OVERLOADING OF the Gerrards Cross air rights tunnel crown caused a section of the structure to fail, Network Rail confirmed last month.
A spokesman said investigations by tunnel contractor Jackson had proved the collapse over the London-Birmingham line was caused by overloading of the crown of the vault.
'For whatever reason, not enough fill had been placed to the sides of the tunnel before material was heaped on top of the crown. The collapse was that simple, ' he said.
Jackson was creating a 320m length of tunnel on the line when the collapse occurred.
It was using fi ll to cover a concrete vault created in a railway cutting to house the track. The aim was to build up the ground around the tunnel to create the site for a Tesco supermarket and car park (GE December 2004).
Fill operations on the £20.3M project were about two-thirds complete when a 30m section of the new tunnel collapsed at 7.30pm on Thursday 30 June (GE August 2005).
It is unclear who was directly responsible for the collapse. Network Rail said that determining responsibility was the role of the Health & Safety Executive, which is still investigating.
Jackson removed 32 damaged elements immediately after the collapse and subsequently took out a further 14 concrete segments because they did not respond as expected when fi l was removed from above the crown.
Network Rail said engineering reports by Tesco indicated the remaining tunnel was in a safe condition. Train services on the line have resumed following work to repair damaged track and signalling equipment.