Mechanics of rock discontinuities uner elevated temperatures and pressures

The mechanics of fractures and discontinuities in rocks are relatively well understood for low to medium effective normal stresses. The governing parameters of the mechanics can be split into two components of strength as described in the Mohr-Coulomb model: "cohesion" and friction. The former is not true cohesion but an element of strength that refers to the shearing of major asperities on rock joints while the frictional component of strength relates to the basic friction and the joint roughness. Of these two components the cohesion and the friction element of the roughness can be destroyed with strain, especially when the material is weak or the effective normal stresses are high. In rocks the strain required to reduce the strength of the material to basic friction is normally small (c. 2-3 mm). The result is that small amounts of displacement can result in significant losses of strength. The role of water in this is of course an additional consideration.
In deep underground facilities there is an additional issue to consider and that is the importance of temperature. Most commonly, increased temperatures are the result of the geothermal gradient. However, in facilities that are designed for the disposal of High Level Radioactive Waste (HLRW), there can be an additional thermal component that comes from the heat produced by the waste itself. Such effects are not trivial. Elsworth (1989) noted that significant changes in discontinuity aperture occurred with changes in temperature and Read (2004) commented that given the temperatures likely to be developed in a HLRW storage facility "...the rock mass will experience increased rock stresses and pore pressures, which may contribute to the development of excavation damage and progressive failure." Furthermore, Ghassemi (et al 2006) noted that thermal effects can result in both compressive and tensile stresses being developed, so the distribution of stresses around excavations and discontinuities are complex, which may lead to challenges in understanding the rock-structure interaction.
Pinyol and Alonso (2010a, 2010b) have considered the thermal loading effects on landslide slip surfaces as a mechanism for allowing rapid, large strain sliding to develop. It is a small step to recognise that the processes involved in this are equally as applicable to discontinuity sliding, where the additional thermal loading comes from an external source and the ability to dissipate water pressures are equally constrained.
Therefore, the aims of this research project are to investigate the response of fracture systems in low permeability rocks and to identify the displacements along discontinuities that will develop in response to heating of pore fluids. This project will have three main stages of development:
Stage 1: Establishment of baseline geotechnical properties of low permeability materials. Sample selection will be drawn from mudstones (e.g. The Oxford Clay or the Opalinus Clay) and low permeability crystalline rocks. Artificial fractures will be created in samples for testing in stage 2.
Stage 2: Evaluation of discontinuity behaviour of different materials. Materials and fractures characterised in stage 1 will be places under triaxial compression and tested under the temperature controlled cells at the British Geological Survey and the University of Leeds.
Stage 3: Modelling and performance prediction. Observations of mechanical and fluid pressure responses from the laboratory experiments in stages 1 and 2 will be incorporated into a synthetic rock mass to identify the impacts of heating discontinuities. Upscaling will be achieved by finite element and distinct element modelling.
The successful applicant should expect to work directly with the three main project partners. The applicant will have the opportunity to develop skills in rock mechanics, rock engineering and numerical modelling