Civil Engineering Reference
In-Depth Information
condition. Thus the movement and reduction of shear strength will progress along the slip
surface, eventually leading to failure of the slope. Because of this weakening of the soil dur-
ing the earthquake, it is best to use a weakening slope stability analysis (see Sec. 9.6).
Other Structures.
Slope stability analysis can be used for other types of engineering
structures. For example, the stability of the ground underneath a retaining wall is often ana-
lyzed by considering a slip surface beneath the foundation of the wall.
Effective Stress Analysis.
The pseudostatic slope stability analysis can be performed
using the effective shear strength of the soil. For this type of analysis, the effective shear
strength parameters
c
and
are input into the computer program. The pore water pres-
sures must also be input into the computer program. For the pseudostatic method, it is com-
mon to assume that the same pore water pressures exist for the static case and the
pseudostatic case. Several different options can be used concerning the pore water pres-
sures:
1.
Zero pore water pressure:
A common assumption for those soil layers that are
above the groundwater table is to assume zero pore water pressure. This is a conservative
assumption since the soil will often have negative pore water pressures due to capillary
effects.
2.
Groundwater table:
A second situation concerns those soils located below the
groundwater table. If the groundwater table is horizontal, then the pore water pressures
below the groundwater table are typically assumed to be hydrostatic. For the condition of
seepage through the slope (i.e., a sloping groundwater table), a flow net can be drawn in
order to estimate the pore water pressures below the groundwater table. Most slope stabil-
ity computer programs have the ability to estimate the pore water pressures below a slop-
ing groundwater table.
3.
Pore water pressure ratio r
u
:
A third choice for dealing with pore water pressures
is to use the pore water pressure ratio. The pore water pressure ratio is
r
u
u
(
t
h
), where
u
pore water pressure,
t
total unit weight of the soil, and
h
depth below the ground
surface. If a value of
r
u
0 is selected, then the pore water pressures
u
are assumed to be
equal to zero in the slope.
Suppose an
r
u
value is used for the entire slope. In many cases the total unit weight
is about equal to 2 times the unit weight of water (i.e.,
t
2
w
), and thus a value of
r
u
0.25 is similar to the effect of a groundwater table at midheight of the slope. A value
of
r
u
0.5 would be similar to the effect of a groundwater table corresponding to the
ground surface.
The pore water pressure ratio
r
u
can be used for existing slopes where the pore water
pressures have been measured in the field, or for the design of proposed slopes where it is
desirable to obtain a quick estimate of the effect of pore water pressures on the stability of
the slope.
In summary, the pseudostatic approach utilizes the same cross section and condi-
tions that apply for the static slope stability case. The only additional information that
most computer programs require to perform the pseudostatic method is the seismic
coefficient
k
h
.
9.2.7 Example Problem
The purpose of this section is to present an example problem dealing with the use of the
pseudostatic slope stability analysis based on the method of slices. A cross section
