Civil Engineering Reference
In-Depth Information
FIGURE 9.30
Slope stability analysis for the weakening condition based on zero shear strength of the liq-
uefied soil. The SLOPE/W computer program was used to perform the stability analysis (Geo-Slope 1991).
Figure 9.32 presents a plot of the normalized liquefied shear strength versus the equiv-
alent clean sand (
N
1
)
60
value (R. B. Seed and Harder 1990, Stark and Mesri 1992). The ver-
tical axis is the undrained liquefied shear strength divided by the pre-earthquake average
vertical effective stress
v
o
. The horizontal axis is the equivalent clean sand standard pene-
tration test (
N
1
)
60
value. For those flow failures in clean sand, the horizontal axis represents
the (
N
1
)
60
value from Eq. (5.2). For those sites having silty sands, the (
N
1
)
60
values have
been adjusted (hence the terminology of equivalent clean sand values).
The data points shown in Fig. 9.32 are based on previous flow slide studies. Using the
distance from the initiation of the flow slide to the location where it eventually stopped
moving, a kinetics analysis has been performed to calculate the undrained liquefied shear
strength needed to stop the movement (Olson et al. 2000). However, it has been argued that
perhaps the flow slides stopped moving simply because water was able to drain out of the
soil pores during the flow side, resulting in a reduction in pore water pressure and an
increase in shear strength. Setting aside the issue of whether these studies actually predict
the undrained liquefied shear strength, the liquefied shear strength shown in Fig. 9.32 tends
to be very small, especially at low values of (
N
1
)
60
.
The flow slide analysis presented in Sec. 9.4.3 is approximate. The greatest uncertainty lies
in the determination of the factor of safety against liquefaction for sloping ground. For exam-
ple, the first step is to determine which soil layers will liquefy during the design earthquake
based on the analysis in Chap. 6. Then the factor of safety against liquefaction as calculated
from Eq. (6.8) must be adjusted for the sloping ground condition. Figure 9.24 would be used
