Client HS2 Ltd’s contractor Balfour Beatty Vinci (BBV) has started preparing for the construction of the ‘Marston Box’ bridge near Junction 9 of the M42 in North Warwickshire.

The 10,000-tonne bridge box structure has a base, three walls and top slab and will be built on land next to the motorway. Once built, a raft will then glide it into position. HS2 Ltd said that, whilst the technology is already used on railways, this will be the first box slide of its kind over a motorway in the UK.

Marston Box bridge for HS2

The bridge was originally designed as a traditional structure, which would have meant significant traffic disruption for motorway users, with about two years of reduced lane widths, reduced speed limits and weekend and night closures.

This construction method will see the whole structure built on land next to the motorway and then moved into place in one movement, meaning only two one-week closures of the motorway over a 12-month period.

HS2 Ltd client project director David Speight said: “At HS2 Ltd we’re always looking for innovative ways to reduce our impact on local communities, and this UK-first ‘box slide’ provides a quicker and safer solution. We’re working very closely with National Highways to ensure traffic management plans are in place, with a clearly signed diversion route to minimise any impacts during the motorway closure.”

Chris Hurrell, site agent at Balfour Beatty Vinci, said: “Through forward-thinking methods such as the ‘box slide,’ we are able to significantly reduce disruption for the travelling public, using the skills and expertise of over 100 of our people across a range of disciplines, to cut the motorway closure period to just two weeks over a 12-month period.”

During a one-week motorway closure in the last week of this year, the team will remove the motorway surface, excavate approximately 3m deep across the footprint of the structure, remove a redundant fuel pipeline, and remove an existing drainage system and reinstall it in its new location. Ground improvements will also be made in preparation for the box slide.

Two tower cranes will be erected on land next to the motorway to service the construction of the guide raft and the box structure in early 2022. The guide raft - a reinforced concrete slab - will be constructed first and then the box constructed on top. Part of the jacking mechanism will be cast into the base of the box.

The box slide itself involves a Freyssinet-designed jacking system, which will push the box across on the guiding raft. Once in motion, the box can reach speeds of over 2m per hour, so the whole operation of the box slide should only take four days, with a week closure required for the preparation, box slide and re-opening of the M42.