Contractors have urged Transport for London (TfL) to remain open to the idea of using immersed tube technology for the proposed £600M Silvertown road tunnel in east London.
Project promoter TfL is currently working up designs for the four lane tunnel following a public consultation earlier this year.
Contractors fear that if TfL takes the design too far it will limit the ability for them to innovate, particularly if the client rules out an immersed tube tunnel.
It is understood that a bored tunnel is preferred because it is a more proven technology in the UK and in London specifically. All tunnels under the Thames to date have been bored.
The last major immersed tube tunnel built in the UK was the second Tyne crossing in Newcastle. Previous schemes in the UK - the Medway Tunnel and the Conway Crossing also used immersed tubes successfully.
The project was a success but replicating this in London is complicated by site constraints. The tunnel is on a tight bend in the river Thames which would make shipping movements around the site difficult and there is no suitable nearby site for a reinforced concrete tunnel section casting yard.
But contractors are eager for the option to be kept open at this stage. The ground conditions and relative short length of the tunnel would suit immersed tube construction, they say.
“It may well be that a bored tunnel would come out as best, but for us, we think immersed tube is the way forward,” said one contracting source.
“What we don’t want is for it to be ruled out before contractors are engaged.”
Contractors are currently forming teams to bid for the project ahead of an expected prequalification notice next year. TfL hopes that the tunnel will open in 2021.
The tunnel is almost certain to be bid as a privately financed project, with payment coming through tolls from the new tunnel and the adjacent Blackwall tunnels.
The project is being worked up to cut delays at the Blackwall tunnel by improving connections and offering alternative crossing options.
The tunnel would pass under the Thames inside an area of land that has been safeguarded for this purpose. Unlike the Blackwall tunnel, which would carry most longer-distance traffic, the Silvertown tunnel would carry more local traffic as well as vehicles which are too tall to use the Blackwall tunnel.