Nanaimo officials are investigating options to stabilize a city street after a void more than two storeys deep was discovered beneath the surface.
The City of Nanaimo found a 10-metre by 12-metre coal mine cave-in beneath the surface of Pine Street near Victoria Road, last week. It starts a couple of metres below the surface and plunges eight metres deep.
According to Susan Clift, the city’s director of engineering and public works, the municipality has been drilling in the area to test for mine voids that might collapse when construction crews start to build trenches for a water and sewer replacement project.
Old coal company maps show Nanaimo’s abandoned mine shaft network stretches close to the surface near Pine Street and Victoria Road, creating a potential safety issue.
The recent discovery is the only hole found so far and has triggered the temporary shut down of one block on Pine Street.
Now city officials are looking for a strategy to fill the hole – an extra expense for the utility project.
The cost hasn’t yet been determined.
“We didn’t budget for this, so we are going to have to find the money,” Clift said. “But it won’t be that bad, basically we’d do fewer sewer projects this year then we [were] intending.”
The Harbour City sits on top of a myriad of abandoned mine shafts, but it’s rare for holes to interfere with city construction projects. The majority of voids beneath downtown Nanaimo are too deep beneath the surface to be a worry, said Clift. It’s the top-end of mine shafts on slopes leading to the waterfront that are the most shallow and could collapse city-made trenches.
The void city crews recently found is 1.5 to 2 metres below Pine Street and extends more than two storeys deep. The city’s public works department believes the hole is a result of the mine slowly caving in on itself.
“The purpose in looking for these is really worker safety,” Clift said. “Some of these mines are quite deep and we need to make sure when we have a hole above a mine we aren’t so close … we collapse into it.”
The city is contemplating filling the recently discovered hole with a concrete and soil mix. An official strategy – and its costs – is expected by the end of this week.
Further drilling is continuing along Victoria Road.