Crossrail – Thames Tunnel Portals

Infrastructure

Walsh’s answer to a problem of limited space for machinery, by providing an alternative form of temporary works, was crucial to the success of this project.

The initial design of the temporary works for the tunnel portals was for a closely spaced portal with stiff walers to achieve the very rigorous movement criteria. The Contractor (a Murphy and Hoctief joint-venture) realised that this would not allow for efficient in-situ assembly of the 400m Tunnel Boring Machine (TBM).

Walsh designed a new temporary works solution, opening up the prop spacing using a bespoke system combining large rolled steel sections with fabricated box beams of up to 2m square.

The 1,000 tonne TBM was built in situ, in as few pieces as possible, resulting in very heavy sections being lowered 15m through the propping system. The props needed to be designed for the high potential impact loads for this operations while the overall system needed to be robust enough to withstand the removal of individual elements.

Each portal required 2 levels of propping however given the timing difference of the excavations (the 2 mile tunnel drive took several months) we were able to adapt the Plumstead lower level temporary works for re-use at North Woolwich.

Another challenge was the closeness of the Plumstead Portal to the North Kent Line railway tracks. The whole team worked closely with Network Rail to ensure minimal impact to the train services, with Walsh ensuring minimal deflection of the excavation.

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Key Facts

  • Client: DAM Structures (for Crossrail)
  • Location: Woolwich / Plumstead
  • Year: 2014
  • Summary:
    Temporary works on the portals at either end of the Thames Tunnel. The tunnel enables Crossrail services to Abbey Wood. It runs between Plumstead and North Woolwich and is the only point where the Crossrail route crosses the River Thames.

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