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Slide (4.0)
A White Paper describing our fully featured
limit equilibrium analysis program for slope
stability with integrated fi nite-element
groundwater analysis capabilities
Geomechanics
software solutions
used worldwide by
geotechnical
engineers
A synopsis of slope stability analysis with Slide
Rocscience is very pleased to announce the release of the latest version of Slide 4.0, which
includes a new integrated groundwater analysis module, major enhancements to support
modelling, and an improved toolkit for annotating fi gures generated in the software.
Introduction to Slide
Slide is a program for two-dimensional slope stability analysis developed by Rocscience. Slide can be used to design and/or analyze natural slopes or man-made (engineered) slopes such as cuts, embankments and fi lls (including earth dams and retaining structures such as tie-back walls, and soil nail structures),
and waste dumps formed from mining or industrial by-products.
Slide has the ability to analyze both a single user-defi ned non-circular failure surface and to search for the minimum non-circular failure surface. Composite surfaces containing both a circular and non-circular component can also be analyzed.
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Slide has an appealing and easy-to-use graphical interface, and provides a wide range of modelling and data interpretation functionalities. It calculates safety factors for circular and non-circular slope failure surfaces, using a number of widely used limit equilibrium analysis approaches such as the Bishop, Janbu, Spencer, Corp Engineers 1&2, Lowe-Karafi ath, and GLE methods.
Slide 4.0 also contains dozens of minor improvements that make the program more usable. The simple-to-use model building and editing tools in Slide
provide very convenient means for performing the parametric studies that underlie sound geotechnical engineering practice. The graphical data interpreter gives users a rich set of tools for viewing and analyzing model results.
As a result of the improvements to Slide, engineers now have a software package created specifi cally for performing the tasks involved in analyzing and designing slopes. This document lays out the philosophy governing the development of Slide, and the engineering toolkit it offers to slope designers.
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Philosophy of Slide: the engineering of slopes
The analysis and design of slopes are not trivial undertakings. Each of the several steps needed to solve this class of problems can consume signifi cant effort and resources. Consequently, a toolkit that simplifi es the performance of the tasks is of great benefi t to engineers. This is especially true when all those tools are assembled into one software package, and organized in an intuitive manner that refl ects how real-world design is done.
To develop a cutting-edge program intended for practical, day-to-day design of slopes, Rocscience software developers consulted practitioners in order to fully appreciate their needs, and combined that knowledge with the most recent advances in slope stability analysis and in computer technology.
Company engineers understood that it would be possible to create an effective software tool for practical design only if the goals and tasks of geotechnical engineers were properly appreciated.
“The sciences do not try to explain, they hardly even try to interpret, they mainly make models.
By a model is meant a mathematical construct which, with the addition of certain verbal
interpretations, describes observed phenomena. The justifi cation of such a mathematical
construct is solely and precisely that it is expected to work” - John von Neumann
Slide was developed to give practicing engineers the opportunity to focus on engineering, leaving the tedious and mundane tasks to the program. In the design environment facilitated by Slide, engineers can spend greater amounts of time studying results from different parameter combinations and creating alternative solutions.
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Goals and Philosophy of PracticalSlope Design and Analysis
Engineers perform slope stability analysis for a variety of reasons. The aims of slope stability analysis identifi ed from literature [References 1 – 2] and from interaction with practitioners include the following:
Assessment of the stability of slopes under specifi ed conditions
Evaluation of the possibilities of the failure of slopes.
Determination of the infl uence of proposed changes on a slope
Comparisons of the effectiveness of alternative remedial or preventive measures
Sensitivity analyses for evaluating the infl uence of variations in critical parameters such as geometry, material properties and groundwater conditions on the stability of slopes
Analysis of failures that have already occurred. This helps understand failure mechanisms and obtain in-situ material properties.
Design of remedial or preventive measures for slopes, and
Assessment of the effects of exceptional loadings such as earthquakes on slopes and embankments
Since the purpose of modelling in geotechnical engineering is to gain understanding and to explore potential trade-offs and alternatives, rather than to make absolute predictions, it was recognized that a practical slope stability program must combine computational speed with model creation swiftness and ease. Such a product would allow engineers to perform sensitivity analysis – a most informing approach in geotechnical modelling – in which the response of models to changes in parameters and assumptions is studied.
In creating Slide, developers ensured that they adhered to the philosophy and methodology behind good geotechnical modelling. Sound geotechnical modelling encourages a cautious approach in which engineers consider wide-ranging scenarios in ways that lead to new knowledge or fresh understanding. A modelling framework of this nature can alert modellers to phenomena, situations, or outcomes not previously considered. As well it can help engineers identify areas where more fi eld data or information is required.
The purpose of
modelling in
geotechnical
engineering is to
gain understanding
and to explore
potential trade-offs
and alternatives.
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Modelling Features in Slide
The stability of a slope is infl uenced by factors such as geological conditions (soil and rock layers,
discontinuities, groundwater conditions, etc.), material properties, and geometry. As a rule many of
these factors cannot be defi ned with much certainty. This uncertainty means that engineers must
analyze various possible scenarios in order to avoid surprises and unexpected behaviour.
For Slide to be a benefi cial program for
routine practical design, it was necessary
to equip the program with a wide range
of analysis methods and strength models,
and with tools that facilitate quick
creation of models, and easy and fast
modifi cation of model input parameters/
assumptions.
In this section we shall be describing the
different analysis methods, shear strength
models, and some of the principal tools
available in the program.
Analysis Methods
Slide incorporates the most widely used and accepted limit-equilibrium approaches based on the method of
slices.
Ordinary
Bishop Simplifi ed
Janbu Simplifi ed
Janbu Corrected
Spencer
Corp Engineers 1 & 2
Lowe-Karafi ath, and
Generalized Limit Equilibrium Method (GLE)
The methods implemented in the program are the:
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A note on the Use of VariousAnalysis Methods
Because the formulation of the stability of a slope in limit equilibrium terms is a statically indeterminate problem (the number of equations is less than the number of unknowns), all approaches based on the method of slices use simplifying assumptions. These assumptions limit the range of application of the different methods. To make the best use of his/her time and of Slide’s capabilities, it is important for the engineer to understand the varying strengths and limitations inherent in different slope stability formulations.
Table 1. Limit equilibrium slope stability methods and the equilibrium equations they satisfy [1].
Method Satisfaction of Force Equilibrium Satisfaction of Moment Equilibrium
Horizontal Vertical
Ordinary Method No No YesBishop Simplifi ed No Yes YesJanbu Simplifi ed Yes Yes NoJanbu Corrected Yes Yes NoCorps of Engineers 1 & 2 Yes Yes NoLowe and Karafi ath Yes Yes NoGLE Yes Yes YesSpencer Yes Yes Yes
In this sub-section we provide guidelines, obtained from literature, on the use of the various analysis methods.
The limit equilibrium methods implemented in Slide fall into three main categories:
Methods that satisfy force equilibrium equations
Methods that satisfy moment equilibrium equations, and
Methods that satisfy both force and moment equilibrium equations.
Table 1 below provides a summary of the groupings of the methods in Slide.
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The method selected for analyzing a specifi ed slope problem must be suitable for the slope
conditions being analyzed. Table 2 below is a summary of the conditions under which the various
methods are most suitable. They were compiled from references [3 – 4].
Analysis Method Shape of Slip Surfaces Limitations and Applications
Ordinary Only circular slip surfaces
Highly inaccurate for effective stress analysis of fl at slopes with high pore pressures.
Perfectly accurate for φ = 0 analysis.
Quite accurate for total stress analysis for circular slip surfaces.
Method does not have numerical stability problems.
Bishop Simplifi ed Only circular slip surfaces
Except when numerical instabilities arise, accurate for all conditions (gives practically the same answers as the methods that satisfy all equations of equilibrium).
Method is numerical unstable under certain conditions.
If for a specifi c slip circle the factor of safety obtained from the method is smaller than the factor of safety from the Ordinary method, then it can be concluded the Bishop method experienced numerical diffi culties. In that case the result from the Ordinary method is the more accurate.
Force Equilibrium Methods:Lowe and Karafi athCorps of Engineers 1 & 2Janbu Simplifi edJanbu Corrected
Any shape of slip surface Computed factor of safety values are sensitive to the assumed inclinations of side forces.
Can experience numerical instabilities.
For slopes in cohesive soils, when side forces are improperly chosen, these methods may yield factor of safety values that are about a third larger than the ‘correct’ value.
Force and Moment Equilibrium Methods:Morgenstern and PriceSpencer
Any shape of slip surface Can be deemed to provide the correct answers to most practical problems.
Accurate for any conditions except when numerical instabilities occur.
Give about the same answers. Variation in answers is slightly increased in problems involving non-homogeneous materials.
Table 2. Analysis methods, situations in which they are most applicable, and limitations.
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Location of Critical Surfaces
One of the most important facets of slope stability analysis is fi nding the slip surface, which has the lowest factor of safety. The developers of Slide implemented proven search techniques for locating both circular and non-circular slip surfaces. They help engineers to ensure they have indeed determined critical surfaces. The techniques in Slide are as follow:
Circular surface grid search
Single circular surface defi ned by a centre and radius or by three points on the surface
Slope Search method (allows users to defi ne slope parts through which circular slip surfaces must pass)
Auto-refi ne search (an iterative technique for locating the minimum slip circle that uses the results of a previous iteration to narrow the search area in the next step)
Non-circular block search using random surface generation, and
Non-circular path search using random surface generation.
Non-circular block search technique
Circular surface grid search technique
Strength Models
The shear strengths of the materials that form a slope have signifi cant impact on stability and are required for all limit equilibrium methods. The following strength models are found in Slide:
Mohr-Coulomb
Hoek-Brown
Generalized Hoek-Brown
Anisotropic strength
Non-Linear shear-normal functions
Undrained (φ = 0)
Zero strength
Infi nite strength
Vertical Stress Ratio
Barton-Bandis
Power Curve, and
Hyperbolic.
In the majority of practical slope problems, the greatest uncertainty is associated with the evaluation of shear strength parameters. As a result engineers often have to assess the infl uence of various assumed strength models and parameters on stability. To facilitate this process signifi cant effort was expended creating simple, quick-to-use means for entering strength parameters into Slide. This goal was realized through the use of intuitively organized dialogs.
Slide can
accommodate slope
models consisting
of multiple materials
as well as tension
cracks. It can also
read data fi les from
other slope stability
analysis packages
such as Slope/W
and XSTABL.
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Model Creation Tools in Slide
The model creation unit (Modeller) in Slide has undergone numerous improvements to make it easier to create and modify models. Unlike most slope stability programs, Slide does not impose severe restrictions on the types of geometries that can be modelled, nor on the manner in which boundaries have to be defi ned. It can readily accommodate complex slope and material boundary geometries. This fl exibility and generality, attained through the use of soil cell technology, allows Slide to be used to model a very wide range of geometric confi gurations. Soil cell technology is an adaptation and improvement of techniques that have been very successful in fi nite element programs.
The geometry of models is entered into Slide using CAD-based graphical data technology. Such an approach allows for the interactive building and editing of models. Slide can also import model geometries from Autocad™ DXF fi les. Additional tools in Slide that facilitate smooth model creation and modifi cation include undo/redo functions.
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Feature for ParametricAnalysis and Interpretationof Multiple Files
In parametric and sensitivity analysis, model geometry and parameters are varied and results of alternative scenarios compared or analyzed. To facilitate the comparison of multiple fi les, Slide is equipped with the two features listed below: Quick-Zoom menu option that
scales all windows to the same coordinate system (the feature is also very useful for producing consistent screen captures). Data-Tips, which bring up relevant
model information as the mouse is moved over different entities in a model. For example, when the mouse is moved over a material, its properties such as cohesion and friction angle pop-up.
Other improvements made to the Modeller’s interface:
Display of coordinates for entities in a model (this feature is invaluable for checking whether a model has been correctly described, and comparing that model with another) Interactive pie chart that makes
entering anisotropic strength functions intuitive Availability of pop-up menus on the different entities of a model (right-clicking on a grid, for example, brings up a dialog that allows you to change grid spacing, or move the grid) Context-sensitive help for all dialogs
in the Modeller Defi nition of specifi ed circular
slip surfaces using three points along the surface, making it easier to place such circles in a model.
Slide is equipped with
a Quick Zoom menu
option and Data-Tips
to facilitate the
comparison of
multiple fi les
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Loading
The loads acting on a slope play a
signifi cant role in stability. The location,
magnitude and direction of loads can
either increase or reduce stability. Slide
allows users to model line loads, and
uniform and triangular distributed
loads. Through the use of the seismic
acceleration coeffi cient, Slide can also
accommodate the modelling of the
impact of earthquakes on slopes. Users
can analyze both horizontal and vertical
loads imposed by earthquakes.
Support
A variety of support measures are
employed by geotechnical engineers
to stabilize slopes. The distribution of
loads along reinforcement elements
differs from one support type to the
other. To enable engineers to design
support systems by modelling them
and comparing their effectiveness, the
following commonly used reinforcement
types are incorporated into Slide:
Grouted Tiebacks End-Anchored Support Soil Nails GeoTextiles Micro-Piles, and User-Defi ned Support
(for complete fl exibility).
Line loads, uniform and triangular distribution
loads can be easily implemented in Slide
Slide 4.0 introduces major enhancements in
support modelling by introducing
several new support types
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Data Interpretation and Report Tools in Slide
The numerical results from slope stability analyses guide engineers in comparing possible
scenarios and adopting design approaches. Tools, therefore, that help with the visualization and
interpretation of results are of great importance to slope designers.
Maximize the insight that can be gained from results
Uncover underlying trends in slope behaviour
Extract model parameters that have the most impact on stability
Detect any anomalous situations or behaviour
Test underlying assumptions on parameters such as shear strength models and groundwater conditions.
These goals were achieved mainly through the use of cutting-edge graphics technology. The graphics techniques in Slide empower users with signifi cant ability to gain fresh and sometimes unexpected insight into slope behaviour.
Slide interpreter
generates minimum
factor of safety
contours and colour
codes critical slip
surfaces by their
ranges of safety
In designing the Data Interpreter in Slide,developers aimed at incorporating approaches that help:
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Slide Features that FacilitateReport Generation
At the end of a modelling or design exercise, engineers have to produce reports on their fi ndings. To facilitate the report generation, Slide can output professional quality fi gures of a model, and results summaries. It also supplies users with tools for dimensioning models, and a toolkit for adding annotations such as text, arrows, lines, and boxes to drawings of models.
Images from Slide can be copied to the Windows clipboard for pasting into reports. As well images can be saved to fi le formats that can be easily imported into Autocad™. The program has a single-click option for converting colour images into grayscale captures. Slide also has simple-to-use functions for exporting data and plots directly to Microsoft Excel™ or other spreadsheet or word processing programs.
Slide supplies
users with tools
for dimensioning
models and a
toolkit for adding
annotations
The data interpretation features in Slide include contouring and plotting facilities, and ability to view multiple plots on a screen. The Interpreter generates minimum factor of safety contours, and colour codes critical slip surfaces by their ranges of factor of safety values. It also has tools for plotting essential model results such as the normal and shear stress distributions, strength distributions and pore pressures along slip surfaces.
The colour coding of critical slip surfaces helps engineers to not only calculate the minimum critical surface, but also identify all surfaces with critical factor of safety values less than a specifi ed threshold. Such knowledge is required, for example, for the determination of lengths of anchors and other stabilization methods. Slide supplies an innovative tool for displaying all such critical slip surfaces.
The Interpreter
tools were grouped
and organized
in ways that
complement the
natural pattern-
recognition
capabilities of
users.
Groundwater Calculations for Slope Stability Analysis
Groundwater conditions in a slope have a signifi cant infl uence on stability. Groundwater affects
stability through effects such as generation of positive and negative pore pressures that alter
stress conditions, and changes to the bulk density of slope materials.
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Typically, traditional slope stability programs incorporate groundwater into analysis through consideration of a ‘hydrostatic line’. Although simple to implement, the ‘hydrostatic line’ approach is conservative in some cases, especially in cases when unsaturated zones can arise. In layered soil problems, the approach overestimates pore water pressures in
some zones, affecting the calculation of shear strength at the bases of slices.
The latest version of Slide, version 4.0, incorporates an integrated steady-state groundwater analysis module. The module uses the fi nite element method to calculate groundwater fl ows, pressures
and gradients.
Pore water pressure
calculated from the
new module can
be automatically
incorporated in
slope stability
analysis by Slide.
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Also the module relies on the same fi nite element and automatic mesh generation technology implemented in Phase 2 (a geotechnical fi nite element stress analysis program developed by Rocscience).
The groundwater module in Slide offers users several types of boundary conditions. This allows users to test different hypotheses on groundwater conditions in a slope. This ability
to test different alternatives makes it possible for engineers to bracket the extents of the infl uence groundwater has on slope behaviour.
Users can seamlessly incorporate computed pore pressures into slope stability analysis in Slide, since the groundwater module is completely embedded in the Modeller. As an added bonus, the module can be used for general groundwater analysis.
Boundary conditions can be easily set by
picking either segments or nodes
Future Advancements
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These include probabilistic analysis, fi nite element analysis, and additional commonly used support models. Plans are also in place for including the Sarma and Generalized Wedge slope stability analysis methods into Slide.
Probabilistic Analysis
Probabilistic analysis is receiving increasing acceptance in several areas of geotechnical engineering. In slope stability analysis it has been recognized that the factor of safety is not necessarily a good indicator of the probability that a slope would fail. A slope can have a factor of safety higher than that of another but still have higher threat of failure. This is due to the fact that factor of safety analysis utilizes average values of parameters and thus can mask the wider variation (uncertainty) in the values of the different parameters affecting stability. With probabilistic analysis engineers will be able to assess the risks of failure associated with different slope and design options.
In addition to Monte Carlo simulation methods, Rocscience intends to implement state-of-the-art reliability methods in upcoming Slide versions.
Finite Element Analysis
With the use of soil cell technology, and the implementation of an integrated fi nite element groundwater analysis module, the current version of Slide has the necessary building blocks used in fi nite element analysis. This method of analysis enjoys certain advantages over limit equilibrium methods. These include the ability to model stresses and deformations, ability to consider the history of formation of a slope (for both natural slopes and engineered slopes such as embankments), and no prior assumptions on shape or location of failure surfaces. The prediction of stress and deformation conditions in a slope is very useful for determining the conditions under which laboratory tests for slope materials should be made. The magnitude of deformations is also a better predictor of stability and behaviour than factor of safety values.
Rocscience strongly
believes that it is
important to extend
the Slide capabilities
to include stress and
deformation analysis.
In response to requests from users and to advancements in slope stability technology, a number of
techniques have been designated for implementation in future versions of Slide.
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Due to these advantages and the existence in Slide of the building blocks of the fi nite element method, Rocscience strongly believes that it is important to extend the program to stress and deformation analysis. Finite element analysis of slopes can be performed in Phase2, the fi nite element geotechnical modelling package developed by the company. However, as the application of the method to slope stability analysis becomes more common, it will be a natural step to include the method in Slide.
Additional Support Models
To expand Slide’s range of design appli-cations, subsequent versions will include more support models. It is the goal of the program’s developers to equip engi-neers with more design tools, in addition to the existing analysis methods.
Sarma and Generalized WedgeAnalysis Methods
The classical slope stability methods, described earlier in the document, are mostly useful for the analysis and design of soil slopes. In rock slopes, however, failure surfaces commonly pass through
pre-existing discontinuities such as joints and faults, or through weak material seams. There are plans therefore to add to Slide’s repertoire of methods, analysis approaches that accommodate the failure modes encountered in rock slopes. Two methods that have been identifi ed for implementation are the Sarma and Generalized Wedge analysis techniques.
The Sarma method is well suited for the analysis of complex non-circular slope stability problems requiring non-vertical slices. It allows the strength properties of slice boundaries to differ from those of the slice material. As a result of this feature discontinuities and failure planes can be explicitly modelled by the method.
The Generalized Wedge method has attributes similar to those of the Sarma method. It differs however in the way it calculates factor of safety values. The Generalized Wedge method uses an iterative approach to compute the factor of safety from force equilibrium considerations of wedges. It also ensures that moment equilibrium is maintained.
Concluding Remarks
The program was designed to free slope designers from mundane tasks to enable them to focus their creative energies on the actual engineering aspects of the design process. It allows users to achieve a most fundamental goal of geotechnical engineering design – multiple analyses of problems that yield insight and fresh understanding. Slide will continue to evolve along these lines.
Slide will also seek to bring recent advancements in slope stability analysis from the realms of academic research to the forefront of practical and everyday analysis and design. This assures users access to state-of-the-art knowledge that enhances their productivity and the safety and economy of their designs.
In creating Slide, developers have been guided by the needs of the practising engineer and the
tasks they typically perform when designing or analyzing slopes.
References
1. Abramson, L.W., L.S. Thomas, S. Sharma and G.M. Boyce. Slope Stability and Stabilization Methods. 2001. John Wiley & Sons. New York.
2. Chowdury, R.N. Slope Analysis. 1978. Elsevier, New York.
3 Duncan, J.M., 1996, “State of the art: limit equilibrium and fi nite element analysis of slopes,” Journal of Geotechnical Engineering, pp 577-596.
4. Duncan, J.M. and S.G. Wright. 1980. “The accuracy of equilibrium methods of slope stability analysis,” Engineering Geology, vol. 16, pp 5-17.
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