Combining Terrestrial and Waterborne Geophysical Surveys to Investigate the Internal Composition and Structure of a Very Slow-Moving Landslide Near Ashcroft, British Columbia, Canada

Abstract

A vital section of Canada’s national railway transportation corridor traverses a 7 km-long section of unstable terrain in the Thompson River valley, British Columbia. Landslides in this region have adversely impacted vital national railway infrastructure and operations, the environment, cultural heritage features, communities, public safety and economy since the late 19th Century. To help manage the potential risks associated with railway operations across this active slide zone, field investigations and monitoring of a very slow-moving Ripley Landslide are being undertaken by a consortium of research partners from government, academia and industry. Knowledge of the internal composition and structure of the landslide as interpreted through surficial geology mapping and geophysical surveys provide contextual baseline data for interpreting monitoring results; in addition to understanding mass-wasting processes in the Thompson River transportation corridor. Bathymetry, electrical resistivity tomography, frequency-domain electromagnetic terrain conductivity, ground penetrating radar, seismic refraction, multi-spectral surface wave analyses, and borehole logging of natural gamma, conductivity and magnetic susceptibility all suggest a moderately high relief bedrock sub-surface overlain by a >20 m thick package of clay, silt, till diamicton, gravel containing groundwater. Planar physical sub-surface features revealed in geophysical profiles and logs include tabular bedding and terrain unit contacts. Field observations and geophysical profiles also show curvilinear-rectilinear features interpreted as sub-horizontal translational failure planes in clay-rich beds beneath the rail ballast and lock-block retaining wall at depths between 5 and 15 m below the surface of the main landslide body. The landslide toe extends under the Thompson River where clay-rich sediments are confined to a >20 m deep bedrock basin. The upper clay beds are armoured from erosion by a lag deposit of modern fluvial boulders except along the west river bank where a deep trough has been carved by strong currents. High waterborne conductivity levels indicate discharge of groundwater through the boulder lag. Fluvial incision of the submerged toe slope at the south end of the landslide is observed <50 m west of where critical railway infrastructure is at risk. Integrating data from surficial geology mapping and an array of geophysical techniques provided significantly more information than any one method on its own.