Geotechnical Engineering Online Library

Boom Clay is one of the potential host rocks for deep geological disposal of high-level radioactive nuclear waste in Belgium. In order to investigate the mechanism of hydraulic conductivity variation under complex thermo-mechanical coupling conditions and to better understand the thermo-hydro-mechanical (THM) coupling behaviour of Boom Clay, a series of permeability tests using temperature-controlled triaxial cell has been carried out on the Boom Clay samples taken from Belgian underground research laboratory (URL) HADES. Due to its sedimentary nature, Boom Clay presents across-anisotropy with respect to its sub-horizontal bedding plane. Direct measurements of the vertical (Kv) and horizontal (Kh) hydraulic conductivities show that the hydraulic conductivity at 80 °C is about 2.4 times larger than that at room temperature (23 °C), and the hydraulic conductivity variation with temperature is basically reversible during heating–cooling cycle. The anisotropic property of Boom Clay is studied by scanning electron microscope (SEM) tests, which highlight the transversely isotropic characteristics of intact Boom Clay. It is shown that the sub-horizontal bedding feature accounts for the horizontal permeability higher than the vertical one. The measured increment in hydraulic conductivity with temperature is lower than the calculated one when merely considering the changes in water kinematic viscosity and density with temperature. The nuclear magnetic resonance (NMR) tests have also been carried out to investigate the impact of microstructure variation on the THM properties of clay. The results show that heating under unconstrained boundary condition will produce larger size of pores and weaken the microstructure. The discrepancy between the hydraulic conductivity experimentally measured and predicted (considering water viscosity and density changes with temperature) can be attributed to the microstructural weakening effect on the thermal volume change behaviour of Boom Clay. Based on the experimental results, a hydraulic conductivity evolution model is proposed and then implemented in ABAQUS. Three-dimensional (3D) numerical simulation of the admissible thermal loading for argillaceous storage (ATLAS) III in situ heating test has been conducted subsequently, and the numerical results are in good agreement with field measurements.

In Belgium, the Boom clay was selected as a potential host formation for the disposal of high-level radioactive waste HLW). o demonstrate the suitability of Boom clay for bearing thermal load induced by the HLW, a large-scale in-situ heater test, called PRACLAY heater test, will be conducted in the underground research laboratory (URL) in Mol. Owing to the limitations of the test (a short period of time compared with that considered in a real repository, different boundary conditions, etc.), the test is designed to simulate, in a conservative way, the most critical state and phenomena that could occur in the host rock. The PRACLAY gallery was excavated at the end of 2007; the heating phase will begin in 2010 and will last for at least 10 years. The PRACLAY gallery itself leaves an opportunity to study the possibilities of sealing a disposal drift in Boom clay and testing the feasibility of hydraulic cut-off of any preferential pathway to the main access gallery through the excavation damage zone (EDZ) and the lining with a seal in a horizontal drift horizontal seal). Indeed, this is a generic problem for all deep geological disposal facilities for HLW. An annular seal made of compacted swelling bentonite will be installed in the front of the heated part of the PRACLAY gallery for these purposes. This paper provides detailed considerations on the thermo-hydro-mechanical (THM) boundary conditions for the design of the PRACLAY heater test and the seal test with the support of numerical calculations. It is believed that these important items considered in the PRACLAY heater test design also constitute key issues for the repository design. The outcome of the PRACLAY heater test will be an important milestone for the Belgian repository design

Boom clay formation is a potential natural host rock for geological disposal of high-level nuclear waste in Belgium. Heating pulse tests with controlled power supply (maximum temperature was limited to 85 °C) and controlled hydraulic boundary conditions were performed under nearly constant volume conditions to study the impact of thermal loading on the clay formation. Selected test results of intact borehole samples retrieved in horizontal direction are presented and discussed. The study focuses on the time evolution of temperature and pore water pressure changes along heating and cooling paths, i.e. pore pressure build-up during quasi-undrained heating and later dissipation at constant temperature.