Linking geotechnical and rheological properties from failure to post-failure: The Pointe-du-Fort slide, saguenay fjord, Québec

Abstract

The Pointe-du-Fort submarine mass movement likely took place at the time of the February 5th 1663 earthquake as a sidewall slope failure which generated a mudflow with a run out distance of 1070m and a final flow thickness of 10-15m resting on a slope of 1.4 degrees. The slide involved about 1.95 Mm3 of clayey sediments from an original slope of 24 degrees. The slide took place in normally consolidated sediments composed of stratified low organic Laflamme Sea clay at the base overlain by progressively more organic rich recent sediments. In situ strength testing and sampling on the tidal flat, morphological analysis and remolded strength of the debris lobe can be related to rheological tests to model the mobility of the debris. For the first time, it has been possible to link the mobility of a submarine slide with the characteristics of the sediments at the time of failure with no need to consider water content increase to explain the observed mobility.