Engineers working on the HS2 project have slid a 4,600t viaduct section across the M6 without a full carriageway closure, in what contractors say is a UK first that will reduce disruption for drivers.

The 17‑hour operation carried out over the weekend of 13-14 December completed the three‑stage assembly and installation of the 315m long East deck of the M6 South viaduct, a structure that will carry high‑speed trains to Birmingham and beyond. The move was carried out by HS2’s main works contractor Balfour Beatty Vinci (BBV) in co‑operation with National Highways.

In contrast to an earlier slide on the same structure that required a weekend shutdown of the motorway, the team developed a so‑called “fully restrained” technique for this slide that allowed the final section to be pushed over the live carriageway while traffic continued to flow. Only a slip road on the adjacent M42 was closed during the weekend.

Engineers initially closed the M6 overnight between junctions 4 and 5 on Thursday 11 December to shift the viaduct forward by 12m so both ends of the beam would be supported on concrete piers. On Saturday the structure was then moved across the motorway at roughly 13m per hour using a system of strand jacks. To reduce friction the deck was slid on low‑friction pads made from a material similar to that used on non‑stick frying pans.

The operation marks the halfway point of the M6 South viaduct project. A parallel West deck, which will carry two additional tracks for northbound trains, is due to be assembled and slid into position next year using the same method.

The East deck has been built in stages to limit disruption: an initial 119m section was moved in June over a slip road and the next section, bringing it to 230m in total length, was slid into place at the end of September during a planned closure of both carriageways near junction 4. That September move was completed so efficiently the motorway was reopened more than nine hours ahead of schedule, the contractors said.

The final East deck now spans 315m and weighs around 4,600t in its principal elements, with 82 precast slabs already installed on top of the steel structure to reduce future work over the motorway. Additional elements, including parapets, will be added later with track systems installed in future years.

HS2 and BBV engineers were able to slide the final section of the viaduct over the M6 without closing it thanks to a new ‘fully restrained’ process

Each HS2 viaduct over the M6 is a hollow double‑box structure made from weathering steel, which develops a protective oxidised surface, giving a characteristic “rusty” appearance and reducing the need for repainting. The viaducts are supported on four pairs of concrete piers, the tallest of which is 9.9m. A 4.5m‑high parapet will be fitted on the side facing Chelmsley Wood to mitigate noise from passing trains.

Structural design was provided by BBV’s design joint venture, comprising Mott MacDonald, Systra and WW+P Architects.

The operation will be seen as a test case for keeping major road corridors open while carrying out large‑scale rail infrastructure work. HS2 has faced regular scrutiny for cost increases and delays, and the project’s ability to limit road disruption is likely to remain an important factor for local communities and motorists as construction continues.

Caroline Warrington, HS2 Ltd 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.”

Russell Luckhurst, the BBV engineer leading the delivery of the works, said: “We’re all feeling a huge sense of pride after sliding a 4,600t 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.”

National Highways regional director for the Midlands Victoria Lazenby said: “Our key focus is the impact that these major construction works have on our roads – we must both ensure the safety of road users and minimise the disruption they face.

“So we are delighted that this innovative technique has meant that not only was this enormous structure slid into place without having to close the motorway during the day but also that the total number of closures needed has also been halved.

“We will continue to work with HS2 and their partners to ensure the smooth running of our roads while this huge infrastructure project takes place and support any initiatives which will reduce disruption for drivers and local communities.”

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