FEATURE
This article provides a brief review
of some of the fundamental aspects of
the behavior of drilled and driven piles
during installation and the effects of the
pile installation methods on the longterm
axial resistance of the piles. The
emphasis is on piles installed in soils,
and the response of the soil to the act of
installing the pile. Although the effects of
installation may be difficult to quantify,
an understanding of basic principles in
terms of soil mechanics is fundamental
to developing engineering judgment and
thereby developing and implementing an
effective design, testing and quality control/
assurance program to achieve reliability
in the performance.
Side and base resistance
Foundation engineers typically characterize
the side resistance (side shear, or
side friction) per unit surface area of a
pile, fs, using one of the following broadly
general approaches:
For cohesionless soils: fs = βσ’v,
where
σ’v is the effective vertical stress at a
point along the pile, and
β is a correlation factor related to the
friction at the pile/soil interface and the
ratio of horizontal to vertical stress.
For cohesive soils: fs = αSu,
where
Su is the undrained shear strength, and
α is a correlation factor.
For in-situ test measurements:
fs = C(Nspt, or qcpt, or other),
where
C = a correlation factor with the in-situ
test parameter.
Generally all of the methods for estimating
static axial resistance for either driven
or drilled piles in soil follow some variation
on the above, with the empirical correlation
factors dependent on considerations such as
soil type, estimated soil material properties,
overconsolidation ratio, pile type or material,
pile volume per unit length, installation
technique, length to diameter ratio of the
pile, length of pile below the point in question,
depth below grade, various averaging
techniques, whether the pile is thought to
be plugging and other factors that may be
thought to be important or relevant.
The methods for estimating base resistance
(end bearing, toe capacity, etc.) per
Figure 1: Effects of placement of a pile into a soil mass (from Vesic, 1977)
unit area at the pile base follow a similar
approach with different empirical correlation
parameters.
Why so many different modification
factors? The following sections describe,
from a soil mechanics and construction
perspective, some of the important issues
going on during installation of driven and
drilled foundations that affect the subsequent
axial resistance.
How does pile driving influence
the axial resistance?
As a pile is driven, the soil in the pile location
is displaced by the pile pushing past,
as illustrated by Vesic (1977) in Figure 1.
This action alters both the state of stress
in the ground around the pile and the soil
itself by remolding and/or changing the
fabric and density. This figure by Vesic
includes a lot of things going on in the
ground that are worth our consideration
in a bit more depth.
Densification of sandy soils
In granular soils such as sands, the forcible
displacement of the soil coupled with
vibrations typically act to densify the
ground around the pile. Most “pile bucks”
understand through experience the need
to consider the driving sequence of piles
in a large group because the last ones
will likely drive harder. An example of the
densification effect is described by Ruesta
and Townsend (1997) from the Roosevelt
Bridge in Stuart, Fla.
Ruesta and Townsend performed tests
on the lateral resistance of pile groups
at the Roosevelt Bridge, and included
an investigation related to densification
around driven piles. The Roosevelt Bridge
is founded on four by four groups of
30-inch square prestressed concrete piles
at 3D spacing and driven into sandy soil.
The piles were installed first by jetting
each pile to a depth of 25 feet, followed by
driving the piles to the required driving
resistance. The question of interest was
related to the effect of the jetting operation
on the lateral load resistance of the
piles, and a load testing program was
implemented including lateral tests on
both single and a group of piles. As part of
the research, in-situ tests were performed
in the soil both within the interior of the
pile group and outside the group in an
area unaffected by pile installation.
The results of the in-situ tests illustrated
the densification of the soil near
the piles that is produced by pile driving
operations, even in the soils through
which jetting had been performed. The
relatively loose fine sands of the upper
15 feet were densified most dramatically,
with cone penetration test (CPT) tip
resistance values increased by a factor of
three to four and CPT friction resistance
increased by six to eight. Dilatometer
(DMT) modulus was increased by two
to three times and the horizontal stress
index (KD) increased by four to five times.
Pressuremeter (PMT) modulus increased
by a factor of five to eight.
These types of measurements demonstrate
that any correlation of pre-construction
in-situ test data with pile capacity
will be subject to the changes in the
actual in-situ characteristics of the soil as
influenced by the pile installation.
If saturated sand is to densify, then
pore water must be driven out and this
action presents opportunity for transient
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