“The existence of the underground railway which cuts through the site is disturbing both from a visual point of view and because of the noise it causes. It has long been accepted that it must be covered over if a residential precinct is to be created.”
Chamberlin, Powell & Bon, Architects “Barbican Redevelopment” April 1959
A major practical problem for the construction of the Barbican Estate was that four tracks of railway lines crossed the site in an open cutting. Two were used by London Underground for Circle and Metropolitan lines; and two were used by British Rail for suburban diesel trains for the Eastern and London Midland region of British Rail. The lines ran in a long arc between (what was then called) Aldersgate & Barbican Station and Moorgate Station.
It was necessary to enclose these lines and run them underground. Consideration was given to simply building over them. But that was abandoned because of the condition of the brick retaining walls, which were already a hundred years old and had been damaged by bombing during the war.
In the end, someone came up with the very clever scheme of building a completely new set of rails between Moorgate and Barbican stations. That way, there would be no need to interrupt any rail services except at the very end of the project when the new lines were joined at Barbican and Moorgate. The old rails could then be taken out of commission and absorbed into the Barbican building process.
A completely new set of lines was constructed between the ends of the existing curve, like the string of a bow, so service was only disrupted when the ends were finally joined.
Another reason for this plan was that the new rails could be constructed on with the view to being reducing noise and vibration. Structure-borne vibration was minimised by a specially devised track support isolated from the main structure. The four tracks were laid on a multi-span bridge, consisting of a continuous plate deck 305 meters long, formed from pre-stressed concrete beams with an in situ concrete deck. The 150 mm depth of the deck is separated from the main structure by a layer of rubber bitumen, to further improve the damping characteristics of the deck. The bridge itself was placed on a transverse beam system. This consisted of two U-shaped beams, the upper one inverted, so that the legs faced each other. Rubber bearings were placed between the contact faces of the beam legs to damp out all vibration frequencies above 30 hz. There is provision for them to be inspected and the beams jacked up to allow replacement of the bearings when it becomes necessary.
The new tunnels were built with dense concrete walls and a double roof with a large air gap in the centre to reduce airborne noise.
The connection of the new stretch of tracks to the old, was successfully carried out on Sundays without disrupting the rail traffic.