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Work has begun on all five of the ventilation shafts that will service High Speed 2’s (HS2's) Chiltern Tunnels.
The vent shafts must be completed in time for the tunnel boring machines (TBMs) to break through, with one of two TBMs having just completed the first 1km of its 16km journey.
Designs for the fifth and final ventilation shaft’s headhouse have also been revealed at Chesham Road, which will provide emergency access to the 16km long Chiltern tunnel.
Publication of the designs completes the set of five headhouses on the Chiltern tunnel, with work now begun at all sites and excavation well underway for what will be the longest tunnel on the HS2 project.
Largely hidden behind a hedge and line of mature oak trees next to the B485, the Chesham Road headhouse is designed to fit into the surrounding landscape and its design takes its inspiration from the style of nearby barns and other agricultural buildings.
The single story building will be wrapped in a simple pre-weathered grey zinc roof which will age naturally over time, without loss of robustness or quality, while the whole structure will sit on a simple dark blue brick base.
Below ground level, a 45m shaft will reach down to the twin tunnels providing access for the emergency services. Unlike the other four shafts, Chesham Road will not need to include ventilation equipment because of its proximity to the north portal of the tunnel.
The plans were drawn up by HS2 Ltd’s main works contractor Align JV – a team made up of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – working with its design partners Jacobs and Ingerop-Rendel, and the architect Grimshaw and landscape designers, LDA.
HS2 Ltd project client Rohan Perin said: “The launch of our giant tunnelling machines – Florence and Cecilia – earlier this year were huge symbolic moments for the project, but just as important has been the progress we’ve made at each of the shaft sites to prepare for the arrival of the TBMs over the next three years.”
Align project director Daniel Altier added: “I am delighted with the progress we have made so far at all five shaft sites. We are working to a detailed schedule to ensure that each shaft is ready for the arrival of our two TBMs, currently tunnelling north from the South Portal.”
The Chesham Road shaft was added to the Chiltern tunnel in 2015 to allow the tunnel to be safely extended by 2.6km – a decision which saved 12ha of woodland at the nearby Mantle’s Wood, Sibley’s Coppice and Farthings Wood.
Construction of the tunnel is now well underway. The two giant TBMs that were launched earlier this year to dig the parallel northbound and southbound tunnels have now passed under the M25 and are heading towards the first shaft site near Chalfont St Peter.
Align has already excavated the shaft to a depth of 68m and is working to complete it before the arrival of the first TBM next year.
At the next shaft site near Chalfont St Giles the HS2 team has completed the concrete diaphragm walls for the 46m deep shaft using specialised drilling equipment and will soon begin excavating material from within the concrete walls in order to create the shaft.
The next shaft site is next to the A413 road junction outside Amersham. Work is well underway, withengineers working on the concrete diaphragm walls for the 51m deep shaft.
Further north at the Little Missenden site, alongside the A413, engineers are preparing for the start of wall construction, while at Chesham Road – the last shaft the TBMs will reach – site set up is now well underway.
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