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That Massive Sinkhole in Ottawa Might Be Due to ‘Quick Clay'

City officials haven’t yet pointed to a firm cause.

by Bryson Masse
Jun 8 2016, 8:19pm

Water can be seen in a large sinkhole that formed on Rideau Street next to the Rideau Centre mall is seen on Wednesday, June 8, 2016 in Ottawa. THE CANADIAN PRESS/Justin Tang

Residents of Canada's capital had an interesting morning on Wednesday when an intersection, in what's essentially the middle of downtown Ottawa, collapsed into a massive sinkhole. It claimed the entire width of the main drag of Rideau Street, and swallowed a van, causing power outages, sparking a gas leak and forcing evacuations.

Investigations should uncover reasons for the dramatic collapse, but one factor might have been lurking in the soil under Ottawa for 10,000 years.

While the direct causes were unknown as of a 1:30 p.m. press briefing, ongoing major construction could be partly to blame: both the digging for a new light rail system and an extensive facelift of the Rideau Centre, a nearby shopping mall, have been the source of traffic snarls and frustrations for months. This sort of activity can wreak havoc on the type of soil that covers much of the Ottawa area: Leda clay.

Also known as quick clay, it is unpredictable and has caused other geological disasters in the region. To find its source, you need to back in time to when the Ottawa Valley was submerged in a massive inlet of the Atlantic Ocean called the Champlain Sea.

About 10,000 years ago, the Champlain Sea stretched up the Saint Lawrence river, into southern Quebec and eastern Ontario. The region flooded as glaciers receded after the last Ice Age. Fossils of beluga whales and marine shells that dot the area give hints to the life that once existed there, in its cold salty waters.

A significant consequence is a layer of silty, sensitive clay that blankets the Ottawa region. It's been at the root of landslides, some with deadly consequences. One landslide in 1908 took the lives of 34 people in Notre-Dame-de-la-Salette, about 50km north of Ottawa. In 2010, a house near Montreal was swallowed by a sinkhole, claiming four people.

"It is a geologic issue," says Peter Bobrowsky of the Geological Survey of Canada. "If we didn't have sand and gravel there, if it would have been the good Pre-Cambrian bedrock, you wouldn't be getting any kind of sinkholes. The geology plays a huge role."

Although we still can't define its exact role in the Ottawa sinkhole, quick clay has caused many headaches in local engineering projects. When vibrations or water disturb the sediment, it can liquefy and flow away. This means that, after activity like construction, that the part of the planet you've decided to place your road on might not exist anymore.

Luckily, no one was hurt when the street caved in, and now there's just a massive cleanup to be done.

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