FLOWSLIDES IN SENSITIVE CLAYS OF QUEBEC AND ONTARIO: STATE OF KNOWLEDGE (Invited Presentation)

In Quebec and Ontario, post glacial marine clays were leached from most of their salt content following the emergence of the land over the sea level during the last 12 000 to 8 000 years, creating sensitive clay deposits. Sensitive clays may change from a relatively stiff material into a viscous liquid when broken and remoulded. They are characterised by an extremely high sensitivity and very low remoulded shear strength. Slopes in such material are prone to very large landslides as in Scandinavia (Norway, Sweden and Finland), Alaska and British-Colombia along the North Pacific coast, and North East America (Quebec and Ontario in Canada, Maine, Vermont and New-Hampshire in USA). Approximately 90 % of the Québec’s population is settled on lowlands within the former limits of the various marine embayments that covered the eastern part of Canada (Champlain, Glodthwait, Laflamme, Tyrrel and Iberville seas). On the Quebec territory, about 80 % of the reported landslides on a year base are concentrated in these lowlands, posing a major challenge regarding public safety and landslides risk management.

The present study is based on an ongoing historical inventory of landslides in the sensitive marine clays of Québec and Ontario since 1840 at the Quebec Ministry of Transportation, and on current research projects at Laval University. Landslides of more than one hectare in area are classified as “Large Retrogressive Landslides (LRL)” in the Quebec province. Based on inventory, one LRL occurs approximately every two years. This inventory also indicates that LRL’s can be classified mainly in two types based on their in failure and post-failure mechanisms and characteristics, namely flowslides and spreads.

The state of knowledge on flowslides in marine sensitive clays of Quebec and Ontario will be presented. This includes the explanation of the failure mode, and the list of elements or factors that reveal, favor, aggravate, and trigger this type of landslide. An emphasis will be held on criteria necessary for the development of a flowslide. Statistical data on dimensions, retrogression and propagation distance of debris material will be presented. An overview of the mapping and mitigation methods applied in Quebec in connection with these phenomena will end the presentation.