FAILURE MECHANISM OF SPREADS IN SENSITIVE CLAYS

Spreads are large landslides occurring in Eastern Canada sensitive clays. They result from the extension and dislocation of the soil mass above the underlying remoulded material forming a shear zone. This process generally leads to a crater filled with blocks of more or less remoulded material having horsts and grabens shapes. Horsts are blocks having a sharp tip pointing upward and grabens are blocks with a flat top surface. They represent 38% of large landslides occurring in the Province of Quebec, the rest being sensitive clay flows (57%) and unidentified large landslides (5%). Regular stability analyses give too large safety factors when back calculating these landslides. These methods are therefore not suitable for understanding this particular type of landslide and another failure mechanism is needed.

Recent works suggest that sensitive clay spreads are formed by two distinct processes: (i) initiation and propagation of a quasi-horizontal failure surface by progressive failure mechanism and (ii) dislocation of the above soil mass in horsts and grabens. According to this failure mechanism, the failure surface is initiated near the toe of the slope and the strain-softening stress-strain shear behaviour of the soil is used to redistribute shear stress along it. As failure propagates, the soil mass above the shear zone is horizontally unloaded and may dislocate in horsts and grabens. It is considered that the development of the shear zone is followed, at some distance, by the dislocation of the above soil mass and that both processes are essentially independent.

A finite elements numerical procedure has been elaborated in order to simulate spreads, taking into account progressive failure mechanism. The procedure was used to back calculate the 1994 spread at Sainte-Monique, Quebec, Canada, involving firm to stiff slightly overconsolidated sensitive clay, and the 2005 spread at Saint-Barnabé, Quebec, Canada, involving stiff overconsolidated clay. In both cases, the initiation and extent of the failure surface observed on site can be explained by progressive failure mechanism. In addition, dislocation of the soil mass in horsts and grabens may be explained by the mobilisation of the active strength of the soil.