Crossrail looks set to abandon the ill-fated bespoke conveyor and hopper muck away system that catastrophically collapsed at Westbourne Park near Paddington last month.
Instead it will use a more reliable manual process using a front loading shovel.
Speaking exclusively to NCE this week, Crossrail programme director Andy Mitchell said that provided the manual muck away system could load rail wagons and keep pace with tunnelling, he “would take some convincing” to return to the original hopper system.
“It is a medium-term solution but in reality, once we have a robust solution there is a real question over whether we would want to return to the original conveyor system,” he said.
The two tunnel boring machines (TBMs) on the Western Running Tunnels ground to a halt last week after the contractor ran out of room on site to stockpile arisings. The first restarted on Monday this week and Mitchell said he expected the second TBM to restart next Monday.
Mitchell added that by this time the current temporary operation to muck away by road would be replaced by a permanent solution using the railway.
“We have been mucking away by road for the last five days to create a sufficient buffer [of stockpile space],” he said.
He added that his priority was to create sufficient space on site to ensure that “once tunnelling restarted it would not have to stop again” due to lack of storage space on site.
But he said putting more lorries on London’s roads was undesirable, he said.
“The rail muck away system will be up and running a week on Monday,” he said, speaking to NCE last week. “We have already cleared enough space on site to keep the first TBM running all week [even without a road muck away operation]. We are now building bins on site to allow front loading machines to load the rail wagons.”
Mitchell also said that, despite losing around three weeks following the collapse, he was confident the project would be back on its original tunnelling programme by March next year.
“In the big scheme of things there should not be any impact on our programme. Of course it would have been better if it had not happened but we still have capacity and I believe that by March we will be back on the original programme,” he said.
120m of tunnel a week
He also pointed out that the TBMs were capable of running at 120m a week against a programmed average figure of around 100m.
Mitchell added that calling a halt to the TBM work had not been a total waste of time as the contractor had been able to use the opportunity to install a California crossing - or temporary passing place - behind one TBM. This will speed supply train operations in future and its installation, he said, would have required the TBMs to be shut down at some point in the programme.
Crossrail’s technical investigation into why the hopper for the original spoil conveyor system collapsed is still on-going. Mitchell said that so far it had not identified any obvious faults with the system. He added that metallurgists were now examining the bolts to try to shed some light on the problem.
And although Mitchell’s team is closely involved with the recovery process and finding a solution to protect the programme, he pointed out that responsibility for the failed equipment rested squarely with contractor BFK, comprising Bam Nuttall, Ferrovial and Kier.
“It is a performance specification,” said Mitchell.
“The conveyor system was provided by a specialist company and used standard designs and configuration. But it is the contractor’s issue - while we heavily specified the TBMs, the conveyor system is not something that we should have specified.”
Mitchell said the priority remained getting the tunnelling back up and running. Finding a solution for the muck away to enable this to happen was, he said, a major step forward.
He also played down the potential for knock-on impacts of the hopper collapse on other tunnelling worksites
“We are in the process of building the muck away solution for the Limmo [Peninsular] site,” he said, referring to the east London launch site for TBMs which will tunnel towards Canary Wharf and Farringdon (see right).
“Once we have the TBMs lowered in this weekend it was always the plan to get those conveyors up and running by January. It is not the same company supplying the systems and other than the fact that they both use conveyors they are very different.”
Mitchell defended the contractor’s original decision to use the complex conveyor and hopper system to load trains.
“In hindsight - and that is always easy - it still made total sense [to use the conveyor system],” he said, pointing out that the system had been working well and, at the time of the collapse, was handling muck direct from the TBM rather than from the stockpile.
“There was nothing unusual about the operation at the time,” he added.