Soil Strength and Slope Stability

J. Michael Duncan, Stephen G. Wright, Thomas L. Brandon

• Publisher: Wiley
• Year: 2014
• Print ISBN: 9781118651650
• eBook ISBN: 9781118917954
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

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 17^th 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.

Chapter 1
Introduction

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:

1. 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.
2. 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 2
Examples 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.

2.2 Examples of Slope Failure

2.2.1 T...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Foreword
• Chapter 1: Introduction
• Chapter 2: Examples and Causes of Slope Failures
• Chapter 3: Soil Mechanics Principles
• Chapter 4: Stability Conditions for Analysis
• Chapter 5: Shear Strength
• Chapter 6: Mechanics of Limit Equilibrium Procedures
• Chapter 7: Methods of Analyzing Slope Stability
• Chapter 8: Reinforced Slopes and Embankments
• Chapter 9: Analyses for Rapid Drawdown
• Chapter 10: Seismic Slope Stability
• Chapter 11: Analyses of Embankments with Partial Consolidation of Weak Foundations
• Chapter 12: Analyses to Back-Calculate Strengths
• Chapter 13: Factors of Safety and Reliability
• Chapter 14: Important Details of Stability Analyses
• Chapter 15: Presenting Results of Stability Evaluations
• Chapter 16: Slope Stabilization and Repair
• Appendix A: Slope Stability Charts
• Appendix B: Curved Shear Strength Envelopes for Fully Softened Shear Strengths and Their Impact on Slope Stability Analyses
• References
• Index
• End User License Agreement