The project deals with the question: "why and how some landslides transform into debris-flows, while most of them stabilize?". Though seldom, debris-flows are a major source of property damage at the European scale, especially in mountain areas. Today the management of debris-flows is quite easily achieved for watersheds where debris-flows result regularly from continuous erosive processes. In the converse case, when debris-flows originate from a landslide, the problematic around the protection alters substantially: a debris-flow activity induced by the landslide transformation can occur suddenly in areas where such an activity has never occurred. There is currently no sound scientific method for properly estimating the probability of landslide fluidisation. An unclear point is that not all landslide produce debris-flow. In most cases, the landslide experiences a significant creep behaviour, then decelerates and finally stops moving. However, in a limited number of cases, the landslide accelerates suddenly and gives rise to a debris-flow. This is particularly true for landslides developed in fine soft rocks where excess pore pressures have often been observed. The hypothesis is that local lateral compression of the landslide body occurs during motion and induces untrained loading and excess pore pressures. The aim of the project is to develop a methodology to improve the prediction and prevention of debris-flows arising from landslides in fine-grained, muddy soils. The research is organised around three objectives:
(1) Conceptualise and model (with the aid of laboratory tests and field observations) the influence of excess pore pressure on debris-flows initiation;
(2) Validate and assess the reliability of the mechanistic model on the laboratory tests and on the field scale of instrumented landslides;
(3) Assess critical threshold conditions for the fluidisation of complex landslides, through scenario modelling.
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