Limit States Design of Deep Foundations - Purdue University

244 Pages·2012·1.05 MB·English

Final Report

FHWA/IN/JTRP-2004/21

Limits States Design of Deep Foundations

by

Kevin Foye
Graduate Research Assistant

Grace Abou Jaoude
Graduate Research Assistant

Rodrigo Salgado
Professor

School of Civil Engineering
Purdue University

Joint Transportation Research Program
Project No. C-36-36HH
File No. 6-14-34
SPR-2406

Conducted in Cooperation with the
Indiana Department of Transportation
and the U.S. Department of Transportation
Federal Highway Administration

The contents of this report reflect the views of the authors who are responsible for the
facts and accuracy of the data presented herein. The contents do not necessarily reflect
the official views or policies of the Federal Highway Administration or the Indiana
Department of Transportation. This report does not constitute a standard, specification,
or regulation.

Purdue University
West Lafayette, Indiana
December 2004
S
TECHNICAL ummary

Technology Transfer and Project Implementation Information

INDOT Research

TRB Subject Code: 62-1 Foundation Soils December 2004
Publication No.: FHWA/IN/JTRP-2004/21, SPR-2406 Final Report

Limit States Design of Deep Foundation
Introduction
Foundation design consists of selecting and ranges for the values of the load factors. These
proportioning foundations in such a way that limit values are compared with those proposed in the
states are prevented. Limit states are of two types: Codes.
Ultimate Limit States (ULS) and Serviceability For LRFD to gain acceptance in
Limit States (SLS). ULSs are associated with geotechnical engineering, a framework for the
danger, involving such outcomes as structural objective assessment of resistance factors is
collapse. SLSs are associated with impaired needed. Such a framework, based on reliability
functionality, and in foundation design are often analysis is proposed in this study. Probability
caused by excessive settlement. Reliability-based Density Functions (PDFs), representing design
design (RBD) is a design philosophy that aims at variable uncertainties, are required for analysis. A
keeping the probability of reaching limit states systematic approach to the selection of PDFs is
lower than some limiting value. Thus, a direct presented. Such a procedure is a critical
assessment of risk is possible with RBD. This prerequisite to a rational probabilistic analysis in
evaluation is not possible with traditional working the development of LRFD methods in geotechnical
stress design. The use of RBD directly in projects engineering. Additionally, in order for LRFD to
is not straightforward and is cumbersome to fulfill its promise for designs with more consistent
designers, except in large-budget projects. Load reliability, the methods used to execute a design
and Resistance Factor Design (LRFD) shares most must be consistent with the methods assumed in the
of the benefits of RBD while being much simpler to development of the LRFD factors. In this study, a
apply. LRFD has traditionally been used for ULS methodology for the estimation of soil parameters
checks, but SLS's have been brought into the LRFD for use in design equations is proposed that should
framework recently (AASHTO 1998). allow for more statistical consistency in design
Load and Resistance Factor Design inputs than is possible in traditional methods.
(LRFD) is a design method in which design loads The primary objective of this study is to
are increased and design resistances are reduced propose a Limit States Design method for shallow
through multiplication by factors that are greater and deep foundations that is based on a rational,
than one and less than one, respectively. In this probability-based investigation of design methods.
method, foundations are proportioned so that the In particular, Load and Resistance Factor Design is
factored loads are not greater than the factored used to facilitate the Limit States Design
resistances. methodology. Specifically, the objectives of the
In order for foundation design to be study are to 1) provide guidance on the choice of
consistent with current structural design practice, values for load factors; 2) develop
the use of the same loads, load factors and load recommendations on how to determine
combinations would be required. In this study, we characteristic soil resistances under various design
review the load factors presented in various LRFD settings; 3) develop resistance factors compatible
Codes from the US, Canada and Europe. A simple with the load factors and the method of
first-order second moment (FOSM) reliability determining characteristic resistance.
analysis is presented to determine appropriate
62-1 12/04 JTRP-2004/21 INDOT Division of Research West Lafayette, IN 47906
Findings
This research was able to develop a to show that the CAM procedure tends to stabilize
systematic framework for the assessment of the reliability of design checks completed using
resistance factors for geotechnical LRFD. Several particular RF values even when the uncertainty of
steps comprise this framework: a) the design the soil at a site is different from that assumed in
equations are identified; b) all variables showing in the analysis.
the design equation are decomposed to identify all The primary objective of this study is to
component quantities; c) probabilistic models for propose a LRFD method for shallow and deep
the uncertain quantities are developed using all foundations that is based on a rational,
available data; d) reliability analysis is used to probability-based investigation of design
determine the limit state values corresponding to a methods. Since resistance factor values are
set of nominal design values at a specified dependent upon the values of load factors used, a
reliability index; e) resistance factors are method to adjust the resistance factors to account
determined algebraically from the corresponding for code-specified load factors is presented.
nominal and limit state values. Resistance factors for ultimate bearing capacity
In order for LRFD to fulfill its promise are then computed using reliability analysis for
for designs with more consistent reliability, the shallow and deep foundations both in sand and in
methods used to execute a design must be clay, for use with both ASCE-7 (1996) and
consistent with the methods assumed in the AASHTO (1998) load factors. The various
development of the LRFD factors. In this study, a considered methods obtain their input parameters
methodology for the estimation of soil parameters from the CPT, the SPT, or laboratory testing.
for use in design equations is proposed to allow Finally, designers may wish to use design
for more statistical consistency in design inputs methods that are not considered in this study. As
than is possible in traditional methods. This such, the designer needs the capability to select
methodology, called the conservatively assessed resistance factors that reflect the uncertainty of the
mean (CAM) method, is defined so that 80% of design method chosen. A methodology is proposed
the measured values of a specific property are in this study to accomplish this task, in a way that
likely to fall above the CAM value. We were able is consistent with the framework.
Implementation
The resistance factor results of this study could the CAM method would not require additional
be used to develop future LRFD codes for efforts than those already common in soil
geotechnical design. As a first step towards investigations. It is easily applied and is
implementation, Purdue University and INDOT demonstrated in the design examples in this
are organizing a workshop to educate designers study report.
on the principles and application of the In summary, the key areas of
resistance factors and their associated design implementation are
methods. This workshop will form the basis for • to hold a workshop on LRFD to
INDOT designers to explore the use of these introduce geotechnical engineers to the
methods in support of code development. It is application of LRFD to foundations
important to note that in order for LRFD to • the use of the Conservatively Assessed
fulfill its promise for designs with more Mean procedure to improve the
consistent reliability, the soil investigation repeatability of soil property
forming the basis of a geotechnical design must assessments
be consistent with the interpretation methods the shift to the use of factored loads and
assumed in the development of the LRFD resistance factors in the assessment of design
factors. Thus, the concept of the CAM method resistances for foundations.
must be implemented as the first component of
the LRFD methodology. The implementation of
62-1 12/04 JTRP-2004/21 INDOT Division of Research West Lafayette, IN 47906
Contacts
For more information:
Indiana Department of Transportation
Prof. Rodrigo Salgado
Division of Research
Principal Investigator
1205 Montgomery Street
School of Civil Engineering
P.O. Box 2279
Purdue University
West Lafayette, IN 47906
West Lafayette IN 47907
Phone: [phone]
Phone: [phone]
Fax: [phone]
Fax: [phone]

E-mail: [email protected]
Purdue University
Joint Transportation Research Program
School of Civil Engineering
West Lafayette, IN 47907-1284
Phone: [phone]
Fax: [phone]
62-1 12/04 JTRP-2004/21 INDOT Division of Research West Lafayette, IN 47906
TECHNICAL REPORT STANDARD TITLE PAGE
1 . Report No. 2 . Government Accession No. 3 . Recipient's Catalog No.
FHWA/IN/JTRP-2004/21

4. Title and Subtitle 5. Report Date
Limit States Design of Deep Foundations
December 2004
6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.
Kevin Foye, Grace Abou Jaoude, and Rodrigo Salgado
FHWA/IN/JTRP-2004/21

9. Performing Organization Name and Address 10. Work Unit No.
Joint Transportation Research Program
550 Stadium Mall Drive
Purdue University
West Lafayette, IN 47907-2051

11. Contract or Grant No.
SPR-2406

12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered
Indiana Department of Transportation
State Office Building Final Report
100 North Senate Avenue
Indianapolis, IN 46204
14. Sponsoring Agency Code

15. Supplementary Notes

Prepared in cooperation with the Indiana Department of Transportation and Federal Highway Administration.

16. Abstract
Load and Resistance Factor Design (LRFD) shows promise as a viable alternative to the present working stress design (WSD) approach to
foundation design. The key improvements of LRFD over the traditional Working Stress Design (WSD) are the ability to provide a more
consistent level of reliability and the possibility of accounting for load and resistance uncertainties separately. In order for foundation
design to be consistent with current structural design practice, the use of the same loads, load factors and load combinations would be
required. In this study, we review the load factors presented in various LRFD Codes from the US, Canada and Europe. A simple first-
order second moment (FOSM) reliability analysis is presented to determine appropriate ranges for the values of the load factors. These
values are compared with those proposed in the Codes. The comparisons between the analysis and the Codes show that the values of load
factors given in the Codes generally fall within ranges consistent with the results of the FOSM analysis.
For LRFD to gain acceptance in geotechnical engineering, a framework for the objective assessment of resistance factors is
needed. Such a framework, based on reliability analysis is proposed in this study. Probability Density Functions (PDFs), representing
design variable uncertainties, are required for analysis. A systematic approach to the selection of PDFs is presented. Such a procedure is
a critical prerequisite to a rational probabilistic analysis in the development of LRFD methods in geotechnical engineering. Additionally,
in order for LRFD to fulfill its promise for designs with more consistent reliability, the methods used to execute a design must be
consistent with the methods assumed in the development of the LRFD factors. In this study, a methodology for the estimation of soil
parameters for use in design equations is proposed that should allow for more statistical consistency in design inputs than is possible in
traditional methods.
Resistance factor values are dependent upon the values of load factors used. Thus, a method to adjust the resistance factors to
account for code-specified load factors is also presented. Resistance factors for ultimate bearing capacity are computed using reliability
analysis for shallow and deep foundations both in sand and in clay, for use with both ASCE-7 (1996) and AASHTO (1998) load factors.
The various considered methods obtain their input parameters from the CPT, the SPT, or laboratory testing.
Designers may wish to use design methods that are not considered in this study. As such, the designer needs the capability to
select resistance factors that reflect the uncertainty of the design method chosen. A methodology is proposed in this study to accomplish
this task, in a way that is consistent with the framework.

17. Key Words 18. Distribution Statement
Load and Resistance Factor Design (LRFD); Geotechnical
Engineering; Foundation Design; in-situ testing; Reliability- No restrictions. This document is available to the public through the
Based Design (RBD); Probability. National Technical Information Service, Springfield, VA 22161

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 234

Form DOT F 1700.7 (8-69)
i
ACKNOWLEDGEMENTS

Bryan Scott and Bumjoo Kim, former Purdue Graduate students, were responsible
for writing the material on load factors. Bryan Scott also made significant contributions
to the assessment of shallow foundation uncertainty and resistance factors. Their work is
greatly appreciated as it is a significant contribution to this final report.
i
ii
TABLE OF CONTENTS

Page
LIST OF TABLES ........................................................................................................... iv
LIST OF FIGURES ....................................................................................................... viii
CHAPTER 1. Introduction ................................................................................................. 1
1.1 Background ............................................................................................................ 1
1.2 Study Objectives .................................................................................................... 3
1.3 Report Organization ............................................................................................... 4
CHAPTER 2. Assessment of Current Load Factors .......................................................... 5
2.1 Introduction ............................................................................................................ 5
2.2 Load And Resistance Factor Design (LRFD) and Limit States ............................. 6
2.3 Load Factors Proposed By LRFD Codes in the US, Canada, And Europe ........... 7
2.4 Simple Reliability Analysis ................................................................................. 13
2.5 Selection of Parameters Used in the Analysis ..................................................... 20
2.6 Comparison Between Results and Load Factors in the Codes ............................ 22
2.7 Future Development of Geotechnical LRFD Design ........................................... 24
2.8 Summary and Conclusions .................................................................................. 25
2.9 Notation ................................................................................................................ 27
CHAPTER 3. Methodology to Determine Resistance Factors ........................................ 39
3.1 A Rational Framework for Evaluating Resistance Factors .................................. 40
3.2 Tools to Assess Uncertainty ................................................................................. 41
3.3 Tools to Assess Resistance Factors ...................................................................... 49
iii
3.4 Summary .............................................................................................................. 55
CHAPTER 4. Assessment of Variable Uncertainties for Shallow Foundations ............. 57
4.1 Assessment of Uncertainty in Bearing Capacity of Footings on Sand ................ 57
4.2 Assessment of Uncertainty in Bearing Capacity of Footings on Clay ................. 70
4.3 Summary .............................................................................................................. 74
CHAPTER 5. Assessment of Resistance Factors for Shallow Foundations .................... 75
5.1 Calculation of Resistance Factors ........................................................................ 75
5.2 Characteristic Resistance ..................................................................................... 85
CHAPTER 6. Design Examples for Shallow Foundations .............................................. 91
CHAPTER 7. Assessment of Design Methods for Deep Foundations .......................... 103
7.1 LRFD Design of Piles ........................................................................................ 103
7.2 Design of Piles in Sand ...................................................................................... 106
7.3 Design of Piles in Clay ...................................................................................... 114
CHAPTER 8. Resistance Factors for Deep Foundations on Sand ................................. 121
8.1 Assessment of Variable Uncertainties for Deep Foundations on Sand.............. 121
8.2 Assessment of Resistance Factors ..................................................................... 161
CHAPTER 9. Resistance Factors for Deep Foundations on Clay ................................. 175
9.1 Assessment of Variable Uncertainties for Deep Foundations on Clay .............. 175
9.2 Assessment of Resistance Factors ..................................................................... 185
CHAPTER 10. Design Examples for Deep Foundations .............................................. 195
CHAPTER 11. Summary and Conclusions ................................................................... 209
LIST OF REFERENCES ............................................................................................... 217
viii
LIST OF TABLES

Table Page

Table 2.3.1 Load Factors..................................................................................................29
Table 2.3.2 Load Factors and Gravity Load Combinations..............................................30
Table 2.3.3 Load Factors for SLS.....................................................................................31
Table 2.5.1 Ratio of Mean to Nominal Load and Coefficient of Variation......................32
Table 2.5.2 Values of Ratio of Mean to Nominal Load and Coefficient of Variation used
for the analysis...................................................................................................................33
Table 2.6.1 Comparison of the Values of Load Factors...................................................34
Table 3.2.1 Values of Number of Standard Deviations (Nσ) (after Tippett 1925)..........44
Table 4.1.1 COVs, Bias Factors and Distribution Types for use in a Probabilistic
Analysis of Bearing Capacity on Sand and Clay...............................................................60
Table 4.1.2 COVs, bias factors and distribution types for bearing capacity factors for use
in reliability analysis of footings on sand using the CPT..................................................69
Table 4.1.3 COVs, bias factors and distribution types for bearing capacity factors for use
in reliability analysis of footings on sand using the SPT...................................................69
Table 4.2.1 Uniform Distribution Bounds on N s d for varying embedment ratios for use
c c c
in a Probabilistic Analysis of Bearing Capacity on Clay (Salgado et al. 2004)................74
Table 5.1.1 Recommended RFs for Bearing Capacity on Sand and Clay........................81
Table 6.1 CPT q log statistics..........................................................................................94
c
Table 6.2 Bearing Capacity Factors in Sand Example.....................................................95
Table 6.3 Results of CPT Design Example on Sand and Clay.........................................98
Table 7.2.1 Summary of Selected Design Methods for Reliability Analysis in Sands...113
ix
Table 7.3.1 Values of α and K for use with Aoki and Velloso (1975) direct design

method..............................................................................................................................117
Table 7.3.2 Values of F1 and F2 for use with Aoki and de Alencar Velloso (1975) direct
design method..................................................................................................................117
Table 7.3.3 Summary of Selected Design Methods for Reliability Analysis in Clays...119
Table 8.1.1 Summary Statistics for Composite Uncertainty of Modulus G ..................157
Table 8.2.1 Summary Table for the Design of Deep Foundations in Sand. .................162
Table 8.2.2 Results of Resistance Factor Evaluation for Property-Based Shaft Capacity
of Closed Ended Piles in Sand for ASCE-7 Load Factors...............................................165
Table 8.2.3 Results of Resistance Factor Evaluation for Property-Based Base Capacity of
Closed Ended Piles in Sand for ASCE-7 Load Factors...................................................167
Table 8.2.4 Results of Resistance Factor Evaluation for Direct Base Capacity of Closed
Ended Piles in Sand for ASCE-7 Load Factors...............................................................169
Table 8.2.5 Results of Resistance Factor Evaluation for Property-Based Shaft Capacity
of Open Ended Piles in Sand for ASCE-7 Load Factors.................................................170
Table 8.2.6 Results of Resistance Factor Evaluation for Property-Based Base Capacity of
Open Ended Piles in Sand for ASCE-7 Load Factors......................................................171
Table 8.2.7 Results of Resistance Factor Evaluation for Direct Shaft Capacity of Open-
Ended Pipe Piles in Sand for ASCE-7 Load Factors.......................................................171
Table 8.2.8 Results of Resistance Factor Evaluation for Direct Base Capacity of Open
Ended Piles in Sand for ASCE-7 Load Factors...............................................................172
Table 8.2.9 Results of Resistance Factor Evaluation for Property-Based Base Capacity of
Drilled Shafts in Sand for ASCE-7 Load Factors............................................................173