High-alpine rock slides controlled by pre-existing geological structures and brittle rock mass fracturing

Abstract

Deep-seated, high-alpine rock slides frequently occur in highly schistose, fractured, anisotropic rock masses. Many studies have shown that pre-existing geological structures are decisive for a rock slide’s initiation and kinematics, as they provide weakness zones that may be reactivated in the rock slide process. Besides this structural pre-disposition, internal deformation processes by brittle rock mass fracturing play an important role in the evolution of a rock slide. Nonetheless, the effect of multiscale rock mass fracturing due to the rock slide process is yet to be fully understood. Especially, as it is challenging to measure, characterize, and to numerically model these processes. In our contribution, we present three deep-seated rock slides located in the European Alps in heavily foliated, fractured rock masses with failure volumes above 500.000 m3 each. Focusing on these case studies, we investigate the internal deformation processes with a combined approach, comprising field mapping, laboratory testing, remote sensing, and numerical modelling. During extensive geological field surveys, we mapped the geomorphological rock slide features and characterized the structural framework of each study site, yielding geometrical models of the rock slides. This provided the basis for our 2D distinct element modelling studies using UDEC, backed by lithological and rock mechanical laboratory investigations. Whilst UDEC allows for modelling large displacement of blocks bounded by pre-existing discontinuities, it lacks the capability to simulate fracture initiation and propagation of new failure paths within intact blocks, thus neglecting brittle rock mass fracturing. We circumvent this constraint by tessellating the intact rock mass into random polygons – referred to as Voronoi elements. Here, we adapted the original Voronoi technique by assigning an asymmetry to the Voronoi elements, characterized by an elongated axis to consider rock mass anisotropy related to schistosity. By applying this approach, we modelled the fractured, anisotropic, metamorphic rock masses as a combination of pre-existing, field-related structures within a matrix of small, asymmetric Voronoi elements. In order to confirm the model outputs, we used terrain and deformation data derived from various remote sensing techniques – e.g. satellite based synthetic aperture radar, terrestrial laser-scanning (Riegl VZ 4000) and several campaigns of unmanned aerial vehicle photogrammetry. In our study, we were able to reproduce the failure mechanism and kinematics of all three rock slides in accordance with our remote sensing deformation data. Thereby, the asymmetric Voronoi tessellation proved to be feasible in reproducing the brittle rock mass fracturing processes in remarkable agreement with our observations in the field. Thus, our results show, how the formation and kinematics of deep-seated rock slides are controlled by the reactivation of pre-existing geological structures and brittle rock mass fracturing. In doing so, our integrated field, laboratory, and numerical modelling approach further contributes to a better understanding of rock slide initiation and kinematics in complex geological media.