Unravelling the Critical Role of Rock Mass Fracturing in an Extensive High-alpine Rockslide

The initiation of rockslides on metamorphic rock slopes is often linked to the reactivation of pre-existing structures, accompanied by the progressive formation of new fractures over time. To demonstrate the crucial role of these progressive rock mass fracturing processes, we present an active rockslide within an anisotropic, fractured, foliated metamorphic rock mass, involving a failure volume of approximately 670,000 m³. The rockslide is located on the mountain ridge of the Mittlerer Burgstall (MBug, 2933 m a.s.l.), adjacent to Austria’s highest peak, the Großglockner. During the maximum glacial extent of the Little Ice Age, the MBug was a nunatak that was completely surrounded by the Pasterze Glacier. However, it has experienced rapid deglaciation in recent decades. To unravel the critical role of rockslide-related fracturing on the MBug, we applied an integrated methodological approach, encompassing field surveys, remote-sensing campaigns, laboratory analyses, process reconstructions, and a twofold numerical modelling approach.

The field investigations comprised geological and structural surveys. Laboratory analyses, including powder X-ray diffractometry and microscopic analysis, were conducted to determine the mineralogical composition and microstructures of the outcropping lithologies. Direct shear tests completed the rock mass characterization and helped to evaluate the shear strength properties of a critical shear zone. By multitemporal drone-photogrammetry campaigns performed annually since 2019, we reconstructed the rockslide process and derived high-resolution digital terrain models. The rock mass characterization and the process reconstructions further served as input parameters for our twofold numerical approach, which included discrete element (DEM) and finite discrete element modelling (FDEM). By utilizing the advantages of each approach, we study the effect of rock mass fracturing in the rockslide process and validate the model results with our process reconstructions.

The preliminary results show that the MBug exhibits a compound rock sliding mechanism, with steep fractures in the head area and a shallower dipping shear zone at the rockslide foot. The compound rockslide involves an active wedge bounded by the steep head fractures and a passive wedge that slides along the critical basal shear zone. In this compound architecture, rock mass fracturing is crucial, especially in the transition zone between the active and the passive wedge. This was reproduced in both DEM and FDEM numerical approaches and validated with the process reconstructions. Based on this comprehensive data basis, we discuss the crucial role that progressive rock mass fracturing has in this compound rockslide, which formed on a recently deglaciated, heavily foliated, metamorphic rock slope.