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Table 8.6: Summary of PGA values
LOCATION
EARTHQUAKE SCENARIOS
M w = 6.2 M w = 7.0 M w = 8.5 M w = 9.0
Soil (Crest of Levee) 0.16g 0.18g 0.11g 0.12g
Bedrock (Sandy River Sandstone) 0.29g 0.38g 0.12g 0.14g
8.6 LIQUEFACTION ANALYSIS
The liquefaction analyses performed in this study consisted of: (1) computing the cyclic shear
stresses in the soil deposits for each scenario earthquake (CSR EQ ), (2) calculating the cyclic shear
stress required to initiate liquefaction in the cohesionless soils (CRR), (3) comparing the results
of steps 1 and 2 to assess the liquefaction susceptibility of the soil, and (4) utilizing the results of
step 3 to estimate the cyclic shear strength of the soil.
8.6.1 Determination of the Cyclic Stress Ratio
The determination of earthquake induced cyclic shear stresses can be estimated in two ways: (1)
by performing a simple empirical analysis developed by Seed and Idriss (1971, 1982, updated
summary by Youd and Idriss, 1997), or (2) by performing a more in-depth, site-specific dynamic
ground response analysis (SHAKE91). The cyclic shear stresses computed from the SHAKE91
analyses were used for this example problem. The computed cyclic shear stresses used in the
liquefaction analyses are the “equivalent uniform” ( eq. uniform ) shear stresses. The maximum shear
stress ( max ) is reduced by the reduction factor (n given in Equation 8-1) using Equation 8-2 to
account for the irregular and transient nature of the shear stress time history. The eq. uniform is then
normalized by the effective overburden stress ( vo ′) to produce the CSR induced by the
earthquake as given in Equation 8-3. Figure 8.17 shows the CSR versus depth for the magnitude
6.2 earthquake motions employed in this study.
τ
n *
eq.uniform
τ max
(8-2)
CSR
τ
eq.uniform
σ
vo
'
(8-3)
8.6.2 Determination of the Cyclic Resistance Ratio
As defined in Chapter 3, the CRR describes the ability of the soil to resist the cyclic loading
induced by the earthquake. The CRR can be determined through empirical relationships based
largely on SPT and/or CPT penetration resistance, or laboratory tests. Both methods were used in
this project and they are discussed in the following sections.
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