Composite rock slope kinematics at the current Randa instability, Switzerland, based on remote sensing and numerical modeling

Kinematic analysis of slope instabilities in brittle rock is crucial for understanding the reaction of the rock mass to external forcing factors. In steep terrain, inaccessibility often limits collection of relevant data and remote sensing techniques must be applied. This is the case at the current Randa rock slope instability in southern Switzerland, where a total volume of about 5–6 million m3 moves at a rate of up to 30 mm/yr. A large portion of the unstable rock mass is exposed in an 800 m high inaccessible cliff; the main scarp of the May 1991 rock slope failure. Between 2005 and 2007, a comprehensive suite of remote sensing techniques, including photogrammetry, LiDAR, and GB-DInSAR, was combined with 3D geodetic measurements to characterize the rock mass structure and displacement patterns. Photogrammetry and LiDAR data were measured simultaneously from a helicopter using a system allowing for oblique view angles, which provided optimal observations of the steep rock cliff. We used these datasets to map large-scale structures and extract their orientation and minimum persistence, as well as to characterize the 1991 failure scarp. The northern part of the May 1991 failure surface shows a transition from stepped planar sliding at the base, to failed rock bridges in the center, to tensile failure close to the vertical head scarp. Kinematic analysis of the discontinuity sets in the currently moving rock mass shows that both toppling and translational sliding are feasible failure mechanisms. Toppling is more likely for steep faces above 2200 m, whereas translational failure is more likely in the lower portion of the instability. Interpretation of GB-DInSAR displacement maps revealed similar kinematic behavior, and also allowed identification of a basal rupture zone and lateral release plane bounding the instability. Displacement vectors derived from geodetic surveying provided new insights into the 3D kinematic behavior of the instability. All information extracted from different data sets were integrated in a conceptual model, which was then investigated with 2D numerical simulation using the discontinuum code UDEC. The numerical models were able to reproduce the hypothesized kinematic behavior well.

Research Highlights
► New remote sensing and 3D geodetic data from the Randa instability are presented.
► The 1991 failure surface shows both shearing and tensile failure of rock bridges.
► 3D kinematics is deduced from structural data and geodetic displacement vectors.
► Two kinematic modes, toppling and translational sliding, govern the instability.
► Numerical models using UDEC support the suggested kinematics.