Integrated back and forward analysis of rock slope stability and rockslide runout at Afternoon Creek, Washington

Abstract

Hazard assessments involving large rock slopes are often problematic, given the influence of geology on failure kinematics and the subsequent influence of the failure kinematics on the rockslide runout. The 2003 Afternoon Creek rockslide in northwest Washington is one such example, where 750 000 m³ of rock slid from a steep ridge harmlessly into Afternoon Creek. However, topographic and structural controls at the source area unexpectedly redirected a small volume (<10%) down the opposite side of the ridge along a much steeper travel path, impacting the highway below. Postfailure investigations indicate that the slope still presents a danger. To address this, a framework was developed that links back analyses with forward modelling of failure initiation and runout. Field mapping and data collection were specifically tailored for these analyses. Advanced numerical modelling was used to assess likely rockslide source areas, volumes, and controlling features. These were then used to guide a series of three-dimensional runout simulations to model rockslide travel path, reach, and velocities. The results show that a rockslide originating from the ridge could occur again and that the topography would again direct a small percentage of material down the backside which would reach the highway below.