Balfour Beatty VINCI engineers on the HS2 project have carried out what is believed to be a UK first by sliding a 4,600-tonne viaduct across the M6 without closing the main carriageway during the operation, cutting disruption for motorists.

The 17-hour manoeuvre completed a three-stage programme to assemble and install the 315-metre East deck of the M6 South viaduct, which will carry high-speed trains towards Birmingham and the north.

Following an earlier slide that required a weekend closure, the team worked with National Highways to devise a 'fully restrained' method enabling the final section to move safely over live traffic. It is thought to be the first time this technique has been deployed on a UK motorway.

To set up the move, the M6 was shut overnight between Junctions 4 and 5 on Thursday 11 December so the structure could be advanced by 12 metres, ensuring both ends of the huge beam were fully supported on concrete piers for the weekend slide.

On Saturday, the viaduct was pushed across the motorway at around 13 metres per hour, with only the M42 slip road closed during the works.

The milestone marks the halfway point in constructing the M6 South viaduct. Over the next year, the same approach will be used to install the parallel West deck, which will carry southbound trains.

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Russell Luckhurst, the Balfour Beatty VINCI engineer leading the delivery of the works, said: "We're all feeling a huge sense of pride after sliding a 4,600-tonne viaduct into its final position this weekend. The third and final slide of the East deck viaduct was delivered over a live motorway for the first time in the UK, making this achievement even more special.

"Using this 'fully restrained' technique meant we were able to keep disruption to an absolute minimum. Our focus will now turn towards the neighbouring West deck viaduct, which will be launched in multiple phases throughout 2026, as well as the East deck finishing works."

Caroline Warrington, HS2 Ltd's Head of Delivery, said: "Along the HS2 route we are pioneering new approaches to engineering and construction in order to deliver more efficiently and with less impact on our neighbours.

"We believe this fully restrained slide was a first for the country, but most importantly it means we've been able to cut in half the number of times we've had to close the motorway. I'd like to thank everyone who worked so hard to make the operation a success."

The viaducts are being constructed in stages to maximise efficiency and reduce disruption: each section is assembled to the side and then pushed over the motorway, with the next piece added behind. In the latest phase, the team used strand jacks (winches) to pull the deck across low-friction pads made from a material commonly found on non-stick frying pans.

Each viaduct comprises a hollow, double-box weathering steel structure. Its protective oxidised surface gives a distinctive ‘rusted’ finish and eliminates the need for regular repainting, reducing maintenance requirements.

Four pairs of concrete piers support the decks, the tallest rising to 9.9 metres. A 4.5-metre-high parapet will be installed on the Chelmsley Wood side to limit noise from passing trains.

The deck was launched with 82 precast slabs already in place on top of the steelwork, further reducing the number of future road closures needed to complete the viaduct.

The structural design was delivered by Balfour Beatty VINCI’s Design Joint Venture, comprising Mott MacDonald and SYSTRA, alongside WW+P Architects.