How long will the steel tunnel wall on the deep level underground railway in London last?

Every Evening after the Tube closes down, engineers walk the line not just chesking the track, but also the tunnel walls. If there are any fault spotted then it is reported and the engineers come in as soon as possible to repair them. If a tunnel fails then its does not get a daily safety certificate meaning that no train can run through it. In answer to your question though...If the tunnels were not inspected and everything was left to nature. The life of the tunnel would depend on the nature od the ground arround it, so say it goes through mudstone and there was a lot of water seepage then the tunnel could last about 5-20 years, before defects in the metal started to weaken and then it would be about 20 years onwards before the tunnel fail. Do not worry though as the tunnels in the underground network are inspected daily!

They aren't actually walls - they are steel rings, forming the tunnel. They are regularly checked and were any to be showing signs of corrosion and possible failure they could be removed independently. They will not all fail at once, if they fail at all. At the present time vast sums are being spent - probably not enough and not soon enough, I agree - on maintenance and updating of the whole network. It is an enormous job - one of the reasons Metronet went belly up in the summer. The Northern Line, that you specifically mention, was not included in the Metronet portfolio, but is the responsibility of Tubelines. A map is available at http://www.tubelines.com/explorer/tubelines_map.pdf showing the work being undertaken on the Northern (amongst others)

Like others have said, there's a rolling program of inspection, maintenance and repair, nothing's left to collapse through old age. As a matter of interest, about 50 miles away from here is a wrought iron railway bridge put up by Brunel & opened in 1859. It is said to be in much better condition that the modern road bridge opened alongside in 1961. The Victorians knew a thing or two about massive engineering, when they built something it was meant to last for centuries.

never, they will always be able to upkeep them with modern methods, my god father used to be architectural maintenance for London underground and he has retired after 45 years in doing so. He reckons they will outlive me and you

2 YEARS, 27 DAYS, 9 HOURS, AND 14 MINUTES.