Editorial: Thermal Behaviour of the Ground

This is the second issue of a Symposium in Print that brings together a collection of papers from different sectors which share a common theme of thermal behaviour. The papers have been grouped together according to their position in the cycle of geotechnical research—characterisation, modelling, application and full-scale testing. 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. (2009), Åkesson 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).

Åkesson et al. (2009) describe an analysis of an experiment on a confined sample of bentonite. The interesting aspect of this paper is that it describes tests at elevated temperatures. This affects the soil moisture retention curve leading to the conclusion that vapour diffusion was a key influence during the transient stage. They found good agreement between their model simulation using the CODE_BRIGHT formulation and the experimental results.

Cui et al. (2009) undertook a series of tests on Boom Clay, a stiff heavily overconsolidated clay that is the host rock for a deep repository for high-level nuclear waste in Belgium. They investigated the effect of thermal consolidation/expansion created by the changes in pore pressure as a result of expansion of water under high temperatures. The expansion or contraction depended on the degree of overconsolidation, though it proved difficult to separate out consolidation from creep for the long term tests at low rates of strain.

The other papers on characterisation (Tarnawski et al. (2009) and Uchaipichat and Khalili (2009)) looked at aspects of the thermal characteristics of sands and silts. It has often been assumed that the thermal conductivity of soils is empirically related to the water content and degree of saturation. Tarnawski et al. (2009) reviewed the empirical relationships between thermal conductivity and degree of saturation for sands and argued that the mass fraction of sand is a key correlation parameter for quartz sands. They were able to show that for many soils this is the case. This implies that empirical relationships can be developed relating thermal conductivity to mineral content rather than degree of saturation.

Uchaipichat and Khalili (2009) undertook a programme of triaxial tests on compacted silt including temperature-controlled soaking and desaturation tests, temperature- and suction-controlled isotropic consolidation tests, suction-controlled thermal loading and unloading tests, constant-water-content thermal loading tests and temperature- and suction-controlled shear strength tests, and compression and unloading stress paths, at different values of overconsolidation ratio. This led to a number of conclusions including the facts that stresses are induced by temperature changes which could lead to failure, thermal softening occurs and the critical state line is independent of temperature.

There were two interesting full scale experiments looking at the mass energy balance at the surface of a landfill cover (Blight, 2009) and the heat distribution beneath structures on shallow foundations (Thomas and Rees, 2009). Blight (2009) challenges the view that evaporation from landfill surfaces in South Africa exceeded the average annual rainfall implying that there would be little leachate generated. If that assumption is correct then there are significant savings in construction and operational costs. Blight (2009) undertook a series of experiments to establish the temperature change of specimens embedded in the ground surface and establish the evaporation from the surface during a 24 h cycle. The conclusion was that heating the soil provided most of the energy for evaporation and hence the assumption about leachate was reasonable but not for the reasons given.

Thomas and Rees (2009) briefly describe two experiments in which temperature and pore pressure changes were recorded beneath buildings on shallow foundations (Hasegawa et al., 1987; Thomas and Rees, 1998). These provided the data to validate numerical analyses of the heat dissipation, the main aim of the paper. They were able to demonstrate that their model predicted the seasonal variation of temperature and, importantly, they were able to show the significant influence the groundwater table had upon the dissipation of heat.

Two papers (Gens et al., 2009; Thomas et al., 2009) describe full-scale tests for the storage of high-level nuclear waste. These were substantial experiments that were conducted over a period of time. Gens et al. (2009) describe the performance and interpretation of a five-year-long large-scale test in Spain simulating the storage of nuclear waste canisters in a tunnel excavated in granite. The canister was sealed into the tunnel by a compacted bentonite barrier. The numerical predictions using a thermal hydromechanical model proved to be reasonable despite the many assumptions. Some of these assumptions were validated at the end of the test by exposing the thermal barrier.

Thomas et al. (2009) describe an analysis of a full-scale test in Canada (AECL, 1994) on a canister representing high-level nuclear waste in a sand/bento-nite buffer within the host rock, granite. The heating period lasted over two years during which temperature and pressure were monitored at various locations within the buffer and host rock. A thermomechanical analysis produced a reasonable prediction of the temperature variation but tended to overestimate the horizontal pressures.

Thus this selection of papers in the two issues represents a range of international views on the thermal behaviour of the ground, how that behaviour can be modelled and how it behaves in practice. The efforts of the authors, assessors, reviewers and publication team have led to this successful compilation of interesting papers covering thermal behaviour of the ground.