Geotechnical Research Medal, Institution of Civil Engineering (London)

Low-permeability clayey rocks, currently considered as suitable host rocks for high-level nuclear waste disposal, pose a challenge for the determination of thermal, hydraulic and mechanical properties. A thermo-hydraulic stainless steel cell, capable of testing undisturbed soil or rock cores 70 mm in diameter and 100 mm long, has been developed and used to identify thermal properties (conduction coefficient, heat capacity, thermal expansion), permeability (derived from gas and water tests) and swelling behaviour. The cell is operated in a constant-temperature bath. Miniature pore water pressure transducers and thermocouples are installed in different positions of the specimen. Strain gauges attached to a reduced-thickness ring section of the cell provide information for swelling pressure determinations. Temperature changes are applied by a thin heater centred in the axis of the specimen. A computer code was specifically written to perform and control experiments. Air permeability is determined by means of a pulse decay technique, whereas liquid permeability is directly interpreted at steady-state conditions. However, back-analysis of the complete measured response of the specimen (pore pressures, temperatures and swelling pressures) provides the best procedure for identifying properties. A testing protocol involving a sequence of undrained and drained heating tests has been developed to determine, in an ordered way, the thermal properties and the permeability. The procedure requires the concourse of a coupled thermo-hydro-mechanical code in order to simulate test results. The program CODE_BRIGHT, developed at UPC, was used for these purposes. The procedure was applied to the identification of the thermal properties of Opalinus clay shale.

Peer Reviewed

Award-winning