Lithological control on the kinematic pattern and the location of slip surfaces in a large clayey landslide (Trièves, French Alps)

39 pages (espace double)Although it has been studied for more than 30 years, the large clayey Avignonet landslide (western French Alps) exhibits a heterogeneous kinematic pattern which is still poorly understood. Conducting electrical resistivity tomography profiles over the whole landslide has allowed the presence of a superficial coarser layer to be detected in its upper north-western part. The extension of this more permeable layer matches the zone exhibiting low slide velocities values (lower than 2 cm/yr),suggesting a lithological and hydrogeological control on the landslide activity. The presence of this coarser layer was not detected before, because investigation efforts concentrated on the southern inhabited areas where displacement rates are the highest (10 cm/yr to 50 cm/yr) and the slope is only composed of laminated clays. Combined interpretation of seismic reflected waves and geological data has also evidenced the presence of a 10m thick sandy alluvial layer interbedded in the clayey sequence. Comparison between reflectors and slip surfaces detected in boreholes suggests a significant role of this coarse layer on the location of the deepest slip surface at about 40m. This study highlights the control of both vertical and horizontal lithological variations on the Avignonet landslide process and the necessity of combining various investigation techniques for understanding the complexity of large landslide mechanism

Plastic strain accommodation and acoustic emission during melting of embedded particles

Melting point phenomena of micron-sized indium particles embedded in an aluminum matrix were studied by means of acoustic emission. The acoustic energy measured during melting increased with indium content. Acoustic emission during the melting transformation suggests a dislocation generation mechanism to accommodate the 2.5% volume strain required for melting of the embedded particles. A geometrically necessary increase in dislocation density of 4.1 x 10^13 m^-2 was calculated for the 17 wt% indium composition.Comment: 4 pages, 3 figures, 1 tabl

Caractérisation hydrogéophysique d\u27un dépôt fluvioglaciaire. Evaluation de l\u27effet de l\u27hétérogénéité hydrodynamique sur les écoulements en zone non-saturée

L\u27urbanisation croissante engendre une pression anthropique accrue sur les sols et les ressources en eau souterraine. La préservation durable de ces milieux requiert une meilleure compréhension des flux d\u27écoulement dans la zone non-saturée, prenant en compte les hétérogénéités du sous-sol, notamment naturelles, à l\u27échelle de l\u27ouvrage urbain. Cette étude vise à caractériser à cette échelle les hétérogénéités sédimentaires et hydrodynamiques du dépôt fluvioglaciaire de l\u27Est lyonnais, afin d\u27évaluer l\u27influence de ces hétérogénéités sur l\u27écoulement en zone non-saturée. Par une approche hydrogéophysique, couplant une caractérisation sédimentologique des structures et textures sédimentaires, une investigation géophysique par radar géologique et panneau électrique, et une caractérisation des propriétés hydrodynamiques, un modèle hydrostratigraphique tridimensionnel du dépôt fluvioglaciaire a été défini au niveau d\u27une parcelle élémentaire d\u27un bassin d\u27infiltration d\u27eaux pluviales. Ce modèle traduit l\u27hétérogénéité sédimentaire et hydrodynamique à l\u27échelle de l\u27hydrofaciès. Des mesures de teneurs en eau à trois profondeurs dans le dépôt, couplée à une modélisation numérique des écoulements non-saturés à partir du modèle hydrostratigraphique, a permis d\u27évaluer le comportement hydrodynamique du dépôt fluvioglaciaire lors d\u27une phase d\u27infiltration. Les résultats montrent l\u27influence de la saturation initiale du dépôt sur les écoulements dans la zone non-saturée. L\u27hétérogénéité de l\u27écoulement est notamment associée à la présence de chemins préférentiels. L\u27approche hydrogéophysique utilisée est prometteuse pour la caractérisation hydrostratigraphique de dépôts sédimentaires hétérogènes

Caractérisation hydrogéophysique d'un dépôt fluvioglaciaire (évaluation de l'effet de l'hétérogénéité hydrodynamique sur les écoulements en zone non-saturée)

L urbanisation croissante engendre une pression anthropique accrue sur les sols et les ressources en eau souterraine. La préservation durable de ces milieux requiert une meilleure compréhension des flux d écoulement dans la zone non-saturée, prenant en compte les hétérogénéités du sous-sol, notamment naturelles, à l échelle de l ouvrage urbain. Cette étude vise à caractériser à cette échelle les hétérogénéités sédimentaires et hydrodynamiques du dépôt fluvioglaciaire de l Est lyonnais, afin d évaluer l influence de ces hétérogénéités sur l écoulement en zone non-saturée. Par une approche hydrogéophysique, couplant une caractérisation sédimentologique des structures et textures sédimentaires, une investigation géophysique par radar géologique et panneau électrique, et une caractérisation des propriétés hydrodynamiques, un modèle hydrostratigraphique tridimensionnel du dépôt fluvioglaciaire a été défini au niveau d une parcelle élémentaire d un bassin d infiltration d eaux pluviales. Ce modèle traduit l hétérogénéité sédimentaire et hydrodynamique à l échelle de l hydrofaciès. Des mesures de teneurs en eau à trois profondeurs dans le dépôt, couplée à une modélisation numérique des écoulements non-saturés à partir du modèle hydrostratigraphique, a permis d évaluer le comportement hydrodynamique du dépôt fluvioglaciaire lors d une phase d infiltration. Les résultats montrent l influence de la saturation initiale du dépôt sur les écoulements dans la zone non-saturée. L hétérogénéité de l écoulement est notamment associée à la présence de chemins préférentiels. L approche hydrogéophysique utilisée est prometteuse pour la caractérisation hydrostratigraphique de dépôts sédimentaires hétérogènesThe increasing urbanization gene rates an anthropogenic pressure on subsoil and water resources. Their sustainable management require a better understanding of water flows in the vadose zone, taking into account their heterogeneities, in particular natural, at the local scale This study aims at characterizing at this scale the sedimentary and hydraulic heterogeneities of the glaciofluvial deposit located east of Lyon in order to evaluate the influence of these heterogeneities on the unsaturated flows. By a hydrogeophysical approach, relying on a sedimentological characterization of the sedimentary structures and textures, a geophysical investigation using ground-penetrating radar and electrical resistivity, and a characterization of the hydraulic properties, a three-dimensional hydrostratigraphic model of the glaciofluvi deposit was defined on a reduced zone of a stormwater infiltration basin. This model shows the sedimentary and hydraulic heterogeneities the hydrofacies scale. Measurements of water contents at three depths in the glaciofluvial deposit, coup led to a numerical modeling of the unsaturated flow from the hydrostratigraphic model, allowed to evaluate the hydraulic behavior of the glaciofluvial deposit du ring an infiltration. The results show the influence of the initial saturation of the deposit on the unsaturated flows. Heterogeneous flows are linked to preferential flow paths. The hydrogeophysical approach followed in this study is promising for the hydrostratigraphic characterization of heterogeneous sedimentary deposits.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF

Lithological control on the kinematic pattern and the location of slip surfaces in a large clayey landslide (Trièves, French Alps)

39 pages (espace double)Although it has been studied for more than 30 years, the large clayey Avignonet landslide (western French Alps) exhibits a heterogeneous kinematic pattern which is still poorly understood. Conducting electrical resistivity tomography profiles over the whole landslide has allowed the presence of a superficial coarser layer to be detected in its upper north-western part. The extension of this more permeable layer matches the zone exhibiting low slide velocities values (lower than 2 cm/yr),suggesting a lithological and hydrogeological control on the landslide activity. The presence of this coarser layer was not detected before, because investigation efforts concentrated on the southern inhabited areas where displacement rates are the highest (10 cm/yr to 50 cm/yr) and the slope is only composed of laminated clays. Combined interpretation of seismic reflected waves and geological data has also evidenced the presence of a 10m thick sandy alluvial layer interbedded in the clayey sequence. Comparison between reflectors and slip surfaces detected in boreholes suggests a significant role of this coarse layer on the location of the deepest slip surface at about 40m. This study highlights the control of both vertical and horizontal lithological variations on the Avignonet landslide process and the necessity of combining various investigation techniques for understanding the complexity of large landslide mechanism

Geophysical characterization of the lithological control on the kinematic pattern in a large clayey landslide (Avignonet, French Alps)

International audienceLithology variation is known to have a major control on landslide kinematics, but this effect may remain unnoticed due to low spatial coverageduring investigation. The large clayey Avignonet landslide (French Alps) has been widely studied for more than 35 years. Displacement measurements at 38 geodetic stations over the landslide showed that the slide surface velocity dramatically increases below an elevation of about 700 m and that the more active zones are located at the bottom and the south of the landslide. Most of the geotechnical investigation was carried out in the southern part of the landslide where housing development occurred on lacustrine clay layers. In this study, new electrical prospecting all across the unstable area revealed the unexpected presence of a thick resistive layer covering the more elevated area and overlying the laminated clays, which is interpreted as the lower part of moraine deposits. The downslope lithological boundary of this layer was found at around 700 m asl. This boundary coincides with the observed changes in slide velocity and in surface roughness values computed from a LiDAR DTM acquired in 2006. This thick permeable upper layer constitutes a water reservoir, which is likely to influence the hydromechanical mechanism of the landslide. The study suggests a major control of vertical lithological variations on the landslide kinematics, which is highlighted by the relation between slide velocity and electrical resistivity

Sedimentary and hydraulic characterization of a heterogeneous glaciofluvial deposit: Application to the modeling of unsaturated flow

International audienceFlows in the unsaturated zones of alluvial deposits are often poorly understood at the scale of structures such as stormwater infiltration basins. The heterogeneous nature of the sediment inherent to this type of formation can lead to preferential flows. The difficulty of quantifying them lies in obtaining hydrodynamic parameters representative of the soil structure in-situ, when the latter is inaccessible. This study proposes a method of estimating the hydrodynamic parameters of the lithofacies composing a glaciofluvial deposit (east Lyon, France) and their integration in a 2D flow model to model flow in the unsaturated zone. The approach developed comprises a sedimentological study based on the Miall code, carried out to determine the analogy between the lithofacies of the study site and those of two glaciofluvial reference sites currently in the process of formation (Chamonix, France; Breidamerkurjökull, Iceland), thereby permitting easy access to all the lithofacies identified on the study site. The water retention and hydraulic conductivity curves were obtained on the reference sites by using Beerkan type infiltration tests for finely textured materials, completed by the Arya and Paris model and data from the literature for coarser materials. Thus hydrofacies corresponding to all the lithofacies were charac- terized. These hydrofacies were used to model a drainage phase on the study site at the scale of a trench (13.50 m long × 2.50 m deep). The model highlighted the importance of the role played by the spatial distribution of hydrofacies on flows, with a pronounced capillary barrier effect of the very coarse lithofacies and a sand lens leading to funneled flows. By assuming that there is a direct link between the sedimentary lithological heterogeneity and the unsaturated hydrodynamic properties of the materials composing its different structures, it was possible to perform a detailed hydrodynamic characterization of the unsaturated zone that could then be used to model flows, and in particular highlight preferential flows

Improvement of 2D ERT measurements conducted along a small earth-filled dyke using 3D topographic data and 3D computation of geometric factors

International audienceIn the framework of earth-filled dykes characterization and monitoring, Electrical Resistivity Tomography (ERT) turns out to be a commonly used method. 2D sections are generally acquired along the dyke crest thus putting forward the question of 3D artefacts in the inversion process. This paper proposes a methodology based on 3D direct numerical simulations of the ERT acquisition using a realistic topography of the study site. It allows computing ad hoc geometrical factors which can be used for the inversion of experimental ERT data. The method is first evaluated on a set of synthetic dyke configurations. Then, it is applied to experimental static and time-lapse ERT data set acquired before and after repair works carried out on a leaking zone of an earth-filled canal dyke in the centre of France. The computed geometric factors are lower than the analytic geometric factors in a range between -8% and - 18% for measurements conducted on the crest of the dyke. They exhibit a maximum under-estimation for intermediate electrode spacings in the Wenner and Schlumberger configurations. In the same way, for measurements conducted on the mid-slope of the dyke, the computed geometric factors are higher for short electrode spacings (+18%) and lower for lower for large electrode spacings (-8%). The 2D inversion of the synthetic data with these computed geometric factors provides a significant improvement of the agreement with the original resistivity. Two experimental profiles conducted on the same portion of the dyke but at different elevations also reveal a better agreement using this methodology. The comparison with apparent resistivity from EM31 profiling along the stretch of the dyke also supports this evidence. In the same way, some spurious effects which affected the time-lapse data were removed and improved the global readability of the time-lapse resistivity sections. The benefit on the structural interpretation of ERT images remains moderate but allows a better delineation of the repair work location. Therefore, and even if the 2D assumption cannot be considered valid in such a context, the proposed methodology could be applied easily to any dyke or strongly 3D-shaped structure using a realistic topographic model. It appears suitable for practical application