COMPARISON OF STABILITY ANALYSIS OF SOIL BENCH SLOPE USING LIMIT EQUILIBRIUM TECHNIQUE AND NUMERICAL MODELLING

1. COMPARISON OF STABILITY ANALYSIS OF SOIL BENCH SLOPE USING
LIMIT EQUILIBRIUM TECHNIQUE AND NUMERICAL MODELLING
MINING ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY RAIPUR

2. Outline
1. Introduction
• Factors affecting slope stability
• Types of failure
• Factors to be considered for assessment of stability
• Slope stability assessment
2. Oasys
• Soil movement and stability solutions
• Components of the user interface
• How Oasys works
• Calculation of factor of safety using Oasys
3. FLAC slope
• Steps involved in FLAC slope
• Analysis procedure in brief
4. Problem Statement
5. Comparison of the results
6. Conclusion
7. References

3. • Stability analysis of slopes forms a vital component of various opencast mining
operations throughout the life cycle of the project.
• It is performed to assess the safe design of a soil bench slope and the equilibrium
conditions by calculating factor of safety.
• Assessment of stability can be performed by limit equilibrium technique and
numerical modelling.
• OASYS is based on LEM technique and FLAC SLOPE in numerical modelling.
INTRODUCTION

4. Factors Affecting Slope Stability
Slope failure occurs when the downward movements of material due to gravity and
shear stresses exceeds the shear strength. Therefore, factors that tend to increase the
shear stresses or decrease the shear strength increase the chances of failure of a slope.
Different processes can lead to reduction in the shear strengths of rock mass. Following
factors affect slope stability
• Geology and geological structure
• Ground water
• Lithology
• Dynamic forces
• Method of mining and equipment used
• Angle of internal friction

5. Factors to be Considered in Assessment of
Stability
 Ground Investigation
 Most Critical Failure Surface
 Tension cracks
 Factor of Safety
 Progressive failure
 Pre-existing failure surface

6. Slope Stability Assessment
LEM TECHNIQUE
• For the analysis of stability of a slope, incipient failure along a potential slip
surface is assumed.
• The material above the slip surface is considered to be a free body.
• Equation concerning force equilibrium or moment equilibrium of the potential
sliding mass are formulated.
• The solution provides the estimate of the total force available to resist and to
induce sliding.
• For a sliding surface on which there is an effective normal stress σ acting, the
shear strength τ developed on this surface is given by:
τ = c + σ tan φ

7. Slope Stability Assessment
Numerical Modelling
• Numerical models use numerical time-stepping procedure to obtain the models
behaviour over time.
• It divides the rock mass into zones, each zone is assigned a material model and
properties.
• It can also incorporate geologic features such as faults and ground water,
providing more realistic approximations of behavior of real slopes than analytical
models.
• These models are used to simulate rock slope as well soil slope with complex
conditions.

8. • Numerical methods may be divided into three
approaches:
 Continuum modelling: Discontinuties are
special case.
 Discontinuum modelling: Assemblage of
rigid or deformable blocks of rock.
 Hybrid modelling: Combined analyses
Slope Stability Assessment

9. OASYS
 OASYS has been essentially designed to inspect the slope stability, with an
option to incorporate soil reinforcement.
 It can also be used to study earth pressure and problems related to bearing
capacity.
 The program can examine both circular and non-circular failures, thereby
enabling calculations to be carried out for soil & rock slopes.

10. Soil Movement & Stability Solutions
 The performance of any exaction bellow ground depends on their material
and structural integrity.
 The design of deep excavations and tunnels in crowded cities is a technical
challenge for engineers.
 In these environments, engineers often need to look at the effects both below
and above the ground.
 OASYS provides the tools needed to do this quickly and effectively.

11. Soil Movement & Stability Solutions (continued…)
 Conventional programs and spreadsheets are time-consuming and difficult to
check.
 OASYS is the industry standard, analysing and displaying the results in clear
graphics.
 OASYS offers a quick and accurate way to predict soil displacement due to load.
This program predicts displacements in a soil mass due to vertical and horizontal
loads.
 It provides a robust way to study a slip surface to find factors of safety against
failure, and to check the improvements from reinforcement. The software
performs two-dimensional slope stability analysis to a variety of methods and
presents the results in a clear graphical format.

12. Components of the User Interface
The major components of Slopes’ user interface are-
 The Gateway,
 Table Views, Graphical Output,
 Tabular Output,
 Toolbars, menus and input dialogs.

14. How OASYS works…
 Simple material and strata geometry may be entered to define a
section through the slope. The program then generates a finite
element mesh from this information and groundwater conditions and
slip circles to analyse
 The analysis then calculates pore pressures within the soil mass using
a finite element analysis method, and follows with a slope stability
analysis using the limit equilibrium method by dividing the slip circles
into a number of slices
 The strength of the materials is represented by specifying cohesion
and an angle of shearing resistance and ground water conditions are
set by specifying groundwater boundary conditions.
 Any combination of reinforcement, consisting of horizontal geotextiles
or horizontal or inclined soil nails, rock bolts or ground anchors, can
be specified.

15. Procedure for Finding out the Factor of
Safety
• From the Start menu the program is opened.
• On the Start-up screen select the option to "Create a new data file“.
• General file information is added.
• Select the required Units for data entry and presentation of the calculations via the
Data/Units option from the program menu or via the gateway.
• Select the type of analysis, direction and type of slip via General Parameters

16. • The analysis method and related data are selected
• The material is defined along with their properties.
• Strata are defined & material is assigned to each stratum
• Slip surface data is defined & also center / grid and the radius for circular slips
defined.
• The data is analyzed& a warning/ error messages are shown if the data are
inappropriate.
• After analysis the Print Selection Dialog will be displayed if analysis is successful.
Click OK to see the Tabular Output.
• The Graphical Output View gives a graphical representation of the strata, slips and
grid centers and their results

17. FLAC SLOPE
• The modeling of geo-engineering processes involves special considerations
and a design philosophy different from that followed for design with
fabricated materials.
• Analyses and designs for structures and excavations in or on rocks and soils
must be achieved with relatively little site-specific data, and an awareness
that deformability and strength properties may vary considerably.
• It is possible to use FLAC SLOPE directly in design if sufficient data, as well as
an understanding of material behavior, are available.

18. FLAC SLOPE Contd..
• FLAC SLOPE provides an alternative to traditional “limit
equilibrium” programs to determine factor of safety.
• FLAC SLOPE is a mini-version of FLAC SLOPE that is designed
specifically to perform factor-of-safety calculations for slope
stability analysis.

19. Steps involved in FLAC Slope
STEP – 1
Defining the aim of the model analysis
The level of detail to be included in a model often depends on the purpose of
the analysis.
For example, if the objective is to decide between two conflicting mechanisms
that are proposed to explain the behavior of a system, then a crude model may
be constructed, provided that it allows the mechanisms to occur.

20. STEP – 2
Conceptual picture of the physical system is created
It is important to have a conceptual picture of the problem to provide
an initial estimate of the expected behavior under the imposed
conditions.

21. STEP – 3
Construct and run simple idealized models
When idealizing a physical system for numerical analysis, it is more efficient
to construct and run simple test models first, before building the detailed
model. Simple models should be created at the earliest possible stage in a
project to generate both data and understanding. The results can provide
further insight into the conceptual picture of the system;

22. STEP- 4
Problem-specific data are put together
• Details of the geometry (e.g., profile of underground openings, surface
topography, dam profile, rock/soil structure)
• Locations of geologic structure (e.g., faults, bedding planes, joint sets)
• Material behavior (e.g., elastic/plastic properties, post-failure
behavior)
• Initial conditions(e.g., in-situ state of stress, pore pressures,
saturation)
• External loading(e.g., explosive loading, pressurized cavern)

23. STEP- 5
A series of detailed model runs are prepared
When preparing a set of model runs for calculation, following aspects are needed
to be considered :
• How much time is required to perform each model calculation?
• The state of the model should be saved at several intermediate stages so that
the entire run does not have to be repeated for each parameter variation.
• Are there a sufficient number of monitoring locations in the model to provide
for a clear interpretation of model results and for comparison with physical
data?

24. STEP – 6
Perform the Model Calculations
It is best to first make one or two model runs split into separate sections before
launching a series of complete runs. The runs should be checked at each stage
to ensure that the response is as expected. Once there is assurance that the
model is performing correctly, several data files can be linked together to run a
complete calculation sequence.

26. Analysis Procedure in Brief
• Model stage: Each model in a project is named and listed in a tabbed
bar in the Models stage.
• Build stage: For a specific model, the slope conditions are defined in
the Build stage.
• Solve stage: In the Solve stage, the factor of safety is calculated
• Plot stage: After the solution is complete, several output selections are
available in the Plot stage for displaying the failure surface and
recording the results.

28. Problem
Statement

29. FOS calculation by OASYS

30. FOS calculation by OASYS
Creating the input

31. FOS calculation by OASYS
Analysis Progress

32. FOS calculation using OASYS
Without water table
FOS = 1.674

33. FOS calculation by OASYS
Adding water table
Ground water coordinates

34. FOS calculation using OASYS
Including water table
FOS = 1.644

35. FOS calculation by FLAC SLOPE
Building the model

36. FOS calculation by FLAC SLOPE
Properties input in the Define Material Calculating Factor of safety

37. FOS calculation using
FLAC SLOPE
Without water table

38. FOS calculation by FLAC SLOPE
Adding water table
• Go to the Build stage and click on the Water button. A horizontal line with
square handles is shown in the Water tool.
• A horizontal line with square handles is shown in the Water tool.
• We position this line to match the location of the water table
• we go to the Solve stage, select the medium-grid model and press the Solve
FoS button.

39. FOS calculation using
FLAC SLOPE
Including water table

40. Comparison of Result
Software FOS without
water table
FOS including
water table
OASYS 1.674 1.644
FLAC SLOPE 1.59 1.47

41. Comparison of LEM technique and
Numerical Modelling

42. Conclusion and Interpretation of result
• Factor of safety of soil bench slope bench decreases due to presence of ground water
table which is mainly due to increase in bulk density of bench material.
• Significant differences in factor of safety are obtained between FLAC SLOPE and
OASYS methods in spite of similar input parameters and geometrical condition.
• In case of OASYS, factor of safety obtained is slightly higher as compared to FLAC
SLOPE.
• Stability analysis in numerical modelling gives a more conservative estimate for
factor of safety as compared to LEM technique.

43. Scope of future work
• In major project field data from opencast mines will be collected and slope
stability analysis will be done using FLAC SLOPE and OASYS.
• Along with water table other parameters like effect of geological disturbances,
blasting and cohesion and friction angle will be carried out.
• Results obtained from these analysis will be compared with analysis done using
these software packages as well as with other software based on three
dimensional analysis such as FLAC 3D etc. to compare the sensitivity and utility
of different software.

44. References
• Duncan C. Wyllie and Christopher W. Mah. Rock Slope Engineering Civil and mining
4th edition. Spon Press 270 Madison Avenue, New York, NY 10016.
• OASYS (Slope 19.0) user manual.
• Itasca (2012), FLAC SLOPE User’s Guide (Version 7.0), Minneapolis: ICG
• HAO Fengshan, WANG Lei. Application Study of FLAC in Analysis of Slope Stability,
College of Civil and Traffic, Liaoning Technical University, Fuxin, Liaoning, 2011.
• Christianson Mark. 2012. FLAC SLOPE 7.0 - A Simple Bench Slope Demo. URL:
(https:// www.youtube.com/watch?v=2oDxTriFlGU)
• Goodman, R.E. (1975), Introduction to Rock Mechanics, John Wiley & sons, U.S.A.