It is said to be the biggest infrastructure project in Europe. Backers of HS2 say that it will transform transport in the UK and help redress the north-south divide. Trains running at around 320km/hr, with a top speed of 360km/hr, are forecast to cut journey times between London and Birmingham by half an hour to just 51 minutes. A second phase will extend the lines to Manchester and Leeds.

On 15 April 2020, the consortia responsible for each of the four main sections of Phase 1 were given the go-ahead in the form of a ‘notice to proceed.’ That unlocks an enormous amount of value and allows contractors to sign deals with subcontractors and suppliers.

Tunnels and Caverns                                                   

All the lines and tunnels are new. Phase 1 of the project runs 225km from Euston Station in London to new stations at Birmingham airport and in Birmingham itself. En route there are 44km of twin-bore tunnel and 8km of cut-and-cover tunnel. HS2 prefers to call the latter ‘green’ tunnels, for reasons not clearly explained but presumably connected to public relations.

Altogether 20% of the route is underground. The longest of the bored tunnels is the 16km Chiltern tunnel; there is the 13km Northolt tunnel and a further 7km of twin-bore tunnel that will take the line into and out of Euston. A short stretch at Long Itchington Wood, and the 6km Bromford tunnel leading into the new Birmingham station make up the twin-bore total.

There are also two large cavern excavations. One at Euston, the other on the outskirts of London, at Old Oak Common (OOC), planned to become one of the busiest transport hubs in the country, where passengers will be able to change between Crossrail, HS2 and the Great Western main line. The OOC station box will be 900m long by 60m wide and 16m deep.

Four joint ventures were originally awarded contracts by HS2 in July 2017:

  • SCS Railways (Skanska Construction UK, Costain and Strabag) has two contracts together worth ?3.3bn, for the Northolt tunnels and the Euston tunnels and approaches;
  • Align JV (Bouygues, Sir Robert McAlpine and Volker Fitzpatrick) has a ?1.6bn contract which includes the Chilterns tunnels;
  • BBV JV (Balfour Beatty and Vinci Construction) has contracts worth ?4.8bn including the 1.5km Long Itchington Wood tunnel, which could be bored or mined, and
  • EKFB JV (Eiffage, Kier, Ferrovial and BAM Nuttal) (Carillion was involved originally but collapsed in January 2018, its place taken by Ferrovial and BAM Nuttal). Contract includes 5km of green tunnels and 30 million m3 of excavation.

The notice to proceed means that finalised designs are now being worked on. The high running speeds of the trains are central, even to fundamental considerations such as the diameters of tunnels.

Dealing with Pressure Waves

Eddie Woods is HS2’s Head of Tunnelling for Phase One. “At Euston we have 7.55m-diameter tunnels, as the trains are moving slowly” he says. “By Victoria Road crossover box they have picked up speed and we have 8.1m diameter. And the diameter of the Chiltern tunnels, where the trains are at maximum speed, is 9.1m.

“Aerodynamic pressures are what matter here. There are Euro directives on passenger comfort criteria, and HS2 has its own standards as well. Even in standard-speed trains you feel the pressure-wave when two trains pass in a tunnel; when they are passing at 360km/hr the shockwave could do real damage to ears.” There are crosslink passages, for safety and evacuation, between the twin tunnels; one might imagine that these would relieve the air pressure ahead of a fast train, but Woods explains that the notion is incorrect.

“On the Thames Tunnel we thought ‘why not leave the cross-tunnels open to reduce the transit pressure?’ It seemed that we were onto a winner. But a fast train causes a pressurespike ahead of it that reflects off the open end of the tunnel and comes back to hit the train. When two trains pass an open crosslink tunnel, in different directions and perhaps asymmetrically, all sorts of pressure spikes are set up. To avoid that, the crosslink tunnels are kept sealed, rather than open, except in emergency. The aerodynamics is actually very complex. Lots of modelling has been done on it, notably by Arup, which set up 1/80 scale models of tunnels and shot model trains through them powered by bungee cords at various speeds.”

Passengers will barely be aware of the different tunnel diameters. The consequences of another aerodynamic effect will be more visible. The world’s first high-speed rail line, Japan’s Shinkansen bullet train, suffered from tunnel boom, the loud noise made by the pressure shockwave that occurs when a train exits the tunnel. The latest generation of trains running on it have a highly aerodynamic profile, with an excessively long-looking nose, to try to reduce it. Indeed tunnel boom has become a principal limitation to increased train speeds on the line. France’s TGV is similarly affected. The solution for HS2 is to install porous tunnel exits – concrete portals, with holes in the sides, to disperse the compression wave more gradually. “I compare them to the silencer on a gun,” says Woods. “Again, a lot of aerodynamic modelling has gone into them. On the Chiltern tunnels only the lowest levels of noise are acceptable, because it is a naturally quiet rural area.”

As for construction, the challenges fit the scale of the project. They are not small. Jacques Didier is director of tunnels’ construction for Align JV, which is responsible for the Chiltern tunnel and its five shafts, some of which are up to 60m deep.

“It is a very challenging project” he says, “and all of us working on it know how important it is for the UK.”

Flint Stones

The geology – chalk with flint – is one of the factors influencing the choice of TBM. In its document for the public outlining benefits and costs of tunnelling options (‘High Speed Two: A Guide to Tunnelling Costs’), HS2 had originally envisioned a traditional slurry machine, with EPBMs for the tunnels in London clay: ‘EPBMs are typically used in the geological deposits beneath London and are likely to be also used for the Bromford tunnel in Birmingham. A ‘slurry’ machine is expected to be used through the chalk of the Chilterns’ it says. Those expectations have been revised. The Chilterns have not been tunnelled through before, so detailed experience of its geology was lacking; and, in the event, slurry machines will not be used for it.

“Chalk has flints” says Woods. “Chalk is soft but flint has a strength of 600MPa. To put that in context, weak concrete has a strength around 20MPa, and the concrete section linings for the tunnels come in at 60MPa; so the flints have ten times that strength. Chiltern flints can be big, up to 600x300mm, but they are brittle. They break up when they hit the cutters of a boring machine. The crushers grind them, but they do give a lot of wear to an EPB machine.”

“The chalk is highly fractured, with high water circulation” says Didier. “The aquifer is particularly sensitive with very high water quality, and it is important not to disturb the ecology of the chalk streams. So the choice of TBM was driven by the requirement not to pollute the aquifer. That is a key challenge.

We needed strong measures to contain the slurry at the face.” The polymer additives that are used with a normal slurry machine to help support the face were therefore unacceptable.

“So we are bringing in new technology. Instead of a conventional shield (slurry) or EPB, we shall use a variable density tunnel boring machine.”

Variable Density TBM

Also known as a multi-mode TBM, the variable density machine can increase the density of slurry at the face using fine sand particles in suspension. “You can adjust the density to control water loss in fractures” says Didier. “That way you avoid any pollution. We have used them before, in Hong Kong and in Kuala Lumpur, where there were karsts and shallow cover. We developed our expertise there. It is higher technology, but it has become very user friendly. It is very efficient when facing variable geology, as we shall experience here.” TBM maker Herrenknecht describes the technology as unique.

“Without major mechanical modifications, the machine can switch between four different tunnelling modes directly in the tunnel. This means that geological and hydrogeological changes along the alignment can be managed with extreme flexibility.” “Thirty per cent of the tunnel length is above the water table, 70% is below” says Woods. “For that too, the variable density TBM is well suited.”

The HS2 document referred to above gave as an assumption that TBM breakdowns and timeouts for maintenance would increase, through wear and tear on the machines, as the project progressed. That may be inevitable, but management and tender procedures, and lessons learned on previous projects, says Woods, have reduced such risk to a minimum. He has been in charge of tunnelling also for HS1 (the fast St Pancras to Channel Tunnel link) and for Crossrail, so he speaks from experience.

“On HS1 we were exposed to risk, and to minimise that risk we gave specifications for the tunnel machines that the contractors were to use. We said they had to be highperformance types. A contractor who bid to build with low-spec machines that he would push to near the limit wouldn’t have got the contract.

“And we did everything we could possibly do to minimise ground movement. On HS1, and on Crossrail, and also on the Tideway scheme (where I was also chief engineer) there was some extreme engineering. They all went close to infrastructure that we did not want to disturb. You can spend money on putting right failures, or you can minimise the movement of the ground and avoid that. It was an approach which was very successful on HS1 and we have also used the same approach here on HS2.

“From all of those large projects lessons have been learned, so now the approach is almost standard. We have high-performance specifications, but we work them out with contractors to suit their method of working because, at the end of the day, they have ownership of the machine, and that machine must not fail and it must protect the infrastructure.”

Portal Logistics

For the Chilterns tunnels, both TBMs will be launching from the south portal, just inside the M25 at Rickmansworth. “It is a big main site” says Didier. The standard popular unit of measurement of large sites these days is the football pitch, and this is an 80-football-pitch area. It contains not just the portals, and the entrance spaces for the TBMs, but the slurry treatment plant and the concrete plant that will be manufacturing the tunnel lining segments – and each of those alone is a very major undertaking. The system and logistics have been designed to minimise impact on the environment.

“The spoil at the excavation face goes into a slurrifying box and is then pumped back to the south portal to the treatment plant” says Didier. “The length of the tunnel means that we need 18 slurry pumps for each of them. Again, having so many is a first. Supplying them takes 12MVA of power, which, to put in context, is twice the power requirements of the TBMs. The total power bundle is 44MVA, which I calculated yesterday is enough to power 9,000 houses. Again this is the first time we have used so much power. The scale is huge.”

So is the slurry plant. “It is a massive plant, the biggest in the world I think” says Didier. “To have capacity to treat the spoil from two TBMs, 10 thousand tonnes a day need to be treated, and all of it will then be put directly to use as landscaping on the site. So there will be no transporting of spoil away from the south portal, which means no need for the huge numbers of trucks that would otherwise be moving it on local roads.”

The plant to make the concrete lining segments is also on-site at the portal. “Again this is to minimise road transport. Each segment is 2m long, and weighs 8.5t, with seven segments to a ring. There will be 16,000 rings, which makes 112,000 segments, to be installed over three years, at 85t per ring when you include mortar and accessories. That is quite a saving on road transport.”

“An innovation is that we will be using continuous boring to improve production; the TBM will not stop each time a new segment is installed. It can install the rings at the same time as it is excavating. It has 14 pairs of rams, which are retracted two at a time to erect the structure. That is completely new. It is a concept that needs sophisticated control of TBM parameters. Development and testing of the equipment is being done in Germany now, and we will be using it on site next year.”

The primary feature of the Chilterns tunnels of course is their length. “There are two big challenges for such a long drive” says Didier. “One is logistics. Lots of vehicles will be moving in the tunnel; we have given a lot of thought to developing a new way of working for the safety of our employees. We are separating people from moving plant. For example we have a robot inside the TBM back-up carriage to remove the wood spacers from between the concrete ring segments. It also installs the steel dowel connector pins between the segments, another task that is normally done manually. The robot does these tasks without exposing anyone to risk. This, too, is new in the world.”

Tunnelling Options

The Euston tunnels and cavern are through London clay, not chalk, and present very different challenges. One, of course, is that Euston is urban and congested. “We have come up with a very robust methodology for that” says Woods. “The original idea was to use cut and cover, but it was realised that it would have a huge impact on residents, and on the running of the existing railway. It would have meant six years of piling operations in front of people’s houses. That was not acceptable.

“So that led to a tunnelling option. There will be a retaining wall; then a series of small-diameter tunnels (next to each other and intersecting) will be dug 4.5m below the existing railway route to form the line of the arch of the roof.

These will be filled with concrete, to give the cavern roof; then the rest of the cavern will be excavated below them. It is very controllable; the risk is low; and London clay stands up very well. Network Rail are very supportive of the method because it means they can run their existing railway without interruption throughout the operation.”

Other underground operations include the cavern-sized excavation at the ‘super-hub’ interchange station at Old Oak Common, and the adjacent Victoria Road Crossover Box which will house the points to allow trains to switch tracks.

“The box has an interesting design” says Woods. A series of concrete circles and arches with beams across and joining them give a strong roof and large uninterrupted space below it and have led to its nickname, ‘the Caterpillar.’ The box will be 25m below ground and be 130m long with three headhouses at ground level for maintenance and emergency access as well as a separate ancillary shaft. During construction, the box will also be used to launch two of the four TBMs digging from Old Oak Common towards Ruislip. An innovation is a proposal to use waste from trains passing through the Victoria Road box to heat new homes around the site.

Tunnelling is due to start early next year. The scheduled completion date, for Phase One, is between 2028 and 2031. Watch this space for news of progress.