Sandy Soil Caused Rideau St. Collapse: Sinkhole Report

The secret city sinkhole report blames the collapse of Rideau Street on sandy soil conditions, a source who has seen the report says.

The source who requested anonymity provided details of the sinkhole a substantial time ago but The Bulldog withheld the information so as to protect the person’s identity.

In the highly technical report, the City of Ottawa has a picture of the sinkhole with a large city water main intact but no soil surrounding it. After the sinkhole appeared the city water main eventually collapsed unleashed a large torrent of water into the collapsed light-rail tunnel that was being dug beneath Rideau Street at the time the sinkhole appeared.

The city maintains in the voluminous report that no city infrastructure cause the collapse of Rideau Street.

“City infrastructure had nothing to do with the sinkhole,” the source said.

The municipality has withheld the controversial report because it says it fears the information in the document could be used by lawyers representing organizations suing the city. However all evidence must be exchanged between opposing lawyers before the case goes to court. That means secrecy of the sinkhole report would be breached by legal process.

The Rideau Street sinkhole was the second major one to appear in the construction of the $2.3-billion Confederation Line. Both sinkholes, the previous one appeared on Waller Street, didn’t cause any loss of life or major injuries. Water inundated the tunnelling and set back construction of the huge plan. Nevertheless the city still maintains the project is on-time and on-budget.

In the early stages of light-rail planning, tunnelling was shortened by about a third because the city discovered, through bore holes, sandy soil in Sandy Hill.

Interestingly Rideau Transit Group has decided not to bid on Stage 2 of the $6-billion Ottawa LRT project. That’s highly unusual because organizations that start huge infrastructure projects usually finish subsequent stages because they know best the technical configuration of the part already completed.





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11 thoughts on “Sandy Soil Caused Rideau St. Collapse: Sinkhole Report

  1. So the sandy soil in the area of Sandy Hill was responsible! Now who would ever have suspected the presence of sand in that area?? Was the first sinkhole at Laurier and Waller also caused by Sandy Hill’s sandy soil? Do we have any info on the cause of sinkhole #3, also along Rideau Street?

    Given the amount of time they spent drilling and doing soil tests along Rideau between Sussex and Dalhousie before the tunnelling ever got underway, how could they not know what was under the street? Or did they know and decided to take the gamble because they wanted to bring the train to Rideau Street instead of following the Transitway route and having the Rideau Centre stop on the south side of the shopping centre? You just have to look at the map of the LRT route to see the huge loop they took to do this which, of course, added to the cost and killed an Elgin St station.

    There was a canal as early as 1842 that ran between the Rideau Canal and the Rideau River called the By Wash that crossed Rideau Street at Musgrove which is where the main door of The Bay is now (see the map at: Was this or any other previous area features taken into account during the planning of the LRT?

    Is the City or RTG going to guarantee the safety of the tunnel between Elgin and where it emerges at the university? If so, do they base that guarantee on the same studies that said it was safe to use the methods they did to tunnel under that area? That and a toonie will buy you a coffee.


    1. Beach fronts, sand castles and shifting dunes are a few examples of the unstable nature of sand. You do not have to be an expert or civil engineer to know this. Preliminary soil drilling along the route should have shown the risk. The true mystery lies in why build where the risk is so high and probably continues after construction is done.


      1. Bruce,
        I don’t know if a 25 year old expert was used or was it a 45 year old with years of experience?
        There are experts and then there are EXPERTS.


        1. Decisions are often made by politicians, with supporting evidence sought afterwards, and evidence that doesn’t support the decision discarded or ignored. In short, the many limitations of the LRT in general, and its route in particular (as articulated on the Bulldog on innumerable occasions) are part of the legacy of Jim Watson’s mayoralty.


  2. Ken,
    The basis information, about where the city is planning to hang its hat, could have been released by the city.

    Here is where the lawyers and insurance companies will hang their hat : Why was there no soil around the intact water main?

    This is going to be an interesting and expensive issue. Before it gets resolved heads may roll.

    Your readers are lucky that you are letting in some light.


  3. What this report appears to tell us is that the sinkhole was not caused by a catastrophic failure of the water main at issue. Does the report state, categorically, that the sink hole was not caused by a longer term, small leak in a water main, or in a nearby sewer line, or …? Does the report state, categorically, that the sink hole was caused by vibrations from the nearby tunneling, and that the sandy soil conditions could not withstand those vibrations?

    The only way we will find out is if Mayor Watson finds his dictionary and looks up the meaning of the words “open, transparent and accountable”.


    1. Ron,
      Just to be more specific – when I referred to using a dictionary and when you refer to a dictionary we must be clear : We want them to use a Merriam- Webster dictionary not an Erdogan- Trump one.
      Chaz :)


    2. It would be funny (funny-sheesh, not funny-haha) if it turned out that, in the course of drilling to take the soil samples along Rideau, they poked a hole in a water or sewer line just big enough that the tunnelling vibrations shook things loose, leading to the sinkhole. I don’t know if that drilling was done by RTG or if it happened before they took over the project.


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