In a landmark engineering feat for Britain's new high-speed railway, a colossal 4,600-tonne viaduct deck was successfully slid into position over the M6 motorway without requiring a full weekend closure – a first for the UK.
A Pioneering Slide Over Moving Traffic
The complex 17-hour operation, completed on Saturday, saw the 315-metre East deck of the M6 South viaduct gently pushed across the busy carriageway at a rate of 13 metres per hour. This critical manoeuvre was the culmination of a meticulous three-stage process.
Led by HS2's main works contractor, Balfour Beatty VINCI (BBV), and developed in close collaboration with National Highways, the team pioneered a 'fully restrained' method. This innovative technique allowed the final slide to proceed safely above the flow of traffic, whereas a previous slide had necessitated a complete weekend shutdown of the main carriageway.
Precision Planning Minimises Disruption
To prepare for the main event, engineers initially closed the M6 overnight between Junctions 4 and 5 on Thursday 11 December 2025. During this brief window, the viaduct structure was shifted forward by twelve metres. This preparatory move ensured both ends of the massive beam were fully supported on concrete piers ahead of the weekend slide.
During the main operation on Saturday, disruption to road users was kept to an absolute minimum, with only the M42 slip road closed. The successful execution marks the halfway point in constructing the M6 South viaduct, which will carry high-speed trains to Birmingham and beyond.
Engineering and Design for the Future
The viaduct is a feat of modern engineering and sustainable design. Each deck is formed from a hollow double-box structure made of weathering steel. This special steel develops a stable, protective layer of oxidisation, giving it a distinctive rusty hue and significantly reducing future maintenance by eliminating the need for regular repainting.
The structure is supported by four pairs of concrete piers, the tallest measuring 9.9 metres high. To protect nearby communities, a 4.5-metre-high parapet will be installed on the side facing Chelmsley Wood to mitigate noise from passing trains.
In a further move to limit future road closures, 82 precast slabs were already installed on top of the steel structure before the slide. Over the coming year, the entire process will be repeated to slide a parallel West deck into position alongside the first, completing the twin-track structure for southbound services.
