The discovery of a two-storey deep mine collapse beneath a south Nanaimo road has city officials on the hunt for other potential weak spots in its network of underground coal tunnels.
Nanaimo city staff members will be reviewing coal maps and testing soil beneath roads for mine tunnels at high risk of collapse, according to Mayor John Ruttan, who said the safety issues and costs of the Pine Street mine collapse have been an eye opener for local officials.
The new action comes on the heels of an announcement that city workers are about to start an estimated $240,000 in remediation work on a developing sinkhole found a couple of metres beneath Pine Street.
The multi-staged plan, six weeks in the making, is expected to provide a roadmap for future mine repairs.
According to city staff members, Nanaimo’s abandoned 19th century mines are slowly disintegrating, leaving voids beneath the surface. While most mines are on private property, they say it becomes a municipal concern when tunnels are shallow and voids open close to the surface in public areas. Shallow caverns have the potential to create sinkholes.
The issue is a worry for politicians, who have called on staff members to launch a city-wide search for potentially dangerous and weakened tunnels beneath roads.
Ruttan said the hope is the City of Nanaimo can identify and address risks before they become a public hazard or an expensive burden on taxpayers.
Previously the city checked for signs of sinkholes on local roads and drilled near infrastructure work to ensure there were no collapsed mines, but only took action on issues it could see.
“Until something catastrophic happens like this, suddenly [it’s] when people say ‘Oh my God, maybe we better do a review of what we’ve got there and what the risks are there’,” he said of the reason for the new approach.
The coal void beneath south Nanaimo’s Pine Street was discovered during preparation work for a sewer and water line replacement, triggering the temporary closure of the street Oct. 10.
Stephen Ricketts, the city’s construction manager, said the void had reached a point where the city had to do something before heavy traffic could continue driving over the road. While it wasn’t “thin ice,” vehicles had the potential to spur a future sinkhole, he said.
City staff members are now planning to test a new strategy that could reopen the road in mid-December and provide a guide for future mine remediation.
Crews will pump cement into a drill hole this week to seal off the collapsed mine chamber, ensuring work doesn’t compromise stability in other parts of the tunnel. The multi-phased project will also include taking off the road top, stabilizing the loose soil with grout and adding new concrete columns. Ricketts said work is specialized and no other method was cheaper.
A more proactive approach to identifying and fixing weakened mines, however, could offset future expenses for mine remediation, according to Ruttan.
It could also prevent any more surprises. The Pine Street void was unexpected, said the city’s mayor, who points out that if another collapse happens suddenly it could present a serious danger to the public.
“[Collapsed mines are] a potential hazard and that’s why we are putting a very serious effort in to find out what the risk is and what else can be done,” Ruttan said.
No schedule or costs have been announced for the new review.