Comparison between qualitative rockfall risk rating systems for a road affected by high traffic intensity

Abstract. The paper deals with the assessment of the rockfall risk for a road stretch, in southern Italy, affected by high traffic intensity. Three qualitative rockfall risk rating systems (QRSs) which use an exponential scoring with a base of 3 were employed, and then the results were compared. The used methods are the following: the Rockfall Hazard Rating System, a modified version of this method already proposed in the past by one of the authors, and the modified version of the Colorado Rockfall Hazard Rating System. The studied road stretch is about 11 km in length and is part of a very tortuous road flanked by rock slopes characterised by complex geostructural and geomechanical layouts. The road was subdivided into 56 sections, defined so as to have – as much as possible – homogeneous geological characteristics. By means of the three QRSs, it was possible to ascertain that high levels of rockfall risks are due to the lack of ditches, a very limited percentage of decision sight distance (PDSD) values and a small roadway width, whereas a subordinate factor is the hazard caused by rockfalls. Several positive and negative aspects arising from the use of the employed methods are highlighted and discussed.