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 con