KEITH BOWERS | INTERVIEW
Above SCL works over the SGI platform tunnel at Tottenham Court Road station
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.
Above SCL tunnelling for LU at Tottenham Court Road’s Northern Line concourse “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.”
February 2026 | 39
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45
