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The giant hole below Allendale Road is seven metres wide and starts about five storeys underground, said Epcor Thursday.

The hole was discovered during an inspection of a manhole near Allendale Road and Calgary Trail, leading to the closure of Allendale Road and two nearby right lanes of Calgary Trail to ensure public safety, the city-owned utility said in a news release.

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Analysis shows the cavern starts about 16 metres under the road and extends down for seven metres, with an estimated size of 700 cubic meters.

This volume is about the same as 120 truck loads of concrete, the release said. One online source indicates this space is also more than one-quarter the size of an Olympic-sized swimming pool.

An opening created in a 1.2-metre-diameter drainage trunk line that runs through the site was plugged Thursday, allowing crews to start filling the big void with a self-compacting mixture of sand and cement in a process that should be finished by the middle of next week.

Once that work is finished, Epcor plans to install a shaft down to fix the damaged line, which will probably take three months.

One lane of eastbound Allendale Road will likely be opened in the last week of August after the shaft is set up.

A depression in westbound Allendale Road thought to be unrelated to the big hole will probably be repaired and the lanes reopened in two or three weeks.

gkent@postmedia.com

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