Two EPBMs of 6.04m o.d. are procured from NFM Technologies for excavation of the twin running tunnels that will extend the London Underground Charing Cross branch of the Northern Line 3.2km further south west to Battersea. Selection of the French-based manufacturer breaks a long cycle of success in London for Herrenknecht, which supplied all eight machines for Crossrail as well as machines for the Lee Tunnel and the London cable tunnels

The extension – together with two new cut-and-cover underground stations at Nine Elms and the terminus at Battersea, south of the Thames – has been long in the planning, but funding the estimated £1 billion cost from the public purse has always been a stumbling block.

With planning permission granted by Wandsworth Council for a huge residential and commercial development in Battersea comprising 16,000 new homes, as well as office and leisure facilities that will create up to 25,000 jobs, agreement has been reached with the developer of the site for a ‘planning gain’ contribution of £250 million towards the project.

The Greater London Authority, funded by the UK government, will use public money to finance the balance of the capital and financing costs – which will be clawed back to the public purse under a specially set up scheme that will enable the local council to set aside a proportion of business rates that it collects each year to pay back money from the public purse.

A public inquiry to consider TfL’s Transport and Works Act Order (TWAO) application to extend the Northern Line was completed in December 2013 and a final decision was made by the UK Secretary of State in Autumn 2014. A TfL spokesman said: “The proposed start of construction for the Northern Line extension is Spring 2015. Subject to funding and permission to build and operate the extension, the new stations at Nine Elms and Battersea could be open by 2020.”

Fig 1. 3.2km Northern Line extension to Battersea

Selection in July 2014 of Laing O’Rourke/Ferrovial Agroman as preferred bidder by TfL, the operator of the London Undergound, advanced the project significantly. The joint venture beat off a rival bid from Bam Nuttall/Balfour Beatty in the third and last negotiation stage of the bidding process. Two other joint ventures – the Bechtel/Strabag JV and the Costain/Dragados/McAlpine JV – were eliminated at the detailed evaluation of proposals stage.

Guide design for the project, upon which project owner Transport for London based its bid evaluations, is by the owner’s retained engineering consultant, Halcrow, now part of CH2M-Hill. TunnelTalk has been able to examine detailed documentation presented to the planning inquiry by Halcrow’s Technical Director of Tunnel Engineering, Jonathan Gammon.

Two EPBMs of approximate diameter of 5.8m were originally specified for excavation of the twin running tunnels between a new underground station at Battersea on the alignment’s southwestern end, through a new underground station at Nine Elms, ending at separate permanent ventilation and emergency access shafts that will be constructed just short of either side of the Kenington Loop (Fig 1). The loop is the turning circle tunnel that currently forms the end of the Kenington branch of the Northern Line. The TBMs will be extracted via the 13.5m diameter shafts.

The most complex part of the project is creating the link with the Kenington Loop, which will be achieved using the step plate methodology. In order to form a connection between the end of the TBM drives and the northern and southern sides of the loop, SCL construction will be used. In order to minimise settlement around both the SCL headings and the step plate junction, excavation of two gallery tunnels of 3.5m i.d. will be constructed from each of the access shafts, back towards the loop, parallel to the SCL alignment (Figs 2 and 3). From these gallery tunnels compensation grouting will be applied as well as groundwater control measures.

Sprayed concrete lining (SCL)

Spheroid graphite iron (SGI) lining

By excavating in this manner disruption to existing services on the Northern Line can be kept to a minimum until the final stages of construction. At locations where mixed soils are encountered, or in situations where groundwater can freely enter excavation, use of spheroidal graphite iron (SGI) linings is proposed as an alternative to SCL. The linings are placed in bolted rings, made up of segments formed of SGI. Although similar to precast concrete rings by way of their appearance inside the tunnel, their strength and their weight place them at an advantage where access or the use of equipment is limited or where the geometry of the lining is complicated.

The TBMs will launch from the northeastern end of a 120m long x 22m wide x 15.5m high crossover box to be constructed using the SCL method adjacent to the cut-and-cover diaphragm wall construction of Battersea Station. Additionally, at the far end of the station, two run-on tunnels are to be excavated, also using the SCL method. These will act as an emergency length of run-off track away from the station platforms, as well as affording storage space for trains that may need to be temporarily taken out of service for mechanical reasons.

The alignment runs mostly through London Clay, at an average depth of about 20m. Spoil from the running tunnels and from the excavation of the station boxes will be transported via a covered conveyor belt system at Battersea to the nearby River Thames, for onward disposal by barge. Disposal of the spoil from excavation of the Kenington shafts will be via road.

The i.d. of the running tunnels was initially designed at 4.75m, but had to be widened to 5.2m to accommodate a walkway access for emergency crews and passengers in case of emergency. By doing this TfL has been able to dispense with a previously planned permanent emergency shaft half way along the alignment.