It has been fenced off for almost four years now, but work is finally underway to repair a sinkhole in Three Sisters Mountain Village.
The hole opened up in 2010 on a pathway adjacent to Dyrgas Gate in the subdivision, and is an airshaft known as B14 that was dug to access the No. 4 coal seam of the No. 4 mine in 1938.
While TSMV was built in an area with significant undermining issues from the coal mines, significant work was done to identify and mitigate the old workings.
Gerry Stephensen, who was chief engineer for the last seven years of the mine’s operations, said 300 specific underminings were identified and mitigated prior to development, including the 56-metre deep B14 shaft.
However, the exact location of the shaft was not known, so the area was designated municipal reserve instead of being developed for housing.
“We are pretty sure the shaft was abandoned in the 1950s before the closure of the No. 4 mine because it was actually replaced by a bigger ventilation access, which would have done a better job than the rather primitive B14,” Stephensen said. “The shaft was probably filled with debris, anything that was handy, and filled right up to the surface.”
Municipal engineer Blair Birch said part of the reason the sinkhole has not been filled in yet is the issue of whose responsibility it is – the municipality or the province.
“There was always the issue of who is responsible for it and how do we get it fixed, so the Town was successful in getting some funding from Alberta Municipal Affairs to go toward mitigation of the sinkhole,” said Birch, adding last year’s flood also delayed the project.
Norwest Corporation, the contractor hired by Canmore to do the work, had crews on site this week, including Bertram Drilling Corp., which is subcontracting drilling to try and find the exact location of the shaft. Work done earlier in August included ground-penetrating radar and seismic measurements in order to narrow down the area where the shaft may be.
But finding the shaft is only the first step, Norwest vice-president of geology Ted Hannah said. Once that is accomplished, the company will make recommendations for mitigation measures to the municipality.
“The scope of our work right now is to locate the shaft as best we can to determine the nature of rock material on each side of the shaft so we know how strong that foundation material is and then to recommend mitigation options that could be used to fix this problem,” Hannah said. “Once mitigation options are put forward to the Town of Canmore, the Town of Canmore has to decide where to proceed from there.”
He said they are “reasonably confident” they have located the shaft and are targeting a specific area to drill 20-metre deep holes for geophysical logging to be done.
“Four of the holes are specifically targeted to the shaft and we are anticipating and almost hoping we run into some voids in there,” Hannah said. “What we really would like to be able to do is put a video camera on a cable down the plastic casing and look around inside and see the condition of the rock.”
Stephensen said over the past 50 years several factors would affect the shaft, including rotting away of the timber frame it was originally built with, and water.
He said water levels in the shaft go up and down by 20 metres depending on the time of year. The drying and wetting of the rock forming the wall of the shaft is “bound to have deteriorated over time.”
“Of course, once the shaft was abandoned, nobody would be inspecting it and nobody would be maintaining it,” Stephensen said.
Over the last 15 years there have only been two mine works that have formed sinkholes out of the 300 identified and mitigated. Stephensen said both the Dyrgas Gate hole and the one that formed on Bow Valley Parkway were water-related.
“In both cases, water has played a role in causing them to fail. On this one, we believe a water line burst and a hell of a lot of water was released into the sealed shaft,” he said, adding the parkway sinkhole resulted from a ruptured water line.
