OVERLOADING of the Gerrards Cross air rights tunnel crown caused a section of the structure to fail, Network Rail confirmed this week.
A Network Rail spokesman said that investigations by tunnel contractor Jackson had proved that the collapse was caused by overloading the crown of the vault.
'For whatever reason, not enough fi l 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 Network Rail's London-Birmingham 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.
Fill operations on the £20.3M project were approximately two thirds complete when a 30m section of the new tunnel collapsed at 7.30pm on Thursday 30 June (NCE 7 July).
It is unclear who was directly responsible for the collapse.
Network Rail said that determining who was responsible was down to the Health & Safety Executive, which is still investigating.
Jackson removed 32 damaged elements immediately after the collapse and took out 14 additional concrete segments a fortnight ago because they did not respond as expected when fill was removed from above the crown (NCE 4/11 August).
Network Rail said that engineering reports by Tesco indicated that the remaining tunnel was in a safe condition and that work had started to repair damaged track and signalling.
Train services were expected to resume by the weekend as NCE went to press.