Ancient fault rocks – not just a thing of the past

How structural geology is influencing low carbon energy  

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Four years ago, in the midst of my PhD research I often found myself wondering what the point of it was. Here I was doing all the things I enjoy – geological mapping, structural geology, data analysis and writing. Yes, the subject was interesting, and I got to look at some cool fault rocks. Yes, I was studying an ancient world-famous fault – the Highland Boundary Fault in Scotland. Yes, I was developing specialist knowledge in structural geology, my research was getting published,  I was meeting a range of people and I was getting the opportunity to collaborate with world leading experts. But I often struggled to understand its relevance and applicability. I was studying an ancient fault that was no longer active. Why did my research matter and who really cares?

Even from a young age I’ve always found structural geology and geological faults fascinating. I remember in primary school being the first one to put my hand up for being able to identify a fault on a map. So, when the opportunity arose to do a PhD studying faults and their mechanical properties within the Faults and Fluid Flow Research Group at the University of Strathclyde, I couldn’t say no. Although admittedly faults were not something that was covered in much detail during my Integrated Master’s degree, so I was still pretty clueless!     

For my PhD I studied the internal structure of plate boundary faults. It’s not often that you get to actually ‘see’ inside a plate boundary fault because they are so big and hence difficult to study. One way to take a look is to drill through the fault, which is expensive and gives only a snapshot of the internal structure of the whole fault. Luckily, a rare opportunity to study the variability of an ancient plate boundary fault exists close to where I was studying in Scotland – and without the need for expensive drilling projects: the Highland Boundary Fault. The Highland Boundary Fault is one of four large ancient faults in Scotland (the others being the Great Glen Fault, Moine Thrust and Southern Uplands Fault), all of which are terrane boundaries i.e. an ancient plate boundary. Of these terrane boundaries, the Great Glen Fault and Highland Boundary Fault are the most distinctive landscape features. The Great Glen Fault forms the dramatic gash of the Great Glen. The Highland Boundary Fault marks the distinctive change from Lowland to Highland scenery.

For my PhD, firstly I mapped the Highland Boundary at Stonehaven (a bonnie seaside village in North East Scotland), then with permission from Scottish Natural Heritage myself and a team of geologists from the University of Strathclyde went to ‘dig out’ the fault at several locations at the Stonehaven site. By ‘digging out’ the fault at five different locations we were able to deliver a level of detail on the variability of an internal fault core structure of a major plate boundary fault that has rarely been seen before. We were amazed to discover a remarkable sequence of fault rocks within the Highland Boundary Fault. The exposures within the fault gouge were remarkable. There are not that many places where you can pick up and mould in your hands the clay that was produced hundreds of millions of years ago by rocks grinding together along a fault plane.

While these discoveries were all very exciting (and have been summarised in a Geoscientist article) it wasn’t until I started working as an engineering geologist at the consultancy firm, COWI, that I finally understood the applicability of my research. Faults are important to engineering geologists, geotechnical engineers, hydrogeologists, earthquake engineers and anyone who needs to build anything on or near faults (or dig through them!). As evident at the Highland Boundary Fault, different rocks, with different mechanical properties can be found within a fault zone which impacts the bulk strength, permeability and rock mass quality, which are critical parameters for the design of any structure. For instance, for net zero technologies such as geothermal energy and carbon capture and storage, the permeability and connectivity of fractures in the fault damage zone will be paramount to the design. However, for rock engineering and tunnelling, the presence of weak gouge in the fault core and increased fracturing in the damage zone will be important for deciding on the correct support for the works.

As part of my job I have had the opportunity to work on several engineering projects in the UK and elsewhere including High Speed 2 (HS2) , the tender design for Sha Tin Caverns in Hong Kong and The West Link (Västlänken) Project in Sweden . However, one of the most exciting projects for me has been Coire Glas – a proposed pumped hydro storage scheme in the Scottish Highlands that is being developed by SSE Renewables. Coire Glas (meaning the “green hollow” in Gaelic) will be the first large-scale pumped storage scheme to be developed in the UK for more than three decades and it will more than double the UK’s existing electricity storage capacity. The scale of the project is unprecedented in the UK in terms of size, technicalities and location. Underground construction will take place nearly a kilometre below ground, while the over 90-metre-high dam will become the largest in the UK. The underground cavern that needs to be excavated to house the power plant will be equivalent to the size of the largest cathedral in Scotland: Glasgow Cathedral (i.e. BIG, for comparison approximately 145 elephants could squeeze into Glasgow Cathedral!) It’s both fascinating and rewarding to be able to work on a project of such a scale in my own country! 

Part of my role on the project has been undertaking the engineering geological mapping of the site in collaboration with project partners Stantec, SSE and the BGS, developing a ground model and designing an extensive ground investigation, allowing for the analysis of a huge volume of geotechnical data before construction begins. This is all the more necessary given the site’s proximity to one of the other large faults in Scotland – the Great Glen Fault; a fault line so prominent it can be seen from outer space. To collect the data, an exploratory tunnel will be excavated from the shores of Loch Lochy through the Great Glen Fault down to where the underground cavern will reside (approx. 700 m depth), collecting samples as it progresses. Once the tunnel has reached cavern level, further boreholes will be drilled laterally, some over 300 metres in length, to collect further data to investigate the rock conditions at depth, inform the design and help manage risk. Similar assessments will also be executed where the dam foundations will be located. The geotechnical data will inform on the potential risks and challenges posed to delivering the project. One of the greatest risks being drilling and constructing tunnels/infrastructure through one of the largest faults in Scotland!

My work on the Highland Boundary can be used to understand the composition and mechanical properties of faults in future tunnelling projects such as Coire Glas, which has a total capital value greater than £1 billion. So here I am being able to apply my scientific knowledge and skills obtained during my PhD to infrastructure development and net-zero applications. Faults and their mechanical properties will always be a hot topic (they are important to anyone who has to dig or build through them). It also just goes to show that structural geology skills and knowledge (e.g., rock mechanics, field work, microstructures, tectonics, fault growth & linkage and 3D/4D visualisation) and the study of ancient rocks is applicable and relevant to modern net zero aligned energy transition applications!