TECHNICAL | SOFT GROUND TUNNELLING
Right:
Segment storage for TBMs at Old Oak Common launch and support site
Geology & spoil removal The geology that the TBMs will be tunnelling through consists of London Clay. “At the cavern starting point we are above the water
table, but we are on a down gradient and the depth of the table varies along the route, but it is below us all the way,” says Edmonds. “And, the clay is in any case impermeable.” Michael Wilson is the TBM pilot supervisor. “The water
table is below us the all the way – if it was up a little bit, it’d be easier to cut because it’d be moist,” he says. Foam is added to the muck to prevent the clay swelling and clogging the extraction screws. “If we get the foam right, it’s a magic job for us. Once we get it right then we can get it going automatically – foam first and then we advance. It’s beautiful.” “The geology itself is fairly straightforward,” says
Edmonds. “The major complication of this part of the project is that we are excavating under a city, with all the constraints that that brings of space to lay out equipment and of infrastructure overhead that must be taken into account. It is not the geology, it is the physical constraints round the site that make it difficult.
“The route goes under the Grand Union Canal and
then takes us 66 metres below Primrose Hill. That is the deepest we go.” At the launch, in the Old Oak Common cavern, there
is approx 15m cover, below the surface rail lines, but the vertical alignment is then downward – with 3% the steepest gradient – “so we do gain depth fairly rapidly,” he adds. “There is a huge quantity of assets – buildings, many
of them Listed, and other infrastructure – that we have to look out for all the way through. And, the asset control and condition monitoring we have to do on all of those structures is a massive challenge. Near Euston there are a lot of historic railway structures; we needed engineers’ assessments on those. “And, before that, we pass under Kensal Green
Cemetery,” he adds. The cemetery opened in 1833 and contains many large Victorian monuments. “Some of the monuments were already leaning, and we shall be setting them upright. It is a public site, so we have put scaffolding around some. We are having to check every monument there. Ironically enough, Isambard Kingdom Brunel’s is one of them.”
Right:
Schematic layout of HS2 tunnels and caverns at the end of the Euston Tunnel, in the Euston area, north central London
20 | May 2026
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