HS2 tunnel portal headwall completed

Preparing the tunnel portal headwall at Chalfont Lane.

Work on Britain’s new HS2 highs-peed line is continuing to make progress as  main works contractor Align JV – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick – has completed the 17-metre-high headwall and ground reinforcement at what will become the south portal of the 10-mile long Chiltern tunnel.

The contractor spent seven months excavating more than 160,000 cubic metres of material in order to create a level surface from which HS2’s first giant tunnelling machines will launch early next year.

Weighing in at 2,000 tonne and 170 metres long – the two tunnel boring machines (TBMs) are being built by world-leading German tunnelling specialists Herrenknecht and due to arrive at the site, near to the M25, later in the year.

Each tunnel-boring machine, seen here during manufacture, will weigh 2,000 tonnes.

Alongside the Align JV excavation, a specialist team of subcontractors, including Roadbridge and KV JV (a joint venture of Keller Group and VSL International), constructed the headwall, through which the TBMs will break to begin their three-year long tunnel drive.

More than 636 ‘soil nails’ – some up to 20 metres long – were driven into the wall and connected to the concrete lining to reinforce the ground behind the headwall. These nails, consisting of steel or glass-fibre reinforced polymer tendons, were inserted using a specialised drilling rig in a delicate operation that can take up two hours for each one.

The finished headwalll is 17 metres (56 feet) high and dwarfs nearby machinery.

This reinforcement is required to hold the ground and the surface of the wall in place as the TBMs begin to break through.

Mark Clapp, HS2.

Mark Clapp, HS2’s senior project manager for the C1 section of the route, welcomed the achievement: “The completion of the headwall and ground reinforcement is a major step towards the start of tunnelling.

“Safety is, of course, our top priority. The new covid-19 processes and procedures put in place by Align ensured that we were able to keep to schedule while keeping everyone safe – and I’d like to thank the whole team for pulling together during what has been a difficult few months for the whole construction industry.”

Daniel Altier, Align JV.

Daniel Altier, Align project director, added: “The completion of the soil nailing is a very visual example of how we are preparing our South Portal site for the arrival of the TBMs later this year. It is now clear for all to see where the TBMs will start their journey.

“This has been a great team effort by all concerned, considering the challenges caused by Covid-19, including HS2, Align and our colleagues at Roadbridge and the KVJV.”

Align is delivering the C1 portion of HS2 phase 1, which includes the 10 mile long (16km) twin bore Chiltern tunnel and the 3.4km Colne Valley viaduct. The work is being led by an integrated project team based at Chalfont Lane, just inside the M25.

Work was paused for four days at the start of the Coronavirus (COVID-19) lockdown in order to deep clean the work site and put in place new hygiene and social distancing procedures. in line with government advice and industry best practice.

Once work is complete, the whole construction site will be landscaped with material excavated from the tunnels and trees planted so as to blend it in with the surrounding countryside.

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