Report - Oregon State Library: State Employee Information Center ...

8.7 EVALUATION OF INITIATION OF LIQUEFACTION
After the CSR caused by the earthquake and the CRR were determined, the potential for
liquefaction was evaluated. At elevations where the CSR is greater than the CRR, liquefaction is
likely to occur. The factor of safety against liquefaction (FS L ) was evaluated at specific depths
with the Equation 8-5. The CRR values used in the FS L calculations were from the CPT-based
evaluation proposed by Robertson and Wride (1997) for sandy soils (SP/SM), and the laboratory
cyclic triaxial cell testing on the fine-grained (ML) soils. Profiles of FS L as a function of
elevation for each scenario earthquake were completed (Figure 8.19). A cross-section showing
zones of soil that are expected to liquefy (FS L < 1.0), and zones expected to have significant
excess pore pressure generation (1.0 < FS L < 1.4) during the M w 8.5 earthquake is shown in
Figure 8.20.
CRR
FS l
CSR
(8-5)
8.8 DETERMINATION OF CYCLIC SHEAR STRENGTH
In order to assess the seismic stability of the levee, it was necessary to estimate the residual shear
strength of the soils prone to excess pore pressure generation. The loss of soil shear strength is a
function of the magnitude of excess pore pressure generation. The following categories delineate
the various stages of soil shear strength reduction due to liquefaction.
FS L > 1.4: Excess pore pressure generation is considered negligible and the soil does not
experience an appreciable reduction in shear strength (CDMG 1997). In this case, the
drained shear strength is computed using the standard Mohr-Coulomb strength equation.
1.0 < FS L < 1.4: Partial excess pore pressure generation will have an effect on soil
strength and should be addressed. The magnitude of the pore pressure generation is a
function of FS L and soil type (Marcuson et al. 1990).
FS L < 1.0: Soils are expected to experience full pore pressure generation and residual
shear strengths should be applied (Seed and Harder 1990; Stark and Mesri 1992;
Ishihara 1993; Baziar and Dobry 1995).
8.8.1 Partial Excess Pore Pressure Generation (1.0 < FS L