sidewalks, thrusting of sidewalks and curbs over
fault rupture of the 1971 San Fernando, California,
streets, breakage of utility lines, and the like.
earthquake (moment magnitude 6.6), which was of the
reverse- or thrust-fault type (see Figure F-1). More
(2) Figure F-4 illustrates the consequence of loss of
than 1.8 m (6 feet) of combined vertical and horizontal
foundation bearing capacity that occurred during the
displacement occurred along the surface trace of the
1964 Niigata earthquake in Japan. As shown,
fault during the San Fernando earthquake.
apartment buildings experienced large settlements and
tilts due to liquefaction of the underlying soil.
b. Soil liquefaction. Soil liquefaction is a
phenomenon in which a soil deposit below the
(3) Liquefaction-induced lateral movements can
groundwater table loses a substantial amount of
occur on extremely flat slopes, less than 1 percent in
strength due to strong earthquake ground shaking. The
some cases. The potential for lateral movements is
reason for the strength loss is that some types of soil
increased if there is a "free face," such as a river
tend to compact during earthquake shaking and this
channel or the sloping shoreline of a lake or bay,
tendency for compaction will induce excess pore water
toward which movements can occur. The hazard of
pressures which, in turn, causes strength reduction in
lateral spreading is illustrated diagrammatically in
the soil. Recently deposited (i.e. geologically young)
Figure F-5. Figure F-6 illustrates the effect of lateral
and relatively loose natural soils and uncompacted or
spreading on a building during the 1989 Loma Prieta
poorly compacted fills are potentially susceptible to
earthquake; the movements pulled the structure apart.
liquefaction. Loose sands and silty sands are
particularly susceptible. Loose silts and gravels also
c. Soil differential compaction. Differential
have potential for liquefaction. Dense natural soils and
compaction refers to the densification of soils that may
well-compacted fills have low susceptibility to
occur due to strong earthquake ground shaking. As
liquefaction. Clay soils are generally not susceptible,
noted above, densification can occur with time
except for highly sensitive clays found in some
following liquefaction as soil excess pore water
geographic locales.
pressures dissipate. In soils that are above the
groundwater table and thus not susceptible to
(1) Potential consequences of liquefaction include:
liquefaction, densification can occur as the strong
(1) reduction or loss of foundation bearing strength,
ground shaking occurs. Loose natural soils and
which can lead to large structure settlements due to
uncompacted and poorly compacted fills are
shear failure in the weakened soils; (2) flotation of
susceptible to densification. If densification does not
lightweight structures embedded in liquefied soil;
occur uniformly over an area, the resulting differential
(3) differential compaction, due to soil densification as
settlements can be damaging to structures. In general,
excess pore water pressures dissipate, that can lead to
the amounts of movement associated with the hazard of
structure differential settlement; (4) horizontal
differential compaction are less than those due to
movements due to lateral spreading or flow sliding of
liquefaction-induced bearing capacity failure or lateral
liquefied soils, which can lead to total and differential
spreading.
lateral movements of structures; and (5) increased
lateral pressures on retaining walls for liquefied soils.
Other manifestations of liquefaction can also occur and
d. Landsliding. Landsliding can occur due to the
may or may not pose a risk to structures. Sand boils
loss of soil strength accompanying liquefaction, as
are common surface manifestations of liquefaction, in
mentioned above. However, landsliding can also occur
which the liquefied soil under pressure is ejected to the
in soils and rocks on hillside slopes in the absence of
ground surface through a vent and forms a conical-
liquefaction, due to the inertia forces induced by the
shaped "sand boil" deposit around the vent. Although
ground shaking. Consequences of landsliding include
sand boils are usually not a cause of damage to
differential lateral and vertical movements of a
structures, the ejection of subsurface materials in a sand
structure located within the landslide zone, or landslide
boil may pose a settlement hazard to an immediately
debris impacting a structure located below a landslide.
adjacent structure. Another phenomenon
An example of a structure within a zone of earthquake-
accompanying liquefaction is ground oscillation, in
induced
which the ground overlying liquefied soil experiences
large-displacement transient oscillations that can result
in extensional and compressional ground failures such
as opening and closing of fissures, buckling of
F-5
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