Rockfall net fence are a widely adopted rockfall risk mitigation measures, suitable for the great majority of the cases. Nevertheless, in particular complex morphologies, the trajectories of the possible detached blocks can be anomalous, with very high values of both the kinematic parameters of passing height and kinetic energy. In this case, a double line of net fence can be a convenient solution. In this case, the upper line is conceived as a fuse element that intercepts a percentage of blocks at least lowering them, while the lower line stops the remaining part. In the framework of partial safety factors design approach, a design method conceived by the Author is herein explained and tailored for a practical application in the common design practice, i.e. with the common trajectory softwares. An example of application on a real site is provided, showing the importance of performing a set of trajectory analyses to optimize the design of the whole system.

Keywords: double line rockfall barreier, net fence, probabilistic trajectory analysis, mitigation measures.