Landslide stability analysis using UAV remote sensing and in situ observations

Abstract

An approach consisting of different methods is applied to determine the geometry, relevant processes and failure mechanism that resulted in the failure of the Charonnier landslide in 1994. Due to their hazardous nature landslides have been a relevant research topic for decades. Despite that individual landslide events are not as hazardous or catastrophic as for example earthquakes, floods or volcanic eruptions, they occur more frequent and are more widespread (Varnes, 1984). Also in the geological formations of the Terres Noires, in south-east France, it is not necessarily the magnitude of events, rather their frequency of occurrence that makes mass movements hazardous. An extremely wet period, between September 1993 and January 1994, caused a hillslope in the Haute-Alps district to fail. Highly susceptible Terres Noires deposit near Charonnier River failed into a rotational landslide, moving an estimated 107,000 m3 material downslope. Precipitation figures between 1985 and 2015 show a clear pattern of intense rainstorms and huge amounts of precipitation in antecedent rainfall. This suggests that the extreme event on January 6 with 65 mm of rain after the wet months of September, October and December caused the sliding surface to fail. A total of 36 soil samples and 22 saturated conductivity measurements show a decreasing permeability with depth and the presence of macro-pores in the topsoil, supplying lateral flow in extreme rainfall events and infiltration with antecedent rainfall periods. Conventional remote sensing observations (e.g. satellite or aerial), with centimeter resolution, over this relatively vegetated landslide remain challenging due to its relative small size. Therefore, an UAV platform with a compact camera was used to capture the current elevation, structure and geomorphological characteristics at the Charonnier landslide with a 6-centimeter resolution elevation model. Multi View workflow and the Structure from Motion process were used to derive a digital surface model from images, with an accuracy of 10 cm in the vertical direction and 8 cm in the horizontal. Making it an easy and affordable to use remote sensing approach with accuracy and resolution comparable to other remote sensing approaches. The results gave insight in the current stability of the Charonnier landslide and can in the future be used to assess its dynamics. Vegetation remains a challenge for many remote sensing techniques, by excluding vegetation points from the texture generation phase in Agisoft a representation of the terrain was created, used to estimate the rupture surface. Due to bias in the control points the overall model quality couldn’t be fully assessed, but the results suggest that the tested approach can be an alternative to for example LIDAR techniques. A total of 29 shear strength tests were performed to capture the materials’ in situ shear strength properties. A friction angle of 30.2° and 33.1 combined with an effective cohesion of 7.6 and 6.0 kPa for parent for parent and slump material respectively suggest that slump material has higher shear strength. Combining all these observations with the best estimate rupture surface, the slip4ex analytical model allowed the assessment of the stability and rupture surface, suggesting a current stable situation at the Charonnier landslide. Using one homogeneous layer and a water table depending on the monthly precipitation it was possible to derive the critical water depth at the estimated location of the sliding surface. Precipitation events with higher return periods than recorded in the past 30 years would be required to reactivate the landslide in its current shape because of a rising groundwater table. Creep and erosion by the Charonnier River are altering the slope stability at an unknown rate as we speak, suggesting that the stability will change in the near future. All together the use of UAV remote sensing combined with more conventional research methods allowed for the complex stability analysis of the Charonnier landslide in relation to precipitation and sub-surface hydrology.