The two tunnel boring machines that set off from just inside the M25 motorway are now three-quarters of their way under the Chilterns as they head north out of London.
At the tunnel entrance just inside the M25 motorway
The two TBMs are digging two tunnels for the HS2 railway and have now reached the Little Missenden ventilation shaft, more than three-quarters of the way through their 10-mile drive under the Chilterns.
The two machines set off at staggered times in early 2021 and have now caught up with each other. Each machine is a 170m-long self-contained underground factory, digging the tunnel, lining it with 56,000 concrete segments to form rings and grouting them into place as it moves forward.
Designed specifically for the geology of the Chilterns, the TBMs have already excavated approximately 2 million cubic metres of chalk and flint. The soil will be used to landscape the tunnel entrance into a chalk wildlife area, replacing the monoculture fields that had been there before. Chalk grassland used to be widespread across the hills of southeast England and is considered a habitat of international conservation significance with just 700ha left across the Chilterns.
As well as digging and lining the tunnels, engineers have also completed the excavation of five shafts that will provide ventilation and emergency access near Chalfont St Peter, Chalfont St Giles, Amersham and Little Missenden with an intervention shaft at Chesham Road.
The 35m deep shaft near Little Missenden – which the TBMs have now reached – is next to the A413 about half a mile from the Buckinghamshire village. A ‘headhouse’ will be built on top of the shaft to house ventilation and safety equipment, designed to resemble local farm buildings with new planting to help blend it into the surrounding landscape.
Little Missenden vent shaft site with A413 in the background (c) HS2
The two TBMs are operated by, Align – a joint venture formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.
It’ll take about three years to complete construction of the tunnel, and about three minutes for HS2 trains to pass through when they’re carrying passengers.
HS2 currently has five TBMs in the ground, with a further five due to be launched over the coming years. Together they will create 64 miles of tunnel between London and the West Midlands including major tunnels on the approach to London and Birmingham.
You say three year project that began in early 2021, I assume they’re saying Easter 2024ish for completion?