Two giant tunnel boring machines have been delivered to a construction site in West Ruislip, where next year they will start drilling HS2 tunnels towards the HS2 station at Old Oak Common.

The 1,900-tonne machines were delivered to HS2’s last month in an operation that required a huge amount of logistical coordination, with support from the Police and National Highways. The largest components of the machines were transported by road from Essex ports on vehicles that could support the extra-wide loads.

The two machines have been designed specifically to bore predominantly through Lambeth group (a set of geological rock strata), chalk and London clay, and are automated to install 4,220 tunnel segment rings each, to create each 5-mile tunnel. A team of approximately 40 people are now working to assemble the tunnel boring machines (TBMs), preparing for the launch in 2022.

In addition to assembling the TBMs, a lot of work has gone into preparing the launch site. A new 33 kilovolt 20MVA cable power supply had to be dug from a substation near Rayners Lane under the streets to bring enough electricity to the West Ruislip to power the TBMs as they chew through the ground. Once the tunnelling is complete, the power supply will be used to power operational equipment within the tunnels.

The site is also unusually shallow for launching a tunnel boring machine, as they tend to need a lot of weight above the tunnel they are digging to help keep it stable, and here there’s barely 4 metres of soil above the top of the launch site. That’s required a lot of reinforcement to stabilise the soil at the tunnel site, and also the addition of landfill ballast to the top to add the necessary mass to prevent “tunnel floatation”, which where the tunnel starts to shift upwards because the weight of the tunnel is less than ground it displaced, and there’s not enough weight above the tunnel to hold it down.

There were also a lot of ground surveys along the route to make sure it’s made up of the sort of clay they want to drill through and there are no unexpected outcrops of rock or chalk, and it was during these surveys that they discovered the “Ruislip Bed” in 2018. This layer of unexpected black clay, found some 33 metres below the surface is thought to have been formed from densely wooded marshes on the edge of a sub-tropical sea that covered London around 56 million years ago.

At the time of the discovery, Dr Jacqueline Skipper, a geological expert from Geotechnical Consulting Group discussing the newly discovered Ruislip Bed said that it “would have been formed during the Paleocene period, which was a time of intense change, with new animals evolving following the extinction of the dinosaurs. Most of Southern England was covered by a warm sea and this clay helps us to pinpoint where the coastline was.”

It’s a useful side effect of large construction sites, is that while they are of necessity temporarily destructive to create space for what’s to be built, they are also often unique opportunities for archaeology and science to discover new things.

The two machines now being assembled in West Ruislip will be the first of six TBMs that will bore 13 miles of twin bored tunnels underneath London where HS2 services will run. They will join eight other machines that will operate on Phase One of the HS2 project between London and the West Midlands, which will dig 64 miles of tunnels in total.

As the date for tunnelling to begin draws closer, HS2 will be seeking to name each TBM and has already begun that process, working with local schools in Hillingdon where the TBMs will be launched. In early 2022, a shortlisted set of names will be put to a public vote, to help choose the names for HS2’s fourth and fifth TBM, after Florence, Cecilia and Dorothy who have already started their journeys underground.


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  1. SteveP says:

    Nice explanation. I assume as with most other TBMs, they will be “parked” off to the side of the final bore in a permanent grave once the work is finished?

    • SteveP says:

      The “Northolt” tunnels between Ruislip and Old Oak Common are being “dug from both end” (so 4x TBM’s) to meet in the middle at (IIRC) a place called “Greenway Park” roughly 700m to the East of Greenford Tube station next t the Royal Mail building. There a vent shaft and electrical feeder station is being built. I understand the plan is to lift out (at least) the cutter heads and cans there and truck them away rather than bury them.

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