Numerical modelling of slope damage in large, slowly moving rockslides: Insights from the Downie Slide, British Columbia, Canada

The deformation of rock slopes is associated with the formation and accumulation of internal and external features (such as tension cracks, rock mass bulging and dilation, rockfall, etc.) that can be comprehensively referred to as “slope damage”. In this paper, we use a 3D distinct element numerical modelling approach to investigate the role of shear zone morphology and groundwater pressure on the displacement and slope damage accumulation at the Downie Slide, a large, extremely slowly moving rockslide in British Columbia (Canada). First, we briefly review the external slope damage features that can be observed in an airborne laser scanner (ALS) dataset, allowing four slope damage domains (upper, central, northern, and southern domain) to be interpreted within the slide area, based on the orientation and type of features. Using the same ALS dataset we construct a 3D model of the present-day slope, explicitly including the two shear zones along which displacements occur, in order to investigate their role in the later stage of the landslide evolution. We assign a strain-softening constitutive model to the slide body, in order to account for the decrease in material properties due to damage accumulation. Virtual inclinometers are also implemented in the model, allowing for the comparison of simulated and observed displacement direction along the shear zones. The progressive deformation and failure of the slope is then simulated both assuming dry and wet conditions, in order to examine the role of pore water pressure, and the morphology of the upper and lower shear zones on the magnitude, orientation, and distribution of displacements. For each numerical model, the simulated slope damage features are recorded, by analyzing the zone volumetric strain and failure state, and compared with the type and orientation of features observed in each of the interpreted slope damage domains, thus allowing the numerical results to be better constrained and validated. It is clearly demonstrated that the orientation and distribution of slope damage and displacements observed in both the surface ALS and the subsurface borehole inclinometer data can be well reproduced in the 3D numerical models. Numerical modelling results show that the principal factor controlling the spatial distribution of slope damage at the Downie Slide is the morphology of the lower shear zone, whereas a negligible role is played by the upper shear zone morphology. We also observe that models incorporating a groundwater table display larger displacements, without significant effects on the orientation and distribution of simulated slope damage. This paper demonstrates that an analysis of slope damage is very important for understanding the mechanisms underlying the behavior of large landslides and should be a fundamental step in the comprehensive characterization of any major slope failure.