Experiences with Slope Instability in Lake Saskatchewan Clays in the Saskatoon Area

ABSTRACT: Glacial Lake Saskatchewan, formed during Wisconsinian deglaciation of southern Saskatchewan, deposited considerable depths of deltaic and lacustrine sediments that exhibit an increase in clay content and plasticity with depth, typical of the sedimentary sequence associated with an “on-lapping” delta. The geotechnical properties of these materials and their associated impacts on the stability of the slopes of the South Saskatchewan River Valley have been studied extensively. Of particular interest is the Beaver Creek Landslide near the southern limits of Saskatoon, located within the deeper lacustrine clay sequences. Natural processes have continued to precipitate slope instability within the Lake Saskatchewan plain as riverbank development has progressed since the earlier studies. Choosing the appropriate shear strength for these clay sediments is a perennial question for geotechnical slope stability analysts, since it is clear there are variances in the properties of the clay sediments within the Lake Saskatchewan basin. This paper will focus on addressing the strength of Lake Saskatchewan clays by presenting further analysis of the Beaver Creek Landslide, supported by six additional case histories in which the strength of the clay was determined by both laboratory testing and back-analysis. The results will be compared to published values for similar sediments, with discussion on the application of this data to new slope stability analyses and remedial design.