A section of concrete in a new viaduct set to carry the HS2 railway between London and Buckinghamshire is being removed, a contractor has admitted.

Align said it was breaking concrete out of a small section of the two-mile (3.4km-long) Colne Valley Viaduct currently being built over waterways between Hillingdon and the M25 near Denham.

The concrete is being removed temporarily to enable cabling to be re-tightened in between two sections. The concrete will then be replaced.

Work is being carried out to rectify issues in the internal structure of what will be the UK’s longest railway bridge, Align told residents on Monday in a letter seen by the Local democracy Reporting Service.

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The contractor said the repair work had halted the progress of the 700-tonne bridge-building machine known as a ‘launching girder’, which is building the viaduct.

Its letter read: “As you may be aware, the launching girder for the Colne Valley Viaduct has now reached the Grand Union Canal next to the Harefield Marina.

“The girder was due to start moving over the Grand Union Canal this week, however we have had to pause this while urgent remedial works are carried out inside the viaduct.”

The letter informed residents that the ‘breaking out’ of concrete may create noise, and that the work would take place during the night and may last until February 22.

Construction of the viaduct began in early 2021 and passed the halfway mark in November 2023.

A spokesperson for HS2 Ltd said: “We need to repeat cable tensioning work on the most recently completed span of the Colne Valley viaduct. To do this, first we must drill out the span’s concrete join.“Work began on 19 February. We hope to complete it this week, and then construction work will continue as planned.”