Tracking the mystery of the N.S. sinkhole: ‘We need more information’

OXFORD, N.S. — Nova Scotia’s sinkhole remains largely mysterious, and officials are bringing in ground-penetrating radar and other technology in a bid to better understand what is happening — and how big it might grow.

The sinkhole’s growth has slowed considerably, but provincial geologist Amy Tizzard said “little mudslides” are occurring at the edges, and cracks are continuing to appear in nearby pavement in an Oxford park.

“You can’t say it’s contained or under control because it’s Mother Nature and she’ll do what she wants,” said Tizzard.

“We are monitoring it to see if we are seeing any patterns in the development and then use that information to predict what the risk would be in the rest of the area around here.”

A large spruce tree fell into the sinkhole overnight, according to town spokesperson Linda Cloney.

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First reported July 28 as a small hole about the size of a dinner plate, it was measured Tuesday at 32.6 metres by 38.7 metres — larger than an NBA basketball court.

Tizzard said officials are measuring existing cracks on nearby land and looking for new ones, and using a drone to collect high resolution imagery of the site.

“So far we are using high accuracy GPS equipment that will tell us if there’s any subtle variations either vertically and horizontally on the ground surface … and we’re mapping out cracks in the pavement and in the forest around the sinkhole and monitoring how fast those cracks are growing, opening up,” Tizzard, regional geologist with the Department of Energy and Mines, said Monday.

“We’re looking to, and hopefully will start this week, employing some geophysical methods like ground penetrating radar and electrical resistivity tomography and any other geophysical technique that might be suitable for mapping out any fissures or fractures or caverns in the sub-surface.”

Tizzard said radar and other technology is “en route” to the site on the outskirts of Oxford, a sleepy town of about 1,000 people that calls itself the wild blueberry capital of Canada.

The sinkhole has swallowed trees, picnic tables and part of a nearby parking lot at the Oxford and Area Lions Parkland.

The wider area is underlaid with gypsum, a soft rock that is prone to the development of fractures, fissures and caves, Tizzard said.

“Just to the west of here there are dozens of known sinkholes and they’re either flooded and ponded and vegetated — there’s nothing that shows an active process like this one does,” she said at the scene Monday.

“All around the hole we still see continued signs that the hole is propagating.”

Cloney said in a news release Tuesday afternoon that the water level in the hole had risen by 11 centimetres in the previous two days.

Another tree near the edge of the sinkhole may be removed to mitigate damage to the parking lot, she said.

Tizzard said she can’t predict what will happen next, or speculate on whether a gas bar and Tim Hortons across Main Street from the park are in any danger.

“We need to gather more information about what’s going on on the pavement here … and all around the sinkhole to figure out if there are more cavities or how big is this cavity underground that we’re dealing with,” she said.

On Monday afternoon, Tizzard knelt in the parking lot with a tape measure to show the progress of one crack near the edge of the sinkhole.

“This crack is Number 12, and we’ve been monitoring this one since last Saturday and it’s grown from two millimetres to about 25 millimetres in the past nine or 10 days,” she said.

“It’s an active site and it remains unstable, so we continue to monitor these things and keep the public away, and safe, from the site.”

The spectacle has been drawing curious onlookers to the small town, located roughly 30 minutes from the New Brunswick border, even causing minor car accidents.

Officials always go into the sinkhole site in groups of at least two, and ropes and life jackets are kept nearby, Tizzard said.

The site is monitored 24 hours a day by a security guard and video surveillance.

Rob Roberts, The Canadian Press

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