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NATIONAL COOPERATIVE HIGHWAY NCHRP REPORT 701 RESEARCH PROGRAM Project 24-21 Systematic, well-designed research provides the most effective ISSN 0077-5614 approach to the solution of many problems facing highway ISBN 978-0-309-21351-6 administrators and engineers. Often, highway problems are of local Library of Congress Control Number 2011937067 interest and can best be studied by highway departments individually or in cooperation with their state universities and others. However, the © 2011 National Academy of Sciences. All rights reserved. accelerating growth of highway transportation develops increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of COPYRIGHT INFORMATION cooperative research. Authors herein are responsible for the authenticity of their materials and for obtaining In recognition of these needs, the highway administrators of the written permissions from publishers or persons who own the copyright to any previously American Association of State Highway and Transportation Officials published or copyrighted material used herein. initiated in 1962 an objective national highway research program Cooperative Research Programs (CRP) grants permission to reproduce material in this employing modern scientific techniques. This program is supported on publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, a continuing basis by funds from participating member states of the FMCSA, FTA, or Transit Development Corporation endorsement of a particular product, Association and it receives the full cooperation and support of the method, or practice. It is expected that those reproducing the material in this document for Federal Highway Administration, United States Department of educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission Transportation. from CRP. The Transportation Research Board of the National Academies was requested by the Association to administer the research program because of the Board’s recognized objectivity and understanding of NOTICE modern research practices. The Board is uniquely suited for this The project that is the subject of this report was a part of the National Cooperative Highway purpose as it maintains an extensive committee structure from which Research Program, conducted by the Transportation Research Board with the approval of authorities on any highway transportation subject may be drawn; it the Governing Board of the National Research Council. possesses avenues of communications and cooperation with federal, The members of the technical panel selected to monitor this project and to review this state and local governmental agencies, universities, and industry; its report were chosen for their special competencies and with regard for appropriate balance. relationship to the National Research Council is an insurance of The report was reviewed by the technical panel and accepted for publication according to procedures established and overseen by the Transportation Research Board and approved objectivity; it maintains a full-time research correlation staff of by the Governing Board of the National Research Council. specialists in highway transportation matters to bring the findings of The opinions and conclusions expressed or implied in this report are those of the research directly to those who are in a position to use them. researchers who performed the research and are not necessarily those of the Transportation The program is developed on the basis of research needs identified Research Board, the National Research Council, or the program sponsors. by chief administrators of the highway and transportation departments The Transportation Research Board of the National Academies, the National Research and by committees of AASHTO. Each year, specific areas of research Council, and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturers’ names appear herein solely needs to be included in the program are proposed to the National because they are considered essential to the object of the report. Research Council and the Board by the American Association of State Highway and Transportation Officials. Research projects to fulfill these needs are defined by the Board, and qualified research agencies are selected from those that have submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Research Council and the Transportation Research Board. The needs for highway research are many, and the National Cooperative Highway Research Program can make significant contributions to the solution of highway transportation problems of mutual concern to many responsible groups. The program, however, is intended to complement rather than to substitute for or duplicate other highway research programs. Published reports of the NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM are available from: Transportation Research Board Business Office 500 Fifth Street, NW Washington, DC 20001 and can be ordered through the Internet at: http://www.national-academies.org/trb/bookstore Printed in the United States of America

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The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. On the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Charles M. Vest is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, on its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. Charles M. Vest are chair and vice chair, respectively, of the National Research Council. The Transportation Research Board is one of six major divisions of the National Research Council. The mission of the Transporta- tion Research Board is to provide leadership in transportation innovation and progress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multimodal. The Board’s varied activities annually engage about 7,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individu- als interested in the development of transportation. www.TRB.org www.national-academies.org

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COOPERATIVE RESEARCH PROGRAMS CRP STAFF FOR NCHRP REPORT 701 Christopher W. Jenks, Director, Cooperative Research Programs Crawford F. Jencks, Deputy Director, Cooperative Research Programs David A. Reynaud, Senior Program Officer Megan A. Chamberlain, Senior Program Assistant Eileen P. Delaney, Director of Publications Natalie Barnes, Senior Editor NCHRP PROJECT 24-21 PANEL Field of Soils and Geology—Area of Mechanics and Foundations Donald Dwyer, New York State DOT, Albany, NY (Chair) Ernest Brandl, Schnabel Foundation Company, Houston, TX John Byrne, Ground Support PLLC, Woodinville, WA Bryan K. Dias, Washington State DOT, Olympia, WA Jerry A. DiMaggio, FHWA (Retired), Washington, DC Richard Endres, Michigan DOT, Lansing, MI Thomas E. Scruggs, Georgia DOT, Forest Park, GA Sunil Sharma, University of Idaho, Moscow, ID John Steward, Vancouver, WA Michael Adams, FHWA Liaison G. P. Jayaprakash, TRB Liaison AUTHOR ACKNOWLEDGMENTS The research contained in this report was sponsored by the American Association of State Highway and Transportation Officials (AASHTO) and was managed by the National Highway Cooperative Research Program (NCHRP) under Project 24-21. The author, Dr. Carlos A. Lazarte, P.E., G.E., is thankful to the panel of the research project for their constructive comments and valuable suggestions. The guidance, review, and support provided by Dr. Gregory Baecher of the University of Maryland, College Park, Mary- land, and Dr. James Withiam of D’Appolonia, Monroeville, Pennsylvania, are greatly appreciated. Dr. Baecher also provided graphics on reliability that are used in this document. The late Mr. Victor Elias was instrumental in the initial planning of this project. Victor’s legacy in the field of soil nail walls has been inspiring to us. The author is also thankful to the various soil nail contractors and colleagues who pro- vided load test data for this project, particularly Drill Tech & Shoring of Antioch, California; Mr. John R. Wolosick, P.E. of Hayward Baker, Inc., Alpharetta, Georgia; Dr. Withiam of D’Appolonia, Monroeville, Pennsylvania; and Mr. Darrin Beckett, P.E., of the Kentucky Transportation Cabinet, Frankfort, Kentucky. In addition, the input of the members of AASHTO Committee T-15 on Substructures and Retaining Walls to a project update presentation is appreciated. Several professionals from Geosyntec Consultants con- tributed to this project, including Dr. Chunling Li, P.E.; Mr. Michael F. Houlihan, P.E.; Dr. Paul Sabatini, P.E.; Dr. David Espinoza, P.E.; Mr. Joo Chai Wong, P.E.; Dr. Lucas de Melo; and Dr. Steve Dapp, P.E. The help of Ms. Lynn Johnson in the preparation of the manuscript is appreciated.

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FOREWORD By David A. Reynaud Staff Officer Transportation Research Board This report contains proposed specifications for the design and construction of soil- nailed retaining structures. Despite their advantages in cut applications, these structures are not available to some state DOTs, due to the lack of guidance for their use in AASHTO’s standard specifications based on load and resistance factor design (LRFD). This report will be of interest to geotechnical engineers and construction managers, who would like to pro- mote a more common utilization of soil nailing. The soil-nailing method of earth retention is the preferred retaining wall option for many cut applications, because their advantages may include cost, speed of construction, con- struction flexibility, and aesthetics. Federal Highway Administration (FHWA) Demonstra- tion Project No. 103 developed comprehensive design and construction manuals for tem- porary and permanent soil-nailed structures. These FHWA soil-nailing manuals contained a detailed design protocol for allowable stress design (ASD) and a preliminary load and resistance factor design (LRFD) approach. The AASHTO Standard Bridge Specifications, the AASHTO LRFD Bridge Design Specifi- cations, and the AASHTO LRFD Bridge Construction Specifications do not include guidance for soil-nailed structures. In the absence of AASHTO LRFD specifications, some state departments of transportation will not use soil-nailed retaining structures. Given the advan- tages of soil-nailed structures, there is a need to develop proposed standard design and con- struction specifications for soil-nailed structures for incorporation into the AASHTO LRFD Bridge Design and Construction Specifications. The objective of NCHRP Project 24-21 was to develop these proposed LRFD design and construction specifications for soil-nailed retaining structures. To accomplish the project objective, the research agency, Geosyntec Consultants, used the existing FHWA guidelines on soil nailing, conducted a comprehensive review of current soil-nailing design and con- struction guidance for both ASD and LRFD specifications, and drafted proposed LRFD design and construction specifications. The research team subsequently identified, evalu- ated, and calibrated a range of resistance factors, based on the level of detail and confidence in the accuracy of the site investigations for multiple soil nail wall (SNW) project scenarios. These resistance factors were used with current AASHTO load factors to design SNWs using LRFD methodology and compared to SNWs designed using ASD methodology for the same project scenarios to demonstrate equivalence.

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CONTENTS 1 Summary 3 Chapter 1: Background 3 1.1 Introduction 3 1.2 Problem Statement 4 1.3 Research Objectives 4 1.4 Report Organization 5 Chapter 2: Research Approach 5 2.1 Introduction 5 2.2 Review of Design and Construction Procedures for Soil Nailing 5 2.3 Compilation of Soil Nail Load-Test Data 5 2.4 Development of Resistance Factors through Calibration of Load-Test Data 5 2.5 Comparisons of Designs Based on the LRFD and ASD Methods 5 2.6 Proposed LRFD Design and Construction Specifications 6 Chapter 3: Findings and Applications 6 3.1 Overview 6 3.2 Review of Current LRFD Practice 6 3.2.1 Historical Development of LRFD 6 3.2.2 Overview of Uncertainty in Design of Structures 7 3.2.3 Overview of the ASD Method 7 3.2.4 Overview of the LRFD Method 9 3.2.5 Resistances and Loads as Random Variables 11 3.2.6 Approaches for Calibration of Resistance and Load Factors 13 3.2.7 Steps to Perform the Calibration of Resistance Factors 15 3.3 Review of Current U.S. Soil-Nailing Practice 15 3.3.1 Introduction 15 3.3.2 Basic Description of Soil Nail Walls 16 3.3.3 Main Components of Soil Nail Walls 18 3.4 Limit States in Soil Nail Walls 18 3.4.1 Introduction 20 3.4.2 Service Limit States 23 3.4.3 Soil Failure Limit States 25 3.4.4 Structural Limit States 32 3.4.5 Seismic Considerations in Extreme-Event Limit States of Soil Nail Walls 33 3.4.6 Design for Service Limit States (Displacements)

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34 3.5 Development of Resistance and Load Factors for Soil Nail Walls 34 3.5.1 Introduction 34 3.5.2 Common Load Factors in Earth-Retaining Structures 37 3.5.3 Resistance Factors for Sliding, Basal Heave, Overall Stability, and Seismic Limit States 38 3.5.4 Resistance Factors for Structural Limit States 39 3.5.5 Preliminary Values of Resistance Factors for Nail Pullout 40 3.6 Development of Soil Nail Test Pullout Resistance and Load Databases 40 3.6.1 Introduction 41 3.6.2 Soil Nail Bond Resistance: Influencing Factors and Typical Values 46 3.6.3 Background of Soil Nail Load Testing 49 3.6.4 Database of Soil Nail Pullout Resistance 50 3.6.5 Database of Soil Nail Loads 54 3.7 Calibration of Pullout Resistance Factors 54 3.7.1 Introduction 54 3.7.2 Description of Calibration Process 60 3.7.3 Results 63 Chapter 4: Conclusions and Suggested Research 63 4.1 Conclusions 65 4.2 Suggested Research 67 References 70 Abbreviations 72 Symbols A-1 Appendix A: Proposed LRFD Design Specifications for Soil Nail Walls B-1 Appendix B: Proposed LRFD Construction Specifications for Soil Nail Walls C-1 Appendix C: Soil Nail Test Pullout Resistance Database D-1 Appendix D: Comparison of ASD- and LRFD-Based Designs of Soil Nail Walls Note: Many of the photographs, figures, and tables in this report have been converted from color to grayscale for printing. The electronic version of the report (posted on the Web at www.trb.org) retains the color versions.