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Falmouth sinkhole not necessarily a fluke: geologist


Published on September 7, 2017

A detailed map of Nova Scotia shows that gypsum and limestone (blue and yellow) are prevalent in West Hants, meaning there’s an increased risk for sinkholes.

©Courtesy of the Nova Scotia Department of Natural Resources

FALMOUTH, N.S. — A sedimentary geologist is cautioning people from brushing off Falmouth’s sinkhole as a one-time event.

Dr. Elisabeth Kosters, a sedimentary geologist and former professor at Dalhousie and Acadia universities, said she doesn’t think the sinkhole that occurred in Falmouth on Sept. 3 should be dismissed as a one-off.

“I think this should serve as a cautionary event; we shouldn’t just lightly say ‘oh it’s a one-off, it won’t happen again,’” Kosters said. “Maybe we should have a really good look before we start developing anything new in this particular area and consult with the regional geologist and do a risk assessment.”

The Municipality of West Hants posted a statement on their website saying the site is being studied further, but preliminary evaluations show that the sinkhole was an isolated, natural event.

“It’s a new, very big home, probably built in the last 15 years, so who knows what building this home did,” Kosters said. “This should certainly serve as caution to a future builder in the same location or in the vicinity.”  

Kosters looked up the area on a geologic map on the government of Nova Scotia’s website, confirming her suspicion that the area is comprised largely of limestone and gypsum, which can be at risk of sinkholes.

“The map says there’s limestone, gypsum and there’s also a lot of faults in that area,” she said. “That terrain has a bumpy, hilly expression and that’s typical for gypsum, limestone terrain that has suffered from dissolution.”

Kosters explains that dissolution is when the rocks that underlay the area are exposed to the elements and disintegrate, compromising the structural integrity of the land.

“If you drive around the terrain, you’ll see dissolution a lot, for example along Highway 101 between exits 5A and 5 on both sides, you’ll see little pocket holes, which is where the rock has dissolved and little craters have formed,” she said.

“The new trail that is developed near the Avondale Sky Winery, there was some gypsum mining there, and the trail wraps around all of these sinkholes which is really illustrative of what happens when this rock is exposed to the elements,” she said. “It’s riddled with zones of weaknesses and water creeps into those zones, it’s a natural process.”

Kosters said the gypsum mines are located pretty farm from Falmouth, so it’s unlikely that mining had a direct impact on this particular sinkhole.

“We don’t know what was done in terms of the foundation, or diverting waterways, to affect the circulation through the rock below their house,” she said. “But to say it’s a one-off and it won’t happen again, I would caution that.”

Kosters said she would like to see a geologist from the provincial Department of Natural Resources study the area to provide a second opinion.

 

DNR geologist weighs in

Bob Ryan, a geologist with the Nova Scotia Department of Natural Resources, said the department would not be involved with studying this specific case.

“We get reports of sinkholes and we recommend that if it happens on your property that you seek geotechnical expertise,” he said. “Our expertise is in the science and the geology, not the geotechnical or engineering end of things.”

Ryan said there are approximately 800 documented sinkholes in the province, but he says there’s likely many more that are undocumented. In the Falmouth area alone, Ryan said there are approximately a dozen documented sinkholes.

“They’re not an uncommon phenomenon, especially in the gypsum areas,” he said. “But this is probably the only time I’ve seen it actually cause damage to a house. I’ve seen barns and sheds and yards open up, but never heard of an incident as devastating as the one in Falmouth.”

Ryan said the fact that the house in question was built on a slab likely made the structural damage more severe.

He said sinkholes come in two forms, naturally-forming and man-made.

“Any time you have a rock-type like gypsum, or in some cases limestone, that are easily dissolved, the groundwater going through those formations create pathways for the water and caverns under the surface,” he said.

“A sinkhole is simply when one of those caverns collapses,” he explained.

“If a water main breaks and a road collapses around it, people call those sinkholes, or other man-made events, like the collapse of an old mine shaft, but from a naturally occurring point of view, those sinkholes are usually caused by gypsum and limestone,” he said.

With the heavy presence of gypsum in parts of Hants County, Ryan said the risk of seeing sinkholes form is higher.

“Any place where gypsum is present at the bedrock or surface, you run a high risk of sinkholes,” he said. “That doesn’t mean if you build your house on gypsum it’s going to fall in; the odds are in your favour, but there is a risk involved.”

Ryan said an updated sinkhole risk map will be released in early 2018, which will be distributed to planners and municipalities.

Current information on sinkholes in Nova Scotia can be found here.