Civil Engineering Reference
In-Depth Information
10.3.4 Summary
As discussed in the previous sections, the liquefaction of soil can affect the retaining wall in
many different ways. It is also possible that even with a factor of safety against liquefaction
greater than 1.0, there could still be significant weakening of the soil, leading to a retaining
wall failure. In summary, the type of analysis should be based on the factor of safety against
liquefaction FS
L
as follows:
1.
FS
L
1.0 : In this case, the soil is expected to liquefy during the design earthquake, and
thus the design pressures acting on the retaining wall must be adjusted (see Sec. 10.3.2).
2.
FS
L
2.0 : If the factor of safety against liquefaction is greater than about 2.0, the pore
water pressures generated by the earthquake-induced contraction of the soil are usually
small enough that they can be neglected. In this case, it could be assumed that the earth-
quake does not weaken the soil, and the pseudostatic analyses outlined in Sec. 10.2
could be performed.
3.
1.0
FS
L
2.0 : For this case, the soil is not anticipated to liquefy during the earthquake.
However, as the loose granular soil contracts during the earthquake, there could still be a
substantial increase in pore water pressure and hence weakening of the soil. Figure 5.15 can
be used to estimate the pore water pressure ratio
r
u
for various values of the factor of safety
against liquefaction FS
L
. The analysis would vary depending on the location of the increase
in pore water pressure as follows:
●
Passive wedge:
If the soil in the passive wedge has a factor of safety against lique-
faction greater than 1.0 but less than 2.0, then the increase in pore water pressure
would decrease the effective shear strength and the passive resisting force would be
reduced [i.e., passive resistance
r
u
)].
●
Bearing soil:
For an increase in the pore water pressure in the bearing soil, use the
analysis in Sec. 8.3.
●
Active wedge:
In addition to the pseudostatic force
P
E
and the active earth pressure
resultant force
P
A
,
include a force that is equivalent to the anticipated earthquake-induced
pore water pressure.
P
p
(1
10.4 RETAINING WALL ANALYSES FOR
WEAKENED SOIL
Besides the liquefaction of soil, many other types of soil can be weakened during the earth-
quake. In general, there are three cases:
1.
Weakening of backfill soil:
In this case, only the backfill soil is weakened during
the earthquake. An example would be backfill soil that is susceptible to strain softening
during the earthquake. As the backfill soil weakens during the earthquake, the force exerted
on the back face of the wall increases. One design approach would be to estimate the shear
strength corresponding to the weakened condition of the backfill soil and then use this
strength to calculate the force exerted on the wall. The bearing pressure, factor of safety for
sliding, factor of safety for overturning, and location of the resultant vertical force could
then be calculated for this weakened backfill soil condition.
2.
Reduction in the soil resistance:
In this case, the soil beneath the bottom of the wall
or the soil in the passive wedge is weakened during the earthquake. For example, the bearing
soil could be susceptible to strain softening during the earthquake. As the bearing soil
weakens during the earthquake, the wall foundation could experience additional settlement,
a bearing capacity failure, sliding failure, or overturning failure. In addition, the weakening of
