Numerical modelling of downward progressive failure in sensitive clays: the case of the 1976 landslide at Saint-Fabien, Québec

ABSTRACT: The Saint-Fabien landslide occurred in December 1976, 270 km north-east of Quebec City, Canada. Passive and active failure zones were observed in the debris, indicating a downward progressive failure mechanism, unusual in Eastern Canadian clays. Conventional stability analysis was performed at the site and a factor of safety of 1.43 was obtained. This means 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 progressive failure initiation and propagation with the finite element code BIFURC, developed at the Norwegian Geotechnical Institute.