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Artist's impression of the Small Dean Viaduct.
Artist's impression of the Small Dean Viaduct.

Small Dean Viaduct

The striking 345 metre long Small Dean Viaduct just south of Wendover will carry the HS2 line across the A413, Small Dean Lane and the Chilterns rail line. After crossing the viaduct, northbound trains will pass the town of Wendover in a short tunnel designed to reduce its environmental impact.

A shared foot and cycle path will also be delivered by HS2 alongside the main road as it passes under the viaduct between the A413 London Road roundabout and the houses near Rocky Lane. This will eventually form part of the Misbourne Greenway project designed to link Wendover with Great Missenden.

Viaduct construction

Engineers have successfully completed the longest and heaviest single-stage viaduct deck slide on the project to date.

Over four days, the massive 4,500-tonne deck was carefully slid into position over a road and railway near Wendover, Buckinghamshire. Unlike traditional methods, the entire concrete and steel deck was assembled to full length before the slide began, with most of the precast concrete deck slabs installed in advance. While this added weight to the operation, it will significantly reduce time required for later construction phases.

To execute the slide, engineers used specially designed PTFE-coated pads—similar to non-stick frying pan surfaces—to minimize friction between the deck and its temporary steel bearings. A precise cable system gradually moved the deck forward at speeds of up to 10 metres per hour.

The achievement follows four years of extensive preparation, including site clearance, utility diversions, road realignments, and the construction of the abutments and six supporting piers.

With the deck now in position, engineers will begin the next crucial stage—carefully lowering it by 60cm onto its permanent bearings. This delicate operation will take around two months, with the deck being lowered 20cm at a time, pier by pier, until it reaches its final position.

Small Dean Viaduct is one of only two locations in the Chilterns where the HS2 route will remain above ground. The structure, supported by distinctive Y-shaped piers, stands just six metres above the road.

Beyond the viaduct itself, the project will also include the realignment of Small Dean Lane and the A413, alongside subtle landscaping to blend the infrastructure seamlessly into the surrounding environment.

Viaduct design

Designed to be seen from close-up 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 6 metres above the road, supported by five ‘Y’ shaped piers.

The viaduct’s beams will be made from weathering steel, which naturally ages to a characteristically dark russet finish to echo the natural tones of the surrounding landscape. The pale concrete parapet and noise barrier along the top of the viaduct is designed to throw the dark steel into shadow, helping to give the appearance of a lighter and thinner structure. The piers are also designed to be as slender as possible.

Structurally, the viaduct beams will use an innovative ‘double composite’ approach, like the nearby Wendover Dean Viaduct. This involves two steel beams sandwiched between two layers of reinforced concrete to create a super strong but light weight span, dramatically cutting the amount of embedded carbon in the structure. The side of the beams will also have deck stiffeners to help manage the structural forces while allowing for a lighter weight beam.

Key facts

  • Length: 345m
  • Height: 6m
  • Number of piers: 5
  • Construction approach: ‘Double composite’ steel beams for strong but lightweight spans.

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