Rockfall impacts on sheltering tunnels: real-scale experiments

In this paper the results of a series of rockfall impact tests on a sheltering tunnel are discussed. A reinforced-concrete (RC) sphere (mass 850 kg, diameter 0·9 m) was dropped (falling height ranging between 5 m and 45 m) on a tunnel along a road in the Dolomites. This experimental programme represents the extension of a previous one, performed at Politecnico di Milano (Campus Bovisa), when the same RC sphere was dropped on a circular pool filled with a loose sand stratum. The new programme is characterised by larger falling heights and the monitoring of the mechanical response of the shelter. A series of instruments have been employed to measure impact forces, stress propagation within the absorbing soil stratum on top of the shelter, and its deflection. The results, which are in good agreement with available literature data, are presented with the aim of highlighting the features of the complex impact event and discussing the engineering factors to be considered for conceiving a realistic and reliable design approach. The main conclusion is that the impact force is unaffected by the presence of the plate, and that the stress increment on the structure is slightly reduced by the structure deflection, which can be neglected for design purposes without unreasonable levels of conservatism being introduced.