Hole Picture

Canada is no Florida when it comes to sinkholes and the damage they cause. However, on this side of the border, geology experts note that vehicles can be damaged or destroyed when road surfaces cave in as a result of either natural or human activities.

“We are fortunate in one sense, because (in Canada) we don’t have the hazard level that we have in Florida,” suggests Stephen Evans, a geological engineering professor at the University of Waterloo whose areas of expertise include landslides. “We don’t have the enormously dense population, both in terms of people on the ground and in terms of highway networks,” Evans explains.

Florida is susceptible to karst, a “geological condition in which subsurface carbonate rock layers, often comprised of limestone or dolomite, dissolve and leave behind an underground void,” states a report released in December by Irvine, California-based CoreLogic Inc. “Sinkhole activity and subsequent property damage will continue to be a substantial risk” across the United States, the report notes.

Although Canada does not have a similar geology that would cause “as many catastrophically formed sinkholes” as in Florida, “we do have environments where so-called sinkholes develop because of subsurface erosion or subsurface solution,” Evans says, citing as an example parts of the Rocky Mountains.

Another way sinkholes form in Canada is when silt – deposited at the end of the last ice age – is eroded, he explains. “Both railways and highways would be subject to these hazards.”

The Transportation Safety Board of Canada (TSB) cites two incidents involving freight trains: one in 1997 not far from Pointe au Baril, Ontario, near the eastern shore of Georgian Bay; another near Lytton, British Columbia.

“In both accidents, the underlying soil foundations had become saturated due to the spring run-off and a large portion of the roadbed had been eroded from beneath the rails,” notes a TSB report. “In both accidents, the trains plunged into the depressions. Neither the train crews nor their respective railway control centres were aware of the track bed failures before the accidents.”

In parts of central British Columbia and western Ontario, Evans says small cave systems develop because of underground erosion, and “we might have a road collapse or a railway cave in.” In this type of situation, “the hazard to life itself is pretty significant, I would think, to the driver or to the crew of the train.”

A more recent sinkhole – local news reports estimated the hole measured five by three metres with a depth of more than a metre – that resulted in damage to vehicles occurred last March near Montreal-Trudeau Airport in Dorval, Quebec.

“It was a shallow sinkhole, but it swallowed two cars,” Abouzar Sadrekarimi, a professor of civil and environmental engineering at Western University in London, Ontario, reports. “From what we have seen, the sinkholes in Canada happen often by man-made or construction activities.”

Sadrekarimi, a geotechnical engineering expert who co-wrote several papers on liquefaction, says a sinkhole forms when a void or disruption under the ground leads to a collapse of surface material. “The sinkhole would basically start as a small cavity and then expand, most often, with the action of water seeping through and eroding the soil with it,” he explains. “At some time, the cavity becomes large enough that the soil above that cavity is so thin that it’s no longer able to arch across the cavity.”

A Year in Review, posted on Environment Canada’s website, cites a number of events that resulted in sinkholes: the July 8 deluge in the Greater Toronto Area, with videos capturing sinkholes opening up; in June in Fort McMurray, Alberta, where fast-moving waters pockmarked road surfaces with sinkholes; and the spring flooding in Ontario’s Cottage Country, causing a huge sinkhole on Highway 11 that forced traffic to be detoured.

Detailed statistics on sinkhole-related auto damage claims are hard to come by, but one traffic safety expert suggests the related damage could be significant. “In some areas, the drop can be as much as 10 feet, so the whole orientation of the vehicle changes,” notes Brian Patterson, president and chief executive officer of the Ontario Safety League. “It’s so abrupt, that it would be like hitting a wall,” Patterson says.

He suggests that sinkhole accidents are more common at night or when there is no surface water. “Sometimes when they occur, there is already surface water bubbling, so people become aware of the change,” Patterson points out.

One incident where bubbling water from a broken watermain provided a warning to drivers was in downtown London, Ontario shortly after 3 am on October 31, 2007. “A car approached it, but at least (the driver) recognized that it was obviously something that you should stay away from, because the water level was starting to cover the intersection,” reports John Lucas, director of water and wastewater for the City of London.

“Even places not commonly associated with sinkholes can experience the formation of these subsurface voids, such as when an underground water pipe breaks and the flowing water erodes a cavern below the surface,” CoreLogic notes in its recent analysis.

The London incident caused “quick and devastating erosive effects on the sandy soils within 10 metres of the failure location,” notes a staff report later submitted to the city’s environment and transportation committee.

“Once in a while, a sewer will collapse internally and materials are being washed into the sewer, and those materials keep coming from above until eventually it gets to the road surface,” Lucas points out. However, road patrols usually identify potential problem spots before they become significant, he says.

Ontario Regulation 239/02, the Minimum Maintenance Standards for Municipal Highways, requires that municipalities inspect roads on a regular basis, he notes.

“Where you’ve got large stretches of roadway, municipalities have to take a more serious look at culverts and where they’ve directed water” underneath roads, Patterson recommends.

In Canada, Sadrekarimi says that highways tend to be vulnerable to sinkholes as a result of the material used in their bases. “For construction of the highway, we need to use cohesionless and granular material to provide the good base for it,” he explains. “If we use, for example, a clay material or an impermeable soft material, you wouldn’t get a strong base, and over time, it may deform.”

But the disadvantage of those granular materials is that they are “susceptible to erosion by water seepage,” Sadrekarimi continues.

“In a case where you have, for example, leakage from a watermain or a sewage pipe, or we have heavy rainfall, this seepage could actually erode this cohesionless soil, and form a cavity under the ground,” he says. “So if this continues unnoticed, it could become larger… and become a sinkhole.”