Elevations had first been ascertained
in the early works by boreholes;
now, as the tool approached theoretical
bedrock, the force on the auger
and progress rates were more closely
followed. “Once cuttings started
being noticed on the tools, the geotechnical
designer and consultant,
Thurber Engineering, verified the
consistency of bedrock across all the
teeth on the tool,” said Beaveridge. If
there was uncertainty, further excavation
was required until all parties
were satisfied that the true bedrock
elevation had been determined. At
that point we had to continue vibrating
to ensure a target of 100 to 200
millimetres of embedment of the
casing; to seal the liner against the
bedrock and prevent any soil from
entering the shaft during the works,”
and to ensure the positive water head
in the casing could be maintained.”
“Working in two shifts 20 hours
a day, six days a week, we could get
a liner down to the bedrock within
48 hours of commencement,” he
said, and an average of about half a
metre per 10-hour shift for the rock
sockets, which were drilled using
core barrels of different sizes and
increasingly larger augers.
Once the drilling was finished,
shafts had to be fully cleaned. For
this, GFL rigged a tremie pipe about
six inches in diameter with a larger
ring that fit around the outside of
the pipe, with teeth along the bottom
that prevented the end of the tube
from hitting the bottom of the shaft.
This was attached to an air hose
from a compressor along the outside
of the tremie pipe. A crane lifted this
device and dropped it to the bottom
of the drilled shaft.
“We’d poured in some flocculent
and let that settle overnight,
then the air from the compressor
would cavitate the water at the
bottom, causing all the sediment to
be thrown up the tube – essentially
a giant vacuum,” Beaveridge said.
Only when the pH and contaminant
levels were within regulation limits
could the water be put into on-site
sedimentation ponds.
Once drilled, the shafts
required stringent airlifting
“The airlifting was a very stringent
requirement,” said Beaveridge. “We
couldn’t have more than 12.7 millimetres
of sediment along the bottom
of the three-metre-diameter shaft,
on average, no more than seven
millimetres, measured by means
of the SQUID Shaft Quantitative
Inspection Device test.” Once
cleaned, the cylindrical rebar cages
were dropped in and concreted.
Balancing the height of the hose
while tremie-pouring concrete into
the 35-metre-deep shafts was another
challenge, he says. Each shaft required
about 262,000 litres of concrete. The
tremie hose had to maintain a minimum
of 1.5 metres of embedment
into the concrete as it came up.
“Pump the concrete too quickly
and the hose is going to want to push
itself out of the concrete, which can
be a huge issue,” said Beaveridge,
resulting in a water pocket or zone
of impure concrete, which entails
massive repair works or even
COVER STORY
Foundation work on the
U.S. side, the Ambassador
Bridge in the background
CONTINUED ON PAGE 14 MALCOLM DRILLING 12 Issue 4 2021 www.pilingcanada.ca
/www.pilingcanada.ca
