Articles 10.6 to 10.9 for different foundation types, are needed. 

3.3.2.3 AASHTO LRFD Article 10.6: Spread Footings 

• Article 10.6.1. General Considerations. For footings on soils, a uniform bearing pressure 

and effective dimension (B′ and L′) should be considered to evaluate the bearing resistance 

and settlement. 

• Article 10.6.2. Service Limit State Design. This article covers the methods used to estimate 

settlement of footings on soils and rocks as well as overall stability. Methods of estimating

bearing resistance at the service limit state are also discussed. 

• Article 10.6.3. Strength Limit State Design. Geotechnical resistances needing evaluation 

include the bearing resistance of soils and rocks, eccentricity, and sliding. The overturning 

stability in the ASD platform has been replaced with the eccentricity limit under LRFD. 

Eccentricity provisions are different from those in ASD because they are based on factored 

loads. 

• Article 10.6.4. Extreme Event Limit State Design. Geotechnical resistances needing

evaluation include bearing capacity, eccentricity, sliding resistance, and overall stability. 

3.3.2.4 AASHTO LRFD Article 10.7: Driven Piles 

• Article 10.7.2. Service Limit State Design. Addresses settlement (including settlement due 

to downdrag loads), horizontal displacement, and lateral squeeze. 

• Article 10.7.3. Strength Limit State Design. Covers the following geotechnical resistances: 

axial compression, uplift, and lateral resistances of a single pile and a group of piles. It also 

briefly describes the structural resistances (axial, bending, and shear) of steel, concrete, and 

timber piles. 

• Article 10.7.4. Extreme Event Limit State Design. The nominal resistances considered are 

similar to those for the strength limit. 
3-14

There have been significant changes in the AASHTO LRFD specifications for driven piles 

(2010a) as compared to the ASD specifications. The p-y method is now specified for lateral 

loading analysis. The Nordlund-Thurman method is now included in the static analysis methods.

Requirements for drivability analysis are discussed (see Article 10.7.8). New diagrams for 

computing settlement of pile combinations are provided. New articles have been added: Article

10.7.3.2, “Pile Length Estimate for Contract Documents,” and Article 10.7.6, “Determination of 

Minimum Pile Penetration.” Other changes between ASD and LRFD for driven piles are 

discussed in chapters 5 and 6. 

3.3.2.5 AASHTO LRFD Article 10.8: Drilled Shafts 

Single and group shaft resistances are discussed in this article. The geotechnical and structural 

resistances covered are similar to those for piles (except without drivability analysis). AASHTO 

LRFD recommends the use of the Strain Wedge model in the lateral loading analysis of largediameter and relatively short, shafts that have a tendency to rotate rather than bend. Design 

methods for cohesive and granular intermediate geomaterials (IGM; materials between soils and 

rocks) are now included. The most recent O’Neill and Reese method (1999) replaces the Reese 

and O’Neill method (1988) for soils. 

3.3.2.6 AASHTO LRFD Article 10.9: Micropiles 

This article was added in the AASHTO 2007 Interim Revisions. Its technical content is mainly 

based on the FHWA micropile design guidance (2005). 

3.3.2.7 AASHTO LRFD Appendix A of Section 10 

This appendix discuses the seismic analysis and design for foundations. See Article 3.1.2 of this 

manual for other AAASHTO references on this topic. 

3.3.2.8 Nominal Geotechnical Resistance Losses 

Only the foundation nominal geotechnical resistance, Rn, that would be available to support the 

applied foundation loads during the bridge entire design life should be considered in the design 

(Σγi Qi ≤ ∑φi Rni). Hence, Rn should not include resistances that may not be available during the 

design life of the structure to support the applied loads. The sources of geotechnical resistance 

losses (GL) that should not be included in determination of Rn are downdrag, scour, liquefaction, 

and future increases of groundwater level (GWL). These will decrease the available foundation 

geotechnical axial and lateral resistances at all limit state, thus leading to longer deep 

foundations. Note that a zone within the soil layer will contribute to only one type of loss: 
3-15

downdrag, scour, or liquefaction. The GWL expected over the life of the structure should be 

considered in the geotechnical design of foundations. With the static analysis methods, Rn can be 

estimated directly without the need to estimate GL. With the pile field axial design methods, 

estimation of GL is necessary to determine the required axial geotechnical resistance the pile 

needs to be driven to in the field. In this case, future increases in the GWL beyond the level at 

the time of driving the pile should be considered in the estimation of GL. Other sources for GL 

are discussed next.

Downdrag (AASHTO Articles 10.7.3.7 and 3.11.8). Assume zero axial resistance (Rn=0) in and 

above the lowest layer contributing to downdrag. To compute Rn below the depth contributing 

to downdrag, consider only the foundation base and sides resistances of the soil layers located 

below the lowest layer contributing to downdrag. The GL is the nominal side geotechnical 

resistances of the soil layers located in and above the lowest layer contributing to downdrag 

(same as the DD loads). Hence, according to AASHTO LRFD, downdrag effect at ALL axial 

limit states should be applied twice: as an additional load (as discussed previously) and as an 

additional lost nominal geotechnical resistance (the loss is applicable to the geotechnical limit 

states). At the service limit, the DD effect will increase foundation settlement due to additional 

loads and lost geotechnical resistance. To address the DD effect increase the length of deep 

foundations for the axial compression geotechnical limit states and increase the size (diameter) 

of the deep foundation for the structural axial compression limit states. 

Scour. The consequences of changes in foundation conditions (e.g., removal of scoured soil 

layers) resulting from the design flood (100- year event) for scour shall be considered at the 

strength and service limit states (AASHTO Articles 3.7.5, 2.6.4.2, 10.7.3.6). The consequences 

of changes in foundation conditions due to scour resulting from the check flood (500- year event) 

for bridge scour and from hurricanes shall be considered at the extreme event limit states 

(AASHTO Articles 2.6.4.2, 3.7.5., 10.5.5.3.2, and 10.7.4). With these two types of floods, 

identify the scour depths due to degradation scour and due to contraction scour, both occur over 

a large area, and the scour depth due to local scour that occurs in a limited area around the 

pile/abutment (Hannigan et al., 2005). Add these three depths to determine the lowest depth, 

Dscour, for scour conditions. Assume zero axial resistance, Rn for all soil layers that will be lost 

due to scour (along Dscour). For computation of Rn for the soil layers located below the scoured 

soil layer (below Dscour), assume zero vertical effective stresses ONLY along the depth of scour 

due to degradation and contraction, but not along the lower depth due to local scour. In 

combination with extreme event limit states EQ, IC, CV, or CT, the commentary AASHTO 

article 3.4.1 suggests to consider only the scour due degradation or one-half of the total scour. 

AASHTO Article 2.6.4.2 recommends to place the bottom of spread foundation below the check 

flood scour elevation, and to place top of deep foundation below the estimated contraction scour 

depth. 
3-16

Liquefaction. In the 2009 AASHTO LRFD interims, the new Article 10.5.4.2 “Liquefaction 

Design Requirements” was added to the code specifications with valuable information in the 

corresponding commentary article. A thorough discussion of the assessment and consideration of 

liquefaction in the design of drilled shafts (also applicable to driven piles) is described in 

chapters 12 and 16 of the new FHWA Manual on Drilled Shafts (Brown et. al., 2010). If the 

assessment of liquefaction potential identifies subsurface zones where liquefaction is likely, the

following principles will govern the design: 

• Assume zero axial resistance (Rn=0) in and above the liquefied zone. To compute Rn below 

the liquefied zone, consider only the foundation base and sides resistances of the soil layers 

located below the liquefied zone. Deep foundations must, therefore, be sufficiently deep to 

develop adequate resistance in the geomaterials located below the zone of liquefaction. 

• Consider a lower lateral resistance value for the liquefied soil layer. The p-y curve should be 

selected based on the soil strength properties corresponding to the liquefied state. 

• Post-liquefaction settlement would cause downdrag, which would increase foundation axial 

compression loads (see AASHTO LRFD commentary to Article C3.11.8) and reduce the 

available axial compression geotechnical resistance. The downdrag forces due to liquefaction 

should not be combined with downdrag considered in the strength limit. 

• Consider the horizontal forces on the foundation due to the lateral spreading (movements) of 

a sloped soil mass due to liquefaction. Deep foundations may be designed with the intent to 

restrain the lateral movement of a soil mass during earthquakes to prevent an overall stability 

(or soil slope) failure and prevent any damage to the deep foundation. 
4-1

CHAPTER 4

CALIBRATION METHODS FOR GEOTECHNICAL RESISTANCE FACTORS

This chapter describes the two calibration methods for geotechnical resistance factors—fitting to 

ASD, and reliability analysis—to the extent needed by DOTs for the local calibration of their 

resistance factors, and then describes the methods employed to develop AASHTO’s geotechnical

resistance factors. 

Both calibration by fitting and reliability analysis discussed herein are demonstrated for the axial 

compression resistance determination methods at the Strength I load combination but the general 

principles presented in this section can be used to calibrate any kind of resistance or 

displacement determination method with any load combination. 

 

4.1 CALIBRATION BY FITTING TO ASD METHODS

A simplified version of the governing LRFD and ASD design equations at the strength limit for 

bridge foundations is 

LRFD: Qf ≤ φ Rn ; ASD: Qs ≤ Rn/FS (4.1)

Where Qf and Qs are, respectively, the summation of factored loads and unfactored loads, and Rn

is the summations of all nominal (or ultimate in the ASD) resistances assuming similar φ for 

various sources of resistances. 

This calibration requires the factor of safety (FS) for the ASD resistance determination to be 

calibrated and the average weighted load factor, γave, defined as the mean ratio between the 

factored loads (used in LRFD design) and the unfactored loads (used in ASD design, using a 

load factor of 1 for each load), or Qf = γaveQs. The same values of loads should be used to 

compute the factored and unfactored loads. Resistance factor from calibration by fitting is 

computed as φ = γave/FS, which leads to factored resistance = γave × allowable ASD capacity. 

This implies that the factored resistances and factored loads in LRFD design are increased by the 

same ratio from the service loads and allowable capacity used in ASD design. With this 

calibration, the outcomes of the calibrated LRFD geotechnical design method will be similar to 

the results of the ASD geotechnical design method and the true margin of safety will remain 

unknown. Some ASD methods employed by DOTs provide allowable design loads (or allowable 

capacity) directly, rather than estimating it as the nominal resistance divided by the safety factor. 

In this case, a reasonable FS can be assumed for estimation of the nominal resistance from these 

methods, but the end result for the calculated factored resistance will be independent of any 

selected FS value. 
4-2

For the axial compression resistance determination methods, at the strength limit, with a load

factor of 1.25 for dead loads (DL) and of 1.75 for live loads (LL), γave can be computed as 

γave = [1.25(DL/LL) + 1.75]/[(DL/LL) + 1], and γave is approximately 1.4 for a typical DL/LL 

ratio of 2 to 3. Note that the DL/LL ratio could vary from State to State depending on a State’s 

typical bridge types and span lengths, and this may lead to a slightly different γave value. 

Example. For an allowable stress design (ASD) method, the safety factor is 2.8 and its 

prediction of the allowable resistance for a foundation at depth of 20 ft is 300 kips. With 

calibration by fitting, the calibrated resistance factor is 1.4/2.8= 0.5, and the equivalent factored 

resistance is 1.4 x 300= 420 kips. 

4.2 RELIABILITY ANALYSIS OF DATA COLLECTED AT LOAD TEST SITES

Two types of calibrated resistance determination methods (called also design methods) will be 

discussed in this manual: 

I. Static analysis methods (resistance determined in the design phase). For example, the shaft 

nominal axial tip resistance in rocks is computed as Rn = 2.5Abqu, where Ab is the shaft base area 

and qu is the unconfined compressive strength of the rock mass. These methods have two main 

components: a) soil/rock strength properties collected from a subsurface exploration program 

(e.g., qu) during design, and b) analytical models (or equations) used to calculate Rn. 

II. Field dynamic analysis methods (resistance determined in the field). These methods are 

used with driven piles to determine their axial static compression geotechnical resistance, Rn, in 

the field either at the end of driving (EOD) conditions or, after sufficient time has passed, at the 

beginning of redrive conditions (BOR) conditions (See Table 4.1). Setup leads to an increase in 

pile resistance over time between EOD and BOR conditions, and this time is called restrike time.

The resistance factors for these methods can be calibrated either at EOD conditions, when EOD

resistances are used in the calibration, or at BOR conditions, when BOR resistances are used in 

the calibration or at both EOD and BOR conditions (see Table 4.1). These methods include 

dynamic testing with signal matching (referred to here as dynamic load test), wave equation 

analysis methods, and the FHWA modified Gates and Engineering News (EN) dynamic 

formulas. They have two components: 

• Analytical model. 

• Pile driving information needed in the analytical model, mainly the hammer developed 

energy or stroke (and hammer efficiency in some methods) and the penetration resistance, 

Nb, defined as the number of hammer blows needed to drive the pile 1 inch, expressed as 

blows per inch, or bpi. 
Example: In the Engineering News (EN) dynamic formula (AASHTO, 2010a), the 

geotechnical nominal static resistance during driving assuming zero geotechnical losses is 

determined during driving as Rn (Kips)= 12Ed/(s + 0.1), where Ed (ft.-tons) is the developed 

hammer energy and s is the pile permanent set in inches per blow, equal to 1/Nb. 

Table 4.1 Demonstration for Determination of Resistance in the Field with Pile Dynamic 

Analysis Methods 

Time Depth 

(ft)

Hammer Blow 

Count Stroke 

(ft)

Geotechnical Resistance 

(kips) (bpi)

Driving 10 1 5 15

Driving 15 1.5 6 75

Driving 27 3.5 6.5 160

Driving 32 4.5 6.5 190

End of Driving 

(EOD) Condition

33 5 6.5 220 

Restrike or Waiting Time

Begin of Redrive 

(BOR) Condition 33 10 6.5 320 

Reliability analysis involves three steps, Step 1: compilation of information at load test sites, 

Step 2: statistical analysis, and Step 3: reliability Analysis. This section will initially describe 

these three steps and then discuss the use of reliability calibrated results to evaluate the 

economics and improve accuracy of the calibrated design methods. 

4.2.1 Compilation of Information at the Load Test Sites 

At the load test sites, collect for each test foundation (for same soil, see Table 4.2):

• Measured foundation axial nominal geotechnical resistances, Rm, from the load test and 

all the conditions used to obtain them, including size, type, and methods for construction and 

quality control of test foundation, and load testing, type, procedure and results. 

• Predicted foundation axial geotechnical resistances, Rn, from the calibrated method and 

all the conditions considered to predict them, including the procedures to determine and 

select strength properties with the static analysis methods, and driving information for the 

dynamic analysis methods (e.g., type and efficiency of hammer, penetration resistance, BOR 

or EOD conditions). Describe in details the analytical expression used to predict the 

4-3 
4-4

resistance and any assumptions used in this expression (e.g., plugged or unplugged 

conditions with the static analysis methods, input parameters for the wave equation analysis). 

The conditions employed to obtain the measured and predicted resistances on test foundations at 

the load test sites should be similar to the conditions considered in the design and construction of 

production foundations as will be discussed in Section 5. 

Table 4. 2. Demonstration Example: Statistical Analysis of Bias Resistance Data 

# of 

data 

Location SPT-N for 

the Base 

Material 

Base Resistance 

(base area, Ab = 1 ft2) 

Bias Resistance = 

Measured Resistance 

/Predicted Resistance Predicted 

Resistance from 

the Design 

Method = N Ab 

Measured 

Resistance 

from Load 

Test

bpf Kips Kips

1 Colorado 5 5 4.5 0.90

2 New York 22.5 22.5 20 0.89

3 Florida 15 15 12 0.80

4 California 16.5 16.5 23.5 1.42

5 Egypt 10 10 15 1.50

.

.

n = adequate # 

of load tests

Assuming Normal 

Distribution:

Resistance Mean Bias (λ ) 1.10

Standard Deviation 0.33

COV 0.30

4.2.2 Statistical Analysis to Account for Design and Construction Uncertainties 

For demonstration purposes, Table 4.2 presents sample data (not actual data) for calibration of a 

design method that predicts the axial base resistance for a drilled shaft as Rn (kips) = NAb, where 

N is the measured SPT-N value and Ab is the shaft base area. The bias resistance is defined as 

Rm/Rn, where Rm is the measured resistance from the static load test. The bias resistance data are 

fitted to a probability distribution curve (see Figure 4.1) that is expressed analytically with a 

mean bias, λ, and coefficient of variation, COV. Assuming normal distribution curves for the 

bias resistance data, the resistance mean bias is computed as λ = ∑(Rm/Rn)/n, where n = number 

of data, and COV can be computed as demonstrated in Table 4.2.
4-5

Bias Resistance (Rm/Rn) 

Figure 4.1. Example Histogram and Frequency Distributions of Bias Resistance Data 

(NCHRP Report 507) 

The geotechnical resistance factor of a design method, φ, is mainly a function of its COV and λ

(Figure 4.2). 

• COV measures the variability of the method in predicting the resistance. The smaller the

COV, the larger the calibratedφ, and more accurate, precise, and economical is the 

calibrated method. 

• Resistance Mean Bias, λ, measures the tendency of the design method to underestimate or 

overestimate the resistance, Rn, relative to the resistance measured by the static load test, 

Rm. For the same COV, the closer λ is to 1, the more accurate the design method is, and as 

λ increases, φ increases. The value of φ could even exceed 1 (see Figure 4.2) with large λ

values (when the design method significantly underestimates the resistance measured by 

the load test). 
4-6

Figure 4.2. Resistance Factor as a Function of Resistance Mean Bias, λ, and COV (from 

NCHRP Report 507)

The statistical parameters λ and COV account for the following sources of uncertainty (errors 

and variations) in the predicted resistance, Rn (see Table 4.2): 

• Errors in the analytical model (or Rn = NAb in Table 4.2) that predicts the nominal 

geotechnical resistance. This error would include the error resulting from the selection of 

input parameters in the analytical model. The more these input parameters (SPT-N values in 

our example) and the weight they represent in the analytical model (N vs. N2) correlate with 

measured resistance from load tests, the smaller this error, and the more efficient is the 

calibrated design method. It would be expected that the design methods that employ soil 

properties obtained from in situ tests rather than laboratory tests, or from cone penetration 

test (CPT) rather than SPT, would be more efficient. 

• Inherent variability of the measured soil properties due to: 

o Inherent (natural) variation in subsurface materials due to the natural characteristics of 

soil, where soil properties within the same stratum (e.g., dense sand) vary from point to 
4-7

point. Due to this natural variation, the average value of soil/rock properties measured at 

various points in the vertical direction are used in the calibration to predict the foundation 

nominal geotechnical resistance, Rn. COV will include this variation because the 

measured and predicted resistances are obtained for the same soil/rock layer.

o Inherent (natural) variation in geotechnical testing methods (e.g., the SPT in our 

example). Repeatability of test results under “almost identical” soil conditions have been 

used to assess the extent of this variation. For example, the use of the automatic hammer 

with the SPT will lead to more consistent N values and larger resistance factors than 

traditional hammers. 

• Inherent variability of the driving systems in the field dynamic analysis design methods for 

driven piles (similar to the variation of testing methods in the static analysis methods). 

• Errors and variations with methods for construction and QC/QA. The better and more 

consistent these methods are, the larger the measured resistance from the load tests would be, 

leading to larger resistance factors. Future research should consider the influence of 

construction method and QC/QA method on the efficiency of the design method, which 

would be increased with better methods for construction and QC/QA. 

• Errors and variations of the load testing procedure, which will lead to errors and variations in 

the measured resistances from load tests. This emphasizes the importance of using quality 

load test data in the calibration of resistance factors. 

4.2.3 Reliability Analysis to Determine Resistance Factors

The following information is needed in the reliability analysis (see Figure 4.2): 

• Similar given Information for all calibrated resistance determination methods, including 

the target reliability index β (index of probability of failure), Load factors for DL (1.25) and 

LL (1.75), DL/LL ratio (often assumed to be 2), and λ and COV of dead and live loads 

(given). 

• Resistance λ and COV of the calibrated resistance determination method that will be 

calibrated (discussed in the previous section). Different design methods for prediction of Rn

have different combinations of λ and COV, depending on their accuracy and precision, 

which lead to different φ values for these methods (see Figure 4.2). 

The uncertainties on the load side (represented by load factors) and resistance side (represented 

by resistance factors) are tied together through a target reliability index β. For the same 

reliability index, as the load factors increase, the calibrated resistance factor would increase. This 

rule is true for the calibration by fitting but for the same safety factor. Three reliability analysis 

methods have been reported in the literature ((Allen et al., 2005; Paikowsky et al., 2004) to 
4-8

determine φ that vary in terms of sophistication and accuracy. The simplest method is called the 

“First Order Second Moment (FOSM),” which seems to generate conservative resistance factors. 

The advanced methods are the “First Order Reliability Method (FORM)” and ”Monte Carlo” 

method. 

4.2.4 Evaluation of Economics and Improvement of Accuracy of the Calibrated Design 

Method

Table 4.3 lists the results of reliability-based calibration for some of the static and field methods 

for driven piles as published in NCHRP Report 507 (Paikowsky et al., 2004). 

Table 4.3. Resistance Factors for Driven Piles Design Methods from NCHRP Report 507 

NCHRP Report 507

Design Method # of 

Cases λ COV φ Efficiency φ/λ

Static Analysis Methods 

Nordlund method: H-pile, sand 19 0.94 0.4 0.46 0.49

λ-method: concrete pile, clay 8 0.81 0.51 0.32 0.39

α-Tomlinson method 18 0.87 0.48 0.36 0.41

α-API method: concrete pile, clay 17 0.81 0.26 0.54 0.67

FHWA CPT method: concrete 

pile, mixed soil 30 0.84 0.31 0.51 0.6 

Field Dynamic Analysis Methods

Dynamic load 

test

EOD 125 1.63 0.49 0.64 0.4 

BOR 162 1.16 0.34 0.65 0.56

Wave Equation 

Analysis

EOD 99 1.66 0.72 0.39 0.24 

BOR* 99 0.94 0.42 0.43 0.46* 

FHWA 

modified Gates EOD 135 1.07 0.53 0.38 0.36 

*Calibration results for BOR conditions are reported in Appendix B of NCHRP Report 507, where φ = 0.49 for a 

reliability index (β) of 2 and φ = 0.40 for β = 2.5, so it is estimated that φ = 0.43 for β = 2.33

The determination of the resistance mean bias, λ, in addition to the resistance factor, φ, is one of 

the key advantages of statistical/reliability analysis over calibration by fitting to ASD. These two 

parameters, λ and φ, can be used to evaluate the economics and improve the accuracy of the 

calibrated resistance determination methods (called also design methods in this manual) as 

discussed next. 
4-9

Economics of the calibrated resistance determination methods is function of its predicted factored 

resistances, which correlate with its efficiency defined as φ/λ. The larger the efficiency of the 

calibrated method, the larger the factored resistance, leading to either shorter pile lengths or a 

smaller number of piles. It is a common misinterpretation to associate more economical design 

methods as those with higher values of φ (or lower values of FS). The efficiency of a method 

cannot be related directly to the resistance factor, φ, because φ is also affected by the bias of the 

method (whether it over- or under-predicts capacity on average). The efficiency of the design 

method correlates with its COV (see Figure 16 in NCHRP Report 507). The more precise (lower 

COV) a design method is in predicting resistances, the higher its efficiency (φ/λ), and the more 

economical the method is. (See the efficiency results for different pile design methods in Table 

4.3.) The static load test, which has a λ of 1 and efficiency equal to its resistance factor (around 

0.7), is the most economical and efficient design method. The λ of the normal probability 

distribution curve for the bias resistance data should be considered in computing the efficiency of 

the design method even if λ and COV of the lognormal distribution curve for the bias resistance 

data are employed to determine φ. 

The variations in λ among different calibrated design methods lead to the variation of the 

predicted resistance generated by these methods at the same location in the soil mass (see 

Figure 4.3). The static load test is the most accurate design method with λ= 1. The accuracy of 

the calibrated design method increases as its λ gets closer to 1. Accuracy of the calibrated 

design method can be improved by adjusting its λ to 1 so that its resistance predictions on the 

average would be similar to those measured by the static load test. This can be accomplished by 

adding a multiplication factor= λ to the analytical expression of the calibrated method. 

Example: Reliability calibrated results for pile design methods 1, 2 and 3 are: 

 

Design Method φ λ φ/λ

1. Rn (Kips) = 4 AbSu 0.4 0.8 0.5

2. Rn (Kips) = 8 AbSu 0.2 0.4 0.5

3. Rn (Kips) = 3.2 AbSu 0.5 1.0 0.5

All of these design methods would generate similar pile lengths because they all have the same 

efficiency. Method 3 is the most accurate because its mean bias is the closet to one. The accuracy 

of all methods can be improved by multiplying their analytical expression by the mean bias, 

resulting in analytical expression as that for Method 3. 
Figure 4.3. Pile Nominal Bearing Resistances, Rn, from Various Pile Design Methods 

 

0

10

20

30

40

50

60

70

80

0 100 200 300 400

Pile Nominal Bearing Resistances (Kips)

Depth (ft)

Beta method

Static load test

Gates-EOD

Dynamic load test

4-10

4.3 AASHTO’S LRFD CALIBRATION METHODS

In developing the AASHTO LRFD platform, reliability analysis was considered when a reliable, 

adequate number of load test data were available. Such data were available for the calibration of 

the axial resistance determination methods for a single drilled shaft and a driven pile. These data 

are compiled and analyzed in NCHRP Report 507 (Paikowsky et al., 2004). 

Allen (2005) finalized most of the resistance factors for the AASHTO LRFD 2006 Interims for 

bridge foundations based on the results of NCHRP Reports 343 (Barker et al., 1991) and 507 

(Paikowsky et al., 2004), and additional calibration work. These AASHTO’s geotechnical 

resistance factors (φ) were developed by fitting to ASD and use of statistical/reliability analysis. 

When significantly different φ values were generated using different approaches, engineering 

judgment was considered to establish the final AASHTO LRFD φ values, considering the quality 

and quantity of load test data. For the bearing and sliding resistances of spread footings, the 

data used in NCHRP Report 343 (Baker et al., 1991) for reliability-based calibration of φ are 

limited and were developed from small-scale model tests. The AASHTO LRFD resistance 

factors for spread footings are very close to the resistance factors implied from calibration by 
4-11

fitting to ASD estimated using a γave of around 1.4 and the safety factor reported in the AASHTO 

Standard Specifications (2002). 

For micropiles, resistance factors for all resistances (including axial compression) were obtained 

through calibration by fitting to ASD methods (AASHTO 2010a).

4.3.1 AASHTO’s Resistance Factors for Driven piles and Drilled Shafts 

For the uplift resistance, the limited number of load test data for uplift resistance reported in 

NCHRP Report 507 justified the selection of lower φ values for uplift resistance than for axial 

compression resistance. For the horizontal resistance, presently, a φ value of 1 is employed 

for the horizontal geotechnical resistance of a single pile or pile group in AASHTO LRFD, 

which is not equivalent to that implied from calibration by fitting to ASD. Changes are expected 

in the future to address this issue. For the block resistance, the authors of this manual think that 

resistance factor was developed based on calibration by fitting using a γave of around 1.4 and the 

safety factor reported in the AASHTO Standard Specifications (2002). 

Methods to determine compression resistance of a driven pile and a drilled shaft. Resistance 

factors were initially selected mainly based on the reliability analysis developed in NCHRP 

Report 507 (Paikowsky et al., 2004). For drilled shafts, the analyzed cases in this report were 

grouped by soil type, design method, and construction technique. Reliability-calibrated results 

are presented for each combination. For static analysis methods of driven piles, the analyzed 

cases in this report were grouped by soil and pile types and design method. Reliability-calibrated 

results are presented for each group. For field dynamic analysis methods of driven piles, the 

analyzed cases were grouped only by design method (not by soil and pile types as with the static 

analysis methods). Reliability-calibrated results are presented for each design method. In 2009, 

AASHTO approved many changes to AASHTO LRFD Section 10, primarily addressing the 

redundancy issue with the design of driven piles, addressing the handling of site variability when 

selecting resistance factors for static load tests and dynamic testing, and adjusting resistance 

factors for the wave equation analysis method. These changes were implemented to reflect 

successful past ASD practices and the need to use engineering judgment when appropriate. 

These methods are impacted with the transition from ASD to LRFD, and therefore the 

focus of Chapters 5 and 6 will be on these methods. 
5-1

CHAPTER 5 

CALIBRATION CONDITIONS AND ASSESSMENT OF SITE VARIABILITY

This chapter describes the conditions that the DOTs need to adhere to when they adopt 

AASHTO’s LRFD geotechnical design methods and provides recommendations to the DOTs to 

consider with they develop local LRFD geotechnical design methods. As discussed in the 

previous chapter, reliability calibration of the AASHTO LRFD resistance factors is considered 

mainly for the axial compression geotechnical resistance determination methods of a driven pile 

and a drilled shaft at the strength limit. Therefore, these methods could be impacted by the 

transition from ASD to LRFD, and the focus of this chapter will be on the conditions to consider 

with these methods. Other geotechnical design methods will not be impacted because their 

resistance factors were mainly developed through calibration by fitting to the ASD methods 

furnished in the AASHTO Standard Specifications. As discussed in the previous chapter, a 

certain acceptable level of project site variability in the horizontal direction called natural or 

inherent site variability is considered in the calibration of resistance factors. This chapter 

concludes with recommendations to evaluate and address the project site variability, which need 

to be considered with both AASHTO and local LRFD design methods. 

It is assumed here that DOTs will adopt and use AASHTO LRFD loads in the design with both 

AASHTO LRFD and locally developed geotechnical LRFD design methods. As discussed in 

Chapter 2, the methods used to compute loads have been improved in AASHTO LRFD, and 

these changes (which resulted in increased loads in some cases) would be applicable to ASD 

design if updates to the Standard Specifications were to be continued. 

5.1 AASHTO LRFD CONDITIONS 

The DOTs need to justify any deviation from these conditions when they adopt AASHTO’s 

LRFD geotechnical design methods. 

Many of the conditions and parameters described in this section are presented in NCHRP Report 

507 (Paikowsky et al., 2004) and Article 10.5 of AASHTO LRFD (2010a). AASHTO’s 

resistance factors are calibrated for certain axial compression resistance determination methods 

for a driven pile and a drilled shaft at the strength limit only. It is possible that some welldeveloped axial compression strength limit design methods are not covered in AASHTO LRFD. 

Future development of reliability-based resistance factors is an ongoing process, and it is 

expected to cover new design methods for all limit states, other resistances, and new types of 

foundations (micropiles). Refinement of the existing reliability-based resistance factors is also 

expected in the future. As the AASHTO LRFD Specifications continue to be applied for bridge 
5-2

foundation design, the AASHTO’s resistance factors are expected to be refined in the future to 

reflect the current standard of practice and results of new reliability-based research studies. 

 

5.1.1 Design Conditions 

These include the conditions employed in the reliability calibration to predict resistances, Rn, at 

load test sites. 

5.1.1.1 Soil and Rock Properties 

The DOTs need to adhere to the minimum guidelines for a subsurface exploration program as 

described in AASHTO LRFD Article 10.4.2 to establish reliable soil/rock properties for the 

design and construction of foundations. 

In the reliability-based calibration, average measured soil and rock properties at each boring 

were used in the calibration of resistance factors. Use of conservative or lower bound design 

properties can negate some of the efficiency provided by LRFD principles and can result in 

overly conservative designs. AASHTO LRFD Article 10.4 acknowledges that some soil/rock 

deposits have natural variations in consistency and strength with depth, and recommends in this 

case either presenting the property value as a function of depth, or dividing the deposit into more 

than one layer. As will be discussed in Section 5.3, AASHTO’s resistance factors account for a 

certain acceptable level of site variability across the site (horizontal site variability) called natural 

or inherent site variability. 

AASHTO’s reliability resistance factors are calibrated for certain groups of geomaterials: Sand, 

clay, and rock (for all foundation types), and cohesive and cohesionless intermediate 

geomaterials (IGM; for drilled shafts). Specific methods to determine and select the design 

soil/rock properties are considered in the calibration of resistance factors. These methods are 

described in AASHTO LRFD Articles 10.4 and 10.5 and in more detail in NCHRP Report 507 

(Paikowsky et al., 2004). As an example for the use of static pile analysis methods for clays, 

AASHTO LRFD Article 10.5.5.2.3 states: “If the soil cohesion was not measured in the 

laboratory, the correlation between the SPT N value and su by Hara et al. (1974) was used for 

calibration. Use of other methods to estimate su may require the development of resistance 

factors based on those methods.” See also the requirements in this article for obtaining the 

friction angle for use with the Nordlund/Thurman method. 
5-3

5.1.1.2 Design Methods for Driven Piles 

In the ASD axial design of a driven pile, the resistances predicted from a static analysis method

are used with the safety factor of a field method to estimate the pile length in the design phase. 

The required field resistance is estimated based on the safety factor of the field method. The pile 

length must be finalized in the field at a depth where the measured resistance from the field 

method exceeds the required resistance. This is no longer the case with LRFD, where the 

geotechnical resistance factor,φ, is calibrated for both 

• Static analysis methods, φstat (resistance predictions from these methods are used in the 

calibration of φstat) 

• Field dynamic analysis methods, φdyn (resistance predictions from these methods are used in 

the calibration of φdyn) 

The reliability calibration of resistance factors for the static analysis methods was totally 

independent of the reliability calibration of the field dynamic analysis methods. Therefore, and 

although AASHTO recommends using static analysis methods only in cases where the dynamic 

analysis methods are deemed unsuitable for field verification of the pile nominal bearing 

resistance, we believe that the static analysis methods can be used to finalize the pile length in 

the design phase as long as site variability is addressed. 

The AASHTO’s resistance factor for any static analysis method is applicable to different pile 

types (influence of pile type on the resistance factor is not considered). 

Conditions with the field dynamic analysis methods. See Table 4.1 for the definition of EOD 

and BOR conditions. NCHRP Report 507 (Paikowsky et al., 2004) resistance factors for design 

methods for driven piles (static analysis and field dynamic analysis methods) are listed in Table 

4.3. 

According to AASHTO LRFD, if relaxation is possible in the foundation soils, then 

determination of the BOR resistance after a sufficient time is a must and should be considered in 

the design. If setup will be directly considered in the field design method, then setup must be 

verified in the field after a specified length of time (called “restrike time”) by measurement of 

the pile BOR resistance. AASHTO LRFD provides recommendations for restrike times for 

different types of soils. 

The AASHTO’s resistance factor for any field dynamic analysis method is applicable to different 

pile, soil, and driving system types. This is because the influence of pile and soil types on the 

resistance factor is not investigated in the reliability calibration (NCHRP Report 507). In the 
5-4

NCHRP Report 507, information on the restrike time and range of penetration resistances (blows 

per inch) considered in the calibration is not clear. 

For the dynamic analysis methods, the same safety factor is used in ASD design for EOD and 

BOR conditions. This is not necessary correct with the reliability calibration where the calibrated 

resistance factor for EOD conditions can be different than for BOR conditions (see Table 4.3).

AASHTO LRFD dynamic analysis methods (AASHTO 2010a) can be divided into two groups: 

• EOD methods: where φ is calibrated using EOD resistances. With these methods, EOD

resistances (assuming no relaxation) need to be measured and employed to determine pile 

length (no direct benefits from setup). In this case, setup should not be considered in the pile 

design. Methods in this category include the FHWA modified Gates and EN dynamic 

formulas. These dynamic formulas are calibrated in the NCHRP Report 507 only at EOD 

conditions (see Table 4.3). According to AASHTO LRFD, a lower φ should be used with 

these dynamic formulas at BOR conditions (to benefit from setup) than the ones provided in 

AASHTO LRFD for EOD conditions. However, no guidelines are provided in AASHTO to 

determine this lower φ. Dynamic testing, in lieu of dynamic formulas, is recommended to 

verify the resistance at BOR and benefit from setup. 

• EOD and BOR methods: where φ for these methods were calibrated using both EOD and 

BOR resistances. To benefit from setup, use the BOR resistance factors in the design and 

measure the BOR resistance in the field. In the NCHRP Report 507, both the dynamic load 

test and the wave equation analysis method were calibrated at EOD and BOR conditions (see 

Table 4.3). For these two methods, the efficiency (economics) is significantly improved if 

setup is measured and verified at BOR conditions (see Table 4.3; note the larger efficiency at 

BOR than at EOD). These methods underestimate the resistance at EOD conditions because 

soil setup is not considered (see Table 4.3; note the large mean bias at EOD). For these two 

methods, AASHTO LRFD seems to suggest that the same AASHTO’s resistance factor can 

be used for EOD and BOR conditions. 

5.1.2 Construction and Load Testing Conditions 

These include the conditions employed in the calibration to measure resistances, Rm, at load test 

sites. 

Construction Conditions: AASHTO’s φ are calibrated for certain pile and shaft diameters (<2 ft 

for piles; <6 ft for drilled shafts). Hence, AASHTO’s resistance factors are not applicable to 

large size and high capacity deep foundations. AASHTO LRFD (2010a) suggests using the 
AAHSTO’s resistance factors with caution for design of piles of significantly larger diameter. 

AASHTO recommends static load testing for piles larger than 2 ft. 

The AASHTO’s resistance factors for the drilled shaft design methods are applicable to various 

methods for construction and quality control (influence of methods for construction and quality 

control on resistance factor is not discussed). Larger resistance factors for driven piles are 

recommended in AASHTO LRFD with increased level of quality control as will be discussed in 

the next chapter. To use AASHTO LRFD design methods, the DOTs need to employ good 

methods for construction and quality control based on: (a) AASHTO LRFD Bridge construction 

specifications (2010b); (b) FHWA manuals for design, construction, and inspection of various 

foundation types; and (c) the DOT’s current construction specifications, which presents the 

DOT’s experiences, practices and judgment with respect to construction of foundations. 

Load Testing Conditions: AASHTO’s reliability resistance factors are calibrated using the 

quick load tests (AASHTO, 2010a). Hence, it is assumed in the calibration that the resistance 

predicted during design will not change during the entire design life of the foundation. The 

designer needs to accommodate any possible reduction of this resistance during the design life of 

the structure (e.g., creep, softening and deterioration of resistance due to water or environmental 

factors). Also, AASHTO’s resistance factors were calibrated for certain definitions of the 

geotechnical nominal resistance measured from load test. Davisson’s failure criterion is selected 

for driven piles. For drilled shafts, the nominal resistance is defined as the load associated with a 

displacement of 5 percent of the shaft diameter, if plunging of the shaft cannot be achieved. The 

calibrated resistance factors would change if these definitions are changed. 

5.1.3 Calibration Analysis Methods and Parameters 

AASHTO’s φ are calibrated for the strength I load factors/load combinations. This may be 

changed in the future, especially with the calibration of lateral geotechnical resistance 

determination methods where the dead and live loads are not the dominant loads in the design.

Specific statistical/reliability analysis methods (FORM method) and assumptions (Lognormal 

distribution for bias resistance data) are considered in the calibration. Changes to these methods 

or to the assumptions used in these methods would change the calibrated resistance factors. 

Specific target reliability index values (β) were selected in the reliability analysis with 

consideration of the foundation redundancy. Resistance factors would increase if the target β for 

calibration is reduced in the future. Redundancy for a group of deep foundation elements means 

that as one element is overstressed, the other elements can take on additional loads so that the 

foundation group will remain stable (see Figure 5.1). According to NCHRP Report 507

(Paikowsky et. al., 2004) redundant group foundations are defined as having at least five piles or 

5-5
5-6

shafts in the group, and a group with a small amount of redundancy is assumed for number of 

deep foundations between 2 and 4. In this report, the reliability-based calibration of resistance 

factors for the axial compression geotechnical resistance determination methods is based on β = 

2.3 (which corresponds to Pf = 1/100) for a number of driven piles ≥ 5, andβ = 3.0 (Pf = 1/1,000) 

for number of shafts between 2 and 4. These β values were selected to achieve two purposes: (a) 

to generate resistance factors close to those obtained through calibration by fitting to ASD 

methods, and (b) to have a level of reliability or safety for foundations consistent with those 

employed in the structural design of bridge superstructures, where a reliability index of 3.5 has 

been used. The reliability of foundation groups will be greater than the reliability of individual 

foundation elements. Hence, a lower reliability index such as those listed above can be used for 

redundant foundations compared with those used for superstructures. 

Figure 5.1. Redundancy of a Group of Deep Foundation Elements (from NCHRP 

Report 507) 

The requirements to address foundation redundancy for driven piles in AASHTO LRFD (2010) 

is covered in article C10.5.5.2.3 (commentary) to allow for some engineering judgment in 

addressing foundation redundancy. For a small pile group, it is recommended in this article to 

reduce φ by 20 percent. Based on engineering judgment, the number of piles in a small pile 

group can range from one to four (our interpretation of Article 10.5.2.3). A pile group with five 

piles or more can be called redundant. For a single shaft, it is recommended in AASHTO Article 

10.5.5.2.4 (not commentary article) to reduce φ by 20 percent to address the lack of redundancy. 

5.2 SELECTION OF CONDITIONS FOR LOCAL CALIBRATION 

The following AASHTO LRFD calibration conditions are appropriate to consider both with local 

calibration by fitting to ASD and based on reliability analysis: 

• Strength I load factors/load combinations (can be changed if justified) 

• Extent of subsurface exploration program per AASHTO LRFD Article 10.4.2 
• Use of average measured soil and rock properties in the design. 

• Certain acceptable level of variability across the site called natural or inherent site variability

• A reduced resistance factor with lower foundation redundancy 

• Account for future changes of the foundation nominal geotechnical resistances 

• Good methods for construction and quality control as discussed before.

With calibration by fitting to ASD, adopt design conditions similar to those used in the previous 

ASD geotechnical design method. For example, continue the use of the same ASD testing 

methods to determine soil/rock properties. 

DOTs should document all conditions considered in the local reliability calibration of resistance 

factors based on load tests and request that the DOT foundation designers adhere to or are aware 

of them when using these local design methods. 

Four categories of conditions to consider in the local reliability development of LRFD 

geotechnical design methods are discussed next. 

5.2.1 Conditions from AASHTO’s Reliability Calibration 

To enable direct comparison of the locally developed resistance factors with those suggested by 

AASHTO, consider the following AASHTO’s conditions discussed before (a) Load testing 

conditions, (b) reliability index. Article 10.5.5.2.1 of AASHTO LRFD reads: “If the resistance 

factors provided in this article are adjusted to account for regional practices using statistical data 

and calibration, they should be developed using the β values provided above, with consideration 

given to the redundancy in the foundation system.” Document and justify any deviation from 

these AASHTO’s conditions. 

5.2.2 Statistical and Reliability Analyses

Consider one of the two advanced reliability methods to generate larger resistance factors 

(FORM and Monte Carlo). As Figure 4.1 suggests, researchers found that lognormal probability 

distribution curves better fit the bias resistance data (Allen, 2005; Long, 2009). In fitting the bias 

resistance data to a lognormal probability distribution curve, fit the lower range of bias resistance 

data (when predicted Rn > Rm), which is the most critical portion of the bias resistance data for 

the accurate calibration of resistance factors. Some researchers (Long, 2009) have employed the 

lognormal best fit of the tail bias resistance data in calibrating resistance factors, and this led to 

higher resistance factors than using the lognormal best fit of all bias resistance data, as was done 

in the NCHRP Report 507 (Paikowsky et al., 2004). 

5-7
According to Article 3.5.4 of NCHRP Report 507 (Paikowsky et al., 2004), higher resistance 

factors will be obtained if only the available actual bias resistance data are used to calibrate the 

resistance factors. Use of a fitted probability distribution function in the reliability analysis may 

lead to the consideration of new extrapolated bias resistance data in the analysis. If an adequate 

number of bias resistance data can be obtained, it is suggested to limit the calibration of 

resistance factors to the measured range of bias resistance data and NOT extended beyond that 

range. 

5.2.3 Local Design and Construction Conditions 

The calibration should reflect the local design and construction practices. Cover local soils and 

rock formations that may not be covered in AASHTO calibration. Cover local design methods, 

including those not covered by AASHTO and local construction practices. Cover local methods 

to determine and select soil/rock properties, including those not endorsed by AASHTO. As 

discussed before, DOTs should consider improvements of their geotechnical design and 

construction practices at the same time as they’re transitioning to LRFD. The DOT needs to 

consider new and efficient geotechnical design and construction practices that would improve the 

local practices. Consider methods recommended by AASHTO and FHWA technical manuals, 

and new promising methods reported in the literature or used by other DOTs. The reliability 

calibration can be used to compare the local and new design methods and select the more 

economical methods as will be discussed in the next chapter. The local reliability calibration will 

capture the value and justify the consideration of better methods for construction and quality 

control in the local practices (leading to large φ). 

The efficiency of local reliability calibrated design methods can also be improved by narrowing 

the range and ensuring consistency of calibrated conditions as discussed next. 

Drilled shafts: consider the influence of type of construction method and type of quality control 

method on the calibrated resistance factor. NCHRP Report 507 (Paikowsky et. al., 2004) 

examined the influence of construction methods for drilled shafts on the calibrated resistance 

factors. For sand, the efficiency of the design method is higher with the use of casing than with 

the use of slurry. The calibrated resistance factor varies from 0.28 with the slurry method to 0.73 

with the use of casing. For clays, AASHTO (2010a) recommends considering local experience 

with the geologic formation and construction practices in the selection of resistance factors. To 

ensure quality of constructed deep foundations, NCHRP Report 507 suggests testing all 

production shafts for major defects after construction. 

Static Analysis Methods for Driven Pile. Consider pile and soil type in the local calibration of 

resistance factors. Calibrated φ results were reported in NCHRP Report 507 (Paikowsky et. al., 

5-8
2004) for various soil and pile types, but AASHTO’s resistance factors are reported for various 

soil types. Allen (2005) recommended that designers be aware of the NCHRP Report 507 

results and the influence of pile type on the resistance factors, which can be significant. Consider 

new static analysis methods not described in AASHTO in the local calibration. NCHRP Report 

507 calibration results suggest that some of the new static analysis methods are more efficient 

than some of the field methods (see Table 4.3). For example, the α-API method described in the 

FHWA manual on driven piles (Hannigan et. al., 2006) was calibrated in NCHRP Report 507 

and found to be very efficient (see Table 4.3). In the calibration for open-ended piles, calibrate 

the resistance factors assuming both plugged conditions and unplugged conditions, and select the 

conditions with larger efficiency. 

Dynamic Analysis Methods for Driven Piles. Calibrate φ at EOD and BOR conditions. The 

efficiency of the field dynamic analysis method would increase if φ is calibrated at BOR 

conditions as indicated in the calibration results for the wave equation analysis and dynamic load 

test (Table 4.3). Consider pile and soil types (and type of driving system if possible) in the 

calibration of φ. For example, it would be preferable to calibrate a dynamic analysis method 

specifically for closed-end pipe piles driven into cohesive soils with certain common driving 

systems than to calibrate this method for use with all pile types; all soil types, and all driving

systems as was performed in the NCHRP Report 507 calibrations. The AASHTO’s resistance 

factor for any dynamic analysis method is calibrated to different types of piles, soil, and driving 

systems. This could reduce the overall efficiency of the methods in order to be applicable to 

different conditions. Employ consistent restrike time and procedure as employed in the DOT’s 

practices and a small range of penetration resistances for better calibrated results (this range at 

EOD conditions can be different than for the BOR conditions). It is acceptable to consider in the 

calibration restrike times and procedures, and ranges of penetration resistances as a function of 

the soil and pile types as long these conditions are observed in the installation of production 

driven piles. 

5.2.4 Conditions at Load Test Sites 

Load test information needed for calibration can be obtained from existing local load test results 

obtained in the state. These can be supplemented with load test data obtained from other states, 

especially neighboring states with similar field and construction conditions, and by reviewing 

records of load tests reported in the literature. NCHRP Report 507 (Paikowsky et al., 2004) has a 

robust database of load tests for driven piles and drilled shafts. Additionally, to get valuable load 

test information for calibration, consider development and execution of a program of new local 

load tests, especially in large projects where load testing can be very cost-effective. For selection 

of existing load test data or planning of new local load tests, the DOTs need to ensure that the 

calibrated conditions at the load test sites on test foundations (disused below) cover and be 

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5-10

consistent with the local conditions considered by the DOT in the design and construction 

of its production foundations. This will determine the number and details of load tests 

selected for calibration. The DOTs should document all the conditions employed to obtain the 

measured and predicted resistances on test foundations at load test sites so that the DOT 

engineers adhere to in the design and construction of production foundations. These conditions 

are presented next. 

• The soil type and its range of strengths. For example, if the calibrated design method is for 

sand that ranges in its friction angle from 25 to 40 degrees, then several load tests should be 

conducted at sites that have sand with a friction angle ranging from 25 to 40 degrees. If the 

design method is for different soil types, then all soil types and their ranges of strengths 

should be tested. 

• Foundation types and sizes. If the calibrated design method and its resistance factors are 

intended to be applicable to different pile types and sizes, then all these pile types and sizes 

should be load tested. 

• Foundation construction and QC/QA methods. The methods employed to construct the test 

foundation at load test sites need to be similar to those employed in the construction of 

production foundations. 

• Details of the analytical expression employed to predict resistances at load test sites and any 

assumptions used in this expression so designer can follow in their design. For the static 

analysis methods, the conditions include the procedure to determine soil/rock strength 

properties. If plugged conditions for piles are assumed in the design method, then plugged 

conditions should be assumed in predicting the resistance at the load test sites. For the pile 

field dynamic analysis methods, the conditions include BOR and/or EOD conditions,

restrike procedure and time, type of driving systems, and ranges of penetration resistances. 

The restrike time and procedure employed to predict BOR resistance at the load test sites 

should be similar to the waiting time and procedure employed by the DOT to measure BOR 

resistance. The load tests should also cover the types of driving systems commonly used by 

the DOT, and should be limited to the range of penetration resistances allowed by the DOT 

in the installation of its production piles. 

• Load Testing Conditions. Consider those described in AASHTO’s conditions, and type of 

load tests (e.g., static, Osterberg), which may have an influence on the calibrated φ, and “load 

test time” defined as the elapsed time between the end of construction of test foundations and 

load testing. The resistance factor would increase with the increase of measured resistance. 

Hence, select in the calibration the longest practical load test time to capture increases 

(common with setup) or even reduction (relaxation) of resistances. The calibrated load test 
5-11

time should be consistent among the calibrated cases. It is acceptable to consider different 

load test times in the calibration of resistance factors for different combinations of soil and 

pile types. 

 

5.3 ASSESSMENT OF SITE VARIABILITY

Site variability occurs due to the presence of subsurface soil or rock deposits that are 

geologically different across the project site in terms of layer thickness, loading history, and 

engineering properties. The focus here is on site variability due to variation in the measured soil 

or rock properties across the site. This site variability can be quantified by calculating the COV 

of the measured soil and rock properties across the site, as will be discussed later. 

AASHTO LRFD commentary to Article C10.5.2.4 (AASHTO, 2010a) suggests that the site 

variability is not an issue if it does not exceed the inherent variability, COVinherent, of the 

subsurface materials and testing methods. This is in agreement with the reliability analysis 

discussed in Section 4.2, where the calibrated φ accounts for this inherent variability, COVinherent. 

In this manual, the site or a zone in the site are is defined as “uniform” if the computed COV for 

the site/zone, COV, is smaller or equal to COVinherent of the site/zone. 

Assessment of site variability requires the evaluation of site variability and if needed addressing 

it. 

5.3.1. Evaluation of Site Variability 

Initially, perform subsurface exploration program in accordance with AASHTO LRFD Article

10.4.2 to establish reliable design and construction soil and rock properties at locations of 

various foundations. Based on the measured soil properties at various borings, ignore the 

variability of soil and rock layers that would not contribute to resistance (e.g., soft surface soil 

layers, or when the foundation resistance is derived from competent rock, the variability of the 

soil layer above the rock is not an issue). For the soil/rock layer (s) that contribute to foundation 

geotechnical resistance, determine its inherent variability, COVinherent, and evaluate its variability 

across the site, COV as discussed next

5.3.1.1 Determination of Inherent Site Variability 

The COVinherent can be estimated by the project geotechnical based on the results of subsurface 

exploration program, the type of testing employed in the subsurface exploration program, 

judgment and past experience. Also, consider the COV reported by Duncan (2000) in Table 5.1. 

for common soil properties, where COV for the SPT results ranges from 15 percent to 45 
5-12

percent, and it is much larger than COV for the Cone Penetration Test (CPT) results. Finally, 

consider recommendations of of the NCHRP Report 507 (Paikowsky et al., 2004) where the 

upper limit for the low site variability is defined as 25%. It seems appropriate to assume COV= 

25% corresponds to the inherent site variability. The disadvantage of this approach is that one 

COVinherent= 25% is assumed for all soil types and testing methods. 

Table 5.1. Values of Coefficient of Variation for Geotechnical Properties (after Duncan, 

2000) 

5.3.1.2 Determination of Site Variability 

The evaluation of site variability requires analysis of the measured soil properties at various 

borings from the subsurface exploration program. For demonstration purposes (see Table 5.2), 

only one sand layer is assumed to contribute to foundation resistance, and its variability across 

the site will be evaluated. Table 5.2 shows the measured SPT-N values for this sand layer in 

terms of # blows per foot (bpf) at three borings located at the east abutment, center pier, and west 

abutment. 
5-13

Table 5.2. Determination of Site Variability, Step 1: For Each Boring, 

Obtain the Average Property Values Pile

Soft clay layer: not 

considered in the 

design, so no need to 

address its variability

East 

Abutment Pier West Abutment

SPT N-Values (bpf)

 

Sand Layer

6 6 13 

 

6 14 

 

9 6 15 

9 9 14 

10 7 12 

Average N value (bpf) 8 7 14 

Boring 1 Boring 2 Boring 3

Step 1. In the vertical direction, obtain the average soil and rock properties for the soil layer in 

each sounding or boring (see Table 5.2). Note that average soil property values are used in the 

calibration of resistance factors. Use of conservative or lower bound design properties can 

negate some of the efficiency provided by LRFD principles and can result in overly conservative 

designs. AASHTO LRFD Article 10.4 acknowledges that some soil/rock deposits have natural 

variations in consistency and strength with depth, and recommends in this case either presenting 

the property value as a function of depth, or dividing the deposit into more than one layer. 

Step 2. Determine the mean property value, standard deviation, and COV for the average 

property values measured at various borings (see Table 5.3). If the COV is lower than the COV 

for the inherent site variability, use the mean soil/rock property value to develop a single 

geotechnical design profile for the entire project site, since site variability is not issue. If it is not, 

compute the lower limit property value as the mean value minus the standard deviation (SPT-N = 

6 blows per foot in our example shown in Table 5.3) and move to step 3. 
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Table 5. 3. Determination of Site Variability, Step 2: Analyze Average Property Values 

Obtained from Various Borings 

Boring Results

Boring Location Average Corrected N Value (bpf)

Boring 1, East Abutment 8

Boring 2, Pier 7

Boring 3, West Abutment 14

Analysis of Average Property Values (N)

Mean Property Value 10

Standard Deviation 3.3

Coefficient of Variation (%) 34

Lower Limit Property Value 6

Step 3. According to AASHTO LRFD (commentary to Article C10.4.6.1), initially conduct a 

sensitivity analysis with the mean property value (SPT-N = 10 bpf) and lower limit property 

value (N = 6 bpf) to assess their influences on the geotechnical design. If the sensitivity analysis 

shows minimal influence (e.g., the foundation depth or width is almost the same using the mean 

property value and the lower limit property value), conclude the analysis. Otherwise, site 

variability is an issue and should be addressed in the design. 

 

5.3.2 Addressing Site Variability 

The simplest way to address site variability is to design all foundation in the project site using 

the same conservative geotechnical design values (property values and resistance factors) as 

demonstrated next. In the static analysis methods, select conservative property value (lower limit 

property value, SPT-N = 6 bpf; see Table 5.3). In the static load test and dynamic analysis 

methods, perform tests at the locations where the soil/rock strength is relatively small (where 

SPT-N= 6 bpf). In this case, there is no need to change φ in the design. The other alternative is to 

select conservative φ value as a function of the level of site variability (low, medium, or high) 

and the number of tests. This later approach is recommended in the NCHRP Report 507 

(Paikowsky et al., 2004) and allowed by AASHTO LRFD (2010a). However, these approaches 

are not economical! Better approaches to address site variability are presented next. 

Initially, it is suggested to finalize the design (size) of production foundations in the field based 

on the soil and rock conditions encountered during construction. This is possible with some field 

design methods, like the dynamic formulas, where all production piles are driven to a driving 

criterion determined before driving the pile. In this case, the pile will penetrate more deeply at 

the zones with weaker soils than at the zones with stronger soils. This would lead to varying pile 

lengths across the entire project site for the support of the same foundation factored load, so site 
5-15

variability is addressed in these methods. Coupled or in lieu of this approach (if is it not 

feasible), and based on AASHTO LRFD (2010), it is suggested to drill more and deeper 

borings (AASHTO LRFD article 10.4.2) and divide the project site into smaller “uniform” 

zones where the subsurface conditions for each zone are relatively uniform. More and deeper 

borings may be needed for better assessment of the site variability in the vertical and directions 

and for locating boundaries of different uniform zones. In this manual, uniform zone has COV 

smaller than its COVinherent. A uniform zone could even be limited to a single substructure unit.

For example, the project site in Table 5.2 can be divided into two smaller zones: Zone A for the 

east abutment and the pier (mean SPT-N value = 7.5 bpf) and Zone B for the west abutment. 

Another test hole should be drilled at the west abutment in order to ensure that there is no 

significant variation in the soil properties in that zone and to determine the mean N-value. Then, 

we suggest to design foundations in each zone based on the measured soil properties in that zone. 

Applications of the approaches recommended above in addressing site variability are discussed 

next.

Static Analysis Methods. For each uniform zone, use the mean property value for that zone in 

design with no changes to φ. Design foundation in each zone based on soil properties in that 

zone. This would lead to varying foundation lengths across the project site for the support of the 

same foundation factored load. Also with these methods, it is suggested to finalize design in the 

field based on the soil and rock conditions encountered during construction. As discussed by 

Brown et. al. (2010), the design of drilled shafts should be based on some criterion of a minimum 

length of embedment into a certain bearing stratum. This would require the shaft tip elevation to 

be finalized during construction, and this elevation could be different from what is provided in 

the design plans. If the number of test holes is adequate to divide the project site into uniform 

zones as suggested before, the difference in the tip elevation between the plan and field would be 

minimal. It is also recommended to require one boring per shaft for rock-socketed shafts to 

provide site-specific information on depth to rock and quality and strength of the rock along the 

side and beneath the tip of the shaft. Where extreme site variability is expected over a short 

distance or with depth, multiple and deeper borings may be justified at a single shaft location 

(e.g., large-diameter shafts in karstic areas). 

Field Dynamic Analysis Methods for Driven Piles. Develop the boundaries of the uniform 

zones through the boring results in the design phase, and finalize these boundaries in the field 

with test pile results. Use the recommended φ (no change) to estimate the required resistance. At 

a minimum, consider a test pile for each uniform zone and drill to a depth below the required 

depth or resistance (to capture vertical variation in the influence zone). Based on the test pile 

results, develop driving criteria and order pile lengths for the production piles in each zone. The 
5-16

driving system employed with the test pile should be the same as that employed for the 

production piles. 

Static Load Tests. After the project site is divided into uniform zones as previously discussed, 

select the locations and number of load tests. For locations of load tests, consider the zone with 

lowest and highest and average soil and rock strength, and where heaviest loads will be applied. 

Investigate if the measured resistances from the load tests at different zones suggest that there is 

site variability (as suggested by the results of the measured soil properties). If a static load test 

will not be performed in each zone, remember that the static load test results reflect the 

resistance in the zone where it was performed. Calibrate the static analysis methods or field 

dynamic analysis methods to predict resistances at zones where static load tests were not

performed. Then, use this calibration, engineering judgment, and past ASD experience to 

extrapolate the load test results to other zones where load tests were not conducted. In this 

extrapolation, you may need to consider a resistance factor smaller than the resistance factor for 

load tests. In some cases, extrapolation of load test results to other zones may not be possible 

(i.e., if the type of soil/rock deposit varies significantly across the project site). 
 

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CHAPTER 6

SELECTION OF LRFD GEOTECHNICAL DESIGN METHODS

As discussed before, DOTs have three options for the selection of LRFD geotechnical design 

methods: i) Adopt AASHTO’s LRFD methods; ii) Develop local LRFD methods by fitting to 

ASD methods that have track records of long-term success, and iii) Develop local LRFD 

methods through reliability analysis of information collected at load test sites. Chapters 4 and 5 

described how these three options can be implemented by the DOTs. This chapter evaluates and 

compares these three options in order to assist the DOTs in the selection of the most appropriate 

option. 

Reliability calibration of the AASHTO LRFD resistance factors is considered mainly for the 

axial resistance determination methods of a driven pile and a drilled shaft at the strength limit. 

Therefore, these methods could be impacted by the transition from ASD to LRFD, and this 

chapter focused on them. Other geotechnical design methods will not be impacted because their 

resistance factors were mainly developed through calibration by fitting to the ASD methods 

given in the AASHTO Standard Specifications.

6.1 COMPARSION OF 2010 AASHTO LRFD AND AASHTO STANDARD 

This section compares the 2010 AASHTO LRFD geotechnical resistance factors (2010a) with 

those implied from calibration by fitting to the ASD methods given in the 2002 AASHTO 

Standard Specifications (2002). This comparison will help DOTs to determine the impact of 

AASHTO LRFD geotechnical design on their practices.

6.1.1. Drilled Shafts 

AASHTO LRFD resistance factors for drilled shafts supported by sand, IGM, and rocks are close 

to those implied from AASHTO Standards. For shafts in clays, AASHTO LRFD resistance 

factors are lower than those implied from AASHTO Standards. This difference is attributed to 

local geologic conditions and construction practices not considered in the reliability calibration 

(Allen, 2005). 

6.1.2 Driven Piles 

In chapter 5, two differences between AASHTO LRFD and AASHTO Standards are discussed. 

First, in the AASHTO Standards (2002), the pile length must be finalized in the field based on 

the geotechnical resistance measured from the “field resistance determination method” (also 
 

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called in this manual “field design method”). With AASHTO LRFD, the static analysis methods 

can also used to finalize the pile length in the design phase as long as site variability is 

addressed. Second, for the dynamic analysis methods, the same safety factor is used in ASD 

design for EOD and BOR conditions (see Table 4.1 for definition of these two conditions). This 

is not necessary correct with the reliability calibration where the calibrated resistance factor for 

EOD conditions can be different than for BOR conditions (see Table 4.3).

Comparison of the Pile Field Design Methods. This comparison assumes uniform sites/zones 

(low site variability) as discussed in Section 5.3. 

Static load tests. A geotechnical resistance factor of 0.75 is recommended in AASHTO LRFD, 

which is larger than 0.7 implied from the ASD with a safety factor of 2 (AASHTO Standards, 

2002). A large φ of 0.8 is recommended in AASHTO LRFD if dynamic testing is conducted on a 

minimum of 2 percent of production piles, but no fewer than two piles. This means that the use 

of static load tests in AASHTO LRFD would lead to more savings than in ASD. 

Dynamic testing with signal matching. If the dynamic testing is limited to 2 percent of the 

production piles, but no less than two piles, a resistance factor of 0.65 is recommended in 

AASHTO LRFD, which is very close to the resistance factor of 0.62 implied from the ASD with 

a safety factor of 2.25. Higher geotechnical resistance factor of 0.75 is allowed in AASHTO 

LRFD if dynamic testing is conducted on 100 percent of production piles. Hence, with 

AASHTO LRFD, there is potential for savings with this method. 

It is clear from the above comparisons that AASHTO LRFD rewards increased level of quality 

control. 

 

FHWA modified Gates dynamic formula. The AASHTO LRFD resistance factor for this 

method (0.4) corresponds to the safety factor recommended in the ASD (3.5). 

Engineering News (EN) dynamic formula. Assume no geotechnical losses from scour or 

downdrag in the comparison discussed next. In the ASD, the form of this EN formula for the 

allowable capacity is 2/[Ed(s + 0.1)], and its equivalent factored resistance (multiplied by the 

average load factor of 1.4) is 2.8/[Ed(s + 0.1)]. In the LRFD, the nominal resistance form of this 

EN formula is 12/[Ed(s + 0.1)], which is six times the allowable form (a safety factor of 6 is 

assumed to get this form from the allowable form). The resistance factor for the LRFD nominal 

resistance form is 0.1 because this form has i) a small resistance mean bias, λ, suggesting this 

form significantly overestimates the resistance when compared to the resistance measured from 

load tests), and ii) a large COV of 0.92, suggesting large variability and low efficiency of this 

form. The form for the factored resistance of the AASHTO LRFD EN formula is 1.2/[Ed(s + 
 

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0.1)]. Based on the above, it can be concluded that the AASHTO LRFD form for the EN 

formula, when compared with the allowable ASD form used by many DOTs, will lead to a 

smaller factored resistance and more costly designs (longer and larger number of piles). 

AASHTO suggests that the Gates formula is preferred over the EN formula. 

Wave Equation Analysis (WEAP). The current AASHTO LRFD φ value of 0.5 for this 

method is developed based on calibration by fitting to ASD. This resistance factor can be used if 

local experience or site-specific test results are employed in the selection of the wave equation 

soil parameters, and if field verification of the hammer performance is performed. This 

resistance factor is larger than the reliability-calibrated φ for this method (around 0.4; see Table 

4.3) recommended in the NCHRP Report 507 (Paikowsky et al., 2004) and adopted by AASHTO 

until 2010. This difference is attributed to the use of default WEAP input parameters for hammer 

and soil in the reliability-based calibration, without consideration of local experience or sitespecific tests, as is required with the use of a larger resistance factor (φ = 0.5). 

6.2 COMPARISON OF AASHTO LRFD AND LOCAL ASD METHODS

It is common for DOTs to select the AASHTO LRFD design methods for service limit states (all 

foundation types), spread footings and micropiles (all limit states), and the geotechnical 

resistances for deep foundations except the axial compression resistance of a single driven pile or 

drilled shaft. Deviations from the AASHTO LRFD have been reported by DOTs for the axial 

compression resistance determination methods for a single driven pile and a drilled shaft. These 

deviations, most likely existed even when those agencies used the AASHTO Standard 

Specifications, and are often attributed to i) Local ASD methods are more economical than those 

provided by AASHTO LRFD, and ii) Local ASD practices are not covered by AASHTO LRFD

in terms of design methods, special soil/rock formations, and local testing methods to determine 

soil/rock design properties. 

DOTs need to compare AASHTO LRFD and local ASD methods before selecting LRFD design 

methods based on calibration by fitting. Two criteria can be considered in this comparison: 

• Based on reliability. Advantages of reliability calibration are discussed later. Some 

AASHTO design methods were calibrated based on statistical/reliability analysis. At a 

minimum, it is recommended that DOTs select the AASHTO LRFD design methods if 

they will lead to slightly higher foundation costs. 

• Based on economics. Select the geotechnical design method that would either add no 

costs (so that costs would be similar to those for existing ASD design), or would generate 

savings. This comparison is discussed next. 
 

6-4

AASHTO LRFD (2010a) provides several alternative geotechnical design methods. For 

comparison purposes, select a method from AASHTO based on its feasibility for use within the 

DOT’s current practices and economics (based on its efficiency, φ/λ, as discussed in Section 

4.2). In the comparison, the AASHTO LRFD loads (AASHTO, 2010a) should be used with both 

the AASHTO LRFD and local ASD geotechnical design methods. As discussed in Chapter 2, the 

methods used to compute loads have been improved in AASHTO LRFD, and these improved 

methods would be applicable to ASD design platform if updates to the Standard Specifications 

were to be continued. 

Two approaches for the economical comparison of AASHTO LRFD and local ASD methods are 

suggested. In the 1st approach, compare results of the ASD and LRFD platforms on actual 

projects. The differences in the foundation sizes between the two approaches can be due to

differences in the structural design (not addressed), or/and differences in the geotechnical design 

(our focus). With this approach, it is important to account for any hidden conservatism in the 

ASD methodology, where the loads could be overestimated and the resistances underestimated. 

In LRFD, the load and mean resistances computed per prescribed methods should be considered 

in the design and any uncertainties involving loads and resistances are accounted for by the load 

and resistance factors. In the 2nd approach, compare results of the ASD and LRFD geotechnical 

design methods. For the service limit design methods, compare the predictions for foundation 

displacements generated by the ASD method and the AASHTO LRFD method. For the strength 

limit design methods, compare the predictions for the foundation factored geotechnical resistance 

generated by the ASD method and the AASHTO LRFD method. The factored geotechnical 

resistance of an ASD method can be determined by multiplying its allowable capacity by the 

average load factor, γave, (around 1.4) as discussed in Chapter 4. The comparison suggested in 

the 2nd approach becomes more complicated if the input information (e.g., soil and rock design 

properties) needed for the local and AASHTO design methods are different.

Demonstration: 

In 2006, the Geotechnical Office of the Illinois Department of Transportation (IDOT) decided to 

abandon its modified form of the EN dynamic formula and adopt the FHWA modified Gates 

dynamic formula as presented in AASHTO LRFD. IDOT started an investigation to compare its

ASD modified EN formula with the AASHTO LRFD FHWA modified Gates formula to assess 

the impact of this transition on the department’s practices. The FHWA assisted with this 

investigation where it is assumed for simplification that there are no geotechnical losses from 

scour or downdrag, and the units for loads and resistances are in kips. 

IDOT used a modified form of the EN dynamic formula to predict the pile allowable capacity

during driving as 1.33Ed/(s + 0.1). The form for the factored resistance that this dynamic formula 

would generate is obtained by multiplying its ASD form by 1.4, leading to 1.86Ed/(s + 0.1). For 
 

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the FHWA modified Gates formula, the form for the nominal static geotechnical resistance, Rn, 

is 1.75(Ed)

0.5log10 (Nb) - 100, and the form for its factored resistance is φ [1.75(Ed)0.5 log10 (Nb) - 

100]. IDOT investigated the use of φ = 0.4 (as recommended by AASHTO) and higher φ values 

of 0.5 and 0.6. The objective was to select a resistance factor for use with the Gates formula that 

would not increase the pile quantities compared with the department’s previous practices. IDOT 

has long-term experience and success with driving piles using the modified EN dynamic 

formula. 

 

The results of the comparison between the ASD modified EN formula and the LRFD FHWA 

modified Gates formula for various hammer energies and penetration resistances of 10 bpi and 5 

bpi are listed in Table 6.1. Based on these results, IDOT decided to use a resistance factor of 0.5 

with the FHWA modified Gates, higher than the 0.4 recommended by AASHTO. According to 

the results in Table 6.1, this would lead in some cases to the use of larger hammers and longer 

piles (when the factored resistance from AASHTO LRFD is smaller than from the ASD modified 

EN formula), and in some cases to smaller hammers and shorter piles than have been required in 

the past with the EN dynamic formula. 

Table 6.1. Economical Comparison of ASD and LRFD Design Methods Conducted by 

FHWA for Illinois Department of Transportation 

Developed Factored Resistance (Kips) for Penetration Resistance of 10 bpi

Hammer 

Energy 

(Ib-ft)

ASD modified 

ENR formula

FHWA 

 φ= 0.4 

Modified Gates per AASHTO LRFD

φ= 0.5 φ= 0.6

14,000 131 126 157 188

20,000 187 158 197 237

25,000 233 181 227 272

30,000 280 202 253 304

35,000 327 222 277 333

40,000 373 240 300 360

50,000 467 273 341 410

60,000 560 303 379 454

6.2.1 Selection of Local ASD Design Method to Develop the LRFD Design Method

The DOT is responsible for this selection and should provide in its LRFD design manual

justification for the local calibration of resistances and selection of resistance factors different 

from those recommended by AASHTO (2010a). In this case, the AASHTO LRFD loads should 
 

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be used in the design, but, as discussed in Section 5 of this manual, some of the conditions 

considered in the ASD platform should be continued (e.g., ASD procedure for the estimation of 

soil and rock design properties). 

It is acceptable to combine locally calibrated design methods with AASHTO LRFD design 

methods as long as all applicable limit states are evaluated. This approach may require two 

different procedures to evaluate soil and rock properties, AASHTO LRFD procedures and local 

testing procedures. It is recommended that DOTs adopt the AASHTO LRFD testing procedures 

wherever possible. Through research studies, develop correlations between the results of local 

testing methods and AASHTO’s testing methods for the determination of soil and rock 

properties. Update the locally calibrated LRFD design methods based on this correlation.

6.3. ADVANTAGES OF LOCAL RELIABILITY CALIBRATION 

The development of LRFD geotechnical methods based on local reliability calibration of data 

collected at load test sites is recommended because it has many advantages discussed in this 

section.

6.3.1 Advantages of Reliability Calibration over Calibration by Fitting 

Some of the following advantages are demonstrated in Chapter 4. 

• With calibration by fitting, uncertainties in loads and resistances are treated equally, with one 

safety factor selected mainly based on past experience and engineering judgment. The true 

level of risk or probability of failure is unknown and can be small or large. Under reliabilitybased LRFD design, load and resistance factors are evaluated based on scientific methods 

and are related to each other or tied through a prescribed reliability index selected to have a 

level of reliability or safety for foundations consistent with those employed in the structural 

design of bridge superstructures 

• In the reliability calibration, design and construction uncertainties are considered and value 

of better geotechnical design or better construction of foundations can be captured in the 

calibrated resistance factors. 

• The reliability calibration provides parameters to evaluate the economics and improve the 

accuracy of the calibrated design methods. 

• With calibration by fitting to ASD methods, potential benefits are limited because practice 

won’t improve upon what was done previously and possible conservative design methods 

would continue. 

6.3.2 Advantages of Local over AASHTO Reliability Calibrations 
 

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The AASHTO LRFD reliability-based resistance factors were developed based on static load 

tests conducted at various locations in the United States and abroad where geological conditions 

and testing, design, and construction practices vary. This approach would lead to relatively large 

COV values and conservative φ values to be applicable and fit various conditions and practices 

(“one size fits all”). The local reliability calibration based on static load tests is recommended to 

account for a State’s specific geology and its testing, design, and construction practices. This 

would narrow the range of calibration conditions and lead to more economical and reliable 

designs. The reliability calibration of several local and AASHTO LRFD resistance determination 

methods would generate resistance factors, φ, resistance mean bias, λ, and efficiency, φ/λ; for all

these methods. These results would enable a DOT to compare all these methods and select or 

develop the most economical and accurate resistance determination method. Local reliability 

calibration is expected to lead to more economical and accurate LRFD geotechnical design

methods than AASHTO LRFD methods and identify and address any conservatism of local ASD 

methods. The results of load tests can also be employed to evaluate various displacement 

determination methods and select or develop the most appropriate method. 

As will be discussed in the next chapter, it is suggested that DOTs always consider load tests in 

the preliminary design for large bridge construction projects. The results of load tests generate 

more accurate design information and, in some cases, significant savings to the project. More 

important, as discussed above, load test data can be used in the local reliability calibration to 

develop more reliable, accurate and possibly economical LRFD geotechnical design methods. 

6.4. SUMMARY AND CONCLUSIONS 

For immediate implementation of LRFD, the DOTs need to consider two options for selection of 

LRFD geotechnical design methods: adopt AASHTO’s LRFD methods or develop LRFD 

methods by fitting to local ASD methods that have track records of long-term success. These two 

options are fast and easy to implement. To select the most appropriate option, compare these 

options based on economics and reliability as discussed in this chapter. For the long-term 

implementation of LRFD, it is suggested that the DOTs develop local LRFD geotechnical design 

methods through reliability calibration of information collected at load test sites. This option will 

require an initial investment of more time and resources but the return will be significant in the 

future as it will lead to development of more reliable, accurate, and possibly economical design 

methods than the two short-term options discussed above. 

When properly implemented, using LRFD methodology for geotechnical design will lead to 

savings or to equivalent foundation costs as compared with the ASD practices. The increases in 

foundation costs that have been reported by some agencies using LRFD design are primarily due 
 

6-8

to increases in design loads and improvements to the design methods and requirements that 

would also be applicable to the ASD platform if the AASHTO Standard Specifications continued

to be updated. 

The DOTs need to keep updating their LRFD design specifications based on future interims of 

AASHTO LRFD. Improvements to the AASHTO LRFD platform will continue in the future 

based on applied research studies, the results of additional load tests, and the experience of the 

highway community with implementing this methodology. The LRFD reliability calibration 

process is dynamic: it allows for continued refinement of the resistance factors with more data. 

In the future, it is expected that the geotechnical resistance factors will be increased and the 

focus will be shifted to the calibration of the service limit design methods.
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CHAPTER 7 

DEVELOPMENT OF LRFD DESIGN GUIDANCE FOR BRIDGE FOUNDATIONS

This chapter provides a roadmap to assist DOTs with the development of LRFD Design 

Guidance that consists of LRFD design specifications and delivery processes for bridge 

foundations. 

7.1 DEVELOPMENT OF LRFD DESIGN SPECIFICATIONS 

7.1.1 Materials Needed for Development 

Development of the LRFD Design Manual by the DOT should follow the sequence presented in 

the AASHTO LRFD Design Specifications. First, develop sections to cover the LRFD design 

specification on loads and on hydraulic and structural designs based on AASHTO LRFD 

Sections 1 to 8 (AASHTO, 2010a). We strongly recommend that DOTs accept AASHTO’s 

loads, load factors, and load combinations at various limit states (see AASHTO LRFD Section 

3). Then, develop a section on LRFD design specification for bridge foundations based on 

AASHTO LRFD Section 10. This section should refer to the earlier sections on loads and on 

hydraulic and structural design, as is the case with AASHTO LRFD Section 10, which refers to 

AASHTO LRFD Sections 1 to 8. 

To develop the LRFD design specifications for bridge foundations based on AASHTO LRFD 

Section 10 (2010a), the DOTs need to implement Steps 1 to 4 of the implementation plan 

presented in Chapter 2, and address these steps as discussed in Chapters 3 to 6. It is important to 

review the references presented in Chapters 3 to 6, which include the DOT current ASD design 

technical references for foundations, the most updated version of AASHTO LRFD Section 10 

and related sections referenced within Section 10, FHWA and NCHRP manuals, and LRFD 

design manuals from neighboring and lead states. The outcomes for implementation of Steps 3 

and 4 are the changes needed to transition from ASD to LRFD (as discussed in Chapters 3, 4, 5, 

and 6). Finalize the LRFD design methods to address all applicable structural and geotechnical 

limit states for all foundation types. Chapters 4, 5, and 6 provide suggestions on how to finalize 

the LRFD geotechnical design methods). It is important to identify and justify “Exceptions” from 

AASHTO LRFD Section 10 —deletions, additions, or modifications—and provide justifications 

for these exceptions: long-term successful past ASD experience/engineering judgment, research, 

and local issues not addressed in AASHTO LRFD. It is suggested that DOTs inform AASHTO 

of these exceptions. The AASHTO Subcommittee on Bridges and Structures has a process for 

resolving specification issues and identifying the need for special studies. Communication with 
7-2

SCOB should be through the State bridge engineer, who would contact AASHTO. For more 

information, visit http://bridges.transportation.org/?siteid=34&pageid=339. 

7.1.2 Roles and Responsibilities of Various Groups in the DOT

All members of a DOT’s LRFD Implementation Committee (structural, geotechnical, hydraulic, 

and construction) should be responsible and work together for the development of the LRFD 

Design Specifications for foundations. 

The geotechnical and hydraulic groups should also work together to develop the specifications 

related to scour. AASHTO LRFD Section 2 discusses the stability of foundations due to check 

flood (a flood not to exceed a 500-year event or an overtopping flood of lesser recurrence 

interval) and design flood (a flood of a 100-year event or an overtopping flood of lesser 

recurrence interval). Scour due to a check flood should be investigated under the Extreme Event 

limit state, and scour due to a design flood should be investigated under both the strength and 

service limit states. The FHWA Hydraulic Engineering Circular (HEC) 18 guidance is routinely 

used to evaluate scour depth on a national basis, but it is generally understood that in some cases 

the scour equations in this guidance yield overly conservative scour estimates because they were 

developed for granular soils. For additional guidance on scour in rock, refer to the FHWA 

Memorandum on scourability of rock formations

(www.fhwa.dot.gov/engineering/hydraulics/policymemo/rscour.cfm), and to Article 10.4.6.6 of 

AASHTO LRFD. Deviations from the HEC 18 guidance on the estimation of scour depth, 

reduction of scour depth through the use of countermeasures (e.g., riprap), and the assumption of 

no scour at the abutments should be justified appropriately as previously discussed. 

Additionally, geotechnical and hydraulic engineers should agree on a process to identify the 

highest groundwater level (GWL) expected over the life of the structure for consideration in the 

geotechnical design at various limit states. The difference between this GWL and the GWL at the 

time of design (or at the time of pile driving for the pile field design methods) leads to future 

axial and lateral geotechnical resistance losses (GL) that should be considered in the 

geotechnical design. A flood would elevate the GWL. It is suggested that the geotechnical 

engineer considers the: a) elevation for design flood provided by the hydraulic engineer in the 

determination of the highest ground water table (GWT) elevation needed in the analysis of 

service and strength limit states; and b) elevation for check flood provided by the hydraulic 

engineer in the determination of the highest GWT elevation needed in the analysis of the extreme 

event limit state with check flood. 

The structural group will be responsible to develop specifications on foundation loads and 

structural design based on AAHSTO LRFD Section 10
7-3

 

The structural and geotechnical groups need to address common foundation design issues, 

such as: 

• Earth loads, including vertical and lateral earth loads, downdrag loads, uplift loads due to soil 

swelling, and lateral spreading and downdrag loads due to liquefaction, and the soil and site 

information needed to estimate earthquake loads. 

• Evaluation of the foundation factored loads that maximize the force effect for the 

geotechnical resistances at the strength limit.

• Design methods to evaluate both the structural and geotechnical limit states under lateral 

loading (e.g., the p-y method, strain-wedge method, and the Broms method). 

• The service limit for all foundation types. Note that foundation displacements develop from 

both the structure and the geomaterials around the foundation. While the foundation factored 

loads and tolerable displacements are finalized by the structural engineer, the geotechnical 

engineer often computes the foundation displacements.

• For each foundation at each applicable limit, agree on the geotechnical and structural design 

requirements, and the structural and geotechnical resistances that need to be evaluated. 

7.1.3 Contents of the LRFD Design Specifications

The geotechnical group should take the lead for development of the entire LRFD design 

specifications for bridge foundation and be responsible for the development of the geotechnical 

design specifications emphasized herein. As a starting point, the DOT may consider conversion 

of its ASD design specifications to LRFD (easier) or consider appropriate LRFD Design 

Specifications developed by other DOTs. In this development, the DOT must address all issues 

discussed in AASHTO LRFD Section 10 and applicable to the DOT and cover the local issues 

not covered in AASHTO. Suggestions to develop various components of these specifications are 

presented next. 

Introduction (Based on AASHTO LRFD Section 10, Articles 10.5.1 to 10.5.4, Chapter 3 of this 

manual). In this section, address the limit states specific to foundations and their governing 

LRFD design equations. Present the governing ASD equations for foundations and discuss the 

similarities and differences between ASD and LRFD. 

Note: AASHTO LRFD Articles 10.5.1 to 10.5.4 describe the structural and geotechnical 

resistances that should be addressed under each limit state, which we recommend to cover for 

each foundation under LRFD Design Methods discussed later. 
7-4

Design Soil and Rock Properties (Based on AASHTO Article 10.4 and FHWA Manuals). In 

this section, cover the procedures to determine and select the soil/rock properties needed for the 

foundation geotechnical design methods at all applicable limit states (discussed later) and for 

construction of foundations. If these procedures are described in the DOT’s LRFD Geotechnical 

Manual, make a brief reference to this manual in the LRFD Design Section on Foundations. Note 

that the AASHTO LRFD specifications allow for consideration of local experience and specific 

geology in the selection of soil and rock properties. Start by presenting the “Informational 

Needs” (as in AASHTO LRFD Article 10.4.1). The DOTs need to adhere to the minimum 

guidelines for a subsurface exploration program as described in AASHTO LRFD Article 10.4.2 

to establish reliable soil/rock properties for the design and construction of foundations. 

Emphasize that the level of subsurface exploration in AASHTO LRFD Table 10.4.2-1 should be 

considered a minimum and should be increased based on past experience, the degree of site 

variability, and the importance of the structure. Next, define groups of geomaterials (in terms of

measurable material properties if possible) for which geotechnical design methods are furnished 

in the LRFD design section. In addition to sand, clay, and rock, AASHTO provides static 

analysis methods for the design of drilled shafts supported by cohesive and cohesionless 

intermediate geomaterials. For each group of geomaterials, present the methods to determine the 

soil/rock properties from laboratory and in situ tests (as in AASHTO LRFD Articles 10.4.3 to 

10.4.5). Cover also selection of design properties for each group of geomaterial (as in AASHTO 

LRFD Article 10.4.6): 

• Summarize procedures for test interpretation (e.g., the use of computer programs to estimate 

the p-y curve for sand from the friction angle)

• Assessment of site variability (see Chapter 5) 

• Selection of deformation and strength properties of soil and rock mass. Discuss the 

influence of special types of loads on these properties (wind load, seismic load, vessel 

impact load). Cover selection of properties for mixed local soils (see AASHTO LRFD) if 

they exist. 

• Assessment of liquefaction and scour potential (under design and check floods). 

• Soil properties needed in the estimation of earthquake loads. 

• Properties of soil and rock needed for construction of the foundation. 

Tolerable Movement Criteria (Based on AASHTO LRFD Article 5.5.5 and the 2006 FHWA 

Soils and Foundation Manual). The vertical and lateral displacements and rotation of the 

foundations lead to movements of the bridge superstructure at critical locations (e.g., the girder 

seat). For example, movement at the top of a pier would be amplified by differential settlement 

and rotation of the foundations. The permissible foundation movements should consider the type 

and configuration of the structure. AASHTO LRFD and FHWA manuals suggest the 

development of project-specific movement criteria for foundations as a function of tolerance of 
7-5

the bridge and the structures around the bridge (approach slab), rideability, economy (cost of 

future maintenance and repair), safety (clearance), and aesthetics. Therefore, each DOT should 

establish final guidelines for these movement criteria that are appropriate to its bridges, field 

conditions, and design requirements. 

AASHTO LRFD indicates that transient live loads may be omitted in the time-dependent 

settlement analysis of foundations bearing on cohesive soils. Note also that the goal of the 

settlement analysis should be to determine the settlement of the bridge that influences its 

performance, not the settlement of the foundation. Therefore, certain loads should be considered 

in the estimation of the bridge settlement, not all the loads that lead to settlement of the 

foundation (see the 2006 FHWA Soils and Foundations Workshop Reference Manual). 

Resistance Factors (Based on AASHTO LRFD Article 10.5.5 of AASHTO LRFD and Chapters 

4 and 5). Present the resistance factors for all LRFD foundation design methods, and the methods 

considered in the developments of the geotechnical resistance factors. Provide justifications for 

selection of resistance factors different from those recommended by AASHTO. Present the 

conditions employed in the development of the resistance factors (either the AASHTO LRFD φ

values or those developed locally). Emphasize that the designer need to adhere to these 

conditions in their design and justification is needed for any deviations from these conditions. 

LRFD Design Methods at All Applicable Limit States (Based on AASHTO LRFD Articles 

10.6 to 10.9, FHWA references discussed in this manual, Chapters 4 to 6 of this manual). For 

each foundation type (as in AASHTO LRFD Section 10), start by developing a General Section 

(e.g., AASHTO LRFD Article 10.7.1 for driven piles) that describes situations and conditions to 

be considered by the foundation designers in the selection of this foundation type. This will help 

the foundation designers select the most appropriate and economical foundation type. Also, in 

this section, cover design issues and requirements applicable to all limit states. Then, and for 

each foundation type, develop Sections for Service Limit Design, Strength Limit Design, and 

Extreme Event Limit Design (as in AASHTO LRFD Section 10). For each limit state design (as 

in AASHTO LRFD Section 10), discuss the design requirements, including the structural and 

geotechnical resistances that needs to be evaluated. Briefly describe the foundation loads and 

structural resistances, and refer the reader to other sections in the DOT’s LRFD Design Manual 

for more specific details. As in AASHTO LRFD, elaborate more on some special earth loads 

(downdrag, uplift). Finally, present methods for estimating and evaluating the foundation 

nominal geotechnical resistances, Rn (or displacements for the service limit state). It is 

acceptable to reference AASHTO LRFD for these methods, but these references should be very 

specific since AASHTO LRFD often provides several design methods. Briefly describe the 

methods referred to in AASHTO LRFD and add specific directions for their use in local design 
7-6

practices. At the end, cover special design topics, such as those covered in Articles 10.5 to 10.7.9 

of AASHTO LRFD for driven piles. 

Construction Design Issues. The design specifications on bridge foundations shall briefly 

describe the foundation construction issues needed for the design of foundations and refer the 

reader to the foundation construction specifications on bridge foundations for more specific 

details. These construction specifications need to be consistent with the design specifications for 

bridge foundations and based on: i) The DOT’s current construction specifications, which 

presents the DOT’s experiences, practices and judgment with respect to construction of 

foundations; ii) The most updated version of AASHTO LRFD Bridge Construction 

Specifications (2010b); iii) FHWA manuals for design, construction, and inspection of various 

foundation types; and iv) the construction methods and inspection/testing methods for QC/QA 

procedures considered in the calibration of resistance factors. 

In the LRFD design specifications for each foundation type, briefly describe the methods for 

construction and QC/QA assumed in the geotechnical design methods and the expected results 

from the QC/QA procedures (e.g., cleaned shaft bases for drilled shafts). This is needed to ensure 

that the assumed foundation geotechnical resistances in the design are met by the constructed 

foundations. For consideration of a construction or QC/QA method that is of lesser quality but 

still acceptable, the design section should describe the procedure for adjusting the foundation 

nominal geotechnical resistance value (not the resistance factor). Examples for drilled shafts:

reduce the unit side resistance when permanent casing will be used, and reduce (neglect) base 

resistance when a cleaned base of the shaft hole could not be ensured. In many cases, the 

foundation design needs to be finalized during construction and this should be described in the 

design guidance. This would be the case when the foundation geotechnical design requires or is 

based on dynamic analysis methods for driven piles, static load tests, field verification of rock 

quality and strength, and minimum length of embedment of drilled shafts in the bearing soil/rock 

layer. 

A project’s design and construction specifications should address the possibility of construction 

and design problems encountered during construction, and the corrective measures needed 

during construction to address them. Foundation construction involves an inherent risk of 

encountering conditions that differ from those anticipated due to the complexity and variability 

of natural earth and rock formations and materials. Differing site conditions” (DSC) is a common 

source of contractor claims on highway construction projects. Federal law requires that a DSC 

clause be incorporated into all federal-aid highway projects. Geotechnical Engineering Notebook 

Issuance GT-15 was prepared to provide guidance to design and construction engineers to 

address DSC (see http://www.fhwa.dot.gov/engineering/geotech/policymemo/gt-15.pdf).
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7.2 DEVELOPMENT OF LRFD DESIGN DELIVERY PROCESSES 

Good references for development of LRFD design delivery process for various foundation types 

are: 

• The NHI LRFD Manual on foundations (NHI, 2005) and FHWA LRFD Manuals (e.g., the 

2010 Drilled Shaft Manual). 

• Arizona DOT Design Memos (see http://www.ncsconsultants.com/). 

• Washington State DOT LRFD Geotechnical Manual (see 

www.wsdot.wa.gov/fasc/EngineeringPublications/Manuals/GDM/GDM.htm). 

7.2.1 Overview 

The DOT needs to develop LRFD-based design delivery process for each foundation type 

consistent with its LRFD design specifications. For every type of foundation, translate the LRFD 

geotechnical and structural design specifications at various limit states into design steps and for 

each step define who will do what, when, and how.” All members of a DOT’s LRFD 

Implementation Committee (structural, geotechnical, hydraulics, and construction) should be 

responsible for the development of the foundation LRFD design delivery processes and work 

together to address all applicable structural and geotechnical limit states. Define the role of each 

group of the design and construction teams (structural, geotechnical, hydraulics, and 

construction). Identify the information needed from each design step and how it will be used in 

the following steps. Require the involvement of the geotechnical engineer in all phases of project 

development, from planning (i.e., site selection) to construction. Define the coordination, 

interaction, and meetings among the design and construction personnel throughout the project 

development, especially to resolve design and construction issues. And finally, employ advanced 

computer programs available for the analysis and design of foundations at all limit states. These 

programs can analyze several load combinations quickly and more accurately than conventional 

methods. 

Specific recommendations for the roles and responsibilities of various groups in the DOT are 

presented next.

7.2.2 Role of the Construction Group

A foundation design should not be considered complete until foundation construction has been 

successfully completed. The role of the construction engineer is essential in finalizing design 

during construction (as discussed in the previous section) and ensuring that all field design 

assumptions for construction methods and quality of construction are met during construction.

The construction engineer needs to report construction problems and work with the design team 
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to develop corrective measures. Finally, the construction engineer needs to be involved in the 

Implementation of LRFD and document any problems encountered (as discussed previously). 

7.2.3 Roles of the Geotechnical and Hydraulic Groups

Many DOTs determine the scour depth of deep foundations at both the design flood and the 

check flood in the early stages of the project and before the subsurface exploration program is 

performed. We suggest changing that and initially perform the subsurface exploration program. 

Then, the project geotechnical and hydraulic engineers need to work together to finalize the 

scour depth at all limit states, and the GWT at all limit states with consideration of the elevation 

of the design and check floods. 

 

7.2.4 Roles of Structural and Geotechnical Groups

7.2.4.1 Preliminary Design Phase 

Some DOTs perform the subsurface exploration program in the early stages of project 

development, before the exact location of the bridge is finalized, and some DOTs perform this 

investigation after the bridge’s exact location is finalized but also after the types of foundations 

for the bridge are selected. There are problems with both alternatives. For major structures, the 

new FHWA Manual on Drilled Shafts (Brown et. al., 2010) 2010) recommends that DOTs divide 

the field exploration program into two phases: a preliminary phase (based on a few borings and 

geophysical tests) that provides sufficient information for planning and optimizing the 

subsequent, more comprehensive investigation phase. A staged investigation program provides 

sufficient information for preliminary design (including preliminary selection of foundation 

types) and defers much of the cost of the site investigation until the structure’s exact location and 

layout is finalized. The preliminary phase can be performed during the Environmental Impact 

Statement phase of the project. 

After the preliminary subsurface exploration program is completed, it is suggested that the entire 

design team (structural, geotechnical, construction, and hydraulic engineers) meet and agree on 

the types of foundations and design methods that need to be evaluated in the preliminary and 

final design phases. Always consider spread footings as a viable foundation system, and a static 

load test as an alternative design method. The subsequent design steps should determine if spread 

footings and a static load test are appropriate or not. 

For all the alternative foundation types, the structural engineer should provide the following 

preliminary design information to the geotechnical engineer: 
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• Preliminary foundation factored loads (or a range of factored loads) at all applicable limit 

states. For deep foundations, this would include the range of factored axial and lateral loads 

that will be supported by a group of deep foundations and by a single deep foundation at each 

applicable limit state. 

• Preliminary foundation sizes and any foundation structural information that the geotechnical 

engineer may need in the preliminary design. This would include the length of a shallow 

foundation, sizes of driven piles, and diameters of drilled shafts. The sizes of the deep 

foundations (e.g., shaft diameter, or HP 12 x 53 for a driven pile) explored in the design 

phase are often selected based on their structural capacities and expected loads. 

For all the alternative foundation types, and based on the results of the subsurface investigation 

and analysis, the geotechnical engineer needs to prepare the following information: 

• The design soil/rock properties that are needed in the geotechnical design of foundations at 

all applicable limit states (including information on scour and liquefaction). 

• Information needed by the structural engineer for estimation of the earth loads and 

earthquake loads.

• Information on subsurface conditions necessary for the construction of foundations. 

• The foundation geotechnical design methods and their resistance factors (with the static load 

test as one of the alternative design methods). 

In some DOTs, the geotechnical engineer has no role in the foundation design. However, it is 

recommended that the geotechnical engineer also perform a preliminary design to assist the 

structural engineer with selection of trial widths for shallow foundations and trial depth and 

number of a group of deep foundations as suggested next. 

 

For shallow foundations on soils at a given depth, develop a “Bearing Resistance Chart” 

relating foundation effective width to the factored axial geotechnical resistance available to support 

the applied axial factored loads at both the strength and service limit states. This chart may be 

developed at different elevations to allow selection of the most cost-effective footing 

depth/elevation.

 

For a single pile or shaft at the strength limit, develop a plot relating foundation penetration 

depth, L, to the factored axial or lateral load that can be supported at that depth, Qf. This plot 

should cover the expected range of foundation factored loads acting on a single pile/shaft. For a 

single driven pile, establish the maximum axial factored load a single pile can support, Qfmax, and 

the maximum penetration depth to which the pile can be safely driven, Lmax. For a single deep 
7-10

foundation element under lateral loading, establish the maximum lateral factored load that the 

shaft or pile can support and its corresponding depth. 

As an example, the predicted top axial factored load, Qf, vs. pile length from various design 

methods up to Qfmax and Lmax are shown in Figure 7.1. This figure demonstrates the benefits of 

static load testing (e.g., shorter piles or a smaller number of piles). Other benefits of the static 

load tests were discussed previously and include providing data for the calibration of resistance 

and displacement determination methods. The geotechnical engineer needs to discuss these 

benefits with the structural and construction engineers and finalize the decision of whether to 

include a load test for the project. If it is feasible, conduct the static load test in the design phase. 

 

0

10

20

30

40

50

60

70

80

0 50 100 150 200 250 300 350 400

Pile Top Factored Axial Load, Qf, Kips

Depth (ft)

Beta Method

Gates, EOD

Dynamic load Tes, BOR

Static Load Test

Structural Strength Lmit

`

Figure 7.1. Example of the Information the Geotechnical Engineer Needs to Develop in the 

Preliminary Design

Note: The initial penetration length of a group of deep foundations needs to be estimated based 

on the limit state that most likely would control or govern the penetration length of a single 

pile/shaft, which is in most cases is the strength limit state, but in some cases could be the service 

limit state or one of the extreme event limit states. 
7-11

7.2.4.2 Final Design Phase 

The information provided in Phase I will help the structural engineer to select the initial trial 

width for shallow foundations, size and type of driven piles, diameter of shafts, and depth and 

number of a group of driven piles and drilled shafts. This and the cost information for the 

alternative foundation types and for the load test will help in the selection of the most costeffective and appropriate foundation type and deciding if conducting a load test is appropriate. In 

the final LRFD design, the structural engineer needs to ensure that all applicable foundation 

structural and geotechnical limit states are met. The geotechnical, hydraulic, and construction 

engineers should have a formal role in the review of the final design. 
8-1

REFERENCES

AASHTO (2009a). “AASHTO Guide Specifications for LRFD Seismic Bridge Design.” 1st

Edition, American Association of State Highway and Transportation Officials,

Washington, D.C.

AASHTO (2009b). “Guide Specifications and Commentary for Vessel Collision Design of 

Highway Bridges.” 2nd Edition, American Association of State Highway and 

Transportation Officials, Washington, D.C.

AASHTO (2010a). “AASHTO LRFD Bridge Design Specifications, 5th Edition with 2010 

Interim Revisions.” American Association of State Highway and Transportation 

Officials, Washington, D.C.

AASHTO (2010b). “AASHTO LRFD Bridge Construction Specifications, Third Edition with 

2010 Interim Revisions.” American Association of State Highway and Transportation 

Officials, Washington, D.C.

AASHTO (2004). “AASHTO LRFD Bridge Design Specifications, 3rd Edition with 2005 and 

2006 Interim Revisions.” American Association of State Highway and Transportation 

Officials, Washington, D.C.

AASHTO (2002). “Standard Specifications for Highway Bridges, 17th Edition.” American 

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

Allen, T. (2005). “Development of Geotechnical Resistance Factors and Downdrag Load Factors 

for LRFD Foundation Strength Limit State Design.” Federal Highway Administration, 

Washington, D.C., Publication No. FHWA-NHI-05-052. 

http://www.wsdot.wa.gov/biz/mats/Geotech/ResistanceFactorsforFoundationsFinal2_05.

pdf

 

Allen, T.M., Nowak, A.S., and Bathurst, R.J. (2005). “Calibration to Determine Load and 

Resistance Factors for Geotechnical and Structural Design.” Transportation Research 

Circular E-C079, Transportation Research Board, Washington, D.C., 83 p. 

http://onlinepubs.trb.org/onlinepubs/circulars/ec079.pdf

Barker, R.M., Duncan, J.M., Rojiani, K.B., Ooi, P.S.K., Tan, C.K., and Kim, S.G. (1991). 

“Manuals for the Design of Bridge Foundations.” NCHRP Report 343, Transportation 

Research Board, National Research Council, Washington, D.C., 308 p. 

Brown, D. (2010). “Drilled Shafts: Construction Procedures and Design Methods,” Federal 

Highway Administration, Washington, D.C, Publication No. FHWA-NHI-10-016. 
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Duncan, J.M. (2000). “Factors of Safety and Reliability in Geotechnical Engineering.” Journal of 

Geotechnical and Geoenvironmental Engineering, Vol. 126, No. 4, pp. 307-316.

Hannigan, P.J., Goble, G.G., Likens, G.E., and Rausche, F. (2006). “Design and Construction of 

Driven Pile Foundations,” National Highway Institute, Federal Highway Administration, 

Washington, D.C., Publication No. FHWA-NHI-05-042, 968 p.

Long, J.H., Hendrix, J., and Baratta, A. (2009). “Evaluation/Modification of IDOT Foundation 

Piling Design and Construction Policy.” Illinois Department of Transportation, Report 

No. ICT-09-037, UILU-ENG-2009-2008. 

http://ict.illinois.edu/Publications/report%20files/FHWA-ICT-09-037.pdf

NHI (2005). “LRFD for Highway Bridge Substructures and Earth Retaining Structures.” Federal 

Highway Administration, Washington, D.C., Publication No. FHWA-NHI-05-094. 

O'Neill, M.W., and Reese, L.C. (1999). “Drilled Shafts: Construction Procedures and Design 

Methods.” Federal Highway Administration, Washington, D.C., Publication No. FHWAIF-99-025, 758 p.

Paikowsky, S.G., Birgisson, B., McVay, M., Nguyen, T., Kuo, C., Baecher, G., Ayyub, B., 

Stenersen, K., O’Malley, K., Chernauskas, L., and O’Neill, M. (2004). “Load and 

Resistance Factor Design (LRFD) for Deep Foundations.” NCHRP Report 507, 

Transportation Research Board, Washington, D.C., 126 pp.

http://www.trb.org/news/blurb_detail.asp?id=4074

Reese, L.C., and O’Neill, M.W. (1988). “Drilled Shafts: Construction Procedure and Design 

Methods,” FHWA Publication No. FHWA-HI-88-042, 564 pp.

Sabatini, P.J., Tanyu, B., Armour, T., Groneck, P., and Keeley, J. (2005). “Micropile Design 

and Construction: Reference Manual.” National Highway Institute, Federal Highway 

Administration, Washington, D.C., Publication No. FHWA NHI-05-039

Samtani, N.C., and Nowatzki, E.A. (2006). “Soils and Foundations: Reference Manual.”

National Highway Institute, Federal Highway Administration, Washington, D.C., 

Publication No. FHWA-NHI-06-088, 462 p.