A TRANSPORT historian believes the "Tesco tunnel" collapse was caused by saturated building material becoming too heavy for the structure.

John Healy, from Chesham, said Incinerator Bottom Ash (IBA), used to cover the tunnel's metal skeleton, may have become saturated with rain water and caused the collapse.

Mr Healy likened the fiasco to the Welsh Aberfan disaster in 1966 when 144 people died after a coal slag tip became waterlogged. It slid down a hill and engulfed a school.

Mr Healy explained that IBA has similar properties to slag, a waste product from mines, and is prone to becoming unstable.

He said: "I think the IBA mixed with water became unstable. It was lucky that it happened now, before the store was finished.

"This is something of a failure. It is a lesson that should have been learned from a disaster that happened 40 years ago in Wales.

"One remembers the disaster of Aberfan, we know what happened there. There may be an element of divine retribution for the protesters mixed in as well."

The type of tunnel used at Gerrards Cross is known as a "cut and cover" and has been successfully used for many years.

A top structural engineer confirmed that they are extremely reliable and not "potential death traps".

John Lane, a member of the Institute of Civil Engineers, said the method was reliable and that accidents were rare.

He said: "Provided it is designed perfectly there is absolutely no problem. If you think about a tunnel under the Alps, or a tunnel under the Severn or the Channel Tunnel, they are subjected to pretty significant pressure so there is no problem with that.

"The only other factor is if there is an inaccurate design."

Mr Healy says tunnels have been successfully used in this area and are still standing.

He said: "This is not the first time that the railway has used a cut and cover method of tunnelling.

"When the railway was built just over 100 years ago, Lords Cricket Ground was taken up and a tunnel was built over the ground.

"The cricket pitch was then relaid. This is all still standing, it has never collapsed.

"This one is still standing because it was built properly."

Mr Lane confirmed that: "History has shown the cut and cover technique to be reliable and capable of dealing with the pressures bestowed on it."

In its simplest form, cut and cover is where a trench is created and roofed over, with strong beams supporting the tunnel to ensure it doesn't collapse.

Another engineer told the Free Press that when problems did arise, it was often because workers had not interpreted the developer's plans accurately.

However, the engineers building the "Tesco tunnel" believe that water is not to blame for the collapse.

A spokesman said: "Any structure in this country needs to be fully designed to cope with the weather conditions. It certainly wasn't a factor from a design point of view."

Read more:Tesco Tunnel Collapse