“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. The existing route was curved. The new set of lines would be constructed between the ends, like the string of a bow. That way, there would be no need to interrupt any rail services except at the very end of the project when ‘the bow was strung’ – i.e. when the new lines were joined to the service at Barbican and Moorgate, and the old rails taken out of commission and absorbed into the Barbican building process.
Another reason for this plan was that it provided an opportunity to sound-proof the rails. The new Underground lines would be running close to the Barbican Arts Centre and it would not be acceptable for the Adagietto in Mahler’s Fifth Symphony to be interrupted every three minutes by Circle Line trains. It would also affect the rents the City Corporation could expect to obtain from flats bothered by vibrations from trains. Laying new rails on a new site provided an opportunity to take all necessary steps to reduce noise and vibration.
Over Arup and Partners carried out a study of vibration on rail lines and brought in two acoustic specialists as consultants. These were Dr P. Grootenhuis, and Dr Parfitt from the Building Research Station at Watford. Their proposed solution was to mount the railway tracks on rubber bearings which would reduce the amount of vibration reaching the ground and the structure of the buildings.
This is a more technical version. 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 tunnels themselves 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 network.
When the tunnels had been completed – but before the new tracks were connected to the Underground system – the City’s Court of Common Council held a banquet there, thus re-enacting an earlier famous banquet held on completion of the Brunel’s Thames Tunnel in 1843. The City grandees never let an opportunity for a good banquet go to waste.