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Tunnelling: Northern link

London Underground’s biggest Tube extension since the 1990s is taking shape south of the Thames, as the Northern Line moves into Battersea

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Two TBMs were launched in Battersea, south London, in February this year to construct 3.2km long tunnels for the Northern Line Extension (NLE) on the London Underground.

There is always a lot of excitement surrounding the start of tunnel construction: the awesome sight of colossal tunnel boring machines (TBMs) being lowered deep underground. There is also the anticipation that, once rested and connected up, they will embark on a relentless journey through virgin earth to create new subterranean passageways. With each machine being ceremoniously named and designed specifically for that tunnel, soil type and the particular logistics of that site, TBMs attract a lot of attention.

Two TBMs were launched in Battersea, south London, in February this year to construct  3.2km long tunnels for the Northern Line Extension (NLE) on the London Underground.

The existing Northern line runs roughly north-south through London and the extension will take its Charing Cross branch beyond Kennington, where trains can turn around on the Kennington “Loop”, to a new station at Nine Elms and another at Battersea Power Station.

The NLE will take the form of two 5.2m diameter tunnels which spur off the Kennington Loop. The extension is the first major addition to the network since the Jubilee line was extended in the 1990s.

It is needed to support regeneration of a 200ha area of land which hugs the south bank of the Thames between Chelsea Bridge and Lambeth Bridge and includes Vauxhall, Nine Elms (home to the New Covent Garden Market and flower wholesalers) and Battersea (with its iconic power station building).

Rampant redevelopment of these areas will result in around 25,000 new jobs and 20,000 new homes.

TBMs Helen and Amy began their six month long drives towards Kennington in late March and early April respectively. Launch of the TBMs was itself nine months in the planning, according to design and build contractor Ferrovial Agroman Laing O’Rourke joint venture. The project has been seriously considered by client Transport for London (TfL) since 2008 when it commissioned the Vauxhall Nine Elms Battersea Opportunity Area Transport Study.

The two 650t TBMs were built by NFM Technologies in France and will drive through London Clay at depths of up to 26m to construct the north and southbound tunnels of the NLE. Tunnel designer is Mott MacDonald.

With TBM tunnel construction being such an automated process, the keener engineering eye is drawn to the part of the NLE where more hands-on methods of construction are being employed.

At Kennington, where the extension connects to the existing Northern line, operatives are literally using spades to mine the final 60m to 65m of tunnel.

“We’ve mechanised processes as far as possible when we are tunnelling away from the railway. When we are working closer to it, we can’t risk machinery hitting [the existing tunnels] so more traditional mining techniques using pneumatic tools, clay spades and timber shoring are used,” says TfL project manager Melanie Barker.

And here is the really interesting part: trains are continuing to run on the Kennington Loop while the spur is being excavated around it.

There is just 25mm of cast iron tunnel lining between the live trains passing every three to four minutes and the tip of a spade.

Tunnel construction here, 17m below ground, involves digging just enough soft London Clay to install timber shoring to hold up the excavation.

This tunnel is then lined using 600mm wide spheroidal graphite iron ring sections and grouted in. The hand dug section advances at about 2.5m a week, compared with the TBMs which sink through up to 30m a day. Mining has just reached the half-way mark. 

The hand dug length of tunnel also accommodates the transition in tunnel diameter from the 5.2m diameter NLE tunnel, the 9.5m diameter cavern as it encloses the Kennington Loop, and the 6.5m diameter tunnel that wraps around the original 4m Northern line tunnel before it connects to it.

Extensive temporary works support the existing Kennington 

Transitions and crossovers

map

Trains on the Charing Cross branch of the Northern Line currently terminate at Kennington where passengers alight and trains turn around on the “Kennington Loop”. 

The Bank branch of the Northern Line continues south from Kennington to Morden. The two new Northern Line Extension tunnels will spur off the Kennington Loop. 

The northbound tunnel will pass through a ventilation and emergency access shaft at Kennington Green and curve past the Kia Oval cricket ground before continuing to the new station at Nine Elms and terminating at Battersea Power Station.

The southbound tunnel makes a similar journey, but passes through a shaft at Kennington Park. The two 14m diameter shafts were constructed using secant piles for their upper portion and SCL lower down where they integrate with the running tunnels.

The route alignment attempts to be as smooth as possible to facilitate a comfortable ride for passengers and enable trains to achieve optimum line speeds.

The TBMs were launched at Battersea in a substantial underground crossover box structure which was long enough to accommodate the complete 100m length of each TBM. The box will contain crossover sections of track to give TfL the flexibility to direct each Northern Line Tube train to different terminating platforms.

Together, the TBMs will excavate over 300,000t of earth and install nearly 20,000 precast concrete segment rings as they advance.

Excavated material will be transported out of the tunnels on conveyor belts, where they will surface in the crossover box and pass directly into barges on the River Thames. From there they will be taken to Goshems Farm in East Tilbury, Essex, to create farmland. Using river transport means that more than 40,000 lorry journeys can be avoided.

 

Nine Elms Station 

nineelms plunge col and insitu stitch

nineelms plunge col and insitu stitch

Top down construction methods have been used for Nine Elms Station

Nine Elms station is a two storey building above ground with four levels of basement. Demolition of the former

supermarket car park site began in March 2015, followed by construction of a 28m wide, 140m long and 24m deep box using 1.5m diameter secant piles which are 22m to 52m long. Intermediate reinforced concrete bearing piles 52m long with 28m long plunge columns create a 9m by 9m structural grid in the basement.

Precast concrete beams up to 1.2m deep will span between the plunge columns and station box while precast concrete slabs, measuring about 800mm at their thickest will complete floor construction. Trains will effectively pass through the third basement level.

Design and build contractor Ferrovial Agroman Laing O’Rourke (FLO) joint venture and station designer Mott MacDonald worked together on the stations for Nine Elms and Battersea Power Station to produce designs that made maximum use of prefabricated components.

Drawing on Laing O’Rourke’s design for manufacture and assembly (DfMA) methodology to speed up and simplify the construction process, the box will be excavated using top down construction techniques using permanent precast concrete beams to prop the excavation.

Temporary steel beams are more commonly used to support excavations of this size where they span between a secant pile wall’s capping or waling beam.

These temporary members would typically be sequentially removed as permanent beams are installed. At Nine Elms, however, the precast beams, will take the role of props during excavation and form the permanent structure thereafter. They will be installed using a gantry beam. A cast insitu concrete “stitch” will connect the cruciform of precast beams where they meet the plunge columns. 

When GE visited the site, piling was complete and excavation had reached basement one level on the east end of the site, while ground beams were still being positioned on the west end. There is extensive monitoring for movement in existing and new structures throughout the project, especially during excavation.

FLO project engineer Channari Penh says that working closely with structural engineers to develop the “stitch” detail between the precast beams and plunge columns using 3D models helped optimise the heavily reinforced connection.

“The insitu stitch gives us flexibility in making the connection between the precast beam and plunge column. Practicing on a mock-up at Battersea also allowed steel fixers to refine installation to achieve a right-first-time fix,” she says.

There are strict tolerances for this connection to ensure loads are distributed evenly without inducing undesirable shear forces.

The junction, plunge columns and bearing piles have also been designed to support future multistorey oversite developments.

Building information modelling (BIM) has been used to develop the construction programme, key construction sequences, such as the top down excavation phasing at Nine Elms, and to extract quantities. Penh and Barker agree that it has also been a valuable visual tool for communicating the project to community liaison groups and to share ideas with the project team.

“The interfaces on this project are what make it most interesting,” comments Penh. “Using the model to visualise the transition between the main tunnel drive and SCL section provided an important starting point for discussions on this complex interface.”

Since the TBMs will drive under the Nine Elms station site, secant piles at either end of the box have been detailed to be shorter. The TBMs will pass though London Clay beneath basement excavation and continue on their journey to the Kennington shafts. The BIM model was used to optimise sequencing so that basement excavation at Nine Elms does not clash with the TBM passing underneath. The tunnel under Nine Elms station will be broken out later to create the platform level.

Nine Elms station’s structure has been designed for temporary construction loads, loads arising from completion of the station in 2020 and for future oversite development. “This posed additional structural design challenges as there were many more critical load cases to consider,” comments Mott MacDonald package lead for Nine Elms station Emma Hale.

The columns which will eventually support the oversite development are noticeably beefier steel members embedded in heavily reinforced concrete columns.

“BIM has made it much more straightforward to see what the structure and architecture can accommodate [to satisfy these different scenarios],” says Hale. “It has also made coordinating services easier.” 

The firm is the lead station designer responsible for MEP and civil and structural engineering.

“Integrating all the systems that allow the Tube to operate and making sure that the structure works around them, makes this project particularly interesting,” says Hale.

She cites ventilation fans, weighing 10t each that must be positioned in the basement where there is sufficient clear space. As a result, they are orientated either horizontally or vertically to suit constraints.

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