A rare case of downward progressive failure in eastern Canada: the 1976 Saint-Fabien landslide

Abstract

The Saint-Fabien landslide occurred in December 1976, 270 km northeast of Quebec City, Canada. The landslide took place in a valley filled with glaciomarine sensitive clay covered by peat bog deposits and presenting an upwards hydraulic gradient. Unique features of this event are the ground surface and the failure surface that are both close to the horizontal. In addition, passive and active failure zones were observed in the debris after the landslide, indicating a possible downward progressive failure mechanism, rarely observed in Eastern Canadian clays. Possible triggers of the event are: the continuous exploitation of the peat downslope and (or) the roadworks going on along Road 132 at the time of the event, which is crossing the landslide area upslope. Conventional stability analysis for the site gives a factor of safety of 1.43, indicating that limit equilibrium analysis cannot explain the formation of the failure surface observed at Saint-Fabien. The initiation and the propagation of the failure were analysed through 1D numerical modelling using the concept of progressive failure. The methodology consists of two steps: first the calculation of the initial stresses in the slope with the finite element software PLAXIS 2D; and second, the modelling of initiation and propagation of the progressive failure along the potential failure surface with the finite element code BIFURC, developed at the Norwegian Geotechnical Institute. Results from the modelling with BIFURC show that the concept of progressive failure is able to explain the extent of the landslide observed at Saint-Fabien with parameter values based on what was measured in situ.