Excavation work has begun on the first of five ventilation shafts that will serve the HS2 tunnel under the Chilterns.
HS2 Ltd has produced a video showing the latest work on the shaft, which is located near the village of Chalfont St Peter and will be 78 metres deep when completed. The design will see it topped by a headhouse designed to look like nearby agricultural buildings.
The shaft will reach both tunnels in the twin-bore system, with fans and other machinery being used to aid air flow to maintain the temperature, extract smoke if a fire occurs and enable easier access for emergency services in the event of a major incident.
Work is being carried out by some major equipment, including a 120-tonne drilling rig being used to excavate the walls, with concrete being poured in to create the wall panels. Further chalk extraction is now underway to dig down to the level of the tunnels and 18,000 cubic metres of material will be used in landscaping nearby to create new chalk grassland habitats.
As elsewhere, the tunnels themselves are being excavated by a pair of two huge boring machines, which have been given female names in accordance with tradition – in this case Florence and Cecilia.
Commenting on the project, JS2 project director Rohan Perin said: “The excavation at Chalfont St Peter shows how much progress we have already made in delivering the Chiltern tunnels.”
He added that the headhouse of the shaft is among 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”.
Demand for landscaping equipment hire will be high all along the HS2 route, as the various cuttings, bridges and tunnels are put in place for the new lines.
These needs will vary from place to place; for instance, some of the tunnels will be under urban areas, such as north-west London, the centre of Crewe and south Manchester.
However, these only make up a minority of the tunnels, of which 64 km of length will be built just on the stretch between London and Birmingham, mainly under the Chilterns in Buckinghamshire. Overall, more than 80 miles (129 km) of tunnels will be built on the network, of which 13 miles will be under London. Others will be in Warwickshire, Northamptonshire and the West Midlands.
The methods utilised to excavate these tunnels will be different. While some will be constructed using boring machines to dig deep underground, others will be much shallower and constructed using the cut-and-cover method. The latter will be used in the Midlands tunnels and at Wendover in Buckinghamshire.
Anyone familiar with the London Underground will have encountered both kinds of tunnel, with the oldest parts of the system featuring sub-surface lines dug in a cut-and-cover fashion, with square carriages and more shape.
By contrast, the deep tunnels cut by boring machines are curved and the trains shaped accordingly. This technique emerged in the late 19th century, subsequently prompting the nicknaming of the system “the Tube”.