The BTS James Clark Medal for 2025 was awarded to Dr Keith Bowers for, as the citation reads, “his considerable contribution to the UK & International Tunnelling profession, especially his exceptional dedication to nurturing the next generation of tunnellers”. Dr Bowers is a Director at Cowi. He has also been Head of Profession for Civil Engineering and Built Environment at Transport for London (TfL) and Head of Tunnels for London Underground (LU). Currently he is Tunnels & Systems Director for the new Lower Thames Crossing, a major piece of infrastructure which will tunnel under the Thames estuary between Kent and Essex and allow transport to flow north and south without touching London itself.
We talked at a fortunate moment. It was the day after the late 2025 Budget, in which the Chancellor, Rachel Reeves, announced £891 million in funding for the Lower Thames Crossing (LTC), of which Dr Keith Bowers is Project Director for the tunnels. It has been an eventful year for him in other respects: in April, after twenty-odd years in the planning and four years in the building, the Silvertown tunnel opened. That also goes under the Thames to the east of London. Bowers was technical lead during the procurement phase of that project. So there were milestones in two major crossings of the Thames, two major long-term easings of London’s transport problems, and Bowers was in on both of them.
“The 900-odd million was the last missing piece of the public funding for LTC,” he says. “The intent has been that the main delivery phase will be done with private investment but the government has paid for the planning and will also support the early construction work, up to a point in time probably two years away. After that it becomes a private-funded venture.”
Silvertown has been his other major decade-long project. It consists of twin bored road tunnels under the river and the Greenwich peninsula to relieve the appallingly-congested (and aging – one bore dates back to 1897) Blackwall tunnel.
He has been involved with it since 2014 and was technical lead during the procurement phase, including the competitive dialogue.
“Silvertown took quite a while in the gestation” he says. “We had some interesting challenges during its development but it has come through probably quicker than some.” Crossrail, now the newly-opened Elizabeth Line, is an obvious parallel – and one on which he was engaged both on behalf of TfL as the future maintainer and also for Crossrail itself as an expert panel member. Crossrail was approved in 2007, began construction in 2009, and opened in 2022. “Some of these things are very slow burn,” says Bowers.
For Crossrail2, which was a separate TfL programme, he was the initial Head of Engineering when TfL set up the organisation in 2015. Planning permissions and consents for such projects are of necessity not an overnight process.
“You do have a lot of stakeholders. It is not simplistically just the people who live next door. There are all sorts of community interests.
“The system in our Western democracy puts great emphasis on giving people the opportunity to express their views. The downside is that sometimes we can take a very long time to do it. It takes a long time to consult. So that, plus the time it takes to assemble the finance for a project, can make it frustrating from the point of view of those wanting to get the benefits of delivering something.
“It is something the current government has been picking up, recognising that there is a challenge in the infrastructure sector. They are going to look particularly at the planning requirements we have. But even if we get the planning system perfectly slick these things do take a while to build and there is sometimes disruption during the build so you can understand why the public mood is different at different stages.”
But as he points out, people do tend to like these projects once they are finished. As Head of Tunnels for London Underground, Bowers contributed to major constructions and upgrades at, among others, Kings Cross, Tottenham Court Road and Bank stations while a few hundred thousand commuters were using them to get to work each day. Getting tunnelling machinery in there and using it throughout the rush hours without closing down the stations or interrupting people on their daily commute must have been every bit as complex as the actual digging.
“London Underground has long valued what they call ‘lost customer hours’. That means they plan work to avoid disrupting people’s journey as much as possible,” he says. “And in those big station jobs, particularly the ones in the centre of the city, an awful lot of work goes into working out how to do it with the minimum of disruption.
“At Bank, for example, we didn’t start where the jobsite was. We put a shaft down and tunnelled in to the site to minimise the amount of movement we had to do right in the centre.
“For the big urban jobs there are two key problems that you tend to run into every time. One is getting through the planning process – that can be hard work. And the other big one for a tunnelling job is usually not how to get your equipment in but how to bring the spoil out. The largest volume of material you are moving is generally the spoil, and an enormous pile of spoil in the middle of the city is a bit of an embarrassment. If you can solve those two issues, of consent and of getting the spoil away, you have actually addressed a lot of the challenges.
“So consent is the first hurdle: balancing long-term public utility against cost, environmental damage, what people who live there want, what people who don’t live there but will use the infrastructure want, and so on. There are obvious considerations, and also less obvious ones.
“The business case for the Lower Thames Crossing has a number of positive impacts, but obviously one of them is around the relief of the existing transport system,” he says. “One of the things people don’t realise is that a very high proportion of the traffic across the Thames is freight. It is heavy goods vehicles rather than cars; and that’s partly because an awful lot of the UK’s freight actually comes in through the ports in the South East. The Channel Tunnel and Dover feed in heavy goods vehicles that aren’t necessarily going to Kent or Surrey.
They could be going to the West Midlands or somewhere to the north; so there is a big demand for freight movement across the Thames.
“But if you look at the Thames Estuary a bit more widely you have in some ways quite similar areas each side of it, in Kent and in Essex. You might expect there to be quite a lot of movement of people between those places, of people going to work or whatever. The freight doesn’t have much choice. It pretty much has to get across the Thames. But people do have a choice: they make that choice by not getting a job the other side of the river. And yet Tilbury on the north bank and Gravesend on the south bank are only about a mile apart. Why would people not want to live in one and work in the other? The answer, fairly self-evidently, is that there’s a river in the way and it’s not very easy to cross it.
“So we have the nationwide issue of getting goods from south to north; and the local issue of allowing people to travel to the county next door. If we remove the constraints on movement we potentially help solve both.
“You get into things like that as part of the benefits of a Lower Thames Crossing, and that’s a good example of planning nowadays. We very rarely develop these transport infrastructure schemes with just one objective in mind. We look much more than we used to at what we get returned in social value, the community benefits. By adjusting the infrastructure you spin off business opportunities.
Someone running a business in Gravesend previously had no clients in Tilbury; he could only look south. When the crossing opens he can look north as well as south: all of a sudden you double his market.
“As well as those kinds of considerations we can do a bunch of other community things off the back of a big infrastructure investment. We are intending to create new parklands, plant comfortably over a million trees, create new habitat areas. That is a project requirement. We will want to design it properly in ecological terms to get the best balance of species, so you suddenly find that in developing your tunnel scheme there is a whole relationship with Natural England [the government body responsible for the environment] and various agencies for birds and different types of wildlife. All of these groups have proper and legitimate interests that you want to serve; you end up with a fairly complex set of stakeholders.
“So infrastructure planning today looks at a much wider community than simply the people who actually want to make the journey through your tunnel. We have to look at social value, which is the legal term but it includes all of the wider range of things.
“Biodiversity and carbon footprint are two of the big ones but there are many others. Another big one for us at the moment is around skills and training, because although the LTC is close to London it goes through areas that are definitely at the lower end of national deprivation scales.
“We are going in there to do a job, so we shall need people to do the job. We see a lot of benefit in providing local employment; so that has taken us into working with local authorities, local colleges and educational institutes and so on to look at the whole skills and training agenda.
“The delivery phase is at least six years, which is big enough and long enough to justify some reasonably substantial investment, but then to make that investment truly effective you really do need to focus around skills that are relevant to us, certainly, but actually skills that are generally in somewhat short supply across the construction sector and that will be of benefit long after the tunnel is completed.
“So on both sides of the river at the moment we are developing skills hubs which are basically training facilities.
The first of them are just in the very early stages of operating. Obviously, our hope and our intent is that the project will then benefit from having a more skilled local community, but then afterwards it may well be that people in the community are up-skilled, to use the jargon, and then go and gain employment somewhere in the construction industry not actually directly driven by the project.
“It is an example of the way you can use the opportunity of a big project with a big investment to generate a wider social impact. And you can argue, I think perfectly reasonably, that there’s a social responsibility to it. We are using large sums of public money so we should be trying to extract as much benefit to the community as we can. It is very easy to focus in on the tunnel, which is a very impressive piece of engineering, but we can’t allow ourselves to be totally fixated on digging the hole. Some of us find digging big holes quite satisfying but there is a lot of depth and breadth of other things we can achieve as part of it.”
Recognising that, let us return for a moment to the hole. It is a large one, or to be accurate, two large ones. The Lower Thames Crossing is going to be twin tunnels, 4.25km long of which 1.3km is underwater. Of the total, 4km will be TBM drives, with the balance being short cut-and-cover sections at the ends.
There will be 27 cross passages, driven with a smaller TBM. The main bores are planned as a massive 15m (49ft) internal diameter, which calls for TBM size of very slightly under 16.5m. In other words the diameters will be huge.
“If you go back to the early parts of the underground system in London, they were 9ft or 10ft in diameter and were then enlarged to 12ft, which became the standard for a long time. A modern metro tunnel will be bigger: we’d probably go for about 5m diameter and we use that extra space to do things like putting in walkways. So if you plot diameter against year of construction there is a steady upward trend.”
A trend which LTC exemplifies in spades. “We are roughly 15 metres internal diameter, 16.5 on the outside. In round figures that is substantially bigger certainly than any other TBM tunnels you’ll find anywhere near. There are a handful now around the world at that sort of size. China has two or three on the go. There is the Alaskan Way in Seattle. But 16 metre tunnels are still relatively unusual and they do provide some different opportunities in construction.”
Opportunities rather than problems? He thinks so. “The actual TBM operation scales up with diameter pretty well. The operations at the front end – shoving the shield forward, building the rings, bringing in the segments and grouting them – all of that is much the same whether your tunnels are 5 metres or 15.
“But the large diameter does start to give us some real opportunities to do things a little bit differently. It is because the traffic envelope you need to run cables and pipes and MSVs through to service your TBM is not 15 metres diameter so it only takes up half of your tunnel instead of threequarters.
You need a certain proportion of that cross-sectional area to run your MSVs and all the rest, but you don’t need the full size so we can segregate the space within it quite early and use that spare space for other useful things.”
One of those things is building the cross-passages.
“Historically we have built cross passages concurrently while we’ve been driving the TBMs in the main bore, but we’ve had to build them with fairly traditional mining techniques by a fairly small gang with fairly small plant. For Lower Thames, there is space for maximising the mechanisation on these things.
“Effectively you have space for a rather smaller TBM which you drive from one bore to the other. Because it is only a short drive – the bores are about 16 metres apart – the TBM can do its job on umbilicals, and you can arrange the backup longitudinally along the side of the main bore. Even so it amounts to a fair bit of kit but if your parent tunnel is 15 metres internal diameter you can devote half of that space to the backup for your mini TBM and you can still have your MSVs for the main bore moving without restriction through the other half.
“Possibly a bigger opportunity is in fitting out the other systems in the scope of the contract, the equipment that make the tunnel finally useful: the road surface, the lighting and ventilation and communications and control systems and so on. Perhaps unusually, all of that sits within the same contract as the actual driving of the tunnel – and that means that you can integrate all those things within the one contractor’s programme, which obviously simplifies matters.
“So what we would like is to do as much of the early systems fit out, and possibly even some of the testing and commissioning of those systems, while we are still driving the main bores.
Usually you wait until the TBMs have finished, then you lay the road works, then you put in the electrical, mechanical and control systems. But with 15 meters to play with we can do an awful lot of that while the TBMs are still working.
“The way to achieve it is by building the road deck structure close behind the TBM. The road deck provides a very robust segregation between what is on top and what is underneath. In our case what is underneath is tall enough to drive a double-decker bus through, so we are talking a really big space. Our plan is to put a lot of that equipment underneath the road and install it while the TBM is still running, perhaps a couple of kilometres ahead. We have created a suitable sterile space for a lot of work, which is a more traditional programme would have had to wait for the TBM to finish.
“Along the length of the tunnel there are a number of plant rooms below the deck which are logical break points; the idea is that we build the road deck up to the first of them and then put a bulkhead on the end of the lower deck, and so we will have created a completely separate environment, sealed off for the dust and dirt and liquids and additives that go with the TBM and that you wouldn’t want to get on your brand-new sensitive electronics. So you have a clean, dry, sealed-off space underneath the road deck where you can start fitting out the power supplies and the transformers and the cabling for lighting and control systems and ventilation fans.
“So that’s a really good example of how we can actually get really tangible benefits from the very large size of the bore.
“The diameter also means that the user experience will be quite different to those in old and generally rather cramped tunnels. The intent is to create an environment where there is free flowing 70 mile an hour traffic in three lanes; it would be much closer to what you would experience on the open road, which is very different to what we have at the current crossings. There are safety considerations as well: if there’s an accident in one lane, it is more manageable when you still have two lanes working and more space for emergency services to get there.
“We shall be using a single TBM which will drive both bores. Two TBMs in parallel was an option we discussed for a while; it is the safer, more conservative assumption for planning but when we brought the contractor on board they were always clear that they saw advantage in doing this with one machine.
Distance is key here: our distances are sitting in the middle, close to the tipping point. Each bore is four kilometres so the total drive is 8 kilometres, and in principle 8 kilometres is perfectly doable with a modern machine. And obviously for us it briefly pops out into daylight halfway through when it finishes the first bore and is turned round for the return, so you get a good chance to look at it and maintain it at half time.
“On balance we concluded that a single machine offered a number of advantages. Clearly it will take slightly longer to drive one machine there and back than it would take to drive two side by side but that’s not the only consideration. The intensity of the work, the service these machines demand because of their size, the rate of spoil production, the demand for electricity consumption – all those sort of things are very, very large here. And obviously if you do two machines side by side you have to do twice as much as of all of that. You need a slurry treatment plant that is twice the size just for starters.
“So when you when you work it through, if you are confident that one machine can last the distance then there are actually very substantial advantages in just running a single system. It is less intensity of work, less capital cost. Very importantly there is substantially less carbon as well. So there are a lot of boxes that you can tick.
“And ultimately you focus your team on running one machine, not two, which has to be a less stressed task at the end of the day.”
Slightly related to that, one of the early discussions he was involved with in the Silvertown tunnel was deciding whether to construct it as a bored tunnel or an immersed tube.
“That was interesting. I had two periods of close involvement with Silvertown and that was the earlier one, around 2014.
Transport for London’s planning service had been developing the project but it had effectively become stalled because of that issue. At the time my role strictly was with London Underground rather than with TfL” [The former deals with the metro, the latter with buses, trams and the entire transport network] “and so in a sense I was outside the camp that was developing the scheme directly so they asked me for an outside view.
“The way we did it was by establishing an industry panel of suitably skilled people. We had a little group of eminent names, both from contractors and consultants, plus me from a client, and we did a review of the whole thing.
It wasn’t a totally straightforward decision but in the end it was a clear recommendation that we gave.
“If you look around the world, a lot of estuary and river crossings are done in immersed tubes. One of the fundamental reasons is because an immersed tube is inevitably shallower which means that your portal ramps at either end can be shorter. So they take up less land and you can start them much closer to the riverbank or even build them out into the estuary and thus reduce the length of your tunnel. Therefore they cost less.
“But with Silvertown a couple of big factors negated that. One was that the route on the Greenwich side was always going to go underground across the peninsula to have its portal over near the Blackwall approach road. It was never going to be close to the riverbank, so you couldn’t get the advantage of a shorter tunnel.
“The other factor was essentially environmental. The riverside obviously has a history of industrial usage. The Greenwich Peninsula used to host one of the world’s biggest gas works. That raised all sorts of issues. If you go digging around out there too long you’ll probably find contamination and have the risk of a few bits of ordnance so it’s not a particularly easy environment for a big surface excavation. There is a whole bunch of risks.
So in simplistic terms the environmental considerations and the fact you had to have a long tunnel anyway removed the advantage of an immersed tube.” Going back to the Lower Thames Crossing, are there any particular technical problems connected with the geology? If it were a normal diameter tunnel would it be a reasonably straightforward one, or particularly challenging? “There are some geotechnical challenges there, quite significant ones that are not that closely related to the diameter” he says.
“The geological challenge is largely at the North end. The area where the tunnel comes up would have been part of the floodplain of the Thames if you had gone there a thousand years ago. It would have been salt marshes on the edge of the river covered over at the occasional high tide. It is essentially marshland, and the marsh is deep. You have tens of meters of alluvium and peats. Below that you have a bit of terrace gravel, then below that you have the chalk.
“So you have to deal with those more challenging materials, and the problem is that they are saturated and extremely weak. And then they are nicely topped off by landfill because the site has been used as a dumping ground since Victorian times. The landfill we have in the tunnel area is, ironically, mainly from historic tunnelling projects. It is the spoil from building London’s underground.
“Which means that it is not as badly contaminated as you might have thought, but it is certainly an issue in terms of physical properties because it wasn’t placed in an engineered fashion: it was just tipped on top of the marsh. My colleagues like to call it blancmange on top of jelly, or possibly the other way round. So our first issue to deal with in engineering terms is how to create very large structures within this very, very soft ground. The North portal and the ramp down to it are large, of the order of 60 metres wide and a few hundred metres long, essentially a diaphragm wall and a base slab. So immediately you have to ask ‘What might we need to do to the ground in order to be able to safely construct this big volume in this very soft and somewhat variable material?’ Similarly for the first part of the tunnel there is a ring stability issue.
“There are probably some practical tunnelling issues as well. It is difficult to point a TBM in the right direction if the ground is too soft because it may develop a tendency to dive. So there is a recognition that we must do some fairly substantial management of the ground risk. There will be a lot of activity in the first couple of years of the programme to address that.
“And pretty much everything needs ground management. As well as those structures there are things like the segment factory, the slurry treatment plant, all the rest of the site set-up. We need to create roads and get utilities in there. All of those things generate demand for ground treatment of one sort or another.
“We are doing some trials early in the New Year to start testing some of the thinking on mixes. The focus for the tunnel and for a number of the major structures is on soil mixing techniques to increase the stiffness of the ground. That’s likely to be in conjunction with band drains for dewatering. We will almost certainly use piled foundations for things like the carousels in the segment factory. Piles are obviously expensive and carbon hungry; the route to minimising both has been to look for dual use of those foundations wherever we can.
“So what we will do is build the segment factory in the position where ultimately we have to put part of the permanent highway.
By doing that we can put in the permanent foundations, we will use them first for the temporary facilities, and after that we will use them for the permanent highway. That way we substantially reduce the amount of foundation work in the area. We are due to get started in the summer of 2026 on the real bulk of it.
“I have always been keen to push the idea that what you do on the current job and on the job after that cannot just be the same as what you did before. I have done a range of things and I’d like to think that some of the changes that have come in are part of what I have pushed through.
“Lower Thames Crossing is not necessarily the most complex of tunnels, but it is very big and that brings its challenges. It remains interesting.”
