2D Limit Equilibrium Slope Stability Analysis.doc

2D Limit Equilibrium Slope Stability Analysis

Slide 5.0 is the most comprehensive slope stability analysis software available, complete with sensitivity, probabilistic and back analysis capabilities. This 2-D program combines an attractive, easy to use CAD based graphical interface with a wide range of modeling and data interpretation options that enable you to perform analyses more thoroughly, more quickly.



Using Slide 5.0, you may determine the probability of failure and reliability index for either the deterministic failure surface with the smallest factor of safety, or for the entire slope. Sensitivity capabilities allow you to easily determine the effect of any parameter on the factor of safety. Determine which parameter has the most effect and optimize your slope remediation based on this knowledge.

Slide 5.0 is the only slope stability software to include built-in steady-state unsaturated groundwater analysis capabilities using the finite-element method. Calculate pore-pressures, heads, discharge using geometry from your slope stability analysis. Meshing is all automatic and done at the click of a button. Pore pressures are seamlessly integrated into your slope stability analysis.

Slide 5.0 easily models complex slope geometry. Draw the interface as you would in any CAD or drawing program or import a scanned image of your slope and digitize over it. Man-made and natural slopes with complex layering, soil lenses, and inclusions are easily modeled. Earth dams with complex core and shell geometries are also easily modeled.

The simple to use editing tools provide a convenient method for performing parametric studies. The graphical data interpreter provides a rich set of tools for the convenient display of model results. With Slide 5.0, you can very quickly and easily create a model, perform a stability analysis, and interpret the results.

What's New in Slide 5.0?
See Slide v5.0 Product Sheet

Features

Documentation

Sensitivity Analysis

In Slide, sensitivity analysis allows the user to determine the "sensitivity" of the safety factor to variation in the input data variables. This is done by varying one variable at a time, while keeping all other variables constant, and plotting a graph of safety factor versus the variable.

Pick sensitivity analysis in the project settings.



Define the range to vary the parameter over. In this case vary the horizontal seismic coefficient between 0 and 0.2.



Run the analysis and plot the distribution of factor of safety with respect to the variable (in this case seismic coefficient).

 

Sensitivity Analysis: Multiple Variables

The user can also generate plots with multiple parameters to determine which has the most influence on the factor of safety. In this case, the horizontal axis will be in terms of Percent Change. The Percent Change is the relative difference between the Minimum value of a variable (0 %) and the Maximum value of a variable (100 %).

Pick the variables to plot.



Sensitivity plot of multiple variables.

 

Sensitivity Analysis: Determine Parameter Value For Defined Factor of Safety

The Sampler option allows you to obtain exact values of points on the sensitivity curves. This can be done graphically or by entering the desired value in a dialog. This allows you to determine an exact parameter value which gives a specified factor of safety (eg. FS = 1.0) or the factor of safety which corresponds to a specified parameter value.

Sensitivity graph with the sampler (the horizontal dotted line). A cohesion value of 33.69KPa will yield a factor of safety equal to 1.0.

 

Probabilistic Analysis

Slide now includes extensive Probabilistic Analysis capabilities for the statistical analysis of slope stability using Monte Carlo or Latin Hypercube simulation techniques. Virtually any input parameter in the model can be defined as a random variable. Any combination of the following can be used as random variables:

1. Material properties (e.g. cohesion, phi, etc…)
2. Support properties (e.g. out-of-plane spacing, anchor capacity, etc…)
3. Seismic Load coefficients
4. Magnitudes of Line loads, and Distributed Loads
5. Location of the water table
6. Location of the tension crack
7. Tension crack properties (e.g. depth of water in the tension crack, etc)

The random variables can be assigned any of the following distributions:

1. Normal
2. Uniform
3. Triangular
4. Beta
5. Exponential
6. Lognormal
7. Gamma

All distributions can be truncated (e.g. minimum and maximum values). Correlation coefficients can be specified (e.g. to correlate cohesion and phi for Mohr-Coulomb materials).

Probabilistic results are displayed using Histogram, Cumulative, Scatter plots. All statistical data can be easily exported to Excel or the clipboard for further analysis.

Slide can perform the probabilistic analysis using two different methods:

1. Assume that the failure surface is the deterministic global minimum
     surface and do the sampling and safety factor calculations only on this
     surface. The probability of failure and reliability index is then calculated for
     this surface.

2. Or, Slide can search for a new global minimum for each set of random
     variable samples -- with this option you can view the "band" of critical
     surfaces, corresponding to different sets of random variables. The
     probability of failure for the slope is determined by taking the number of
     simulations with a global minimum less than one and dividing it by the
     total number of simulations.

Advantages of Probabilistic Analysis

Depending on the uncertainty that may be present in your site conditions, two slopes with the same factor of safety can have different probabilities of failure. Performing a probabilistic analysis will determine the probability of failure for your slope, which will give you a much better representation of the level of safety in your design.

Whether your current work requires you to perform a probabilistic analysis or you simply want more confidence in your design, performing a probabilistic analysis will only improve your slope stability analyses.

For an introduction to the use of probabilistic analysis, including an example of probabilistic analysis being applied to a slope stability investigation, please refer to chapter 8 of Hoek's notes, Factor of safety and probability of failure. Also see the Slide Tutorial Manual - Part 2, for several examples of probabilistic analysis using Slide.


 

Back-Analysis using Probabilistic Analysis

Probabilistic analyses can also be used for performing a back analysis to determine material properties or groundwater conditions. If you have a slope that has already failed, you can use the failure geometry and the implicit factor of safety (<= 1.0, implied by the failure) to determine material properties or groundwater conditions.

You simply specify a large range for the unknown properties (e.g. friction angle), run the analysis, and see where the plot crosses the factor of safety axis at a value of 1.0. For an example of how a probabilistic analysis could be used in this manner, please refer to the following article (published in a 2002 RocNews newsletter):

http://www.rocscience.com/roc/ProjectSpeight.htm

 

Probabilistic Analysis: Monte-Carlo and Latin-Hypercube Simulation

Slide provides two statistical sampling methods - Monte-Carlo and Latin-Hypercube - for Probabilistic analysis. For both methods, you can set the Number of Samples for the probability simulation. Latin-Hypercube sampling allows you to get more accurate results with fewer samples thus greatly improving the speed and accuracy of the solution.

Select the sampling method from the pull-down menu



 

Probabilistic Analysis: Statistical Distributions

In Slide, a wide range of statistical distributions are available for defining the probability density functions of your random variables. These include the statistical distributions which are most commonly used in geotechnical engineering analysis.

To define a parameter as a random variable, first select one of the seven possible choices for Statistical Distribution: normal, uniform, triangular, beta, gamma, exponential or lognormal.

 

Select the Statistical Distribution from the pull-down menu



Once a statistical distribution has been selected, you will then be able to enter the mean, standard deviation (if required) and relative minimum and maximum values. All distributions are automatically checked to ensure unrealistic values (eg. negative) are not used.

Enter the mean and relative minimum and maximum values for the exponential distribution



 

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