HS2 begins excavation for ‘barn design’ tunnel vent shaft

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Engineers working on HS2 have now begun excavating the first of five shafts that will provide ventilation and emergency access to the ten-mile tunnels under the Chiltern hills

The structure was designed by HS2 Ltd’s main works contractor Align JV, working with its design partners Jacobs and Ingerop-Rendel, and the architect Grimshaw and landscape designers, LDA.

The 78m deep shaft near the village of Chalfont St Peter will be topped with a headhouse inspired by nearby barns and other agricultural buildings to help it fit into the surrounding landscape.

Below ground level, the shaft will reach down to the twin tunnels, with fans and other equipment designed to regulate air quality and temperature, remove smoke in the event of a fire and provide access for the emergency services.

Set back from the road, the single-story building will be wrapped in a pre-weathered grey zinc roof with doors and vent openings picked out in a dark bronze colour and a simple blue brick base.

A 120 tonne drilling rig with a specialised cutting head was used to excavate the walls of the shaft, before concrete was poured in to form each of the 16 wall panels.

Once the tunnel boring machines (TBMs), named Florence and Cecilia, have passed through, the team will build the internal vent shaft structures, basement works and install the equipment.

Around 18,000 cubic meters of material is expected to be excavated from the shaft, with the chalk set to be reused to landscape the site once construction is complete, taking trucks off locals roads and helping to create new chalk grassland habitats. 

Before the start of construction, a temporary access road was built to link the site with the A413, Amersham Road. This has allowed construction traffic to avoid the centre of Chalfont St Peter and stopped heavy goods vehicles (HGVs) driving past Robertswood School and the Epilepsy Society.

A new temporary marshalled lorry holding area has also been established on the A413 near Gerrards Cross.

HGVs can be held in this newly surfaced layby until their approach to the vent shaft site is clear – helping to reduce congestion and eliminate queueing at the site entrance.

Protecting landscape

Rohan Perin, HS2 Ltd’s project client director said: “The excavation at Chalfont St Peter shows how much progress we have already made in delivering the Chiltern tunnels.

“Topped with a headhouse inspired by the style of local barns and agricultural buildings, the shaft is one of the few elements of the tunnel which will be visible above ground level and shows just how seriously we are taking our responsibility to protect the landscape and natural environment.”

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