‘THESE THINGS HAPPEN’
LRT dig likely had a role in sinkhole
A root cause report makes it clear that LRT tunnelling work in soft ground likely contributed to the massive sinkhole forming on Rideau Street last June.
Wet, sandy conditions and groundwater pressure above the top of the tunnel pushed soil down on June 8, 2016, after workers below constructed a reinforcing “pipe umbrella” support system, which had disturbed the soil, the report says. A water main broke as the supportive soil washed away.
The city released the report Wednesday after receiving access to information requests, including those from the Postmedia, for the document earlier this year.
The city hired McMillen Jacobs Associates for the sinkhole investigation.
Around 10:30 a.m. on June 8, 2016, a massive crater opened up on Rideau Street, just east of Sussex Drive, and an exposed underground pipe was spewing water into the sinkhole. A minivan fell into the hole and it’s now encased in concrete under the road.
Steve Cripps, director of the O -Train project, said it was a unique situation, but the soft ground conditions in that area were intimately known by Rideau Transit Group’s tunnelling experts.
“This was no surprise,” Cripps said in an interview. “There was a lot of geotechnical investigation in that area. Everyone knew this. RTG knew this. They took appropriate precautions. (RTG has) world-class tunnelling experts as part of that team. They took appropriate steps, but there are risks with tunnelling and these things happen.”
Cripps noted the report didn’t come to any one reason why the sinkhole happened.
The report says there was evidence of ground movement on the surface of Rideau Street in the days before the sinkhole. A photograph dated June 6, 2016, illustrated a separation between the sidewalk and a curb.
Cripps said tunnelling normally causes ground movements, which are monitored constantly by several devices, so evidence of the shifting wouldn’t be a surprise.
“The ground is moving when they tunnel and that’s just a natural occurrence,” he said.
It’s obvious the water main didn’t break before the sinkhole happened since the Rideau station west entrance excavation would have had water pouring into it if there’d been a pipe break, Cripps said.
Indeed, an expert’s close examination of the 305-millimetre pipe, which ran on the south side of Rideau Street, concluded the failure was caused by an “external force” from above, not an overpressurization. A second expert saw no evidence in photographs of a pre-existing leak or crack that caused the break.
The report notes the RTG’s “sequential excavation method” of tunnelling is generally less risky moving from soil to bedrock, but at some point a decision was made to also tunnel from west to east — from bedrock into the soil.
The collapse started between 10:15 a.m. and 10:30 a.m. Workers in the tunnel started noticing high water flows and ground instability at the tunnel face and they evacuated. No injuries were reported. The sinkhole in the heart of Canada’s capital received international attention.
It wasn’t until Dec. 15 that Rideau Street reopened to buses and pedestrians.
Until now, no one has drawn a line between the sinkhole and tunnel work.
When the root cause report was finished in December, the city decided against releasing the document to the public because of the legal claims related to the sinkhole.
The city assured council, though, that it wasn’t at fault for the sinkhole.
Thirty-two businesses filed claims for compensation as of Wednesday. The city claimed $1.4 million for emergency remediation work, plus another $780,000 recently for other costs, such as legal and consultant fees.
It was all sent to the LRT project’s insurance company to sort out. The insurer hadn’t made a decision on the claims as of Monday, the city said.
The $2.1-billion Confederation Line LRT between Tunney’s Pasture and Blair station is scheduled to open in 2018. RTG has told the city the project remains on track to open next year.
The 2.5-km tunnel is now completely excavated and either has a permanent liner or a temporary one as workers continue to build it.
“The tunnel is perfectly safe and perfectly reinforced,” Cripps said.
Cripps lauded RTG for a quick response to the sinkhole to make sure everything around the crater was safe.
“I think that RTG got in there with 3,000 cubic metres of concrete fast stabilized buildings and stabilized soil,” Cripps said. “They should be commended for their response, actually. They did a really good job.”
They took appropriate steps, but there are risks with tunnelling and these things happen.