Quantitative risk assessmenent for buildings due to rock- falls: some achievements and challenges

An application example of Quantitative Risk Assessment due to rockfalls for a developed area is presen-ted in this paper. The methodology aims at the calculation of the risk for buildings which are situated at the bottom of a rockfall prone slope and may be impacted by rock blocks and of the global risk by aggre-gation of the results. It can be applied at either site-specific or local scales and it is analytical. Frequency of the rockfall events has been obtained from historical records and dendrochronology. The probability of a rockfall reaching the developed area is obtained by trajectographic modelling. A key issue is the consi-deration of fragmental rockfalls. Falling rock masses are expected to break apart after first impacts on the ground, leading to individual blocks that will follow independent paths. Different impact energy le-vels may lead to four potential damage states: 1. non-structural damage, 2. local structural damage, 3. partial collapse, and 4. extensive to total collapse. For every building, the risk is expressed in terms of the annual probability of loss and it is the sum, for all rockfall magnitudes, of the products of the rockfall frequency with the conditional probability of a rock block reaching the building with a certain kinetic energy sufficient to cause a specific state of damage and its associated vulnerability. The details of the proposed methodology are presented here through an application example in the Principality of Andorra. Mots clefs: Rock falls, Quantitative Risk Assessment, Pyrenees.