Refinement of the volcanic slope rating approach for determining slope stability in volcanic rocks of the Canary Islands

Due to their genesis, volcanic rocks present some singularities that make their geotechnical characteristics significantly different from other, more common types of rock masses, such as sedimentary and metamorphic rocks. The formation mechanisms of volcanic rocks are varied, rapid and generally high energy. These processes give this type of rock geotechnical behaviour and geomechanical properties that are completely different from those of nonvolcanic materials, due to their high heterogeneity and anisotropy. In volcanic rock slopes, applying widely used geomechanical classifications to assess the quality of any rock mass present several challenges due to their distinctive and singular characteristics, which detract from their validity. For this reason, the Center for Studies and Experimentation of Public Works (CEDEX) of the Ministry of Public Works, under the agreement signed with the Ministry of Public Works and Transport of the Government of the Canary Islands, carried out during 2017 and 2018 an exhaustive analysis and reviews of existing geomechanical classifications and their applications to volcanic terrains. They developed a new geomechanical classification called VSR (volcanic slope rating), based on data obtained from 85 slopes across the Canary Islands, which allowed for the estimation of the stability of volcanic rock slopes by evaluating seven geotechnical parameters. This article presents the work conducted to review and calibrate this tool, based on the study of 94 slopes of different lithotypes throughout the Canary Islands (lavas, pyroclasts, heterogeneous), culminating in the proposal of a new classification for pyroclastic massifs (with new parameters and different weightings) and the adjustment of the current classification for the other slopes (regarding the Surface Regularity parameter). Therefore, the VSR geomechanical classification was applied to assess the stability of these 94 slopes, revealing a strong fit for Types A and C (R² > 0.97), while the tool overestimated stability in Type B slopes, prompting a new classification proposal for this category.