Complex earth slides in the Thompson River Valley, Ashcroft, British Columbia

Eleven retrogressive, multiple, translational earth slides have occurred along 10 km of the Thompson River Valley between the communities of Ashcroft and Spences Bridge in south-central British Columbia, Canada. Historic accounts suggest that some of these slides formed in the late 1800s and have been active ever since. Geotechnical studies have been carried out for the five most active of these earth slides since the early 1980s. The rupture surfaces of the earth slides followed highly plastic, overconsolidated non-swelling clays within a Pleistocene stratigraphic unit that consists of up to 45 m of rhythmically-bedded silt and clay glaciolacustrine sediments. Whereas reactivations of these landslides during the last 35 years have resulted in very slow movements (slower than 400 mm/yr), river down-cutting and river bank erosion may cause rapid to very rapid movements (rate between 1.8 m/hr and 5 m/s) by retrogressions of the slides on current rupture surfaces or movements on new deeper rupture surfaces. These earth slides reactivate in late summer and early fall. The reactivations appear to be caused by a drawdown mechanism in response to overpressure in the slope during drops in the levels of the Thompson River after high flows or by erosion of the Thompson River banks during floods.