Valorisation des boues de lavage de gravière dans le domaine du stockage d'eau potable, d'eau chaude et de la rétention de substances polluantes
The present study on the recovery of gravel washing muds is placed in the context of geology applied to engineering and environment, which is a field of research done at the Laboratory of Geology (GEOLEP) of the Swiss Federal Institute of Technology in Lausanne (EPFL). Washing muds from alluvium or crushed rock constitute an industrial waste product whose utilization, for the time being, is very limited and whose disposal is an environmental problem. The recovery of this by-product can be envisaged through the exploitation of the intrinsic properties of this slightly sandy silt: low hydraulic conductivity, homogeneity and isotropy. The basic idea would be to use this material in the installation of underground containment dams for the management and protection of groundwater. A whole series of samples coming from different types of alpine geological deposits (sensu lato) has been collected, according to the type of geological deposit and to its petrology, in order to characterize the muds with a view to their potential utilization. This a priori typology has been tested by specific tests on the muds. In conjunction with the results of other studies, it has been possible to select a type-mud, representative of the lemanic muds. In order to better understand the behaviour of the muds according to their envisaged applications, it has been necessary to analyse several properties: - Mineralogical properties: The X-rays analysis of the samples has determined the muds' mineralogical composition and the distribution of the potentially swelling clay minerals. These compositions have been replaced in the context of the clays' general properties, so as to better understand the narrow correlations existing between their structure and their physical and chemical properties. The role played by the smectites during the swelling phase in presence of water has been particularly underlined. - Mechanical properties: Tests show that the type-mud used can be assimilated to a rather clayey silt, containing between 25 and 30% of particle sized clays. Its consistency limits put this silt in the CL category according to the USCS grading. Tests in consolidation cells have shown that this clayey silt is highly deformable (mv=0.046·10-3 m2/kN). - Hydraulic properties: Hydraulic conductivity resulting from testing the samples is exceedingly low, around 10-8 to 10-9 m/s. To take into account the scale factor, large physical models have been used to better assess the behaviour of the silt in a vertical hydraulic barrier. These large scale tests show poorer performances if compared to tests executed on the samples. In particular, they have demonstrated the important role played by the variations of the water content in a mass of clay, and the muds sensitivity to desiccation through the appearance of drying cracks. These cracks contribute to a reduction in the performance of the dam. This could be reversed by rehumidification. However, despite desiccation the dam's hydraulic conductivity remains low. To avoid a too drastic fall of performance due to water saturation reduction, for the installation of an "out of water table" curtain, it is necessary to have a large curtain, highly watertight and insensitive to desiccation. - Thermal properties: Thermal tests simulating geothermal storage in a high conductivity aquifer have shown an increase of conductivity in relation to temperature, which practical corresponds to Darcy's law. Conductivity remains sufficiently low so as to allow the conductive part to prevail on the convective part, and thus confirming the usefulness of washing muds in the stabilization of the heat "bubble". It is a very efficient solution which can be applied wherever conductivity is too high to allow geothermal storage. - Geochemical retention properties: Leakage tests of contaminants have shown that washing muds can be used as a barrier near landfill sites. They can reduce the contaminants' flux coming out of the landfill and going into groundwater. The study of the muds' properties and in particular of this type-mud, has allowed us to better understand the correlations existing between the different characterization parameters. It particularly emphasizes the prevailing role played by the consistency, the deformability and the soil suction on the target parameters of this study, namely hydraulic conductivity, thermal conductivity and sorptive capacity. Suggested fields of application: drinking water storage, geothermal storage and creation of watertight conditions around contaminated sites. Several types of curtains and of installation technologies are presented here, according to prevailing geological conditions. When compared to other methods, containment with gravel washing muds presents many advantages, in particular, an economically interesting recovery of this industrial by-product. The socio-economical and environmental impact is important and should contribute to the development of further containment activities.