On the thermal consolidation of Boom clay

When a mass of saturated clay is heated, as in the case of host soils surro unding nuclear waste disposal at great depth, the thermal expansion of the constituents generates excess pore pressures. The mass of clay is submitted to gradients of pore pressure and temperature, hydraulic and thermal flows , and changes in its mechanical properties. In this work, some of these asp ects were experimentally studied in the case of Boom clay to help predict t he response of the soil, in relation to investigations in the Belgian under ground laboratory at Mol. Results of slow-heating tests with careful volume change measurements showed that a reasonable prediction of the thermal exp ansion of the clay-water system was obtained by using the thermal propertie s of free water. Despite the density of Boom clay, no significant effect of water adsorption was observed. The thermal consolidation of Boom clay was studied through fast-heating tests. A simple analysis shows that the hydrau lic and thermal transfers are uncoupled. Experimental results from fast-hea ting tests showed that the consolidation coefficient does not change signif icantly with increased temperature, due to the opposite effect of increasin g permeability and decreasing porosity. The changes of permeability with te mperature were investigated by running constant head measurements at variou s temperatures. An indirect analysis, based on estimation of the coefficien t of volume change m(v), showed that the indirect method of estimating the permeability from consolidation tests should be considered carefully. Intri nsic permeability values were derived by considering the change of the visc osity of free water with temperature. A unique relationship between the int rinsic permeability and the porosity was observed, with no dependence on te mperature, confirming that the flow involved in the permeability test only concerns free water.