then ground modification beyond the immediate
most cost-effective way to mitigate a liquefaction
building area may need to be considered. This is
hazard if the densification process can be undertaken
because the potential for lateral spreading movements
without adverse effects on adjacent structures
beneath a building may depend on the behavior of the
(e.g., potential effects of settle ment or vibration).
soil mass at distances well beyond the building as well
Figure F-21 illustrates the technique of vibro-
as immediately beneath it. Thus, measures to prevent
replacement in which a vibrating probe is inserted into
lateral spreading may, in some cases, require
the soil at close spacings and gravel or crushed rock is
stabilizing large soil volumes and/or constructing
also placed at the vibration locations to create a dense
buttressing structures that can reduce the potential for
gravel column surrounded by densified in-placed soil.
or the amount of lateral movements.
(3) Different types of grouting that can be
(6) Modifications to the structure or its foundation
considered include permeation grouting, compaction
may also be considered to mitigate the consequences.
grouting, and formation of grouted soil columns.
If the predicted movements are small, the structure can
Permeation grouting involves injecting chemical grout
be strengthened to resist the deformations. The
into liquefiable sands to essentially replace the pore
foundation system can be designed to reduce or
water and create a non-liquefiable solid material in the
eliminate the potential for large foundation
grouted zone. The more fine-grained and silty the
displacements, for example by using deep foundations
sands, the less effective is permeation grouting.
to achieve bearing on a deeper, non-liquefiable strata.
Compaction grouting involves pumping a mixture of
Alternatively, a shallow foundation system can be made
soil, cement, and water into the ground to form bulbs of
more rigid (for example by a system of well-reinforced
grouted material. The formation of these bulbs
grade beams or mats between isolated footings) in
compresses and densifies the surrounding soil, thus
order to reduce the differential ground movements
reducing its liquefaction potential. However, the
transmitted to the structure.
amounts of densification that can be achieved may be
limited because static compression is less effective than
(7) Conceptual schemes to mitigate liquefaction-
vibration in densifying sands. Compaction grouting
induced settlement or bearing capacity reductions are
must be done carefully to avoid creating unacceptable
illustrated in Figure F-22. Conceptual schemes to
heaving or lateral displacements of adjacent structures
mitigate liquefaction induced lateral spreading are
during the grouting process. The mixing or injection of
illustrated in Figure F-23. Remediation methodologies
grout locally beneath foundation locations can also be
are discussed in more detail in a number of
accomplished to form stabilized columns of soil to
publications, including Mitchell (1981), Ledbetter
transfer vertical foundation loads to deeper
(1985), National Research Council (1985), ASCE
nonliquefiable strata.
(1997), Department of Defense (1997), and Ferritto
(1997b), Mitchell et al. (1998).
(4) Drain installation (e.g., stone or gravel
columns) involves creating closely spaced, vertical
columns of permeable material in the liquefiable soil
d. Soil differential compaction. For cases of
strata. Their purpose is to dissipate soil pore water
predicted significant differential settlements of a
pressures as they build up during the earthquake
building, mitigation options are similar to those for
shaking, thus preventing liquefaction from occurring.
mitigating liquefaction potential beneath a building.
To achieve the objective of high permeability in the
These options include modifying the soil or
gravel column, it must be constructed by a method that
groundwater conditions beneath the building, designing
avoids contamination by a mixing of the gravel with the
the structure to withstand the ground movements, or
surrounding soil. Permanent dewatering systems lower
modifying the foundation system by deepening or
groundwater levels below liquefiable soil strata, thus
stiffening.
preventing liquefaction.
(5) All of the above techniques can potentially be
applied beneath the building area to prevent the
occurrence or reduce the extent and effects of
liquefaction. If the assessed consequences of
liquefaction are reduction of bearing capacity and/or
building settlements, these measures should be
sufficient. However, if a potential for significant
liquefaction-induced lateral spreading exists at a site,
F-39
Previous Page
