Reliability-Based Geotechnical Resistance Factors for Axially Loaded Micropiles (2022)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

2022 N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP RESEARCH REPORT 989 Reliability-Based Geotechnical Resistance Factors for Axially Loaded Micropiles J. Erik Loehr Andrew Z. Boeckmann Dan Ding University of Missouri Columbia, MO Subscriber Categories Bridges and Other Structures • Geotechnology Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transporta- tion results in increasingly complex problems of wide interest to high- way authorities. These problems are best studied through a coordinated program of cooperative research. Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 ini- tiated an objective national highway research program using modern scientific techniques—the National Cooperative Highway Research Program (NCHRP). NCHRP is supported on a continuing basis by funds from participating member states of AASHTO and receives the full cooperation and support of the Federal Highway Administration (FHWA), United States Department of Transportation, under Agree- ment No. 693JJ31950003. The Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine was requested by AASHTO to administer the research program because of TRB’s recognized objectivity and understanding of modern research practices. TRB is uniquely suited for this purpose for many reasons: TRB maintains an extensive com- mittee structure from which authorities on any highway transportation subject may be drawn; TRB possesses avenues of communications and cooperation with federal, state, and local governmental agencies, univer- sities, and industry; TRB’s relationship to the National Academies is an insurance of objectivity; and TRB maintains a full-time staff of special- ists in highway transportation matters to bring the findings of research directly to those in a position to use them. The program is developed on the basis of research needs iden- tified by chief administrators and other staff of the highway and transportation departments, by committees of AASHTO, and by the FHWA. Topics of the highest merit are selected by the AASHTO Special Committee on Research and Innovation (R&I), and each year R&I’s recommendations are proposed to the AASHTO Board of Direc- tors and the National Academies. Research projects to address these topics are defined by NCHRP, and qualified research agencies are selected from submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Academies and TRB. The needs for highway research are many, and NCHRP can make significant contributions to solving 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 research 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 by going to https://www.mytrb.org/MyTRB/Store/default.aspx Printed in the United States of America NCHRP RESEARCH REPORT 989 Project 04-40 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-68687-7 Library of Congress Control Number 2022935131 © 2022 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and the graphical logo are trade- marks of the National Academy of Sciences. All rights reserved. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this 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, FTA, GHSA, NHTSA, or TDC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for 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 from CRP. NOTICE 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 by the National Academies of Sciences, Engineering, and Medicine. The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research and are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; the FHWA; or the program sponsors. The Transportation Research Board does not develop, issue, or publish standards or speci- fications. The Transportation Research Board manages applied research projects which provide the scientific foundation that may be used by Transportation Research Board sponsors, industry associations, or other organizations as the basis for revised practices, procedures, or specifications. The Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturers’ names or logos appear herein solely because they are considered essential to the object of the report.

The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, non- governmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. The National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. John L. Anderson is president. The National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. The three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. The National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.nationalacademies.org. The Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. The mission of the Transportation Research Board is to provide leadership in transportation improvements and innovation through trusted, timely, impartial, and evidence-based information exchange, research, and advice regarding all modes of transportation. The Board’s varied activities annually engage about 8,000 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 individuals interested in the development of transportation. Learn more about the Transportation Research Board at www.TRB.org.

C O O P E R A T I V E R E S E A R C H P R O G R A M S CRP STAFF FOR NCHRP RESEARCH REPORT 989 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs Waseem Dekelbab, Associate Program Manager, National Cooperative Highway Research Program Amir N. Hanna, Senior Program Officer Emily Griswold, Program Coordinator Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications NCHRP PROJECT 04-40 PANEL Field of Materials and Construction—Area of General Materials Heather Z. Shoup, Illinois Department of Transportation, Springfield, IL (Chair) Rodrigo A. Herrera, Florida Department of Transportation, Tallahassee, FL Arash Khosravifar, Portland State University, Portland, OR Mary Ellen Bruce Large, Deep Foundations Institute, Eighty Four, PA Brett S. McKiernan, Connecticut Department of Transportation, Newington, CT Ching Tsai, Ardaman and Associates, Inc., Baton Rouge, LA; Louisiana Department of Transportation & Development (formerly) Sung Min Yoon, Texas Department of Transportation, Austin, TX Jennifer Nicks, FHWA Liaison Nancy M. Whiting, TRB Liaison AUTHOR ACKNOWLEDGMENTS The authors gratefully acknowledge the many individuals and companies that provided information for use in the project, including micropile load test data, information on geology, structural characteristics, and general project details.

This report proposes modifications to Section 10 of the current AASHTO LRFD Bridge Design Specifications that, if implemented, will provide design flexibility that will help designers achieve target reliability most cost-effectively for a broad range of project conditions. The infor- mation contained in the report will be of immediate interest to state bridge and geotechnical engineers and others involved in the different aspects of bridges and structures. Micropile foundations are small-diameter (typically less than 12 inches) drilled and grouted non-displacement, reinforced elements that can be constructed in situations of low-headroom, restricted-access, obstructions, and vibration-sensitive sites. However, current micropile foun- dation design procedures do not incorporate reliability-based geotechnical resistance factors, and performing local calibrations would require large amounts of data and resources that are unlikely available for individual state departments of transportation. There was a need to develop reliability-based geotechnical resistance factors and speci- fications that can be used for various design and construction methods. These factors and accompanying specifications should provide an increased level of confidence in the design and help highway agencies in making decisions regarding the use of micropile foundations. Under NCHRP Project 04-40, “Reliability-Based Geotechnical Resistance Factors for Axially-Loaded Micropiles,” the University of Missouri was charged with developing reliability-based geotechnical resistance factors for axially loaded micropiles and related spec- ifications. To accomplish this objective, the researchers reviewed literature pertaining to the reliability of micropiles and other deep foundation elements, especially with regard to the influence of load testing; compiled a database of load test measurements; performed related analysis; and developed approaches for estimating micropile bond resistance. Based on the findings of this work, the research team proposed changes to Section 10 of the current AASHTO LRFD Bridge Design Specifications. This report summarizes the work performed in the project. An appendix is available for download by going to the website of the National Academies Press (www.nap.edu) and searching for NCHRP Research Report 989: Reliability-Based Geo technical Resistance Factors for Axially Loaded Micropiles. F O R E W O R D By Amir N. Hanna Staff Officer Transportation Research Board

1 Summary 3 Chapter 1 Background 3 1.1 Micropile Classification and Use 3 1.2 Micropiles at the Strength Limit State 3 1.2.1 Micropile Design for the Strength Limit State 5 1.2.2 Micropile Reliability for the Strength Limit State 8 1.3 Micropiles at the Service Limit State 8 1.3.1 Micropile Design for the Service Limit State 9 1.3.2 Micropile Reliability for the Service Limit State 10 1.4 Reliability and Load Tests 10 1.4.1 Bayesian Updating of Foundation Resistance 12 1.4.2 Reliability of Micropile Resistance Based on Load Tests to Failure 13 1.4.3 Reliability of Micropile Resistance Verified by Proof Load Tests 13 1.4.4 Current AASHTO Provisions for Design Considering Load Tests 14 1.5 Reliability of Deep Foundations in Redundant Groups 17 1.6 Target Reliability 19 Chapter 2 Methods for Establishing Nominal Unit Bond Resistance 19 2.1 Data Collection 19 2.2 General Design Approaches for Axially Loaded Micropiles 20 2.3 Selection of Condition Ranges for Empirical Design Methods 21 2.4 Presumptive Models for Micropile Bond Resistance 21 2.4.1 Characterization of Mean and Uncertainty for Presumptive Models 23 2.4.2 Bayesian Updating for Presumptive Models 24 2.4.3 Presumptive Model for Cohesive Soils 26 2.4.4 Presumptive Model for Clean Sand 27 2.4.5 Presumptive Model for Gravelly Sand 28 2.4.6 Presumptive Model for Silty or Clayey Sand 29 2.4.7 Presumptive Model for Argillaceous Rock 31 2.4.8 Presumptive Model for Limestone 32 2.4.9 Presumptive Model for Karstic Limestone 33 2.4.10 Presumptive Model for Sandstone 34 2.4.11 Presumptive Model for Gneiss 34 2.4.12 Presumptive Model for Granite or Basalt 34 2.5 Predictive Models for Micropile Bond Resistance 34 2.5.1 Predictive Model for Cohesive Soils 38 2.5.2 Predictive Model for Clean Sand 39 2.5.3 Predictive Model for Gravelly Sand 40 2.5.4 Predictive Model for Silty or Clayey Sand 41 2.5.5 Summary of Recommended Predictive Models C O N T E N T S

42 2.6 Design Based on Site-Specific Load Tests 42 2.6.1 Establishing Nominal Resistance from Load Tests 43 2.6.2 Characterizing Uncertainty in Nominal Resistance from Load Tests 45 Chapter 3 Probabilistic Calibration of Resistance Factors 45 3.1 Target Probability of Failure 46 3.2 General Calibration Procedure for Strength Limit States 47 3.3 Resistance Factors for Presumptive Design Methods at Strength Limit States 49 3.4 Resistance Factors for Predictive Design Methods at Strength Limit States 49 3.4.1 Resistance Factors for Predictive Unit Resistance in Cohesive Soils 51 3.4.2 Resistance Factors for Predictive Unit Resistance in Sandy Soils 52 3.5 Resistance Factors for Design Considering Site-Specific Load Tests at Strength Limit States 52 3.5.1 Probabilistic Calibration for Unit Resistance from Site-Specific Load Tests 52 3.5.2 Practical Considerations for LRFD Considering Load Tests 53 3.5.3 Simulation of Resistance Factors for Load Tests to Failure 55 3.5.4 Simulation of Resistance Factors for Proof Load Tests 58 3.5.5 Evaluation of Alternative Resistance Factors 62 3.5.6 Recommended Approach for Design Based on Site-Specific Load Tests 63 3.6 Resistance Factors for Redundant Micropile Groups 66 3.7 Load Factors for the Service Limit State 69 Chapter 4 Recommended Design Methods 69 4.1 Recommendations for Design Using Presumptive Unit Bond Resistance 70 4.2 Recommendations for Design Using Predictive Unit Bond Resistance 71 4.3 Recommendations for Design Using Unit Bond Resistance from Load Tests 73 4.4 Recommendations for Service Limit State Design 73 4.5 An Illustrative Example 76 Chapter 5 Summary and Recommended Research 76 5.1 Summary and Conclusions 78 5.2 Recommendations for Future Research 79 References A-1 Appendix Data Collection and Load Test Database (Web-Only) ATT-1 Attachment Proposed Changes to the AASHTO LRFD Bridge Design Specifications Note: Photographs, figures, and tables in this report may have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.nap.edu) retains the color versions.