The city’s utility manager says a cracked pipe was the cause of the sinkhole that closed St. Albert Place this week.
City crews continue to work this week to patch a large sinkhole discovered on St. Anne Street on Monday.
It looks like the hole was caused by a fracture in a 200-millimetre drinking water line under the street, said Brian Brost, utilities manager for the City of St. Albert.
“Water normally follows the path of least resistance,” Brost said, which in this case was the dirt around a stormwater line next to the pipe.
Water burst out of the pipe, washed away the dirt and rushed into the stormwater line through a crack where it connected to a manhole. Some 5,000 cubic metres – about two Olympic swimming pools – of chlorinated water washed into the Sturgeon River before city crews stopped the flow.
The water created a large gap in the soil that was about 15 metres long and a metre deep, visible on the surface as a roughly 1.5-by-1.5-metre depression in the street, Brost said.
Crews informed Alberta Environment and applied de-chlorination agents to minimize the impact of the water on the Sturgeon, Brost said. They have since fixed the cracks in the storm and freshwater lines and are now filling in the hole in the street.
Sinkholes are not unheard of on city streets. There was one on Eldorado Drive last May, for example, and another on St. Anne Street in July 2009.
Virtually all sinkholes in the Edmonton region are caused by water pipe breaks, said Hugh Donovan, St. Albert resident and head of the City of Edmonton’s materials testing lab, which works to create better asphalt for streets. He typically sees about one a year on the job.
“Some of them we don’t find until it’s really accidental,” he said.
There was a three-metre deep one on the Yellowhead 15 years ago that they didn’t notice till a jackhammer disappeared down it, for example. There was another that turned up during the demolition of the Rat Hole underpass – they only found that one when they noticed a pile of gravel had sunk 1.5 metres below the street over the weekend.
Sturgeon County sometimes gets sinkholes from the many old coalmines beneath its surface. A six-metre wide, 7.5-metre deep sinkhole caused by an old mine and heavy rain swallowed the green of the 12th hole at the Cardiff Golf and Country Club back in 2001, for example.
While parts of the U.S. can get huge sinkholes caused by water-soluble rock –such as limestone – we don’t have that kind of rock or problem here, Donovan said.
Leaks and cracks in pipes can be caused by poor construction, old pipes and freeze-thaw cycles, Donovan said. Brost suspected the latter in the St. Anne case.
Sinkholes can be dangerous if they collapse suddenly, and have swallowed cars in the past, Donovan noted. Most roads are flexible enough to sag before they break, however, creating an obvious dip.
There’s no easy way to detect sinkholes before they do damage except for ground-penetrating radar, which is expensive, Donovan said.
“Unless the road starts to subside, we don’t really know it’s there.”
Donovan advised residents to report any subsidence in roads they notice to public works officials, as they may be sinkholes.
“It’s as simple a thing as a dip in the road that wasn’t there last week.”
Brost had yet to calculate the cost to fix the sinkhole, but said it should be fully repaired within 10 days, depending on weather and subsurface conditions.
