On The Thermal Consolidation Of Boom Clay

TL;DR

This study investigates the thermal consolidation behavior of Boom clay under heating, revealing that permeability changes with temperature are primarily due to free water viscosity, and that thermal and hydraulic flows are uncoupled.

Contribution

It provides experimental insights into the thermal expansion, permeability, and consolidation of Boom clay, highlighting the independence of thermal and hydraulic processes and the relationship between permeability and porosity.

Findings

Thermal expansion of Boom clay can be predicted using free water properties.

Permeability changes with temperature are mainly due to water viscosity effects.

Thermal and hydraulic flows are uncoupled during heating.

Abstract

When a mass of saturated clay is heated, as in the case of host soils surrounding nuclear waste disposals 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, to hydraulic and thermal flows, and to changes in its mechanical properties. In this work, some of these aspects were experimentally studied in the case of Boom clay, so as to help predicting the response of the soil, in relation with investigations made in the Belgian underground laboratory at Mol. Results of slow heating tests with careful volume change measurements showed that a reasonable prediction of the thermal expansion of the clay-water system was obtained by using the thermal properties of free water. In spite of 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 hydraulic and thermal transfers are uncoupled. Experimental results from fast heating tests showed that the consolidation coefficient does not change significantly with increased temperature, due to the opposite effect of increasing permeability and decreasing porosity. The changes of permeability with temperature were investigated by running constant head measurements at various temperatures. An indirect analysis, based on the estimation of the mv consolidation parameter, showed that the indirect method of estimating the permeability from consolidation tests should be considered carefully. Intrinsic permeability values were derived by considering the change of the viscosity of free water with temperature. A unique relationship between the intrinsic permeability and the porosity was observed, with no dependence on temperature, confirming that the flow involved in the permeability test only concerns free water.