The interplay between phyllosilicates fabric and mechanical response of deep-seated landslides. The case of El Forn de Canillo landslide (Andorra)

Deep-seated landslides are among the most devastating natural hazards on earth, usually involving a rigid rock mass sliding over a weak, clayey shear band rich in phyllosilicates. The mechanical response of this shear band to the loading of the overburden is, therefore, critical for the stability and the evolution of a landslide. We hereby show that this mechanical response is strongly associated with mineralogy and microstructure of clay minerals forming the shear band, and vice versa. 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 processes determining the failure mechanism of a large deep-seated landslide. The case study chosen is the El Forn landslide located in Andorra Principality (Eastern Pyrenees). Its shear band is formed of black shales of the Silurian period. Using core samples of this landslide, we have performed mineralogical and microstructural analyses (XRPD, SEM-EDS, and MicroCT), combined with mechanical tests (liquid limit, plastic limit) to study the interplay between the internal texture of the material of the shear band and its mechanical response. Our results show that the highest mechanical alteration of the material occurs at the center of the shear band, where the phyllosilicates are perfectly aligned parallel to the shearing direction. The alignment of the crystals and their face-to-face contact increases the plasticity index of the material and reduces its porosity. Hence, the shearing movement rearranges the contacts of the phyllosilicate grains inside the shear band, reducing the resistance of the material and promoting the slip of the overburden.