Civil Engineering Reference
In-Depth Information
TABLE 15.3
Summary of Analyses for Soil Weakened During the Earthquake—Slope Stability
Topic
Discussion
Types of
slope failures
The material types, minimum slope inclinations, and the different types of
earthquake-induced slope movement for rock and soil slopes are presented
in Tables 3.1 and 3.2. The threshold earthquake values needed to cause slope
movement and the relative abundance of earthquake-induced slope movement for
rock and soil slopes are summarized in Tables 9.1 and 9.2.
Liquefaction
for
sloping
ground
The analysis for liquefaction discussed in Table 15.2 is for level-ground sites.
When the slope inclination is greater than about 6° and the relative density
D
r
of
the soil is less than about 45, consider a reduction in the factor of safety against
liquefaction (see Sec. 9.4.2).
Slope
stability—
flow slides
Determine if the slope will have mass liquefaction, zonal liquefaction, or
liquefaction of layers or seams of soil. Because of the destructive effects of flow
slides, use a conservative approach and assume the liquefied soil will have zero
shear strength. The type of analysis versus soil type is summarized in Table 5.4.
Effective stress analyses:
Use an effective stress analysis if the slope consists
of cohesionless soil. For the liquefiable layer(s), use
r
u
1.0 or use
0 and
c
0. For all other layers, use effective stress parameters (
and
c
). Since it
is an effective stress analysis, the earthquake-induced pore water pressures must
be estimated. Use Fig. 5.15 to estimate
r
u
based on the factor of safety against
liquefaction.
Total stress analyses:
Use a total stress analysis if the slope consists of
cohesive soil with liquefiable sand layer(s). For the liquefiable sand layer(s), use
0 and
c
0. For all other layers, use total stress parameters (
s
u
or
and
c
).
If there are soft clay layers that will be susceptible to cyclic softening during
significant earthquake shaking, then use ultimate or fully softened undrained
shear strength values. The analysis does not include pore water pressures because
it is a total stress analysis.
If the factor of safety is 1.0 or less, then a flow slide is likely during the
earthquake. If the factor of safety is greater than one, there could still be lateral
spreading of the slope because of the layer(s) of liquefiable soil.
Slope
stability—
lateral
spreading
As discussed in Sec. 9.5.2, the lateral spreading equations will usually predict
a significant amount of horizontal ground displacement for the condition of
liquefaction at a sloping ground site. For example, using Eq. (9.5) with a
slope inclination of 5% and only a one-foot (0.3 m) thick layer of liquefiable
soil, the calculated lateral movement due to lateral spreading is 2.3 ft (0.7 m).
A steeper slope and/or thicker layer of liquefiable soil will result in even
greater values of lateral movement due to lateral spreading. Since most roads,
utilities, and foundations are weak in tension, they are unable to accommodate
this large amount of lateral movement. Remedial measures will likely be
needed, such as the densification of the potentially liquefiable layer and use
of strengthened foundations.
Slope
stability—
cyclic
softened
cohesive soil
In this case, there is no liquefaction of the soil comprising the slope, but instead
there is cyclic softening of soft cohesive soil that has a high water content. The
loss of undrained shear strength is typically due to an earthquake-induced increase
in pore water pressure.
For significant earthquake shaking, use a total stress analysis and use the ultimate
or fully softened undrained shear strength values. The analysis does not include
