6 months after Rideau Street sinkhole, cause remains open question

Six months after a gaping sinkhole suddenly turned one of Ottawa's busiest downtown streets into a chaotic danger zone of hissing gas and gushing, brownwater, neither the city nor the light rail construction consortium tunnelling below know what caused the catastrophe.

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Both the city and Rideau Transit Group are endeavouring to find out how the June 8 Rideau Street road collapsehappened, and the results of their separate investigations should be available in a matter of weeks, according to the city's director of the rail office, Steve Cripps.

Did a city water mainrupture, causing the sinkhole, or did the tunnelling work below trigger the collapse, causing the water main rupture? Zeroing in on that answer is key to determining who'son the hookfor the multi-million dollar repair bill.

"Completely rebuilding that road and all the services that are in it will be a substantial cost," said Cripps."It would be millions. It's hard to say if it's tens [of millions]."

New drone footage

New drone video obtained by CBC News offers never-before-seen views of the aftermathof the sinkhole at the centre of the twin investigations.

So far RTG has paid for all of the repairs, including millions of dollars for truckloads of concrete poured to stabilize the area.

According to an incident notification report filed just an hour after the sinkhole occurred amongmore than 100 pages of city documents about the sinkhole obtained by CBC News through a municipalaccess to information requestthe water main rupture resulted from the collapse, rather than the reverse.

"At approximately 10:40 am a sinkhole formed at Rideau running tunnel opening on the surface of Rideau St. As a result a water and gas main broke at the corner ofRideauand Sussex," wrote the project's safety administrator Allison Simons and project safety manager PeterWolters.

ButCrippssaid thatearly assessment doesn't carry any weight.

"On the day it happened no one is going to be in the position to assess the cause," saidCripps. "We're six months later and we're still working on it. So I would say it might be just a choice of words at the time."

City using its own experts

While RTG has hired an independent company to conduct its investigation into the cause of the sinkhole, the city is using its own experts already on the municipal payrollthe same collection of technical advisers, including tunnel and geotechnicalspecialists, overseeing the first phase of Ottawa's LRT project.

"They know the project, they know the history of the area, they're very familiar with the soils of the area," said Cripps."So the city didn't see a need to go independent. It's not something that would compromise their investigation by being associated with the city. They're simply looking at the data and making engineering assessments on what they're seeing."

Cripps said both RTG and the city are covered under the project's insurance plan, however there's no guarantee the insurer will pick up the whole tab for the sinkhole repair. The insurer is also expected to carry out its own investigation.

There are separate bills for fixing the storm sewer, two broken water main valves and other utilities that service nearby businesses, but none of those figures have been released to the city,Crippssaid. (Rerouting traffic around the site cost the city $14,662, including overtime and temporary barricades,in the days immediately after the event, records show.)

Reopening expected mid-month

RideauStreet has beenclosed to all traffic since mid-October while crews work around the clockto reinforcethe ground where the sinkhole occurred so the final 20 metres of the tunnel underneath can be completed.

"Out of a 2.5-kilometre tunnel it's the only area that isn't in fairly solid bedrock," said Cripps."So it's always been the riskiest part. But once that sinkhole happened it took soil that's been there for 10,000 years and just disturbed it, and loosened it and moved it all around. So now you've got soil that wasn't quite as stable as it was."

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The city's goal is to reopenRideauStreet to buses and taxis by Dec. 16, Cripps said.

Until then, investigators continue to pore over extensive amounts of data from the day of the sinkhole and the weeks leading up to it.

"There's thousands of bits of information and they all need to be analyzed," said Cripps."So it's a huge undertaking."

The investigations into the cause of the sinkhole are expected to be completed by the end of the year, and have not delayed the project's overall completion targets, the city said.

Rideau Transit Group did not make anyone available for an interview.

"The cause of the sinkhole is still under investigation and we have no further information to provide," wrote Peter Lauch, the technical director of RTG in an email to CBC News.