Large Landslides in Sensitive Clay in Eastern Canada and the Associated Hazard and Risk to Linear Infrastructure

Abstract

The Saint Lawrence Lowlands in eastern Canada contain extensive deposits of marine soils deposited in post-glacial seas during and following the retreat of the most recent continental glacier. These marine soils include silt and clay deposits known collectively as Champlain clay. When the pore fluid in these marine deposits has changed over time to a lower salinity, the clay can become very sensitive, or demonstrate substantial strength loss after reaching the peak strength with sufficient strain under undrained load conditions. Sensitive clay soils are subject to a peculiar type of very large landslide that typically involves great extents of nearly horizontal ground, usually occurring suddenly and without warning. These landslides tend to be described as “retrogressive” in the literature and practice, implying that they develop as a series of successive small failures that advance rearward until a final stable position is reached. The work of this thesis is organized into four different themes, with an overall objective of understanding the hazard and risk associated with large landslides in sensitive clay to linear infrastructure such as railways. The first theme, documented in Chapter 2, develops a number of spatial relationships between specific physiographic and geologic features and landslide occurrence or absence, as determined through air photo analysis and a review of the literature. The second theme, documented in Chapter 3, presents the construction of a digital database of large landslides in sensitive clay in eastern Canada, for the purposes of studying landslide susceptibility, hazard and risk. The third theme, documented in Chapters 4 and 5, presents and defends a novel mechanical model for development of these large landslides. This model suggests the landslides develop progressively, rather than retrogressively, and the science of fracture mechanics is employed to substantiate the model. The fourth theme, documented in Chapters 6 and 7, synthesizes the findings of the earlier themes and presents a methodology for estimating landslide susceptibility in Champlain clay. That approach is then extended to develop an understanding of the hazard. The concluding chapter extends that work to present an initial appreciation of landslide risk to railways.