Seismic Settings

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 Advanced checkbox and select the Compute Ky for all failure surfaces option.

When this option is selected, Slide2 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.

See Tutorial - Seismic Analysis for an example of how to use the Seismic options.

Newmark Displacements

To enable this option, select the Advanced checkbox and select the Newmark Displacements option.

A sliding block is used to estimate a slope behavior during earthquakes and analyze permanent deformation. Rigid block can either be analyzed with decoupled or fully coupled assumptions. You can first define or import seismic earthquake records, then Slide will use the input records to analyze the results. Engine is based on the SLAMMER program which has been incorporated into Slide2.

When you run a Slide2 model using Newmark analysis, you will obtain the Newmark displacement value for each slip surface, with the critical surface (maximum displacement) highlighted.

In the Newmark Analysis Options dialog:

For the Analysis type combo:

Choose types of analysis: Any or all types of analysis can be performed simultaneously. Check the boxes next to the types of analysis desired.

• Rigid Block: This type of analysis can be conducted so as to allow only downslope displacement or to allow both upslope and downslope displacement.
• Coupled and Decoupled: Conducting coupled and (or) decoupled analysis of a flexible sliding block requires specifying several slope properties: (1) the shear-wave velocity of the material above the slip surface, (2) the shear-wave velocity of the material below the slip surface (commonly taken as rock), (3) the damping ratio of the material within the slip surface, and (4) the reference strain used to define the nonlinear modulus reduction and damping curves of the soil within the slip surface (the reference strain is the shear strain associated with G/Gmax=0.5 and the default is 0.05 percent). The type of soil model, linear elastic or equivalent linear, also must be specified. The equivalent linear analysis is the most complex and requires the greatest computing time.

For more information regarding coupled/decoupled analysis, please refer to paper by Jibson (2011) in the following sections :

• 4.1 Types of permanent-displacement analysis
• 4.1.1 Rigid-block analysis
• 4.1.2 Decoupled Analysis
• 4.1.3 Coupled Analysis

Scaling

You can choose this option to use the original acceleration vs. time seismic record by selecting 'Do not Scale'. Or, you can apply a factor to acceleration in the record by selecting 'Scale record by a factor of' option and enter a factor you want to apply to the acceleration.

DISPLACEMENTS COMPUTED USING:

There are 5 possible options for how the acceleration-time history is used to compute displacements.

(1) normal polarity (the “top” or "positive accelerations"),

(2) inverse polarity (the “bottom” or "negative accelerations"), and

(3) the average of the two polarities. (Average accelerations)

(4) the maximum of (1) and (2). (Maximum positive/negative)

(5) All Accelerations.

The maximum of 1 and 2 is the default.

Slammer: http://pubs.usgs.gov/tm/12b1/

Reference: Jibson, R.W., Rathje, E.M., Jibson, M.W., and Lee, Y.W., 2013, SLAMMER—Seismic LAndSlide2 Movement Modeled using Earthquake Records (ver.1.1, November 2014): U.S. Geological Survey Techniques and Methods, book 12, chap. B1, unpaged.

See Tutorial 28 Seismic Analysis for an example of how to use the Seismic options.

Staged Pseudo-Static Strength

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 project setting in Slide2 must be used. Along with turning on this option, the Duncan Wright Wong 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. Discussion of the application of the method is in chapter 10 of the same text.

The staged pseudo-static option also allows you to use the Army Corp 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 method when doing a Staged pseudostatic strength analysis.