'Unnerving': Oxford could face much bigger sinkhole problem

A Lions Club park and community centre in Oxford, N.S., will remain permanently off limits because of the risk of new sinkholes forming in the area.

Geological testing of the property has revealed extensive potential for further ground collapse. One area of concern is about 10 metres from Oxford's Main Street. Also, data will soon be released showing a network of sinkholes that begin in Oxford and cross the Trans-Canada Highway to Springhill.

"Mother Nature didn't give us any warning that this was going to happen, and that's the challenge with this," said town CAO Rachel Jones.

A sinkhole on the Lions Club property opened suddenly in August 2018, swallowing trees and picnic tables. It eventually filled with salty water, and is now approximately the size of an Olympic swimming pool.

This is not the result the Lions Club was wishing for. 

"We were all hoping for news that we could remediate the hole and reopen the park," said Robert Moores with the Oxford and District Lions Club.

"It was basically gutting. All of the members of the club are now concerned about, can we find a place where we can continue to provide services to the community?"

Geological study

The findings came from a $100,000 study by international engineering firm GHD. Engineers did seismic tests and electrical conductivity tests to determine underground topography of the area, which consists mostly of gypsum, salt, gravel and sandstone.

Groundwater can dissolve those materials, opening spaces underground.

"The net effect is that there could be many caverns at deeper levels," Moores said.

He said it's too risky to allow people to use the park or community centre at the site. 

That conclusion negates the possibility of filling in the current sinkhole, which geologists guess is 15 to 20 metres deep and would require 14,000 to 19,000 cubic metres of material to fill.

Next steps

Moores said the Lions Club is still deciding what to do with its building. 

One option is to sell it for the value of recoverable parts, such as windows and the heating and cooling systems. 

A second would be to take it apart and move it to a new site. Moores said it's not clear how much that would cost.

He said the cost of building a completely new centre would be roughly $500,000.

No matter what happens, he said the Lions Club is determined to move forward. 

"When we had our meeting to discuss this, I anticipated that there would be some dejection within the group," he said. 

"But it seems like everyone was universally of the view that we needed to find a way to continue to do the things that we do."

Future monitoring

Jones said news of a potential sinkhole just metres from Oxford's Main Street is "unnerving," but useful. 

"At least we have some of that information and we know where we can start to watch," she said.

"Our sewer and water lines, there's power lines going through there, so it was really important to understand what type of risk might be there for our public infrastructure."

Monitoring could include regular surveying to track any changes to the surface, and overhead photography using drones.

The ground over the existing sinkhole subsided over several weeks before the eventual collapse.

Implications for the Trans-Canada Highway?

Jones said the province will soon release LIDAR data showing a network of sinkholes that begins in Oxford, crosses the Trans-Canada Highway to Springhill, and then loops back to a different part of her town.

She said there have been subsidences, or sinking ground, near an off-ramp from the Trans-Canada near Oxford, and she wants further investigation.

She said the town wanted the provincial Transportation Department to piggyback on the geological survey.

'We would have the equipment and the expertise and the manpower mobilized to do the testing," she said. "And they could perhaps do some testing to at least get a benchmark there, to see if there were challenges or areas of concern, which didn't happen."

Jones notes that a sinkhole near the Trans-Canada Highway could affect all traffic moving between Nova Scotia and New Brunswick.