Hinkley Point C – Tunnelling and Marine Works

Balfour Beatty is delivering one of the most complex marine engineering projects currently taking place in the world to support the construction of the first new nuclear power station built in the UK in a generation.

The project will see the construction of three tunnels under the seabed that will supply the two reactors at Hinkley Point C with cooling water and then discharge it back into the Bristol Channel.

Our experts will use state-of-the-art technology to excavate a total of nine kilometres of tunnel, which will be lined with 38,000 concrete segments forming 6000 lining rings.

The tunnels will be connected to the seabed by vertical shafts and capped with large intake and outfall heads, each weighing close to 5000 tonnes, that allow sea water to pass into the tunnels.

Both the concrete segments and the heads are under manufactured to exacting specifications at a purpose-built state-of-the-art facility at Balfour Beatty’s site in Avonmouth, Bristol.


Three Tunnel Boring Machines will use rotating cutting wheels to excavate two 3.5km intake tunnels and one 1.8km outfall tunnel. As the machine cuts through the rock it will line the tunnel with a ring of concrete segments.

The first of the Tunnel Boring Machines commenced tunnelling in September 2019 and continue for 12 months to construct the first Intake Tunnel.


of tunnel will be excavated


Concrete segments will be used to line the tunnels


The length of the largest outfall and intake heads

A further 800 metres of underground access and service tunnels are also being excavated and reinforced using a method known as sprayed concrete lining.

Outfall and intake heads

The tunnels will be capped with four intake and two outfall head structures. The largest heads will be 44-metres long (roughly the length of four double-decker buses), around eight metres high and weigh in at just under 5,000 tonnes.

These structures are the most complex and dense reinforcement structures across the Hinkley Point C Project.


A complex dredging campaign will prepare the seabed for the installation of six tunnel heads.  The heads will be transported by barge to the Hinkley Point C site from their construction site in Avonmouth. They will then be lowered into place by two of the largest marine cranes in the world, operating from barges bigger than a football pitch.