Failure probability of rockfall net fences subjected to ageing: a reliability- based approach for risk reduction

Net fences, or rockfall flexible barriers, are rockfall risk mitigation structures that have an inherent failure probability. Analysing the variability of the phenomenon and the uncertainties associated to the prediction of the trajectories of the potentially falling blocks, the Authors have found that the adoption of non site-specific partial safety factors, as suggested in Italian and Austrian rules and the European Guidelines, would lead to a wide range of failure probabilities. Hence, they have introduced a new approach tailored for any slope that implements a time-integrated reliability-based analysis. All the previously discussed analyses have been performed on new structures, considering a Dirac-delta distribution for the capacity of the barrier. The present contribution deals with the introduction of ageing phenomena (e.g. corrosion of components, untightening of connections, etc) or the activation of the energy dissipating devices into the evaluation of the failure probability of the barrier. Numerical and analytical modelling and recent tests on artificially corroded barrier components have shown an overall reduction of energy dissipation capacity. Based on the assessment of the current state of a net fence, an appropriate distribution that accounts for the reduction of the capacity and the uncertainties in the evaluation of the ageing parameters is introduced, and the effects on the failure probability of the system are quantified. An example of application is included, showing how risk reduction can be efficiently achieved by considering the effective failure probability of the net fences