Pore pressure transients in brittle translational rockslides

Abstract

In a recent paper (Loew and Strauhal, Pore pressure distributions in brittle translational rockslides. In: Italian Journal of Engineering Geology and Environment, Book Series (6): International Conference on Vajont 1963-2013 – thoughts and analyses after 50 years since the catastrophic landslide, pp 181–191, 2013) we showed how variable extents, depths and hydraulic properties of slope parallel sliding zones and fractured rockslide masses can explain most of the observed steady-state hydrogeologic features of translational rockslides, such as seepage faces or pore pressure distributions. In this paper we extend this analysis to transient conditions, controlled both by annual recharge variations from snowmelt and summer rainstorms and hydropower reservoir level fluctuations. We apply a geometrically simple 2D numerical model to investigate and explain characteristic transient pressure variations, as observed in highly instrumented rockslides at the Mica Reservoir in British Columbia. We characterize pore pressure transients and seepage forces above and below major sliding zones at different elevations above the reservoir level. Sliding plane geometry or toe constraints caused by thick alluvial deposits at the valley bottom explain why reservoir level fluctuations significantly impact slope movements and stability, even though the induced pressure variations are relatively small and only impact smaller parts of the rockslide rupture planes.