FEATURE
HARTMAN
ENGINEERING 1.716.759.2800 | www.hartmanengineering.com
4910 Ransom Road, Clarence, NY 14031-2141
Our firm has many years experience taking on the challenges of difficult design problems in the
specialized area of cofferdams, retaining walls, foundation support, and damaged structures
pile that deteriorate with time, leading
to a time-dependent increase in the horizontal
stress acting on the pile sidewall.
Figure 4 presents data from horizontal
stress measurements using pressure
cells embedded into the pile sidewall.
The measurements shown are for
depths ranging from 24 to 60 feet below
grade and were made on a 9.25-inch-wide
square concrete pile. He describes these
effects as “stress relaxation”; however, the
axial resistance is not relaxing because
the effect of these changes in stress are
correlated to long-term increases in measured
static axial resistance in sands during
periods of up to two years.
Considerable setup is known to occur
in clay soils both during the period of pore
pressure dissipation and reconsolidation
around the pile, which can take months
or longer for clays of high plasticity, and
for years thereafter. The effects of aging
beyond the reconsolidation phase may
be attributed to chemical bonding and/or
long-term increase in horizontal effective
stresses as describe above. Karlsrud (2014)
reported measurements of side resistance
in clays increasing by as much as 55%
during a period between three months
and two years after driving, and a few
older tests suggest that the side resistance
may continue to increase for at least two
decades after pile installation.
Base resistance
Piles driven to bear on hard rock can
develop a base resistance that is generally
limited only by the structural strength of
the pile and the stresses in the pile during
driving. For piles bearing on rock,
the problem is therefore not so much a
geotechnical problem as a structural/
construction problem to install the pile
without damage.
Piles driven to bear on very dense or
cemented materials may also develop
very high base resistance, and the presence
of such a bearing layer may override
considerations of side resistance, setup,
etc., if strong bearing can be achieved.
These types of soils are characterized by
standard penetration test (SPT) values at
refusal (i.e., 50b/6in or more) or refusal
during cone penetration testing (CPT).
The uncertainty for foundation engineers
in these conditions is primarily the stratigraphy
of the project site and the depth
to the bearing stratum. There can be
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.
Continued on page 68
66 | ISSUE 4 2020 www.piledrivers.org
/www.hartmanengineering.com
/
