MYSTERY surrounds why the "Tesco tunnel" dramatically collapsed with what could have been devastating consequences.

The 320-metre long concrete tunnel was built over a cutting to create land for a new 35,000 sq ft supermarket and car park.

Structural engineers, tunnel specialists and the Health and Safety Executive are now investigating what caused the 30-metre long section to give way.

One rail worker at the site suggested the recent bad weather may have been the contributing factor.

"The extra weight of all that rain may have caused the earth to give way," he said.

"It takes about a minute for trains which have stopped at Gerrards Cross to pass through the tunnel by the time they have gathered enough speed. It's pure luck that this wasn't a much more serious incident," he added.

The tunnel, or concrete vault, is made up of more than 300 steel-reinforced "ribs" which are around 350mm to 400mm thick.

A hinge is located at the crown with pulverised fuel ash used as part of the backfilling operation.

Whatever the cause, the thousands of tonnes of rubble and concrete which fell blocking the track and could take daysand maybe weeks to clear.

Robin Gisby, Network Rail director of operations and customer services, said: "Our engineers have an enormous task in front of them to restore services on the line.

"We have now taken charge of the site and will be working night and day on the task, with several thousand tonnes of soil to be cleared."

Tesco, and their building contractor Jackson Civil Engineering. could face a huge compensation bill from Chiltern Railways, Network Rail, and passengers.

A spokesman from Jackson said: "Prior to the collapse, there was no indication of a problem and at this stage it would be inappropriate to speculate as to why this happened."