Rock bridge control on the failure mechanism of a rock fall in a metamorphic rock mass

How to implement rock bridges and rock bridge failure in slope stability analysis is an ongoing discussion within the rock mechanic and landslide community. Although there has been intensive research over several decades, there is still a lack of knowledge on how to measure intact rock bridges on rock slopes, how to quantify their impact on rock mass strength, and how they affect the initial failure mechanism. Therefore, we present the analysis of a rock fall case study located in the alpine environment of southern Salzburg (Austria), where a rock slope composed of a polymetamorphic rock mass hosted three rock fall events in the year 2019. The primary aim of this study is the reconstruction of the multiphase failure event and the investigation of the influence of the discontinuity network with its intact rock bridges on the initial failure mechanism.

In our study, we performed a detailed reconstruction of the rock fall process by helicopter-borne event documentation. Moreover, we identified the rock fall failure mechanism by analysing a video capturing the first rock fall event.

Furthermore, we developed a high-resolution digital surface model of the complex post-failure topography by unmanned aerial vehicle photogrammetry (UAV-P) with real-time kinematics (RTK). Based on this model, we map the location, orientation and persistence of pre-existing discontinuities and identify failed intact rock bridges on the rupture surface of the unstable rock slope.

Additionally, we conducted point load and direct shear tests in the rock mechanic laboratory. We applied the former on block specimens to derive the uniaxial compressive strength of the intact rock. The latter allowed us to estimate the Mohr-Coulomb shear strength properties of intact rock and of failure planes, which formed sub-parallel to foliation planes in course of the test procedure.

After the third rock fall event of 2019, a ground-based interferometric synthetic aperture radar (GbInSAR) was installed for 166 days to monitor the actual deformation of the rock slope. We analysed the obtained deformation data at mm resolution to detect zones of ongoing slope movements.

Finally, we integrate the topographical and geological model, the structural inventory, and the geomechanical properties into a 2D numerical model based on the distinct element method (UDEC). We use Voronoi tessellation to allow the development of any failure path within intact rock bridges. By varying the persistence of pre-existing discontinuities and the shear-strength properties of rock bridges, we study the impact of rock bridge location, spatial distribution, and strength on the initial failure mechanisms of the rock slope. We validated the distinct element model by comparing its outcome with the essential characteristics of the rock fall observed in the event reconstruction and deformation monitoring.

By this integrated approach of methods applied to a polyphase rock fall process, we show that the initial rock fall failure mechanism is sensitive to the spatial distribution of rock bridges and their assigned shear strength properties.