Évaluation de la résistivité d'une digue remplie de terre avec une tête hydraulique permanente (Canal de Roanne à Digoin, France): effets 4D sur la surveillance 2D ERT

3rd Internat. Workshop on Geoelectrical MonitoringGELMON 2015, Vienna, AUT, 24-/11/2015 - 26/11/2015International audienceSeveral geophysical and geotechnical techniques were tested on an earth-filled canal dike in the framework of a French research program (labelled DOFEAS). The aim of this program was to test the ability of geophysical techniques, first, to image the body of the dike and to detect weak zones associated with known leakages and, second, to monitor the evolution of geophysical parameters (mainly electrical resistivity) with time in relation with the evolution of water circulation and internal erosion. The chosen site is an earth-filled dike (3 to 4m high) relying upon Jurassic marly limestones. The dike contains a permanent hydraulic head and exhibits 2 leakage zones in the study area. Repair works, conducted after the canal was emptied, allowed to visually identify the seepage entries (a few tens of cm each in diameter) within the canal. Drillings, in situ and laboratory tests allowed to build a hydrogeotechnical model of the site. Probes were installed in boreholes to monitor the water table variation over time. Active geophysical experiments including seismic refraction and tomography (P- and S-waves), surface wave inversion (SWI) and electrical resistivity tomography (ERT), were used to characterize the geometry of the sub-surface. Concerning the time-lapse ERT survey, 128 electrodes, spread each 1 m, were permanently installed on the dike crest. Measurements were conducted using a Wenner-Schlumberger protocol(around 3600 measurements). 7 sequences were acquired during 2 months prior, during and after repair works. These works induced variations in the water level within the canal. The combined effects of the complex 3D geometry of the dike and of the varying water level were numerically determined to correct the geometric factor k. Compared to the analytical k, the geometric factor corrected from the topographic effects induced complex resistivity variations between 8 and 18%, depending on the electrode spacing but also on the water level into the canal. Furthermore, numerical simulations also showed that, depending on the water level into the canal, the electrical current density within the dike could also be affected by a factor of up to two. This indicates that a more or less important part of the measured resistivity carries information which originates from the water. However, a full 3D correction of the measured apparent resistivity would require to know the 3D resistivity distribution within the ground.The time-lapse inversion of the ERT data allowed to locate the main pipe within the dike, at a depth of around 3.4m, just above the interface between the bedrock and the dike body. This location was in agreement with the visual location of the seepage. Corrected resistivity were found to be more realistic than with the analytical k factor and allowed a better detection of the main pipe. However, the second and smaller pipe was not detected, probably because of a too low contrast. These results confirm the applicability of calibrated geophysical techniques for assessing earth dike geometry. They also suggest that the correction for the 3D to 4D effects provides a better detection of anomalies (i.e. seepage pipe) evolving with time

De l'utilisation de méthodes géophysiques pour caractériser les hétérogénéités de dépôts quaternaires alluvionnaires. Intérêt pour l'infiltration des eaux pluviales

Colloque avec actes et comité de lecture. International.International audienceStormwater infiltration basins are generally built on geological formations with large values of hydraulic conductivity. Such is the case for quaternary alluvial formations, among which are glaciofluvial deposits. Two geophysical investigation methods, namely ground-penetrating radar and electrical tomography, were tested on an infiltration basin built on a glaciofluvial deposit. Geophysical profiles were calibrated on a trench wall dug in the glaciofluvial formation. A sedimentological study coupled to geophysical measurements behind the trench wall highlights the three-dimensional architecture of the deposit (paleochannels and high energy deposit). A typology of geophysical facies connected to the sedimentary characteristics was thus established. A simple estimation model of saturated hydraulic conductivities was used to quantify the hydraulic properties of glaciofluvial lithofacies. This study shows that sedimentary heterogeneities generate a strong hydraulic heterogeneity, potentially leading to preferential flows. The orientation of paleochannels is a privileged flow direction. Heterogeneity at the scale of lithofacies must be taken into account in the numerical models of water flow underlying stormwater infiltration basins. A preliminary geophysical investigation, coupled to a localized knowledge of alluvial stratigraphy, can be used to define a realistic hydrostratigraphic model

Effect of the settlement of sediments on water infiltration in two urban infiltration basins

International audienceThe sealing of surfaces in urban areas makes storm water management compulsory. The suspended solids from surface runoff water accumulate in infiltration basins and may impact on water infiltration. This paper describes a study of the effect of the settlement of sedimentary layers on the water infiltration capacity of two urban infiltrations basins. In situ water infiltration experiments were performed (1) to quantify the effect of sediment on water infiltration at local scale and (2) to derive the complete characterization of the unsaturated hydraulic properties of sediments and subsoils by using the BEST algorithm. These properties were used to model the effect of sediments on 1D water infiltrations at basin scale, simulating real operating conditions for three model rainfalls (shower, rainfall and light rainfall) and the full year 2008. The results clearly prove that sediments reduce local water infiltration capacities, due to their lower saturated hydraulic conductivities (Ks). Moreover, numerical results have shown that this finding could have a drastic impact on water infiltration at the basin scale through increasing the number and duration of water pondings. From the technical point of view, this study underlines the need for efficient monitoring of infiltration basin sedimentation and its impact on water infiltration capacity

Référentiel technique digues maritimes et fluviales

Généralement, en France, les digues de protection contre les inondations sont des ouvrages hydrauliques de faible hauteur sur terrain naturel