Europe’s largest infrastructure project, Crossrail, is moving closer to its phased opening in May 2017, leading up to full opening in December 2018.

When that happens, Crossrail will connect Reading and Heathrow in the west with Shenfield and Abbey Wood in the east. Crossrail will add 10 per cent capacity to London’s rail network, will serve 40 stations – 10 of which will be new – and will carry an estimated 200 million passengers each year.

This monumental project will have a profound effect on the way millions of people visit or transit through London every day, so it’s all the more remarkable that, for the average man or woman in the average street in the UK, it’s been pretty invisible.

There are exceptions, of course. In 2014 an excellent three-part BBC documentary profiled the project and some of the individuals working on it. And earlier this year the Prime Minister David Cameron and Mayor of London Boris Johnson featured in the news as they celebrated the completion of tunnelling following the final breakthrough at Farringdon station.

The fact that this huge endeavour has caused relatively few ripples is a great source of pride to the teams responsible for its construction, particularly as asset protection has been at the top of the agenda.

Roger Mears, project manager for the C305 eastern running tunnels contract and Greg Reichmann, project manager for the C300/410 western running tunnels, agree that third party asset protection both above and below ground has been fundamental to Crossrail’s success.

Among the many infrastructure obstacles thrown up, Mears cites having to tunnel under the Canning Town viaduct as an example.

Here, the Dragados Sisk joint venture (DSJV) implemented a very sophisticated system of jacking and monitoring to compensate for ground movement.

Existing utilities also posed significant challenges: "We’ve bypassed a number of large diameter sewers by a very few metres so we’ve had to carry out sewer lining operations, which are challenging scopes of works in their own right," said Mears. "Trying to agree what the nature of the mitigation is and then installing it ahead of the TBM passage generates a whole new package of works."

Further west, concerns from "a very important stakeholder" were of an altogether more cultural tone.

"We tunnelled under the Barbican Centre and they were very concerned about the noise impact on their concert halls from both the tunnelling and the subsequent railway operations," said Mears.

"They were quite nervous during the days, weeks and months prior to us tunnelling under, so much so that we understood when the New York Philharmonic Orchestra was going to start performing. Their fear was that concert goers would be able to hear the underlying rumblings of a huge TBM in the gaps between the beats in the music, which would be a terrible disruption."

The team was so con_ dent that noise would be minimal that, in fact, the programme of works wasn’t altered.

"What we did do was change to a different type of track bed both for the construction railway laid behind the TBM and for the permanent works," said Mears. "A section of track slab will be on bearings which will damp down any vibration and prevent noise from the railway operations.

"For C305 we built the full track bed and the construction railway as we tunnelled and used a different sleeper design and we’ve had no complaints from the Barbican Centre throughout our works."

Quite the opposite, in fact. The Barbican Centre was so impressed with Crossrail they commissioned their own artwork to mark the passing of the TBM underneath the estate (this can be viewed at http://blog.barbican.org. uk/2015/04/pipe-dreams).

Underneath the western running tunnels the infrastructure included 18th century wells at Tottenham Court Road and Bond Street stations. "If they had been charged with water and been in a slightly different location they could have been a really big issue from a structural stability point of view," said Reichmann.

"But our processes and tunnelling excavation sequences and methods take account of those types of obstacles and we only go forward at 1-1.5m at a time in those advances so that, if we do come across something, we haven’t suddenly exposed a large section of unsupported ground."

Culturally significant buildings are very thick on the ground in this part of London. In Soho Square, for example, one of the capital’s oldest buildings, the Grade I listed House of St Barnabas, had to be closely monitored for ground settlement. And access for vintage cars worth many hundreds of thousands of pounds had to be maintained at Bonham’s auction house in New Bond Street.

Both of these challenges were highlighted by the BBC documentary – as was the captivating tale of ‘the eye of the needle’ at Tottenham Court Road station where the Northern Line and Central Line of the London Underground intersect. Crossrail needed to link to this hub and negotiate sewers, pipes, cables, the Northern Line platforms and two escalators. The only option was to drive the 900 tonne TBM 850mm above the live running tube line and 350mm below the escalators.

However, Reichmann and Mears are modest about these more publicised achievements.

"The Tottenham Court Road location was tighter than you would want but in terms of managing the TBMs alignment to get it to where you wanted it to be within normal tolerances, the guys have got the competence to do that," said Mears.

"They are able to steer the TBM very precisely, assuming the laser surveying technology works – which it has done."

"The BBC picked up on the things they could dramatise easily but there were so many smaller white knuckle moments that didn’t come with the same drama to other stakeholders," added Reichmann.

"There were things like getting a Thames Water main monitored and secured before a TBM went under it, or getting the approvals from external stakeholders and asset owners before we started excavation of a new section of tunnel. Those one-off interfaces are very intensive in terms of the amount of time and human capital spent on trying to resolve issues.

"And working in the middle of Mayfair, where you have residents and businesses to keep happy is a challenge because the residents don’t want you working at night and the businesses don’t want you working during the day, so we’ve had to employ practical means to make sure we don’t impact either.

"Stakeholders are rightly concerned about their assets and overcoming those trust hurdles is as demanding as the physical works."

This is where the unsung heroes in the background have really come into their own, said Reichmann.

"Take our compensation grouting team as an example," he said. "There are five grout shafts at Bond Street, seven at Tottenham Court Road and one at Fisher Street [there are 22 in total]. The fact that you didn’t hear much about their work was because it was a success. It’s those types of activities that can stop a project if they go wrong."

Reichmann and Mears agree that Crossrail’s smooth progress has also largely been down to the "really high-spec" TBMs employed on the project.

"Crossrail wrote the specs for the TBMs and gave those to the contractors to adapt and procure," said Mears. "There was no debate about leaving them to try to find the lowest possible cost solution – it was a case of ‘this is what you need, this is what you are going to get’. That has been a good lesson."

Headline grabber
While the eight TBMs have undoubtedly been star performers, both project managers point to the SCL work as being perhaps the most rewarding when it comes to a sense of achievement.

"The running tunnels tend to be the prizewinners when it comes to getting media attention because they’re big," said Reichmann. "The TBM purists of the world will kill me for this but I find the SCL is a lot more demanding from a technical perspective."

The largest SCL caverns for Crossrail were built at Stepney Green where two drives meet – the Z drive that goes out to Pudding Mill Lane, near Stratford, and the Y drive that leads on to the drives that run down to Abbey Wood.

The caverns are 17m at their widest point, twice the diameter of the running tunnels, and have an access shaft for ventilation and emergency access and egress.

"This was a very challenging piece of construction work because it couldn’t be done in a single drive," said Mears.

"It was done in a double side wall drift, so the left and right central sections of the tunnel were advanced to a certain point and then the central heading was brought up to enlarge it to its maximum."

Forming the tunnel lining at the point of application, as opposed to building a concrete ring in factory conditions created different challenges.

"You have got to be really spot on and the materials have to be perfect," said Mears. "The SCL is a sensitive chemical mix including retarders and accelerators that can be affected by the ambient temperature, storage of the accelerators, the wetness of the aggregates, where the cement has come from and so on. It’s a really delicate balance and takes a lot of control and competence to make sure you have the right product and the right workforce who know how to apply it.

"The caverns were built in about 18 months, which is quite remarkable and, again, a big success story".

Crossrail has provided a showcase for SCL and Reichmann pointed out that the project has pioneered different profile and excavation control techniques that have been used in the past but have been improved.

"The technique of using beamers to control excavation on the King’s Cross project has been improved and all the contractors are adopting that technology to improve the product they can offer," he said.

"There is much to learn from the materials that are being used on Crossrail, as well," continued Reichmann. "This is the first time that a sprayed waterproof membrane of this magnitude has been used in this environment. And just spraying a secondary lining of this scale after we’ve already installed a primary lining that will last tens of years without any settlement is remarkable."

Mears points to another example of new techniques being applied to old methods, this time where SCL turned out not to be the preferred option.

"On C305 we built our cross passages using SGI segments rather than SCL. There was a lot of debate about it but DSJV wanted to go with segments because of the surety of the products – they felt more confident in the programme if they could advance the heading knowing another segment was waiting for them.

"This presented another challenge in terms of manhandling these very large segments, so they worked with their supply chain to design and build an erector structure that sits in the cross passage. I’ve seen films of London Underground construction where the men had to lift the segments on their backs and force them into place but now that process has been mechanised. I’ve never seen it done before."

Mears and Reichmann both pay tribute to the contractors they’ve worked with on their respective contracts – DSJV for the eastern running tunnels and Bam Ferrovial Kier (BFK JV) for the western.

"DSJV have done a really excellent job – and I’m sure Greg will say the same about BFK JV in the west," said Mears. "They’ve been really collaborative and innovative in working out ways to build the job as efficiently as possible.

"They haven’t just done the bare minimum, they’ve been open to looking at taking on whatever challenge sits with us as a client’s risk, as opposed to a contractor’s risk and have worked hard to help us mitigate those. They’ve really stepped up to the plate for the good of the job and it’s the main ingredient in the success of the project."

Mears and Reichmann also agree that, in many respects, the knottiest challenges have arisen at the interfaces between the many different contracts and sub-contracts that have made up the mammoth Crossrail project.

"The western package was originally two contracts," said Reichmann. "C300 was the running tunnels and C410 was the station platform tunnels and concourse tunnels. BFK JV proposed bringing those contracts together in a solution that was beneficial to Crossrail and from a programming perspective.

"There was a lot of planning at tender time to make sure those interfaces worked but, as with any construction project of this scale, as time goes on programmes change and suddenly some of the elements that used to fit, don’t any more.

"The big, ongoing construction challenge is making sure those elements dovetail – such as starting work on the Bond Street and Tottenham Court Road platform tunnels at the same time as you’re trying to strip out a TBM.

"You work together to make sure you can accommodate any changes but, again, like any project, if you were doing it again there would be things you would change and improve to make sure those interfaces were really rock solid," said Reichmann.

"The interface between contractors is challenging," agreed Mears. "A lesson learned for everyone on Crossrail and definitely for myself is that you have to make sure you minimize those interfaces and have them at sensible points in three-dimensional space.

"It obviously makes it very difficult when you have to transit a TBM through a box that was built by someone else. It’s easy to say with hindsight that maybe we could have structured some things differently but take your mind back to the when the job was first let – it was a difficult time in the industry and Crossrail was conscious it didn’t want to burden one contractor but wanted to spread the work evenly among companies who were capable of doing the work."

Of course, there is still an enormous work to be done before passenger trains start running through the Crossrail link but there is a sense that now the tunnelling is finished, from a risk perspective at least, the most challenging part of the project is complete.

And, with that, comes a huge sense of pride in the achievement thus far.

"Some of the platform construction is complete and the platform edge screen doors are going in so it’s all coming together," said Reichmann. "Seeing that is when you think ‘we were a part of that, we created the space to allow this to happen’. It’s very rewarding."

"Crossrail is remarkable in the amount of tunnelling that has been carried out through this densely populated city with many historic assets without any serious impact on any railway operations or buildings," added Mears. "We’ve done it without any serious ground failures or collapses or any issues working with TBMs or SCL. We’ve excavated more than three million tonnes of central London and sent it out to Wallasea in Essex [where a wetland nature reserve is being created] and it’s all been done in three years."

While Crossrail construction hasn’t caused any major ripples in the public consciousness, the project has raised the profile of the tunnelling community.

"Crossrail has done an excellent job of making sure we have a positive image," said Mears. "And the BBC documentary was brilliant – the number of people who now ask me about my job and say it sounds really interesting and that what we’re doing is great is terrific."

And, of course, while the British public may have improved their knowledge about tunnelling, the learning experience for those actually working on the project has been second to none.

"Thousands of people have now worked on Crossrail and it’s been like a university of construction professionals," said Mears.

"Crossrail has really championed apprenticeships, which has got to be the way forward for this industry. And, as for myself, it’s really accelerated my learning. I’ve learned an infinite amount of how to build these complex tunnels, from dealing with technical challenges such as settlement and ground water, to managing contracts. It’s been fantastic, incredible."

With the Crossrail experience bolstering their skill sets, many of the workforce have now moved on to other projects. Several have moved to the Middle East to work on Riyadh and Doha metro projects while others have stayed closer to home and are already working on the Thames Tideway scheme – Greg Reichmann included.

There is also the prospect of High Speed 2 (HS2), the rail link between London and the West Midlands, to look forward to and, perhaps, Crossrail 2, which, if it goes ahead, will provide a north/south route across London.

And, looking at this wider picture, the success of Crossrail to date should ease the way of other major infrastructure projects, both in the UK and elsewhere.

"Big infrastructure projects in the past have had a tarnished reputation of being late, over-budget and of not really delivering on the vision they were meant to at the start," said Reichmann. "But in London we’ve had Heathrow Terminal 5, the Olympics and now this. It gives so much credence to the likes of Crossrail 2 and HS2 because we’ve got the skills, the reputation and the knowledge to go and do them. There is a lot more confidence about putting money into infrastructure now – and that’s a big positive."

Mears, who has spent five years of his 12-year career on Crossrail, is in full agreement. "The start of my career was a time when civil engineering projects were late, over budget and filled with acrimony between client and contractor," he said. "To date, all our contracts have been settled amicably and the project is going to be delivered on time and on budget, so that is a step change," he continued. "This is how a tunnelling job through a city should be"