Intermediate shafts and headhouses are required at key points along the route of the tunnel. The tunnel boring machines will link these points as it progresses along the routes.

The shafts and headhouses are needed for the following reasons:

• Health and safety – it is essential to have access points to ensure the safety of our workforce during construction. The shafts are also used to access the tunnel for maintenance work once the tunnel is built and operational.

• Ventilation – ventilation equipment, in the form of fans, is required at some sites to cool the cables and regulate the temperature inside the tunnel.

The headhouses will be designed in order to minimise their visual impact on the local area. For example, materials are being chosen to complement the surrounding environment in agreement with the local council.

In February 2011, we embarked upon a seven-year, £1bn infrastructure project to rewire the Capital, the first major investment in the electricity transmission system in London since the 1960s. The project, known as London Power Tunnels, saw the construction of 32kms of underground tunnels, running from Hackney in the east to Willesden in the west, and from Kensal Green to Wimbledon in the south. The newly built 10 transmission circuits can now carry some 20% of the Capital’s electricity demand and stretched out they would run all the way around the M25.

Key facts about London Power Tunnels phase 1

• The depth of the tunnels varies between 20-60metres. The tunnels go under tube lines, canals and rivers.

• 192km of 400kV cable was installed in the tunnels, along with 30km of 132kV cable.

• There were 14 access shafts dug to support the tunnelling work, which was completed in 2015.

• Over 700 people were employed at the busiest time on the project, with forty apprenticeship tunnelling roles created over the project’s lifespan. In total, it took over 6 million people hours to complete LPT.

• We recycled 99% of the material excavated during the construction of the tunnels, with the excavated 500 cubed metres of London clay enough to fill Arsenal’s Emirates stadium, which is just down the road from the project’s Highbury substation.

• We are proud of the positive legacy we left with phase 1 of the London Power Tunnels project where we engaged with over 30,000 students; it is our aspiration to build on this through phase 2.