In mountainous areas, tunnel portals and job sites for tunnel construction are often located in areas prone to rockfall events. Providing the safety of roads close to tunnel portals, subjected to rockfall risk, is an unavoidable issue that should be taken into account. In this paper, a risk management approach has been introduced to evaluate rockfall risk on the road stretch at the entrance and exit of tunnels. The event tree method is improved to achieve a flexible solution to calculate the rockfall risk for tunnels. Furthermore, some design abacuses are developed to provide the input parameters related to the event tree. Afterwards, a parametrical analysis is presented to explain the methodology. The elaborated approach can be useful to choose best technical solutions and design protection devices against rockfall.

1 Introduction

The rock block collapse from slopes is one of the most frequent hazards for public transportation systems in mountainside areas. The effects of these instabilities can led to vehicle damages, injury or death of drivers and passengers as well as economic loss due to road closure (Duncan & Norman 1996).

Referring to tunnelling engineering, sometimes tunnel portals and job sites for tunnel construction are located in areas prone to rockfalls and the rock block detachments can occur while the tunnel is in service or during construction.

In order to deal with this issue, several geometry configurations of a rock slope prone to rockfall overhanging a tunnel portal have been defined and the results of trajectographic analysis have been analysed. The rockfall trajectographic analysis has been performed to evaluate the number of rock blocks that can reach the road and their percentage. This analysis has been done with a 2D method and a lumped mass model has been utilized. The geotechnical restitutional parameters (kn, kt, ϕ) are set as standard values available in technical literature (Giani 1992, Hungr & Evans 1988, Azzoni et al. 1995). The goal of this research is to provide a design procedure and suggest somemeasures to prevent the falling rock blocks toward the road such as rockfall barriers and/or artificial protruding tunnels (Fig. 1a,b,c).

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