DFI Magazine January/February 2020 - 75

Pile Driving Analyzer (PDA) test - sensor setup

Polycarbonate tube protecting the thermocouples

Finally, thermal testing was performed to expand the
knowledge and understanding about the heat generation and
temperature profile of cushion blocks during pile driving. This type
of testing and its results are especially impactful when approached
from the perspective of testing a viscoelastic material. Three
thermocouples were fitted inside each of the tested cushion blocks
- in the volumetric center, near the middle of the bottom face and
near to the cylindrical face at the mid-height of the cushion. The
thermocouples were secured using special tubes, plasticine and
industrial-grade tape. The leads from each of the thermocouples
was connected directly to signal amplifiers/conditioners, which
were placed on the hammer's casing located less than 1.0 m (3.3 ft)
away. The leads were then connected to a constant voltage source
and a data acquisition (DAQ) interface, which was transmitting the
converted digital data to the LABView software. The virtual
instruments further conditioned the signal and transcribed it into a
spreadsheet format along with the corresponding timestamps.
Despite the procedures being duly followed, the results were
quite ambiguous. The results of the testing performed using the
classic nylon and classic aluminum blocks indicated somewhat
similar results, as do the three-piece and five-piece blocks. The
maximum temperatures attained during the 20 minutes of pile
driving were around 40° C (104° F) and 90° C (195° F) for the
classic nylon and classic aluminum blocks, respectively. Heat

transfer theory can be used to explain the temperature being greater
for the nylon-aluminum block than for the nylon-air block.
In the prototype cushion blocks, the measurements were taken
closer to the aluminum layers, which allowed for more accurate
readings. The classic nylon cushion, however, isolates the internal
heat, which results in misleading thermal readings. This hypothesis
was further confirmed using cooling/heating ratios. While heating
(measured in °C/hammer blow) is visibly greater for the prototype
blocks than for the conventional components (0.047° C/blow
compared to 0.012° C/blow), the cooling was consistent and similar
across all of the cushion blocks (about 0.004).

Conclusions
There are many unknowns associated with pile driving, and anyone
who has experience with metal piles being driven into the ground
can surely relate. Despite not focusing much on the ground
conditions, we can still improve how the machinery works and how
to properly maintain the rigs. A cushion block is but one part of a
pile driver, a machine based on deceptively simple principles, but,
as we have shown, has a significant effect on the pile driving process.
Recently, Dawson Construction Plant employed the favorable
five-piece design on one of its rental fleet hammers and the effects
are under further investigation at the time of the writing of this
article. It may take a few months before meaningful data is analyzed
DEEP FOUNDATIONS * JAN/FEB 2020 * 75
DFI Magazine January/February 2020

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