Long Term Monitoring of a Retrogressive Landslide in Sensitive Clay Using Low Altitude Digital Photogrammetry
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Champlain Sea Clay, also known as Leda Clay, is a glaciomarine clay that was deposited between 12,500 and 10,000 BP within the limits of the Champlain Sea transgression during the retreat of the Laurentide ice sheet. Isostatic rebounding of the region raised the deposits above sea level, leading to the development of deep river valleys throughout watersheds in the Ottawa region. The river banks comprised of Champlain Sea Clay have been shown to be highly susceptible to retrogressive failures due to the soil’s sensitivity. A long term monitoring campaign was conducted to quantify slope deformation prior to the initial failure and successive retrogressive failures of a slope in the Ottawa region. Digital photogrammetry (DPG) was selected to create five 3D models of the site with varying temporal resolutions. A helium filled blimp unmanned aerial vehicle (UAV) followed by a Cinestar8 octocopter UAV were used as low altitude camera platforms to carry a digital single lens reflex (DSLR) camera for image acquisition of the slope. A total station was used for the collection of ground control point (GCP) coordinate information. A unique dataset detailing the evolution of a retrogressive failure was obtained and orthographic images, site cross sections, GCP displacements and digital elevation models were created. The results displayed how monitoring methods capable of accurately capturing sub 10cm ground surface movements can be used to capture the onset of progressive failure in sensitive soils such as Champlain Sea Clays. With the addition of subsurface soil layering geometry, soil layer properties and hydrogeological monitoring data to the DPG data, a full investigation into slope failure can be conducted to further understand the development of retrogressive landslides in Champlain Sea Clay slopes.