The main goal of most soil improvement techniques used for reducing
liquefaction hazards is to avoid large increases in pore water pressure during
earthquake shaking. This can be achieved by densification of the soil and/or
improvement of its drainage capacity.
Vibroflotation involves the use of a vibrating probe that can penetrate
granular soil to depths of over 100 feet. The vibrations of the probe cause
the grain structure to collapse thereby densifying the soil surrounding the
probe. To treat an area of potentially liquefiable soil, the vibroflot
is raised and lowered in a grid pattern. Vibro Replacement
(right, HB) is a
combination of vibroflotation with a gravel backfill resulting in
stone columns, which not only increases the amount of densificton,
but provides a degree of reinforcement and a potentially effective means
Densifiction by dynamic compaction is performed
by dropping a heavy weight of steel or concrete in a grid pattern from heights
of 30 to 100 ft. It provides an economical way of improving soil for mitigation of
liquefaction hazards. Local liquefaction can be initiated beneath the drop point
making it easier for the sand grains to densify. When the excess porewater pressure
from the dynamic loading dissipates, additional densification occurs. As illustrated
in the photograph, however, the process is somewhat invasive; the surface of the soil
may require shallow compaction with possible addition of granular fill following
As described above, stone columns are
columns of gravel constructed in the ground. Stone columns can be constructed
by the vibroflotation method. They can also be installed in other ways, for
example, with help of a steel casing and a drop hammer as in the Franki Method.
In this approach the steel casing is driven in to the soil and gravel is
filled in from the top and tamped with a drop hammer as the
steel casing is successively withdrawn.
Installing compaction piles is a very effective
way of improving soil. Compaction piles are usually made of prestressed concrete or
timber. Installation of compaction piles both densifies and reinforces the
soil. The piles are generally installed in a grid pattern and are generally
driven to depth of up to 60 ft.
Compaction grouting is a technique whereby a slow-flowing water/sand/cement mix
is injected under pressure into a granular soil. The grout forms a bulb that
displaces and hence densifies, the surrounding soil
(right, HB). Compaction
grouting is a good option if the foundation of an existing building requires
improvement, since it is possible to inject the grout from the side or at an
inclined angle to reach beneath the building.
Liquefaction hazards can be
reduced by increasing the drainage ability of the soil. If the porewater within
the soil can drain freely, the build-up of excess pore water pressure will be
reduced. Drainage techniques include installation of drains of gravel, sand or
synthetic materials. Synthetic wick drains can be installed at various angles, in
contrast to gravel or sand drains that are usually installed vertically. Drainage
techniques are often used in combination with other types of soil improvement techniques
for more effective liquefaction hazard reduction.
For information on other soil improvement techniques, see these
to related web sites.
A number of methods
can be used to verify the effectiveness of soil improvement. In-situ techniques
are popular because of the limitations of many laboratory techniques.
Usually, in-situ test are performed to evaluate the liquefaction
potential of a soil deposit before the improvement was attempted. With the knowledge of
the existing ground characteristics, one can then specify a necessary level of
improvement in terms of insitu test parameters. Performing in-situ tests after
improvement has been completed allows one to decide if the degree of improvement was
satisfactory. In some cases, the extent of the improvement is not reflected in in-situ
test results until some time after the improvement has been completed