Seventy metres down, in deep incognito beneath a disguised ventilation shaft in the Chilterns countryside, lies HS2’s buried treasure: two 10-mile tunnels, built to avoid an area of outstanding natural beauty, eerily spectacular in gleaming concrete.
They are, laments a staffer on the high-speed railway scheme, what all of the route should look like by now: pristine, fully constructed, and just waiting for a railway to run through them.
The ballooning cost of HS2, and the construction delays that have dogged the project, have provoked much soul-searching and bluster about a supposed national inability to build infrastructure.
Main tunnelling was completed last year, and with the final shafts, the construction phase is now officiallyfinished/ Photograph: Graeme Robertson/The Guardian
From mothballed worksites scarring central London to abandoned routes in the Midlands and the north, plenty has not gone to plan. But this first look inside the completed 9m-diameter tunnels – their ventilation shafts the final piece in the jigsaw – shows what civil engineers with a clear remit can do.
This, ironically, was one of the first areas of dispute and redesign of the railway’s route. Uproar over the potential disruption – in Conservative marginals, no less – saw extended tunnelling agreed before HS2 was anywhere near the statute books.
While politicians continued to mull the final route, and scrapped designs for Euston, construction work here took off at the start of the pandemic. The compound housing HS2 Ltd’s main works contractor for this section of the line, Align JV – a joint venture between Bouygues, McAlpine and VolkerFitzpatrick – grew to about 1,800 people at its peak.
The compound also served as the base to build the stunning Colne Valley viaduct, a 2.2-mile rail bridge skimming the nearby reservoirs. A concrete production plant was set up on site for the viaduct and then the 112,000 tunnel segments required. Two tunnelling machines brought in from Germany worked 24 hours a day, seven days a week, staffed in 12-hour shifts over a period of 33 months from the first launch in 2021.
The stunning HS2 Colne Valley viaduct. Photograph: Leon Neal/Getty Images
Main tunnelling was completed last year, and with the final shafts the construction phase is now officially finished. Mark Clapp, HS2 Ltd’s head of civil engineering for the tunnel, says those who delivered a project of this scale and complexity on schedule should “feel certain that their hard work will stand the test of time”.
But building infrastructure is far from popular with local people, and Clapp is quick to reel off the mitigations, including rerouting work traffic, as well as installing a slurry treatment plant on site for the millions of tonnes of excavated earth. “We took the chalk out of the spoil and reused it to landscape the area to create more calcareous grassland – it’s a rare and declining habitat, and we’ve added to it.”
The ventilation shafts – required to be large enough for emergency and maintenance access – were also designed with local impact in mind. At ground level, the headhouses, by the village of Chalfont St Peter, resemble small agricultural buildings – masking a cavernous basement big enough to house giant fans and machinery, that might have stood several storeys above ground.
The design also allowed the shafts’ volume to be minimised, according to chief engineer for HS2, Mark Howard: “A lot of the conversation was about the disruption, as well as what the headhouses look like. The more you excavate, the more you bring out – and double the amount of spoil means double the amount of lorries on the road.”
The tunnel portals hold further innovation: a funnelled opening designed to stop the sonic boom that high-speed trains could otherwise produce when entering at 200mph. Japanese bullet trains have a tapered 18 metre-long nose to avoid the boom when emerging from tunnels, but for physical and economic reasons, the UK could not do the same: “That protruding piece would just smack into the old Network Rail infrastructure. And it takes up the space of about 40 seats,” says Howard.
The HS2 section under the Chilterns. Photograph: Graeme Robertson/The Guardian
Lab testing ensued to understand micro pressure waves, which involved shooting miniature trains on elastic bands through tubes – resulting in the construction of 200m-long perforated concrete funnels to stop the boom.
Eventually, from these portals, trains should emerge in just three minutes about 10 miles north of here. But that three-minute ride could take another decade to start. Next comes the installation of mechanical, electrical and plumbing equipment. Rail systems, including track and overhead electrical equipment to power trains, will follow. Years of testing will follow that.
And first, the reset of the entire HS2 build awaits. The progress on the Chiltern tunnels makes them outliers, even in what was once just phase 1, from London to Birmingham. The original £32.7bn budget, then including the Leeds and Manchester lines, is expected to be revised to beyond £80bn at current prices.
A spokesperson for HS2 Ltd said: “Our chief executive, Mark Wild, has been clear that overall delivery of HS2 has been unacceptable, and he’s committed to ending the project’s cycle of cost increases and delays.
“Over the last year he has led a comprehensive review, which has scrutinised every part of HS2 – creating a framework to build the railway efficiently and for the lowest reasonable cost. This means putting the construction programme back into the right sequence, transforming HS2 Ltd into a leaner delivery company, rebooting our relationship with the supply chain and driving significant improvements in productivity.”
With more than a year having elapsed since Wild took over as chief executive in December 2024, the promised reset itself appears tardy. But after the wholesale changes made by previous governments, culminating in the overnight axing of phase 2 to Manchester, ministers have pledged to give Wild time to produce what they hope will be a decisive plan.
The finished tunnel. Photograph: Graeme Robertson/The Guardian
Wild’s initial assessment, sent to the transport secretary last March, was that hopes that HS2 would be up and running by 2033 were unachievable. Since then, Wild and the Department for Transport have been working on what can be done when, and at what cost.
The leadership team has been revamped, including the hiring of ex-London Transport commissioner, Mike Brown, as chair. But hundreds of other corporate roles have been axed in favour of frontline civil delivery jobs. Meanwhile, commercial renegotiation with the supply chain, to hammer out contracts that do not simply load ballooning costs back on to the taxpayer, is continuing this year.
Last week, ministers said a line from Birmingham to Manchester to link with Northern Powerhouse Rail was still “an intention” – even if they stressed it would absolutely not be another part of HS2.
Wild, whose last job was to steer Crossrail to open the Elizabeth line on a reconfigured budget and schedule, may have a tougher task to restore the progress and reputation of Britain’s high-speed railway. But when it opens, there may again be plenty to marvel at.
