Civil Engineering Reference
In-Depth Information
FIGURE 6.9
Relationship between cyclic resistance ratio and corrected CPT tip resistance values for clean
gravel and silty gravel for
M
7.5 earthquakes. (
From Stark and Olson 1995, reprinted with permission of
the American Society of Civil Engineers.
)
curve, the cyclic resistance ratio is obtained. The factor of safety against liquefaction is then
calculated by using Eq. (6.8) (or FS
CRR/CSR). Note that Fig. 6.10 was developed for
a moment magnitude
M
w
earthquake of 7.5. The magnitude scaling factors in Table 6.2 or
Fig. 6.7 can be used if the anticipated earthquake magnitude is different from 7.5.
An advantage of using the shear wave velocity to determine the factor of safety against
liquefaction is that it can be used for very large sites where an initial evaluation of the lique-
faction potential is required. Disadvantages of this method are that soil samples are often not
obtained as part of the testing procedure, thin strata of potentially liquefiable soil may not be
identified, and the method is based on small strains of the soil, whereas the liquefaction
process actually involves high strains.
In addition, as indicated in Fig. 6.10, there are few data to accurately define the curves
above a CRR of about 0.3. Furthermore, the curves are very steep above a shear wave veloc-
ity of 200 m/s, and a small error in measuring the shear wave velocity could result in a sig-
nificant error in the factor of safety. For example, an increase in shear wave velocity from
190 to 210 m/s will essentially double the CRR. Because of the limitations of this method,
it is best to use the shear wave velocity as a supplement for the SPT and CPT methods.
