Building Design and Construction Handbook - Merritt - Ventech!

SOIL MECHANICS AND FOUNDATIONS 6.91
(‘‘Design and Construction of Post-tensioned Slabs-on-Ground,’’ 2d ed., Phoenix)
provides typical anchorage details for the tendons.
4. Shallow Foundation Alternatives. If the expected settlement for a proposed
shallow foundation is too large, then other options for foundation support or soil
stabilization must be evaluated. Some commonly used alternatives are as follows:
(a) Grading. Grading operations can be used to remove the compressible soil
layer and replace it with structural fill. Usually the 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.
(b) Surcharge. If the site contains an underlying compressible cohesive soil
layer, the site can be surcharged with a fill layer placed at the ground surface.
Vertical drains (such as wick drains or sand drains) can be installed
in the compressible soil layer to reduce the drainage paths 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.
(c) Densification of Soil. Many different methods can be used to densify loose
or soft soil. For example, vibro-flotation and dynamic compaction are often
effective at increasing the density of loose sand deposits. Another option 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.
(d) Floating Foundation. A floating foundation 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.
6.8.2 Deep Foundations
Probably the most common type of deep foundation is the pile foundation. Table
6.15 presents pile type characteristics and uses. Piles can consist of wood (timber),
steel H-sections, precast concrete, cast-in-place concrete, pressure injected concrete,
concrete filled steel pipe piles, and composite type piles. Piles are either driven into
place or installed in predrilled holes. Piles that are driven into place are generally
considered to be low displacement or high displacement, depending on the amount
of soil that must be pushed out of the way as the pile is driven. Examples of lowdisplacement
piles are steel H-sections and open-ended steel pipe piles that do not
form a soil plug at the end. Examples of high-displacement piles are solid section
piles, such as round timber piles or square precast concrete piles, and steel pipe
piles with a closed end.
A cast-in-place pile is formed by making a hole in the ground and then filling
the hole with concrete. As shown in Fig. 6.42, in its simplest form, the cast-inplace
pile consists of an uncased hole that is filled with concrete. If the soil tends
to cave into the hole, then a shell-type pile can be installed (see Fig. 6.42). This
consists of driving a steel shell or casing into the ground. The casing may be driven
with a mandrel, which is then removed, and the casing is filled with concrete. In
other cases, the casing can be driven into place and then slowly removed as the
hole is filled with concrete.
Figure 6.43 shows typical pile configurations.