Assessment of rockfall risk along roads

This paper contains a method for the analysis of rockfall risk along roads and motorways. The method is derived from the Rockfall Hazard Rating System (RHRS) developed by Pierson et al. (1990) at the Oregon State Highway Division. The RHRS provides a rational way to make informed decisions on where and how to spend construction funds. Exponential scoring functions are used to represent the increases, respectively, in hazard and in vulnerability that are reflected in the nine categories forming the classification. The resulting total score contains the essential elements regarding the evaluation of the degree of the exposition to the risk along roads. In the modified method, the ratings for the categories ditch effectiveness, geologic characteristic, volume of rockfall/block size, climate and water circulation and rockfall history have been rendered more easy and objective. The main modifications regard the introduction of Slope Mass Rating by Romana (1985, 1988, 1991) improving the estimate of the geologic characteristics, of the volume of the potentially unstable blocks and the underground water circulation. Other modifications regard the scoring for the categories decision sight distance and road geometry. For these categories, the Italian National Council's standards (Consiglio Nazionale delle Ricerche - CNR) have been used (CNR, 1980). The method must be applied in both the traffic directions because the percentage of reduction in the decision sight distance greatly affects the results. An application of the modified method to a 2 km long section of the Sorrentine road (no 145) in Southern Italy was developed. A high traffic intensity affects the entire section of the road and rockfalls periodically cause casualties, as well as a large amount of damage and traffic interruptions. The method was applied to seven cross sections of slopes adjacent to the Sorrentine road. For these slopes, the analysis shows that the risk is unacceptable and it should be reduced using urgent remedial works.