DFI Magazine January/February 2020 - 72

Current Issues - Problem Stated
The research on cushion blocks and the
internal mechanics of pile drivers, in
general, is very limited. It seems to have
mostly come to a halt at the handbook level.
While the entire machine serves its
purpose, there seems to be a lack of
optimization and only a few modifications
or improvements now and then. If the pile
driver on the jobsite doesn't drive your
piles, a larger machine will be used and
hopefully succeeds. Sometimes, the overall
infeasibility of such action is considered
(e.g., energy consumption and potential
damage to piles/machinery) but not always.
It makes sense - the job has to be done.
Furthermore, if the design of the equipment
is to maintain a competitive edge, each
manufacturer is forced to protect the
sensitive information pertaining to and
obtained from the equipment. It may be
that disclosing data collected from the
machines' on-board data acquisition (DAQ)
system is impossible; however, manufacturers may still be well suited to collaborate
and share their respective findings to
advance the overall pile driving industry.
Originally, wooden blocks were used as
a cushioning material during a pile driving
operation. However, as the machines
became larger, the cushioning material
became too costly to maintain. Due to the
internal friction and when the impact force
and rate become high enough, the blocks
catch fire quickly after commencing pile
installation. As such, wooden cushioning
blocks are now considered impractical,
since they last for no more than a few
installations of driven piles, even when
using smaller pile drivers. Other materials
used contemporarily for cushioning are
canvas phenolic (sometimes sandwiched
with aluminum), steel wire/rope, nylon,
aluminum blocks and various metal
springs (disk, plate, etc.).
There were very few resources to build
upon when the research on cushion blocks
started. There was already some
speculation about the two materials
Dawson Construction Plant had been using
- Aluminum 6061 and Nylon6 (PA6),
which seem to represent two extreme cases.
Aluminum cushions are very durable and
resilient. They deliver a large portion of the
imparted energy to the pile and don't
72 * DEEP FOUNDATIONS * JAN/FEB 2020

Conventional aluminum cushion block after catastrophic failure

provide damping nor attenuation. Their
main failure mode is brittle fracture, which
poses a danger of catastrophic failure. In
larger pile drivers, it had been noticed that
the aluminum blocks are even more
susceptible to cracking, presumably due to
3D phenomena, such as non-unidirectional
stress wave propagation. The risk isn't great
since the initial cracks are easy enough to
spot during visual inspections. However,
the cushions also emit high pressure levels
of noise, which limits their use to non-noise
restricted projects.
On the other hand, nylon exhibits
intrinsic damping properties. Due to its
viscoelastic nature, the peak impact force is

attenuated, and the blow softened.
However, the driving performance of the
hammer doesn't deteriorate compared to
the (elastic) aluminum counterpart. On the
contrary, the anecdotal evidence has shown
an improvement in efficiency, less noise
generated and less damage to the
machine/pile. However, the material isn't
flawless. It was reported that, in cases of
more intense driving, the very center of the
cushion block had melted and, subsequently, the component sheared through
a nearly perfect conical plane. Nylon heats
up due to stress-strain hysteresis - for
each blow, a portion of energy is converted
to heat. In the hardest soils, the cushion

Conventional nylon cushion block after melting shearing catastrophic failure
DFI Magazine January/February 2020

Table of Contents for the Digital Edition of DFI Magazine January/February 2020
