Next to the M25 in northwest London is a huge railway construction site that won’t connect with either of the two railway lines that run on either side of it and yet when it’s finished it will significantly increase the number of trains that commuters will be able to catch on those other lines.

This is HS2’s south portal site, and the largest of the railway sites that will build the new HS2 railway that aims to massively increase space on the railway for more commuter and regional train services.

It’s a dual-site, building a viaduct towards London as well as sending two tunnel boring machines (TBMs) under the Chiltern hills. The first of the TBMs was launched a few months ago, and is now outside the M25, with the second TBM now eating its way into the tunnel.

As someone who has been on a number of building sites in urban areas, the main thing that strikes you, apart from it being 3 miles from the nearest public transport, is how huge the site is. Walkways that are usually narrow here can be wide, and a walk around an urban building site that normally takes 10 minutes here needs a minibus to get to different areas.

The cutting for the TBMs and later railway tunnel portal.

This huge site is both needed to be huge because of the amount of work that will be carried out here, but also because pandemic notwithstanding, Britain’s railways are bursting at the seams during rush hours and something needs to be done to cope with demand.

That demand will recover post-pandemic for while some businesses can function remotely all the time, as many more are finding out, they want staff back in the office, at least some of the time. An increase in working from home was always expected to happen even before the pandemic and is factored into most transport plans, and the pandemic is generally thought to have brought forward existing online/WFH trends by about 5-years. That still means a lot of commuters, and unless rail capacity is increased, it’ll mean people are still being squashed into trains every day.

HS2 will take all the long-distance trains off the lines they share with commuters, put them on their own line, which then releases incredible amounts of capacity on the commuter and regional railways for everyone else to use.

So, here in northwest London, two tunnels are now being dug under the hills to carry those intercity trains. But it’s taken a couple of years of preparation work to get to the point where the TBMs can start digging their tunnels.

A deep cutting had to be constructed to lower the TBMs down to the level of the tunnel entrance, and will later become the portal for the high-speed railway. All the facilities for staff had to be built on-site, and the factory that will be making the concrete rings to line the tunnel. Also, a second factory will be making the parts for the viaduct heading into London.

To bring equipment and materials onto the site, they are conveniently next to the M25 motorway, so two slip roads have been built connecting the motorway to the construction site. For motorway nerds, as the two slip roads connecting the M25 to the construction site are for works access only, Highways England didn’t grant them a formal Junction number.

The advantage of the motorway entrance also means they’ve been able to keep the heavy lorry traffic away from the nearby towns on the other side of the construction site.

A staff entrance on that other side of the construction site was originally expected to bring staff in by buses, but the pandemic made that impossible so they had to build more car parks and let people drive to work. They also have a large two-storey welfare and office building, which gained a third floor to help with social distancing.

The site, being until recently mainly above ground preparations and with a lot of staff working in single-driver trucks and lorries wasn’t as badly affected by the pandemic as it could have been, but now that tunnelling has started and also more people are working inside large warehouse buildings, it’s starting to become more of an issue. No delays so far though.

First the tunnels

As the ground they will be tunnelling through is mainly chalk and flints the spoil from the cutting head is mixed with water to form a chalky slurry that can then be pumped back down the tunnel to the outside, where the flints are filtered out, the slurry dried out to remove the chalk and the water reused.

By the most remarkable coincidence, the area that’s been dug down to create the entry portal for the TBM’s is where a series of old, and very much smaller, chalk pits used to be. Since those pits closed the area has been arable farmland. Now it’s a deep cutting in the chalk landscape, with the huge concrete retaining wall that will later become the railway portal, but today gave the tunnel boring machines something to grip onto as they started digging.

The two TBMs are, using the International Standard for Measuring Things, roughly the same length as 16 London buses.

Just as tunnel boring machines are given names, in this case, Florence and Cecilia, there is a tradition that tunnels portals should have an icon of Saint Barbara, Patron Saint of Mines on them during construction.

St Barbara in a niche between the tunnels

The staff who work on the TBM work 12-hour shifts, with typically several days working followed by time off work. For their convenience, there’s even a small hotel on-site where they can stay while working shifts. The TBMs also include their own canteen area, toilets (yes, for both men and ladies), and in another innovation, an on-site grout factory that makes the grout used to join the concrete tunnel segments.

Everything the TBM needs to do its job has to be delivered down the tunnels using vehicles specially designed for the job. As the tunnel gets ever longer, they expect towards the end of tunnelling, that the journey from the building site to the tunnel head will take up to an hour each way.

That causes a problem, as at the moment the service vehicles drive along the bottom of the curved tunnel which means only one vehicle can drive down the tunnel at a time. They need more than that, but to pass each other would need a wider flat surface. Which is soon to be built.

The curved bottom of the tunnels will be filled with concrete to form a flat road surface, and in order to be able to pour wet concrete while still allowing vehicles to drive over it, they’ve developed a movable bridge that will follow behind the TBM up the tunnel. Under the bridge, a new roadway will be poured, while the service vehicles drive over the top. When the tunnels are completed, the roadway will become the bed for the train tracks. The use of a moving bridge allows the concrete bed to be built even as the tunnel is being dug – often that happens afterwards.

Service vehicle heading into the tunnel – they have drivers cabs at both ends as they can’t turn around in the tunnel.

One thing which is sort of possible to show in photos but photos don’t fully do it justice is just how incredibly big these tunnels are. They are massive, and everyone on the site visit I attended let out audible gasps when we got up close to them.

The steel pipe provides ventilation into the tunnel.

TBMs are bespoke built for each project, but they also progressively get better with each generation of design, and HS2’s TBMs are the first large scale deployment of semi-continuous tunnelling. Conventionally, a TBM digs out space at the front then the whole cutter head is pushed forward leaving a space behind in to which a ring of concrete is assembled, and repeat. HS2’s TBMs are able to push forward off the bottom three segments of the tunnel ring even before the remaining four segments have been installed. That speeds up the tunnelling.

TBM Cecilia slowly pushing into the tunnel face. The small orange blobs are humans, for scale.

They’ve also introduced a couple of robots to improve safety. Normally there have to be humans in the segment installation area to install dowels into the segments to hold them in place and remove the wooden spacer bars that separate the concrete segments in storage. These are normally done by humans, but working in a confined space where precision timing is needed can be hazardous, hence the development of robots to do the job instead.

Amongst all the construction site noise, there’s a constant here that tells everyone that things are working fine – the sound of small stones tinkling as they flow down a long pipe. These are the flintstones that have been dug up by the TBMs and are crushed before being sent down the pipe along with the chalk slurry.

Nearby chalk is already piling up under shed roofs ready to be used for landscaping the area after all the construction work has finished.

The back of the first TBM is just visible in the tunnel.

The tunnel segment factory

Also nearby are two huge hanger-like sheds — the concrete batching plants and the mould making factories.

The viaduct factory

Each tunnel ring is made up of seven segments. Each ring is identical, but with a slight angle on one side. If the tunnel needs to curve or dip slightly, as each of the tunnel rings are installed they rotate them around just a little a bit and that angle causes the tunnel to slowly curve around or down as needed.

Segment mould

Just as robots have been introduced on the TBM, they’ve been added here as well. One of the most unpleasant jobs in a concrete factory is cleaning the mould each time it’s used. Now a robot does that. They also have a robot that finishes off each mould when they are filled with concrete. In between them, lots of humans are greasing moulds, filling moulds and checking moulds.

Unlike say the Crossrail segments factory where the moulds are all laid out in rows, here a conveyor belt moves them between stages of being cleaned, filled, and then cured before the completed concrete segments are removed to be stored.

The concrete is pretty routine for tunnel segments, being a fibre-reinforced mix, but in some areas where extra strength is needed, they have reinforced steel cages that can be added to the moulds. These are typically for use where the tunnels meet ventilation shafts or side passages.

Preassembled rebar

Since production started, they’ve built up a 3-month supply of segments, and the aim is to maintain that buffer until they get close to finishing so that there’s never a supply shortage of tunnel rings for the tunnellers.

Collecting segments to take to the tunnel

After they leave

Although most of the work over the past few years has been to prepare the site for tunnelling, they are already preparing the site for when they leave.

The area, once a series of monocultural arable fields of limited benefit to wildlife will be turned into a large chalklands based wildlife park, which will also be open to the public to wander around. Resuing the chalk dug up by the tunnel boring machines in this way not only expands otherwise shrinking calcareous grasslands, but it also eliminates the need to remove the spoil to another location.

But, you can’t just pile chalk rubble upon the ground, and expect it to sit there. You need to prepare the ground first, and that’s what is already underway in some nearby former chalk pits which will be landscaped. They have to add a lot of drainage channels and put down supporting gravels, and then they can slowly start building up the chalk valleys that will later become the wildlife haven they want to leave behind.

Preparing the land for 20 metres of chalk to be piled on top.

All this work is needed to deliver something the UK’s railway networks are clamouring for — more rail tracks. HS2 will take most of the intercity trains off the current railway, leaving vast amounts of capacity for more regional and commuter trains.

When HS2 opens, for example, Chiltern Railways whose tracks run either side of the construction site are likely to have more capacity to run more trains more often, so while the people living near this HS2 construction site won’t get a new station, they will be getting a lot more trains at their existing station. All because a new railway took all people travelling between cities, whether for meetings or to visit families and friends, and put them onto their own dedicated railway to create more space on the existing railway.

But back to this piece of the HS2 jigsaw — it’ll take around 3 years to dig those two tunnels, and when the railway opens, it’ll take a high-speed train just 3 minutes to pass right through them.


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  1. Phil Johnston says:

    Excellent stuff – my only question is how Chiltern Trains will get more capacity? I get the WCML etc getting more capacity for local/stopping trains, but there aren’t much in the way of intercity trains on Chiltern?

    • ianVisits says:

      There are some intercity trains on Chiltern, but the main benefit is how the whole UK rail network can be reshuffled around to maximise the way trains are used to boost regional and commuter lines. The specifics for each line are still being worked out, but if you don’t need to run trains from London to Marylebone anymore, then that releases more capacity for commuter services.

    • James says:

      You’re probably right that the capacity benefits for the Chiltern line will be marginal (unless they add an intermediate station or two on HS2 in the Chilterns, as they have done on rural stretches of the TGV network in France), but the new line will probably all but kill off the CR Marylebone to Snow Hill service, with the West Coast’s Euston to New Street becoming the new slower/cheaper alternative. If that happens, there should be some more room for commuter trains on the line, albeit not loads.

  2. Richard says:

    Having lived very near the tunnel portal most of my life I, in Buckinghamshire, I had to look up the boundary as I didn’t think the site was in London. It’s bang on the Bucks boundary, but actually in the most southern tip of Hertfordshire, which did surprise me! 🙂 it’s a very odd boundary that seems to encompass the site perfectly.

    Great article though! Thanks!

  3. Tink Hefferon says:

    Amazing how they dig the tunnel today canteen, toilets,back in 70s we had our food bought to the face on the loco hard graft 12hr shifts week of days then nights, kilroes, streeters, kinear moody, I think I’ll see if I can get the start down there 😀

  4. James Sargeant says:

    The steel ventilation pipe looks massive. As the TBM moves slowly forwards (15m per day?) does the ventilation pipe get an extension? Does the 200m long TBM have a good onboard store of 2m? diameter sections for the ventilation pipe? And what is carried in the other smaller section pipes please? Maybe water in and sewage out? There must be a team of expert plumbers onboard.

    • Pete D says:

      Water in, Slurry (water plus excavated material) out.

    • Neil says:

      While Crossrail used to publish a progress map for the TBMs, HS2 is not, and there could be security reasons for that given the Stop HS2 antics. However, it would be great to have a report on how well the first TBM Florence, is performing, in terms of physical progress and TBM operation.

    • ianVisits says:

      Stop HS2 activists won’t be able to stop a TBM that’s 30 metres under the ground.

    • mark bates says:

      All the services are extended as the machine drives forward re the ventilation feed that will be on and with the TBM for the whole drive

  5. Ace says:

    A significant benefit of course that the chiltern line will gain is the passive provision being made for services to terminate at old oak common. As well as creating additional London terminating capacity I think this would end up being very popular by proving better connections than Marylebone allowing onward travel to HS2, crossrail for the city and Heathrow, London overground services and GWML services – and access to the jobs being created in the ‘canary wharf of the west’
    I don’t think all the funding is locked down yet but suspect the strategic value of this link will see it provided for by the time the full HS2 network is complete

  6. Neil says:

    Your response does you no credit. My point is that HS2 communication strategy now has to take account of Anti-HS2, in the way it focuses on promoting aspects of the project. Protesters crave the oxygen of publicity, and a lot of attention on TBM progress may well encourage them to find ways of disrupting these project elements.
    I take it that you have not asked HS2 how, what, and when they will inform us of the TBM progress; and how actual TBM progress and performance compares to expectation.

  7. Sir Philip westward OBE says:

    Will the hs2 ever be built up the Lancashire Preston or Lancaster
    And HS3 up the Carlisle and Glasgow

  8. Clive Broadhead says:

    The TBM’s should be digging the InterCity tunnels between Leeds and Manchester as a first priority to achieve the promised up levelling.

  9. Mr James Emerson says:

    Ian, as a nearby resident and neighbour of the landowner where the tunnel starts you’ve provided an informative article that’s fresh and with facts that have been missing from the News/Media.
    You described the technology well and I became more fascinated as I read more. Thanks for your article.

  10. Kevin M says:

    Hi Ian.

    Great page, how did you manage to organise the visit?

  11. Winston says:

    Seriously hope once the first leg is done the public/government see the value of it and keep the plan for a leg up to Leeds. Look at all these other countries with highly complex high speed trains, nobody protests when they decide to build another tunnel. The problem is that the protesters minds are so short sighted and cant see that there will always be a need for more network.

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