01-40_WC_149_JUL-AUG26_PT - Flipbook - Page 19
Within the highly specialized world of tunnel boring
machines, Anna is categorized as an earth pressure balance
machine, essentially designed to carve a path forward using
a rotating cutter-head. As Anna excavates soil in front, it
conditions the tunnel muck in a pressurized excavation chamber
where chemical additives, including foam polymers, turn it
into a thick paste that’s deposited behind the machine using a
screw conveyor. The tunnel muck is then taken to the surface
for disposal. “The tunnel is 50 metres below the Fraser River,
so there’s significant ground pressure,” Threadkell says, noting
an absolute imperative to prevent ground movement and
shift overhead. “Ground loss can impact utilities, roads and
structures on the surface, so this was a very real risk because
we were tunnelling under a very dense urban environment,
particularly once we reached New Westminster.” Automated
geotechnical stations monitored ground levels round-theclock to alert the project team about any unexpected ground
movement, though Threadkell says no significant shifting was
noted during construction.
As Anna progressed underground, essentially crafting
the tunnel space, crews installed a small temporary rail line,
similar to ones used in mines, to ferry people and materials
and abet the installation of large ringed precast concrete liners,
in 1,400 kg segments, designed to permanently support and
reinforce the tunnel’s structural integrity. Threadkell describes
the work as meticulous, with a total of 1,800 rings installed
at a pace of roughly 15 each day. While Anna was boring its
way underneath the river, crews built the exit shaft in New
Westminster so that the machine could be immediately hoisted
out when its journey ended.
After two years of crunching through challenging ground
conditions, Anna emerged from below and the project was
deemed well past its midway point. With the most demanding
work safely completed, crews set about installing the 2.6-metrediameter steel watermain in segments, with sections of
pipe placed and welded to seal it along the tunnel’s entire
length. Crews worked in reverse this time, beginning in New
Westminster and removing sections of rail as they inched
backwards, reaching the Surrey shaft in mid-2026. After this
final retreat, crews used a cellular concrete backfill to fill the
annulus between the welded steel pipe and the precast concrete
segments that they had installed during the initial excavation.
Once work inside the tunnel was completed, crews set about
excavating and building large valve chambers at both shaft sites
to house mechanical equipment. Threadkell says these will
connect with the region’s water transmission network when
the project team is ready for commissioning. This work is
currently in progress, and when construction is finished Metro
Vancouver will plant trees and restore both shaft sites to provide
green space for community use. “We’re currently tracking final
completion in 2028,” Threadkell says.
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Threadkell describes the tunnelling itself as the project’s
most complicated phase because it is the phase of the
work that carries the most risk. During otherwise routine
maintenance on the boring machine’s cutting tools, which
wore significantly as the tunnel excavation progressed, divers
performed a procedure known as a hyperbaric intervention
to safely access the cutter-head. “Specialized divers enter
the excavation chamber under a pressure equivalent to
the ground pressure to avoid face instability,” Threadkell
explains. “It’s basically the same principle you’d find with
divers working at the bottom of the ocean—for example
doing work on a North Sea oil rig.” The amount of time
divers have to complete any repairs or maintenance is limited
in order to avoid decompression sickness, he adds.
To keep water from inundating the tunnel, crews installed
pumps near the Surrey entry shaft, and the tunnel boring
machine also has an outer shield made of steel that offered
protection. “Before the back of the shield reaches a concrete
precast segment, that (space) is grouted to ensure no water
enters the tunnel itself,” Threadkell explains. “It’s a sequence.
The machine excavates, it’s protected by its shield and, as
it advances, the space between the precast segments and
the ground is injected with grout to prevent water ingress
entering the tunnel.” Furthermore, because the pumps run
on electricity, backup generators ensure continuity should a
hydro outage occur.
During excavation, the project team also had to guard against
significant ground movements, which could potentially impact
structures above ground such as the SkyTrain light rail. The
route carefully threaded its way between residential towers and
other tall buildings in order to minimize potential impacts, and
Metro Vancouver developed a monitoring plan ahead of time
and engaged all stakeholders. The method of boring the tunnel,
itself, was also designed to minimize ground movement, and
crews installed warning systems to alert crews to any changes in
ground conditions.
“These tunnels are all designed to withstand a 1-in-10,000year seismic event,” Threadkell says. “It’s designed so that
there would not be a risk to the workers themselves during
construction.”
The bottom line with tunnel projects, Threadkell says, is
developing an effective risk management process early on.
“Ensure that you’re prepared for a wide variety of different
ground conditions that can be encountered, that you have a
robust system for performing hyperbaric interventions, and
that you have an effective process for managing relationships
with stakeholders and keeping them well informed,” he says.
“On any large capital project like this, a lot comes down to how
you manage and mitigate risk. We had a very effective plan and
we were prepared. We’ve mitigated the different risks that we
forecast on this project as part of the planning process.”
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