Rockfall Reconnaissance and Stability Assessment: Frenchman Coulee, WA
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The Sunshine Wall, a 30-meter tall outcrop of Columbia River Flood Basalt, is a popular climbing wall within Frenchman Coulee, located in central Washington. This study investigates the stability of the columnar basalt that makes up the wall, to educate climbers on how this slope fails and provide qualitative estimates of how close portions of the wall are to failure. Because the wall predominantly consists of columnar basalt, the most probable mode of large-scale slope failure is toppling. Some portions of the wall are “entablature,” a highly fractured basalt which presents a significant risk with regard to rockfall. The stability of the Sunshine Wall was assessed using geomechanical rock mass classification systems, geometric measurements, and kinematic analyses. Data were collected using traditional field methods along with digital photogrammetry, from which I produced a high-density 3D point cloud and digital model of the wall. By combining traditional field methods and modern technology, a thorough and organized investigation was conducted. Data obtained by implementing these methods includes rock strength, rock mass ratings (quality), and kinematic analyses, which indicate that the columns are most likely to fail in direct toppling. Further, a simple qualitative center-of-mass analysis was applied to determine how close these columns may be to failure. Although many of the columns are stable, some are precariously balanced and may be unstable. In summary, the high strength of the intact rock, rough fracture surfaces, and lack of steeply dipping daylighting joints add to the stability of the slope. Low quality rock zones, areas with high densities of randomly oriented fractures, and the presence of potentially weak underlying material add to the instability of the slope. Climbers should use caution while belaying under zones of entablature, which are delineated in this study. They should also be aware of the conditions that lead to elevated risks of toppling failure. These conditions include columns that lean significantly away from the slope creating a large gap at the top of the colonnade, columns that are narrow and tall, and the presence of lower quality rock coinciding with the point at which a column might rotate away from the slope. To monitor the toppling hazard, the movement of key columns could be evaluated by repeatedly measuring the gaps at the top of the colonnade over time. These measurements will help to evaluate the significance of this hazard and whether or not these failures are slow and continuous or unpredictable and sporadic.