abandonment of the shaft. “We’d
pour about a cubic metre of grout to
prime the line, then start to pump
the concrete, keeping the tremie
hose in the same location until the
concrete reached three metres above
the bottom of the hose. At that point
we’d start lifting, then stop in midoperation,
tie the tremie hose off to
the side of the liner, remove a 15-
to 20-metre section of the tremie
pipe, reconnect it and then start
pumping again.”
Twisting and pushing the
casings into and back out of
the ground
On the U.S. side, to combat the high
artesian pressure Malcolm extended
its casings 20 feet out of the ground
and kept a fluid head on top of the
shaft, working at extreme elevations
while pouring shafts and setting rebar
cage, Glider said. “All the things we’d
normally do right at grade.”
“The Canadian rock formation
had a little different characteristic,
so their rock sockets were a little
bit shorter,” said Glider. On the
American side “it was about 100-
feet of overburden soil down to the
top of rock, then an approximately
21-foot-deep rock socket, so 121-feet
of total length.”
Malcolm drilled its 12 main span
shafts and six backstay shafts to the
bedrock using a casing oscillator, a
piece of equipment made by Leffer
GmbH in Germany, that pins into
the cranes and drills. The casing
oscillator clamps around the complete
circumference of the casing
pipe and uses an oscillating twisting
force and huge hydraulic rams to
twist, push and pull the casing into
and then back out of the ground.
“You’re talking about drilling a
nine-and-a-half-foot-diameter hole
21 feet down into intact, strong
limestone,” said Glider. “We used
our Bauer BG 50, the biggest drill
in North America, a 640,000-pound
drill rig. And for these size shafts,
the oscillators’ extraction force is
about 800 tons.”
Then 140,000-pound rebar cages,
which had been built on the ground,
were picked up with a 300-ton
crane, tripped and set vertically into
the main tower shafts. “These shafts
were designed to have the utmost
capacity for their size because the
load needed for the bridge,” he said.
“It takes bigger equipment to
do it” this way, Glider says, and
Malcolm had to install 35 24-inchdiameter
steel reaction piles to
stabilize the seawall against its
massively heavy equipment. These
reaction piles, plus the additional
effort needed to install drilled
shafts using the oscillator method,
were offset by the significant
material costs that this technique
saves, since Malcolm was able to
extract almost all of its steel casing
back out of the ground during
concrete placement.
“It’s also a speed thing,” he said.
“We were installing and pouring two
three-metre shafts a week, which is a
pretty extraordinary pace.”
Along with healthy competition
between the U.S. and Canadian
teams, there was cooperation and
learning, Beaveridge says. At one
point, when Malcolm was having
much better production rates, “they
sat down with GFL and had a couple
of chats,” pointing out that reducing
the density of the teeth on the tools
would increase the force on each
individual tooth, “which allowed us
to progress at a higher rate.”
GFL, which commenced project
work in May 2019, was almost demobilized
by the time Covid-19 hit,
and Malcolm poured its last shaft
on March 26, 2020, says Glider,
before most Covid-19 restrictions
were imposed.
Some aspects of the project were
definitely unique, Glider said, “But
we knew our challenges ahead of
time and planned our work accordingly,
put the right equipment on
the job and had one of our best
superintendents in the country
running the show. I think we successfully
executed the job without
any major surprises.”
COVER STORY
Casing installation
by Malcolm Drilling
MALCOLM DRILLING 14 Issue 4 2021 www.pilingcanada.ca
/www.pilingcanada.ca
