Rockfalls risk assessment along a E45 motorway section in South Tyrol (Italy)

The E45 motorway in South Tyrol (Italy) is exposed to rockfalls in many locations. For this reason, a significant number of rockfall risk reduction measures (nets, barriers, etc) has been progressively installed since its construction. Planning of further mitigation and monitoring measures can benefit from the assessment over a large-area and at adequate scale, of the exposure to rockfalls and of the associated risk, a piece of knowledge that this study has provided for a 13.5 km motorway section. First, susceptibility mapping has been carried out using bivariate statistical methods with supporting evidences from an inventory of rockfalls occurrences covering the period 1993 to 2020. This has allowed to define potential rockfall detachment zones located upslope the E45. For each zone, rockfall runout modelling with RocPro3D software by considering 0.5 m and 2.0 m blocks diameter and high-resolution Lidar DTM has allowed to assess potential interactions between rockfall and different motorway structures (i.e. viaduct piers, tunnel entrances and road embankments). Spatial-temporal frequency of 2 m diameter rockfall (i.e. n° of rockfalls per year and unit area) has been assessed on the basis of the inventory of rockfalls occurrences and of the overall extent of slopes resulting highly susceptible to rockfalls. On such basis, the expected rockfall triggering frequency (n° events/year) in each source area has been assessed by considering its extension.  Hazard has been assessed by using an heuristic matrix-based approach that combines frequency and geomechanics (expressed by the GSI) of the rock masses. Rockfall spatial impact frequency, energy and bounce height determined by runout models have been used to establish exposure and vulnerability (i.e. expected damage level) of the motorway infrastructures. Finally, risk has been evaluated in function of hazard and vulnerability (by using combination matrices tailored to each type of interaction of rockfall – on infrastructures taken into consideration. Results allowed us to determine and map that, out of the total 13.5 km motorway section considered, about 1.5 km for 0.5 m diameter blocks, and about 3.2 km considering 2.0 m diameter blocks, should be considered at high to very high rockfall risk. This result is also relevant with respect to the identification of priorities for more in-depth slope-scale surveys and monitoring of rockfalls in the perspective of further structural and non-structural mitigation measures implementation.