Ripley Landslide: the geophysical properties of a slow-moving landslide near Ashcroft, British Columbia

Landslide hazards in the Thompson River valley, British Columbia adversely impact vital national railway infrastructure and operations, the environment, cultural heritage features, communities, public safety and the economy. Field investigations and monitoring of the very slow-moving Ripley Landslide, 7 km south of Ashcroft, indicates movement across the main body, with the greater displacement at the south end of the slide near a lock-block retaining wall separating Canadian National (CN) and Canadian Pacific (CPR) rail tracks. 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 and understanding mass-wasting processes in the Thompson River transportation corridor. Bathymetry measurements, 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 and gravel. Planar physical sub-surface features revealed in field observations, geophysical profiles and borehole logs include tabular bedding and terrain unit contacts, in addition to curvilinear-rectilinear features interpreted as sub-horizontal rotational-translational slide surfaces in clay-rich beds beneath the rail ballast and retaining wall at depths between 5 m and 15 m below the surface of the main landslide body. Geophysical data presented support field observations and borehole logs that show sub-surface glaciolacustrine unit boundaries are gradational rather than sharply defined. Geophysical profiles show that clay-rich glacial deposits are the units most likely to contain failure planes. 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. Waterborne conductivity measurements indicate groundwater discharge at three zones across the submerged landslide toe. 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.