(a) Richards et al. (1993) suggest that, in addition
to strength reductions accompanying high excess pore
e.
Landsliding. Prior to performing engineering
water pressures and liquefaction, lateral inertial forces
analyses to assess landslide potential, the data gathered
in the soil may reduce the bearing capacity of a shallow
in the screening stage should be supplemented if
foundation system, thereby affecting the settlement
necessary. More detailed geologic reconnaissance and
performance of the foundation. However, the
mapping may be needed. If preexisting landslides were
importance of this phenomenon in comparison to the
identified at the site in the screening stage, subsurface
geologic hazards addressed in this appendix is not yet
investigations may be required to assess the slide
clearly demonstrated by case histories. The
geometry. Geotechnical data should be reviewed to
phenomenon should be considered when evaluating
assess the engineering properties of the subsurface
foundation bearing capacities as part of a seismic
materials in the slope(s). If sufficient data are lacking,
rehabilitation design process.
supplemental field and laboratory testing may be
required. For slopes located in stiff, nonsensitive clays,
(10) Consequences of liquefaction -- increased
dry sands, and saturated sands that do not liquefy or
lateral pressures on walls. Behind a wall, the buildup
lose their strength during earthquake shaking, the
of pore water pressures during the liquefaction process
increases the pressure on the wall. This pressure is a
stability of the slopes can be evaluated using either
static pressure which reduces with time after the
pseudo-static analysis or deformation analysis
earthquake as pore pressures dissipate. Ebeling and
procedures. The deformation behavior of slopes that
Morrison (1992) provide procedures for assessing
liquefy is addressed in paragraph F-4c.
effects of variable amounts of pore pressure buildup on
the lateral pressures behind walls. In addition, the
(1) Pseudo-static analysis procedure. The
Ebeling and Morrison (1992) procedures cover the
pseudo-static analysis can be used in the initial
transient, dynamic pressures on walls induced by
evaluation. In the pseudo-static analysis, inertial forces
earthquake ground shaking. Both types of increases in
generated by the earthquake are represented by an
lateral pressures due to earthquakes may influence the
equivalent static horizontal force (seismic-coefficient)
behavior of retaining walls, although most cases of
acting on the potential sliding mass. In this analysis,
retaining wall failures during earthquakes have been
the seismic coefficient should be equal to the peak
associated with liquefaction of loose sand backfills
ground acceleration in the vicinity of the slope. The
behind waterfront retaining walls. Department of
factor of safety for a given seismic coefficient can be
Defense (1997) presents design procedures for steel
estimated using limit equilibrium slope stability
sheet pile walls based on the procedures developed by
Ebeling and Morrison (1992).
methods. A computed factor of safety greater than one
indicates that the slope is stable and further evaluations
(11) Consequences of liquefaction -- flotation of
are not required. A computed factor of safety of less
buried structures. The potential for flotation of a buried
than one indicates that the slope will yield and
or embedded structure can be evaluated by comparing
deformations can be expected. In this case, an estimate
the total weight of the buried or embedded structure
of the expected slope deformations should be made
with the increased uplift forces occurring due to the
using the procedures described below.
buildup of liquefaction-induced pore water pressures.
(2) Deformation analysis procedures. Simplified
d. Differential compaction. The procedures
procedures for estimating deformations of slopes
described by Tokimatsu and Seed (1987) are suggested
during earthquake shaking are based on the concept of
for estimating earthquake-induced settlements due to
yield acceleration originally proposed by Newmark
densification of saturated and unsaturated cohesionless
(1965). Newmark's method has been modified and
soils. Other procedures can be used if justified. The
augmented by several investigators (Goodman and
principal soil parameter required for evaluations using
Seed, 1966; Ambraseys, 1973;
the Tokimatsu and Seed (1987) method is the
normalized Standard Penetration Test (SPT) resistance,
(N1)60, in blows/foot. Appendix G provides an
example of the application of this methodology. It is
noted that the procedure provides an estimate of the
total earthquake-induced settlement at a site for a given
soil profile. The differential settlement must then be
assessed based on considerations of soil variability and
other factors.
F-29
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