Characterisation of a volcanic residual soil and its implications for large landslide phenomena: application to Tenerife, Canary Islands

Large landslides are common processes during the evolution of volcanoes and individual events can exceed several cubic kilometres in volume. Volcanic slope failures are a significant risk for the neighbouring population due t o their huge volumes and great runout distances, Around the Canary archipel ago, a total of seventeen deposits of large landslides have been found, and on Tenerife, seven large landslides have affected the subaerial and submar ine morphology during the last similar to6 Ma. However, the causes of such mass movements ale still poorly understood. This work analyses the events a round the Canary Islands and focuses on the ones that occurred on Tenerife in order to obtain new insights into the mechanisms of large volcanic lands lides. The study is divided into a first part that includes site investigat ions examining the general features favouring large-scale failures at volca noes. The second part describes the laboratory tests used to analyse a resi dual soil that may be the potential slip surface of the slides on Tenerife. The site investigation revealed that regional tectonics and the climate ha ve a significant influence on the spatial distribution of the landslides. M oreover, morphological and geological features such as deep fluvial canyons , a high coastal cliff and persistent dike intrusion may favour the initiat ion of slope failure. A typical residual soil sample from the lateral scarp of the La Orotava amphitheatre on Tenerife was studied by carrying out sta ndard laboratory tests. The microstructure was analysed using environmental scanning electron microscopy and a particular bonding was found. This bond ing was also detected by the geotechnical tests. Consolidation tests and di rect shear tests revealed that the mechanical behaviour of the residual soi l changes greatly if the bonding of the soil is broken. The bonded structur e generally fails when the effective normal stress surpasses the yield stre ngth of the bonding. In the case of large volcanic landslides with thicknes ses up to several hundred meters, the high overburden easily exceeds this y ield strength and generates a broken bonding. Therefore, volcanic residual soils, such as the one analysed in this study, are perfect candidates for t he potential failure surfaces of large volcanic landslides. Referring to th e La Orotava events, we assume that residual soil layers and morphological, geological and climatic features reduced the slope stability to critical c onditions, whereas a strong earthquake associated with a caldera collapse e pisode may have finally triggered the landslide. The results obtained indic ate that the residual soils play an important role in affecting the stabili ty of volcano slopes and their destabilising influence significantly favour s large-scale sliding. We suggest that the results obtained from this study can be applied to other locations since volcanic residual soils are common in volcanic areas. (C) 2001 Elsevier Science B.V. All rights reserved.