FEATURE
One should not anticipate that a calculation method
that has been used with great reliability in the alluvial
clays of New Orleans would necessarily be used with the
same reliability in the Yorktown formation in Norfolk, Va.
or in the coastal sand deposits of Long Island, N.Y.
Huey P. Long Bridge widening project in
New Orleans were constructed using full
length segmental casing with a cutting
shoe equipped with teeth at the bottom.
The oscillator machine works the
casing back and forth as it extracts the
casing during concrete placement, with
the result that the cutting teeth leave
a roughened surface at the interface,
as evident in the photograph in Figure
8. This photograph shows the surface
of one of a group of nine-foot diameter
shafts that were uncovered in the cofferdam
excavation during construction
of the footing; the herringbone pattern
in the exposed concrete surface results
from the casing teeth during oscillator
extraction of the casing.
Although the surface of a drilled foundation
may be relatively rough, the pile/
soil interface is affected by the construction
operations. The soil at the interface
is sheared and remolded by the passage
of the tools during drilling; remolding
from shearing action may be more obvious
with a continuous auger and is more
likely to affect the interface with clay
soils. In addition, the interface of a drilled
shaft can be affected by residual effects
of drilling fluid. For example, published
experimental results (e.g., Brown, 2002
and Brown and Muchard, 2002, Lam et
al, 2010, ) have shown that mineral based
(bentonite) drilling fluids can significantly
affect the side resistance as compared
to synthetic polymer fluids.
The base resistance of drilled foundations
is also sensitive to construction
operations. Compared to the residual
force at the base of a driven pile, the
soil at the base of a drilled pile cycles
through an unloading during excavation,
followed by a replacement stress from
the column of fresh concrete. The contact
at the base can be contaminated by drill
cuttings if the base of the excavation is
not adequately cleaned, or by mixing of
the grout and cuttings at the toe of a CFA
pile. Effective construction practices can
mitigate many of the deleterious effects
on base resistance, but good quality control
coupled with rigorous inspection is
essential for consistent quality assurance.
So how can the axial resistance of
driven and drilled foundations be
most reliably predicted?
If nothing else, the above discussion
should remind the reader to maintain
a healthy respect for the uncertainty in
our ability to reliably estimate the axial
capacity of deep foundations of all types.
On top of the uncertainties about the variability
in ground conditions, we have the
uncertainties related to the effects of pile
installation, which vary with construction
technique and soil type. For these
reasons, our estimates of axial resistance
should be just that: estimates. A successful
design requires some verification that
the method of estimating axial resistance
is reliable, usually through load testing of
representative piles.
It may not be a surprise that hundreds
of alternative calculation methods have
been proposed for estimating the capacity
of driven piles and drilled foundations
of various types, ranging from theoretically
based models to simple empirical
correlations with in-situ tests. Why so
many? Undoubtedly because of the lack
of successful use of any single method
across a wide spectrum of possible conditions.
However, many engineers have
developed predictive methods that are
used with confidence for specific types
of piles for specific local geologic conditions;
the reliability of such methods are
invariably connected to load test verification
and a history of use.
HEAVY MARINE & WATERFRONT CONSTRUCTION
30 Church Street
P.O. Box 0811
Liberty Corner, NJ 07938-0811
office: 908.580.0200
fax: 908.580.0880
www.trevcon.com
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