LRT tunnelling likely cause of Waller Street sinkhole: city
By David Reevely and Meghan Hurley, OTTAWA CITIZEN February 21, 2014 9:30 PM
OTTAWA — Tunnelling under Waller Street for the city’s light-rail project is the only explanation anyone has yet for why a sinkhole opened up near the intersection of Waller and Laurier Avenue, deputy city manager Nancy Schepers said Friday afternoon.
A section of road about three metres by three metres collapsed around 1 a.m. just south of Laurier. Waller is closed there for the rail project and isn’t travelled, though the city closed the nearby intersection while the cause of the collapse is investigated.
Leaking pipes, typically water mains, are the most frequent cause of sinkholes, as slow trickles of water wear away at underground soil and then the surface falls in all at once. But the only pipe running through the area is a small sewer and there’s no indication there was anything wrong with it until the collapsing ground broke it, according to the city.
“At this point, we cannot confirm its root cause,” Schepers said of the sinkhole at a news conference 12 hours after the collapse. The tunnelling is the obvious thing to look to, she said.
For now, there’s no reason to expect the sinkhole to spread, she said. “Monitoring equipment has confirmed that the impact is localized and the geotechnical team has not identified any safety concerns at this point.”
The first sign of trouble came at about 10 p.m. Thursday, according to Tim Stewart, the construction director for the city’s rail contractor, Rideau Transit Group. Miners, who’d just started 24-hour-a-day work on the tunnel there the previous day, noticed “material” falling from the top of the face where a massive digging machine called a roadheader was clawing away at the earth under Waller. They stopped digging and watched for a short time, and when the sand and debris kept coming down, they pulled the roadheader back and got out, Stewart said.
Nobody was hurt. Once the sinkhole stopped growing, crews filled it with concrete to stabilize it, work that needed to be done before inspectors could get close enough to start figuring out exactly what happened.
“It’s early for us. We’re continuing to analyze what happened and why it happened,” Stewart said. “We have ensured that our operation is safe.”
The roadheader (“Crocodile Rouge” is the nickname given to this one) is safely away from the collapse, he said.
The east portal for the rail tunnel is being dug in the soft ground of Sandy Hill and it’s been the subject of constant assessments and reassessments as tunnelling has progresses, with engineers determining just how much support the roof of the tunnel needs as the digging goes on. In that spot, the roof of the tunnel had been supported with horizontal pilings and a layer of “shotcrete,” a form of concrete that’s sprayed on with a hose, Stewart said. The tunnelling is 15 to 20 metres from hitting the hard bedrock that’s under most of downtown Ottawa, where the digging is expected to get simpler.
Tunnelling at the west portal near Bronson Avenue and at a central shaft near Queen and Lyon streets is already in bedrock and is continuing, Schepers said. The mess at the east portal is Rideau Transit Group’s problem for now, she said, because unless it turns out the city was somehow at fault, the contract with the construction consortium makes risks associated with the tunnelling part of the $2.1-billion deal.
“RTG is fully engaged in first containing and mitigating the impact, secondly in identifying the cause, and finally in putting forward a plan to resume tunnelling operations in the area when it’s safe to do so,” Schepers said.
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