3d Geologic Modeling and Probabilistic Stability Analyses Support a Rock Face Stabilization Project at Boundary Dam, Pend Oreille River, Washington State

摘要

Seattle City Light's Boundary Dam is a 340-foot high double curvature concrete arch in a steep, narrow canyon on the Pend Oreille River in northeastern Washington. Original site explorations included over 100 borings and exploratory adits to support design and construction of the dam, the large underground powerhouse, the forebay, penstocks, draft tubes and extensive access and diversion tunnels. Recent work at the dam includes laser scanning of both aboveground area and underground works. Rock bolting of large potential slide blocks and underground excavations was completed both during construction and later. However, continued concerns over the stability of the steep rock face on the right abutment and the lack of an integrated understanding of the site geology prompted development of a computer-based, 3D geologic model. The model integrates the laser scans with the original exploratory borings, geologic maps, and information gleaned from an extensive library of construction photographs. The model allows a site-wide, 3D visualization of the spatial relationships of the rock types, through-going joint planes, topography, and the numerous project excavations and constructed features. The right abutment was given specific attention, and the geometries and possible release planes for large, potentially removable blocks near the dam were defined in the model. Kinematic analyses demonstrated little risk to the dam. However, the work did lead to concerns regarding the stability of a pier supporting the access bridge from the dam face to the right abutment. Field work and probabilistic stability analyses led to the decision to stabilize the pier foundation.