SOIL CHARACTERIZATION AND HYDROGEOLOGICAL MODELLING OF AN EPISODIC RETROGRESSIVE LANDSLIDE IN CHAMPLAIN SEA CLAY
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Champlain Sea clay is a sensitive marine clay deposited within the St. Lawrence lowlands during the later stages of the Wisconsin glaciation. The sensitive clay has been known to transform from a relatively brittle material to a liquid mass when subjected to shear distortion. As a result, large-scale retrogressive landslides are common within natural slopes in Eastern Ontario and Quebec. Multiple retrogressive failures have been observed along the slopes of the Mud Creek river valley, however, these failures display rather unique behaviors, involving limited retrogressive distances, and episodic retrogressive events. The mechanisms involved with these failures are not yet adequately understood. Therefore, a laboratory testing program was initiated to place the geotechnical properties of the Champlain Sea clay deposits at Mud Creek in context with those of more renowned large-scale retrogressive events. Results of this study indicate that the Mud Creek field site is heavily overconsolidated, and that strength properties at Mud Creek are similar to those of large-scale retrogressive failures. As such, it is concluded that the small heights of the river banks at Mud Creek, in combination with relatively high undrained shear strengths, do not allow for undrained conditions to develop after initial failure events. Therefore, retrogression at Mud Creek can be described as a series of drained failures, the frequency of which is determined by the seasonal shear deformations caused by elevated total heads induced by spring thaws. To understand the groundwater regime at Mud Creek, hydrogeological modelling was conducted on two separate spring melt events. These models successfully match the in-situ field measurements (±30mm) for two separate geometries while maintaining all material properties constant and varying only the magnitude of infiltration. Investigations into potential triggering mechanisms indicated that instances of two rapid snowmelts, separated by less than two weeks, resulted in a worst case scenario for slope stability. This triggering mechanism was observed to be present prior to the retrogressive failure events at Mud Creek in 2012 and 2013.