In the United Kingdom there are exciting times ahead for those with an interest in all things that lie beneath the ground, as we look for ways to provide long term energy storage in the form of pumped storage hydro. On the journey to net zero, and Clean Power by 2030 with closing date for Ofgem’s Cap and Floor mechanism just passed (9 June 2025) the prospects seem even more real. The cap and floor regime provides a minimum revenue floor for LDES operators to manage high capital costs and long build times, while the revenue cap lowers costs for consumers. The UK Government introduced the scheme to encourage LDES investment and appointed Ofgem as the regulator based on our interconnector cap and floor experience.
With career-long exposure (arguably life-long) I examine specifically in terms of what we will – and should – leave behind. Our positive lasting legacy : in engineering excellence, reduced emissions, and some very social and human considerations.
Underground mining and construction continues to be active throughout the United Kingdom with exciting things happening across the nation. North of the Highland Boundary Fault, in the strong rocks of the Highlands of Scotland, the last major boom in underground construction arguably spanned 100 years from 1870. Principally for Scotland’s hydro generation schemes for domestic power and aluminium production, these construction activities all-but ceased by 1976. In more recent times, activities that continued these skill sets included Connonish Gold Mine (Stirlingshire, Scotland), Lochaline Silicia Mine (Lochaber, Inverness-shire) and the Duntallich Mine (Aberfeldy, Perthshire) with construction activities limited to Glensanda’s various supporting projects (Highlander etc.) (Morvern, Argyll) and Glendoe (by Fort Augustus, Inverness-shire) – this last being the only modern day hydro project with significant underground construction. Otherwise, these skills were lost to other national or international projects, as workers sought opportunities suited to/ which matched their specialisms, with many never to return to the area.
The last pumped storage hydro project with significant underground construction in these areas was Foyers Pumped Storage, completed in 1976. The scheme converted the 1876 conventional hydro scheme (associated with aluminium smelting) to pumped storage hydro, with this, the most major recent example, now falling well outside the lifetimes of many current professionals in the field.
Renaissance in pumped storage?
In 2025, plans to meet our net zero commitments, and the ambitions for Clean Power by 2030: we approach a potential renaissance in pumped storage hydro, and as a result in vast underground construction in several locations. Firstly, developments are planned that will be similar in scale to the existing Cruachan Power Station (AKA Hollow Mountain, located at Dalmally, Oban, Argyll, Scotland) and Foyers Power Station (Foyers, Inverness, Scotland).
Moreover, the largest generating capacity schemes being explored are Coire Glas and Fearna (both Invergarry, Inverness, Scotland), Glen Earrach (Invermoriston, Inverness-shire, Scotland), Earba (by Laggan, Inverness-shire, Scotland), and Bailliemeanoch (by Dalmally, Oban, Argyll). Additionally, the recent report by Biggar Economics (2023) identifies various other schemes which total 6GW potential generating capacity. These include the expansion of the existing Cruachan Scheme, adding a further 600MW of generating potential there.
ESO Futures Scenarios 2023 report identifies 15GW of energy storage projects in pipeline which will again inevitably involve aspects of underground construction and tunnelling in the form of pumped storage hydro for long term energy storage. At the British Hydropower Association Pumped Storage Hydro event (July 2024, London), a future scenario of requiring long duration storage of 12GW by 2035 and 20GW by 2050 the majority from pumped storage hydro, as a proven and stable technology, was tabled.
Excited and experienced as we are about this sector and its somewhat resurgence, in looking ahead to these projects, I think it is important that we look to the past for information on solutions we can use, adapt and improve upon. Not just in terms of construction and operational solutions (across all engineering realms) but considering too the human and social impacts as we move thousands of people to their place of work, house them, feed them, support their families, educate their children, and look after them (medically as well as regarding safety in their workplace).
Thankfully, there are many oral records, photographs and even cine film documenting construction solutions and celebrating past milestones reached which augment the formal documentation. Just as we are today wary of social media efficacy, we need to look at these records through a corrective lens and imagine what could additionally be true or instructive – for instance plaques and group photos to tunnel driving records achieved don’t tell the tale of hand-arm-vibration syndrome which plagued many featured in those photographs during the years that followed . As you may expect, records are not collated in one central repository, so archiving processes and levels of completion vary, frequently offering an uneven picture if taken in isolation.
Regarding written research and testimony, Emma Wood’s well-researched “The Hydro Boys” is an essential read, giving an easily digestible overview, and a good lead into the history of the construction of a hydro scheme. “Tunnel Tigers” is Patrick Campbell’s own recollections of his experience in the 1950s as he travelled from Co. Donegal with many hundreds of others to work in Scotland’s tunnels, albeit with his author’s note emphasising that each worker will not have had the same experience. Critical reading for comprehending the socio-political setting is the “The Hydro” by Peter L Payne , described as a study of the development of major hydroelectric schemes undertaken by the North of Scotland Hydro Electric Board – the bibliography to the rear of the book is extensive and useful. Although we have developed in the decades since these accounts, the parallels to the modern energy security setting are plain to see.
Unbeknownst to many, useful research resources are available at the tip of our fingers. Digital archives at the National Library of Scotland and Historic Environment Scotland can be accessed from anywhere in the world which aid in research and planning of subterranean projects. Additionally, our professional institutions maintain archives – with perhaps more available in proceedings and journals than we extend ourselves to publish in the modern day . Yet more valuable knowledge can be found in the extensive archive and records maintained by local libraries, in asset owners’ offices, and on the dusty shelves of many consulting engineers (as they were known then) – not to forget in archive boxes of constructors of the time (or on the shelves of the businesses who acquired their trading workbank).
Outdated as these may sound, faced with the challenge of maintaining some of the existing hydro assets over the last 30 years I’ve had the privilege to access many physical locations that few people ever have done – perhaps many do not know they even exist. This access is not always easy, and in many cases convoluted. For example, as the last revolution evolved, some tunnels were specifically sized to accommodate the Series 1 Landrover (itself a revolution) but of course, these days 4×4 vehicles have become larger and no one wishes to take petrol driven equipment underground these days – unimaginable in previous times. Somewhat surprising to modern sensibilities, in most of the aqueducts, adits and tunnels of the era it seems that maintenance activities were considered to be unlikely, so provision for access has been restricted to the mere aperture of a manhole inspection cover. This gets ever more complicated when there can be miles between access points, and when as well as people, we need to get materials, plant and equipment to a worksite. These practical access difficulties that we have faced should encourage everyone to think about how both routine maintenance and inspection activities can be accessed beyond the present.
Challenges
Creating underground space can be expensive, particularly where an existing asset is generating; i.e., it’s not just the construction cost, it’s also the addition of the possible loss of revenue during downtime. As time has passed, standards have changed and many items of mechanical and electrical equipment have evolved in efficiency and size. When looking to make changes, it is often the case that new equipment is slotted into unaltered underground spaces (why would you) but to the detriment of safe, effective access.
Each type of underground construction poses its own challenges when it comes to inspection and maintenance, although each have been addressed in every era according to the knowledge at the time. In unlined or partially lined tunnels and aqueducts careful consideration needs to be made of not only how it is dewatered but consider the effect that dewatering has on the structure, the speed at which it is sensible to dewater and the time it might take to make the tunnel invert safe for transit of people or plant. However anticipated it may be, it is slightly disconcerting to hear roof blocks falling into partially drained and draining tunnel, although perhaps our predecessors averted this surprise by avoiding maintenance altogether.
Consider how water tight do you wish your structure to be for maintenance crews, should there be an interlock and/or a double penstock – the speed, depth and volume of through flows effects methodology and can also have swift ecological impacts in the event of something being accidently discharged.
Unchanged for decades, and perhaps obvious but often overlooked is the fact that accessing a flat-bottomed structure is much easier on the feet than one on a radii, also much easier on the wheel bearings, axles and track growsers for plant that may need to access as space. Creating a concrete invert (as a minimum) makes walking a much more pleasant and safer experience; protecting this invert from hydraulic plucking by ensuring a suitable thickness and reinforcing should be considered. In case it’s not plain many of the hydro assets tunnels and aqueducts were unlined.
The reality of almost all high-pressure tunnels is they will be completely round, fully lined in either concrete or steel – it’s important to think ahead to whether a completely round solution suits the future working environment – even though it’s usually best for the transient analysis. The roughness co-efficient carefully considered at time of newness requires an eye on long-term consistency: consider how this can be maintained in environs where, generally, peat fibres will begin to coat the lining, creating what can only be described as the bobsleigh effect for anyone entering this space, especially as they will ordinarily be on a gradient. Sliding all-ways. Even if there is to be no intrusive maintenance activity, there will come a time when the ROV survey will ascertain that it’s time for some lining cleaning.
The majority of these large schemes will be constructed over the course of many years – perhaps approaching a decade. Constructors in Scotland will face both of its Seasons: Winter, and June (Summer). Decade-long projects have time to build-in winter resilience and establish their setup, as seen in past projects. Maintenance activities will not necessarily have this luxury. Upper reservoirs will likely be above the potential snow line for all but three months of the year; any activity will be affected by horizontal snow, hail, constant rain, and low temperatures. Suddenly, come May, it’s usual to experience a month-long drought and almost overnight, you are not battling torrents – you have no water and dust rises everywhere you travel. In Summer, midges will also affect team productivity and morale. There is nothing quite like the feeling of helplessness as masses of these tiny creatures surround you. We all need to remember this from our planning desks while sheltered from the elements, where hopefully we are not being bitten.
In every case maintenance underground will require light, electrical supply, water supply and air movement. What a delight it would be that these services were built into tunnels and underground spaces we need to access at some stage to perform maintenance. If these services are not built-in then laydown areas to accommodate supporting services should ideally be positioned at handy locations adjacent to portals and/or access points, and maintained. There should be places to park and turn vehicles too. Thinking ahead with net zero in mind, it would be much preferable to see grid connections and water supply to these points too, saving on the burning of diesel in generators and in transporting potable water from sources, ordinarily, far away.
A lasting legacy
Many on the construction phase operations in the last revolution readily deployed narrow gauge railway infrastructure – some temporary and rudimentary, but many robust and potentially long lasting. The narrow gauge railway for British Aluminium Company’s Puggy Line can still be traced from near the centre of Fort William all the way to Loch Treig, for instance. Some of this route is now encompassed in Nevis Range Ski Centre, and other parts in cycle paths. Considering that this was a legacy linear asset which was largely ignored through the 20th Century, what an asset it could have been for the many visitors to Lochaber had the route been still passable along its entire length. In fact, many of the historical linear routes, tracks, trails, aqueducts, pipe races, tailraces and dams have become attractions, points of interest, places to visit and use for access. In building new, we must consider the legacy assets we leave behind for others to find, use and enjoy. Remembering that The Golden Eagle Statue which sits beside Glendoe’s Reservoir has become a destination for many visitors.
Becoming a destination, as many schemes have, attracts visitors. Visitors need facilities. In the areas where these future developments are planned, visitor accommodation is already only sparsely available. Consider then the impact too of construction phase operations, typically 500 to 1000 personnel with the specialist skills perhaps not readily on tap in these areas – potentially personnel descending from across the globe. They too will need places to stay, eat and play during the period of their contribution. No doubt some will decide to settle for the duration, whereas others will treat the locale more like a dormitory. Perhaps with overlapping and concurrent construction activities, there may be three of four of these projects being constructed at any one time, thus potentially 3000 directly-employed personnel and many other supporting services. Decisions key to communities’ wellbeing need to be made – deciding on the balance of permanent housing to expand these villages and townships, and temporary accommodation that will be removed at the end. Unlike Patrick Campbell’s experience, these won’t be open huts where beds sit top to tail and almost all “hotbedding” – we can’t ask that of our people in the 21st Century; can we?
Without a doubt communities will be changed forever by these major plans to decarbonise our energy needs. We need to ensure we leave a positive legacy harnessing the construction capability for long term wider benefits. Or, put another way, to ensure that constructing these invaluable assets for our future is done without detriment as we rush to realise our net-zero aspirations.
Given the published locations of some of the projects, Tulloch, Roy Bridge and Spean Bridge Railway Stations appear ideally placed to support possible mass transit of personnel. However, with total annual passenger numbers less than the labour pool of one project I suggest there is likely to be a need for some infrastructure improvements to accommodate possible footfall. Fortunately, in support of managing fatigue, all these stations are served by the Caledonian Sleeper, allowing personnel to travel in relative comfort while contributing less CO2 on their journey to creating an energy storage resource which further reduces emissions. Better still, there is Invergarry Station – unfortunately no longer connected to mainline but temptingly, the track bed and abutment, and piers of principal structures still exist. After construction is completed could these heritage resources continue to be used and made available?
A heritage asset that is already being considered for use is the Caledonian Canal. Through the Great Glen we benefit from the existence of the Caledonian Canal effectively connecting the North Sea to the Atlantic. This will no doubt become an essential linear asset supporting these projects as a few rely on its main water bodies, Loch Lochy, Loch Oich and Loch Ness for their lower reservoir. Would Thomas Telford have used it the way it is today for projects today – how can it or crafts be sympathetically adapted to meet these upcoming challenges and future proof maintenance activities.
Lastly in our hierarchy of linear transit on net zero journey should be road transport. Unfortunately the A85, A82, A83, A9 and A86 are all currently ill-equipped to cope with construction demands. The local trunk road network is currently occupied by the products of the north west highlands heading to market. Forest Products, Fish and Aluminium Ingots all head southwards deploying the apparent agility of road haulage – but at what cost? Our maintenance teams compete for space and availability on the highway with freight but also with visitors and other people travelling for work. The comparatively short distance from Glasgow, or Edinburgh to Fort William taking much longer than it could – and you need to keep your wits about you. Could these projects become a further incentive for trunk road improvements?
With the potential to begin soon this possible revolution could generate employment for 100s of people in construction phase, and continued employment in operation so perhaps most important of all is spreading the news to those in schools and colleges that there opportunities for the young.
These huge underground projects can and will create a lasting legacy, as did those constructed by our predecessors. Of this, perhaps the Net Zero goals are the most pressing now, but we nevertheless need to think carefully about what the lasting physical and human legacy will look like.
Angus MacGregor is a Registered Ground Engineering Professional at BAM Ground Engineering
An earlier version of this article appeared in IOM3 Materials Works October 2024.
