Rock mass characterisation and distinct element modelling of a deep-seated rock slide influenced by glacier retreat

Abstract

The active deep-seated Marzellkamm rock slide is situated in the high mountainous region of Tyrol (Austria), which has been subjected to the most recent deglaciation since the Little Ice Age (LIA). This study presents the investigation of initial rock slide formation and deformation mechanisms with respect to the structural geological inventory and the recent glacier retreat of approximately 2.2 km in length and 160 m in thickness since 1893. An integrated approach comprising field-based rock mass characterisation, glacier reconstruction based on historical maps (1893–1949) and remote sensing data (1953–2017), multi-temporal morphological analysis (1971–2019), and 2D distinct element modelling was applied.We show that deformation features mapped on the surface are reproduced by numerical modelling, and that (i) rock slide formation and slab development originates along pre-existing fault zones, (ii) a sliding mechanism occurs along a predefined fully persistent, curved but non-circular basal shear zone, and (iii) meso-scale fractures (e.g. joints) have only a minor impact on rock slide formation.Glacier reconstruction and deformation analysis show that increased slope displacements are directly associated with accelerated deglaciation rates between 1893 and 2017. Stable slope conditions were obtained by modelling as long as half to one third of the lower slope was covered by glacier ice, i.e. before the 1969 glacier stage. Rock slide movement is assumed to have initiated during the basal shear zone formation from 1949 onwards. Triggered by the general sliding movement, shear displacement along pre-existing fault zones led to the formation of rock slide structures and the separation in individual slabs and blocks.Our study shows that detailed geological mapping and deformation monitoring in combination with numerical modelling techniques can create a promising rock slide model.