Tunnelling halted for a day as investigation launched into cause of sudden accident.
Overloading is thought to have been the most likely cause of the collapse last week of a hopper on Crossrail’s muck-away conveyor at Paddington, tunnelling experts told NCE this week.
An unexpected build-up of spoil in the hopper is thought to have caused the collapse at 11.30am last Thursday as material from Crossrail’s eastern running tunnels was being loaded into waiting freight train wagons.
The sudden collapse of the steel frame structure supporting the hopper, which transfers tunnel boring machine (TBM) spoil from the conveyor to wagons, halted tunnelling and prompted the one line of the adjacent Great Western Line to be suspended for the day.
Tunnelling experts told NCE that failure or malfunction of the discharge mechanism - known as the tripper - most likely caused spoil to build up in the hopper. This appears to have caused it to fall out of position and result in the collapse of the support frame below.
“The logic is the hopper was either overloaded or under designed - I suspect it was the latter,” said independent tunnel consultant and former British Tunnelling Society chairman David Fawcett. “It’s the first time I’ve seen a tunnel conveyor system collapse - it’s not that common.” He attributed the likely overloading to an operation error.
The collapse happened at Crossrail’s Westbourne Park site near Paddington. The hopper was receiving spoil from Crossrail’s TBM when it collapsed.
Contractor BFK (a Bam Nuttall/ Ferrovial Agroman/ Kier Construction joint venture) immediately halted its two western running tunnel TBMs. One is some 1km into its journey east to Farringdon and other has just begun. Crossrail launched a “thorough and full” investigation.
A Crossrail spokesman was unable to confirm the cause of the failure.
“During the transportation of excavated material from the tunnels to the freight trains, the hopper, which feeds excavated material into wagons at Westbourne Park, tilted into an individual wagon,” he said. ” No injuries resulted.”
Independent safety expert John Anderson also agreed the likely cause was overloading.
“It’s very unusual - normally things collapse within the tunnel,” Anderson told NCE.
The conveyor system runs from inside the western tunnels, to the tunnel portal at Royal Oak and then along the length of the Westbourne Park site.
It is understood that recent problems with the commissioning of the conveyor and loading system forced Crossrail to stockpile spoil from the main TBM muck-away conveyor on site rather than loading straight onto wagons. But, the problems had been overcome and the stockpile wasbeing drawn down with excavators loading the wagons.
The collapse means that the site will now return to stockpiling muck until the hopper and conveyor are repaired. A 500t crane removed the damaged hopper and conveyor section over the weekend.
Fawcett said Crossrail’s conveyor system was “bespoke”, highlighting the fact that any tunnelling in London requires the spoil to be moved by rail prompting the development of the bespoke system.
“Usually you take the spoil away on lorries or narrow gauge railway,” added Fawcett. Crossrail’s system uses standard gauge wagons running on the main national heavy rail network to take the spoil to the Wallasea Island nature reserve in Essex.
He also dismissed suggestion that recent heavy rain could have contributed.
Crossrail was unable to give a date by which the loading system would be up and running again and said only that it was preparing a plan to repair the damage and bring the full conveyor system back into use in “due course”.
It also played down impact on the tunnelling programme. It said work continued over the weekend and would do so for the rest of the week.