A 17-metre high headwall at what will become the south portal of the 10-mile long HS2 Chiltern tunnel has been completed.

The headwall has been finished ahead of the arrival of HS2's first giant tunnelling machines.

A specialist team has spent seven months excavating more than 160,000 cubic metres of material to create a level surface from which the two enormous machines will launch early next year to dig under the Chilterns.

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

A team of subcontractors have also built the 17-metre tall headwall - which the TBMs will break through to begin their three-year tunnelling drive.

To reinforce the ground behind the headwall, more than 636 ‘soil nails’ – some up to 20 metres long – were driven into the wall and connected to the concrete lining.

Mark Clapp, HS2 Ltd’s C1 Senior Project Manager said: “Once complete, HS2 will transform rail travel across the UK, offering fast, reliable and low carbon journey options for millions of people across the country.

"The completion of the headwall and ground reinforcement is a major step towards the start of tunnelling and delivering on that goal."

Daniel Altier, project director at Align, which will build the first phase of the controversial railway line including the Chiltern tunnel and Colne Valley viaduct, 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."

Work at the site was paused for just four days at the start of the coronavirus lockdown in March so the site could be deep-cleaned and new social distancing measures put in place.

HS2 has said the whole construction site will be landscaped with trees planted to try and "blend" it with the surrounding countryside.