Back Analysis: Support Design

Given a required factor of safety, Slide will now compute the reinforcement load to achieve that factor of safety for all surfaces analyzed. Whether you are required to design for a particular factor of safety or you would simply like a good starting point for your support design, you will find Slide's new back analysis feature saves you lots of time. Using the back analysis feature, coming up with a preliminary design for your support is much faster. Rather than a lengthy, iterative, trial-and-error process, you simply enter a factor of safety (and an elevation to apply the load), run the analysis and then a reinforcement load is shown in the Interpreter. You can also use this feature to gain confidence in your existing design. You can run the back analysis feature and verify that the support you have designed exceeds the reinforcement load that is calculated by Slide's back analysis feature.




The location of the support force displayed in the Modeler




After analysis, the Interpreter will display the force required to equal or exceed the specified factor of safety for all slip surfaces. It will also show the surface that requires exactly this force to meet the factor of safety. Both passive and active support forces can be displayed.

Back Analysis: Using Sensitivity or Probabilistic Analysis

Sensitivity Analysis or Probabilistic Analysis can be used for the Back Analysis of material properties, groundwater conditions and other slope parameters. 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 the probable values of slope parameters at failure.

A Sensitivity analysis can be used for this purpose if you are simply considering a single variable for your back analysis (for example: the cohesion of one material or a load magnitude).

A Probabilistic analysis allows you to consider more than one variable at the same time for a back analysis. For example, if you had a weak layer with material properties that were not well known, you could specify Uniform distributions for the Cohesion and Friction Angle of the layer between minimum and maximum possible values. After running the Probabilistic analysis, create a Scatter Plot of Cohesion versus Friction Angle for the material. On the Scatter Plot, use the Highlight Data feature to highlight all of the data within a narrow range of Safety Factor = 1 (eg. 0.99 to 1.01). This would then indicate all possible combinations of Cohesion and Friction Angle which combine to yield a failure condition for the slope.

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/products/slide/Speight.htm