The A46 Kenilworth Bypass reopened was reopened to traffic 30 hours ahead of schedule after HS2 engineers slid a massive 14,500-tonne concrete box structure into position beneath the road.

The huge box – which weighs around the same as two new Royal Navy destroyers – is the heaviest of its kind in Europe.

It will allow high-speed trains between London and Birmingham to pass under the dual carriageway on their approach to the new Interchange station near the NEC.

HS2’s designers opted for a box structure, that could be built alongside the road and slid into place in one operation to avoid two years’ worth of speed restrictions and lane closures that would have been necessary to build a more traditional bridge design.

The complex three-part operation was led by HS2’s main works contractor, BBV – a team made up of Balfour Beatty and VINCI – working closely with National Highways and timed to coincide with lighter traffic during the school Easter holidays.

Cameron Thompson, HS2 head of delivery, said: “It’s amazing to see the A46 box in position and I’d like to thank everyone who’s worked so hard to get the road open early.

“This record-breaking slide could not have been achieved without the years of meticulous planning and preparation that the team put in, working closely with our supply chain and National Highways.

“While the vast majority of the bridge is now complete, we’ve still got a few weeks’ of lane closures to finish the central reservation and wingwalls – and I’d like to thank drivers in advance for their patience during these works.”

The first stage involved the demolition of a 200m long section of the old road and the excavation of around 35,000 cubic metres of material to make space for the box to be pushed into place. This material – mostly earth and sandstone – was stockpiled on site for reuse later.

The second stage involved pushing the 42m wide structure into position. This was achieved over 10 hours. Specialist engineers used an innovative jacking mechanism called Autoripage, designed by specialist civil and structural engineering company Freyssinet, to push the box across on a guiding raft at an average speed of 6.5 metres per hour for a total distance of 64 metres.

Fifteen enormous jacks were used to guide the box into position, allowing engineers to gradually adjust the direction of travel with millimetre precision.

A series of holes in the base of the box allowed engineers to inject bentonite – a clay-based material – into the space between the box and the ground. This acted a lubrication to help ease the sliding operation.

During the final stage of the operation, the embankment which carries the road was carefully reconstructed, the new carriageway put into place and the road was reopened to traffic at around midnight last night – 30 hours ahead of schedule. The road reopened at midnight on 29 April.

To allow time for the completion of the central reservation and wingwalls on either side, one lane on either side of the road will be closed until Thursday 31 July.

Elsewhere in the West Midlands, progress is also being made on the Delta Junction which will take trains into Birmingham city centre, with the first deck recently completed on the River Tame West Viaduct.

Two giant tunnelling machines under London have also recently completed their drives, excavating the five-mile-long Northolt tunnels.