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CSR should be modified for the presence of large overburden pressures and for
static shear stresses; however, these concerns were not used in the levee analyses
consistent with the recommendations contained in Youd and Idriss (1997).
Therefore, the CSR can be calculated with the following equation:
CRR
M x
CSR
M x
MSF
CSR
M 7.5
(8-4)
where MSF is the magnitude scaling factor (Table 8.8 from Figure 3.4). It should
be noted that the MSF for M w 9.0 was extrapolated from recommended design
curves (Youd and Idriss 1997) and is considered very approximate, pending
further study.
Table 8.8: Magnitude Scaling Factors, MSF (Youd and Idriss 1997)
EARTHQUAKE MAGNITUDE
6.2 7.0 8.5 9.0
MSF 1.80 1.25 0.75 0.65
8.6.2.2.2 Cone Penetration Test (CPT)
In the Proceedings of the NCEER Workshop on Evaluation of Liquefaction
Resistance of Soils (Youd and Idriss 1997), the workshop participants were unable
to reach a consensus on a single CPT-based criteria for evaluating liquefaction
resistance. Therefore, methods proposed by Robertson and Wride (1997) and
Olsen (1997) were used in the liquefaction resistance assessment of the levee to
evaluate the differences between the two procedures. It was determined herein
that the CRR values calculated by CPT-based methods are, on average, smaller
and thus, more conservative than SPT-based methods.
The following assumptions were made in the liquefaction hazard assessment.
For the 100-year flood event, the soil is saturated to the crest of the levee due
to infiltration, perched groundwater conditions, and capillary rise of pore
water. For the summer flow conditions, the groundwater table was held at the
river stage [El. 2.1 m (7ft)].
Thin soil layers [~ 0.6 m (2 ft)] were assumed to be discontinuous and their
influence on the slope stability was assumed to be negligible.
The analysis was performed assuming level ground conditions.
Figure 8.18 provides a plot of CRR versus elevation that compares the results
from the methods proposed by Olsen (1997) and Robertson and Wride (1997).
Gaps in the log produced in accordance with the Robertson and Wride procedure
represents those zones of soils that had I c > 2.6, which indicates a high fines
content and, according to Robertson and Wride, low liquefaction hazard.
Although the relative trends of the CRR values computed using the two methods
look somewhat similar, several points warrant further review. First, in the upper
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