Evaluation de la dessiccation, du retrait et de la fissuration de materiaux silteux peu plastiques

Fine-grained soils are often characterized by a tendency to shrink when subjected to a high suction. The decrease of the specific volume is related to a water content reduction, and this may result in the formation of desiccation cracks.;Tailings from hard rock mines are fine-grained materials that tend to interact with the environment when exposed to natural conditions. The effective management of hydrogeological and environmental issues that may arise from tailings impoundment requires an understanding of their behavior during drying, shrinkage, and cracking.;A number of studies have been carried out on compressible clayey materials to assess their shrinkage behavior. However, the properties of fine-grained tailings classified as low plasticity materials differ from those of clayey soils. This project aims at evaluating the factors that influence the process of desiccation, shrinkage and cracking of low plasticity silts, with an emphasis placed on tailings from hard rock mines.;In the first phase of this study, shrinkage tests were conducted on various tailings. Two main types of shrinkage tests were conducted in the laboratory: water retention tests using a pressure plate apparatus and free shrinkage tests in moulds of varying thicknesses. For the latter, the moulds and procedures were designed so that the tests are conducted with no (or little) drainage of the sample; this makes evaporation the key process that controls water loss during drying. The shrinkage tests are performed on samples resting on a thin plastic film that practically eliminates frictional (shear) stresses on the walls of the mould. Comparing results from shrinkage tests and water retention tests shows a good correlation, while the two combined approaches gives a fairly complete characterization of the response of tailings undergoing desaturation. The main results are presented in the form of different hydrogeotechnical relationships, which include the well known shrinkage curves and the water retention curves, with the resulting characteristic parameters. The shrinkage tests were conducted using different initial water contents to assess the effect of this factor on each tested material, which included the Bulyanhulu mine (Tanzania) tailings (as the reference material) and four other types of tailings. Additional shrinkage tests were conducted on tailings with a binder to evaluate the contribution of cement on the deformation (shrinkage) and water retention.;A second phase of the study focused on defining the onset of desiccation cracks in relation with the prevailing conditions, by performing restrained desiccation tests. These tests were carried out using the same setup, with the same moulds, as the free shrinkage (no cracking) tests, in which a wire mesh is placed at the bottom. This modification restrains shrinkage and tends to induce cracks by limiting horizontal displacement at the base of the specimen undergoing drying. Once the first crack is initiated, the specimen is weighed to obtain the water content at the onset of cracking. The water retention curve of the material is then used to estimate the critical suction for crack initiation.;A numerical code is also used to simulate the evolution of water content during the drying process and the related development of suction over time. This leads to the definition of the time at which the material begins to desaturate. This time, called the critical air entry time, is very important when there is a need to prevent the entry of air (oxygen) into reactive (sulfur-rich) tailings, to control the formation of acid mine drainage (AMD). The numerical results are successfully compared with those from the laboratory tests. This helped validate the simulations.;In the last part of this project, the shrinkage characteristics of the different materials are analyzed and discussed. The experimental data are used to extend an existing analytical model that can predict the shrinkage functions of these tailings on the basis of their physical properties (such as particle size and void ratio). The results from this study should be useful for a better planning of tailings disposal under conditions that favor drying and desiccation.