Ottawa sinkhole struck in unstable 'rock valley' flagged by engineers - Ottawa - CBC News

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By Alistair Steele, CBC News Ottawa, June 9, 2016

The sudden road collapse that left a gaping hole in the middle of a street in downtown Ottawa Wednesday morning occurred over a vein of sand, silt and fractured rock so unstable that workers completing the light rail tunnel below had to inch along cautiously for fear of causing a cave-in.

The sinkhole occurred in the same place engineers conducting a geotechnical survey for the light rail project in 2011 discovered a 120-metre wide "bedrock valley" stretching under Rideau Street, just east of Sussex Drive.

http://www.cbc.ca/news/canada/ottawa/ottawa-sinkhole-rock-valley-1.3623008