ANALYSIS OF THE FAILURE MECHANISM OF SPREADS IN SENSITIVE CLAYS (Invited Presentation)

Spreads are large landslides occurring in sensitive clays. They generally lead to crater filled with blocks of more or less remoulded material having horsts and grabens shapes. Regular stability analyses are not suitable for understanding this type of landslide and another failure mechanism is needed to understand the conditions leading to this type of landslide.

Recent work applied a 1D finite element numerical procedure, taking into account progressive failure mechanism, to study spread. The procedure was used to back calculate four detailed cases of spreads that occurred in sensitive clays and analyse the main factors influencing the failure mechanism.

This analysis indicates that sensitive clay spreads are formed by two distinct processes: (i) initiation and propagation of the failure surface by progressive failure mechanism and (ii) dislocation of the 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 unloaded and may dislocate in horsts and grabens. In all cases, the initiation and extent of the failure surface observed on site can be explained by progressive failure mechanism and the dislocation of the soil mass in horsts and grabens may be explained by the mobilisation of the active strength of the soil. It is considered that the development of the shear zone is followed, at some distance, by the dislocation of the above soil mass. Parametric study shows that, all other factors being the same, a decrease in large deformation strength (and thus an increase in sensitivity) increases the propagation distance. The rate at which the strength decreases from the peak to its large deformation value influences both the initiation of progressive failure and the retrogression distance. The more rapidly the strength decreases, the smaller is the necessary disturbance to initiate progressive failure and the larger is the retrogression distance.

This analysis confirms the application of progressive failure to spreads and shows the need to assess the strain-softening behaviour of the soil mobilized during initiation and propagation of spreads.