Use of Rockfall Rating Systems in the Design of New Slopes

Abstract

One typically uses different means to evaluate a highway rock slope depending on whether it exists currently or is in design. For example, the Rockfall Hazard Rating System (RHRS) and derivatives are commonly used to evaluate existing slopes and inform decision makers who are managing rock slope inventories. In contrast, kinematic and limit equilibrium analyses and methods based on observation and probability, such as Ritchey Ditch Criteria, Rockfall Catchment Area Design (RCAD), and the Colorado Rockfall Simulation Program (CRSP), are typically used to provide information for decision making when designing new slopes. Is there good reason for this difference? This paper raises this challenge and proposes that rating systems are not just good for existing inventories; they are good tools for the design of new and rehabilitated slopes. Some of the challenges in using a rating system for design are addressed and the importance of distinguishing risk from hazard is highlighted. Finally, the paper demonstrates how rating systems can help us move towards and define a standard of practice for rock slope design in Colorado and other mountainous environments, and it discusses the challenge of establishing and applying an appropriate standard.