Monitoring Slope Movement Using Unmanned Aerial Vehicles

ABSTRACT: Slope movement post precipitation events are a well-documented and understood phenomenon. The increase in moisture, if able to infiltrate the hill slope, can lead to the buildup of excess pore water pressure resulting in a decrease in shear strength of the soil. This phenomenon results in surficial failures that can slowly degrade the stability of a hill slope and eventually lead to failure. Monitoring this using traditional surveying techniques can prove to be time consuming and difficult due to the sheer number of points required to accurately capture the slope movement. To overcome this, this study sought to use unmanned aerial vehicle (UAV) techniques and equipment to monitor existing slope movement and failures. A study area in Rapid City, South Dakota around the South Dakota School of Mines and Technology (SDSMT) campus, which included multiple existing slope failures, was used. To monitor slope movement, photogrammetric imagery was taken of the study area both prior to and after precipitation event. Drone imagery was used to construct structure from motion (SFM) models to monitor slope movement by comparing previous flights to one another. All areas of interest or failures documented from the flights were individually ground truthed to ensure accuracy of the imagery. From this study it was found that seepage areas were a high indicator of slope failure of exposed ground surfaces and SFM models were able to accurately capture and document slope failures that had occurred after precipitation events. 1. INTRODUCTION Seasonal weather cycles result in the development of anisotropic stresses within the surface layer of earthen structures, resulting in altered engineering properties and the development of an active layer of soil1. These weathering cycles include iterative shrink-swell with wetting and drying and freeze-thaw as well as times of acidic water, organic acids, and other chemical weathering processes. As the degree and severity of weathering cycles increases, this active layer propagates downward and results in near surface (upper 5-m) failures, weakening the overall integrity of geotechnical earth structures, resulting in increased erosion rates2-5, slope failure5, and compromised hydraulic conductivity in dams and levees2,7-9. These alterations in the engineering properties of earthen structures results in unforeseen or premature failure for slopes sensitive to changes in properties and conditions in the active zone (3 to 5-m), especially materials in the upper 1-m sensitive to seasonal freeze-thaw. Therefore, it is understood that seasonal weathering cycles are a key contributor to obscelense of earthworks.