FEATURE
Environmental or public relations aspects of
pile installation such as noise and vibration,
real or perceived, often influence some
public agencies to avoid pile driving.
a rational means of verification that the
axial resistance of production piles is
achieved, subject to limitations (set-up
time, full mobilization of resistance, pipe
pile plug behavior, etc.). Static, dynamic
and rapid load testing can be performed
on CFA piles in the same way, although
routine testing of production piles is not
a routine part of verification as for driven
piles. Bi-directional load cell testing of
drilled foundations enables loading of
large diameter to substantial loads of a
magnitude that may be impractical with
conventional testing.
The point of this discussion is that
the reliable prediction of any deep foundation
element requires performance
verification through field measurements,
and fortunately for foundation engineers,
such measurements are obtainable with
current technology. The key to success is
that field load testing be performed with
an understanding of the specifics of both
the unique ground characteristics for the
project and for the specific installation
details pertaining to the piles.
So how does the axial resistance
of driven and drilled foundations
compare?
The multitude of factors influencing performance
preclude a definitive broad general
answer to this question. However, an
understanding of the principles reviewed
in the preceding pages may provide some
ideas, when coupled with a lot of experience
in testing different types of piles. The
author’s general thoughts are as follows,
which are necessarily very general, subjective
and reflective of personal experience.
And, of course, there are often factors
other than load-carrying efficiency
that affect the selection of the type of
deep foundation.
The worse the ground conditions, the
more conditions generally favor driven piles
over drilled foundations. Loose or weak
soils present stability problems for drilled
foundations and these represent potential
construction risks as well as inefficiency.
More granular soil deposits tend to
favor driven piles because of the beneficial
effects of the ground improvement
associated with displacement piles and
the potential relaxation associated with
drilled foundations.
Stronger cohesive or cemented soils
favor drilled foundations; the favorable
conditions for borehole stability combined
with the benefits of sidewall roughness
can promote excellent side resistance.
Other major factors affecting foundation
type selection include:
• Lateral loads and flexural strength
demands; large diameter drilled shafts
often benefit from demands associated
with high seismic loading or liquefaction
induced lateral spreading,
although large diameter steel pipe can
also be efficient in these circumstances.
• Marine conditions: over-water work
favors the simplicity of driving piles
unless expensive underwater cofferdam
work is required.
• Congested sites and nearby structures
tend to favor large diameter drilled
shafts if the footprint associated with
the foundation can be reduced to a single
shaft. However, a group of smaller
piles provides redundancy.
• Many piles to be installed close proximity
for a footing can favor driven piles
if staging of cast-in-place piles complicates
construction sequencing.
• Environmental or public relations
aspects of pile installation such as
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