Civil Engineering Reference
In-Depth Information
TABLE 15.5
Summary of Analyses for Soil Not Weakened by the Earthquake (
Continued
)
Topic
Discussion
Slope
stability—
pseudostatic
method for
landslides
The pseudostatic method can be used for existing landslides where the shear
strength along the entire slide plane is equal to the drained residual shear strength
r
. Use an effective stress analysis and use the same residual shear strength
r
and pore water pressures for the static case and the pseudostatic analysis.
For large landslides, Seed (1979b) recommended
k
h
0.10 for sites near
M
8.5 earthquakes, and the
acceptable pseudostatic factor of safety is 1.15 or greater.
6.5 earthquakes,
k
h
0.15 for sites near
M
Slope
stability—
Newmark
method
If the pseudostatic factor of safety is less than 1.0, the Newmark method [Eq. (9.3)]
can be used to estimate the slope movement. The Newmark method assumes no
deformation of the slope if the pseudostatic factor of safety is greater than 1.0.
The Newmark method should be used only for slopes that will deform as an
intact massive block, and not for those cases of individual soil particle movement
(Sec. 9.3).
Retaining walls
If there is no weakening of the soil beneath the retaining wall footing, in the passive
zone, and behind the wall, the usual recommendation is to allow a one-third
increase in footing bearing pressure and passive resistance for seismic loading
conditions. Use the pseudostatic method with the value of the seismic coefficient
k
h
a
max
g
. Use Eqs. (10.7), (10.8), or (10.9) to determine the pseudostatic
force
P
E
or
P
AE
and calculate the factor of safety for sliding, factor of safety for
overturning, location of resultant vertical force, and maximum bearing pressure.
An acceptable factor of safety for sliding and overturning is usually 1.1 or greater.
Deep
foundations
When the material will not be weakened by the earthquake, a one-third increase
in pile or pier capacity is usually allowed for seismic analyses. Check to make
sure the deep foundation will not punch downward into a weaker underlying
layer.
15.5
SUMMARY OF MITIGATION MEASURES
Mitigation measures are covered in Part 3 of this topic. In summary, mitigation measures
can be divided into four broad categories, as follows:
1.
Avoidance:
Where the potential for failure is beyond the acceptable level and not
preventable by practical means, the hazard should be avoided. Examples include geo-
logic hazards, such as surface fault rupture, regional subsidence, and tsunami (Chap. 3).
Developments should not be built in active fault zones or low-lying areas subjected to
regional subsidence or tsunami. For example, the
International Building Code
(2012) in
Appendix M states that buildings that represent a substantial hazard to human life in the
event of failure and essential facilities shall be prohibited within a Tsunami Hazard Zone.
2.
Prevention:
The goal of prevention is to remove or improve the in situ soil so that
the amount of foundation movement during the earthquake is within acceptable values.
Commonly used methods include soil replacement, water removal, site strengthening, and
grouting (Table 12.1). Other prevention measures include dynamic compaction methods,
compaction piles, compaction with vibratory probes, and vertical gravel drains (Sec. 12.3.3).
For slope stability hazards, the stability of the slope can be increased by flattening the
slope, removing unstable or potentially unstable material, and controlling groundwater
