GEOPHYSICAL SURVEY RESULTS, INTERPRETATION AND IMPLICATIONS FOR AN ACTIVE LANDSLIDE IN THE THOMPSON RIVER VALLEY, B.C

A vital section of the Canada’s national railway transportation runs through the Thompson River valley in southern British Columbia, Canada. The mountain valleys of western Canada have challenged the development and operation of railways since the late 19th Century. In the 21st Century, pronounced negative economic and environmental repercussions can occur when 20th Century infrastructure and rail service is disrupted by landslide activity. To better understand and manage landslide geohazards along this section of the rail corridor, an international research group is studying a small, slow-moving complex landslide that is adversely impacting railway infrastructure. GPS and InSAR monitoring indicate movement across the main body, with greatest displacement along the south end in the vicinity of a lock-block retaining wall. Terrain mapping combined with electrical resistivity tomography, fixed frequency electromagnetic, ground penetrating radar and seismic refraction surveys indicate a high relief bedrock sub-surface overlain by a 10 m to >60 m thick package of clay, till and saline groundwater-rich gravel and diamicton. Planar sub-surface features revealed in geophysical profiles include tabular bedding and terrain unit contacts. Profiles and exposures in railway embankments also suggest buried glacially-rafted bedrock blocks and multiple planar-curvilinear features interpreted as sub-horizontal translation failure planes and vertical tension fractures in clay-rich beds in the main body of the slide, beneath the rail ballast and retaining wall. Integrating data from surficial geology mapping and an array of geophysical techniques provided significantly more information than any single assessment and monitoring technique on its own. Knowledge of the internal composition and structure provides contextual baseline data for interpreting results from on-going monitoring programs at this landslide, and for understanding slope instability elsewhere in the Thompson River valley.