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SSR: Frequently Asked Questions

This page contains the answer to commonly asked SSR-related tech support questions.

Rocscience has also written papers on Shear Strength Reduction, and links to these papers are provided below.

The papers below, from RocNews, also discuss SSR.

What are some general guidelines for carrying out an SSR analysis?

The following are a few things to keep in mind when carrying out an SSR analysis in RS3:

  • Relevant tutorials: SSR Tutorial.
  • Use 10-noded Tetrahedra for an SSR analysis
  • If material properties are set to zero, this can cause numerical issues
  • The first thing to check when doing an SSR analysis is the Critical SRF value beside the Results workflow tab. You should be looking for color of the text where green colored text represent converged solution of SRF values and red text refers to diverging results.
  • Even for 10-noded Tetrahedra there can be mesh sensitivity, so always be mindful of this.
  • It is recommended to use both Slide3 and RS3 to properly judge the factor of safety of a slope.
  • In an SSR analysis, both the peak and residual strengths are reduced for a particular SRF value. Residual values are used after failure of the material.
  • The displacements at any stage of an SRF analysis are the displacements you would see if you reduced the strength of the in situ material by that SRF factor.
  • A pin at the edge surface of the model is preferable to a roller when doing SSR analyses, particularly if the boundary is close. If rollers a used, the failure surface can propagate along the external boundary since it is in essence a frictionless, cohesionless surface.
  • Before doing an SSR analysis on a multi-stage model, one should always do the model without SSR first.

Can SSR analysis be done for tunnels?

We have used SSR analyses for slopes only, not for underground excavations/tunnels. However, there are people who have used SSR for such analyses.

The following comments are from Evert Hoek regarding the use of SSR in tunneling design (comments are from 2009):

I have thought a great deal about the possible role of SSR in tunnel design and I am afraid that I fail to see much future for it other than that it provides another interesting number. The practicality of tunnelling requires the designer to start off with a number of common sense questions that determine the methods that are best suited to providing a reliable and practical solution. These questions include the nature of the rock mass and in situ stress field and, as a consequence, the most likely failure modes; the method and sequence of excavation and the constraints imposed by this method on the support systems that can be used; the influence of groundwater on the design (for example whether the lining is leaky or impermeable); the role of time (creep and rock deterioration) on the response of the rock mass around the tunnel and the sensitivity of surface structures above the tunnel on the permissible subsidence and tilt. Each underground structure is unique and it is important to determine which of the factors listed above, or others, will dominate the design. There is no single magic bullet for tunnel design and it is usually a combination of methods that provide a number of quantified responses which then have to be assessed as to their adequacy to meet the over design requirements. In some cases these requirements may be dictated by national codes or guidelines.

It is very difficult to see where a "factor of safety" determined by SSR fits into this scheme of things. In fact, it does not answer any of the questions outlined above and the number arrived at may or may not have any physical significance. The sequence of failure, if it occurs, can be adequately assesses by means of the normal pi or modulus reduction techniques that we employ. I always set maximum displacements as the background screen and add failure indicators and boundary displacements. If I am using rockbolts I look at the axial loads in the bolts and whether any portions of the bolts have failed and if I am installing a lining I look at the support capacity plots. For near surface tunnels I look at surface displacements.

Results Interpretation

What is the critical SRF?

The Critical SRF is identical to the Factor of Safety in limit equilibrium slope stability analyses (Slide2 & Slide3). A value of 1.0 means the slope is in a state of limit equilibrium and that any reduction in shear strength will result in failure of the slope. You do not design for a factor of safety of 1.0 unless you are using limit states and factoring input parameters (Eurocode). Design values for slope stability analyses are up to the user and depend on the type of slope and consequences of failure.

If you are looking for guidelines on what factors of safety to use, a suggested reference is John Read and Peter Stacey's book, "Guidelines for Open Pit Design". This book contains typical safety factors for use in design.

How do I interpret the SRF value and the displacements that result from SSR analysis?

The critical SRF value is a global factor of safety similar to the Factor of Safety computed by a limit-equilibrium program. It is recommended to do a limit-equilibrium analysis as well.

The examples shown below are taken from the tutorial RS3: Slope stability analysis (SSR).

Example from RS3: Slope stability analysis (SSR)

What does it mean if the critical SRF calculated is less than 1?

For one stage models, RS3 accurately computes models with SRF or FS less than 1. For multi-stage models, if and only if all stages leading up to the last stage converge is the solution accurate. RS3 uses the stress state from the end of the second last stage as the initial stress state for the SSR analysis. So if it is invalid (due to non-convergence) the SSR analysis will not be correct. There are numerous examples of one stage models with SRF<1 in the Slope Stability Verification Manuals in RS2 for theory.

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