After five years of trying to find a way to fix a sinkhole on a municipal pathway in Three Sisters, engineers are being forced back to the drawing board for a new solution.
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, work was done to identify and mitigate the old workings. In all, 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 and a trail built instead of being developed for housing.
Canmore’s manager of engineering Andy Esarte said mitigation at that time, building setbacks and stabilization at the surface, were sufficient.
“It was deemed to be a medium constraint zone so it was designated municipal reserve,” he said.
The sinkhole appeared after an irrigation line break in the immediate vicinity, but Esarte said it has not been fully confirmed that the water line was the cause of the sinkhole, however, it is suspected to be.
What makes the Dyrgas Gate sinkhole even more complicated an issue for the municipality is that the provincial government does not provide liability for undermining on municipal land.
At the time Three Sisters was beginning development, the province provided undermining liability insurance for the developer through a regulation in the Municipal Government Act and at the time the municipality considered land it would acquire as also being included.
However, after the Dyrgas Gate sinkhole appeared in 2010, the province indicated it only insured third party property.
“That particular scenario was not covered in the undermining regulation because there were no third party loses, which is what it protects,” Esarte said.
Despite the misunderstanding, the province provided Canmore a $600,000 grant to mitigate the sinkhole.
The work to find the ineffective solution included engineering work and then drill work to locate the shaft and provide a geotechnical report that was used by a separate contractor to suggest the mitigation. That contractor was only one of three submitted that came within the scope of the budget.
Esarte said the proposed solution was to use concrete encased steel piles that are drilled into the ground at an angle to create a tepee above the opening of the airshaft and then use grout to fill in the voids.
“We mobilized the site and started doing some exploratory drilling and what we found through that exploratory drilling is that the shaft at the surface of the bedrock is larger than we anticipated and there is quite a bit more voiding in the material,” he said.
Being larger and deeper than originally thought, Esarte said the solution proposed would no longer work and the contractor was asked to demobilize from the site. Some alternative mitigations already considered are significantly more expensive to undertake, including digging out and excavating the shaft before it is capped.
“We want to do a bit more exploration; there may be alternatives that are cheaper,” he said. “We have given approval to mobilize a drill rig to do exploratory drilling in the next couple weeks.
“Then with the remaining budget we are going to look at the options, so the information we get will include recommendations.”
Esarte said once recommendations are received, the Town can determine if additional funding is needed and if so, the issue will likely return to council for consideration in spring.
Over the last 15 years there have only been two mine works that have formed sinkholes out of the 300 identified and mitigated. In addition to the mine shaft sinkhole in Dyrgas Gate, one formed on Bow Valley Parkway several years ago after a water main burst.
