On the roof of the Northern line extension’s Battersea Park Road base is a decked terrace which looks out over the expansive worksite that now surrounds Battersea Power Station. One only has to cast one’s eyes briefly around the site to get an idea of the well-tuned production line that’s required to excavate rail tunnels.
A large rectangular launch shaft sits directly below, surrounded by the precast concrete segments used to line the tunnels and the two cranes that are lifting them in. A conveyor system hangs off the side, transporting spoil from Helen and Amy, the two tunnel boring machines (TBMs) which are currently making their way towards Kennington station – an interchange station for the Charing Cross and Bank branches of the Northern line.
Following tunnelling tradition, both TBMs have been named; one after British astronaut Helen Sharman and the other after aviation pioneer Amy Johnson.
The 3.2km extension will include new stations at Battersea Power Station and Nine Elms. The latter is situated just down the road from the site of the former Nine Elms railway station. This had been a terminus station on the London & Southampton Railway during the brief 10-year period it was open between 1838 and 1848. Since then it has been the location of the L&SWR’s carriage and wagon works and a freight yard. Badly damaged by bombs during the Second World War, it was demolished in the 1960s and the flower section of New Covent Garden Market now stands in its place.
£1.2 billion project
Serious calls for an extension of the Northern line to Battersea were first made in 2007, but it wasn’t until 2014 that the project received government approval. Things moved considerably quicker after that and, later that same year, a joint venture between Ferrovial Agroman and Laing O’Rourke (FLO) was awarded the main construction contract.
The Greater London Authority will borrow up to £1 billion to fund the line’s construction, although the current cost quoted by Transport for London (TfL) is £1.2 billion. The funds to repay the loan will be recouped later on from the local developers set to benefit from the resulting surge in property prices.
The site around the iconic power station is substantial. To the left of the launch shaft is the Battersea station box, beyond that is the junction of the South London Line and Brighton main line. The huge regeneration project currently underway around Battersea, Vauxhall and Nine Elms – dubbed Dubai-on-Thames – will rely on the Northern line extension (NLE), and the TBMs are now operating 24 hours a day, seven days a week, to deliver it.
Articulation joints
During a tour of the worksite, Jonathan Cooper, the project manager overseeing the tunnelling phase, explained the progress that had been made following the launch of the TBMs in March this year. These machines, supplied by French manufacturer NFM Technologies from its factory in Le Creusot, are slightly smaller than those used for Crossrail’s tunnelling programme. However, NLE’s tunnels will still be wider than the existing Northern line tunnels to allow enough room to install an escape walkway.
From Battersea, the twin-bore tunnel will run beneath the Victoria line at Vauxhall and connect up to the Charing Cross branch of the Northern line at Kennington. Two permanent ventilation shafts will also be constructed at Kennington Park and Kennington Green.
As well as running close to the Victoria line, the NLE passes just a couple of metres below the South West Storm Relief Sewer, which is located between the two new stations. As part of the preparatory works for the route, Amey installed eight articulation joints along a 55-metre segment of the sewer to allow it to flex as the TBMs pass beneath.
Helen is now well on her way towards Kennington with Amy following on behind. Excavation of the station boxes at Battersea and Nine Elms is well advanced. Later this year, both TBMs will stop short of the Kennington loop and the final breakthrough will take place during a closure of the Northern line at Christmas.
Learning lessons from Crossrail
Tunnelling programmes are challenging engineering feats, particularly when historic records prove unreliable. In one case, a water well was discovered in the path of one of the TBMs, 10 metres away from where the plans showed it could be. In this kind of scenario, TfL and its contractors have to work quickly to get the tunnelling programme back on schedule and try to minimise any delays to the rest of the programme.
The London clay Helen and Amy are wrestling with is also very similar to the ground conditions that Crossrail’s engineers had to overcome. Indeed, Jonathan said the NLE project has looked to learn lessons from Crossrail.
Helen and Amy are served by six locomotives which help ferry people and equipment from the launch shaft into the tunnels. At any one time, each TBM will have around 15 operators loading concrete segments into the TBM and overseeing its progress.
The TBMs move at a rate of around 50mm a minute and install around 20 precast concrete rings a day – altogether 20,000 segments will make up the tunnel’s lining. Each tunnel ring is made up of five segments and a key; software used to control the TBM can work out what segment it needs next up to three rings in advance.
300,000 tonnes of earth
One of the most important parts of the entire process is the conveyor system that removes spoil from the site. The earth that is excavated is mixed with water and a foaming agent to make it more malleable and easier to transport.
Barges rather than lorries are being used to move spoil from the site, and TfL estimates that around 300,000 tonnes of earth will be excavated during the tunnelling phase. TfL’s figures suggest that transporting the waste on the back of a truck would have added 40,000 lorry journeys to London’s congested road network. Instead, the spoil will leave site via the Thames where it is being taken to Goshems in Essex, to be used to create arable farmland on a former landfill site next to Tilbury power station – a site that was also used to dispose of some of the material excavated for Crossrail.
2020 and beyond
Both TBMs are due to complete their drives in the autumn, at which point they will be returned to the manufacturer and the fit-out of the tunnels will begin.
TfL plans to begin the station fit-out in 2019 and hopes to complete the extension by 2020.
The NLE is viewed by TfL as part of its ongoing upgrade of the Northern line, which has included retrofitting the line with CBTC signalling and upgrading stations including Tottenham Court Road, Bank and Camden Town.
Beyond 2020, there are those who would like to see the NLE taken all the way to Clapham Junction, connecting one of the country’s busiest stations to the London Underground network.
Although it hasn’t been ruled out, TfL has said a further extension wouldn’t be viable without Crossrail 2, which includes Clapham Junction on its proposed route map. A TfL consultation document for Crossrail 2 indicated that any further extension of the Northern line would rely on the project going ahead. Otherwise, the result could be even more congestion on Northern line services.
To give an idea of scale, Crossrail 2’s tunnels will be 10 times as long as NLE, which could well end up being the warm-up act for London’s next mega project.
This article was written by Marc Johnson
