Rockfall hazard mitigation using ditch charts modeled with CRSP-3D

Abstract

Rockfalls on roadways are a serious hazard to users. Many resources are invested in rock slope maintenance and measures of stabilization and protection to mitigate the risk. Catchment areas (ditches) are one of the least expensive and most effective protective measures to contain and restrict rockfall onto roadways. While their effectiveness depends directly on their design criteria, previous studies have mainly been limited and based on empirical studies. Ritchie (Evaluation of rockfall and its control., pp. 13–28, 1963) drew up the first design charts and tables, establishing the impact distance of a rockfall as a function of the slope height and steepness. Though his work is still accepted, it has some significant limitations: his design relies on such a deep, steeply sloped ditch that it reduces road safety, restricts the slope geometry and complicates the maintenance of these catchment areas. Pierson et al. (Rockfall Catchment area design guide. Final Report SPR-3(032), 2001) created new graphic charts based on real rockfall tests carried out on different slope-ditch arrangements, but their research presents certain drawbacks: the examined situations are limited to a specific type of material, shape and possible rock size, the dimensions proposed to obtain certain percentages of rockfall retention are very large and, in most cases, the costs are unreasonably high. This present research complements previous studies by using a CRSP 3D computer simulation model (Colorado DoT, USA) and analyzing a wider number of slope-ditch arrangements and input parameters: (A) 5 talus heights, 5 slope gradients and V-ditches with 3 foreslopes. The highest slopes (≥18 m) have an intermediate 1 m bench at 12 m height. (B) Different kinds of materials are handled: 4 bedrock lithologies, two of them for the slope (hard rock and soft rock), one for the ditch (concrete) and the other for the road pavement (asphalt). The properties of these materials (density, elasticity, roughness) have been established according to the CRSP 3D methodology and adapted to previous empirical knowledge of each material. (C) A wide array of blocks was studied considering various possible combinations of geometries (cube, cylinder, sphere) and sizes (0.31, 0.62 and 0.94 m). A total number of 270 different cases for hard rock and 180 for soft rock have been evaluated. (D) Rocks are released randomly (along the whole slope) and the initial velocity is zero. As a result of the numerical analysis, a set of practitioner-friendly charts were drawn up, not only for infrastructure planning and design tasks, but also to evaluate existing catchment area effectiveness and to reduce rockfall hazard. The proposed design charts offer an estimation of the dimensions required for the ditch, depending on the relation between the optimal stop distance and the cumulative percentage retained along the trajectory, satisfying specific retention requirements (95%).