HS2 railway bridge installed over busy Chilterns road in just one day

Over the weekend, a busy road in the Chilterns changed forever when a huge HS2 railway bridge appeared right over it. On Friday morning, there wasn’t anything there, and on Friday evening, a bridge had appeared.

It wasn’t a surprise, as the road had to be closed for the operation to be carried out and HS2’s contractors have been preparing the site for several years. During that time, they’ve been building the concrete piers that hold the bridge and then, next to the road, building the bridge itself, which would then be slid into place.

On Friday morning, the road was closed to allow the enormous 4,500-tonne preconstructed deck to be pulled over the road to sit on its new piers. The road was closed not because they thought it might go wrong, but mainly to avoid motorists being distracted by the sight of huge red jaws slowly moving over their heads.

The nose of the Small Dean viaduct deck over the A413 (c) HS2

Once the lead section of the bridge crossed the road over the weekend, it kept moving forward — as right next to the road is the Chiltern mainline railway, and both needed to be crossed over the same weekend.

As a construction site, it’s been one of the most complicated places to work. One of the area’s busiest roads is on one side, a fast railway is on the other, and HS2 viaduct piers must be squeezed in around the lot.

The piers assembly August 2024 (c) ianVisits

Ahead of the Big Move, the viaduct’s concrete and steel deck was assembled to its full length before the slide began with most of the precast concrete deck slabs installed in advance of the launch. This increased the weight of the slide but will save time during the later stages of the project.

The whole operation was therefore the heaviest and longest slide of a signal-stage viaduct deck on the HS2 project so far.

The A413 open to traffic after the Small Dean viaduct deck slide Feb 2025 (c) HS2

With the deck now in position, engineers can begin the challenging job of lowering it by 60cm onto the permanent bearings that will support the full weight of the viaduct. The two-month operation will see it carefully lowered by 20cm at a time, pier-by-pier, until the whole deck settles into its final position.

The viaduct is one of only two places in the Chilterns where the HS2 route will be above ground level, with the underside of the viaduct just 6m above the road, supported by the huge Y-shaped piers.

Like the nearby Wendover Dean viaduct, Small Dean’s enormous deck is made from weathering steel, which naturally ages to a characteristically dark russet finish to echo the natural tones of the surrounding landscape.

Both viaducts have innovative ‘double composite’ structures, with layers of reinforced concrete on the top and bottom to create a much more structurally efficient box-like span which substantially cut the amount of embedded carbon in the design.

While final works on the railway bridge are completed, landscaping of the road that will pass underneath it continues. For the first time along this stretch of busy road, there will also be a shared foot and cycle path, which will eventually form part of the Misbourne Greenway project designed to link Wendover with Great Missenden.

CGI concept of the Small Dean Viaduct (c) HS2