Geotechnical characterization of the Saint-Jude clay, Quebec, Canada

Abstract

On May 10th 2010, a landslide occurred along the Salvail River in the municipality of Saint-Jude, tragically killing the four members of a family. The Ministère des Transports du Québec in collaboration with Université Laval carried out a detailed investigation to characterize the soil involved in this landslide. The investigation included field observations, in situ testing, sampling using thin-wall tubes, as well as laboratory tests that enabled to obtain information on the stratigraphy of the deposit and the geotechnical, mineralogical, micro-fabric and physico-chemical properties of the soils involved in the landslide. The stratigraphy and geotechnical properties were found to be uniform around the landslide. The clayey deposit is composed of various minerals dominated by quartz and feldspar, with a clay fraction containing large amounts of illite (or mica-like minerals) and a flocculated fabric. The soil involved in the landslide consists mainly of sensitive grey clay, typical of Canadian Champlain Sea clays, with a liquidity index varying between 2 and 1 from top to bottom of the deposit, intact shear strength increasing linearly with depth from 25 to 65 kPa, and an OCR decreasing with depth from 1.9 to 1.2. High quality samples were also taken using the Laval sampler. Triaxial tests were performed on these samples to characterized the mechanical behaviour of the Saint-Jude clay and its critical and limit states. The critical state is defined by a friction angle in the normally consolidation range of 30.6° and a cohesion of 5 kPa. The limit state is centered around the normally consolidated coefficient of earth pressure at rest line, with a peak strength envelope beyond the critical state envelope and an isotropic limit state equal to 0.7σ’p,typical for Champlain Sea deposits.