Movement triggers and mechanisms of two earth slides in the Thompson River Valley, British Columbia, Canada

Since the construction of the Canadian Pacific Railway and Canadian National Railway main rail lines in western Canada in 1885 and 1905, respectively, both companies have had to contend with 11 large, translational, retrogressive earth slides in the Thompson River Valley south of Ashcroft, British Columbia. The initiation of these slides is associated with the down cutting by the Thompson River through the Quaternary sediments in its valley. The slides move on two subhorizontal weak layers in a glaciolacustrine clay–silt unit within this Quaternary sediment sequence. Transient seepage and stability analyses were conducted for two sample slides, and the results were in agreement with inclinometer and piezometric data. It is concluded that the Thompson River triggers the movements in a drawdown mechanism and (or) erosion mechanism. The Thompson River affects the stability of these slides in three ways: (i) by changing the pore pressure on the rupture surface, (ii) by changing the supporting force on the toe of the slide, and (iii) by changing the geometry of the slides as a result of river erosion. The relative importance of each of these effects depends on the river erosion protection, the depth of the rupture surface, and the amount of river level fluctuation.