The Beaver Creek retrogressive landslide: a reevaluation

Abstract

An analysis of the Beaver Creek retrogressive landslide south of Saskatoon, Saskatchewan was undertaken. Supplemented by data from previous studies of the landslide, information from the installation of four inclinometers and three piezometers was sufficient to describe the field behaviour of the retrogressive landslide and to determine an appropriate procedure for analysis. Additional information was provided by laboratory measurements such as direct shear testing of field samples. The Morgenstern-Price method was used for the stability analyses. Slope indicator measurements made over a three year period starting in August, 1977 clearly indicate the presence of separate blocks within the landslide with the rate of movement increasing from the scarp to the toe. Total movement at the toe is estimated at 175 mm, while movement at the scarp has been 65 mm. The rates of movement vary throughout the year with increases and decreases in the movement rate corresponding to increases and decreases in the piezometric level as measured by piezometers in the landslide site as well as those indicated by an observation well near the site. A common failure plane is located well below the river elevation at the contact between the stratified lacustrine deposits and the underlying tills. The landslide continues to retrogress, as is indicated by the formation of a new block at the scarp. This retrogression appears to be in response to erosion at the toe of the landslide. Stability analyses have shown that for the Beaver Creek landslide, the blocks within the sliding mass can be considered as a singular unit for purposes of computing the factor of safety. However, a series of blocks starting from the toe can also be analyzed in a conventional manner. The shear strength mobilized by the Beaver Creek landslide is approximately 7.0 degrees, plus or minus 1.0 degree, which agrees favourably with the 6.6 to 10.6 degree range of residual shear strength measured in the laboratory. Note:Page 163 appears in the document twice, but page 164 is not included. The second page 163 should be labelled page 164.