FEATURE
DRIVEN Piles are
TESTED Piles
A legacy of
innovation in testing
By Ryan Allin, P.E., Pile Dynamics, Inc.
At a recent deep foundation event focused on the
future of foundation design and construction, I was
startled when a contractor in a panel discussion
stated, “I hope in 10 years’ time we aren’t still load testing drilled
shafts to bedrock.” My immediate thought was, “I hope to goodness
we are testing far more drilled shafts, whether they go to bedrock
or otherwise.”
As an industry, driven piles have long benefitted from load testing
programs, and with the use of dynamic testing, this has been
achieved in an efficient and cost-effective manner. Beyond the
use of load testing programs, each pile driven is, in fact, tested. As
monitoring the blow count is an indirect integrity test and, when
appropriately calibrated through either a static load test or dynamic
test program, is also a reasonably accurate means of assessing
pile capacity. This benefit then extends not only to the test piles
themselves but also to every driven production pile in the project,
leading to PDCA’s slogan, “A Driven Pile is a Tested Pile” – a slogan
that has endured for good reason.
Most Departments of Transportation will use driven piles
as their foundation of choice and the reason is simple: no other
deep foundation will deliver that high level of confidence in performance.
That is largely because almost all driven pile projects
include a robust testing program.
When we review the development of testing programs over the
years prior to the advent of dynamic testing, a project energy formula
(sometimes referred to as a dynamic formula) was developed
over 150 years ago for the situation of drop hammers installing
lightly loaded timber piles. (The ENR formula, which is still in use
in some states, was published in 1888.) The basic principle of these
formulas balances the energy delivered by the hammer with the
energy dissipated into the soil. However, there are many assumptions
on energy losses from the hammer as well as energy losses to
the soil, which generally make these formulas grossly inaccurate. A
large study in the 1930s led to widespread distrust of formulas and
the recommendation to obtain capacity only by static load tests.
The current practice of modern hammers now installing larger and
longer high capacity piles does not fit the original assumptions.
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