New London cable tunnel in the bidding 04 Jul 2019

Karen Martin, TunnelTalk

Design-build construction of the latest cable tunnel under London is scheduled for award in November 2019 and start of construction on the six-year project in 2020. Running 32.5km from Wimbledon in the southwest to Crayford in the southeast, the new tunnel is £1 billion Phase 2 project by National Grid to rewire the capital. It follows successful completion in 2018 of a £1 billion seven-year London Power Tunnels Phase 1 project and adds further to more than 200km of electricity cable tunnels under London since the first was built in 1992.

Route of the new 32.5km cable tunnel
Route of the new 32.5km cable tunnel

The 3m and 3.5m diameter tunnels will run between 10m and 63m deep with most of the alignment at about 30m deep between the existing Phase 1 substation at Wimbledon to a existing site at Crayford (Fig 1). The project will replace three sections of existing power transmission cables that currently run under the surface of local roads. These are:

  • A 12km section from Wimbledon to New Cross, which is expected to be operational by 2025
  • An 18km section from New Cross to Hurst, to be operational by 2026; and
  • A 2.5km section from Hurst to Crayford, scheduled to be operational by 2024.

Intermediate shafts are required at key points along the route with permanent access headhouses required on the shafts at:

  • Kings Avenue in Brixton;
  • New Cross, on and adjacent to an existing substation on the Old Kent Road;
  • Kidbrooke;
  • Eltham, on land owned by UK Power Networks;
  • Hurst, at the existing Hurst substation; and
  • Crayford, close to an existing National Grid site.

Works at the connecting substation site with the Phase 1 network in Wimbledon will involve using the existing headhouse and shaft, and preparing it to receive a Phase 2 TBM from the shaft site at Kings Avenue, Brixton. National Grid intends to build the tunnels and shafts under its permitted development rights, with planning permission required for the headhouses on the shafts not located on National Grid operational land.

Phase 2 continues successful construction of the 32km long Phase 1 project
Phase 2 continues successful construction of the 32km long Phase 1 project

A shotlist of contraction groups believed to have submitted bids a month ago for the project are

  • the Balfour Beatty/Ghella JV
  • a Morgan Sindall/BAM Nuttall JV
  • a JV of Murphy/Hochtief.

    Phase 1 of the project was the first major investment in the high voltage electricity delivery system for London since the 1960s. The project, constructed by the Costain/Skanska JV, runs 32km from Hackney in the northeast to Wimbledon in the south with connections to Willesden and with Kensal Green in the northwest (Fig 2). As well as passing under the route existing underground infrastructure including water and sewer tunnels, and the London Underground and Crossrail transport networks.

    The 3m and 4m diameter tunnels pass at between 20m and 60m below street level, and they host almost 200km of 400kV cable. A series of 14 shafts were sunk along the tunnel route, allowing easy access and ventilation of the heat generated by the cables. Four existing substations were connected to the Phase 1 tunnels with two new ones added. The substations transform the primary 400kV voltage to a lower 132kV or less voltage for passing on to the local network company and delivered to consumers. Phase 1 resulted in 10 new underground transmission circuits that currently carry up to 20% of the power needed in the capital. A significant achievement on Phase 1 included recycling up to 98% of the excavated material.

    In preparing for Phase 2 of the project, National Grid is one of four early adopters to begin implementing the new Project 13 approach to delivering infrastructure. Launched by the Institution of Civil Engineering, Project 13 proposes a different model to deliver and manage infrastructure. It is based on an enterprise, rather than a traditional transactional arrangement, and provides the possibility to draw on best practice from across the industry in order to boost certainty and productivity in delivery, improve whole life outcomes in operation, and supports a more sustainable, innovative, highly skilled industry.

    New tunnel will house high voltage electricity cables
    New tunnel will house high voltage electricity cables

    All contracts for Phase 2 of the project are based on the NEC4 design-build terms and conditions.

    The new network of Phase 1 and Phase 2 tunnels for National Grid are needed to replace existing electricity circuits, which are coming towards the end of their useful lives. The majority of these currently run beneath the road network. In addition, electricity consumption in London is rising year on year and more supply is needed to ensure a safe and reliable transmission network to support current and future needs. Most electricity is transmitted through cables installed just beneath the road surface and work to maintain them is by open trench work in the road. A major advantage of housing new electricity cables in man access tunnels is that future repair and maintenance can be carried out without disrupting traffic, businesses and residents.

    Many countries around the world have come to appreciate the advantages of the increased investment and have followed the UK lead in placing these vital public services in secure man-entry cable tunnels.

References

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