Hinkley Point C – Tunnelling and Marine Works


Balfour Beatty will deliver 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 more than 20 years.

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 40,000 concrete segments. The tunnels will be connected to the seabed by vertical shafts more than 40 metres in depth and capped with large intake and outfall heads that allow sea water to pass through into the tunnels. Both the concrete segments and the heads will be manufactured to exacting specifications at a purpose-built facility at Balfour Beatty’s site at 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 each one weighing over 4,000kgs.

A further 800 metres of underground access and service tunnels will also be also be 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.


A complex dredging campaign will prepare the seabed for the installation of six tunnel heads.  The heads will be lowered into place by two of the largest marine cranes in the world, operating from barges bigger than a football pitch.

Once in place the heads will be piled to seabed and connected to the tunnel shafts. At 13 metres the Bristol Channel has the second highest tidal range in the world, presenting engineers with unique challenges to overcome.

of tunnel will be excavated
Concrete segments will be used to line the tunnels
The length of the largest outfall and intake heads