The Strength Reduction option in the Project Settings dialog allows you to perform a finite element slope stability analysis with Phase2, using the Shear Strength Reduction (SSR) method. For an overview of the SSR method in Phase2 see the Shear Strength Reduction Overview topic.
To enable the Shear Strength Reduction method:
Select the Determine Strength Reduction Factor checkbox, under the Strength Reduction tab in the Project Settings dialog.
You may customize the analysis parameters described below, or use the default values.
Initial Estimate of SRF
The initial estimate of the Strength Reduction Factor for the first iteration of the SSR analysis. By default this is set to 1.
Automatic Step Size
The Step Size is the difference in the Strength Reduction Factor, between two iterations of the SSR analysis.
If Step Size = Automatic (this is the default selection), an appropriate Step Size is automatically determined by Phase2, and may vary from iteration to iteration (i.e. it may increase or decrease between any two iterations).
Tolerance (SRF)
If Step Size = Automatic, the Tolerance (SRF) determines the convergence (end of analysis) for the SSR method. The Tolerance (SRF) is implemented as follows:
When the difference in SRF between two iterations of the SSR method is LESS THAN the Tolerance (SRF), AND...
The Stress Analysis has converged for the SSR iteration with the lower SRF, but does NOT converge for the SSR iteration with the higher SRF, THEN...
The SSR analysis is deemed to have converged. The critical value of SRF is the SSR iteration with the lower SRF.
Custom Step Size
If Step Size = Custom, this allows the user to define the Step Size and Final SRF values. In conjunction with the Initial Estimate of SRF value (see above), this fully determines the number of iterations and the value of SRF at each iteration.
With the Custom Step Size option, the Step Size is uniform and increases monotonically between the Initial SRF and Final SRF values.
With the Custom Step Size option, the Critical SRF is simply the lowest value of SRF for which the Stress Analysis converged (i.e. equilibrium was achieved).
Limit SSR Search Area
The Limit SSR Search Area option allows you to limit the SSR analysis to a particular region of the model. To define the search area:
Select the Limit SSR Search Area checkbox, and select the Define button.
In the dialog which appears, enter the x,y coordinates of opposite corners of a rectangular region, which encompasses the area of the model you wish to analyze.
See the SSR Search Area topic for more information. NOTE: the SSR Search Area can also be defined graphically by selecting Analysis > SSR Search Area > Define SSR Search Area from the menu.
Advanced SSR Options
The following advanced SSR analysis options can be configured by selecting the Advanced button under the Strength Reduction tab in Project Settings.
Reduce number of iterations after failure
By default, Phase2 will automatically reduce the maximum number of iterations allowed for the stress analysis, if failure occurs (i.e. the model becomes unstable and convergence is not achieved), for a given iteration of the SSR analysis. This decreases the required computation time, and generally does not affect the final results of the SSR analysis.
If you do NOT want Phase2 to automatically reduce the maximum number of iterations after failure, then clear this checkbox. Phase2 will then always use the same value for the maximum number of iterations allowed for convergence of the stress analysis, regardless of whether or not failure has occurred for a given iteration of the SSR analysis. In some cases this may improve the accuracy of the SSR analysis. However this will depend on your particular model. If you are uncertain about the SSR analysis results, you can try re-running the analysis with this checkbox turned OFF, to see if this has any effect on the results. In general this will increase the required computation time.
Convergence Parameters
By default Phase2 will automatically determine appropriate convergence parameters (Tolerance, Maximum Number of Iterations, Convergence Type) for the SSR stress analysis. If desired, the convergence parameters can be user-defined by selecting the User Defined Convergence Parameters option. The following convergence parameters can be user defined.
Tolerance (Stress Analysis)
The definition of this parameter is the same as described for the Tolerance parameter in the Stress Analysis (Project Settings) topic. The only difference is that this Tolerance value applies to the Phase2 stress analyses which are computed during each iteration of the SSR analysis.
Maximum Number of Iterations
The definition of this parameter is the same as described for the Maximum Number of Iterations parameter in the Stress Analysis (Project Settings) topic. The only difference is that this value applies to the Phase2 stress analyses which are computed during each iteration of the SSR analysis.
Convergence Type
In Phase2, you may choose either Absolute Energy or Square Root Energy as the convergence criterion used for the finite element stress analysis. For details see the Convergence Criteria document in the Phase2 theory section.