Instability of wet sandy soil, and not a watermain rupture, was the "highly likely" root cause of a massive sinkhole that opened up on Rideau Street 10 months ago, says the city's own report.

"The loss of ground likely occurred very rapidly because the ground had previously been disturbed and loosened" by installing a pipe and excavating for a tunnel entrance, said the report written by engineering consultants McMillen Jacobs Associates.

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

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

Leaking water pipe not to blame: report

Light rail construction crews were tunneling near the intersection of Rideau and Sussex Drive when the sinkhole occurred the morning of June 8, 2016. It grew quickly, swallowing a van and three lanes of traffic. An OC Transpo bus had driven over the intersection just moments before.

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

Crews worked around the clock drilling holes where the sinkhole happened to inject a concrete mixture into the ground to reinforce the area. (Ashley Burke/CBC News)

Ironically, it was a previous leak in the Rideau watermain that led city consultants to determine that leaks weren't the cause of the Rideau sinkhole.

In November of 2015, a watermain leak 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 the Rideau sinkhole, none of the visual clues that would point to a watermain leak were reported.

"In fact, groundwater levels in the vicinity of the sinkhole dropped ... six meters 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 a watermain leak, said Steve Cripps, the director of the city's rail office. "None of those things presented 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 told councillors in a memo last December that the city's infrastructure wasn't to blame for the sinkhole, but kept the actual report confidential for over three months.

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

CBC News had requested the document under access-to-information laws 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 why the city's team of "external technical experts" wasn't able to "pinpoint a singular cause of the event."

Water splashes as soil collapses into a large sinkhole that formed on Rideau Street next to the Rideau Centre on June 8. (Jusin Tang/Canadian Press)

The big question then was: Did a city watermain rupture, and cause the sinkhole, or did the tunneling work for the LRT cause the water main to rupture, and trigger the collapse?

Zeroing in on that answer is seen as the key to determining who's on the hook for 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 used the same collection of experts — technical advisors, tunnel and geotechnical specialists — already working on the first phase of the LRT project, according to Cripp.

Crews jet grouted the final 20 metres of tunnelling below the spot where the sinkhole formed. (Ashley Burke/CBC News)

"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."

Cripps said one of the advantage of just McMillen Jacobs consultants is that the "they know the 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 company is working on its own report, and declined to comment.

Rideau Transit Group has incurred millions of dollars to repair the damage and is filing a claim, and the city has submitted $2-million worth of claims to its insurance company 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 closed Rideau Street for six months.