Slope Stability and Stabilization Methods / Edition 2

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Overview

A major revision of the comprehensive text/reference

Written by world-leading geotechnical engineers who share almost 100 years of combined experience, Slope Stability and Stabilization, Second Edition assembles the background information, theory, analytical methods, design and construction approaches, and practical examples necessary to carry out a complete slope stability project.

Retaining the best features of the previous edition, this new book has been completely updated to address the latest trends and methodology in the field. Features include:

  • All-new chapters on shallow failures and stability of landfill slopes
  • New material on probabilistic stability analysis, cost analysis of stabilization alternatives, and state-of-the-art techniques in time-domain reflectometry to help engineers plan and model new designs
  • Tested and FHA-approved procedures for the geotechnical stage of highway, tunnel, and bridge projects
  • Sound guidance for geotechnical stage design and planning for virtually all types of construction projects

Slope Stability and Stabilization, Second Edition is filled with current and comprehensive information, making it one of the best resources available on the subject-and an essential reference for today's and tomorrow's professionals in geology, geotechnical engineering, soil science, and landscape architecture.

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Editorial Reviews

From The Critics
The new edition of a text which addresses the multidisciplinary knowledge required for the analysis and stabilization of soil and rock slopes. Ten separately authored chapters discuss the geologic feature of slopes; treatment of groundwater and seepage problems, investigative and testing techniques for developing subsurface ground models; slope stability analysis procedures ranging from simple rules of thumb and design charts to the more complex rigorous limit equilibrium methods; stabilization design methods; and issues related to design, construction, and long-term modeling of slopes. Written for graduate courses in geotechnical engineering and geology, this new edition also includes new chapters on shallow failures and stability of landfill slopes and discussions of highway, tunnel, and bridge projects. Annotation c. Book News, Inc., Portland, OR (booknews.com)
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Product Details

  • ISBN-13: 9780471384939
  • Publisher: Wiley, John & Sons, Incorporated
  • Publication date: 11/1/2001
  • Edition description: REV
  • Edition number: 2
  • Pages: 736
  • Product dimensions: 6.42 (w) x 9.21 (h) x 1.78 (d)

Meet the Author

LEE W. ABRAMSON, a vice president of Hatch Mott MacDonald, Inc., is the author of the Handbook on Ground Control and Improvement (Wiley).

THOMAS S. LEE is a senior professional associate with Parsons Brinckerhoff Quade & Douglas, Inc.

SUNIL SHARMA is a professor of civil engineering at the University of Idaho in Moscow, Idaho.

GLENN M. BOYCE is a senior professional associate with Parsons Brinckerhoff Quade & Douglas, Inc.

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Table of Contents

Preface.

Acknowledgments.

About the Authors.

1. General Slope Stability Concepts (Lee W. Abramson).

2. Engineering Geology Principles (Thomas S. Lee).

3. Groundwater Conditions (Thomas S. Lee).

4. Geologic Site Exploration (Thomas S. Lee).

5. Laboratory Testing and Interpretation (Sunil Sharma).

6. Slope Stability Concepts (Sunil Sharma).

7. Slope Stabilization Methods (Lee W. Abramson).

8. Design, Construction, and Maintenance (Glenn M. Boyce).

9. Shallow Failures (Thomas S. Lee).

10. Stability of Landfill Slopes (Lee W. Abramson).

Index.

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First Chapter

Note: The Figures and/or Tables mentioned in this Sample Chapter do not appear on the web.

GENERAL SLOPE
STABILITY CONCEPTS

1.1 INTRODUCTION

The evolution of slope stability analyses in geotechnical engineering has followed closely the developments in soil and rock mechanics as a whole. Slopes either occur naturally or are engineered by humans. Slope stability problems have been faced throughout history when men and women or nature has disrupted the delicate balance of natural soil slopes. Furthermore, the increasing demand for engineered cut and fill slopes on construction projects has only increased the need to understand analytical methods, investigative tools, and stabilization methods to solve slope stability problems. Slope stabilization methods involve specialty construction techniques that must be understood and modeled in realistic ways.

An understanding of geology, hydrology, and soil properties is central to applying slope stability principles properly. Analyses must be based upon a model that accurately represents site subsurface conditions, ground behavior, and applied loads. Judgments regarding acceptable risk or safety factors must be made to assess the results of analyses.

The authors have recognized a need for consistent understanding and application of slope stability analyses for construction and remediation projects across the United States and abroad. These analyses are generally carried out at the beginning, and sometimes throughout the life, of projects during planning, design, construction, improvement, rehabilitation, and maintenance. Planners, engineers, geologists, contractors, technicians, and maintenance workers become involved in this process.

This book provides the general background information required for slope stability analyses, suitable methods of analysis with and without the use of computers, and examples of common stability problems and stabilization methods for cuts and fills. This body of information encompasses general slope stability concepts, engineering geology principles, groundwater conditions, geologic site explorations, soil and rock testing and interpretation, slope stability concepts, stabilization methods, instrumentation and monitoring, design documents, and construction inspection.

Detailed discussions about methods used in slope stability analyses are given, including the ordinary method of slices, simplified Janbu method, simplified Bishop method, Spencer's method, other limit equilibrium methods, numerical methods, total stress analysis, effective stress analysis, and the use of computer programs to solve problems. This book is intended for individuals whodeal with slope stability problems, including most geotechnical engineers and geologists who have an understanding of geotechnical engineering principles and practice.

1.2 AIMS OF SLOPE STABILITY ANALYSIS

In most applications, the primary purpose of slope stability analysis is to contribute to the safe and economic design of excavations, embankments, earth dams, landfills, and spoil heaps. Slope stability evaluations are concerned with identifying critical geological, material, environmental, and economic parameters that will affect the project, as well as understanding the nature, magnitude, and frequency of potential slope problems. When dealing with slopes in general and slope stability analysis in particular, previous geological and geotechnical experience in an area is valuable.

The aims of slope stability analyses are

(1) To understand the development and form of natural slopes and the processes responsible for different natural features.

(2) To assess the stability of slopes under short-term (often during construction) and long-term conditions.

(3) To assess the possibility of landslides involving natural or existing engineered slopes.

(4) To analyze landslides and to understand failure mechanisms and the influence of environmental factors.

(5) To enable the redesign of failed slopes and the planning and design of preventive and remedial measures, where necessary.

(6) To study the effect of seismic loadings on slopes and embankments.

The analysis of slopes takes into account a variety of factors relating to topography, geology, and material properties, often relating to whether the slope was naturally formed or engineered.

1.3 NATURAL SLOPES

Many projects intersect ridges and valleys, and these landscape features can be prone to slope stability problems. Natural slopes that have been stable for many years may suddenly fail because of changes in topography, seismicity, groundwater flows, loss of strength, stress changes, and weathering. Generally, these failures are not understood well because little study is made until the failure makes it necessary. In many instances, significant uncertainty exists about the stability of a natural slope. This has been emphasized by Peck (1967), who said:

Our chances for prediction of the stability of a natural slope are perhaps best if the area under study is an old slide zone which has been studied previously and may be reactivated by some human operations such as excavating into the toe of the slope. On the other hand, our chances are perhaps worst if the mechanism triggering the landslide is (1) at a random not previously studied location and (2) a matter of probability such as the occurrence of an earthquake.

Knowing that old slip surfaces exist in a natural slope makes it easier to understand and predict the slope's behavior. Such slip surfaces often result from previous landslides or tectonic activities. The slip surfaces may also be caused by other processes, including valley rebound, glacial shove, and glacial phenomena such as solifluction and nonuniform swelling of clays and clay-- shales. The shearing strength along these slip surfaces is often very low because prior movement has caused slide resistance to peak and gradually reduce to residual values. It is not always easy to recognize land-slide areas (while postglacial slides are readily identified, preglacial surfaces may lie buried beneath glacial sediments). However, once presheared strata have been located, evaluation of stability can be made with confidence.

The role of progressive failure in problems associated with natural slopes has been recognized more and more as time goes on. The materials most likely to exhibit progressive failure are clays and shales possessing chemical bonds that have been gradually disintegrated by weathering. Weathering releases much of the energy stored in these bonds (Bjerrum, 1966). Our understanding of landslides involving clay and shale slopes and seams has increased largely due to the original work by Bishop (1966), Bjerrum (1966), and Skempton (1964).

1.4 ENGINEERED SLOPES

Engineered slopes may be considered in three main categories: embankments, cut slopes, and retaining walls.

1.4.1 Embankments and Fills

Fill slopes involving compacted soils include highway and railway embankments, landfills, earth dams, and levees. The engineering properties of materials used in these structures are controlled by the borrow source grain size distribution, the methods of construction, and the degree of compaction. In general, embankment slopes are designed using shear strength parameters obtained from tests on samples of the proposed material compacted to the design density. The stability analyses of embank-ments and fills do not usually involve the same difficulties and uncertainties as natural slopes and cuts because borrow materials are preselected and processed. Because fills are generally built up in layers, analyses are required for all steps in the life of the project including:

(1) All phases of construction
(2) The end of construction
(3) The long-term condition
(4) Natural disturbances such as flooding and earthquakes
(5) Rapid drawdown (for water-retaining structures like earth dams)

Debris and waste landfills differ from other engineering slopes in that they typically have an extreme variety of material types, sizes, and characteristics. These materials are extremely difficult to characterize and analyze.

Constructed fills have been used since antiquity with varying degrees of success and failure. In ancient times, they were used to construct earth fill dams for storing irrigation water. One of the oldest recorded earth fill dams is the dam completed in Ceylon in the year 504 BC, which was 11 miles long, 70 feet high, and contained about 17 million cubic yards of embankment (Schuyler, 1905).

It is well known and documented that compaction of soils increases their strength. Tools and methods for compacting soils were developed long before the principles of compaction were discovered in the 1930s. For a long period of time, before the building of the first road compaction roller in the 1860s, cattle, sheep, and goats were used to compact soils. For example, in the United States, the 85-foot high Santa Fe water supply dam of New Mexico was compacted by 115 goats in 1893 (Highway Research Board, 1960).

Although mechanical equipment has been used to compact soil since the late 1860s, the engineering literature prior to the 1930s gave no evidence that anyone had established the relationships between moisture content, unit weight, and the compaction effort, relationships that are now documented as the fundamental principles of soil compaction. Between the 1930s and 1940s, the principles of compaction were widely known and discussed among engineers. The "Proctor curve" was a result of these studies. Following the work by Proctor, numerous investigations and reports were prepared to increase knowledge of compaction principles, which, through modifications and upgrading, resulted in widely used compaction testing standards.

Today, as in the past, earth materials continue to be used for embankment fills and backfill behind retaining structures because of their widespread availability and relative economy. Backfill is compacted in lifts that vary from 6 inches to 3 feet, depending on the types of soils and proposed use. Different types of compactors, ranging from large sheep's foot and vibratory rollers to small hand-operated tampers, have been developed and used to compact soils.

Embankment fills generally consist of:

(1) Cohesionless soils (sands and gravels)
(2) Cohesive soils (silts and clays)
(3) A mixture of cohesionless and cohesive soils, gravels, and cobbles (herein called earth-- rock mixtures).

Organic soils, soft clays, and silts are usually avoided. The range of particle sizes of embankment fills is governed, for economic purposes, by the availability of the materials from nearby borrow areas.

The density of the materials after being excavated from borrowareas is usually very lowand can be increased by compaction with mechanical equipment. In general, when the moisture content of the compacted soil is increased, the density will increase under a given compaction effort, until a peak or maximum density is achieved at a particular optimum moisture content. Thereafter, the density decreases as the moisture content is increased. The variations of moisture contents with density of the compacted soil is generally plotted in curves (Proctor curves) similar to Figure 1.1. The point of 100 percent saturation is called the saturation line, which is never reached since some air (pore space) always remains trapped in the soil material.

Cohesionless and cohesive soils behave differently when being compacted and have different compaction curves under the same compaction effort. Engineering characteristics of fills are discussed in the following sections. Typical engineering properties for compacted soils include maximum dry unit weight (standard compaction), optimum moisture content, typical strength, and permeability characteristics.

In general, soil shear strength varies with soil type and compaction conditions. Samples compacted dry of optimum moisture content appear stronger and more stable than those compacted wet of optimum moisture content. Increasing the compaction effort on soil reduces the permeability by reducing the amount of void space between the soil particles. When soil is compacted dry of optimum moisture content, the permeability is increased with an increase in water content. There is a slight decrease in permeability if the water content exceeds the optimum value.

Cohesionless Fills Cohesionless soils generally consist of relatively clean sands and gravels that remain pervious when compacted. These soils are represented by Unified Classification System soil groups SW, SP, GW, GP, and boundary groups of any of these.

Cohesive Fills Cohesive soils consist of those that contain sufficient quantities of silt and clay to render the soil mass relatively impermeable when properly com-pacted. Unlike compacted cohesionless soils, whose physical properties are generally improved by compaction to the maximum dry unit density, the physical properties of cohesive soils are not necessarily improved by compaction to a maximum unit density. For example, Figure 1.2 indicates that the strength of compacted silty clay decreases with increasing molding water content (Seed and Chan, 1959).

Whether compacted cohesive soils should be placed dry or wet of optimum for the same density depends on the type of construction. During construction of a fill slope, data (Lee and Haley, 1968) indicate that it is better to place cohesive soils dry of optimum. Such data are presented in Figures 1.3 and 1.4. Nevertheless, for earth dam construction, there is an increasing tendency to compact the cores of earth dams on wet of the optimum moisture content to minimize crack development and subsequent formation of seepage channels. A balance must be struck between the resultant lower strength and potential pore pressure problems caused by using a higher initial water content.

By changing the moisture content of compacted clays, a pronounced change in their engineering properties results. The effects of compaction dry and wet of optimum on the shear strength, permeability, compressibility, and structure of cohesive soils are shown in Table 1.2.

Earth-- Rock Mixtures There has been a considerable increase in the usage of earth-- rock mixtures in the fills of high embankments over the last 40 years. Such soils are heterogeneous mixtures of particles that may range in size from large boulders to clay. The mixing of the larger size particles enhances the workability of the soil in the field and increases the overall strength of the soil.

Past research studies by the Bureau of Reclamation (Holtz and Gibbs, 1956) suggested that the strength of earth-- rock mix fill depends on the amount of rock to be mixed with the in situ soils. The strength will increase with the amount of rock until some threshold percentage, for instance about 50 to 68 percent for sand-- gravel mixtures, is reached. Further increase in gravel contents produces little to no increase in strength. Variations in friction angle with gravel contents of earth-- rock mixtures for coarse-grained soils are shown in Figure 1.5.

Embankments on Weak Foundations Embankments are sometimes built on weak foundation materials. Sinking, spreading, and piping failures may occur irrespective of the stability of the new overlying embankment material. Consideration of the internal stability of an embankment-foundation system, rather than just the embankment, may be necessary. A simple rule of thumb based on bearing capacity theory can be used to make a preliminary estimate of the factor of safety against circular arc failure for an embankment built over a clay foundation. The rule is (Cheney and Chassie, 1982)

The factor of safety computed using this rule serves only as a rough preliminary estimate of the stability of an embankment over a clay foundation and should not be used for final design. The simple equation does not take into consideration factors such as fill strength, strain incompatibility between embankment fill and the underlying foundation soils, and fill slope angle. In addition, it does not identify the location of a critical failure surface. If the factor of safety using the rule-of-thumb equation is less than 2.5, a more sophisticated stability analysis is required (Cheney and Chassie, 1982).

Figure 1.6 shows the variations in safety factor, strength, pore pressures, load, and shear stresses with time for an embankment constructed over a clay deposit. Over time, the excess pore pressure in the clay foundation diminishes, the shear strength of the clay increases, and the factor of safety for slope failure increases.

Embankment fills over soft clay foundations are frequently stronger and stiffer than their foundations. This leads to the possibility that the embankment will crack as the foundation deforms and settles under its own weight and to the possibility of progressive failure because of stress-- strain incompatibility between the embankment and its foundation. Design charts developed by Chirapuntu and Duncan (1977), using finite element method analyses, depict the effects of cracking and progressive failure on the stability of embankments on soft foundations. These charts may be used as a supplement to conventional stability analyses. The use of geosynthetic reinforcement in the fill may prevent the initiation of cracking and subsequent failure in these cases. Alternatively, it may be necessary to remove the soft foundation materials or locate the fill at another site.

Peak strengths of the embankment and the foundation soils cannot be mobilized simultaneously because of stress-- strain incompatibility (Figure 1.7). Hence, a stability analysis performed using peak strengths of soils would overestimate the factor of safety. Many engineers perform stability analyses using soil strengths that are smaller than the peak values to allow for possible progressive failure.

Shale Embankments Embankments constructed of shale materials often have slope stability and settlement problems. According to DiMillio and Strohm (1981), the underlying causes of shale fill slope failures and excessive settlement frequently appear to be:

(1) Deterioration or softening of certain shales over time after construction
(2) Inadequate compaction of the shale fill
(3) Saturation of the shale fill

These types of failures have been found to be typical in many areas from the Appalachian region to the Pacific coast. In general, severe problems with shales in embankments are found in states east of the Mississippi River rather than west of the river (DiMillio and Strohm, 1981). Embankments can use fill originating from shale formations successfully if the borrow source is not particularly prone to long-term decomposition and if adequate compaction and drainage are required. In addition, shale embankments should be keyed into any sloping surfaces by using benches and installing drainage measures to intercept subsurface water that may enter the foundation area. Guidelines for design and construction of shale embankments have been established by Strohm et al. (1978).

1.4.2 Cut Slopes Shallow and deep cuts are important features in any civil engineering project. The aim in a slope design is to determine a height and inclination that is economical and that will remain stable for a reasonable life span. The design is influenced by the purposes of the cut, geological conditions, in situ material properties, seepage pressures, construction methods, and the potential occurrence of natural phenomena such as heavy precipitation, flooding, erosion, freezing, and earthquakes.

Steep cuts often are necessary because of right-of-way and property line constraints. The design must consider measures that will prevent immediate and sudden failure as well as protect the slope over the long term, unless the slope is cut for temporary reasons only. In some situations, cut stability at the end of construction may be a critical design consideration. Conversely, cut slopes, although stable in the short term, can fail many years later without much warning.

To a certain degree, the steepness of a cut slope is a matter of judgment not related to technical factors. Flat cut slopes, which may be stable for an indefinite period, are often uneconomical and impractical. Slopes that are too steep may remain stable only for a short period of time. A failure may pose a danger to life and property at a later date. Failures could involve tremendous inconvenience and the expense of repairs, maintenance, and stabilization measures.

Figure 1.8 shows the general variations of factor of safety, strength, excess pore pressure, load, and shear stresses over time for a clay cut slope. The initial shear strength is equal to the undrained shear strength on the assumption that no drainage occurs during construction. In contrast to embankment slopes, the pore pressure within the cut increases over time. This increase is accompanied by a swelling of the clay, which results in reduced shear strength. Thus the factor of safety decreases over time until an unstable condition is reached. This, for the most part, explains why clayey cut slopes sometimes fail a long time after initial excavation.

For cuts in overconsolidated clays, the in situ shear strength is a direct function of the maximum past overburden pressure. The higher the maximum past overburden pressure, the greater the shear strength. However, if the clay is subjected to long-term unloading conditions (permanent cuts), the strength of the clay no longer depends on the prior loading. The strength of a cut slope will decrease with time. The loss in strength is attributed to reduction of negative pore pressure after excavation. This loss in strength has been observed to be a time-dependent function related to the rate of dissipation of negative pore pressure.

In practice, the loss in strength after cuts are made is not easily determined. According to McGuffey (1982), the time dependency of clay cut slope failures can be hypothesized to be a function of the Terzaghi hydrodynamic lag model. The estimated time to failure can be expressed as (McGuffey, 1982)

This model was used with some success by McGuffey (1982) to determine and back-analyze the time for stress release leading to slope failures in clay cuts in New York.

Long-term cut slope stability is also dependent on seepage forces and, therefore, on the ultimate groundwater level in the slope. After excavation, the free-water surface will usually drop slowly to a stable zone at a variable depth below the new cut surface. This drawdown usually occurs rapidly in cut slopes made in sand but is usually much slower in clay cut slopes. Although typical rates and shapes of groundwater draw-down curves have been proposed for cut slopes, none has proved useful for correctly predicting the time or rate of drawdown of preconsolidated clays. The main obstacle to such prediction comes from the difficulty in correctly modeling the recharge of the area in the vicinity of the cut slope.

1.4.3 Landfills

Landfills are a special case where both cut and fill slopes are involved (Figure 1.9) and where the fill materials are much less than optimum. To make matters worse, except for very old landfills, zones of clay barriers and, more recently, geosynthetic barriers (Figure 1.10) are placed between the fill materials and the natural ground, creating an extra zone that must be characterized and analyzed with respect to short-term and long-term stability. Also, the plethora of environmental regulations that are imposed on existing and new landfills places an extremely heavy burden on the engineers and geologists to accurately characterize and analyze the short-term and long-term behavior of landfills for the life of the project and beyond.

Landfills may contain organic materials, tree limbs, refuse, and a variety of debris that are commonly dumped, pushed, and spread by bulldozers, and then compacted by refuse compactors. Compaction of landfills is somewhat different from the compaction of soils, particularly with respect to crushing. Compaction crushes (collapses) hollowparticles, such as drums, cartons, pipes, and appliances, and brings the crushed particles closer together. It may be expected that landfills, compacted at the top only, will be relatively loose. Landfill materials are commonly soft, and large voids can be encountered.

The evaluation of landfill slope stability is similar to the analysis of other types of slope stability problems. Selection of proper values for the strengths of the waste and foundation materials, and of proper shearing resistances along the interfaces within the liner and cover systems, is the most critical part of any stability study. The greatest difficulties and uncertainties are associated with evaluation of the strength and stress-- strain properties of the liner system materials and interfaces, and of the waste fill.

Very little is known about the geotechnical engineering properties of landfills. The paucity of data results in part from the difficulties in credible sampling and testing of refuse. This difficulty is further compounded by the fact that refuse composition and properties are likely to change erratically within a landfill and are also likely to decompose with time.

The unit weight of landfill is highly dependent on a number of factors, including initial composition, compactive effort, decomposition, settlement, and moisture content. Although the density of the landfill will typically increase with depth, considerable variability should be expected within relatively short distances (Landva and Clark, 1987).

The moisture content of landfill materials is also dependent on a number of interrelated factors, such as the initial composition, landfill operating procedures, the effectiveness of any leachate collection and removal systems, the amount of moisture generated by biological processes within the landfill, and the amount of moisture removed with landfill gas (Mitchell and Mitchell, 1992). The water content of the landfills in the United States ranges from 10 to 50 percent.

Shear strengths of the landfills may be estimated by means of:

(1) Laboratory testing
(2) Back-calculation from field tests and operational records
(3) In situ testing

Laboratory samples are usually reconstituted from landfills before they are tested. Direct shear tests have been commonly used to determine shear strength parameters of the landfill materials. Back-calculation of an existing landfill based on field load tests also can be made to estimate the shear strengths of the landfill (Converse et al., 1975). However, the back-calculated strengths are usually conservative by an unknown amount because the back-calculation assumes failure of the slope (that is, a factor of safety equal to 1). Vane shear and standard penetration tests have been used to estimate the shear strength of refuse in a landfill near Los Angeles, California (Earth Technology Corporation, 1988). The shear strength data obtained by these in situ testings may not be representative of the actual conditions because both the vane shear device and the standard penetration sampler are small compared with the inclusions (for example, tires, wood, carpet) that make up the landfill.

True cohesion or bonding between particles is unlikely in landfills. However, there may be a significant cohesion intercept that results from interlocking and overlapping of the landfill constituents. This interpretation is supported by some laboratory test results and the common observation that vertical cuts in landfills can stand, unsupported, to considerable heights (Mitchell and Mitchell, 1992).

Alshunnar (1992) proposed the following design considerations for landfill slopes:

(1) Groundwater conditions before and after construction of the landfill
(2) Subsurface conditions
(3) Construction sequence
(4) Adjacent site conditions and history
(5) Site topography
(6) External loads such as from construction equipment, stockpiles, earthquakes, and so on
(7) Liner geometry and configuration
(8) Filling sequence

It is extremely difficult to reconcile the uncertainties implicit in landfill site characterizations (e. g., landfill material types and characteristics, future land use at and around the site, the probability of severe natural phenomena like earthquakes, tsunamis, sinkholes, etc.) with the guarantees that regulatory agencies require of operators and owners related to siting, operations, and closure. Seed and Bonaparte (1992) stated:

There are some considerable uncertainties with respect to material properties and dynamic response characteristics associated with both: (a) waste fill masses, and (b) base liner systems and final cover systems. As a result, current analysis and design methods are generally based on a sequence of conservative assumptions at various stages of the analyses, with the resulting cumulative level of conservatism generally selected so as to offset the possible impact( s) of uncertainties at each stage of analysis and design. In the opinion of the authors, the compounding of conservative assumptions, both in terms of property/ parameter selection and analysis/ evaluation methodology, at multiple stages during the overall analysis and design process should provide a conservative final result when carefully implemented.

An example of how modern regulations and construction methods of landfills surpassed analytical characterization and design methods is the case of the Kettleman Hills Landfill B-19 Phase IA failure. On March 19, 1988, there was a failure in Landfill B-19, Phase IA at the Kettleman Hills Class I hazardous waste treatment, storage, disposal facility (Byrne et al., 1992). Approximately 580,000 cubic yards of waste and other material had been placed to a height of 90 feet above the base at the time of the failure (Figure 1.11). The entire mass slid a horizontal distance of about 35 feet toward the southeast, and vertical slumps of up to 14 feet along the sideslopes of the landfill were observed after the failure (Figure 1.12). Initial study of the failure indicated that it had most likely occurred within the liner system and identified both geomembrane/ clay and various synthetic/ synthetic interfaces as candidate sliding surfaces. The characteristics of the landfill base and sideslope liners are shown in Figure 1.13. Further study, testing, and two-and three-dimensional slope stability analyses after the failed waste and liner materials were excavated concluded that

(1) The mechanism of failure consisted of slip along multiple interfaces within the landfill liner system.
(2) Low liner interface strengths (residual friction angles as low as 8 degrees) relative to the constructed geometry of the landfill were clearly the underlying cause of the failure.
(3) The predominant surface of sliding during the failure appeared to be the geomembrane/ clay interface of the secondary liner system that apparently behaved in an essentially undrained mode during the approximately one year of waste loading prior to failure.
(4) Undrained shear strength testing of the clay/ geomembrane interface indicated that the shear strength was sensitive to the as-placed moisture and density conditions of the clay.
(5) The calculated factor of safety using a three-dimensional model and residual shear strengths was 0.85. This was consistent with the observed occurrence of large displacements following failure initiation and the attainment of residual strength conditions over the entire slip surface.
(6) The failure demonstrated that specifications for the placement of liner clay must focus not only on achieving specific permeability requirements, but also on developing liner shear strengths that are adequate to support both the interim and final geometric configurations of the landfill.

It should be noted here that three-dimensional analysis was required for this case because of the complex geometry and difficulty in selecting a typical two-dimensional section to analyze. However, as Duncan (1992) states, "The factor of safety calculated using 3D analyses will always be greater than, or equal to, the factor of safety calculated using 2D analyses." This is true for all cuts and fills, not just landfills.

1.4.4 Retaining Structures

Retaining structures are frequently used to support stable or unstable earth masses. The different types of retaining structures, as shown in Figure 1.14, are:

(1) Gravity walls (e. g., masonry, concrete, cantilever, or crib walls)
(2) Tieback or soil-nailed walls
(3) Soldier pile and wooden lagging or sheet pile walls
(4) Mechanically stabilized embankments including geosynthetic and geogrid reinforced walls

Retaining structures are used in seven principal ways, as shown in Figure 1.15. The design of retaining structures requires three primary considerations:

(1) External stability of the soil behind and below the structure
(2) Internal stability of the retained backfill
(3) Structural strength of retaining wall members

1.5 LANDSLIDES

When a slope fails, it is often called a landslide or a slope failure. Several classification methods and systems have been proposed for landslides. The one adopted in this book and most consistently around the world is the one proposed by the International Association of Engineering Geologists (IAEG) Commission on Landslides.

1.5.1 Features and Dimensions of Landslides

Typical features of a landslide are shown schematically in Figure 1.16. The observable features of a landslide are

(1) Crown The practically undisclosed material above the main scarp
(2) Main Scarp A steep surface on the undisturbed ground at the upper edge of the landslide
(3) Top The highest point of contact between the displaced material and main scarp
(4) Head The upper parts of the landslide between the displaced material and main scarp
(5) Minor Scarp A steep surface on the displaced material produced by differential movements
(6) Main Body The part of the displaced material that overlies the surface of rupture
(7) Foot The portion of the landslide that has moved beyond the toe
(8) Tip The point on the toe farthest from the top
(9) Toe The lower margin of the displaced material
(10) Surface of Rupture The surface that forms the lower boundary of the displaced material
(11) Toe of Surface of Rupture The intersection between the lower part of the surface of rupture and the original ground surface
(12) Surface of Separation The original ground surface now overlain by the foot of the landslide
(13) Displaced Material Material displaced from its original position by land-slide movement

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Our reader reviews allow you to share your comments on titles you liked, or didn't, with others. By submitting an online review, you are representing to Barnes & Noble.com that all information contained in your review is original and accurate in all respects, and that the submission of such content by you and the posting of such content by Barnes & Noble.com does not and will not violate the rights of any third party. Please follow the rules below to help ensure that your review can be posted.

Reviews by Our Customers Under the Age of 13

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