Details of the tendering deal have yet to be finalised, but for the contractors who take on the job the risks involved in the pounds 80m project will be high.
After 70 years of scouring since it opened in 1926, only a few metres lie between the riverbed and the decaying roof of the Northern Line tunnel beneath Hungerford bridge. London Underground said it was concerned with the roof and "the whole structural integrity of the tunnel".
A few metres deeper lies the Bakerloo Line river tunnel built in 1906. The ingress of water in both tunnels was diplomatically understated by an Underground spokeswoman as "more than we would like". Water, however, affects the whole Tube system with 3 million gallons being pumped out every day.
Another repair, when planning permission is granted, will be on Brunel's Thames tunnel. Built in 1843 and a listed structure, it "desperately needs work", engineers say. London Underground admits it has operated an only-when-something-goes-wrong fix-it policy for decades. A spokeswoman said: "Everything has an expiry date. Now the time is really up."
Preparing the subterranean river crossing for the next 100 years will involve closing off the Thames tides using two huge, open-topped steel cells to enable work to be carried out in still conditions. Divers operating inside the cells will lower reinforced slabs down on to the riverbed. The arched tunnels will then have their sides and roof encased by the slabs.Reuse content