A Hanmer family got the shock of their lives last month when they discovered what they thought was a sinkhole in their backyard was actually an abandoned mine shaft.
The family has asked their names not be used to ensure no one looks for their home, violating their privacy and potentially putting themselves in danger.
However, the problem has since been addressed under a Ministry of Northern Development and Mines policy dealing with abandoned mines. The province paid the roughly $70,000 it cost to fix the problem. Much of that solution involved pouring concrete — 5 1/2 loads, to be exact – into the hole until the area was safe.
Lesley Cooper, manager of mine rehabilitation and compliance section with the Ministry of Northern Development and Mines, said the homeowner contacted them June 11, and staff were sent out the same day. City crews first secured the area with a fence, which was then re-enforced by the ministry.
While the property isn't in their abandoned mines database, Cooper said it's likely some sort of exploration work had been done there.
“We don't have any record of it being a mine, but that's what we believe it to have been,” she said. “It certainly didn't appear to be a natural occurrence.”
Guido Mazza, the city's director of building services, said they worked with soil and rock experts from Laurentian University who helped determine the property had been used for mining at some point, and to ensure the house wasn't in danger of collapsing.
“It's not unlike a tornado hitting the city,” Mazza said. “The first thing we do is, as the chief building official, go in and ensure public safety. We have the power to prohibit occupancy if need be.”
Mazza and Cooper both praised the way the way city and ministry staff worked together to address the problem as quickly and safely as possible. The best guess is the affected area was an exploratory copper mine shaft from the 1920s.
While exploration today involves drilling and other modern techniques, Mazza said back then, it involved digging a potential ore vein with a “pick and ax” to see where it led. But there are no signs a mine was ever brought into operation.
“We think it's just a localized shaft that was abandoned and, for whatever reason, wasn't put into the database the ministry has,” he said.
The mineral rights on the property don't belong to the homeowners, Copper said, but by 749549 Ontario Ltd., a company not involved in mining. Under provincial rules, mineral rights owners are responsible for remediation costs in these cases, but the province can step in if the owner can't pay for the work, or arrange for it to be done quickly.
The province can then work to recover costs after the problem is fixed. While the ground was too unstable to get an exact picture of the affected area underground, Cooper said it appears it's limited to the homeowner's property, running from the front of the garage to the backyard.
The land had been covered by what's called a crown pillar, she said – basically, a thin layer of rock.
“And over time, that rock became unstable, got thinner and thinner, until it couldn't support things any longer.
“So basically, we filled the hole with concrete,” she said. “Now that the ground has stabilized, the ministry is working to confirm this.”
Crews will remain on site until they're confident the problem has been successfully addressed and the ground is stable. While there are 323 known abandoned mine sites in Greater Sudbury alone, Mazza said he's never dealt with this sort of situation before.
“I've been here since 1993, and this is a first for my department,” he said. “This doesn't happen very often because, thanks to the database, the abandoned mines have been well documented across the (Sudbury) Basin.”