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below the compressible soil layer or the groundwater table can be economically
lowered.
Water Removal. If the site contains an underlying compressible cohesive
soil layer, the site can be surcharged with a fill layer placed at ground surface.
Vertical drains (such as wick drains or sand drains) can be installed in the
compressible soil layer to reduce the drainage path and speed up the
consolidation process. Once the compressible cohesive soil layer has had
sufficient consolidation, the fill surcharge layer is removed and the building is
constructed.
Site Strengthening. Many different methods can be used to strengthen the
onsite soil. For example, deep vibratory techniques are often used to increase
the density of loose sand deposits.
Grouting. In order to stabilize the ground, fluid grout can be injected into
the ground to fill in joints, fractures, or underground voids. For the releveling of
existing structures, one option is mudjacking, which has been defined as a
process whereby a water and soil-cement or soil-lime cement grout is pumped
beneath the slab, under pressure, to produce a lifting force that literally floats
the slab to the desired position. Another commonly used site improvement
technique is compaction grouting, which consists of intruding a mass of very
thick consistency grout into the soil, which both displaces and compacts the
loose soil. Compaction grouting has proved successful in increasing the density
of poorly compacted fill, alluvium, and compressible or collapsible soil. The
advantages of compaction grouting are less expense and disturbance to the
structure than foundation underpinning, and it can be used to relevel the
structure. The disadvantages are that analyzing the results is difficult; it is
usually ineffective near slopes or for near-surface soils because of the lack of
confining pressure, and the danger exists of filling underground pipes with
grout.
Thermal. The thermal site improvement method consists of either heating
or freezing the soil in order to improve its shear strength and reduce its
permeability.
