The original route of the extension was supposed to be from Charing Cross to Aldwych, Ludgate Circus, Cannon St, and eventually through to new tunnels replacing the existing East London line.
An experimental tunnel was built at New Cross in 1972/3 to test the then new Bentonite Shield - the tunnel would later have become part of the extension. Presumably the tunnel is still there. I wonder if it's being used for anything?
Does anyone know anything about this experimental tunnel? Its the first time I've ever heard of it.
The essential things in life are seen not with the eyes, but with the heart. --Antoine de St. Exupery
The purpose of the tunnel was to test a new type of tunnelling machine - the bentonite shield. This type of shield allowed tunnelling to take place in water-bearing ground without having to use compressed air.
At the tunnelling face, slurry was mixed with the ground, and was then pumped back along with the spoil through the shield to a separation plant. Here, the spoil was removed, and the slurry re-used.
The New Cross tunnel was jointly funded by LT and the National Research Design Corporation, and was primarily an experiment, but was built on alignment of the proposed Fleet Line so that it could be used later if necessary. This alignment ran beneath the disused n/b East London Line, parallel to and to the east of Milton Court Road. The length of tunnel driven was 144m, from a hexagonal shaft (19.6m deep) alongside the South Eastern tracks. The machine was delivered to the site on 12 December 1971, and the first drive attempt took place on 24 February 1972.
The method was largely successful, and the amount of surface settlement was much less than by conventional methods. However, the trials also exposed issues with the processes which take place BEHIND the shield, namely the efficiency of removing the spoil. Similar, but much refined, shields were used to dig Contract 107 of the Jubilee Line Extension from Canada Water to Canary Wharf.
As far as I know, the tunnel and shaft remain to this day. The top of the shaft is, I believe, visible as a fenced enclosure next to the South Eastern tracks, on the west side just north of New Cross station. It can also be seen on Google Earth, as a hexagonal imprint just north of the junction of Edward Street/Milton Court Road.