Potential of Rockfall Impacts on Critical Infrastructure During Earthquakes

Abstract

ABSTRACT During seismic events, infrastructure in mountainous terrain is often vulnerable to rockfall impacts. This paper presents a probabilistic approach to evaluating the potential of rockfall impacts on an existing hydropower infrastructure under the design ground motions condition. The evaluation was conducted in three steps: (1) identification of potentially unstable rock blocks whose trajectory might reach the infrastructure, (2) seismic stability analysis to evaluate the potential of these rock blocks falling, and (3) rockfall trajectory analysis to evaluate the potential of falling rock blocks reaching the infrastructure. Once relevant rock blocks were identified, pseudo-static rock stability analysis under the design seismic condition using Block Theory was performed. 1,000 random three-dimensional seismic load directions were generated for each rock block. Joint and rock block surface orientations necessary for the stability analysis were identified with LiDAR information. Using the software "RAMMS::ROCKFALL", rockfall trajectory analysis with 1,000 random initial velocity magnitudes and directions was performed. The peak velocities were estimated from peak ground accelerations selected during the Design Earthquake. Overlaying the results from the stability and trajectory analyses, we identified rock blocks with a potential of failure in case of the design earthquake and, in case of detachment, of impacting the infrastructure due to the applied initial velocities. INTRODUCTION In mountainous regions, structures for hydropower, transportation, or tourism are vulnerable to damage by rockfall. Marginally stable rock blocks above the infrastructure can be destabilized by ground motion during earthquakes. The serviceability of these structures can be critical for public safety; therefore, it is important to assess the rock fall hazard for planning potential mitigation engineering measures. Earthquakes are highly uncertain events, with unknowns in the magnitude and direction of ground motion. The rockfall process itself is also uncertain since many properties, such as the initial velocity of the rock block and its orientation, can influence the final trajectory. This anonymous case study illustrates the considerations of rockfall triggered by seismic events and the resulting risk on infrastructure. Our probabilistic rock fall hazard evaluation involves the following three steps: (i) Identification: Based on available site information and observations, we selected potentially detachable rock blocks greater than 3 meters (m) in size in any direction on the slopes above the infrastructure of concern. We assumed that rock blocks smaller than 3 m are not large enough to cause catastrophic damage to the structure. (ii) Stability Evaluation: For the wedge and plane sliding analysis, we used a pseudo-static approach to assess whether the rock blocks would be unstable under the Design Earthquake (DE) loading condition based on the most recent seismic hazard study for the structure. Peak ground accelerations and random directions were considered. (iii) Trajectory Analysis: When assessing whether falling rocks would hit the structure, we considered random initial velocity direction and magnitude.