Civil Engineering Reference
In-Depth Information
35 percent. Thus experience and judgment will be required in the selection of the value of
K
from Fig. 9.24 for sloping ground conditions.
9.4.3
Stability Analysis for Liquefied Soil
Introduction.
The first step in a flow analysis is to determine the factor of safety
against liquefaction for the various soil layers that comprise the slope. The factor of
safety against liquefaction is based on level-ground assumptions (Chap. 6). Then the fac-
tor of safety against liquefaction [Eq. (6.8)] is adjusted for the sloping ground conditions,
by using Fig. 9.24. If it is determined that the entire sloping mass, or a significant portion
of the sloping mass, will be subjected to liquefaction during the design earthquake, then
the slope will be susceptible to a flow slide. No further analyses will be required for the
“mass liquefaction” case.
For the cases of zonal liquefaction or liquefaction of soil layers or seams, a slope sta-
bility analysis is required. To perform a slope stability analysis for soil that is anticipated
to liquefy during the earthquake, there are two different approaches: (1) using a pore water
pressure ratio
1.0 or (2) using zero shear strength for the liquefiable soil.
Pore Water Pressure Ratio (r
u
1.0)
. The first approach is to assume that the pore water
pressure ratio of the liquefied soil is equal to 1.0. As previously mentioned, the pore water
pressure ratio
r
u
is defined as
r
u
u
(
t
h
), where
u
pore water pressure,
t
total unit
weight of the soil, and
h
depth below the ground surface.
As indicated in Fig. 5.15, at a factor of safety against liquefaction FS
L
equal to 1.0 (i.e.,
liquefied soil),
r
u
1.0. Using a value of
r
u
1.0, then
r
u
1.0
u
(
t
h
). This means that
the pore water pressure
u
must be equal to the total stress
t
h,
and hence the effective
stress
is equal to zero (
u
). For a granular soil, an effective stress equal to zero
means that the soil will not possess any shear strength (i.e., it has liquefied). A pore water
pressure ratio
r
u
of 1.0 for liquefiable soil should be used only when the soil has an effec-
tive cohesion
c
equal to zero, such as sands and gravels.
Note that the pore water pressure ratio determines the pore water pressures within the
soil. Thus the pore water pressure ratio must only be used with an effective stress analysis.
For those soil layers that do not liquefy during the earthquake, the effective shear strength
parameters (
c
and
) and the estimated pore water pressures must be used in the slope sta-
bility analysis.
Shear Strength Equals Zero for Liquefied Soil.
The second approach is to assume that
the liquefied soil has zero shear strength. If a total stress analysis is used, then the liquefied
soil layers are assumed to have an undrained shear strength equal to zero (
s
u
0). If an
effective stress analysis is used, then the effective shear strength parameters are assumed
to be equal to zero (
c
0 and
0).
Example Problem.
The purpose of the remainder of this section is to present an example
problem dealing with a flow slide analysis. A cross section through the slope is shown in
Fig. 9.25. Specific details on the condition of the slope are as follows:
●
Type of analysis:
effective stress analysis
●
Slope inclination:
2 : 1 (horizontal : vertical)
●
Slope height
25 ft
●
Soil types:
1.
Compacted fill:
It consists of dense granular soil having the following shear strength
parameters:
37
and
c
0. The total unit weight of the soil
t
125 lb/ft
3
.
