The 1994 landslide at Sainte-Monique, Quebec: geotechnical investigation and application of progressive failure analysis

Abstract

Abstract:In 1994, a landslide occurred in the municipality of Sainte-Monique, Quebec. The debris of the landslide had graben and host shapes, typical of spreads in sensitive clays. The geotechnical investigation shows that the soil involved is a firm to stiff, sensitive, nearly normally consolidated grey silty clay of high plasticity. This soil exhibits a high sensitivity and a high brittleness during shear and is therefore susceptible to progressive failure. Traditional stability analysis cannot explain this landslide. This gives the opportunity to examine the applicability of progressive failure analysis to this spread. Using the finite elements method, it is demonstrated that the initiation and observed extent of the failure surface are explained by a soil having high brittleness during shear and a large-deformation shear strength close to the remoulded shear strength of the soil. The dislocation of the soil mass can also be explained by the active failure occurring in the soil mass above the failure surface during or shortly after failure propagation. It is therefore numerically demonstrated that progressive failure explains the initiation and the extent of the failure surface of this spread.Key words: progressive failure, spread, sensitive clay, large-deformation shear strength, brittleness.Résumé : Un glissement de terrain est survenu en 1994 dans la municipalité de Sainte-Monique, Québec. Les débris présentaient des formes de horsts et grabens typiques des étalements dans les argiles sensibles. L'investigation géotechnique démontre que le sol impliqué dans ce glissement est une argile silteuse grise normalement consolidée, sensible, de forte plasticité et ayant une consistance ferme à raide. Ce sol présente une sensibilité et une fragilité lors du cisaillement élevées et peut donc être susceptible à la rupture progressive. Les analyses de stabilité traditionnelles n'arrivent pas à expliquer ce glissement. Ceci offre donc l'opportunité d'examiner l'application du concept de la rupture progressive sur cet étalement. À l'aide de programmes d'éléments finis, il est démontré que l'initiation et l'étendue de la surface de rupture observée peuvent être expliquées par un sol ayant une grande fragilité lors du cisaillement et une résistance à grandes déformations près de la résistance du sol remanié. La dislocation de la masse de sol en horsts et grabens est expliquée par la rupture active survenant dans la masse de sol au-dessus de la surface de rupture pendant ou peu après la propagation de la rupture. Il est donc démontré numériquement que la rupture progressive explique l'initiation et l'étendue de la surface de rupture d'un étalement.

The Saint-Jude landslide of 10 May 2010, Quebec, Canada: Investigation and characterization of the landslide and its failure mechanism

“…As explained by Bjerrum (1967), Quinn et al (2011), Locat et al (2011a, 2013, 2015, and Leroueil et al (2012), progressive failure can explain how a failure surface can progress horizontally into an intact soil mass creating a spread. Locat et al (2011aLocat et al ( , 2013Locat et al ( , 2015 associated the development of spreads in sensitive clays with progressive failure by two distinct processes: (i) propagation of the failure surface horizontally into an intact soil mass and (ii) dislocation of the soil mass above the failure surface into horsts and grabens.…”

“…Locat et al (2011aLocat et al ( , 2013Locat et al ( , 2015 associated the development of spreads in sensitive clays with progressive failure by two distinct processes: (i) propagation of the failure surface horizontally into an intact soil mass and (ii) dislocation of the soil mass above the failure surface into horsts and grabens. As explained by Leroueil et al (2012), development of progressive failure requires (i) a geomaterial with strain-softening behaviour, (ii) nonuniformity of stresses, (iii) boundary conditions enabling the slope to deform, and (iv) stresses exceeding the peak shear strength of the soil.…”

“…Conditions for progressive failure seem therefore to have been present and progressive failure could have taken an important role in the initiation and propagation of the main failure surface. In addition, Locat et al (2011aLocat et al ( , 2013Locat et al ( , 2015 explained and demonstrated that when progressive failure is taken into account to understand spreads in sensitive clays, only a small unloading near the toe of the slope can initiate a failure surface resulting in a spread. As mentioned above, it is not clear what the importance of the trigger necessary to initiate the 2010 Saint-Jude landslide was.…”

“…This inclination corresponds to the results of an active failure, as seen in undrained triaxial tests on clay. Horsts seem therefore to be formed by active failure occurring during the landslide as explained by Locat et al (2011aLocat et al ( , 2013Locat et al ( , 2015.…”

Numerical modeling of combined effects of upward and downward propagation of shear bands on stability of slopes with sensitive clay

“…Figures (f) and (g) show that the failed soil mass follow the eroded block with formation of a number of horst and grabens. In this set of analyses, the displacement of the eroded block is stopped at Δ = 45 m, which in the field represents a scenario that the displacement of the failed soil mass is blocked by the opposite side of the river such as the 1994 landslide at Sainte‐Monique in Quebec . A comparison between Figures (f) and (g) shows that at a low s up = 30 kPa the formation of shear bands at large displacements mainly occurs in the initially failed soil mass.…”