HS2’s engineers have celebrated a double milestone in the construction of two huge viaducts that will eventually carry the high-speed railway across a floodplain.

The work, near Brackley, saw the successful completion of a 2,695 tonnes deck slide for the Westbury viaduct as well as construction of the abutments at either end of the nearby Turweston viaduct.

Set low in the landscape to the east of the town, the twin viaducts cross the floodplain of the River Great Ouse which passes under the new high-speed railway twice as it makes its way through Buckinghamshire and West Northants.

Three miles apart and built using similar designs, the two structures use an unusual ‘double composite’ approach, with layers of reinforced concrete above and below the steel beams, creating a super-efficient box-beam.

This approach uses less carbon-intensive concrete and steel than a more traditional design.

The deck of the Westbury viaduct – which stretches for 320m – was assembled in three stages, ranging from 84m to 135m with each one pushed out from the north abutment before the next section was attached behind it.

This six-month long process meant that the weight of the deck increased with each push, up from an initial 1,145 tonnes at the start of work in March - to 2,695 tonnes by the time it reached the south abutment last week.

During the slide, special pads covered in Teflon - a material usually found on the surface of a non-stick frying pan - were used to cut friction between the deck and the temporary steel bearings on top of each of the concrete piers.

Engineers were also celebrating at the nearby Turweston viaduct this week with the completion of the two abutments, which will support either end of the 80m long structure.

Focus will now turn to assembling the deck girders ready for launch of the 591-tonne structure early next year.

HS2’s senior project manager Elizabeth Longinotti said: “It’s been incredible to watch the enormous deck at Westbury come together over the last year and see it so carefully slid into position.

“I’d like to thank everyone who helped make it happen – and I look forward to seeing similar progress at Turweston in the coming months.

“The innovative double composite structure they share has huge potential to cut carbon in construction by enabling more efficient bridges and I hope that the lessons we have learnt can be successfully applied to other projects across the UK in the future.”

Both viaducts are being built by HS2’s main works contractor, EKFB - a team made up of Eiffage, Kier, Ferrovial Construction and BAM Nuttall – with the manufacture and installation of the beams being led by specialists at Eiffage Metal.

With the steelwork now in position at Westbury, engineers can begin the challenging job of lowering the deck 60cm onto the permanent bearings which will support the full weight of the structure.

The two-month long operation will see the steelwork above each pier carefully lowered by 20cm at a time, pier-by pier, until the whole 320m long deck settles into its final position.

The last few months has also seen significant progress at HS2’s other major viaducts, including the key structures that will form the Delta Junction in North Warwickshire and the viaducts taking the railway into Birmingham Curzon Street.

In total, HS2 is building more than 500 bridging structures – ranging from small road bridges to the record-breaking Colne Valley Viaduct which became the longest railway bridge in the UK when the deck was finished earlier this year.