Fears that Crossrail’s Westbourne Park site in west London will be swamped with spoil have forced contractors to suspend work on one of its running tunnels.
Contractor BFK, a joint venture between Bam Nuttall, Ferrovial and Kier, has been stockpiling spoil at the site for two weeks following the collapse last month of the hopper used to transfer excavated material into railway trucks.
Before the hopper collapse, Crossrail and BFK had been hoping to draw down a stockpile which had built up before the conveyor system had been fully commissioned.
The hopper is designed to drop spoil from the tunnel boring machines (TBMs) and from the stockpile into railway wagons which then take it away. It is designed to travel above and along a line of wagons.
It has emerged that the hopper was commissioned on 11 September just two weeks before it collapsed.
The decision to halt tunnelling on one of the drives came as stockpiling space started to run out.
The hopper collapse has led to a build up of spoil at the tightly constrained site near Paddington forcing Crossrail to suspend TBM2 - known as Ada - until further notice, according to Crossrail programme director Andy Mitchell. The other TBM, known as Phyllis has continued working uninterrupted.
“Stockpiling muck is a major challenge we face,” Mitchell told NCE. “I would prefer a smaller stockpile. It gives me contingency.”
Crossrail initially kept both TBMs running following the hopper collapse, but Mitchell decided to concentrate on keeping TBM Phyllis working.
“While we are working on solutions [for the conveyor system] the priority was to keep Phyllis going,” added Mitchell.
Mitchell said he wanted to keep on tunnelling with Phyllis because it had reached the Paddington station box, east of the tunnel portal at Royal Oak.
Allowing it pass through the Paddington box would allow diaphragm wall work for the station to continue, before Ada reaches it.
Ada has stopped about 150m west of the box.
He added the overall tunnelling programme would be unaffected by the hopper collapse.
Before the hopper was commissioned, spoil from the TBMs had been stockpiled at Westbourne Park and then transferred to freight wagons via excavators (see opposite).
BFK had begun to draw down the stockpile before the collapse and Mitchell said he understood the spoil within the hopper at the time of the collapse was from the stockpile and not directly from the TBMs.
Tunnelling experts told NCE last week the likely cause of the collapse was overloading (News last week). Mitchell agreed that this scenario was “possible”. “I hope to have a clearer understanding of what happened next weekend,” he added.
The collapse has triggered claims that conveyor system staff were inadequately trained.
Mechanical and electrical contractor EIS, which was involved with designing and constructing the conveyor system over 18 months - questioned “the level of training for the operation of such a complex conveyor loading system”.
EIS finished its work with tunnelling contractor BKF in July.
EIS said it could not say whether the accident would have happened if its staff had been on site. The conveyor system was provided by TBM manufacturer Herrenknecht’s subsidiary H&E Logistik.
Crossrail disputed claims that conveyor system staff were inadequately trained. “The conveyor system is operated by BFK with training provided by H&E Logistik,” said a Crossrail spokesman.
Conveyor system brought into use after tunnelling started
The spoil conveyor system at Westbourne Park has been progressively built up since the first Crossrail TBM was launched in May. Tunnelling operations at Westbourne Park are constrained by a narrow site and restricted storage space for precast tunnel lining segments and spoil.
The site is on a narrow area of land sandwiched between the Great Western railway and local streets with the A40 Westway running above and alongside it. It incorporates the tunnel portal at the western end, a tunnel lining segments storage area and the spoil conveyor system, which takes spoil above a railway siding so that material can be dropped into a travelling hopper which then fills railway wagons stationed below it.
Crossrail had originally proposed to stockpile the spoil at the western end of the site, along with the concrete segments, west of the portal.
During design development it was decided to move the stockpile location to the east of the portal above the line of the tunnel to allow more “breathing space” according to a source close the project.
The Royal Oak portal approach is only 10m wide at its narrowest point, so the 7.5m diameter TBMs have had to be launched one at a time. The first TBM was launched in May without the ability to directly load spoil onto the freight wagons.
Initially spoil exited the TBM to a small temporary location within the portal approach. From there it was transferred to freight wagons using excavators.
Later the spoil exited the TBM onto a limited system of conveyors which took it out of the tunnel and up to the stockpile.
The full conveyor system with the hopper was only commissioned after the second TBM had started work in August. At that point, spoil was transferred direct from the tunnel faces to the freight wagons.
BFK has also constructed a loop take up conveyor. This is located to west of the Royal Oak portal and extends the conveyor system in 500m increments as the TBMs progress.
Risk assessment is vital for preventing high consequence site accidents
Last week’s hopper collapse at Crossrail highlights the need for firms to take the risk of a highly disruptive accident more seriously, a leading safety expert told NCE.
Construction Industry Research and Information chairman Alan Gilbertson said the sudden collapse of the hopper at Westbourne Park was an example of a low probability high consequence event that is often overlooked by firms.
“Firms need to ask what could go wrong,” said Gilbertson.
“If the hopper is next to a railway then can it be moved. If not then make it more robust with a bigger factor of safety,” he added.
Part of the Great Western Railway line next to the site was closed temporarily immediately after the collapse, although it has since reopened.
Gilbertson said a robust risk assessment should take into account risks such as overfilling a hopper or a conveyor belt break.
“Risk management training in companies is very poor,” he added.
Gilbertson co-authored Ciria’s Preventing Catastrophic Events in Construction report with Loughborough University and the Health & Safety Executive - last year.
Gilbertson has investigated previous hopper collapses on other projects.
“The issue with hoppers and silos is they are often full to capacity in a way buildings are not,” he said
He added that design codes should take this into account.