Shear Strength Reduction Overview

The Shear Strength Reduction option in Phase2 allows you to automatically perform a finite element slope stability analysis, and compute a critical strength reduction factor for the model. The critical strength reduction factor is equivalent to the "safety factor" of the slope.

Shear Strength Reduction analysis of dam

Terminology

The Shear Strength Reduction method in Phase2 is referred to as SSR analysis in the Phase2 documentation. The Strength Reduction Factor is referred to as SRF.

Overview of Method

The basic concept of the Shear Strength Reduction (SSR) method is actually quite simple.

  1. The strength parameters of a slope are reduced by a certain factor (SRF), and the finite element stress analysis is computed.

  2. This process is repeated for different values of strength reduction factor (SRF), until the model becomes unstable (the analysis results do not converge).

  3. This determines the critical strength reduction factor (critical SRF), or safety factor, of the slope.

Performing an SSR analysis in Phase2

The SSR analysis in Phase2 is completely automated. The following steps are required to create an SSR model and run the analysis.

  1. Setting up the model - an SSR model is no different from a regular Phase2 model. However, you should pay particular attention to setting up the appropriate boundary conditions and field stress. Also, there are a few restrictions (e.g. the SSR method cannot be used with all strength models).

  2. Enabling the SSR option - to enable the SSR analysis option, you simply need to select the Determine Strength Reduction Factor checkbox, under the Strength Reduction tab in the Project Settings dialog. You can also configure various parameters for the SSR analysis.

  3. Computing the SSR analysis - if SSR is enabled (Step 2), the SSR analysis will be computed when you select the main Compute option in Phase2. The SSR analysis is computed after the regular stress analysis is complete. If your model is multi-stage, note that the SSR analysis is only computed for the FINAL stage of your model.

See the Phase2 Tutorials for detailed step-by-step guidelines for creating an SSR model, running the analysis and interpreting the results.

Also see the Importing Slide Files topic for information about importing slope models from Slide into Phase2. Slide is a 2D limit equilibrium slope stability analysis program, available from Rocscience.

Material Properties

The SSR method can be used with the following material models in Phase2:

NOTE: the SSR method CANNOT be used if your model uses the Cam-Clay or Modified Cam-Clay material strength models. This may be changed in a future Phase2 upgrade, but at present you cannot perform an SSR analysis in Phase2 in conjunction with these material models.

Joint Properties

If your model includes joints, then by default the SSR analysis is NOT applied to the joint strength parameters. If you wish to apply the SSR analysis to joint properties then select the Apply SSR analysis checkbox in the Define Joint Properties dialog.

Support Properties

If your model includes support, note that the SSR analysis in Phase2 is NOT applied to support properties (i.e. bolts or liners). Support strength parameters are NOT reduced during an SSR analysis, and retain their original values.

Multi-Stage Models

If your model is multi-stage, note that the SSR analysis is only computed for the FINAL stage of your model. It is not computed for intermediate stages. If you want to perform the SSR analysis at an intermediate stage, you will have to reduce the Number of Stages in Project Settings to the desired stage number.

Narrowing the SSR Search Area

By default the SSR analysis considers the stability of the entire model when the analysis is computed. However, there are circumstances when you may wish to focus on the stability of a particular area of the model. This can be accomplished with the SSR Search Area option, which allows you to apply the SSR analysis to a particular region of a model.