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Northern Line Extension Nine Elms to Battersea
Our work focused on finding ultra-low carbon alternatives to Portland cement, which is traditionally used in concrete.
Image credit : Transport for London
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Going further on London’s Northern Line

Some 130 years after our founders Basil Mott and David Hay designed the first deep underground railway lines in London, our engineers delivered a 3km long extension of the Northern Line, with new stations at Nine Elms and Battersea. The Northern Line Extension (NLE) opened in September 2021.

Client
FLO – a joint venture between Ferrovial Construction (UK) and Laing O’Rourke
Expertise
Civil, structural, mechanical, electrical, public health, and rail systems and integration services; architectural design was by our design partner Grimshaw
3km
length of twin-running tunnels
2
tunnel boring machines, called Helen and Amy
20,000
precast concrete segments forming 4000 rings lining the tunnels
300,000t
tunnel excavation waste removed more by up 26 barges a week
845,000t
material excavated overall
25,000
forecast jobs
20,000
new homes

The area it serves, just south of the River Thames and to the west of Vauxhall, was dominated by the vast, derelict, Battersea Power Station and rail yards, and was poorly served by public transport. The NLE was needed to initiate regeneration, unlock investment and breathe new life into the area.

We were lead designer for FLO, the joint venture between Ferrovial Construction (UK) and Laing O’Rourke that delivered the line. We provided multidisciplinary services, including alignment design, civil, structural, MEP, fire, traction power and tunnel ventilation engineering, systems integration, and environmental and planning advice. We brought architect Grimshaw onboard to provide architectural design.

Along with the two new subterranean stations at Nine Elms and Battersea, the project included two major ventilation shafts at Kennington, twin bored and sprayed concrete lined (SCL) tunnels and complex step plate junctions to connect to the existing Northern line, and the provision of all associated rail systems. Construction took six years.

Opportunity

The 3km route was packed with challenges. It passes beneath countless homes and other buildings, including more than 130 listed structures, and crosses below mainline railway tracks and viaducts between Clapham Junction and Victoria, and Waterloo stations. Underground, the tunnels run very close to the existing Victoria and Northern lines, water and gas mains, and electricity and communications cables.

Meanwhile, both new stations support significant over-site development. Complex structural support had to be provided within the station boxes, while providing attractive, passenger-friendly spaces and architectural excellence.

And the junction between the NLE and existing Northern Line tunnel was geometrically, structurally and logistically complex.

We used building information modelling (BIM), possibly more so than on any other transport project to date, to co-ordinate the complex design, achieving safe clearances, avoiding clashes and plan the sequence of construction works. We linked the BIM models to visualisation tools, such as virtual reality (VR) and augmented reality (AR) to enable the client, contractor, user groups and emergency services to explore and examine the design, leading to improvements. Design for manufacture and assembly (DfMA) was used extensively to speed and simplify construction.

Solution

The new station boxes sit 20-25m below ground. To speed construction and avoid extensive temporary works, top-down construction was employed to build them. It enabled the foundations and superstructure to be built simultaneously.

Noise limits for the NLE are particularly strict and we used our in-house ReVERB software to model train vibrations and noise levels to predict their peak and cumulative impacts in residential areas. We validated our predictive models by measuring noise on the nearby Victoria Line, setting a new benchmark for ground-borne noise and vibration modelling.

To ensure the two tunnel boring machines could excavate the twin tunnels without causing damage to nearby utilities and structures we compiled a database of underground assets, representing them in BIM. A virtual construction model of the tunnels, generated from BIM, enabled FLO to hold detailed weekly briefings with stakeholders including Thames Water, Network Rail and London Underground.

We fine-tuned the design, reducing the thickness of the tunnel linings by 30mm, from 280mm to 250mm. This saved about 2700m3 of concrete and 1500t of carbon. We also worked with FLO to design a concrete mix replacing ordinary Portland cement with ground-granulated blast furnace slag from steel manufacturing. This reduced embodied carbon by 80%. Introducing LED lighting for the tunnels and stations produced significant whole-life cost and carbon savings compared with conventional lighting – £2.4M in energy savings and 23,400t of carbon.

Outcome

The new stations have been catalysts for economic and social transformation. Three towers, from 74m to 89m tall, above the station box at Nine Elms contain 479 ‘build to rent’ homes with associated amenity space. A fourth building alongside the station provides commercial and retail spaces. Oversite development above the Battersea NLE station forms part of a mixed-use precinct of residential, retail and commercial units and a new park. Nine Elms and Battersea are new destinations.

The NLE also provides the area’s new residents with swift and easy access to the rest of London.

Overall, the extension is forecast to boost London’s economy by up to £7.9bn, with about 25,000 jobs and 20,000 homes created in the area. Better accessibility and quicker journeys alone will contribute about £1.5bn.

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