Improvements in technology, practice, productivity and safety are all in the pipeline for a tunnelling industry needing to reinforce its reputation for delivery.
Until this month, Britain was perceived as a good place to be as a tunneller. Numerous subterranean projects are underway or actively under consideration. In workload terms, London is the current tunnelling capital of Europe with other parts of the UK close behind.
Yet challenges with Crossrail are threatening the industry’s global reputation for on time, to budget delivery. Improvements in technology innovation will be key.
Test beds for ideas
The list of potential test beds for proving new ideas is long. The Thames Tideway tunnel, already facing time and cost pressures, springs to mind, as do the Lower Thames road crossing and High Speed 2’s (HS2’s) prospective 56km of twin bores.
These and other tunnelling schemes are opportunities to build British expertise while drawing in knowledge and skills from overseas. There is heavy pressure on everyone not just to employ the best of practice but to improve on this.
Andy Alder understands the challenges, having worked on Crossrail where he was chief tunnel engineer and project manager. He is now programme director of Tideway. Thinking holistically about tunnelling projects in the future will bring major gains in productivity, he believes.
It would be better if we thought of each scheme as a whole and advanced it as such
“Tunnels are linear by nature and we tend to build them one element at a time, as logistics permit. It would be better if we thought of each scheme as a whole and advanced it as such. With, for instance, track bed, permanent way, services and so on all closely following tunnel boring and lining.
“This presents the opportunity to radically improve project schedules but needs investment and the procurement model to support this.”
Such a way of constructing tunnels would require innovative changes to tunnelling custom and practice, to include increased mechanisation, remote operation, robotic intervention, greater emphasis on health and safety plus higher training standards.
Some of these are becoming available already, others are on their way or are being contemplated.
Take training for example. Simulation of tunnel boring machine (TBM) operation to train TBM drivers – or pilots, as they are sometimes called – is already taking place; French contracting giant Bouygues perhaps being the leader in development and use of TBM simulators.
“We have more and more TBMs operating and want conformity of operation, conformity of reaction to events,” says Bouygues Travaux Publics business development director, Jérôme Furgé. Bouygues Travaux Publics is one of the partners in the Align joint venture awarded HS2’s challenging Chilterns contract.
“So we have built simulators to replicate – in a general way – changes of soil conditions, ground faults, face collapse and so on, to help teach pilots how to react to or avoid such events. They’re proving very effective as a training tool.”
Bouygues is now moving towards a second stage in which specific projects are simulated. “The intention is that pilots working on a particular tunnel will have a clear idea of the challenges ahead of them. Obviously for this we need good information about the ground.”
Obtaining knowledge about ground conditions is one critical area that has to improve in the future, according to Align project director Daniel Altier.
Eliminating geotechnical uncertainty
“I cannot understand why, as yet, we don’t have the means to get a really comprehensive picture of the ground we’re going to tunnel through,” Altier says. “There is too much uncertainty in tunnelling, especially where we’re having to go deeper, where the easier (to tunnel) upper layers are already occupied.”
Using the tunnel itself to find out what the ground is like should no longer be anyone’s default position.
“It’s much too late,” he asserts. “We need radar or sonar systems, or something not yet devised, to provide absolute foresight of what we will pass through so that tunnellers can anticipate with confidence metre by metre what they will find.”
With greater knowledge of the ground and further development of TBM computerised guidance and operation systems, the activity of TBM drivers could then be reduced to a minimum – to the extent that tunnel boring machines gain a degree of autonomy.
We have built simulators to replicate changes of soil conditions, ground faults, face collapse and so on, to help teach pilots
Such autonomy will undoubtedly be extended in the future to behind tunnel shield operations, such as the erection of tunnel lining segments, with greater use of robotics and remote control. The same applies ahead of the TBM.
Furgé cites one example of where human intervention can beneficially be reduced is in replacing cutters, particularly where face support is being provided by slurry. “Bouygues is developing such robots,” he says.
Greater automation should bring an overall reduction in personnel needed underground while excavation is taking place. Pilots, for instance, could drive their TBMs remotely, guided by CCTV and multiple sensors as well as their enhanced knowledge of the ground.
Tim Smart has extensive experience of major infrastructure projects. He is currently HS2 Ltd’s chief engineer.
He remarks: “Old stagers say you need to be in the ground to ‘feel’ the ground but this won’t really hold as we go forward. With the ability to monitor more effectively, with better instrumentation and even greater reliability of machines, we’ll be able to manage with fewer people under the surface.”
A body of opinion also holds that there should be less plant underground. Altier says that an important area of research has to be into the feasibility of reducing the length of TBMs.
“Today we have TBMs that are 120m long, the train including all the ancillary equipment needed for the TBMs to perform. By why can’t all the functions provided by that equipment, such as the hydraulics, electrical power, lubrication and so on, be provided from the surface via ‘umbilical cords’?” he asks.
“Keep the cutter head and segment erection within the ground but remove everything else.” In other words, opt for small TBMs and thereby greatly ease logistical congestion within the tunnel drive.
Smart stresses that the tunnelling industry has come a long way in recent times with development of TBMs; that these are now being fabricated with enhanced reliability, better able to cope with difficult ground while working at a faster rate.
Old stagers say you need to be in the ground to ‘feel’ the ground but this won’t really hold as we go forward
And they are getting even better. For instance, developments are taking place which maximise and combine the best aspects of earth pressure balance machines and those that employ slurry – namely, variable density tunnel boring machines (VaDTBMs).
Suffice to say, the clue is in the name. The density of slurry in front of a VaDTBM’s cutter head can be thickened or thinned to maintain face support through a wide range of ground conditions and faults.
That they are highly effective is indicated by their pioneering use in Hong Kong and Kuala Lumpur. The choice of variable density machines for HS2’s Chilterns tunnels represents a first for Europe. TBM manufacturer Herrenknecht has reason to believe the application of its VaDTBMs will become universal over time.
Another first, namely “continuous boring” could also be deployed on HS2. This is an innovative process in which the installation of tunnel lining does not interrupt TBM forward movement.
Conventionally, tunnel boring has to stop as each ring of precast concrete tunnel lining segments is installed. It starts again once the ring is in place and the TBM can resume thrusting against the ring circumference.
The plan is to modify the jacks positioned between the ring circumference and the TBM so that these can elongate and maintain thrust at all times. The intention, of course, is to save time. “I believe such innovative thinking is essential on the project,” says Altier.
We will want longer lives for our tunnelling assets and will take steps to ensure this
On the subject of tunnel lining, HS2’s Smart is an advocate for redesigning segments so that they interlock better and require less bolting.
The topic of secondary linings has been taxing engineers on the Thames Tideway tunnel. “We need to prevent groundwater getting into the tunnel when it’s empty and effluent getting out when it’s in use and under pressure,” says Alder.
A secondary lining to back up the primary one is an obvious solution. But how should this be formed? Insitu concrete is a possibility but Alder’s experience with such a lining on another tunnel means he has reservations.
“Concrete had to be pumped long distances and we learned lessons about control of quality and speed of progress,” he says. Membranes of plastics is another option but the favoured one – so far – is precast concrete installed using an innovative custom-made erector.
“We’ve been involved with Herrenknecht conducting full scale trials of the machine. These were completed in January and will help us make a final decision on the right material to use.”
Precast concrete should bring better standards of quality and productivity while also conferring health and safety benefits, Alder adds. The firm impression given is that if Herrenknecht’s erector proves up to the job, then precast will be the choice, all other things being equal.
Alder says: “The water tightness of the primary tunnel lining should actually be sufficient on its own. The secondary lining is as much about securing durability and to protect against abrasion and chemical attack. Thames Tideway tunnel is being designed and built to last.”
Alder predicts that asset monitoring – the kind of oversight, for example, to which bridges are subject – will become more prevalent with tunnels in future.
“We will want longer lives for our tunnelling assets and will take steps to ensure this: including building in durability and embedding technology so that monitoring and maintenance are easier.
“In addition, monitoring the performance of tunnels in service and providing a platform to share the results with the industry could provide real benefits in more efficient lining designs in the future.”
This, he says, would address some of the conservatism inherent in current design methods.
Personnel tracking for safety
Keeping track of personnel and plant during construction is another function currently being revolutionised in tunnelling. Bouygues is well on with its “Tracktivity” system for knowing exactly where people and equipment are located.
“Having such information in real time is increasingly crucial for efficiency and safety,” says Furgé.
“If you’re not happy with productivity, for example, you can trace the course of a machine, to see how the situation can be unlocked or improved, to reduce standing time, for example. And in an emergency, of course, you can see where all your people are.”
Bouygues’ system is based on radio-frequency identification (Rfid); where electromagnetic fields are used to automatically identify and track tags attached to people or objects.
“With chips on helmets, plant and equipment,” Furgé says.
Bouygues is already applying Rfid on a major project in Paris. “The intention is to use it in the UK.” Thames Tideway is also planning to employ the technology during its tunnelling operations to help secure the safety of people and track the location of safety critical assets.
Better health monitoring for operatives
Improved monitoring of tunnel operatives’ health and wellbeing is high on the agenda of HS2.
“There is increased awareness of the benefits of looking after your workforce. Unacknowledged fatigue or ill health means accidents are more likely to happen. Conversely a healthier workforce improves productivity,” says Smart.
In future, much more attention will be paid to this aspect of tunnel working, he believes. “And I’m talking mental health and wellbeing here as well as physical.”
Finally, a recent letter to New Civil Engineer raised an interesting issue. Bruce Latimer referred to the increased use of electric cars and consequential reductions in oxygen consumption and production of CO2 and NO2. He was writing in the specific context of tunnel ventilation and the possibility of making formerly unusable tunnels viable for traffic (Your View, last month).
The expert view is that ventilation will always be required, to control piston effects and extract smoke in an emergency. But the necessity of its provision via expensive shafts may be considerably less.
Already the prospect of mass electric vehicle use is having an impact on design of the Lower Thames Crossing, although admittedly not of the tunnel itself.
The proud claim of its project director Tim Jones is that the tunnel approach will be equipped with the first green rest area in Britain. There, “refuelling” will solely involve charging points, not petrol or diesel pumps. That truly is the future.