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TitlePreliminary analysis of the failure mechanism at the Nomash River Landslide, Vancouver Island, British Columbia, Canada
AuthorSivak, T J; Brideau, M A; Stead, D; Couture, R; Guthrie, R
SourceGeologically Active: Proceedings of the 11th IAEG Congress; by Williams, A L (ed.); Pinches, G M (ed.); Chin, C Y (ed.); McMorran, T J (ed.); Massey, C I (ed.); 2010 p. 691-699
Alt SeriesEarth Sciences Sector, Contribution Series 20100023
Meeting11th IAEG Congress; Aucklaud; NZ; September 5-10, 2010
ProvinceBritish Columbia
AreaNomash River; Vancouver Island; Tahsis
Lat/Long WENS-127.0000 -126.5000 50.0000 49.7500
Subjectsengineering geology; surficial geology/geomorphology; landslides; landslide deposits; slope deposits; slope failures; slope stability; slope stability analyses; Nomash River Landslide
Illustrationslocation maps; stereonets; photographs; diagrams
ProgramNational Guidelines for Natural Hazard Assessment and Mitigation, Public Safety Geoscience
AbstractThe Nomash River is situated within a steep-sided valley approximately 6 km northwest of Tahsis, on the northwest coast of Vancouver Island. A 0.3 Mm3 landslide originating on the summit of a 430-m high rock wall failed on April 25th or 26th 1999, destroying a logging road and temporarily blocking the Nomash River. The landslide escarpment contains two crystalline limestone faces of the Quatsino Formation which are crosscut by mafic dykes and sills. The landslide is thought to have been triggered by freeze and thaw cycles over the preceding days. The objective of this study was to identify feasible failure mechanisms based on field data measurements of discontinuity orientations and characteristics. This was accomplished by conducting kinematic, limit equilibrium surface wedge, and block theory analyses. Based on these techniques a wedge failure is the preferred failure mechanism. Geological Strength Index (GSI) estimates and Unconfined Compressive Strength (UCS) values derived from point load testing were used to characterize the rock mass.