Power cables to move underground across Snowdonia 15 Oct 2020

Jonathan Rowland, TunnelTalk

Tendering is underway for construction of a project to move underground a 3km section of 400kV overhead power lines across Snowdonia in north Wales. It is one of several projects by owner National Grid to remove the visual impact of electricity cable pylons from national parks and areas of outstanding beauty in the UK. According to National Grid, the tendering process is at an early stage with a preferred bidder expected to be announced in early 2021. Early works on site are expected to begin later in 2020 with construction of the main works running through to 2026. The contract is to be awarded on a design-build basis.

Fig 1. Underground route to replace pylons in Snowdonia
Fig 1. Underground route to replace pylons in Snowdonia

The new connection will run from Cilfor on the western edge of the Snowdonia National Park, under the Dwyryd Estuary, to Garth (Fig 1). Construction works will comprise a 4.4m i.d. segmentally lined cable tunnel, two shafts with permanent head houses, and removal of ten existing high voltage electricity cable pylons. The length and depth of the proposed tunnel will be confirmed during the detailed design by the contractor.

In July 2020, the Visual Impact Project received the key planning approval to construct the head houses by the planning committees of both the local Gwynedd Council and the Snowdonia National Park Authority. Further planning approval will be required from both councils when the design-build detailed design has been completed. The project would also be dependent on UK Government regulator, Ofgem, confirming any necessary funding required outside of the £500 million already raised to mitigate the visual impact of power lines.

Ten pylons will be removed and the cables replaced underground
Ten pylons will be removed and the cables replaced underground

According to planning documents, a slurry TBM is anticipated for the 4.4m i.d. drive with the final decision by the appointed contractor. Geology along the route is expected to comprise mudstones, coarse-grained and fine-grained turbiditic sandstones, and quartzose siltstones of the Maentwrog, Ffestiniog and Dol-cyn Afon Formations of the Mawddach Group. In areas of soft or poor ground conditions, the planning application allows for the use of conditioning additives, with the contractor seeking additional approval for their use by Natural Resources Wales when the type of additive is determined.

Excavation is anticipated to progress from the western shaft and take 17 months to complete the drive under the estuary and national park. A total of 135,000m3 of spoil is expected to be excavated by the proposed slurry TBM drive and shaft construction, of which 13,000m3 will be alluvium.

Pyrite was found within the rock formations during ground investigations, which may complicate the reuse of spoil. According to an AECOM study commissioned by National Grid to detail the extent and nature of the pyrite, the spoil is not thought to pose a risk to public health or controlled waters with regards to metals, metalloids and exotics.”

Fig 2. Replacing overhead power lines with underground solutions
Fig 2. Replacing overhead power lines with underground solutions

Shaft diameters will be up to 15m for the western TBM launch site and up to 12.5m on the eastern retrieval site. Depths and construction methods will be determined during detailed design by the contractor.

Water inflows during excavation of the shafts of 137m3/day and 268m3/day are expected for the western and eastern shafts, respectively. According to the planning documents, it is not currently known whether the water inflow will be fresh or saline, due to the proximity of the two shafts to the estuary. Fresh water could be discharged to surface waters close to the shafts; saline water would need to be contained and piped to locations where it could be discharged to brackish waters.

The Snowdonia Visual Impact Project is one of a number of projects being undertaken by National Grid to remove pylons from national parks and areas of outstanding beauty and replace with underground solutions (Fig 2). Current project locations include the Peak District National Park and the areas of outstanding beauty in North Wessex and Dorset. A fifth project in the New Forest National Park was paused in March 2019, after legal complexities made it unlikely that the project would have received planning approval before the funding deadline.


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