National Grid, which owns and manages the electricity transmission network in England and Wales, has commissioned a 3km tunnel in the Snowdonia National Park. Its purpose is simple: to help transform and enhance the natural beauty of the local landscape.
The tunnel will of course carry electricity infrastructure: six high-voltage (HV) cables that form an integral part of the network that provides secure and reliable power supplies to North Wales and beyond.
It is set to replace a section of overhead line and 10 pylons that partly run across the estuary of the Dwyryd River, an area of white sands and natural beauty. Here, the uplands of Snowdonia meet the sea, and not far away is the architecturally unique and popular tourist destination of Portmeirion.
The scheme forms part of National Grid’s Visual Impact Provision (VIP) project. It is a world-first in that it is the first time part of the high-voltage transmission network has been removed purely to enhance the landscape. It makes use of a provision made available by Ofgem (the Office of Gas and Electricity Markets – the nation’s regulatory body for electricity and downstream natural gas) to carry out work that mitigates and reduces the impact of existing high-voltage transmission lines in English and Welsh Areas of Outstanding Natural Beauty and National Parks (see box).
Uniquely, the whole initiative is stakeholder led, with all key decisions being taken by a national Stakeholder Advisory Group (SAG) comprising organisations dedicated to conserving the landscape and countryside throughout England and Wales. Chaired by environmentalist Chris Baines, the group comprises senior level representatives from countryside charity CPRE, the National Trust, the Campaign for National Parks, the Landscape Institute, Historic England, Natural England, and the Ramblers and their Welsh counterparts. It was SAG that singled out the line section to be removed.
Five major projects are currently part of this Visual Impact Provision. One – in the Dorset Area of Outstanding Natural Beauty (AONB), located near Dorchester and the Jurassic Coast World Heritage Site – is the most advanced. Here, 22 pylons and an 8.8km stretch of overhead line are set to start coming down later this year. A site in the Peak District National Park is similarly working to replace a 2km section of overhead line with underground cable which is due to be completed by the end of 2022. A scheme, near Devizes, Wiltshire, in the North Wessex Downs AONB, and another east of Cheltenham in the Cotswolds National Landscape are currently at the planning and early development stages respectively.
Each of these projects is being carried out using advanced cut-and-cover trenches and cable-ducting techniques. But the Snowdonia project is unique in that the cables will be laid in a tunnel excavated by TBM. In all cases, the primary goal is to conserve and enhance the natural beauty, wildlife and cultural heritage of nationally protected landscapes.
The decision to tunnel the project was also taken by the Stakeholder Advisory Group and a group of local technical stakeholders, including sector experts and representatives from the Snowdonia National Park Authority, Gwynedd Council, the National Trust, Cadw, Natural Resources Wales and the local town and community councils. Planning was complex as the two tunnel headhouses which needed planning permission are located in two different planning authorities, but full consent was granted in 2020.
Originally constructed in 1966, the line – spanning just over 3km – forms part of the 400kV electricity route connecting the Pentir substation near Bangor with the former Trawsfynydd Power Station, on a site which includes a 400kV substation. Currently the pylons operate with one circuit at 400kV on one side of the estuary, while the circuit on the estuary’s other side operates at 132kV as part of the distribution network operator’s system.
The section of line identified for removal runs from the village of Garth near Minffordd and crosses the estuary at Penrhyndeudraeth, where it enters the western edge of the Snowdonia National Park. It then continues eastwards to just beyond the small settlement of Cilfor. There is already an existing section of underground cable across the neighbouring Glaslyn Estuary to the west of the project, and the cables in the new tunnel will connect with this.
“That was the case when we originally tendered for the project in late 2019,” says Lawrence Jackson, managing director of Hochtief, which is carrying out the work. But an added benefit has since emerged: “In early 2021, the scope was updated to include the requirement for additional cables in the tunnel and the ability to run the circuits on both sides of the estuary – both circuits at 400kV. This is to allow for future network connections, which might be wind farms off the North Wales Coast.” In which case, green energy supply as well as a greener landscape could result.
The tunnelling and construction contract is a very recent one, awarded on 16 December 2021. Hochtief has signed an NEC Option C contract with National Grid, won by competitive tender. The contract date start was 31 January 2022, with notice to proceed with full Ofgem funding scheduled for 1 July 2022. The project is therefore still in the early stages with further surveys to be carried out this spring, construction to start in 2023 and overhead line removal scheduled for 2029.
Steve Ellison, National Grid Snowdonia VIP’s Senior Project Manager said: “This project has been a long time in planning and National Grid has worked closely with stakeholders and the community in developing these plans. Local people have been very enthusiastic to see the pylons come down and extremely patient in waiting for us to make a start. The effect will be transformational on the landscape but also presents us with an enormous opportunity to greatly enhance the natural environment and to bring additional benefits to the local community through initiatives like our Community Grant Fund.”
TUNNEL FACTS
The tunnel will be 3,370m in length, running from Garth at the western end, under the Dwyryd Estuary to Cilfor on the eastern side. The nearest town is Penrhyndeudraeth. The geology comprises fluvio-glacial deposits of stable mudstone of the Dol-Cyn-Afon formation at the Garth end running into Ffestiniog Flags formation mudstone towards Cilfor, but with mediumand high-inflow fault lines encountered en-route. “The supplier of the TBM has not yet been confirmed” says Jackson, “but it will be a slurry machine. The fault zones and deposits have driven that solution.”
Tunnel access will be made possible by shafts at each end. The drive shaft, at the Garth end will also be the portal for the TBM, which will be lowered down by gantry crane. This shaft is planned at 15m internal diameter and up to 65m deep; groundwater is at ground level. The reception shaft, at Cilfor, is to be similar but of 12.5m internal diameter.
“That shaft is complicated by the fact that it is very close to a fault zone,” says Jackson. “Because of that, it will need ground conditioning. That shaft is deeper as well.” It will excavate through 5m of cohesive superficial deposits, followed by 10m of fluvio-glacial deposits before hitting at around 15m depth a Maentwrog formation zone which continues down to 60 metres.
A foreshunt and backshunt tunnel, for assembly and servicing of the TBM, will be excavated from the bottom of the Garth shaft. “Ground improvement will also be required for those. The shunts are effectively temporary works, to aid construction. On completion, the backshunt will be backfilled, and the foreshunt will be narrowed down to the final tunnel diameter, which is 3.5 metres. There are to be no access shafts en-route. The TBM drive is expected to take about a year.”
Lining will be by concrete segments, 1.2m wide, although the thickness has not yet been determined but will be in the region of 250mm. Jackson explains: “We are foreseeing that a filter press will be used to reduce the muck-away volumes. Using a filter press rather than a centrifuge cuts the power consumption for this part of the tunnelling process by more than 50%. It also reduces muck-away volumes, which further reduces carbon associated with transport.
“Detailed noise assessments carried out at the tender stage, and a temporary headhouse will be employed over the launch shaft to reduce disturbance. We will also be specifying plant that will minimise noise emitted from the tunnelling operation.
“And Hochtief intends to work closely with the designer to reduce embodied carbon in the tunnel lining, in shaft construction and in headhouse materials. Reducing the lining thickness has the potential to cut carbon associated with the concrete by 16%. We are specifying temporary works to minimise the import of construction fill and concrete.
“We shall be using HVO fuel for generators and plant, which significantly reduces carbon associated with the construction process.” HVO – hydro-treated vegetable oil – is derived from plant residues and wastes and can be directly substituted for diesel in engines with no need to modify the engines and with no loss of performance. It is a non-fossil fuel and when sustainably sourced can be carbon-neutral. “Moving from diesel to HVO will reduce carbon emissions for generated power for the TBM and site from 23,500t to 1,700t. And Hochtief will achieve biodiversity net gain by specifying and implementing a detailed landscaping scheme.”
At each end of the tunnel the cables curve upwards, performing a basically right-angle bend to pass vertically up the shaft to the surface, where they are managed into troughs in the sealing end compound. There will be headhouses at each end of the tunnel.
“At the Cilfor end, we have to build quite a large headhouse, and we have also to install the high voltage equipment in the compound. The head house forms a gantry which supports the overhead lines coming in; those drop down into the high-voltage equipment. We marshal them into troughs, from where they transfer to the cables that go down the shaft.
“At the Garth end we have a similar arrangement except that the tunnel cables are not transferring to overhead lines but to existing buried circuits.
“The tunnel will carry twelve cables, six cables to a side, with an access space between them. The access space can take a small tunnel vehicle – basically a scooter – for maintenance or inspection staff, but National Grid is not expecting to have to do that very often. The cables are suspended as catenaries between steel brackets. The spacing between the brackets has not yet been determined but is likely to be between five and eight meters, and the cables sag between them. The cables heat up, to an extent that depends on the current they are carrying, and therefore expand and sag to a greater or lesser extent; of course, we have to allow clearance for the maximum sag that is expected.
Jackson continues: “The cables are multiple-strand, with various sheathing layers of insulation; they are supplied by a specialist subcontractor. But all the high-voltage cabling design is ours at Hochtief; it is in our scope to design, supply, install and commission the high-voltage circuits, so we can hand over to National Grid a fully-operational system.
“Obviously the spacing of the cables is critical, and that depends on the tunnel diameter. Once you have the diameter and the cable spacings, you can do heat-loss calculations, and then you can determine what size cooling fans you need. The fans are sited in the head-house at the Garth end and suck in air through the tunnel from the Cilfor end. If the cables are at full loading – which would only happen if there were to be a fault elsewhere on the grid – the air coming out could be pleasantly warm. There will also be some backup pumps but we are not expecting any water to ingress into the tunnels.
“High-voltage cable tunnels are not actually new” says Jackson. “Sydney has them; we at Hochtief do these tunnels through London. The reason is simply access: you cannot string high-voltage overhead cables through the middle of a major city.”
Is he expecting any particular difficulties or complexities for the geology or the ground? “It is not easy. It has complexity in the geology. If you look at the geology it is all alluvial material, and there are fault zones in it. We are confident that the slurry machine type of TBM that we have chosen is the best for the job.”
Headcount on site will be around 100 or 120 people. “It’s a good project” says Jackson. “It has been championed by stakeholders and the local community and is going to make a huge difference to how the local area looks, restoring the estuary landscape to its spectacular natural beauty.”
“Cable tunnels in themselves as I said are nothing new, but to build one purely to improve the visual effect on the environment – that I think may be a first.”
