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Analysis of Quebec landslide may help identify dangerous zones

Researchers took a closer look into the cause of a deadly landslide that devastated Saint-Jude, Quebec, in May 2010. The study, which was based on investigations that directly followed the event, revealed that the landslide was triggered by a disturbance of highly sensitive clay sediment that is 10,000 years old.

The experts found that the solidity of the underlying clay was compromised by natural causes of the Salvail River, which cuts through the base of the landslide site. Both sediment erosion caused by the river and high groundwater pressure beneath the river contributed to the decreased stability of the clay sediment.

The landslide was determined to have happened in two phases, with two major shifts at two different elevations causing the event.

Dr. Ariane Locat is the lead author of the report and a professor in the Department of Civil Engineering and Water Engineering at Université Laval. She explained that, “Such features have never been clearly identified before in spreads in sensitive clays.”

The research team reconstructed the landslide through field and laboratory tests using what they had learned about the properties of the soil and the shape and positions of the clay debris. They classified it a “large” and “complex spread,” a typical landslide type occurring in sensitive clays. The landslide had a width of 275 meters and a volume of debris of ~520,000 m3, which is the equivalent of the volume of more than 200 Olympic-sized swimming pools.

“Scientific knowledge on spreads in sensitive clays is limited,” said Dr. Locat.  “Investigations of case studies, like the one presented in this study of the Saint-Jude landslide, give us new information and insights on characteristics of spreads and on conditions in which they occur.”

90 percent of Quebec’s population resides among these “quick clays,” which are leftovers from the Champlain Sea. Quebec is more vulnerable to landslides brought on by the disturbance of this sensitive clay sediment than any other natural disaster.

“These studies are essential to the development of tools allowing for better identification of zones potentially exposed to spreads and to protect the population from this natural hazard,” added Dr. Locat.

The study is published today in the Canadian Geotechnical Journal.

By Chrissy Sexton, Earth com Staff Writer

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