Most challenging aspect of High Speed 2’s (HS2’s)C1 Chilterns contract for 21.6km of railway infrastructure is undoubtedly the driving of a twin tube tunnel nearly 16km long.
Ground conditions to be met by the project’s tunnel boring machines (TBMs) are a mixture of chalk, marl, alluvium, gravel and clay with flints. Plus more than the occasional fault line.
“There’s always a risk that we could lose material at the face during boring with earth pressure balance (EPB) machinery or cause ground pollution using conventional slurry machines,” says Daniel Altier, project director of Align, the contractor responsible for C1.
“Which is why we’ll be using two Herrenknecht state of the art variable density TBMs (VaDTBMs) in a first for Europe,” he says. “They’re fantastic in variable ground, having proved themselves in Hong Kong and Kuala Lumpur.
15m to 16m of tunnel per day
“C1’s twin bores are 9.1m in diameter and we’re aiming to achieve progress of 15m or 16m a day with both UK machines. Regardless of the ground conditions we meet.”
Align is a joint venture (JV) of three major contractors: Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.
Altier, a graduate of the prestigious Ecole Spéciale des Travaux Publics in Paris, has worked for Bouygues for 30 years; he embodies the French company’s sense of engineering innovation and excitement. Altier arrived in the UK last summer fresh from completing the immense Liantang tunnel, one of four major projects in Hong Kong on which he honed his skills.
“I’m very proud to be here in England now working for Align and playing a leadership role on HS2,” he says.
So what, exactly, are VaDTBMs? “They’re a mix between EPB and slurry TBMs that allow the slurry density to be adjusted to suit the ground being driven through. VaDTBMs are ideal for dealing with a much wider range of mixed ground conditions as they allow pressure in front of the cutter head to be maintained – including through faults and voids and where there is little cover – to ensure stability at the face.”
When I arrived last summer what was amazing to me was the way Align and staff from client HS2 Ltd were working together
“Such tunnelling versatility is likely to prove crucial for Align on C1. For instance, there is a number of water abstraction sites in the locality – we don’t want ‘thinner’ slurry getting into them and polluting water sources.
“In these areas, with our VaDTBMs, we can increase the density of slurry at the face and prevent pollution occurring,” explains Altier.
Altier is being joined at Align by French tunnelling talent straight from Hong Kong to get the machines working. This includes Didier Jacques, Align’s tunnels construction director. “We need experienced people at the beginning,” says Altier.
“Knowledgeable international key players are being put in place. But once both VaDTBMs are up and running we will be instigating a training programme to bring local people up to speed.”
Technology transfer benefits
Practical technology transfer is one of the principal benefits Altier sees in working in JV; and indeed, working as a JV with other joint ventures involved in the same overall railway project.
“Align’s JV partners bring their own particular expertise to C1 from which others can benefit, while themselves benefiting from knowledge others are contributing to HS2,” he explains.
He cites the use of precast inverts to promote cleaner tunnel working as an example of Align gaining from shared ideas.
Right now, taxing the ingenuity of Altier and colleagues at Align is the challenge of achieving “continuous boring”.
“Our goal is not to stop the TBMs,” he says. Normally, a TBM comes to rest after boring a certain length of tunnel, to allow erection of precast concrete tunnel lining segments to “catch up” with the machine.
Fewer lining rings
There are seven segments to each ring of Chilterns tunnel lining. Conventional practice would have 14 pairs of jacks (two each per segment) maintaining pressure between the TBM and the circumference of segments already in place.
Two pairs of jacks would then be released to allow erection of one segment of the new ring. Once this segment was installed, the jacks would be re-pressurised. Then another two pairs of jacks would be released, the next segment installed, and so on, until completion of the new ring.
“We’re now considering a system where – expressed simply – the 12 pairs of jacks would not just maintain pressure during segment erection in the manner described above but also to physically elongate, giving the TBM something to push against and allow excavation to continue,” Altier says.
“Such continuous boring would have to be fully computerised – there can be no possibility of human error. It has to be safe. Our calculations indicate by not stopping, we could shave a month off our 36 month tunnelling schedule. And time is money.”
Such innovative thinking is essential on the project, believes Altier. Align is committed to providing the best technical solutions combined with the best value.
“We’re helped by being so well integrated with the client. When I arrived last summer what was amazing to me was the way Align and staff from client HS2 Ltd were working together. It’s real integration: there is no difference between the people of Align and those of HS2 Ltd.”
Such close working, especially in the early stages of C1, is extremely helpful in thrashing out practical solutions says Altier. “Take the shafts,” he says. C1 has five vent shafts up to 80m deep to cope with piston effect and provide tunnel ventilation.
“We examined eight options for the design and construction of each shaft. We decided together to go for smaller diameter shafts sited between the twin bores, and for the TBMs to clip them on either side. Four of the shafts are in wet locations and these will be sunk with diaphragm wall cutter rigs; the fifth, drier shaft, is likely to be built within a caisson. The point is, we’ve anticipated likely problems with value engineered solutions.”
Does this constitute the accolade of ‘tunnelling excellence’? “No,” Altier replies. “Tunnelling excellence to my mind is not injuring anyone. We are committed at Align to promoting safety and achieving this definition of excellence.”
- Produced in association with Align