Pierre Potherat’s research while affiliated with École Normale Supérieure de Lyon and other places

A geophysical imaging campaign, including four 950m electrical profiles and four 470m long seismic profiles, was performed on the large Séchilienne landslide with the objective of constraining the depth and the volume of the affected zone. Compared to the undisturbed ground, the moving area exhibits lower velocity and higher resistivity values. Comparison with existing geodetic, geomorphic and geological data (investigation gallery and borehole) allowed showing that these geophysical parameter variations result from intense fracturing and the progressive development of air-filled voids within the moving mass. A maximum thickness of 150m to 200m was found in the most deconsolidated zones. Locally, near-vertical very low resistivity anomalies were evidenced, related to cataclasis zones filled with clayey material. Applying the Wyllie's experimental law, rock mass porosity images were derived from seismic tomography profiles. Porosity between 30% and 4% was found from the top to the bottom of the deconsolidated zone, explaining the deep water table. Considering a porosity threshold of 3.7% for the sound bedrock, the total volume of the Séchilienne landslide was estimated to about 60±10×106m3, a more precise bracket than the previous estimations (20×106 to 100×106m3).

The National Road RN 91 has been threatened for about twenty-five years by a huge landslide, located 25 km south-east to the town of Grenoble (France). If several million cubic meters of rock fall down, the debris will dam the valley. Then the failure of the dam by overtopping and rapid erosion might result in a catastrophic flood and dramatic consequences for human life, environment and economy throughout the valley. The paper presents the hazard assessment based on geological and hydrological surveys, including small scale hydraulic tests, as well as the risk evaluation that has been performed. The risk management relies first upon a high level monitoring and an emergency plan; various mitigation strategies have been considered. KeywordsLandslide–Progressive failure–Séchilienne–France

The 3D displacement fields of the Sedrun landslide located in Switzerland in the Grison canton have been computed for two periods (1973 to 1990 and 1990 to 2003) by combining Digital Terrain Models (DTM) differences and correlation of aerial ortho-images. The displacement fields, which are similar for the two periods, have been compared to the tectonic structures which exist at the surface of the landslide. Zones of major velocity gradient visible on displacement maps correspond to areas marked by strong brittle strain visible in the field. The landslide that currently moves at an average velocity of 10cm/y can be divided in four main parts based on characteristics of displacement field. The upper part is mainly in subsidence. The upper intermediate area shows an apparent uplift and a strong translation in the main slope. The lower intermediate field is in subsidence with a translation. The lower part of the landslide is in apparent uplift with a translation perpendicular to valley of the Drun Tobel. These particularities are discussed in terms of architecture and mechanisms of the landslide. The upper part subsides as a corner limited by normal faults. The intermediate part is affected mainly by toppling and the behaviour of the lower part suggests the existence of a decollement level. Ortho-image correlations and DTM differences appear as an efficient tool to constrain external and internal architecture of landslides.

The active Séchilienne landslide (Isère, France) has been continuously monitored by tacheometry, radar and extensometry devices for 25 years. Indeed, if the 3 mil. m³ of rocks in the active zone named “Ruines” fell down, the debris would dam the Romanche valley. The breaking of the dam by overtopping and rapid erosion would bring a catastrophic flood and other dramatic consequences throughout the valley. Given the rockfall hazard in the most active zone, it is impossible to use targets in this area: Only reflectorless remote sensing techniques can provide information. A time-series of seven Terrestrial Laser Scanner (TLS) point clouds acquired between 2004 and 2007 enable us to monitor the 3D displacements of the whole scanned area, although point coverage is not homogeneous. From this sequential monitoring, the volume of registered collapses can be deduced and the landslide movement along the main geological structures can be inferred. From monitoring associated subsidence and toppling observed on TLS data, it can be deduced that blocks rearrangements are linked to structural settings and that the Séchilienne landslide is complex. To conclude, TLS point clouds enable an accurate monitoring of the evolution of the inaccessible “Ruines” area and, therefore, this device has proven its ability to provide reliable kinematic information, even in areas where on-site instrumentation is infeasible.

Le mouvement de versant de Séchilienne est situé à une vingtaine de kilomètres au sud-est de Grenoble. Les vitesses de déplacement de la partie la plus active dite des " Ruines " sont en augmentation constante depuis le début des mesures en 1985. En 2008, une campagne de reconnaissance par méthodes géophysiques a été mise en oeuvre afin de mieux connaître les limites du volume instable. Les méthodes électriques et sismiques ont été appliquées sur quatre profils disposés sur l'ensemble du versant. Sont présentés ici les résultats de ces investigations et les enseignements en termes de profondeur de roche déconsolidée, notamment dans la zone active des Ruines. Basés sur ces éléments, de nouveaux calculs de volume confirment la première estimation de 3 millions de mètre cubes mobilisables dans cette zone.

Le mouvement de versant de Séchilienne est situé à une vingtaine de kilomètres au sud-est de Grenoble. Les vitesses de déplacement de la partie la plus active, dite des " Ruines ", sont en augmentation constante depuis le début des mesures en 1985. Sur le terrain, les désordres observés attestent de l'activité importante de la zone frontale dite des " Ruines " et délimitent clairement celle-ci du reste du versant. La première partie de cette communication concerne l'évolution de la connaissance cinématique du phénomène. La deuxième partie intégrera les récentes données géologiques, piézométriques et inclinométriques acquises par la réalisation de plusieurs sondages jusqu'à 150m de profondeur en bordure de la zone active.

Depuis 10 ans un prototype de distancemètre radar est en fonction sur le site instable de Séchilienne, dans la vallée de la Romanche (38). Il a été construit à Toulouse par l'ONERA qui en assure la maintenance et le développement et est destiné au suivi des déformations du massif par tout type de temps. Il est également capable d'effectuer des mesures sur sol nu avec une résolution millimétrique, ce qui lui permet de poursuivre la surveillance en cas d'éboulement accompagné de la destruction de plusieurs Capteurs. Des tests à très longue distance ont montré que cette technique pourrait être avantageusement mise en oeuvre sur des sites spécifiques difficilement accessibles, tels que les falaises du Vercors, de Chartreuse ou les remparts de la Réunion, pour mesurer, depuis le fond de vallée, des mouvements très ténus entre blocs et en assurer le suivi jusqu'à la rupture.

Rock mass fall case studies presented in this paper show that rupture occur after a more or less long preparation stage which is link to different parameters such as the rate of displacement or the volume and the morphology of the unstable mass. In all sites efficient safety decision could have been taken early enough, with an appropriate monitoring system, even in the case of cliff collapses that are generally considered as sudden movements impossible to anticipate. Kinematics criteria that allow fixing in advance threshold displacement values for different alarm levels have been proposed.

The slope movement at Sedrun (Switzerland) has been studied with ortho-rectified images. Displacement maps compiled for two periods (1973–1990 and 1990–2003), based on a correlation of the aerial images indicate maximum average slope movements of 60cm/year, similar to those obtained by traditional photogrammetry over the period 1973–1990. The limits of the most active zones determined by image correlation correspond to those obtained by fieldwork. Comparison of the two displacement maps shows an acceleration of the instability of 150% since 1990. This paper demonstrates the value of using more than one technique to help understand the evolution of this long-term instability. Le mouvement de versant de Sedrun (Suisse) a été étudié par corrélation d’images aériennes ortho-rectifiées. Des cartes de déplacements de ce mouvement de versant ont été déterminées sur deux périodes (1973–1990 et 1990–2003). Le mouvement du versant de Sedrun atteint une vitesse de 60cm/an au maximum. Les valeurs de déplacements ainsi déterminées correspondent aux déplacements obtenus par photogrammétrie classique sur la période 1973–1990. Les limites des zones les plus actives correspondent aux observations de terrain alors que d’autres zones, bien que fort remarquables, semblent moyennement affectées. La comparaison des deux cartes d’amplitudes de déplacements montre une accélération de l’instabilité de 150% à partir de 1990. La corrélation optique à partir d’images aériennes d’archives a montré sa capacité à renseigner l’évolution de cette instabilité à long terme. KeywordsLandslide-Sedrun-Image correlation-Aerial images Mots clésMouvement de versant-Sedrun-Corrélation d’images-Images aériennes