RINA - Démonstrateur d’utilisation de l’intelligence artificielle pour la gestion opérationnelle des risques naturels gravitaires géologiques | FEREC 2020

1. Introduction 1.1. Rock falls Rock falls occur when a block of rock detaches from a bedrock and descends by free fall or leaping (Varnes 1978). They are quick and brutal events different from other slope movements. It occurs due to erosion that causes the mass to depart the cliff. Rock fall is a problem when it can reach a target. Rocks that fall in the middle of the forest are not studied, but those which target roads or villages must be studied in order to avoid accidents. This phenomena is frequently studied, it is hard to have accurate predictions due to its infrequent and discontinuous nature (Luckman 2013). Along the years of studies, geomorphologists separated two classes of rock slides based on their origin (Luckman 2013). The first are primary rockfalls, when the volume detaches from the cliff and falls immediately after. The secondary rockfalls occur when there is a declaration between the detachment and the fall. The block detaches from the bedrock, due to weathering and other processes, but remains stationary until another factor causes it to fall. Causes of rock falls Rock blocs fall when the driving force applied on it is greater than the resistance holding it back. It is not always possible to say what the cause is, as it might be a combination of different factors. The final factor is simply a spark that triggered the event that was already on the verge of happening (Varnes 1978). The main factors are briefly outlined in this section. The removal of support disrupts the equilibrium of forces and may create weaknesses that cause blocks to detach with time. This may happen for various reasons like erosion, previous movements of the slope or human activity. Additional charges will also disrupt the stability of the bedrock. Humans can be responsible for this by constructing structures, leaking water from pipelines, sewers, canals and reservoirs, and other activities. Natural agents that create surcharges include, weight of precipitation, pressure caused by rain on cracks, freeze thaw cycles, clay expanding due to hydration, accumulation of volcanic material and vegetation. Another factor is vibrations of the rockbed. This includes natural tremors like earthquakes and also the ones caused by humans like explosions and drilling. Rock falls are also caused by the low resistance of the cliff. This can be caused by its formation or variations with time. Initial weaknesses include: the geological composition, the resistance of the materials, how are the different materials aligned, how they interact with water, presence of faults and other discontinuities, form of the slope and how steep it is. Changes with time are linked to weather conditions. Rain is one of the main factors of erosion and reacts with many different minerals causing them to erode. Even though all of those factors are important causes, three of them are usually described as the main triggering factors: rain, freeze-thaw cycles and earthquakes. This is true for primary and secondary rockfalls, but it is conceived that secondary rockfalls are triggered by a wider range of events (Luckman 2013). 5