FEATURE
uncertainty regarding the potential penetration
into these layers during installation
of low displacement steel piles, such
as H or open pipe sections that may have
a relatively small bearing area. Most engineers
rely on local experience and/or test
pile installations to develop a rational
approach to design and construction.
Summary
The installation of driven piles has a substantial
impact on the fabric, relative density
and state of stress in the soil around
the pile, and the effects of these changes
profoundly impact the axial resistance of
the pile. Furthermore, changes in the soil,
the state of stress and the resulting axial
resistance of the pile occur as a function
of time long after the pile is installed.
How and why does the axial
behavior of a drilled pile differ?
The most common types of drilled foundations
in soil include continuous flight
auger piles (CFA), also referred to as
augered-cast-in-place piles, and drilled
shaft foundations, also referred to as
bored piles or drilled piers. Micropiles
are another common type of drilled
foundation, but less commonly employed
into a soil profile and so will not be directly
addressed in this article. Likewise,
drilled displacement piles (which are
often used as a form of ground improvement
and sometimes used as foundation
piles) will not be included for brevity.
Drilled shaft foundations are very
often installed into rock so that the structural
capacity of a large reinforced concrete
column can be realized. The discussion
in this article will focus on the
behavior of drilled foundations in soil,
which may not be relevant if a drilled
shaft is installed on or into rock. CFA
piles generally cannot be drilled into hard
rock, although they may be successfully
installed into soft limestone or weakly
cemented materials. The term “pile” will
be used generally for both types of drilled
foundations, and the term “concrete” will
also be used generally even though CFA
piles are typically constructed using a
sand-cement grout mixture.
Both CFA and drilled shafts in soil differ
fundamentally from driven piles in
two main respects:
1. The pile is constructed by excavating
and replacing the soil rather than by
displacing it, and
2. The pile/soil interface is affected by
the drilling and casting of concrete
rather than by the soil remolding
around a prefabricated pile element.
When a hole is excavated into the soil,
the stability of the excavation must be
maintained until the concrete placement
is completed. The stability of a properly
constructed hole is maintained in one of
the following ways:
1. The soil has sufficient cohesion or
cementation that it remains stable on
its own, without any internal support,
2. The CFA auger flights remain filled
with soil so that the sidewall does not
continually collapse into the augers,
3. The drilled shaft excavation is maintained
with a support fluid or casing.
Obviously, if inadequate internal
support is not provided and the soil is
not stable on its own, then collapse can
occur resulting in subsidence around the
hole and loosening of the ground that is
intended to support the structure. Such
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