UK Engineering First: HS2 Viaduct Slides Over Live M6 Motorway

Engineers on the HS2 project achieved a UK-first over the weekend by sliding a 4,600-tonne viaduct across a live section of the M6 motorway.

The 315-metre bridge deck, which will eventually carry HS2 trains, was moved during a 17-hour operation on Saturday, 13 December, without the need to close the motorway to traffic below. This marked the final stage of a three-part process to assemble and position the east deck of the M6 South viaduct.

While the previous deck slide required a full weekend closure of the main carriageway, the team—led by HS2’s main contractor, Balfour Beatty Vinci (BBV)—collaborated with National Highways to develop a ‘fully restrained’ method that enabled the viaduct to be slid safely above moving traffic. It is believed to be the first time this technique has been used on a UK motorway.

Initially, the M6 was closed overnight between Junctions 4 and 5 on Thursday, 11 December, to move the viaduct forward by 12 metres, ensuring both ends of the deck were fully supported on concrete piers ahead of the main slide. The remainder of the deck was then pushed across the motorway at a rate of 13 metres per hour on Saturday, with only the M42 slip road closed for the weekend.

This milestone means the M6 South viaduct project has now reached its halfway point. Over the next year, the same technique will be applied to slide the parallel West deck into place, creating the southbound track for HS2 trains.

Caroline Warrington, HS2 Ltd’s Head of Delivery, said:

“Along the HS2 route we are pioneering new approaches to engineering and construction to deliver more efficiently and with less impact on our neighbours. We believe this fully restrained slide is a UK first, but most importantly, it has allowed us to halve the number of motorway closures required. I’d like to thank everyone involved in making this operation a success.”

The M6 South viaduct has been constructed in stages, with each section assembled on one side of the motorway before being slid into its final position. During the latest phase, the structure was moved using strand jacks resting on Teflon pads, which helped reduce friction.

BBV project engineer Russell Luckhurst commented:

“We’re all immensely proud to have positioned a 4,600-tonne viaduct over a live motorway. Delivering the final slide of the East deck using this fully restrained technique has minimised disruption. Our focus will now shift to launching the West deck throughout 2026 and completing the remaining works on the East deck.”

Each viaduct features a hollow double-box steel structure made from weathering steel, which forms a protective oxidised layer, giving it a distinctive rust-like appearance and reducing maintenance needs. Four pairs of concrete piers support the structure, with the tallest reaching 9.9 metres. A 4.5-metre-high parapet will be installed on the side facing Chelmsley Wood to minimise noise from passing trains.

The viaduct was launched with 82 precast slabs already installed on the steel deck, which will further reduce the need for future road closures. The structural design was developed by BBV’s design joint venture, including Mott MacDonald, Systra, and WW&P Architects.