Influence of surficial crusts on the development of spreads and flows in Eastern Canadian sensitive clays

ABSTRACT: Spreads and flows are the two main types of large retrogressive landslides occurring in Eastern Canadian sensitive clays. In spreads, the soil mass mobilized during failure is dislocated in a succession of horsts and grabens leading to a typical ribbed topography in the landslide scar. The failure mode for flows, on the contrary, is characterized by a succession of rotational slides propagating rearward, which requires that clays liquefy during the movement. These mechanisms are now relatively well understood. However, conditions leading to the development of either a flow or a spread are not yet clearly identified. Some numerical results published in the literature suggest that spreads form preferentially when a non-sensitive crust overlying a sensitive clay deposit is present. We examine in this paper whether this result is supported by observations made on several spreads and flows that occurred in southeastern Ontario and Quebec. It is shown that the presence of a crust is likely not a discriminating factor.