Seven Arches Aqueduct, Leeds

The Seven Arches Aqueduct was built in 1842 as a key link in Leeds’s first public water supply system. It is now disused but it is preserved as a Grade II Listed Structure. Close beside it are water mains which carry half of Leeds’s entire water supply and also the Dales Way public footpath.

The aqueduct structure was leaning at an angle of 3° to 4°, almost as much as the Leaning Tower of Pisa. Yorkshire Water asked Thomasons to investigate its stability. Thomasons found that forces in the curved structure were pushing it over sideways and its piers were up to 500mm out of line. It was still moving and Thomasons advised that eventually collapse would be inevitable.

Thomasons developed an innovative solution using prestressing cables passed through the structure from end to end, concealed inside the disused water duct and stressed up to a load of 56 tonnes. It is thought to be the first time that an existing masonry viaduct has been prestressed in this way.

The project was successfully completed on time and within budget. Monitoring showed that the piers straightened by up to 20mm after stressing, indicating that the system is working. The project generated press and TV interest and received a national Historic Bridge Award.