Issue 43

A. Luciani et alii, Frattura ed Integrità Strutturale, 43 (2018) 241-250; DOI: 10.3221/IGF-ESIS.43.19 248 The most important results obtained comparing the six numerical models are: - in models (a), (b) and (d) the modified net fence fulfilled the test at 3000 kJ without failure of any element, while in model (c) at an impact of 3000 kJ the net reaches a plastic strain higher than the maximum recorded during the original model MEL test in the contact area. Therefore, the net is considered failed. Repeating the simulation with an energy of 2900 kJ no failure has been produced. Based on these results the residual efficiency for model (a), (b) and (d) is of 100% while it is of 97% for model (c). Moreover, the simulations show a variation in the behavior of the net fence in terms of maximum elongation. In the damaged models the maximum elongation of the net fence increased, up to the 20% in model (c) (Fig. 6), while the final elongation is almost the same of the original model. This result is very important in terms of correct positioning of the net fence on the slope. The increase of maximum elongation is due to the absence of the upstream cables involving a lower stiffness and lack of constrains of the system. Figure 6: Comparison of the maximum elongation of the original model and of models (a), (b), (c) and (d). - in model (e), the net, the lower longitudinal cables and the two energy dissipating devices of the upstream cables convergent to the central anchorage get to failure for an impact energy of 3000 kJ. These failures were due to the higher stiffness due to the shorter upstream cables. Repeating the simulations with lower energy, the model withstood an impact with an energy of 2400 kJ with residual efficiency of 80%. - in model (f), the longitudinal cables failed at an impact of 3000 kJ. In this condition, the energy dissipating devices of the upstream cables were not activated. This behavior may be explained considering that longer upstream cables involve lower stiffness and so the cables were less charged. Consequently, the energy coming from the impact concentrated on the other elements of the net fence and particularly on the longitudinal cables. The model withstood an impact at 2600 kJ, with residual efficiency of 87%. These results allow to say that after some time of ageing or when the net fence has been not correctly installed the energy it can withstand is lower than that assessed in the ETAG027 classification. Therefore, residual efficiency value should be considered in rockfall risk analysis in order to take into account in this process the deterioration and installation conditions of the net fence. When making the design of a protection by net fences, the choice of the product is usually based on the statistical analysis of the rockfall trajectories and on the evaluation of the rock block size to be stopped. Therefore, based on the statistical evaluation of the computed speeds and height of trajectories in correspondence of the protection device to be installed, it is possible to assess the maximum energy to be stopped and consequently choose the optimal product. After this analysis, it is possible to assess the number of blocks that cannot be stopped and, based on this number, it is possible to assess the residual risk. Risk analyses are usually started taking into account the number of blocks that exceeds the barrier capacity or jumps over it and, consequently, reaches the object to be protected [3]. As a consequence, the risk mitigation is directly affected by the percentage of block stopped by the net fence i.e. its efficiency. An incorrect installation can reduce the ability of the barrier to stop the block and consequently a higher percentage of blocks can pass. The analysis has allowed to quantify this value for a set of frequent defects or ageing conditions. Taking as an example, the procedure proposed by Peila and Guardini [3], the key parameter in the evaluation of the probability in the analysis is the number of rockfall events that can affect the infrastructure per year ( ܰ ௥ ). A protective device reduces the probability of occurrence of the event, i.e. induces a reduction of the number of blocks affecting the road, that can be estimated as