Nothing boring about Hinkley Point C’s latest milestone moment

Another key milestone has been achieved at the Hinkley Point C construction site in Somerset, with the completion of the first of three off-shore tunnels needed for the power station’s cooling-water system.

The first Tunnel Boring Machine (TBM), named Mary, reached the end of her 3.5km journey under the Bristol Channel to complete intake tunnel 1 on 9 December 2020.

Roger Frost, Balfour Beatty Project Director, said: “This is a significant achievement - one that marks another step towards the successful delivery of the UK’s landmark nuclear power station, Hinkley Point C. The unrivalled expertise of our people, combined with our state-of-the-art offsite manufacturing facility in Avonmouth, has made it possible for us to break through the first of three off-shore tunnels. I am immensely proud of the commitment everyone has shown.”

The 3.5km-long intake tunnel 1 was mined at a depth of 33m below the Bristol Channel, using one of three TBMs needed for the project. The TBM, named Mary after prominent palaeontologist Mary Anning, was effectively a moving factory operating underground.

As Mary advanced forward she installed concrete rings, each made up of six segments. The tunnel comprises more than 2,300 of these rings, with nearly 14,000 segments needed to complete them. After each ring was placed, the crew then filled the gap behind it with grout. Around 12,000m3 of grout was used in total over the full length of the tunnel.

The nuclear-standard precast segments were produced at Balfour Beatty’s purpose-built manufacturing plant in Avonmouth and transported to site. Some 38,000 segments will be needed to complete the three tunnels.

Mary’s cutter head removed around 340,000 tonnes of earth, which was passed along seven different conveyors, both belt conveyors and a ‘bucket’-style vertical conveyor, down the length of the tunnel, up and out of the deep dig. From here it was loaded onto trucks and used for landscaping on site.

Parts of the TBM will now be stripped and installed on another machine, with the remainder immortalised under the Channel as a time capsule to the incredible feat of engineering.

Attention now turns to the two remaining tunnels for the cooling-water system. Emmeline, the largest of the TBMs, has just started on her 1.8km journey mining the outfall tunnel, and Beatrice, supplemented by equipment from Mary and responsible for intake tunnel 2, is set to launch early next year.