Carolina Seguí , Esperanca Tauler, Xavier Planas, Jose Moya, Emmanouil Veveakis

Deep-seated landslides are amongst the most devastating natural hazards on earth, typically involving a rigid rock mass sliding over a weak, clayey shear-band. The mechanical response of this shear-band to the loading of the overburden is therefore critical for the stability of a landslide. We hereby show that this mechanical response is strongly linked to the mineralogy and microstructure of the clay minerals forming the shear band. By presenting a detailed mineralogical, textural, and mechanical characterization of a shear band of an active deep-seated landslide we attempt to shed light on the processes determining the failure mechanism of a large deep-seated landslide.
The case study chosen is the El Forn landslide located above the village of Canillo (Andorra). Its shear band is at 29m depth and is formed by black shales of the Silurian period. Using core samples of this landslide, we have performed compositional and microstructural (XRPD, SEM-EDS, MicroCT) analyses, combined with mechanical tests (Liquid Limit, Plastic Limit) to study the interplay between the internal structure of the material that forms the shear band and its mechanical response. The results show that the mechanical properties (porosity and plasticity index) have their highest values inside the shear band, where the clay fabric is perfectly oriented along the shearing direction. Right above and below the shear band, the mechanical parameters have their lowest values, and are accompanied by folded textures (fabric). Finally, outside those areas, the mechanical parameters have intermediate values and the fabric of the minerals is randomly oriented.