They stand at the entrance portals, Cecilia in full view, Florence half-hidden in her tunnel. Even by TBM standards, these are large machines. Their white paint is still fresh, pristine and gleaming. But it will not be so three-and-a-half years from now, by which time they will each have tunnelled, if all goes well, 16km under the Chilterns, emerging near Great Missenden in Buckinghamshire. These are the TBMs that will excavate the Chiltern Tunnels, the longest on the new HS2 route.


Their portal is right beside the M25 motorway. The 136-acre (55Ha) site exudes that air of quiet efficiency common to all good construction sites. It hosts far more than just the TBM launchpads. Two precast factories will produce the lining segments for the tunnels. They are on-site to reduce road transport costs and disruptions. Since there are 112,000 segments to be placed in all, the prospect of those being delivered by road would please no-one who has to use the over-congested M25 at any time in the next three years. Another precasting plant will make sections for the Colne Valley viaduct, which exits from the south of the site and will carry the line over a series of lakes and waterways in towards London. At 3.4km long, it will be almost a kilometre longer than the Forth Railway Bridge. Clearly, nothing about this project is small.

Phase One of the HS2 project comprises the construction of 140 miles (224km) of new, high-speed rail line that will join London to the Midlands. The Chiltern Tunnels, at their deepest points, will be 90m below ground level. The internal diameter of each bore will be 9.1m (30 ft), and there will be five ventilation and emergency shafts en-route.

The TBMs arrived in 300 shipments last year, and have now been assembled. Florence started excavating in May this year and by the time this author visited in early June, only the rearmost of her six gantries was still visible. The rest of her 170m length is under the M25 motorway. Cecilia cannot start until her sister TBM has burrowed under the motorway and is clear on the other side. “Highways England has asked that only one TBM be under the motorway at a time, due to potential impacts on the stability of the ground,” says Mark Clapp, senior project manager. When she does start, Cecilia will tunnel slightly faster to overtake Florence and be first to make the breakthrough. “Florence will be handling the cross-connection works, which will slow her down a little. And the geological data from Florence when she is leading will be passed to Cecilia, which will help her along.” Progress is planned to be at the rate of approximately 15m/day.

“We spent three years working with Herrenknecht on the design of these TBMs,” says James Reilly, Align’s TBM engineer on the project. Align, the engineering JV responsible for this stretch of the new line, is a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick. So many new features are on board the TBMs that they qualify as entirely innovative machines. “I do think that these can be thought of as a step-change in the technology,” he says.

They are, for one thing, the largest TBMs ever to be used on railway tunnelling in the UK. Each weighs in at 2,000t, is 170m long, and bores a 10.26m diameter. The trains that will run through the tunnels they build will be of normal dimensions, but they are travelling at much higher speeds than standard UK trains – routinely at 330km/hr (206mph) and with a top speed of 360km/ hr (225mph). A larger-than-normal gap is therefore needed between train and tunnel to permit air flow around them. But much more than mere size is new.

“These are variable density machines,” says Reilly. “Variable Density TBMs have been used before, of course, but not in the UK.” The geology dictated the choice. “We are excavating through chalk and flint. Classic slurry is low density, and would leak into the chalk fractures, and so into the aquifers; we don’t want to pollute those. Variable density gives us a thicker slurry paste. We actually have a whole new concept of slurry management on the machines.” Details of this are still confidential, but T&TI hopes to go into greater detail at a later date. However, that still leaves plenty of innovations that can be revealed.


Muck disposal is not so unusual, though still worthy of mention. From the slurry box on the TBM, the slurry is sent hydraulically back to the surface where it goes into holding tanks. A total of 24 filter presses remove the water and compress the fines into cakes; the cakes will be used to recreate a new 127Ha (313 acre) landscape and wildlife refuge around the tunnel entrances, of which HS2 Ltd is quite proud. “It will be calcareous chalk grassland, which is a rare and decreasing habitat. Before, it was intensively farmed, which offers far less biodiversity,” said Rob Hutchinson, Align’s head of communications. And since the landscaping is on-site and within the M25 boundary, no road traffic will be generated from the disposal.

To return to the TBMs and the six gantries that make them up. Starting at the front, ten 350kW motors produce 2.1 kiloton metres of torque at the cutterhead. The control cabin, in the lead gantry, has innovative features, but more of that later. Gantry Two contains the Krokodyl and Dobydo robot. “This is one of the world-firsts,” says O’Reilly. “The robot is one of those big developments that we are implementing.”

The segments arrive by MSV. They are taken off by the quick unloader. The segment crane picks up the segment and moves it to the front where it is rotated. Usually, it would then be lowered down to the segment feeder to be fed to the erector; but at this stage, the robot comes in to insert the dowels that will hold the segment in place. Then the segment is moved forward to the segment feeder, but while this operation is happening the same robot is automatically moving backwards and removing the wood timbers that separate the segments during storage and delivery; it picks them up and puts them into a box just behind where the quick unloader is located.

“This operation happens regardless of whether we are mining or stopped. It is a big safety innovation. Historically in the industry, we have people climbing on the segments picking the timbers up. We decided to put this innovation in place to demonstrate that we don’t want anyone climbing anymore; the robot is taking over this operation from an operator on the machine. It allows us to take people out of harm’s way from an area which would otherwise be relatively dangerous. It has been tested in the factory and we have tested it on site, and it works really, really well. It is one of the steps to get the construction industry up to the next level of robotics to improve how we are working.

“They are using robots in the precast facility on the site as well. There, they do the cleaning and the smoothing of the segments in the moulds. So that is yet another use of the technology on the site.”

Gantry Three has emergency refuge chambers, with life support for up to 24 operators, hopefully never to be needed. Innovations on the TBMs mean that a crew of just 17 is the normal operating compliment, but maintenance workers might also be present.

Another first, says Reilly, is in the grouting. “We have the grout plant on the machine itself. Usually the grout is pumped to the machine; here we are batching it on the TBM itself.” The grout manufacturing area is on Gantry Five. “The constituents are brought to the machine. The liquids are pumped, the cement comes up by MSV. We then mix them together, right there on the machine, in a dedicated batching plant and pump it up to the excavating face.

“Again, we brought in the innovation because of the length of the tunnel. It removes the need for a huge pipe behind the TBM; it mitigates the risk of that pipe getting blocked, which might take only two or three hours to clear if it is near the tunnel entrance, but which might take two days to find and clear if it is seven or eight kilometres in, and you don’t know exactly where it is.”


Perhaps the most significant innovation of them all is that Felicity and Cecilia can perform continuous boring. Other TBMs excavate a metre or two, then stop to install a ring of lining segments behind them. When the ring is complete the TBM rams transfer themselves to the new ring and push against it, using it as purchase to drive the cutterhead and the entire TBM forward as excavation commences again.

But Cecilia and Felicity dispense with this stop-start process The dowelling and shape of the concrete lining segments are designed to interlock in such a way as to give both radial and axial strength, even when only part of the ring is complete.

“The TBM can push against half-completed rings” says Reilly, “so it can move even while the ring behind it is being erected. With continuous boring we are able to build the ring and excavate at the same time. This is very innovative. It has been tried before, about twenty years ago, but the technology of the time wasn’t up to it. We have done a huge amount of study at the design stage with Herrenknecht working out what we need to do to be able to achieve this. A huge amount of effort has gone into that part of the operation.”

It is sure to pay dividends. Continuous boring is obviously faster, since the machine does not spend a good part of its time stopped. It is more efficient, since accelerating 2,000t of TBM from zero several times a day takes a lot of energy. And perhaps more unexpectedly, it makes steering the machine both simpler and more pleasant.

“Part of the benefit of continuous boring is that it allows us to implement a different mode of steering,” says Reilly. “Standard procedure is to have potentiometers, six of them, which divide the thrust between the hydraulic rams, and you use that to control how the TBM is being directed.” More thrust on the rams on the left-hand side tends to steer the TBM to the right. “With continuous boring we are able to put directional thrust in place. Instead of having somebody to control the individual potentiometers, we now use a touch screen. We just use a finger to say to the controlling software: ‘We want to go in this direction.’ The software calculates the position where the centre of thrust should be to achieve it, and all the cylinders manipulate themselves accordingly in order to move in that direction. It makes steering the TBM much, much easier, and we have had really good feedback from the operators on how this is working.”.

Ring building is another task that has been automated. “The erector is able to set up the segments without an operator. That has been used before, in Paris, and it has been working so well that we are implementing it here too. It reduces our operators’ workload, and it also reduces the number of people underground.

“Everything that we are implementing now is taking tunnelling to the next level: it is trying to improve safety in the tunnel industry. That is something we are working hard on. We have really spent a lot of time studying the ergonomics of the TBM machine. Simple things such as all the accesses and the egresses, and things that make it a pleasant place to be in and to work in. Sixteen kilometres is an unusually long tunnel for the UK, and it is definitely going to be a challenge, so we must also provide for easy maintenance of the TBM, and to see that people can walk around all the machine easily and in safety.

“We have a good team, and this is only just the start of the journey. We have three years’ worth of tunnelling to go. It is going to be really interesting.”