How sensitive clay underneath Ottawa led to an Orléans landslide

Greg Brooks (blue jacket) and Marc Hinton are researchers with the Geological Survey of Canada and National Resources Canada who study and document landslides in eastern Ontario. Photo by JEAN LEVAC /POSTMEDIA

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Bordering the Ottawa River to the north, a striking and somewhat hidden landscape lies within a forested stretch of Orléans, near the intersection of Jeanne d’Arc Boulevard and Tenth Line Road.

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A series of rolling ridges define the terrain, giving the land a rugged and almost sculpted feel. Today, that uneven ground has become a popular spot for kids to ride their bikes up and down on the earthy waves, likely oblivious to what caused the land to form like this in the first place.

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Yet this area holds a storied past: The ground once gave way in an estimated 500-metre-long sensitive clay landslide that reshaped the landscape in a matter of minutes. Researchers say it’s hard to pinpoint exactly when it happened, but mature trees and thick vegetation now blanketing the ridges suggest the earth has been settled that way for at least a few centuries.

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Researchers at the Geological Survey of Canada (GSC), part of Natural Resources Canada, are working to map and date sensitive clay landslides in the Ottawa Valley while trying to better understand what triggers them and how their impacts might be reduced.

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“Landslides are a naturally occurring thing,” said Greg Brooks, a geomorphologist with the GSC, NRCan. “They’re not some act of God; they’re a process that occurs in our landscape. They’ve occurred many times in the past, and they will occur again in the future.”

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Greg Brooks (blue jacket) and Marc Hinton are researchers with the Geological Survey of Canada and National Resources Canada who study and document landslides in eastern Ontario. Photo by JEAN LEVAC /POSTMEDIA

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Roughly 1,500 sensitive clay landslides have occurred in the Ottawa Valley, though only seven have occurred along the Ottawa River. Tributaries such as the Gatineau, Quyon and Lièvre rivers are among the most common sites, according to Didier Perret, a GSC, NRCan research scientist who has spent years counting and mapping each of these landslides.

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Much of Ottawa and the surrounding valley sits atop unstable marine clay, often called sensitive clay or Leda clay. Within the city, this material is most prominent along the Ottawa River, particularly near Orléans in the northeast and Dunrobin in the northwest, Brooks said.

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This clay dates back to the end of the last ice age, when the Champlain Sea covered what are now the Ottawa and St. Lawrence valleys. Glacier melt combined with saltwater from the ocean and deposited fine clay sediments on the sea floor, explained Marc Hinton, a GSC, NRCan hydrogeologist.

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When the ocean eventually receded and saltwater drained away, the clay’s plate-like particles formed a fragile structure around the salt left behind. Today, much of the Ottawa region is built on what was once the Champlain Sea bed from about 10,000 years ago.

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The clay structure remains highly sensitive and can be disturbed by factors such as riverbank erosion, heavy rainfall, earthquakes or even human activities such as construction blasting, Perret said. This causes the clay structures to collapse abruptly, transforming from a solid into a liquid.