Soil Strength and Slope Stability E-Book

J. Michael Duncan

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 813

Veröffentlichungsjahr: 2014

Beschreibung

The definitive guide to the critical issue of slope stability and safety

Soil Strength and Slope Stability, Second Edition presents the latest thinking and techniques in the assessment of natural and man-made slopes, and the factors that cause them to survive or crumble. Using clear, concise language and practical examples, the book explains the practical aspects of geotechnical engineering as applied to slopes and embankments. The new second edition includes a thorough discussion on the use of analysis software, providing the background to understand what the software is doing, along with several methods of manual analysis that allow readers to verify software results. The book also includes a new case study about Hurricane Katrina failures at 17th Street and London Avenue Canal, plus additional case studies that frame the principles and techniques described.

Slope stability is a critical element of geotechnical engineering, involved in virtually every civil engineering project, especially highway development. Soil Strength and Slope Stability fills the gap in industry literature by providing practical information on the subject without including extraneous theory that may distract from the application. This balanced approach provides clear guidance for professionals in the field, while remaining comprehensive enough for use as a graduate-level text. Topics include:

• Mechanics of soil and limit equilibrium procedures
• Analyzing slope stability, rapid drawdown, and partial consolidation
• Safety, reliability, and stability analyses
• Reinforced slopes, stabilization, and repair

The book also describes examples and causes of slope failure and stability conditions for analysis, and includes an appendix of slope stability charts. Given how vital slope stability is to public safety, a comprehensive resource for analysis and practical action is a valuable tool. Soil Strength and Slope Stability is the definitive guide to the subject, proving useful both in the classroom and in the field.

Leseprobe

Table of Contents

Title Page

Copyright

Preface

Foreword

Chapter 1: Introduction

Summary

Chapter 2: Examples and Causes of Slope Failures

2.1 Introduction

2.2 Examples of Slope Failure

2.3 The Olmsted Landslide

2.4 Panama Canal Landslides

2.5 The Rio Mantaro Landslide

2.6 Kettleman Hills Landfill Failure

2.7 Causes of Slope Failure

2.8 Summary

Chapter 3: Soil Mechanics Principles

3.1 Introduction

3.2 Total and Effective Stresses

3.3 Drained and Undrained Shear Strengths

3.4 Basic Requirements for Slope Stability Analyses

Chapter 4: Stability Conditions for Analysis

4.1 Introduction

4.2 End-of-Construction Stability

4.3 Long-Term Stability

4.4 Rapid (Sudden) Drawdown

4.5 Earthquake

4.6 Partial Consolidation and Staged Construction

4.7 Other Loading Conditions

4.8 Analysis Cases for Earth and Rockfill Dams

Chapter 5: Shear Strength

5.1 Introduction

5.2 Behavior of Granular Materials—Sand, Gravel, and Rockfill

5.3 Silts

5.4 Clays

5.5 Municipal Solid Waste

Chapter 6: Mechanics of Limit Equilibrium Procedures

6.1 Definition of the Factor of Safety

6.2 Equilibrium Conditions

6.3 Single Free-Body Procedures

6.4 Procedures of Slices: General

6.5 Procedures of Slices: Circular Slip Surfaces

6.6 Procedures of Slices: Noncircular Slip Surfaces

6.7 Procedures of Slices: Assumptions, Equilibrium Equations, and Unknowns

6.8 Procedures of Slices: Representation of Interslice Forces (Side Forces)

6.9 Computations with Anisotropic Shear Strengths

6.10 Computations with Curved Strength Envelopes

6.11 Finite Element Analysis of Slopes

6.12 Alternative Definitions of the Factor of Safety

6.13 Pore Water Pressure Representation

Chapter 7: Methods of Analyzing Slope Stability

7.1 Simple Methods of Analysis

7.2 Slope Stability Charts

7.3 Spreadsheet Software

7.4 Finite Element Analyses of Slope Stability

7.5 Computer Programs for Limit Equilibrium Analyses

7.6 Verification of Results of Analyses

7.7 Examples for Verification of Stability Computations

Chapter 8: Reinforced Slopes and Embankments

8.1 Limit Equilibrium Analyses with Reinforcing Forces

8.2 Factors of Safety for Reinforcing Forces and Soil Strengths

8.3 Types of Reinforcement

8.4 Reinforcement Forces

8.5 Allowable Reinforcement Forces and Factors of Safety

8.6 Orientation of Reinforcement Forces

8.7 Reinforced Slopes on Firm Foundations

8.8 Embankments on Weak Foundations

Chapter 9: Analyses for Rapid Drawdown

9.1 Drawdown during and at the End of Construction

9.2 Drawdown for Long-Term Conditions

9.3 Partial Drainage

9.4 Shear-Induced Pore Pressure Changes

Chapter 10: Seismic Slope Stability

10.1 Analysis Procedures

10.2 Pseudostatic Screening Analyses

10.3 Determining Peak Accelerations

10.4 Shear Strength for Pseudostatic Analyses

10.5 Postearthquake Stability Analyses

Chapter 11: Analyses of Embankments with Partial Consolidation of Weak Foundations

11.1 Consolidation During Construction

11.2 Analyses of Stability with Partial Consolidation

11.3 Observed Behavior of an Embankment Constructed in Stages

11.4 Discussion

Chapter 12: Analyses to Back-Calculate Strengths

12.1 Back-Calculating Average Shear Strength

12.2 Back-Calculating Shear Strength Parameters Based on Slip Surface Geometry

12.3 Examples of Back-Analyses of Failed Slopes

12.4 Practical Problems and Limitation of Back-Analyses

12.5 Other Uncertainties

Chapter 13: Factors of Safety and Reliability

13.1 Definitions of Factor of Safety

13.2 Factor of Safety Criteria

13.3 Reliability and Probability of Failure

13.4 Standard Deviations and Coefficients of Variation

13.5 Estimating Reliability and Probability of Failure

Chapter 14: Important Details of Stability Analyses

14.1 Location of Critical Slip Surfaces

14.2 Examination of Noncritical Slip Surfaces

14.3 Tension in the Active Zone

14.4 Inappropriate Forces in the Passive Zone

14.5 Other Details

14.6 Verification of Calculations

14.7 Three-Dimensional Effects

Chapter 15: Presenting Results of Stability Evaluations

15.1 Site Characterization and Representation

15.2 Soil Property Evaluation

15.3 Pore Water Pressures

15.4 Special Features

15.5 Calculation Procedure

15.6 Analysis Summary Figure

15.7 Parametric Studies

15.8 Detailed Input Data

15.9 Table Of Contents

Chapter 16: Slope Stabilization and Repair

16.1 Use of Back-Analysis

16.2 Factors Governing Selection of Method of Stabilization

16.3 Drainage

16.4 Excavations and Buttress Fills

16.5 Retaining Structures

16.6 Reinforcing Piles and Drilled Shafts

16.7 Injection Methods

16.8 Vegetation

16.9 Thermal Treatment

16.10 Bridging

16.11 Removal and Replacement of the Sliding Mass

Appendix A: Slope Stability Charts

Use and Applicability of Charts for Analysis of Slope Stability

Averaging Slope Inclinations, Unit Weights, and Shear Strengths

Soils with

Soils with

Infinite Slope Charts

Soils with and Strength Increasing with Depth

Examples

Appendix B: Curved Shear Strength Envelopes for Fully Softened Shear Strengths and Their Impact on Slope Stability Analyses

Introduction

Measured Strength Envelopes

Equations for Strength Envelope

Impact on Slope Stability

Conclusions and Recommendations

References

Index

End User License Agreement

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Guide

Cover

Table of Contents

Preface

Foreword

Begin Reading

List of Illustrations

Figure 1.1

Figure 1.2

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 2.10

Figure 2.11

Figure 2.12

Figure 2.13

Figure 2.14

Figure 2.15

Figure 2.16

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figure 3.6

Figure 3.7

Figure 3.8

Figure 3.9

Figure 4.1

Figure 4.2

Figure 5.1

Figure 5.2

Figure 5.3

Figure 5.4

Figure 5.5

Figure 5.6

Figure 5.7

Figure 5.8

Figure 5.9

Figure 5.10

Figure 5.11

Figure 5.12

Figure 5.13

Figure 5.14

Figure 5.15

Figure 5.16

Figure 5.17

Figure 5.18

Figure 5.19

Figure 5.20

Figure 5.21

Figure 5.22

Figure 5.23

Figure 5.24

Figure 5.25

Figure 5.26

Figure 5.27

Figure 5.28

Figure 5.29

Figure 5.30

Figure 5.31

Figure 5.32

Figure 5.33

Figure 5.34

Figure 5.35

Figure 5.36

Figure 5.37

Figure 5.38

Figure 5.39

Figure 5.40

Figure 5.41

Figure 5.42

Figure 5.43

Figure 5.44

Figure 5.45

Figure 5.46

Figure 5.47

Figure 5.48

Figure 5.49

Figure 5.50

Figure 6.1

Figure 6.2

Figure 6.3

Figure 6.4

Figure 6.5

Figure 6.6

Figure 6.7

Figure 6.8

Figure 6.9

Figure 6.10

Figure 6.11

Figure 6.12

Figure 6.13

Figure 6.14

Figure 6.15

Figure 6.16

Figure 6.17

Figure 6.18

Figure 6.19

Figure 6.20

Figure 6.21

Figure 6.22

Figure 6.23

Figure 6.24

Figure 6.25

Figure 6.26

Figure 6.27

Figure 6.28

Figure 6.29

Figure 6.30

Figure 6.31

Figure 6.32

Figure 6.33

Figure 6.34

Figure 6.35

Figure 6.36

Figure 6.37

Figure 6.38

Figure 6.39

Figure 6.40

Figure 6.41

Figure 6.42

Figure 6.43

Figure 7.1

Figure 7.2

Figure 7.3

Figure 7.4

Figure 7.5

Figure 7.6

Figure 7.7

Figure 7.8

Figure 7.9

Figure 7.10

Figure 7.11

Figure 7.12

Figure 7.13

Figure 7.14

Figure 7.15

Figure 7.16

Figure 7.17

Figure 7.18

Figure 7.19

Figure 7.20

Figure 7.21

Figure 7.22

Figure 7.23

Figure 7.24

Figure 7.25

Figure 7.26

Figure 7.27

Figure 7.28

Figure 7.29

Figure 7.30

Figure 7.31

Figure 7.32

Figure 7.33

Figure 7.34

Figure 7.35

Figure 7.36

Figure 7.37

Figure 7.38

Figure 7.39

Figure 7.40

Figure 8.1

Figure 8.2

Figure 8.3

Figure 8.4

Figure 8.5

Figure 8.6

Figure 8.7

Figure 8.8

Figure 8.9

Figure 9.1

Figure 9.2

Figure 9.3

Figure 9.4

Figure 9.5

Figure 10.1

Figure 10.2

Figure 10.3

Figure 10.4

Figure 10.5

Figure 10.6

Figure 10.7

Figure 10.8

Figure 10.9

Figure 10.10

Figure 10.11

Figure 10.12

Figure 11.1

Figure 11.2

Figure 11.3

Figure 11.4

Figure 11.5

Figure 12.1

Figure 12.2

Figure 12.3

Figure 12.4

Figure 12.5

Figure 12.6

Figure 12.7

Figure 12.8

Figure 12.9

Figure 12.10

Figure 12.11

Figure 12.12

Figure 12.13

Figure 12.14

Figure 12.15

Figure 12.16

Figure 12.17

Figure 12.18

Figure 12.19

Figure 13.1

Figure 13.2

Figure 13.3

Figure 13.4

Figure 14.1

Figure 14.2

Figure 14.3

Figure 14.4

Figure 14.5

Figure 14.6

Figure 14.7

Figure 14.8

Figure 14.9

Figure 14.10

Figure 14.11

Figure 14.12

Figure 14.13

Figure 14.14

Figure 14.15

Figure 14.16

Figure 14.17

Figure 14.18

Figure 14.19

Figure 14.20

Figure 14.21

Figure 14.22

Figure 14.23

Figure 14.24

Figure 15.1

Figure 15.2

Figure 15.3

Figure 15.4

Figure 15.5

Figure 15.6

Figure 15.7

Figure 15.8

Figure 15.9

Figure 15.10

Figure 15.11

Figure 16.1

Figure 16.2

Figure 16.3

Figure 16.4

Figure 16.5

Figure 16.6

Figure 16.7

Figure 16.8

Figure 16.9

Figure 16.10

Figure 16.11

Figure 16.12

Figure 16.13

Figure 16.14

Figure 16.15

Figure 16.16

Figure 16.17

Figure 16.18

Figure 16.19

Figure 16.20

Figure A.1

Figure A.2

Figure A.3

Figure A.4

Figure A.5

Figure A.6

Figure A.7

Figure A.8

Figure A.9

Figure A.10

Figure A.11

Figure A.12

Figure A.13

Figure A.14

Figure B.1

Figure B.2

Figure B.3

Figure B.4

Figure B.5

Figure B.6

Figure B.7

Figure B.8

Figure B.9

Figure B.10

Figure B.11

Figure B.12

Figure B.13

Figure B.14

Figure B.15

Figure B.16

List of Tables

Table 4.1

Table 5.1

Table 5.2

Table 5.3

Table 5.4

Table 5.5

Table 5.6

Table 5.7

Table 5.8

Table 5.9

Table 5.10

Table 5.11

Table 5.12

Table 5.13

Table 5.14

Table 5.15

Table 5.16

Table 5.17

Table 6.1

Table 6.2

Table 6.3

Table 6.4

Table 6.5

Table 6.6

Table 6.7

Table 6.8

Table 7.1

Table 7.2

Table 7.3

Table 7.4.

Table 7.5

Table 7.6

Table 7.7

Table 7.8

Table 7.9

Table 8.1

Table 8.2

Table 8.3

Table 8.4

Table 9.1

Table 9.2

Table 10.1

Table 10.2

Table 10.3

Table 10.4

Table 11.1

Table 12.1

Table 12.2

Table 12.3

Table 13.1

Table 13.2

Table 13.3

Table 13.4

Table 13.5

Table 13.6

Table 13.7

Table 13.8

Table 13.9

Table 14.1

Table 15.1

Table 15.2

Table 15.3

Table B.1

Soil Strength and Slope Stability

Second Edition

J. Michael Duncan

Stephen G. Wright

Thomas L. Brandon

Cover image: Michael Duncan

Cover design: Wiley

This book is printed on acid-free paper.

Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data

Duncan, J. M. (James Michael)

Soil strength and slope stability / J. Michael Duncan, Stephen G. Wright, Thomas L. Brandon.

pages cm

Includes bibliographical references and index.

ISBN 978-1-118-65165-0 (cloth); ISBN 978-1-118-91795-4 (ebk); ISBN 978-1-118-91796-1 (ebk)

1. Slopes (Soil mechanics) I. Wright, Stephen G. (Stephen Gailord), 1943- II. Brandon, Thomas L. III. Title.

TA710.D868 2014

624.1′51363— dc23

2014004730

Preface

In the nine years since the appearance of the first edition of Soil Strength and Slope Stability there have been significant developments in measurement of soil strength in the laboratory and the field, advances in methods of stability analysis, and development of new techniques for slope stabilization. In situ testing, particularly cone penetration testing, has improved the efficiency of soil exploration and evaluation of soil strength through the use of correlations. Chapter 5, on shear strength of soil and municipal solid waste, is greatly expanded in this edition, providing discussions of the behavior of rockfill, gravel, sand, silt, and clay, as well as compilations of data and typical values of their strengths. This edition also draws together more lessons that have been learned from recent slope failures, such as the failures of I-walls in New Orleans during Hurricane Katrina, and delayed failures that resulted from gradual softening of clays over long periods of time. The purpose of this book is to describe the current state of knowledge on soil strength and slope stability in a form that makes it easily accessible to geotechnical graduate students and professionals.

Development of this book would not have been possible without the assistance of many colleagues, whose contributions to our understanding we gratefully acknowledge. Foremost among these is Professor Harry Seed, who taught all of us and was the inspiration for our lifelong interest in soil strength and slope stability. We are also grateful for the opportunity to work with Nilmar Janbu, who during his sabbatical at Berkeley in 1969 taught us many valuable lessons regarding analysis of slope stability and the shear strength of soils. Our university colleagues Jim Mitchell, Roy Olson, Clarence Chan, Ken Lee, Peter Dunlop, Guy LeFebvre, Fred Kulhawy, Suphon Chirapuntu, Tarciso Celestino, Dean Marachi, Ed Becker, Kai Wong, Norman Jones, Poul Lade, Pat Lucia, Tim D'Orazio, Jey Jeyapalan, Sam Bryant, Ed Kavazanjian, Erik Loehr, Loraine Fleming, Bak Kong Low, Bob Gilbert, Garry Gregory, Vern Schaefer, Tim Stark, Binod Tiwari, Mohamad Kayyal, Marius DeWet, Clark Morrison, Ellen Rathje, George Filz, Mike Pockoski, Jaco Esterhuizen, Matthew Sleep, and Daniel VandenBerge have also contributed greatly to our understanding of soil strength and stability. Our experiences working with professional colleagues Al Buchignani, Laurits Bjerrum, Jim Sherard, Tom Leps, Norbert Morgenstern, George Sowers, Robert Schuster, Ed Luttrell, Larry Franks, Steve Collins, Dave Hammer, Larry Cooley, John Wolosick, Noah Vroman, Luis Alfaro, Max DePuy and his group at the Panama Canal Authority, and Fernando Bolinaga have helped us to see the useful relationships among teaching, research, and professional practice. Special thanks go to Alex Reeb, Chris Meehan, Bernardo Castellanos, Daniel VandenBerge, and Beena Ajmera for their invaluable assistance with figures, references, proofing, and indexing. Finally, we express our deepest appreciation and love to our wives—Ann, Ouida, and Aida—for their support, understanding, and constant encouragement throughout our careers and during the countless hours we have spent working on this book.

Foreword

Slope stability is arguably the most complex and challenging of all the subdisciplines of geotechnical engineering, and is often the least understood. In the first edition of this book, the authors captured the essence of this subject in an authoritative, comprehensive, and informative manner. Since publication in 2005, the first edition has come into widespread use in the profession and has virtually become a classic in the slope stability literature. The authors have certainly done no less in this second edition. Eleven of the 16 chapters have been significantly expanded and/or supplemented with new material. Moreover, the new materials are highly focused on the latest knowledge, experience, and practices that have been developed since the first edition. These new insights will render this second edition a highly relevant and useful volume for practitioners, academics, and students for years to come.

While all the valuable new additions to the book are too voluminous to address in detail here, there are several items in the writer's opinion that are particularly relevant. In Chapter 2, case histories have been added of the New Orleans “I-Wall” failures during Hurricane Katrina, from which much valuable information was obtained. Chapter 3 includes a new discussion of the effective stress envelope for unsaturated soils. Chapter 5 on shear strength has been significantly expanded with new concepts on curvature of strength envelopes and recent correlations of shear strength with various field tests and index properties. In the nine years since publication of the first edition, our understanding of soil strength and its application to slope stability analysis has made significant strides, especially related to fully softened strengths of highly plastic clays. Chapter 5 also includes a detailed discussion of this topic including laboratory testing methods, representation of curved strength envelopes with piecewise linear and power-curve techniques, and application of fully softened strength in slope stability analyses. Chapter 6 includes a presentation on the finite element strength reduction method for calculating the factor of safety of slopes and an update on determination of pore pressures by finite element methods. Chapter 7 includes new finite element solutions to the verification problems and a new verification problem using a curved strength envelope. Chapter 8 on reinforced slopes has been updated to include current FHWA (2009) methods for MSE walls. Chapters 9 and 10 contain updated material on rapid drawdown and seismic slope stability analyses. While this is but a brief discussion of a few of the many new portions of the book, it illustrates the breadth of new valuable material in the second edition.

In keeping with the first edition, the authors have maintained a format beginning with elemental principles that university students can quickly comprehend and moving in a smooth and logical manner to the highly advanced material for even the most experienced user. It is the writer's opinion that the pristine covers of the new second edition publication will soon become ragged and worn in tribute to the widespread relevance and usefulness of this book.

Dr. Garry H. Gregory, Ph.D., P. E., D.GEBoard of Governors of the Geo-InstituteChair of the Embankments, Dams, and SlopesCommittee of the Geo-Institute

Chapter 1Introduction

Evaluating the stability of slopes in soil is an important, interesting, and challenging aspect of civil engineering. Concerns with slope stability have driven some of the most important advances in our understanding of the complex behavior of soils. Extensive engineering and research studies performed over the past 80 years provide a sound set of soil mechanics principles with which to attack practical problems of slope stability.

Over the past decades, experiences with the behavior of slopes, and often with their failure, have led to development of improved understanding of the changes in soil properties that can occur over time, recognition of the requirements and the limitations of laboratory and in situ testing for evaluating soil strengths, development of new and more effective types of instrumentation to observe the behavior of slopes, improved understanding of the principles of soil mechanics that connect soil behavior to slope stability, improved analytical procedures augmented by extensive examination of the mechanics of slope stability analyses, detailed comparisons with field behavior, and use of computers to perform thorough analyses. Through these advances, the art of slope stability evaluation has entered a more mature phase. Experience and judgment, which continue to be of prime importance, are combined with a more complete understanding of soil behavior and rational methods of analysis to improve the level of confidence that is achievable through systematic observation, testing, and analysis.

In spite of the advances that have been made, evaluating the stability of slopes remains a challenge. Even when geology and soil conditions have been evaluated in keeping with the standards of good practice, and stability has been evaluated using procedures that have been effective in previous projects, it is possible that surprises are in store. As an example, consider the case of the Waco Dam embankment and the lessons learned from that experience.

In October 1961, the construction of Waco Dam was interrupted by the occurrence of a slide along a 1500-ft section of the embankment resting on the Pepper shale formation, a heavily overconsolidated, stiff-fissured clay. A photograph of the 85-ft-high embankment section, taken shortly after the slide occurred, is shown in Figure 1.1. In the slide region, the Pepper shale had been geologically uplifted to the surface and was bounded laterally by two faults crossing the axis of the embankment. The slide was confined to the length of the embankment founded on Pepper shale, and no significant movements were observed beyond the fault boundaries.

Figure 1.1 Slide in the downstream slope of the Waco Dam embankment (U.S. government, Corps of Engineers photograph).

The section of the embankment involved in the slide was degraded to a height of approximately 40 ft, and an extensive investigation was carried out by the U.S. Army Corps of Engineers to determine the cause of the failure and to develop a method for repairing the slide. The investigation showed that the slide extended for several hundred feet downstream from the embankment, within the Pepper shale foundation. A surprising finding of the studies conducted after the failure was the highly anisotropic nature of the Pepper shale, which contained pervasive horizontal slickensided fissures spaced about apart. The strength along horizontal planes was found to be only about 40% as large as the strength measured in conventional tests on vertical specimens. Although conventional testing and analysis indicated that the embankment would be stable throughout construction, analyses performed using the lower strengths on horizontal planes produced results that were in agreement with the observed failure (Wright and Duncan, 1972).

This experience shows that the conventional practice of testing only vertical samples can be misleading, particularly for stiff fissured clays with a single dominant fissure orientation. With the lesson of the Waco Dam experience in mind, geotechnical engineers are better prepared to avoid similar pitfalls.

The procedures we use to measure soil strengths and to evaluate the stability of slopes are for the most part rational and may appear to be rooted solidly in engineering science. The fact that they have a profound empirical basis is illustrated by the failure of an underwater slope in San Francisco Bay.

In August 1970, during construction of a new shipping terminal at the Port of San Francisco, a 250-ft long portion of an underwater slope about 90-ft high failed, with the soil on one side sliding into the trench, as shown in Figure 1.2. The failure took place entirely within the San Francisco Bay Mud, a much-studied highly plastic marine clay.

Figure 1.2 Failure of the San Francisco LASH Terminal trench slope.

Considerable experience in the San Francisco Bay area had led to the widely followed practice of excavating underwater slopes in Bay Mud at an inclination of 1 (horizontal) to 1 (vertical). At this new shipping terminal, however, it was desired to make the slopes steeper, if possible, to reduce the volume of cut and fill needed for the stability trench, and thereby to reduce the cost of the project. Thorough investigations, testing, and analyses were undertaken to study this possibility.

Laboratory tests on the best obtainable samples, and extensive analyses of stability, led to the conclusion that it would be possible to excavate the slopes at 0.875 to 1. At this inclination, the computed factor of safety of the slopes would be 1.17. Although such a low factor of safety was certainly unusual, the conditions involved were judged to be exceptionally well known and understood, and the slopes were excavated at this steep inclination. The result was the failure depicted in Figure 1.2. An investigation after the failure led to the conclusion that the strength of the Bay Mud that could be mobilized in the field over a period of several weeks was lower than the strength measured in laboratory tests in which the Bay Mud was loaded to failure in a few minutes, and that the cause of the difference was creep strength loss (Duncan and Buchignani, 1973).

The lesson to be derived from this experience is that our methods may not be as scientifically well founded as they sometimes appear. If we alter our conventional methods by “improving” one aspect, such as the quality of samples used to measure the undrained strength of Bay Mud, we do not necessarily achieve a more accurate result. In the case of excavated slopes in Bay Mud, conventional sample quality and conventional test procedures, combined with conventional values of factor of safety, had been successful many times. When the procedures were changed by “refining” the sampling and strength testing procedures, the result was higher values of undrained shear strength than would have been measured if conventional procedures had been used. When, in addition, the value of the safety factor was reduced, the result was a decision to use an excessively steep slope, which failed. Altering conventional practice and reducing the factor of safety led to the use of a procedure that was not supported by experience.

Summary

The broader messages from these and similar cases are clear:

We learn our most important lessons from experience, often from experience involving failures. The state of the art is advanced through these failures and the lessons they teach. As a result, the methods we use depend strongly on experience. In spite of the fact that our methods may have a logical background in mechanics and our understanding of the behavior of soils and rocks, it is important to remember that these methods are semiempirical. We depend as much on the fact that the methods have worked in the past as we do on their logical basis. We cannot count on improving these methods by altering only one part of the process that we use.

We should not expect that we have no more lessons to learn. As conditions arise that are different from the conditions on which our experience is based, even in ways that may at first seem subtle, we may find that our semiempirical methods are inadequate and need to be changed or expanded. The slide in Waco Dam served clear notice that conventional methods were not sufficient for evaluating the shear strength of Pepper shale and the stability of embankments founded on it. The lesson learned from that experience is now part of the state of the art, but it would be imprudent to think that the current state of knowledge is complete. We need to keep abreast of advances in the state of the art as they develop, and practice our profession with humility, in recognition that the next lesson to be learned may be lurking in tomorrow's project.

The objective of this book is to draw together some of the lessons that have been learned about measuring soil strengths and performing analyses of stability into a consistent, clear, and convenient reference for students and practicing engineers.

Advances in the state of the art and the state of practice have continued since publication of the first edition of this book. Notable are the lessons earned as a result of stability failures caused by Hurricane Katrina, improved techniques for in situ measurement of soil properties, advances in the understanding of when and where fully softened shear strength should be used for design, advances in techniques for pseudostatic seismic stability analyses, better understanding of the long-term behavior of reinforced slopes, and many other developments detailed in this edition.

Chapter 2Examples and Causes of Slope Failures

2.1 Introduction

Experience is the best teacher but not the kindest. Failures demand attention and always hold lessons about what not to do again. Learning from failures—hopefully from other people's failures—provides the most reliable basis for anticipating what might go wrong in other cases. This chapter describes 13 cases of slope failures and recounts briefly the circumstances under which they occurred, their causes, and their consequences. The examples are followed by an examination of the factors that influence the stability of slopes and the causes of instability, as illustrated by these examples.