HS2 releases designs for final Chiltern tunnel rural headhouse

Credit: HS2 Ltd

The reveal of the HS2 headhouse 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.

Chiltern tunnel’s rural headhouse

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 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 below to provide access for the emergency services. Due to its proximity to the North Portal of the tunnel, unlike the other four shafts, Chesham Road will not need to include ventilation equipment.

The plans were drawn up by HS2 Ltd’s main works contractor Align JV, a team made up of Bouygues Travaux Publics, as well as Sir Robert McAlpine, and VolkerFitzpatrick, who are working with its design partners Jacobs and Ingerop-Rendel, the Grimshaw Architects and landscape designers, LDA.

Welcoming the new designs, HS2 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.

“The reveal of the first designs for Chesham Road complete the set of five headhouse designs. As with each of the other rural headhouses, our priority has been to blend the buildings into the surrounding landscape, and I look forward to seeing what the local community thinks of the plans.”

Daniel Altier, Align Project Director 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.

“As is the case with all the shafts, it is important that the headhouse design at Chesham Road fits into the surrounding landscape, therefore we look forward receiving comments on the design from the local community.”

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 12 hectares of woodland at the nearby Mantle’s Wood, Sibley’s Coppice, and Farthings Wood.

Construction of the Chiltern tunnel

Construction of the tunnel is now well underway. The two giant tunnel boring machines (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. An expert team of engineers from Align have already excavated the shaft to a depth of 68m and are working to complete it before the arrival of the first TBM next year.

Other rural headhouse projects

At the next shaft site, near Chalfont St Giles, the HS2 team have now 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 to create the shaft.

The upcoming shaft site is next to the A413 road junction outside Amersham. Work is well underway, with engineers working on the concrete ‘D-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 ongoing.