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conservatively estimated in subsequent analyses to be normally consolidated based on the
results of the consolidation tests, and the DMT and CPT data reduction. In order to
estimate the CRR values for the silts from the laboratory data, the number of uniform
loading cycles anticipated for each design-level earthquake is required. Recent data
relating the number of uniform cycles to earthquake magnitude was utilized (Liu and
Stewart 1999) and are shown in Figure 3.15. It should be noted that the curves were
extrapolated for magnitudes greater than seven. Given the cyclic resistance curves
(Figure 3.11) and the number of uniform loading cycles for the scenario earthquakes
(Figure 3.15), the CRR can be estimated. Table 8.7 shows the equivalent number of
cycles and corrected CRR values for the ML soils for the design-level earthquakes.
Table 8.7: Cyclic Resistance Ratio (CRR) from Lab Test Data for Normally Consolidated, Silty Soils
EARTHQUAKE
MAGNITUDE (M w )
EQUIVALENT NUMBER
OF UNIFORM CYCLES
CYCLIC RESISTANCE
RATIO (CRR field )
6.2 6 0.18
7.0 16 0.16
8.5 39 0.14
9.0 45 0.14
8.6.2.2 Field Tests
The recommended method of characterizing the liquefaction resistance of a soil deposit is
based on the results of in situ tests, due to the disturbance inherent in the sampling and
laboratory testing of cohesionless soils (Chapter 3). The SPT has historically been used
for liquefaction assessments, but the CPT is becoming more common. The CPT-based
methods were used in this example problem (Robertson and Wride 1997; Olsen 1997).
The SPT-based method was used as an independent check on the CPT calculations.
Therefore, a brief discussion on the SPT will be followed by a more in-depth description
of the CPT methods proposed by Robertson and Olsen.
8.6.2.2.1 Standard Penetration Test (SPT)
Seed and his co-workers (1979, 1985, 1986) plotted standardized SPT values
versus CSR for earthquakes of magnitude 7.5 [denoted as (CSR M=7.5 )] for
locations with and without the occurrence of liquefaction (Figure 3.3). This plot
provides a very practical method of estimating the cyclic stress ratio necessary to
initiate liquefaction using normalized SPT values and the percentage of fines in
the sandy soil. Once the in situ penetration resistance of the soil has been
obtained, the chart solution can be used to estimate the minimum cyclic stress
ratio required to initiate liquefaction. Note that this cyclic stress ratio corresponds
to the capacity of the soil to resist liquefaction, and it has been termed the “cyclic
resistance ratio” (CRR) used in the liquefaction hazard analysis. Because
variations in earthquake magnitudes lead to an increased number of earthquakeinduced
shear cycles, the CSR M=7.5 should be modified using magnitude scaling
factors to account for different earthquake magnitudes. In some situations, the
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