Building Design and Construction Handbook - Merritt - Ventech!

6.110 SECTION SIX
into the soil. Based on the mass of soil within the cylinder, the wet density can be
calculated. Once the water content (w) of the soil is obtained, the dry density
(� d) of the fill can be calculated.
Probably the most common type of nondestructive field test is the nuclear
method. In this method, the wet density is determined by the attenuation of gamma
radiation. The nuclear method can give inaccurate results (density too high) where
oversize particles are present, such as coarse gravel and cobbles. Likewise, if there
is a large void in the source-detector path, then unusually low density values may
be recorded.
6.10.2 Site Improvement Methods
If the expected settlement for a proposed structure is too large, then different foundation
support or soil stabilization options must be evaluated. As discussed in Art.
6.8.2, one alternative is a deep foundation system that can transfer structural loads
to adequate bearing material in order to bypass a compressible soil layer. Another
option is to construct a floating foundation, which is a special type of deep foundation
where the weight of the structure is balanced by the removal of soil and
construction of an underground basement. Other alternatives include site improvement
methods, such as the following (see Table 6.21):
Soil Replacement. As indicated in Table 6.21, there are basically two types of
soil replacement methods: (1) removal and replacement, and (2) displacement.
The first is the most common approach and consists of the removal of the compressible
soil layer and replacement with structural fill during the grading operations.
Usually the remove and replace grading option is economical only if
the compressible soil layer is near the ground surface and the groundwater table
is below the compressible soil layer or the groundwater table can be economically
lowered.
Water Removal. Table 6.21 lists several different types of water removal site
improvement techniques. 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 (see Table 6.21). 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 thickconsistency
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