HS2 tunnel progress as ‘Florence’ and ‘Cecilia’ pass Amersham

The builders of HS2, the high speed railway project in England, have confirmed that ‘Florence’ and ‘Cecilia’ – the two giant tunnelling machines digging the project’s longest tunnels – have passed Amersham, 5-and-a-half miles into their 10 mile drive under the Chilterns (16 kilometres in total). The enormous 2,000-tonne machines have spent almost two years excavating the twin tunnels between the M25 London orbital motorway and the community of South Heath in Buckinghamshire. The builders say the tunnels will help the high speed rail project protect the environment while improving connections between London, Birmingham and the North of England.

Much of the HS2 project will be underground, in answer to a variety of environmental and engineering challenges. Each of the project’s tunnel boring machines (TBMs) is a 170-metre-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 first TBMs were launched in Summer 2021 from a site near the M25 and have excavated more than 1.8 million cubic metres of chalk and flint. One tunnel has already been completely bored, with several more works underway.

Excavation of five shafts

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, Little Missenden and an intervention shaft at Chesham Road. The 44-metre deep shaft at Amersham, which the TBMs have now passed, will be in the middle of a road junction just outside the Buckinghamshire town. A ‘headhouse’ will be built on top of the shaft to house safety equipment, with a flint-faced boundary wall and a pre-patinated zinc roof to help match the natural tones of the surrounding landscape.

Once complete, say HS2, trains will pass through the tunnel at speeds of up to 320 kilometres per hour, providing zero carbon journeys between London, Birmingham and the north while freeing up capacity on the existing rail network. “The Chiltern tunnel will take HS2 underground and safeguard the woodlands and wildlife habits above the tunnel as well as significantly reducing disruption to communities during construction and operation of the new railway”, said Martyn Noak, HS2 Ltd’s Head of Tunnel Engineering.

2.7 million cubic metres

Each machine has a crew of 17 people, working in shifts and supported by over 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation. Approximately 2.7 million cubic metres of material – mostly chalk and flint – will be excavated during the construction of the tunnels and used for landscaping. Once construction is complete, the temporary buildings at the south portal will be removed and the site landscaped with around 90 hectares of new wildlife-rich chalk grassland habitats.

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 (103 kilometres) of tunnel between London and the West Midlands including major tunnels on the approaches to London and Birmingham.

Further reading:

• Sweet Caroline comes on stream for HS2’s 30,000 project workers
• HS2: another week, another tunnel
• Industry at large is in condemnation of HS2 Golborne Link cancellation
• Alternatives on the table to UK’s HS2 Eastern Leg
• HS2 controversial progress towards Manchester may displace Network Rail
• Does HS2 threaten the future of Britain’s tilting trains?
• New UK bridge over doubled railway is music to the ears of rail planners