Analysis of the factors affecting rockfall stop-distance to reduce impact risk on roads

Abstract

Costs associated with rockfall risk are high. Plenty of resources are invested on rock slope maintenance and stabilization and protection measures to reduce rockfall hazards on transport infrastructures. However, limited studies are focused to evaluate the relative influence of the different factors (geometrical and material properties) affecting falling rock trajectories and the efficiency (retention capacity) of catchment areas. There are numerous factors that influence not only the characteristics of rockfall motion but also their impact and stop-distance. The Ritchie empirical research (1963) was the first to identify these characteristics and determine the expected impact distance of rockfalls according to the slope geometry. Later works (Pierson et al., 1994) have proved that Ritchie results were not as conservative as previously thought, and also is difficult to apply his ditches on roadways due to the excessive depth (dangerous for vehicles) and width (expensive construction and maintenance). For this reason, the Ritchie ditch has been improved by using computer simulation programs, proposing concrete walls or fences at the edge of the road (Pantelidis, 2010). To optimize the catchment area geometry with more simple and reduced depth, it is necessary a systematic and quantitative analysis about the effect of each factor over the rock stop-distance, applying a simulation model. With this aim, this work applies a computer simulation model CRSP 3D (Colorado DoT, USA) considering 75 different configurations of slope-ditch geometries, 4 types of materials and 9 size and shape combinations of the falling rocks. A total number of 270 different cases for hard rock and 180 for soft rock have been examined. A statistical analysis was performed with the simulated rock stop-distances to assess the different variables affecting rockfall motion. Results show that: (A) lithology is a significant factor of the maximum stop-distance: in soft rock it tends to be a unimodal probability distribution; however in hard rock it seems to have a bimodal distribution. (B) These differences are related to the material properties: elasticity and density. Higher elasticity is related to lower energy loss in bounces, thus the stop-distance is larger. Greater density generates a higher mechanical work due to higher initial potential energy. The combination of both properties implies an amplification of this effect. Finally, increments of the ditch foreslop gradient reduce in a significant way the maximum reach distance of the falling rocks.