Rainfall patterns triggering shallow flowslides in pyroclastic soils

• Hydrological behaviour of a pyroclastic layer subject to different rainfall histories
• Definition of three water storage thresholds to describe effects of rainfalls
• Definition of a framework to characterise rainfall patterns triggering flowslides
• Use of the framework to interpret three flowslide case histories

In the context of landslide-prone pyroclastic soils this paper investigates the physical significance of antecedent rainfalls in relation to the major rainfall event and the influence exerted by evaporation. The work is based on results from tests using a physical model, developed to characterise the hydraulic response of a pyroclastic soil volume subjected to actual meteorological conditions. Rainfall, evaporation, water storage, soil suction and soil volumetric water content were continuously monitored over a meteorological window exceeding two years. Interpretation of the experimental results provides three characteristic values of water storage which are used to explain the physical significance of antecedent and triggering precipitations and shed light on the aspects of major rainfall events triggering landslides.