Prediction of residual rockfall risk in presence of net fences according to the position of the impact

Residual risk assessment in presence of structural mitigation measures represents a challenging issue as the risk in a given time framework, e.g. the annual risk, should be quantified. This presupposes the knowledge of the frequency-magnitude relationship of the considered geohazard. To lower the risk below an acceptable threshold, protective measures are often installed. Structural systems have an inherent failure probability that should be accounted in the computation of the residual risk. Dealing with rockfall hazard, the present work proposes an enhanced version of a reliability-based approach to quantify the failure probability of net fences and the residual risk. Net fences are complex systems, made by assembling several metallic components with different functions. For each system, the performance is certified by standards tests that presuppose that the impact occurs in the center of the fence. Previous works by the authors considered the capacity of the barrier being distributed with a Dirac-delta function at the certified energy absorption capacity. Anyway, the energy absorption capacity of a net fence varies according to impact position and impacting features. This paper proposes a new distribution of the resisting capacity of the barrier based on the different position of the impact thanks to a numerical FEM investigation on a real barrier. An example of application is included. The proposed method thus allows predicting the risk reduction in presence of net fences, accounting for the possible rock impacts. The procedure can provide the basics to be tailored for other geohazards or different structural protective measures.