The Petrofka landslide, Saskatchewan

Abstract

Both abutments and all piers of the Petrofka bridge are on a landslide. The bridge has performed satisfactorily for 23 years, indicating the factor of safety of the Petrofka landslide is greater than unity. This stability is abnormal because landslide slopes on bedrock clays (shale) in this region are notoriously unstable. A dormant landslide is difficult to analyze because the input parameters must represent a condition of limiting equilibrium.The Petrofka landslide is 70 m high, 880 m long, 3000 m wide, and up to 100 m thick. The shear zone is in highly plastic, montmorillonitic clays in an unnamed formation of probable Tertiary age. The Upper Cretaceous Lea Park, Judith River, and Bearpaw formations and the unnamed formation underly the landslide and are restricted to a collapse structure resulting from dissolution of Devonian evaporites.A retrogressive mechanism developed as valley downcutting progressed. The pore-water pressures could be estimated from the hydraulic head in the artesian aquifer of the Judith River Formation. The hydraulic head was maintained at critical as the valley deepened until the Judith River Formation was eroded downstream. The landslide was active for only a few hundred years, between 11 500 and 11 000 years ago.The back-calculated effective angle of friction for the bedrock clay (shale) was estimated at between 5.6° and 6.5°, assuming c′ = 0, resulting in a present-day factor of safety between 1.2 and 1.3. Key words: back analysis, dormant landslide, collapse structure, artesian pressure, historical simulation, residual shear strength.