FEATURE
stress at the pile/soil interface is likely
to change as the ground adjusts to the
conditions just as has been described
and measured on driven piles. Some
CFA contractors apply an overpressure
during pumping to try to restore the post-
construction lateral stresses in the
ground, although this practice can result
in overconsumption of grout and it is not
clear that the effect remains long-term.
In any event, the soil around a drilled
pile cycles through a process of transient
stress relaxation during construction,
as opposed to the transient total
stress increase associated with driven
pile installation.
The pile/soil interface for a drilled
foundation is formed by concrete cast
against the excavated soil surface, a
process which undoubtedly increases
the roughness compared to the pile/
soil interface surface of a prefabricated
driven pile. The actual roughness is difficult
to quantify and is a function of
the drill tooling and the soil characteristics,
but clearly the surface roughness
as exhibited in the exposed secant pile
wall in Figure 7 can contribute to side
resistance of a drilled foundation. This
is not to say that all drilled foundations
would appear so rough, as this feature is
likely to vary widely.
When smooth wall temporary steel
casing is advanced ahead of the drilling
with a vibratory hammer, the frictional
benefits of this roughness may be diminished,
as was demonstrated by the load
testing program on the Cooper River
Bridge project in Charleston, S.C. as
described by Camp et al (2002). However,
small details can make a big difference;
the drilled shafts constructed for the
Figure 7: Photo of exposed surface of drilled shafts in a secant wall
(photo courtesy of Mark Madgett, DBA)
Figure 8: Photo of exposed surface of drilled shaft constructed using
segmental casing (photo courtesy of Malcolm Drilling)
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