Thermo-hydraulic characterisation of soft rock by means of heating pulse tests

Abstract

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 ReviewedAward-winningPostprint (published version

Coupled thermo-hydro-mechanical behaviour of a deep clay

“…Table 1 summarises different theoretical and experimental values of the thermal pressurisation coefficients Ʌ (MPa/ºC) for different geomaterials. The evaluation of the coefficient is usually performed by back-analysis of in situ or laboratory tests [11][12][13][14][15] or directly determined by laboratory experiments [8,10,16,17].…”

Experimental techniques for the investigation of coupled phenomena in geomaterials

“…Figure 1 shows a scheme of a quasiisochoric and axi-symmetric heating cell [3,6], which is used to study heating pulse tests with controlled power supply and controlled hydraulic boundary conditions. Soil dimensions are 75 mm in diameter and 100 mm high.…”

Editorial: Thermal Behaviour of the Ground

“…In this issue there are nine papers covering characterisation and full-scale testing, many of which focus on storage of high-level nuclear waste. Muñoz et al (2009Muñoz et al ( ), Å kesson et al (2009 and Cui et al (2009) investigated the characteristics of host rocks for the storage of high-level nuclear waste and the bentonite barriers between the waste and host rock. Samples of rock (Muñoz et al, 2009) were subjected to heating and cooling cycles with pore pressure and temperature responses being measured within the specimen as the test proceeded for more than a year.…”

“…Muñoz et al (2009Muñoz et al ( ), Å kesson et al (2009 and Cui et al (2009) investigated the characteristics of host rocks for the storage of high-level nuclear waste and the bentonite barriers between the waste and host rock. Samples of rock (Muñoz et al, 2009) were subjected to heating and cooling cycles with pore pressure and temperature responses being measured within the specimen as the test proceeded for more than a year. They were able to stimulate the specimen response to an axial heater by using CODE_BRIGHT, a numerical formulation to simulate the thermomechanical behaviour of ground (Olivella et al, 1996).…”