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The following Seismic Analysis options are available on the Seismic page of the Project Settings dialog.

Compute Ky for All Failure Surfaces

To enable this option, select the Compute Ky for all failure surfaces checkbox.

When this option is selected, Slide3 will compute the horizontal pseudo-static seismic coefficient required to lower the slip surface factor of safety to the Target Factor of Safety. By default the Target FS = 1, but the user can enter a different value if desired. This analysis is done for ALL slip surfaces. For a given slip surface, this is referred to as the critical seismic coefficient. The overall critical surface reported is the surface which requires the LOWEST value of Ky to reach the Target FS.

When you choose this analysis option, the output will be in terms of Ky values rather than Factor of Safety. If all slip surfaces have an initial Factor of Safety less than the Target FS, then the reported critical Ky will simply be zero.

Staged Pseudo-Static Strength

To enable this option, select the Staged pseudostatic strength checkbox, and choose a method:

  • Effective Stress
  • Duncan, Wright, Wong 3 Stage (1990)
  • Army Corp. Eng. 2 Stage (1970)

Because seismic loading is of short duration, it is reasonable to assume that except for certain coarse materials, the soil will not drain appreciably during the period of earthquake shaking. Thus, undrained shear strengths are used for most pseudo-static analyses (with the exception of soils that tend to dilate when sheared and may lose strength after the earthquake as they drain).

In order to model the application of undrained strength in a pseudo-static seismic analysis, the Staged pseudostatic strength option in Project Settings must be used. The Duncan Wright Wong (1990) method of computing the undrained shear strength is recommended. This procedure is nearly identical to the staged rapid drawdown procedure of Duncan Wright and Wong. The first stage is used to estimate the in-situ stress state at the bottom of each slice prior to the earthquake. This stress state is then used to determine the in-situ anisotropic consolidation stress ratio prior to the earthquake. This stress ratio is then used in stage 2 to determine the undrained shear strength during the seismic event. The exact procedure is defined in chapter 9 of Duncan Wright and Brandon, Soil Strength and Slope stability 2nd edition (2014). Discussion of the application of the method is in chapter 10 of the same text.

The staged pseudostatic option also allows you to use the Army Corp (1970) multistage rapid drawdown method to determine the undrained strength during a seismic event.

There is also an Effective stress option which assumes the effective strength prior to the seismic event is representative of the strength during the earthquake. As a result, the static shear strength is used for the seismic analysis.

As mentioned previously, we recommend the use of the Duncan Wright Wong (1990) method when doing a Staged pseudostatic strength analysis.

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