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. In mudstones, the effects of even modest temperature increases is highlighted by Douma et al (2016) who in an investigation of samples from the Whitby Mudstone observed decreases in strength and elastic modulus and an increase in compressibility. It is apparent therefore that the thermal loading is going to impact on the engineering performance of mudstones, and understanding these effects will impact on the development of underground space and how high level (thermogenic) waste can be stored.
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. In the UK such formations include Kimmeridge Clay, Mercia Mudstone and the Lias Group mudstones amongst others. Mudstones are composed of numerous mineralogical components. These include clay minerals, often dominated by illite, and to a lesser extent smectite group minerals. However, there is significant differences in quartz content in UK formations with that variation being seen both spatially and stratigraphically. For example, Armitage et al (2013) described stratigraphic variations within the Mercia Mudstone leading to quartz contents varying from 32% to 66% of the mineral composition. By comparison Callovo-Oxfordian clay formations investigated at the ANDRA Site in France show quartz contents of ranging from 19 to 36% (Gaucher et al 2004).
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.
Therefore, the aims of this project are to:
A1) 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;
A2) Identify the mechanisms that control the response of the rock material which can be used in a process model to predict long term behaviour
A3) 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 successful applicant will work with colleagues at the British Geological Survey to design and perform a series of experiments in which samples will be subjected to a range of thermos-mechanical 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.
The successful candidate will develop skills in weak rock mechanics, thermo-hydro-mechanical (THM) modelling and engineering geology broadly. The student is expected to spend one month of each year working on their PhD while placed with the industrial partner (Arup) and should expect to spend short periods of time at the British Geological Survey in Keyworth, Nottinghamshire. Students without a background in Engineering Geology will be encouraged to follow relevant sections of the MSc Engineering Geology at the University of Leeds.
Applicants should hold, or expect to hold a minimum of an upper second class honours degree (or equivalent) in Geology, Geophysics, Physics, Material Science or Civil Engineering. Applicants who have experience of laboratory work or numerical modelling would be advantageous.
This is a collaborative project between the University of Leeds, Arup and the British Geological Survey. In addition to the University of Leeds staff, Dr Tasos Stavrou (Arup) and Dr Audrey Ougier-Simonin (BGS) are core members of the project team.