Amersham ventilation shaft is one of five critical intervention points along the 10‑mile Chiltern Tunnel, forming an essential part of HS2’s safety and ventilation strategy. Positioned just outside Amersham town centre, the shaft provides airflow management and emergency access for the high‑speed rail network.
Kilnbridge was appointed to support Align JV – the partnership of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick – by delivering an essential external works package for the project.
The shaft is a deep, circular structure located in the middle of a busy roundabout junction. The structure features a headhouse which features perforated zinc cladding to provide a sympathetic architectural finish to a utility structure.
To soften the structure and blend it in with the surrounding architecture, Kilnbridge was tasked with delivering external works, featuring the installation of more than 300 precast units – including T-wall and flint panels – gabion walls, drainage and surfacing.
This unusual setting – the middle of a busy roundabout – demanded exceptional planning and coordination, especially when it came to material deliveries and craning. To deliver the external works package, Kilnbridge had to undertake extensive temporary works design and lifting plans, to ensure the works were delivered safely and within the minimum of disruption to the local community.
A retaining wall system formed the backbone of the project. Kilnbridge installed 197 precast T‑Wall units to stabilise the site and create the primary retaining structure. To achieve this safely and efficiently, the team designed and fabricated a bespoke lifting frame, engineered specifically for the constraints of the roundabout location.
Above the retaining wall, Kilnbridge constructed a traditional flint wall comprising 103 pre cast units. This heritage‑inspired finish was chosen to reflect the character of the local Chilterns architecture, ensuring the new HS2 infrastructure would blend sensitively with its surroundings.
The flint panels feature a concrete frame that sits forward of the flint panels on their left-hand side and slopes to a flush finish in the right top corner. This finish resulted in the panels being an uneven weight across their face, meaning that if lifted using traditional methods they would twist and pose a health and safety risk. As such the team had to develop a further custom lifting frame, to ensure the pre-cast flint panels were lifting safely.
Further earth retention and visual integration were achieved through the installation of 230 metres of gabion baskets. These provided both structural support and a natural aesthetic that complemented the wider landscape.
Alongside this, Kilnbridge delivered a comprehensive drainage and ducting package, installing 800 metres of drainage, gully systems, manholes and 500 metres of channel drains. This infrastructure ensures long‑term water management and serviceability for the shaft and its surrounding environment. The team also installed a lightening protection system comprising of 600 metres of copper cable.
To complete the project, the site was fully landscaped to integrate seamlessly with the existing roundabout layout and new tarmac surfacing was applied to restore traffic routes and enhance the final presentation of the completed structure.
Delivering Quality, Safely
Amersham shaft showcases Kilnbridge’s ability to deliver complex civil engineering works in constrained and high‑profile environments while maintaining exceptional standards of safety, quality and technical innovation. From bespoke lifting solutions to heritage‑sensitive finishes and extensive drainage infrastructure, the project reflects the breadth of Kilnbridge’s capability and the strength of its people.