Dutchman’s Ridge: Results from the analysis of geologic, monitoring and LiDAR-derived data of an extremely slowly moving rockslide

ABSTRACT: Dutchman’s Ridge rockslide is located along the Upper Columbia River, 1.5 km upstream of Mica Dam in British Columbia. It is underlain by a Basal Fault and was recognized during investigations in the 1960s. Displacements up to 25 mm/year of a 115 million m3 rock mass started after reservoir filling, which were identified in a 1984 review. A drainage adit was built in 1986-1988, decreasing water pressures and, subsequently, displacement rates. This paper proposes new boundaries, according to geological and geotechnical evidence, and that the instability is not a single homogeneous mass, but presents several identifiable zones. The slope is dissected by a feature, that could be the superficial expression of Lavender Shear, recognized during adit construction. The geometry and shear strength of the Basal Shear Zone were approximated using Rocscience’s Slide2. Ongoing work using finite element program RS2 explores the effect of increasing water pressures due to a hypothetical loss of drainage capacity.