Hydromechanical assessment of a complex landslide through geophysics and numerical modeling: Toward an upgrade for the Villerville landslide (Normandy, France)
Résumé
The Pays d'Auge (Normandy, France) is impacted by large deep-seated landslides along its coastline. The largest of these landslides, the “Cirque des Graves”, has been studied since the 1980s because of significant stakes in the active zone. To characterize the hydromechanical functioning of the central part of the cirque, which is the most active part and includes the majority of the issues at stake, we performed new investigations that complement prior studies. Through the joint use of high-resolution geophysics (seismic refraction and ERT) coupled with geotechnical knowledge, a geometric model was highlighted on a large profile. Then, the mechanical behavior and slope stability were assessed by finite-difference numerical modeling (FDM) with FLAC2D®. To calibrate the model, a back-analysis was first realized on an eastern profile, for which the geometry and slip surfaces were identified by field surveys. The agreement of the model results on the eastern profile constrained by inclinometry enables a transposition of the geomechanical parameters on the central profile to assess its functioning and to locate the areas subject to mechanical weaknesses. A translational movement was revealed at the upstream end of the landslide, as well as nested roto-translational envelopes in the downstream two-thirds of the landslide. These results allowed robust assumptions with regard to the location of the slip surfaces and the zones undergoing stress-strain. The influence of groundwater was also assessed and discussed with respect to the overall slope stability. This study demonstrates the pros and cons of conventional finite difference modeling, as well as of back-analysis by parametric transposition. The benefits of the joint use of geophysical methods and numerical modeling were also highlighted and discussed.