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3106F.5 Slope or Embankment Stability and Seismically Induced Lateral Spreading
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3106F.5.1 Static Slope Stability
Static stability analysis using conventional limit equilibrium methods shall be performed for site related slope or embankment systems. Live load surcharge shall be considered in analyses based on project-specific information. The long-term static factor of safety of the slope or embankment shall not be less than 1.5.
3106F.5.2 Pseudo-Static Seismic Slope Stability
Pseudo-static seismic slope or embankment stability analyses shall be performed to estimate the horizontal yield acceleration for the slope for the Level 1 and Level 2 earthquakes. During the seismic event, appropriate live load surcharge shall be considered.
If liquefaction and/or strength loss of the site soils is likely, the following shall be used in the analyses, as appropriate:
- Residual strength of liquefied soils
- Strengths compatible with the pore-pressure generation of potentially liquefiable soils
- Potential strength reduction of clays
The residual strength of liquefied soils shall be estimated using guidelines outlined in SCEC [6.4] or other appropriate documents as noted in CGS Special Publication 117A [6.5].
Pseudo-static analysis shall be performed without considering the presence of the foundation system. Using a horizontal seismic coefficient of one-half of the PGA, if the estimated factor of safety is greater than or equal to 1.1, then no further evaluation of lateral spreading or kinematic loading from lateral spreading is required.
3106F.5.3 Post-Earthquake Static Slope Stability
The static factor of safety immediately following a design earthquake event shall not be less than 1.1 when any of the following are used in static stability analysis:
3106F.5.4 Lateral Spreading — Free Field
The earthquake—induced lateral deformations of the slope or embankment and associated foundations soils shall be determined for the Level 1 and Level 2 earthquakes using the associated PGA at the ground surface (not modified for liquefaction). If liquefaction and/or strength loss of the site soils is likely, the following shall be used in the analyses, as appropriate:
- Residual strength of liquefied soils
- Strengths compatible with the pore-pressure generation of potentially liquefiable soils
- Potential strength reduction of clays
The presence of the foundation system shall not be included in the "free field" evaluations.
Initial lateral spread estimates shall be made using the Newmark displacement approach documented in NCHRP Report 611 [6.6] or other appropriate but similar procedures.
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