Thermal loading on mudstones

The long-term geological disposal of radioactive waste has three potential host rocks: crystalline rocks, evaporites and lower strength sedimentary rocks. The latter, which are normally mudstones, are attractive due to their low permeability and plastic deformational characteristics.

The mechanics of UK mudstone successions have been extensively researched (e.g. Cripps and Taylor 1986, 1987, Hobbs et al 2002), but with an emphasis on their strength properties at the near surface, and all such work has been carried out at ambient (room) temperatures. Prior work carried out at the University of Leeds in collaboration with the BGS and Arup suggests that thermal damage in sandstones is evident at modest temperatures ranging from 50o-125oC (Woodman et al., 2021). This suggests that materials subject to heating in this range, and then subjected shearing will mobilise lower strengths than those tested under ambient (20o-25oC) conditions.

Given the proposed uses of mudstone/clay formations as a host rock for the disposal of thermogenic radioactive waste in Europe, there is a need to understand processes affecting mudstones generally, especially where there are different compositional differences such as variations in silt content. There is therefore, a clear need to investigate the mechanics of mudstones under elevated temperatures in order to identify controlling factors on process impacting on long term performance.
This research aims to:
- Investigate experimentally the thermo-mechanical changes in lower strength sedimentary rocks subject to thermal loading and whether there is a threshold level for different levels of damage;
- Identify the mechanisms that control the response of the rock material which can be used in a process model to predict long term behaviour
- Upscale the results of laboratory observations to evaluate damage and identify the extent and consequences of a thermally disturbed zone around the storage of any thermogenic wastes.

In order to meet these aims the PGR will work with the British Geological Survey to design anical loading conditions. This will allow the establishment of baseline properties at the Rock Mechanics and Engineering Geology laboratories at the University of Leeds. The role of moisture content will also be evaluated in order to separate mineral content effects from pore water pressure effects and work to develop a conceptual model of thermal damage based on the technical literature and observational data. Samples will either be collected in the field or, where possible, sourced from core recovered during ground investigations.

Scanning electron microscopy will be used to identify micromechanical effects on a grain to grain scale in order to validate numerical models developed working with colleagues at Arup. Those numerical models will be used to upscale laboratory observations to a rock mass scale to attempt to develop an understanding of the impacts of prolonged thermal loading on the performance of underground space