9780784412787

SLOPE STABILITY THEN AND NOW

J. Michael Duncan, Ph.D, PE, Distinguished M. ASCE

Professor Emeritus, Virginia Tech, 1600 Carlson Dr., Blacksburg, VA, 24060

Abstract: Many changes in geotechnical engineering for slopes and embankments have taken place since the 1966 and 1992 conferences on this subject (ASCE 1966. 1992). Most of the changes are due to the revolutionary influences of computers in nearly every aspect of the field. Highly significant changes include possibilities for very thorough and detailed evaluations of slope stability and performance, use of 3D stability analyses, finite element and finite difference analyses of slope movements, examination of probability of failure, in situ and laboratory measurement of shear strength, improved tools for slope monitoring, and many new and effective methods of slope stabilization. Although the tools we use are much more sophisticated than in 1966, experience, judgment and thorough quality control remain as important as ever.

FIRST, A LOOK BACK

Geotechnical engineering in general, and slope design in particular, has been fortunate to have been guided by many very talented and insightful engineers and engineering geologists who laid the groundwork on which we are still building. Many of those who made important contributions to the 1966 and 1992 conferences (ASCE 1966, ASCE 1992) are no longer with us. Notable among those in 1966 were:

David Henkel stressed the importance of the geological setting in stability of natural slopes.

Juul Hvorslev emphasized fundamental aspects of clay strength, and stresses in embankments and embankment foundations.

John Lowe described the state of the art with respect to analysis of the stability of embankments.

Ralph Peck discussed the state of the art with respect to the stability of natural slopes.

Harry Seed described analyses of earthquake-induced landslides, and the stability of sloping core earth dams.

Bob Whitman discussed the fundamental mechanics of slope stability and the use of computers for stability analysis.

Stan Wilson described instruments for measuring pore pressures and movements in dams, and their use to investigate embankment behavior during and following construction.

Notable among the leaders at the 1992 conference were: Roger Foott wrote about severe cracking that had been observed in the

Sherman Island (California) levees. Lyman Reese described a method for designing drilled shafts to stabilize

potentially unstable slopes. George Sowers described ten case histories of landslides in natural slopes and

developed a grouping according to their predominant motion.

2191Geo-Congress 2013 ASCE 2013

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Others who showed the way in 1966 and 1992 are still active, continuing to guide the development of the field. We are fortunate that geotechnical engineering has benefitted from the work of so many exceptional engineers and engineering geologists. We should strive to continue in their footsteps.

Judging by the proceedings of the 1966 conference on Stability and Performance of Slopes and Embankments, the principal concerns of the profession at that time were slope stability analysis, shear strength of soil, and monitoring of movements in slopes. Those topics continued to be the subjects of many contributions to the 1992 conference, with growing interest in methods of slope reinforcement and ground improvement. Judging by the titles of abstracts received for this conference, these topics are still at the top of the list. Each of these will be examined here. First, however, it is important to review the changes in computer technology which influence every aspect of the field.

TRANSFORMATION OF THE WORK ENVIRONMENT BY COMPUTERS

The environment we work in today is very different from the way things were in 1966. In 1966, most slope stability calculations were done by hand, using a slide rule, paper and pencil, a process unfamiliar to engineers who began their careers in 1980 or later. The alternative to hand calculations, not available to most practicing geotechnical engineers in 1966, was use of a main frame computer. This was revolutionary, but a far cry from the way we are able to use computers today. Engineers had to develop their own computer programs there were no commercially available programs. The computer program and input data, both on punched cards, were submitted to the operators of a computer center. Alphanumeric output (no graphics) was available hours later. It was expensive as well as inconvenient - $300 per hour for mainframe use which would be over $2,000 per hour today.

In a paper at the first conference on stability and performance of slopes and embankments (ASCE 1966), Whitman and Bailey (1966) described what seemed like an unimaginably convenient way to use these mainframe computers. They wrote:

Let us begin by imagining how we might wish to perform slope stability analyses using a computerAn engineer, with experience in the design and analysis of slopes, seats himself in front of a keyboard

Whitman and Bailey suggested that besides the keyboard, the engineer would have a monitor, a light pen, and a pointing device so that graphical as well as numerical input would be possible. Despite how visionary this was, and how incredibly convenient it seemed at the time, reality has now exceeded the fondest hopes of 1966. Neither Whitman and Bailey nor anyone else could anticipate that computers could be so small, so powerful, and so convenient as they are now.

Consider the contrast between then and now: The IBM 7094 mainframe computer at Berkeley in 1966 was capable of performing 3.6 x 108 floating point calculations in an hour, which, as noted above, cost the equivalent of $2,000 today. A laptop computer, available today for about $500, can perform the same number of floating point calculations in 0.11 seconds, at negligible cost.


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Wide availability of this computer power has transformed the environment in which we work. Computations can be done so rapidly that we become impatient if a process requires as long as a minute. Data can be transmitted at the speed of light over the internet, making it possible to deliver a document with intricately formatted text and color graphics anywhere in the world almost instantaneously. In addition, storage of data has become incredibly efficient. One 4 GB flash drive, weighing 30 grams, holds as much information as 27,000 boxes of punch cards weighing a total of 196 tons.

EVOLUTION OF COMPUTER PROGRAMS FOR SLOPE STABILITY

When mainframe computers became available for use in civil engineering, efforts began to develop computer programs to make use of their power, among them programs for slope stability analysis. Before commercial programs were available, the programs were written by the engineers who used them graduate students and practicing engineers who had a strong interest in being able to use computers as a means of learning about slope stability analysis, or who wanted to use the computer to enhance productivity. Most of the programs were written in FORTRAN, and FORTRAN was taught to engineering students (including civil engineering students) at many universities. When PCs with FORTRAN compilers became available in the early 1980s, some of these programs were adapted for use on personal computers. Kai Wong, working at Berkeley, adapted several computer programs that had been developed by Steve Wright and Guy Lefebvre during their doctoral and post-doctoral studies. These batch-process programs were used by a number of firms and geotechnical engineers for a few years. The programs were crude by todays standards, with typed data files for input and only alphanumeric output, but they were used because there were no better alternatives. The developments at Berkeley were undoubtedly paralleled at many universities, and a large number of slope stability programs were in use at that time, each with its own small group of users.

In the three decades since the advent of personal computers, geotechnical engineers have teamed with professional programmers (or have themselves become sophisticated programmers) to develop commercial programs that take advantage of the speed and capacity of the powerful desktop and laptop computers now available. These present-generation programs provide great analytical power, easy-to-use graphical interfaces, computer-aided drafting and word processing features within the stability analysis programs. Used in conjunction with color printing and essentially instantaneous document transmission over the internet, they provide a range of capabilities not even dreamed about in the mid-1980s, let alone in 1966. Whitman and Bailey were indeed visionary when they described in 1966 how we might wish to perform slope stability analyses using a computer, but reality now exceeds that vision by a wide margin. Today we take for granted what no one could imagine in 1966. It behooves us to use these powerful tools carefully, with a thorough understanding of soil mechanics principles, careful judgment, and a high level of quality control to ensure that results are correct and meaningful.

Many commercially available computer programs for soil slope stability are now available. A few are listed in Table 1, but many more can be found by searching


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online. These programs have a wide variety of the features needed to make them useful and efficient for practical application. My personal experience is limited to a few Clara-W for 3D analyses, GSTABL7 with STEDwin, SLIDE, SLOPE/W, and UTEXAS. An appreciable amount of time is needed to become familiar with any of these programs and efficient in their use, because each has its own style and features. Selecting from among available programs the one or two best suited to a particular engineers needs can be difficult. It would be very helpful if a committee of professionals, or a graduate student supervised by one or more experienced engineers, would undertake a thorough review of some of the available programs, and present the results in a Consumer Reports-like format, to assist engineers faced with selection of programs for analysis of slope stability.

TABLE 1. A Sampling of commercially available computer programs for soil slope stability analysis

1 CHASM 9 SLIDE

2 Clara-W (2D and 3D) 10 Slope 2000 3 GALENA 11 SLOPE/W

4 Geo-Tec B 12 Stable for Windows

5 GGU-Stability 13 STABLPRO

6 GSLOPE 14 SVSLOPE (2D and 3D) 7 GSTABL7 with STEDwin 15 TSLOPE (2D and 3D) 8 LimitState:GEO 16 UTEXAS3

Links to these programs and others can be found at geotechnicaldirectory.com Each program has a web page that describes capabilities and availability.

Validity of the results of an analysis is of course the responsibility of the user. No matter which program is used, it is possible that the results will be incorrect if the program is applied incorrectly. With complex programs that have a great number of options, this is always possible. Steve Wright recommends that analyses should be done using two or more computer programs, because the computations are so complex that it is virtually impossible to check results thoroughly in any other way. During the investigation of flood wall failures in New Orleans after Hurricane Katrina, Tom Brandon, Steve Wright, Ron Wahl and Noah Vroman and I used both SLIDE and UTEXAS to perform analyses and confirm results. In the process, we found that neither program had the features necessary to perform analyses for the condition where a crack was formed down the back of a floodwall by hydraulic fracturing. Later investigation showed that SLOPE/W also had to be modified for this previously unforeseen condition. All three programs have now been modified to deal with this type of loading correctly. Had the analyses been performed using a single program, the results might have been erroneously judged correct, with serious consequences for the results of the investigation.


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One of my most interesting discoveries while writing this paper has been the potential of spreadsheets for complex analyses, including searching for critical circular and non-circular slip surface, and probabilistic analyses using the Hasofer Lind method and Monte Carlo simulations (Low 2003; Low and Tang 2004, 2007). Such analyses require that the user be adept at Visual Basic programming to describe complex conditions, and are thus not suitable for general use. However, it is clear that spreadsheets have great power, and may provide computational engines for future generations of user-friendly programs.

Fast computers with large memory and storage have made possible types of analyses that were not possible using hand calculation. Among these are finite element analyses, three-dimensional analyses, and probabilistic analyses, discussed in the following paragraphs.

FINITE ELEMENT ANALYSES

Strength Reduction Method of Stability Analysis. Slope stability analyses can be performed using the finite element method or the finite difference method, by means of the strength reduction technique described by Griffiths and Lane (1999). The soil is modeled as an elastic-perfectly plastic material. Gravity stresses are established within the finite element model of the slope by the gravity turn-on method, where the full force of gravity is applied in one increment. The factor of safety against slope instability is determined by gradually reducing the strength of the soil. As this is done, deformations occur, and at some value of the strength reduction factor, the deformations become very large, and the numerical process will no longer converge. This numerical instability is interpreted as physical instability, and the value of the strength reduction factor when this happens is the factor of safety the factor by which the strength must be reduced to bring the slope to a state of barely stable equilibrium. This is the same definition of factor of safety used in limit equilibrium analyses of slope stability.

An example using the strength reduction method for a hypothetical slope is shown in Figure 1, after Griffiths and Lane (1999). The example shows a rapid increase in displacement as the strength reduction factor approached 1.38, the minimum factor of safety determined by limit equilibrium analysis. The two sets of strength reduction data show the results with and without a compressible foundation layer. The nodal point displacement vectors at the right side of Figure 1 show a failure mechanism that is shaped much like a circular rupture surface.

While it may seem like overkill to use finite element analyses to calculate the factor of safety against instability of a slope, the strength reduction method does have one significant advantage as compared to limit equilibrium analysis: It is not necessary to specify the position or shape of the slip surface. As shown subsequently, this is particularly advantageous for 3D analyses.

Incremental construction of the slope can be modeled. However, with assumed linear elastic soil stress-strain properties, the stresses computed by gravity turn-on or incremental construction are the same. The calculated displacements are not the same, however. If meaningful displacements as well as factor of safety are desired,


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the analysis should model the actual sequence of fill placement or excavation involved in construction of the slope.

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Strength Reduction Factor

Limit Equilibrium F = 1.38

Compressible Foundation

Rigid FoundationNodal point displacement vectors

SRF = 1.4

FIG. 1. Variation of displacement magnitude with strength reduction factor and nodal point displacement vectors for a hypothetical slope.

(After Griffiths and Lane, 1999)

A number of commercially available finite element programs provide options for slope stability analysis using the strength reduction method, as well as for analysis of stresses and movements in earth masses and soil-structure interaction problems. Among these are PLAXIS, SIGMA/W, Phase2 and a number of academic programs. FLAC 7, a finite difference program, also has this capability. These and many other finite element programs can be found through geotechnicaldirectory.com.

Finite Element Analysis of Progressive Failure. Finite element analysis can also be used to study the possibility of progressive failure. As shown in Figure 2, from Filz, et al. (2001), shearing resistance of a geomembrane liner beneath a landfill increases with displacement up to a peak, then decreases with further displacement.

FIG. 2. Progressive failure of waste impoundment on a geomembrane liner. (Filz, et al. 2001)

With this type of shearing resistance, it is not possible to mobilize the peak strength simultaneously at all points along the liner because displacements vary from point to point and with time during placement of the waste fill. By means of finite element analyses that modeled the nonlinear shear stress-displacement curves from


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laboratory interface shear tests, Filz et al. were able to track progressive development of failure as shear occurred between the waste fill and the liner, and at interfaces within the composite liner. Applying this method to the Kettleman Hills landfill, it was concluded that the average shearing resistance at the time when the waste fill became unstable would have been considerably less than the peak shearing resistance, and only very slightly larger than the residual shearing resistance.

Finite Element Analysis of Reinforced Slopes. If the stress-strain properties used in finite element analyses are realistic, and the sequence of events involved in construction and evolution of the slope are modeled realistically, the calculated movements can be expected to be meaningful approximations of the actual movements in the field. Such analyses can be useful in a number of ways, including modeling the performance of reinforced slopes. Limit equilibrium analyses can be used to determine the increase in factor of safety of a slope achievable with a reinforcement force of a given magnitude, but they do not show what force will actually be developed in the reinforcement. Unless the reinforcement is prestressed, the force in reinforcement depends entirely on the movements of the slope, and the consequent strains that occur in the reinforcement. Determining the magnitude of the force that will develop under working conditions requires a soil-structure interaction analysis finite element or finite difference.

A soil-structure interaction analysis was performed as part of the design studies for Mohicanville Dike No. 2, shown in Figure 3. The dike, which was constructed on a very weak foundation, was reinforced with a steel reinforcing mat at the bottom of the embankment (Franks, et al., 1988). As the embankment was constructed on top of the steel mat, and settled on the weak foundation, the embankment and foundation tended to spread laterally, inducing tension in the reinforcement. The resulting reinforcement force stabilized the embankment. Figure 4 shows the calculated and measured tensile forces in the steel reinforcing mat.

FIG 3. Mohicanville Dike No. 2, reinforced with a steel mat at the base of the embankment. (Franks, et al., 1988)


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FIG. 4. Mohicanville Dike No. 2 reinforcement forces. (Franks, et al., 1988)

3D ANALYSES OF STABILITY

3D analyses are needed where a three-dimensional failure mechanism may have a lower factor of safety than the usual 2D failure mechanism. An example is the Kettleman Hills waste landfill, shown in Figure 5. A slope failure in 1988 (Seed, et al. 1990) occurred by sliding along weak interfaces in the composite liner that separated the waste from the ground beneath. Figure 6 shows factors of safety

FIG. 5. Plan view of Kettleman Hills waste landfill (Seed, et al. 1990)


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computed for several cross sections through the waste fill. Normally, for design, only the maximum section would be analyzed, because in most cases the maximum section has the lowest factor of safety. However, this is not true for the Kettleman Hills landfill geometry. It can be seen that the factors of safety for sections K, J, and H are lower than for the maximum section, B or G. The failure mechanism was three-dimensional and the position of the failure surface was known, with the mass moving as shown by the arrows in Figure 5, by sliding within the relatively weak lining. The actual value of factor of safety, F=1.0 at failure, is not represented by any single 2D section, and computing it requires consideration of the equilibrium of the mass as a whole. Seed, et al. (1990) performed what they described as a multiblock analysis, considering the equilibrium of various blocks within the mass, and their interaction. This approach resulted in a factor of safety equal to 0.96, close enough to unity to confirm that the three-dimensional interaction between the various section of the mass were an important factor in this case.

FIG. 6. Factors of safety for six 2D sections through the Kettleman Hills waste landfill (Seed, et al. 1990)

Figures 7 and 8 show an example of a 3D finite element analysis used to determine the factor of safety of a curved section of the Wolf Creek Dam embankment slope by the strength reduction method. The Wolf Creek Dam embankment wraps around the end of the concrete section of the dam, as shown in Figure 7. The slope at the end of the embankment is curved, and a 2D stability analysis does not represent the conditions there. To evaluate the effect of the curvature of the slope on the factor of safety, Jeremic (2010) used the strength reduction method for a 3D finite element analysis of the wraparound section. The 3D finite element mesh shown in Figure 8 was used to model the curved section of the embankment, and a plane strain mesh (shown grey) was used to model the straight section.


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FIG. 7. Wolf Creek Dam embankment.

FIG. 8. 3D finite element mesh for straight and wraparound sections of the Wolf Creek Dam embankment.

Comparison of the critical strength reduction factors for the straight and the wraparound sections showed that the factor of safety for the wraparound section was only 6 percent lower than for the straight section, indicating that the curvature of the embankment in the wrap-around section did not have a significant effect on stability. The advantage of the strength reduction method was that it was unnecessary to describe and analyze a 3D slip surface, which is problematical unless the shape of the slip surface is known a priori, as at Kettleman Hills.

PROBABILISTIC ANALYSES

Probabilistic analyses provide a useful supplement to factors of safety against slope instability. Because these analyses reflect the effects of uncertainties in the information used in stability analyses, they provide a viewpoint on safety that can be helpful to geotechnical engineers and their clients. Computing both factor of safety and probability of failure is better than computing either one alone. Much experience


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is available regarding acceptable factors of safety for slopes, and this experience is useful for judging safety. Estimating probability of failure (or its counterpart, reliability) provides a measure of the uncertainty involved in factor of safety calculations, which enhances our ability to judge their significance.

Vanmarcke (1977) described the fundamental basis for these analyses, and Baecher and Christian published their landmark book on reliability and statistics in geotechnical engineering in 2003. As the value of probabilistic analyses for use in practice has become evident, probabilistic methods have been applied with increasing frequency to practical slope stability problems (Christian, et al. 1994; Duncan 2000; Low 2003; Low et al. 2007; Low 2008).

A variety of probabilistic methods have been rendered more practical and transparent, including the Taylor series technique (Wolff 1994; U.S. Army Corps of Engineers 1997, 1998), the first-order reliability method (FORM), the Hasofer-Lind method (Low, 2008), and Monte Carlo simulation, which can be applied to any analysis done in a spreadsheet using the computer program @Risk (Palisades Corporation, 2012). As noted earlier, these analyses have the added benefit of showing the greatest sources of uncertainty in analyses. It is very important that all significant sources of uncertainty be represented in the analyses.

SHEAR SRENGTH

It has often been said that the three most important things in slope stability analysis are shear strength, shear strength and shear strength. Other things are also important of course, but there is no possibility of correctly assessing the stability of a slope if the shear strength is not evaluated correctly, no matter what type of analysis is performed.

As described by Tom Brandon in his keynote paper to this conference (Brandon 2013), computers have changed testing methods in many ways. Laboratory tests are almost always computer controlled, and both laboratory and in situ test data is almost always collected by automatic data acquisition systems. The result is a reduction in personnel costs, more complete data, and greater efficiency because the automated equipment can operate around the clock. Although testing efficiency and accuracy have improved, the usefulness of the data still depends on sample quality are the samples representative of field conditions, and as nearly undisturbed as possible?

Along with the changes in testing methods that have occurred since the 1960s, there is growing appreciation and improved understanding of the importance of fully softened strength. As described by Terzaghi (1936), and by Skempton (1964, 1970, 1977) and his colleagues, strength loss occurs over years or decades in fissured clays that is not reflected in relatively short term laboratory tests on undisturbed samples. Kayyal and Wright (1991) found that softening also occurs in highly plastic clay embankments where desiccation cracks develop due to cyclic wetting and drying. The gradual softening that occurs is caused by influx of water through the fissures or cracks. Through this process, the strength of the clay is eventually reduced to, or close to, the normally consolidated strength. In cases where softening occurs, tests on undisturbed samples overestimate the long-term strength in the field, and the strength for the fully softened condition is best evaluated by performing tests on remolded


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normally consolidated samples. The applicability, measurement and use of fully softened shear strength are discussed in a paper to this conference (Duncan et al. 2013).

There is also a growing realization that the variation of soil shear strength with effective normal stress is often nonlinear, and a number of slope stability analysis computer programs provide capabilities for using curved strength envelopes in slope stability analyses. The result is more realistic representation of soil shear strength characteristics, especially at low confining pressures, and improved agreement between the positions and shapes of calculated critical slip surfaces and those observed in the field.

INSTRUMENTATION AND SLOPE MONITORING

Stan Wilson, in his paper Investigation of Embankment Performance (Wilson, 1966) listed the purposes for slope monitoring and the instruments in use at that time. The reasons for monitoring slope performance (check behavior during construction, monitor the performance of the completed structure, and obtain basic information for future projects) remain the same today. What has changed significantly as a result of new technology are the instruments being used to monitor performance, and the methods used for collecting, storing, processing, and displaying the data they collect.

The inclinometer developed by Wilson (Wilson, 1962) is still the most frequently used means of monitoring subsurface movements. The operation of inclinometers has been speeded up through the use of data loggers and improvements in the hardware, but obtaining readings is still quite labor intensive. In-place inclinometers (IPIs), available from Durham Geo Slope Indicator (dgsi.info) and others, can be read automatically, reducing labor costs. Shaped Accelerometer Arrays (SSAs), available from Measurand, Inc. (measurand.com), can also be read automatically. SSAs consist of a string of accelerometers spaced at 30 cm or 50 cm along a cable. Both IPIs and SSAs are relatively expensive because more hardware is required, and because the hardware must be dedicated to a single hole (Mikkelsen 2012). The advantages are that readings can be obtained rapidly, and that labor costs are lower.

Vibrating wire piezometers, now used widely, have a number of advantages over earlier types of piezometers. They monitor water pressures with extremely small movement of the pressure diaphragm. As a result, they can be grouted in place and they equilibrate almost instantaneously (Contreras, et al. 2012). The data can be collected automatically on a data logger, or can be transmitted by cellular telephone or other telemetry, and accessed over the internet. Engineers can thus monitor pore water pressures in real time from their office (Mikkelsen 2012).

Surface movements can be monitored using GPS (global positioning system) technology with horizontal accuracy of about 1 mm and vertical accuracy of about 1.5 mm (Bond and Nyren 2012; Shimizu et al. 2012); accuracies of 10 mm vertical and 15 mm horizontal are routinely available. Computerized total stations are able to provide about the same accuracy for points as far as 500 meters apart. Surface movements can be measured with millimeter or sub-millimeter accuracy using inSAR (interferometric synthetic aperture radar). Figure 9 shows this equipment being used to measure the movements within an area of the wall of an open pit mine in South


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Africa. No targets are required for the observations, and movements smaller than a millimeter can be detected (Little 2011).

FIG. 9. Sandsloot open pit mine in South Africa, and mobile radar scanning antenna for inSAR measurements of slope movements.

(M. J. Little, 2011)

SLOPE STABILIZATION

In 1966 consultants and owners were reluctant to accept new technologies for ground improvement or slope stabilization, but the climate is more open to innovation now. MSE walls, soil nailing, light-weight foam fill, column-supported embankments, and reinforced embankments have been used on many projects. Consultants and owners are often receptive to these new methods when they are proposed by contractors they view as reliable, and when construction quality will be thoroughly verified by a comprehensive quality control program. Computers and instrumentation provide data in real time to verify procedures and make comprehensive quality control possible. The climate for acceptance of new methods is also enhanced by design-build contracts, which put the responsibility for success of innovation in the hands of the people who will do the work (Sehn 2012).

In times past, knowledge about and adoption of innovative techniques was usually driven by an individual or a group personally familiar with the techniques, who could judge their suitability for a particular project. A recent SHRP2 (Strategic Highway Research Program 2) project has made information about a wide range of techniques more widely available by organizing a great deal of information about ground improvement techniques (including slope stabilization) in a very convenient computer-based form (Schaefer et al. 2012). This interactive system which contains information for 46 ground improvement techniques, has been called "a comprehensive web-based information and guidance system for ground improvement." Engineers can use the information system: (1) by reviewing a catalogue of the 46 technologies, or (2) by using an interactive system, which identifies the technologies that are most suitable based on the users description of project characteristics. For each technology, the system provides these types of information:

Technology Fact Sheets Photographs Case Histories


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Design Procedures Quality Control/Quality Assurance Procedures Cost Estimating Specifications Bibliography

Having these resources available more or less instantly, at the click of a mouse, saves a great deal of research time. Final selection of a method and final design can proceed quickly with this information readily available. Nonetheless, proper use of this new resource still requires careful evaluation by experienced geotechnical engineers with full appreciation of the actual conditions unique to each specific project and problem.

The potential for efficient use of engineering resources is clear. It seems likely that this innovative system provides a model that could be useful for geotechnical applications beyond the transportation projects for which it was developed.

IMPROVEMENTS IN GEOTECHNICAL ENGINEERING PRACTICE

Improvements in geotechnical engineering practice do not arise automatically from the availability and application of the advances in technology discussed in the preceding pages. Achieving high quality in geotechnical engineering has always required experience, judgment, and comprehensive quality control. It is my belief that when these basic requirements are met, appropriate use of new technologies does lead to improvements in practice.

However, if these basic requirements are not met, problems can arise in even simple projects. The experience at Silver Lake Dam serves as an example.

The spillway capacity of the Silver Lake Dam, in the Upper Peninsula of Michigan, was increased in 2003 to accommodate the probable maximum flood (FERC 2003). To achieve the required discharge capacity, the fuse plug embankment shown in Figure 10 was constructed on the rim of the reservoir. The fuse plug was designed to erode away within about an hour when it was overtopped, providing an additional emergency spillway 250 feet wide and five feet high. The embankment

FIG.10. Silver Lake fuse plug embankment at end of construction, October 8, 2002


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was designed and constructed using a standard cross section developed and tested by the U. S. Bureau of Reclamation.

The embankment shown in Figure 10 was only five feet high. It had been constructed using imported materials that were carefully graded and compacted. The embankment cross section had been tested in the Bureau of Reclamation laboratory and used successfully on other projects requiring fuse plugs. What could go wrong?

Through an error, the elevation of the fuse plug pilot channel (the elevation at which erosion would begin) was designed too low, and the fuse plug embankment was washed away less than year after it was constructed, by a flood far smaller than the probable maximum flood. Furthermore, the fuse plug embankment was constructed on an erodible sand foundation rather than non-erodible rock or concrete. As a result, erosion continued for 25 feet below the base of the fuse plug embankment, releasing virtually the entire reservoir. Consequences included temporary evacuation of about 1800 people from the city of Marquette and about $100 million in damages (ASDSO 2003). Fortunately, no lives were lost.

This case shows that even on a simple project, success can only be assured through judgment, experience and diligent quality control. Omission of any of these can lead to problems.

SUMMARY

Methods of geotechnical engineering for slopes have been transformed since the first ASCE conference on stability and performance of slopes and embankments in 1966, largely because computers have changed so many aspects of geotechnical engineering and geo-construction. While the new tools available for shear strength evaluation, computation, communication, construction, and monitoring have revolutionized the way we work, the need for judgment and the value of experience have not diminished.

Computer programs for slope stability analysis have been developed that can perform analyses and provide results in figures of professional report-quality, in very little time. However, as pointed out many times by Steve Wright, results should not be accepted at face value. They should be checked thoroughly. Because the computer programs now available are so complex, it is virtually impossible to check the results using hand calculations. The only feasible way of checking the results is by using a second computer program to analyze the problem. Wright has shown the value of thorough checking through many examples.

Finite element and finite difference analyses are finding use for several types of analyses of slopes. They can be used to evaluate factors of safety against instability through the strength reduction method, which defines the factor of safety in terms of shear strength the same definition of safety factor used in limit equilibrium analyses. The advantage of the strength reduction procedure is that it is not necessary to prescribe the position or shape of the rupture surface it develops naturally as the shear strength is reduced.

Finite element and finite difference analyses can also be used to estimate displacements in slopes and embankments. If the stress-strain properties assigned to the soil are realistic, and if the sequence of events involved in construction and


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evolution of the slope are modeled realistically, the calculated movements are meaningful approximations of the actual movements in the field. Such analyses can be useful in a number of ways, including modeling the performance of reinforced slopes.

Finite element and finite difference analyses are also capable of tracking changes of stress and strain within a slope that can lead to progressive failure in strain-softening materials. Analyses of this type showed that the shearing resistance that could be mobilized within the liner at the Kettlemen Hills Landfill was only marginally greater than the residual strength because of the nature of the displacements that developed as the waste was placed in the landfill.

Finite element and finite difference analyses provide an effective means for three-dimensional analyses of slope stability because it is unnecessary to locate the critical rupture surface by trial and error. Analyses of this type showed that the factor of safety for the wraparound section of the Wolf Creek Dam embankment was only slightly lower than that for the adjacent straight section of the embankment.

The ability to perform many analyses very quickly has made probabilistic analyses feasible for use in practice. Probabilistic analyses provide a supplement to deterministic analyses of stability, and indicate the most important sources of uncertainty in stability calculations. A variety of analytical methods have been developed, including the Taylor series technique, the Hasofer-Lind method, and Monte Carlo simulation.

Methods of measuring and representing the shear strength of soils have advanced, notably through greater appreciation of the importance of fully softened strength, and the use of curved shear strength envelopes to characterize soil strengths more accurately. Laboratory tests to evaluate shear strength can be performed more quickly, and with less manual labor, using new computer-controlled equipment and data acquisition systems. Although testing efficiency and accuracy have improved, the usefulness of the data still depends on selecting or preparing samples that are representative of field conditions and that are as little affected by disturbance as possible. In situ tests have been made more accurate and efficient through use of electronic data acquisition systems.

Field monitoring has been made more efficient and more accurate through use of computer-controlled instruments. Computerized total stations, GPS, and inSAR provide position data with accuracy as refined as 1 mm horizontally and 1.5 mm vertically, with very little labor required for the measurements. Vibrating wire piezometers provide fast response, accurate pressure readings, and capabilities for automatic recording and transmission of data.

Innovative techniques for slope stabilization and ground improvement are now used widely, aided by more receptive consultants and owners, more thorough quality control procedures, and more frequent use of design-build contracts. A comprehensive web-based information and guidance system for ground improvement developed under a SHRP2 research project provides practically immediate access to information on 46 ground improvement procedures (Schaefer, et al. 2012). This online system is a model for efficient use of geotechnical engineering knowledge that has potential for use in other areas of geotechnical engineering.


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Computers have revolutionized geotechnical engineering for slopes in many ways, freeing engineers to focus on the issues that control stability and performance. When results are achievable so quickly, and when they are presented in such polished-looking forms, it is more important than ever to remember that the most important thing to know about any piece of information is that it may be wrong. Geotechnical engineers need to polish their quality control skills to ensure that the power of computers improves the quality of their work.

ACKNOWLEDGEMENTS

The writer is indebted to many people who have provided valuable assistance with preparation of this paper, and wise counsel over many years. Matthew Sleep, who was a post-doctoral student at Virginia Tech when this paper was written, was very helpful in the literature review. Al Sehn helped the writer to understand the state of practice in geo-construction, and the reasons that the profession is more open to innovation than it was in 1966. Erik Mikkelsen helped the writer understand how the instruments for measuring movements and pore pressures have been changed by computer technology, and how these new tools have led to more efficient methods for monitoring slope performance. Robert Werner was very helpful in explaining the use of shaped accelerometer arrays in in-place inclinometers, and by providing a very instructive example of the use of this new technology. George Filz, Vern Schaefer, and Jim Mitchell gave generously of their time to provide access to, and explain the function of the web-based information and guidance system for ground improvement. They were also very helpful by providing their views on the state of the art and state of practice in slope stabilization and ground improvement, and on the state of practice for slope stability. Jim Mitchell also made other suggestions that extended the scope and improved the accuracy of the paper. Tom Brandon very generously provided photographs of modern equipment for laboratory and in situ testing for the San Diego lecture on this subject, and explained how the state of practice in strength evaluation is being changed by this new equipment. Bak Kong Low made me aware of the potential of spreadsheets for very complex deterministic and probabilistic analyses. Steve Wright, Kai Wong, Tom Brandon, Dan VandenBerge and Garry Gregory have given generously of their time to provide their thoughts and recollections of the evolution of slope stability analysis, and the capabilities of some of the computer programs that are currently available. Pat Lucia helped me to consider the relationship between improvements in technology and improvements in practice, and encouraged me to address this topic directly. All of these talented, capable and generous colleagues have contributed greatly to this paper. This assistance is much appreciated and is gratefully acknowledged.


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REFERENCES

ASCE (1966). Stability and Performance of Slopes and Embankments, Proceedings of an ASCE Specialty Conference, Berkeley, California.

ASCE (1992) Stability and Performance of Slopes and Embankments-II, Proceedings of an ASCE Specialty Conference, Berkeley, California, Geotechnical Special Publication No. 31.

ASDSO (2003) Association of State Dam Safety Officials Water Webster News Summary 11/10/2003, Community Partners, 1000 North Alameda Street, Suite 240, Los Angeles, CA 90012.

Baecher, G.B. and Christian, J.T. (2003). Reliability and Statistics in Geotechnical Engineering. Wiley: Chichester, UK; Hoboken, NJ, 618 p.

Baecher, G.B. and Christian, J.T. (2008). Spatial variability and geotechnical reliability, Chapter 2 (pages 76-133) of Reliability-Based Design in Geotechnical Engineering- Computations and Applications, Taylor and Francis, ed. K.K.Phoon.

Bond, J. and Nyren, R. (2012). Remote monitoring of deformations using Differential Global Positioning System (D-GPS, Geotechnical News, Vol. 30, No. 2, p. 28, June 2012.

Christian, J.T., Ladd, C.C. and Baecher, G.B. (1994). Reliability applied to slope stability analysis. J. Geotech. Eng. ASCE, 120(12): 2180-2207.

Contreras, I. A., Grosser, A. T., and Ver Strate, R. H. (2012). Update of the fully-grouted method for piezometers installation, Geotechnical News, Vol. 30, No. 2, p. 20-25, June 2012.

Duncan, J. M. and Buchignani, A. L., (1973), "Failure of an Underwater Slope in San Francisco Bay," Journal of the Soil Mechanics and Foundations Division, ASCE, 99, No. SM9, pp. 687-703.

Duncan, J. M., (2013) Impacts of Time on the Performance of Reinforced Slopes, Proceeding of the ASCE Geo-Congress, San Diego.

Duncan, J.M. (2000). Factors of safety and reliability in geotechnical engineering. J. Geotechnical and Geoenvironmental Engrg. 126(4): 307-316.

Duncan, J. M., VandenBerge, D. R., and Brandon, T. L. (2013) Fully Softened Strength of Natural and Compacted Clays for Slope Stability, Proceeding of the ASCE Geo-Congress, San Diego.

Fell, R., Hungr, O., Leroueil, S. and Riemer, W. (2000). Keynote Lecture - Geotechnical engineering of the stability of natural slopes, and cuts and fills in soil, GeoEng 2000, Vol.1, Invited Papers, Technomic Publishing, Lancaster, pp.21-120, ISBN: 1-58716-067-6, November 2000.

FERC Independent Consultant Review Panel (2003) Technical Reasons for the Release of Silver Lake Reservoir on May 14-15, 2003, Report to FERC Division of Dam Safety and Inspections, Washington, DC.

Filz, G. M., Esterhuizen, J. J. B., and Duncan, J. M., (2001). Progressive Failure of Lined Waste Impoundments, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 127, No. 10, October, 2001.

Franks, L. W., Duncan, J. M., Collins S. A., Fowler, J., Peters, J. F., and Schaefer V. R. (1988). "Use of reinforcement at Mohicanville Dike No. 2." Proceedings of the Second International Conference on Case Histories in Geotechnical Engineering, St. Louis, Missouri.


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Griffiths, D. V., and Lane, P. A., (1999), Slope Stability Analysis by Finite Elements, Geotechnique 49, No. 3, pp. 387-403.

Jeremic, B. (2010) Comparison of 2D vs 3D Slope stability for the Wolf Creek Dam, Report to the Nashville District, USACE.

Kayyal, M. K., and Wright, S. G. (1991). Investigation of long-term properties of Paris and Beaumont Clays in earth embankments, Center for Transportation Research, University of Texas at Austin, Austin, 134

Little, M. J. (2011). Slope Monitoring Strategy at PPRust Open Pit Operation, International Symposium on Stability of Rock Slopes in Open Pit Mining and Civil Engineering, Vancouver, pp. 211-230.

Low, B.K. (2003). Practical probabilistic slope stability analysis. Proceedings, Soil and Rock America 2003, Cambridge, Massachusetts, USA, June 22-26, 2003, Verlag Glckauf GmbH Essen, Vol. 2, 2777-2784.

Low, B.K. (2008). Practical reliability approach using spreadsheet, Chapter 3 (pages 134-168) of Reliability-Based Design in Geotechnical Engineering-Computations and Applications, Taylor and Francis, ed. K.K.Phoon.

Low, B. K., and Duncan, J. M., (2013) Testing bias and parametric uncertainty in analyses of a slope failure in San Francisco Bay mud, Proceeding of the ASCE Geo-Congress, San Diego.

Low, B.K., Lacasse, S. and Nadim, F. (2007). Slope reliability analysis accounting for spatial variation. Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards, Taylor & Francis, London, 1(4): 177-189.

Low, B.K., and Wilson H. Tang (2004). Reliability analysis using object-oriented constrained optimization. Structural Safety, Elsevier, Amsterdam, 26(1), 69-89.

Low, B.K. and Wilson H. Tang (2007). Efficient spreadsheet algorithm for first-order reliability method. J. Engineering Mechanics, ASCE, 133(12): 1378-1387.

Mikkelsen, P. E. (2012). Personal communication. Palisades Corporation (2012) @Risk computer program for Monte Carlo Simulation,

Ithaca, New York, online at @Risk. Schaefer, V. R., Mitchell, J. K., Berg, R. R., Filz, G. M., and Douglas, S. C., (2012),

Ground improvement in the 21st century: a comprehensive web-based information system, Geotechnical Special Publication No. 226, ASCE Geo-Institute.

Seed, R. B., Mitchell, J. K., and Seed, H. B. (1990), Kettleman Hills Waste Landfill Slope Failure, II: Stability analyses, Journal of Geotechnical Engineering, Vol. 116, No. 4, April, 1990.

Sehn, A. L. (2012). Personal communication. Shimizu, N., Masunari, T., and Iwasaki, T. (2012). GPS displacement monitoring

system for the precise measuring of rock movements, Proceedings of the 12th ISRM International Congress on Rock Mechanics, pp. 1117-1120.

Skempton, A. W. (1964). Long-term stability of clay slopes, Gotechnique, 14(2), 77-102.

Skempton, A. W. (1970). First-time slides in over-consolidated clays, Geotechnique, 20(3), 320324.

Skempton, A. W. (1977). Slope stability of cuttings in brown London clay, Proceedings of the 9th International Conference on Soil Mechanics and Foundation Engineering, 3, 261-270.


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Terzaghi, K. (1936). Stability of slopes of natural clay, Proceedings of the 1st International Conference of Soil Mechanics and Foundations, 161-165.

U.S. Army Corps of Engineers. (1998). Risk-based analysis in geotechnical engineering for support of planning studies. Engrg. CircularNo. 1110-2-554, Department of the Army, Washington, D.C., ^www.usace.army.mil/usace-docs& (27 Feb. 1998).

Vanmarcke, E.H. (1977). Reliability of earth slopes. J. Geotech. Eng. ASCE, 103(11): 1247-1266.

Whitman, R. V., and Bailey, W. A. (1967). "Use of computers for slope stability analyses." ASCE, Journal of the Soil Mechanics and Foundations Division, Vol. 93, No. SM4, July, 475-498. [Also in Stability and Performance of Slopes and Embankments, Proceedings of an ASCE Specialty Conference, Berkeley, California, August 22-26, 1966, pp. 519-542.]

Wilson, S. D. (1962). The Use of Slope Measuring Devices to Determine Movements im Earth Masses, Field Testing of Soils, ASTM STP 322. pp. 187-197.

Wilson, S. D. (1966). Investigation of Embankment Performance, Proceedings of the ASCE Conference on Stability and Performance of Slopes and Embankments, pp. 153-177.

Wolff, T. F. (1994). Evaluating the reliability of existing levees. Report prepared for U.S. Army Engineer Waterways Experiment Station Geotechnical Laboratory, Vicksburg, Miss.

Wright, S. G. (2013). Application of Computer Software for Slope Stability Computations, ASCE Geo-Congress, San Diego.


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