The most overlooked wonder of modern infrastructure might be the tunnels we never see. As we walk around London, we pass over miles of pipes and wiring that bring us water, heat and light as well as phone and internet connections. Most impressive and perhaps least known, are the deep tunnels that carry London’s high-voltage electricity cables. 

The tunnels have a lifespan of over a hundred years

To make such a tunnel, a shaft with a diameter of 50 feet is lowered into the ground to a depth of between forty and two-hundred feet — deeper than the Northern and Victoria lines. It takes up to three months to dig a shaft. A Tunnel Boring Machine is lowered into the shaft and proceeds to drill four-hundred-feet of tunnel per week. This machine not only has to drill through all the various types of rock and geological strata under London, it must tunnel safely under the Thames. Precast concrete tunnel lining is installed behind the boring machine, and the high voltage cable is unspooled from large drums. 

Starting in 2011, tunnels were built running from Hackney to Willesden and from Kensal Green to Wimbledon. That is twenty miles of tunnel providing twenty per cent of the power used in London. One-thousand-three-hundred tonnes of earth were moved every day for three years. Stretched out, the cables in those tunnels would run round the entire M25. The majority of the cables carry 400kV. National Grid is currently spending a billion pounds to add twenty miles of these tunnels between Wimdbledon and Crayford. The tunnels have a lifespan of over a hundred years. 

There will be some forty miles of tunnels under London by 2026. This is the first time that investment has been made on this scale since the 1960s. The Tunnel Boring Machine that went into the ground at the New Cross Substation was called Edith. Edith weighs one hundred and forty tonnes. The machines in north London were called Cleopatra and Evelyn, named by schoolchildren.

All of this work must be done whilst avoiding the existing underground infrastructure as well as projects underways: sewers, water mains, train lines and projects like Crossrail must all be taken into account. London is a crowded place under as well as over ground.

The tunnels have been challenged by underground conditions. The National Grid CEO said in a recent interview that when he went to the bottom of the shaft at New Cross, it was “raining downstairs”. Last year trials were started to automate some of the building process, and a robot creates some of the concrete rings that line the tunnel.

The transformer at Highbury weighs one-hundred-and-thirty tonnes

Cables are typically laid in trenches in groups of three, well-spaced to allow their heat to dissipate. Concrete cable covers are laid on top to prevent damage. There are up to four trenches, with a total of twelve cables in a tunnel, to allow enough energy to pass through. In the London tunnels, cables run along the wall, supported by brackets. Underground electricity cables require special insulation to withstand the high voltage. Overhead cables are air-insulated, but underground cables have aluminium sheaves and plastic covers as well as insulating fluid. The cable has to undergo a year of testing before it is certified as usable. Cable pulling equipment includes rollers, socks, jacks, winches and trailers.

Some modern cables do not require fluid insulation, which reduces maintenance. These have been in use in Germany since the late 1990s and are now being installed in London. Forty miles of cable were supplied for the route between Elstree and St Johns Wood at a total weight of 2,440 tonnes. That cable was contained on sixty-three giant drums. 

Because the cables are incredibly high power, the electricity must pass through a transformer before it goes into a substation, to reduce the voltage from 400kV to 130kV. The transformer at Highbury weighs one-hundred-and-thirty tonnes.

Unlike much of the rest of the country, London has no overhead electricity cables, due to land scarcity. London accounts for twenty per cent of energy use (or more) in Great Britain, so it requires a great deal of cabling. The new tunnels will replace many miles of cable that run beneath the roads, so we will start seeing much less disruption. Repairs will now be possible without having to dig up streets and pavements. 

Instead, engineers will be able to go into the underground tunnels, which they will move through on a bicycle as buses and taxis pass overhead.