An evaluation of the performance of rock fall protection measures and their role in hazard zoning

Rock falls threaten human lives and assets in mountainous regions all over the world. Protection measures are one of the most effective solutions for mitigating rock fall-related hazards and risks; however, their optimal working conditions must be ensured throughout their whole life span, in order for the measures to play their role properly and not to have their performance compromised. This paper presents a methodology for a simple yet effective evaluation of the performance of existing rock fall protections, whose goal is to establish their actual performance capacity and, based on that, whether they can play their mitigation role. The methodology is articulated into four main steps. In the first, data and information about the hazard affecting a site, the current state of existing protections and possible faults/causes of malfunctioning of the protections are collected. The second and third steps evaluate the actual performance capacity of the protections in comparison with their nominal capacity, after the potential influence of the factors degrading the effectiveness, detected in the first step, is considered (i.e. factors reflecting negative interaction between site and measures, structural design issues of the protections, faults and malfunctioning due to lack of maintenance, etc.). These three steps were implemented in a spreadsheet tool, allowing to store relevant data on protection measures collected during a field survey and to perform the evaluation analysis directly on site, semi-automatically, based on the data collected. Finally, once an actual performance capacity is obtained from these evaluations, the last step of the methodology is to compare this capacity to the hazard at the site, in terms of energy and return period of the events at each location of interest, to establish whether the protections can in fact mitigate such hazard or need intervention (reparations, replacement, etc.). A detailed application of the whole procedure is shown, by means of a demonstrative example carried out at a Swiss site where rock fall protections measures were previously installed and hazard zoning maps are available.