Pile dia = 10in
at 22ft depth
Model 4500S
Standard Piezometer
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FEATURE
of an inch into the soil away from the
pile surface. Likely many readers have
observed similar behavior with extracted
sheet piles or steel casings.
The behavior of remolded clays around
a pile and the resulting side resistance
is affected by the prior stress history of
the soil (notably overconsolidation ratio)
and can be influenced by the installation
process, particularly where interruptions
in driving disrupt partial setup in the soil
(Saye et al, 2013).
Residual forces
Residual forces in driven piles are always
present after driving due to the locked-in
resistance from the pile driving operations;
these forces have the effect to
pre-load the pile and lock in some of
the axial resistance at the pile toe with
the result that the axial stiffness of the
pile is increased. As a pile is impacted
and compressed during driving, the soil
locks-in a downward directed side resistance
over the upper portion of the pile
against the upward directed toe force
plus some side resistance in the lowermost
portion of the pile. The actual distribution
depends upon the resistance
at the toe and the distribution of resistance
along the pile length and will likely
change with time after driving. A good
explanation of this effect and observations
in instrumented pile load tests is
provided by Fellenius (2002).
Residual forces are not easy to measure
accurately and reliably, but the true
distribution of side and base resistance
requires accounting for residual forces. A
recent load test described by Ganju et al
(2020) demonstrates the results of a carefully
instrumented test of a 60-foot (18m)
deep 24-inch (0.6m) diameter closed end
steel pipe pile that was impact driven into
gravelly sand. Residual shaft resistance
acted downward in the upper two-thirds
of the pile against the combined upward
shaft resistance and base resistance
below, and these forces were around 10%
of the nominal axial resistance of the pile
as measured in the static load test.
Time effects
The time-dependent change (often
referred to as “setup”) in axial resistance
of driven piles is well documented and
normally related to increasing side resistance
with time. As a result, foundation
engineers often wait for a period of one to
30 inches
water content, %
36
34
32
30
28
26
24
22
20
1.2
1
0.8
0.6
0.4
0.2
0
shear strength, ksf
Figure 2: Data from Flaate (1972) at depth of 22 feet
piles the water content was found to be
consistently around 24% within about
four inches of the pile. Measurements
of undrained shear strength (using fall
cone, vane shear, and some unconfined
compression tests) consistently showed
strength values within four inches of the
pile that were 1.5 to two times the values
measured elsewhere. A graphical depiction
of Flaate’s data at a depth of 22 feet
is illustrated in Figure 2.
These effects are sometimes visually
observable. During a test pile program a
few years ago in the Cape Fear River near
Wilmington, N.C., some 10-inch diameter
steel pipe piles were installed in clay soil
for lateral load testing, after which the
piles were extracted. When the piles were
pulled (using an enormous ringer crane),
the piles were coated in clay, indicating
that the shear failure occurred not at the
pile/soil interface but rather a fraction
64 | ISSUE 4 2020 www.piledrivers.org
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