Exploratory tunnelling at the site of what would be the UK’s first large-scale pumped storage scheme to be developed in 40 years is now complete.

STRABAG UK has completed the 1.2km long exploratory tunnel in what is the latest milestone in SSE’s ongoing development of its Coire Glas project in the Scottish Highlands.

Located at Loch Lochy in the Great Glen, the proposed Coire Glas project would have an installed capacity of 1,300MW and be capable of delivering 30GWh of long duration electricity storage – enough to be able to provide firm, flexible renewable power for three million British homes for up to 24 hours non-stop.

SSE Renewables hopes to make a final investment decision on the project in late 2025 or early 2026, allowing for main construction to commence in the second half of 2026, subject to being successful in the administrative allocation of an investable cap and floor mechanism.

Exploratory works at Coire Glas commenced in December 2022 and are being conducted by mining and tunnelling expert STRABAG UK.

In early 2023, SSE announced a £100m investment boost to further develop Coire Glas to include the construction of a major exploratory tunnel as part of a comprehensive package of site investigation works.

SSE’s technical partners, Stantec and COWI, have designed and supervised the complex ground investigation programme which is being delivered by STRABAG.

The works included the creation of a tunnel approximately five metres high and four-and-a-half metres wide which cuts into the hillside around the proposed location of the underground powerhouse complex.

Construction of the tunnel will allow the project team to gather accurate information on the geological conditions to inform the detailed design for the main works.

Now that tunnel construction is complete, a programme of exploratory drilling and testing will continue for the next three months to further investigate the ground in the area of the proposed underground powerhouse excavations.

Mike Seaton, SSE Renewables’ director of development for Coire Glas, said: “Completing the tunnelling is testament to the hard work and dedication of everyone involved and a positive step forward in de-risking the project.

“The works progressed to plan, and samples of the materials excavated from within the hillside are currently being analysed.

“This phase of the project is an engineering challenge in itself and we are learning a huge amount as we progress the works.

“The findings of the tunnelling works, alongside our wider site ground investigation works completed by Fugro in December 2023, will be used to inform the final design, including the detailed design of underground structures, and will be a key consideration in any final decisions related to the project.”

Managing director of STRABAG UK, Simon Wild, said: “We are absolutely thrilled to have completed the exploratory tunnel at the Coire Glas project.

“To set up a fully operational tunnel construction site from scratch with all the necessary infrastructure in such a remote location has been a huge achievement for our team.

“Our use of the drill and blast technique in particular demonstrates our industry leading expertise.”

STRABAG’s multi-disciplinary team of more than 100 members of staff set up a fully operational tunnel construction site in the remote location 20 miles from Fort William in December 2022.