City installing temporary dam to help determine and repair cause of sinkhole

City crews are working on determining the cause of the sinkhole that opened up at Winnipeg and Carney streets following this week's heavy rain. Crews will have to excavate down approximately 20 feet to look at a large drainage pipe installed the 1970s. Motorists are advised to avoid the area, if possible, as work will continue for at least the next few days. Bill Phillips photo
City crews are working on determining the cause of the sinkhole that opened up at Winnipeg and Carney streets.

The City of Prince George has begun installing a dam near Carrie Jane Gray Park to help determine and repair the cause of a sinkhole at the nearby intersection of Carney Street/20th Avenue and Winnipeg Street/Massey Drive. The project is expected to take several weeks to complete and may result in the closure of the intersection. Currently, only the northbound lanes of Winnipeg Street are closed to traffic.

In recent years, Prince George has had four sinkholes at this location – two in 2014 (one of which was caused by a watermain break) and two in 2018. Engineers believe the cause of the repeated sinkholes to be damage to a large stormwater pipe, likely installed in the 1960s, or to an adjoining manhole.

The stormwater pipe collects run-off from catch basins within the road network and discharges it into the channel in Carrie Jane Gray Park. The water ultimately flows through the Hudson’s Bay Wetland, beneath Queensway, and into the Fraser River. So far, crews have excavated 4-5m down and exposed the top of the stormwater pipe.

“It has not yet been possible to expose the whole pipe and determine the cause of the failure due to water in the pipe and at the bottom of the excavation,” says Dave Dyer, General Manager of Engineering and Public Works. “Therefore, the City will attempt to de-water the excavation and the pipe via a sheet pile dam to allow crews to be able to see both inside and around the pipe to determine where it may be failing and then fix the issue. Sheet piling equipment has been moved to the site to allow construction of the dam to begin.”

Once a sufficient quantity of water is removed from the excavation, crews will use a vactor truck to extract sediment from the storm pipe at the manhole to allow camera or confined space entry to confirm where the pipe failure occurred.

“Once the problem is determined, a repair plan will be confirmed and we will be able to determine how much more we need to excavate the area to fix the repair. We may need to excavate an additional two metres down,” says Dyer. “If that is the case, we will most likely have to extend the top edge of the excavation well into the intersection. This may require the entire intersection to be shut down and more traffic detours to be set up. In that case, we will do all we can to minimize the time it takes to fix the problem.”

Due to the extensive amount of work to be done and as yet unknown factors (e.g. the depth of the groundwater surface, the amount of time it will take to drain the pipe, the exact location and extent of the pipe failure, the extent of the excavation and how far into the intersection it will go), the City estimates that the operation could take as many as six to eight weeks. The City must now also build in contingencies in case of another major storm event to ensure the safety of crews and the public at a very busy intersection.