Work completed on tunnel under Bristol Channel
ATUNNEL that is nearly nine times longer than the Clifton Suspension Bridge and taller than a double decker bus has been dug under the Bristol Channel in little more than a year.
It is the first of three off-shore tunnels needed for Hinkley Point C’s cooling-water system – two of the tunnels bring water in and one takes it out.
The team developing the massive £20 billion nuclear plant on the Somerset coast this month marked the key milestone.
The first tunnel boring machine (TBM) – named Mary – reached the end of its 3.5km (2.18 mile) journey under the Bristol Channel last week to complete intake tunnel 1, less than 15 months after starting in September 2019.
The tunnelling contract at Hinkley, awarded to Balfour Beatty, is valued at more than £200 million.
The front section of the expensive German-built TBM will remain buried 33 metres under the seabed forever as it is wider than the completed tunnel, which has a diameter of six metres. But, many parts are being stripped from it to use on subsequent tunnels.
All TBMs are by tradition named after women – because the patron saint of miners and tunnellers is Saint Barbara.
Primary school children competed to name the four TBMs being used at Hinkley. Their choices were:
» Mary, after renowned Dorset fossil pioneer Mary Anning;
» Beatrice, after Beatrice Shilling, who designed vital parts on Hurricane and Spitfire aircraft;
» Emmeline, after Bristol women’s rights activist Lady Emmeline Pethick-Lawrence
» and Sarah, after Sarah Guppy, who worked with Isambard Kingdom Brunel on the Clifton Suspension Bridge design.
The 3.5km-long intake tunnel 1 was mined at a depth of 33m below the Bristol Channel, with TBM Mary effectively working as a 24-hours -a-day-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 12,000 cubic metres of grout was used in total over the full length of the tunnel.
The nuclear-standard concrete was 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 the massive Hinkley 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.
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.”
Attention now turns to the two remaining tunnels for the coolingwater 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. Both of those digs are also anticipated to take about a year.
TBM Sarah will be used for smaller tunnelling kit during the project, which when completed in the mid-2020s will provide approximately seven per cent of Britain’s electricity.