Tunnel work likely loosened sandy soil, causing Rideau Street sinkhole: Report

Instability of wet sandy soil, and not awatermainrupture, was the "highly likely"root cause of a massive sinkhole thatopenedup onRideauStreet 10 months ago, says the city's own report.

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"The loss of ground likely occurred very rapidly because the ground had previouslybeen disturbed and loosened" by installing a pipe and excavating for a tunnel entrance, saidthe report written by engineering consultantsMcMillenJacobs Associates.

Once the ground beneath thewatermainmoved, there was nothing to support the pipe.

It burst, releasing a torrent of water thatflushed away the surrounding soil, causing the "subsequent catastrophic rupture" of the roadway, according to thereport that was made public for the first time Wednesday.

Leaking water pipe not to blame: report

Light rail construction crews weretunnelingnear the intersection ofRideauand Sussex Drivewhen the sinkhole occurred the morning of June 8, 2016. It grew quickly,swallowing a van and three lanes of traffic. An OCTranspobus had driven over the intersection just moments before.

It tookRideauTransit Group, the consortium building the Confederation Line, weeks to pump water out of the tunnel.

Ironically, it was a previous leakin theRideauwatermainthat led city consultantsto determine that leaks weren't the cause of the Rideausinkhole.

In November of 2015, awatermainleak caused water to bubble up onto the surface of the pavement, as well as a rise in groundwater levels.

The pipe was repaired the same day, although no cause for the leak was determined.

But in the days leading up to theRideausinkhole, none of the visual clues that would point to awatermainleak were reported.

"In fact, groundwater levels in thevicinity of the sinkhole dropped ... sixmeters in response to west running tunnel construction activity in the days leading up to the sinkhole," the city report said.

"There's certain evidence that presents itself" with awatermainleak, said SteveCripps,the director of the city's rail office. "None of those thingspresented themselves this time around. So that allowed the consultant to compare one against the other and say there's really no evidence that city infrastructure contributed to the sinkhole."

Report kept secret for 3 months

The city's solicitor toldcouncillorsin a memo last December that thecity's infrastructure wasn't to blamefor the sinkhole, butkept the actual report confidentialfor over threemonths.

The city maintained that the document could prejudice the insurance claim or any potential lawsuits.

CBC News had requested the document under access-to-informationlaws and on Wednesday, city solicitor and clerk Rick O'Connor released them.

In the Dec. 22 memo, O'Connor said the sinkhole was a "complex event" that was "further complicated" when crews filled the sinkhole with 3,000 cubic metres of concrete to secure and stabilize the site.

Those circumstances are whythe city's team of "external technical experts" wasn't able to "pinpoint a singular cause of the event."

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The big question then was: Did a citywatermainrupture, and causethe sinkhole, or did the tunneling work for the LRTcause the water main to rupture, and trigger the collapse?

Zeroing in on that answer is seen as thekey to determining who'son the hookfor the multi-million dollar repair bill.

City didn't hire 3rd-party investigator

There have also been questions about how the city conducted its investigation.

The city usedthe same collection of experts technical advisors, tunnel andgeotechnicalspecialists already working on the first phase of the LRT project, according toCripp.

"We didn't see this as any conflict of interest," said the rail director. "They're not responsible for the design or oversight of the tunnelling."

Crippssaid one of the advantage of justMcMillenJacobs consultants is that the "they knowthe soil conditions, they know the monitoring reports, so they've got intimate knowledge of this project."

RTG hired an independent company to conduct its investigation into the cause. The companyis working on its own report, and declined to comment.

Rideau Transit Grouphas incurred millions of dollars to repair the damage and is filing a claim, and the cityhassubmitted$2-millionworth ofclaims to its insurancecompany to cover its costs from the sinkhole.

More than 30 businesses have written notices saying they want the city to pay for damages and loss of business they say is the result of the massive sinkhole, which closedRideauStreet for six months.

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