HS2 engineers have completed what is believed to be a UK first by sliding a 4,600-tonne viaduct deck across the live M6 without a full motorway closure. The 17-hour operation installed the 315m East deck of the M6 South viaduct, which will carry northbound high-speed services towards Birmingham and beyond.
The move marked the final stage of a three-phase launch sequence for the East deck, led by main works contractor Balfour Beatty VINCI (BBV) in collaboration with National Highways. Earlier phases required full weekend closures, but the team developed a ‘fully restrained’ sliding method that allowed the final launch to proceed over live traffic, with only the M42 slip road closed.
To enable the technique, the M6 between junctions 4 and 5 was closed overnight on Thursday 11 December to advance the structure by 12m, ensuring both ends of the deck were fully supported on concrete piers ahead of the main slide. The viaduct was then pushed across the motorway on Saturday at a controlled rate of around 13m per hour.
HS2 said the operation halves the number of full motorway closures required for the viaduct works and brings the M6 South structure to its halfway point. The parallel West deck, which will carry southbound HS2 services, is scheduled to be launched using the same method in multiple phases through 2026.
The launch sequence uses strand jacks to winch the deck into position, with the steel structure sliding over low-friction PTFE-based pads similar to non-stick cookware coatings. The ‘fully restrained’ approach keeps the deck continuously supported and controlled throughout the move, reducing risk and minimising disruption to the strategic road network.
Each viaduct comprises a hollow double-box weathering steel superstructure, designed to form a durable, low-maintenance solution by eliminating the need for cyclical repainting. Four pairs of reinforced concrete piers support the decks, with the tallest pier reaching 9.9m.
A 4.5m-high parapet will be installed on the Chelmsley Wood side of the viaduct to provide noise mitigation for local residents once HS2 services are operational. The deck was launched with 82 precast concrete slabs already fixed to the steelwork, reducing the volume of follow-on works that will require further traffic management.
Design for the M6 South viaduct has been delivered by BBV’s Design Joint Venture, comprising Mott MacDonald and SYSTRA with WW+P Architects. HS2 said the staged, off-line assembly and incremental launch strategy is central to its drive to increase productivity, compress programmes and limit disruption on key motorway corridors.
