Cone Penetration Testing and Hydrogeological Monitoring of a Retrogressive Landslide in Champlain Sea Clay

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Potvin, Joshua
landslide , cone penetration testing , piezometers , geotechnical investigation
Champlain Sea Clay (also known as Leda Clay) is a sensitive marine clay that was deposited within the limits of the Champlain Sea transgression during the final retreat of the Laurentide ice sheet. Upon isostatic rebound, the watersheds incised deep river valleys throughout the Ottawa region. These sensitive clay river banks have been shown to be highly susceptible to large retrogressive landslides. A cone penetration testing and hydrogeological program was developed in this thesis to characterize a retrogressive landslide along a creek valley consisting mainly of Champlain Sea Clay. As Champlain Sea Clay has been commonly shown to consist of banded layers, a 2 cm2 piezocone, and 5 cm2, 10 cm2 and 15 cm2 CPTu cones were used to demonstrate that the slightly larger 5 cm2 penetrometer was the most practical size for investigating landslides in Champlain Sea Clay. In doing so, the 5 cm2 cone was capable of high resolution stratigraphic profiling, locating remoulded layers for slip surface detection and characterizing the Champlain Sea Clay landslide near Ottawa. Due to the significant effects of the pore pressure distribution on slope stability and retrogressive behavior, a long term hydrogeological program was initiated which defined the ground water regime and real-time pore pressure data during a retrogressive landslide event. The seasonal change in the ground water regime from rapid snowmelt has shown to be a significant hydrogeological influence on triggering a retrogressive landslide along Mud Creek. With regular monitoring over multiple seasons, the seasonal pore pressure changes can be used to further understand the long term development of retrogressive landslides in Champlain Sea Clay.
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