Warm sunshine has flushed out an ugly hazard on the streets of Red Deer.
Early Wednesday morning, as the pavement began to warm, what started as a small pothole opened into a water-filled sinkhole big enough to swallow a car on 43rd Avenue, about half a block north of 32nd Street.
Gary Gaalaas, whose house is one door down from the site, said he heard noises in his basement at about four a.m.
There was no sign of anything unusual on the street at 8:15 a.m. when he drove away. But a gaping sinkhole had opened up when he returned less than an hour later.
Estimated at about four metres deep, the hole opened up next to a manhole cover in the middle of the street.
Darren Hilsendager, an operator in the city’s water distribution branch of Environmental Services, said the problem started with a leaking water main.
Likely a result of fatigue in the pipe, the leaking water softened earth and gravel around the water main while filling the space with water, said Hilsendager.
Frost at the surface held the pavement in place as water collected in the cavern.
The pavement gave way when the frost started to thaw, he said.
Crews had to shut off water service to 18 homes in the immediate area while the leak and the hole were repaired.
Hilsendager hoped to have repairs completed and water service restored by 9 p.m. on Wednesday, including fixing the leak, filling the hole and patching the pavement.
It can take six to 24 hours to fix this type of hole, depending on what the crews find when they get in there, said Hilsendager.
“It’s a bit of a treasure hunt.”
A more permanent repair will be performed in summer, when there’s no more frost in the ground.
Sinkholes are not common but can occur during colder months when water cannot come through the layer of frost that has formed at the surface, said Tom Warder, the city’s Environmental Services Department manager.
A similar leak occurred a few days earlier on 41st Avenue, two blocks east of the sinkhole that appeared on Wednesday.
In both cases, the water had probably not been leaking for very long, said Warder.
“The water main likely broke within hours, or not too long a period of time, prior to it actually breaking through the pavement. We can’t tell until we actually get down there and see what the cause of the water main break was.”
When the ground is frozen, the leak will tend to erode a cavern. Otherwise, it would come to the surface, he said.
“It’s a combination of events that creates this type of thing.
“It’s really the weight of the soil that brings it down. It’s not a trap waiting for something to drive over them. I suppose, if it happened in a high-traffic area, there is some risk.”
The area where the sinkholes appeared are in older subdivisions that still have cast-iron water mains, said Warder. The cast-iron pipes are subject to corrosion and failure over time.
The city is gradually replacing the pipes, concentrating on areas that have a history of failures or where new pavement is to be laid.
bkossowan@www.reddeeradvocate.com