Civil Engineering Reference
In-Depth Information
TABLE 15.3
Summary of Analyses for Soil Weakened During the Earthquake—Slope
Stability (
Continued
)
Topic
Discussion
Slope
stability—
cyclic
softened
cohesive soil
(
Continued
)
pore water pressures because it is a total stress analysis. If the slope stability
analysis indicates a factor of safety greater than 1.0, the slope could still be
subjected to significant movement because the lateral out-of-slope earthquake
inertial forces have not been included in the analysis.
Slope
stability—
strain-
softened
cohesive soil
Highly sensitive and quick clays are considered strain-softening soils because
there can be a substantial reduction in shear strength once the peak shear strength
is exceeded. A classic example of strain-softening sensitive clay causing a slope
failure is the Turnagain Heights landslide (Sec. 3.5.2). Because of the sloping
environment, the clay was already subjected to a horizontal shear stress. The
additional shear stress induced into the clay layer by the earthquake inertial forces
exceeded the peak shear strength, leading to a significant loss of shear strength
with strain.
For a slope stability analysis, the most important factor is the level of static
shear stress versus the peak shear strength of the sensitive soil and the amount
of additional shear stress that will be induced into the soil by the earthquake.
If the existing static shear stress is close to the peak shear stress, then only a
small additional earthquake-induced shear stress will be needed to exceed the
peak shear strength. Once this happens, the shear strength will significantly
decrease with strain, resulting in substantial lateral movement of the slope. If this
is anticipated to occur, then use the ultimate or fully softened undrained shear
strength values in the slope stability analysis. The analysis does not include pore
water pressures because it is a total stress analysis. If the slope stability analysis
indicates a factor of safety greater than 1.0, the slope could still be subjected to
significant movement because the lateral out-of-slope earthquake inertial forces
have not been included in the analysis.
Slope
stability—
granular soil
with excess
pore water
pressures
In this case, there is no liquefaction of the soil comprising the slope, but instead
there is an increase in pore water pressure as the granular soil contracts during
the earthquake. Use an effective stress analysis and estimate the earthquake-
induced pore water pressures by using the pore water pressure ratio
r
u
from
Fig. 5.15. If the slope stability analysis indicates a factor of safety greater
than 1.0, the slope could still be subjected to significant movement because
the lateral out-of-slope earthquake inertial forces have not been included in
the analysis.
