Rideau Street sinkhole floods 300 metres of LRT tunnel

Engineering experts say 300 metres of the city's light rail tunnel is flooded after Wednesday morning's Rideau Street sinkhole and water main break.

Engineers for the Rideau Transit Group (RTG) went down into the tunnel Friday to assess the damage for the first time since workers were told to drop their tools and leave the site in the minutes after the street above the tunnel fell away, creating a sinkhole five metres deep.

Late Friday afternoon, City of Ottawa manager Steve Kanellakos sent a memo saying they'd discovered the significant flooding.

"Over the next several days, water will be pumped out to allow teams to better assess the impact and plan the path forward," he said.

"RTG staff are assessing conditions and finalizing a timeline for resuming tunneling work."

Kanellakos added that construction crews, despite the flooding, are back to work at the future Rideau Station.

He did not share details of how much water was in the flooded part of the tunnel.

RTG could be liable for costs 

If work done for the LRT project ultimately caused the sinkhole, then RTG is liable to pay for the damage because of the nature of its public-private partnership (or P3) agreement with the city.

RTG was responsible for paying for the sinkhole damage that appeared at the corner of Waller Street and Laurier Avenue in February 2014.

Ottawa Mayor Jim Watson said Thursday that this time it may take months to determine what caused the sinkhole.

"It took us about a month to do the Waller Street [sinkhole investigation] and this one is obviously substantially bigger," he said.

LRT tunneling almost complete

RTG had been digging the 2.5-kilometre light rail tunnel from two ends, with both machines simultaneously moving toward each other.

When the sinkhole opened up the tunnel was almost complete, with the two ends now just 50 metres apart.

The westbound tunnel had made it to the future Rideau Station, which is also under construction but was not damaged by the sinkhole, according to RTG technical manager Peter Lauch.

"We're back working in the station itself," he said. "It wasn't compromised. But on the tunnel, I can't tell you when we're going to start up again, it's going to take a while before we're back in there."

RTG working slow through unstable ground

Lauch said RTG understood they were working in unstable soil through the area and crews were taking it slow.

Rather than excavating at a pace of 4.5 to five metres a day through limestone, as crews did in the eastbound tunnel, the westbound work was creeping along at 0.8 to one metre a day.

CBC reported Thursday that the sinkhole occurred in the same place engineers conducting a geotechnical survey for the light rail project in 2011 discovered a 120-metre wide "bedrock valley" under Rideau Street.

The slow and steady approach will continue after repairs to get the tunnel back in the same shape it was before the sinkhole appeared, Lauch said.

LRT project expected to be finished on time

RTG CEO Antonio Estrada met with city manager Steve Kanellakos Thursday for the first time since the sinkhole appeared. Lauch said they discussed next moves for both the sinkhole repairs and the LRT project.

Now that the sinkhole has been half filled with concrete, on Thursday night RTG began using "shotcrete" — a kind of concrete that can be sprayed — to stabilize the vertical sides of the sinkhole so utility crews can begin reconnecting broken pipes.

By Friday afternoon, workers were back in the partially-filled pit.

As for the road closures on Rideau Street, Elgin Street and Sussex Drive, Kanellakos said they'll stay closed "until the full extent of the impact on surrounding infrastructure can be determined."

More Rideau Street sidewalks are now open again, with north sidewalks only closed from Sussex Drive to the Tim Horton's entrance and south sidewalks from Sussex Drive to Nicholas Street.

Businesses that are still closed will reopen on a case-by-case basis when it's safe, the city said.

Lauch had said Thursday Rideau Street should be back in working order in two weeks.

The tunnel repairs, he said, should not take longer than a month.

RTG does not expect the repairs to delay the 2018 deadline for completing the LRT project.