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Shear Strength Reduction Overview

The Shear Strength Reduction option in RS2 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

Shear Strength Reduction analysis of dam

Terminology

The Shear Strength Reduction method in RS2 is referred to as SSR analysis in the RS2 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 RS2

The SSR analysis in RS2 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 RS2 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 need to select the Determine Strength Reduction Factor check box on the Strength Reduction page 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 RS2. 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 RS2 Tutorials for practice working with an SSR model.

Also see the Importing Slide Files topic for information about importing slope models from Slide2 into RS2. Slide2 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 RS2:

  • Elastic/Plastic
    • Mohr-Coulomb
    • Hoek-Brown
    • Generalized Hoek-Brown
    • Drucker-Prager
    • Discrete Strength Function
  • Slide Models
    • Barton-Bandis
    • Hyperbolic
    • Power Curve
    • Shear/Normal Function
    • SHANSEP
    • Vertical Stress Ratio
  • Anisotropic
    • Snowden Mod. Anisotropic Linear
    • Anisotropic Linear
    • Generalized Anisotropic
    • Jointed Mohr Coulomb
    • Jointed Generalized Hoek Brown

Note: The SSR method CANNOT be used if your model uses any model in any other category in the material properties dialog such as the Cam-Clay or Modified Cam-Clay material strength models within the Softening/Hardening list of models. This may be changed in a future RS2 upgrade, but at present you cannot perform an SSR analysis in RS2 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 check box in the Define Joint Properties dialog.

Support Properties

If your model includes support, note that the SSR analysis in RS2 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. You can also exclude regions with the SSR Exclusion Area option.

RS2 Knowledge Base - SSR

The RS2 Knowledge Base – SSR contains general guidelines for conducting an SSR analysis, answers to commonly asked SSR-related questions and articles published by Rocscience on the subject.

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