Civil Engineering Reference
In-Depth Information
12.3.3 Site Strengthening
Many different methods can be used to strengthen the on-site soil (see Table 12.1). Examples
are as follows:
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Dynamic compaction methods:
For example, heavy tamping consists of using a crane
that repeatedly lifts and drops a large weight onto the ground surface in order to vibrate
the ground and increase the density of near-surface granular soils. Although this method
can increase the density of soil to a depth of 60 ft (18 m), it is usually only effective to
depths of approximately 20 to 30 ft (6 to 9 m). In addition, this method requires the fill-
ing of impact craters and releveling of the ground surface.
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Compaction piles:
Large-displacement piles, such as precast concrete piles or hollow
steel piles with a closed end, can be driven into the ground to increase the density of the
soil. The soil is densified by both the actual displacement of the soil and the vibration of
the ground that occurs during the driving process. The piles are typically left in place,
which makes this method more expensive than the other methods. In addition, there must
be relatively close spacing of the piles in order to provide meaningful densification of soil
between the piles.
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Blasting:
Deep densification of the soil can be accomplished by blasting. This method
has a higher risk of injury and damage to adjacent structures. There may be local restric-
tions on the use of such a method.
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Compaction with vibratory probes:
Deep vibratory techniques, such as illustrated in
Fig. 12.1, are often used to increase the density of loose sand deposits. This method is
considered to be one of the most reliable and comprehensive methods for the mitigation
of liquefaction hazard when liquefiable soils occur at depth (R. B. Seed 1991). Some
techniques can be used to construct vertical gravel drains (discussed below).
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Vertical gravel drains:
Vibroflotation or other methods are used to make a cylindrical
vertical hole, which is filled with compacted gravel or crushed rock. These columns of
gravel or crushed rock have a very high permeability and can quickly dissipate the earth-
quake-induced pore water pressures in the surrounding soil. This method can be effective
in reducing the loss of shear strength, but it will not prevent overall site settlements. In
addition, the method can be effective in relatively free-draining soils, but the vertical
columns must be closely spaced to provide meaningful pore pressure dissipation. If the
drain capacity is exceeded by the rate of pore pressure increase, there will be no partial
mitigation (R. B. Seed 1991).
12.3.4 Grouting
There are many types of grouting methods that can be used to strengthen the on-site soil
(see Table 12.1). For example, to stabilize the ground, fluid grout can be injected into the
ground to fill in joints, fractures, or underground voids (Graf 1969, Mitchell 1970). 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 which literally floats the slab to the desired
position (Brown 1992). Other site improvement grouting methods are as follows:
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Compaction grouting:
A 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 (Brown and Warner 1973; Warner
1978, 1982). Compaction grouting has proved successful in increasing the density of
