Crossrail has overcome many complex challenges and is now about to begin its trickiest phase as contractors prepare to go under the River Thames. Mark Hansford reports.
Crossrail tunnelling contract C310 is unique for many reasons. It is the only contract to feature a tunnel under the River Thames. It is the only contract to use mix-shield slurry tunnel boring machines (TBMs). And, significantly, it is the only contract to feature a portal as well as a tunnel.
More of that later. First an overview. The contract involves the Hochtief/Murphy joint venture driving a 2.6km long twin bore tunnel from its launch portal in Plumstead, south east London, and under the Thames to its reception portal in North Woolwich, just south of London City airport.
Tunnelling is in two distinct phases, with the first phase - from Plumstead portal to a station box at Woolwich - completed last month. The next phase, which began this month, will see the tunnel dip beneath the Thames from Woolwich to the North Woolwich portal.
This phase is the most complex, and arguably the highest-risk tunnelling operation on the whole Crossrail project.
“It is the biggest tunnelling risk on the whole project. So we need to be well prepared”
Andreas Raedle, Hochtief/Murphy
”It is the biggest tunnelling risk on the whole project,” says Hochtief/Murphy technical & risk manager Andreas Raedle. “So we need to be well prepared.”
But the project team takes confidence from the first phase of tunnelling, which finished on schedule and caused a minimum amount of settlement.
“This tunnel was almost the test run for going under the Thames,” adds Hochtief/Murphy construction manager Andy Ingram.
“The average advance was 15m a day which is very good and what we had planned,” notes Raedle. “But you always plan in a tender to be quite optimistic. So we are happy.
“The settlement was also very good at only 20% of predicted settlement,” he adds, “which proves the value of the mix-shield TBM. You get great control of the support pressure.”
This was critical with the tunnel dipping below listed buildings near the Plumstead portal and the Docklands Light Railway running tunnels near the Woolwich station box.
The team opted for two Herrenknecht mix shield slurry tunnel boring machines as it felt they would be best able to deal with the combination of gravelsand chalk that would be encountered.
Direct labour rules at Woolwich
Use of directly employed labour is a passion of Hochtief/Murphy construction manager Andy Ingram and is a major feature of the contract.
It also fits with Crossrail’s stated ambition of training up 3,500 people over the course of the project. It means there are 250 directly employed tunnellers and concreters out on site and another 100 or more trained-up staff in the offices. Around 80% to 85% are from the UK and 15% to 20% are from Germany through Hochtief.
“By directly employing it means the turnover is low and then you start to build a culture - one of quality, safety and high productivity,” explains Ingram. “We built up quite a team from the advance works, particularly in the ground works, and trained them up as tunnellers. It takes effort on the management side but the rewards are there.”
Ingram is a Murphy man, where directly employing staff is in the blood. But direct employment also fits with Hochtief’s company culture, says Hochtief/Murphy technical and risk manager Andreas Raedle.
“In tunnelling in Germany we would usually directly employ,” he says. “We benefit by having our culture and team ethos embedded in the workforce. Most importantly it means we deliver safely and higher quality. We take pride in that.”
“We made the decision for the good of this project that if someone is shown the way to do something properly they wouldn’t do it any other way,” says Ingram. “So you get no cracking of the concrete, and you get no leaking in the tunnel. Now, we have a long-term strategy to not just do this project but other
Mix shield technology is more expensive but in overall scoring of risk it outperformed the alternative earth pressure balance machine option.
They are also the same type of machines as used by the same joint venture on High Speed 1’s Thames Tunnel. Crossrail’s tunnel is a similar length and has similar geology to the HS1 tunnel, built between 2001 and 2004. Crossrail’s Thames Tunnel will be about 15m below the existing river bed.
Each of the Crossrail machines cost £8.5M, so it is perhaps surprising that they will never run simultaneously. While one is boring, the other is either being moved or being prepared for action.
But it is worth it to keep the programme moving along, explains Raedle.
“We have two TBMs but we only run one at a time. It would be beneficial to run them in parallel, but we do not have sufficient power and water at Plumstead to do so,” he explains. “But having two allows for a quicker switch over.”
Subsequently the Slurry Treatment Plant (STP) was only designed to be working for one running TBM.
So while complex, a combination of track record, good planning and good progress makes the team quietly confident about the tunnelling works. That good progress was built on a strong start - and that brings in Plumstead Portal.
Construction of the 433m long portal, tucked away in south east London between the high security Belmarsh Prison and Network Rail’s hugely importantNorth Kent Line, has without question been the most challenging part of the job for the Hochtief/Murphy construction joint venture so far.
“It took over eight months to develop a workable solution with Network Rail to enable the piling to commence as we were outside the standards”
There was little room for manoeuvre close to the North Kent Line, where trains run every three minutes and the stopping distance of passenger trains is 350m from the driver engaging the brakes.
“The proximity of the portal works to the North Kent Line has always been the biggest risk on the project,” says Raedle.
And Ingram adds: “There was a risk that Network Rail would force us to do a lot of our work in track possessions, which would have been a major issue.”
That’s because the portal footprint is less than 3m away from the live railway line and Network Rail regulations prohibited any activities that could potentially lead to something falling on the line.
“Piling so close to the railway has never been done in the UK before,” notes Raedle.
“It took over eight months to develop a workable solution with Network Rail to enable the piling to commence, as we were outside the standards.”
That solution was to have four piling rigs operating on a specially constructed reinforced concrete platform with five cranes on permanent standby to support the rigs, should they threaten to topple onto the railway.
Meanwhile, three full time protection staff were on the railway line, communicating with the crane supervisors.
Crossrail nears the halfway point
Crossrail currently has over 40 active worksites throughout London and the South East with work hitting its construction peak between now and 2015.
The total funding envelope available to deliver Crossrail is £14.8bn. The route will pass through 38 stations and run more than 100km fromMaidenhead and Heathrow in the west, through 21km of twin-bore tunnels below central London to Shenfield and Abbey Wood in the east.
When complete, Crossrail will increase London’s rail-based transport network capacity by 10%, supporting regeneration and cutting journey times across the city. Crossrail services are due to begin through central London in 2018.
Network Rail was convinced, and work was able to progress. The portal was built by HMJV with piling works subcontracted to Cementation Skanska, one of the few things on the job not tackled by directly-employed labour (see box).
The four rigs were put to work to create the box for six months from late 2011 to mid-2012.
At its shallowest, as the line begins to dip below ground level, continuous flight auger piles were used.
After 60m these give way to a 180m section of up to 18m deep secant pile wall, before these give way to a 193m section of up to 22m deep diaphragm wall, beefed up around the portal itself with tension piles and rotary bored piles up to 38m deep.
Two levels of propping, with a 400% factor of safety, were then installed as the box was excavated. An impressive 1,400t of steel was installed in total, with some of the lower props in place for less than four weeks before removal in what Raedle describes as a “very complicated” decommissioning sequence, made all the harder by the limited crane coverage.
It all worked seamlessly. “There were no major incidents and the approach is now seen as best practice,” says Raedle.
“North Woolwich is also a very congested site and we are on the approach to the Woolwich Ferry”
Andreas Raedle, Hochtief/Murphy
Network Rail has now updated its standard and lessons learned have been taken forward to the North Woolwich portal design.
“North Woolwich is also a very congested site and we are on the approach to the Woolwich Ferry,” says Raedle. It is currently being excavated and that work needs to be handed over by the end of October in time for the first TBM to arrive in mid-November.
The early work done at Plumstead Portal was enabled in large part by Crossrail’s use of an optimised contractor involvement (OCI) procurement phase - a tweak on the Highways Agency’s early contractor involvement arrangement.
Raedle is a big fan, with the time phase also allowing time for Hochtief/Murphy to make changes to the design and construction methodology of the tunnels too.
“OCI was very good,” says Raedle.
“We managed to put our ideas in to have safer, more efficient construction methods.”
This included changing the shape of the concrete lining segments to speed installation and changing the construction methodology for the cross-passages, taking out steel girder sections and replacing them with reinforced concrete elements.
“OCI was a good element in the process and there should be more time for it,” reflects Raedle.