Preparation for the arrival of HS2’s first giant TBMs has passed a major milestone with the completion of the 17m high headwall and ground reinforcement at what will become the south portal of the Chiltern tunnel.

A specialist team, led by HS2’s main works contractor Align JV – a joint venture formed of three companies: Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – spent seven months excavating more than 160,000 m3 of material in order to create a level surface from which the two enormous machines will launch early next year. The two TBMs due to arrive at the site, near to the M25, later in the year.

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 the site just inside the M25.

Alongside the excavation, a specialist team of subcontractors including Roadbridge and KV JV - formed from Keller Group plc and VSL International – also constructed a 17m high headwall through which the TBMs will break to begin their three-year long tunnel drive.

In order to reinforce the ground behind the headwall, more than 636 ‘soil nails’ – some up to 20m long – were driven into the wall and connected to the concrete lining. The nails, consisting of steel or glass fibre reinforced polymer tendons, were inserted using a specialised drilling rig in a delicate operation which can take up two hours.

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

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