Cindy Vulliez
Director: Prof. Michel Jaboyedoff
Co-directors: Dr. Antonio Abellán, MSc. Antoine Guerin
External Expert: Marie-Aurélie Chanut
The Sechilienne landslide, which is located in the Romanche Valley (France) is mass movements of about 650 m high and 250 m wide, with a potential volume of 3 million cubic meters of material in the active part. The slope, which is mainly composed of micaschist, is characterized by the presence of a NE-SW sub-vertical fracturing system involved in the destabilization of the area. Several investigations are being performed by different research groups on this landslide, including fieldwork investigations, remote sensing, seismic acquisitions, geochemistry, deformation analysis by extensometers, etc. The rock slope has been continuously moving since the eighties decade, with a growing acceleration during the period 2009-2013 followed by a progressive stabilization during the last years.
In this work, we used different 3D techniques in order to monitor the whole rock slide
displacements. First of all, nine different fieldwork campaigns using Terrestrial Laser Scanning were performed during the last six years to obtain high-resolution point clouds (8 cm point spacing) of the rock slope topography. In addition, we used three Helicopter-based Laser Scanning campaigns carried out in between 2011 and 2014 acquired by the Cerema. Finally, more than 600 photos were taken in April 2015 in order to build a photogrammetric model of the area using Structure-from-Motion (SfM) workflow in Agisoft PhotoScan software. All types of data allowed us having a good spatial vision of the evolution of the whole slope.
A detailed structural analysis was performed from both LiDAR and SfM point clouds using
Coltop3D. Eight joint sets were detected, allowing us to estimate the susceptibility of the slope to three main failure mechanisms: planar sliding, wedge sliding and flexural toppling. Moreover, we carried out the monitoring of the active area by three approaches: comparisons among all the point clouds, tracking of several points in the frontal area, 3D tracking of several homogenous rocky compartments of the rear part using the roto-translation matrix. Large-scale movements (from several dm to more than 10 m) were observed in the active area with a coupling between subsidence and toppling oriented towards the valley. Lateral structures that cut de rear active part also seem to be affected by a clockwise rotation around the topple axis. Once the individual blocks fell during the period 2009-2014, we individualize their volumes and calculated them. We
also determined a frequency-volume relation of events in the slope and estimated the erosion rate of. Finally, we compared the two methods of imaging based on the results obtained.
The acquisition of accurate topographic data taken from LiDAR and SfM has been useful to
characterize the instability, quantifying the displacements and clarifying the failure mechanisms involved in the complex dynamic of the Sechilienne rockslope active part.
