major piece of work on the Gordie
Howe Bridge had to have a full-scale
model of that element performed
ahead of time to prove we were competent
and could execute the means
and methods as described on paper.”
Proving requirements included a
three-metre full-depth test drill
shaft, mock-ups of the footing, the
side span towers, the tie beam and
the post-tensioning and tower mockups,
which were “the equivalent of
a full four metre lift of the tower,
complete with all rebar, formwork
and accessories.”
For their test, Malcolm drilled a
shaft three metres in diameter, “One
hundred feet down to top of rock,
with a 15-foot-deep rock socket,”
identical to one that would be built
to support the actual tower, said
Jim Glider, Malcolm project manager.
Then, to prove the capacity
of both the pile and the bedrock, an
O-Cell test was performed: a rebar
cage, equipped at the bottom with an
O-Cell – a hydraulically driven, high
capacity, sacrificial loading device –
was set into the shaft and concreted;
once the concrete cured to an appropriate
strength, the O-Cell was
hydraulically pressurized to put a
measurable load on the shaft in both
an upwards and downward direction.
“This applied load determines
the movement of the shaft under
the prescribed amount of load,”
Glider said. “The shaft resisted an
equivalent top load of over 38 million
pounds with only millimetres
of movement.”
Coping with deadly gas and
extreme artesian pressure
The limestone rock formation on
both sides of the river contained
hydrogen sulfide, an extremely
deadly gas. The crews on both sides
used H2S gas detectors when working
at the top of the shafts and
casings since “as we drilled into rock,
we ran the risk of it coming up into
the shaft,” said Glider. When these
monitors went off a couple of times
on the U.S. side, “we had to evacuate
the site and follow our safety procedures
to get back onto the site and to
the hole.”
The foundation subcontractors
on both sides of the Detroit River
also had to deal with the challenge
posed by water in the drilled shafts.
Due to the geological formation
found there, the groundwater is
under strong artesian pressure. Such
pressure occurs when the elevation
from which an aquifer’s recharging
water comes is higher than the
elevation of the site’s groundwater
because that water is confined
underground by an overlying impervious
layer of rock. When a shaft is
drilled through this layer, the pressure
caused by the “upstream” water
in the aquifer causes the water in the
shaft to rise above the level at which
it is first encountered at the site.
However, despite the similar conditions,
the Canadian and American
subcontractors used very different
installation techniques for their
piles. On the Canadian side, GFL
drilled the shafts in a traditional
manner, says Beaveridge, approximately
30 metres down through
overburden and then about 4.8
metres further into the bedrock.
COVER STORY
Rebar placement during test shaft installation, June 18, 2019
GORDIE HOWE INTERNATIONAL BRIDGE PROJECT
10 Issue 4 2021 www.pilingcanada.ca
/www.pilingcanada.ca
