DELICATE STRESSING begins next week on a 156 year old tilting masonry aqueduct near Leeds.
Outward lean on each arch of up to 4degrees inevitably draws comparison with Pisa's leaning tower. The tilt is similar to that of the Italian structure, which is also built on weak ground and small stone slab foundations.
But analysis of the lean halts further comparison. Built to a 168m radius, and once forming part of the first modern water supply to the city, the 90m long Seven Arches aqueduct now displays outward pressures on each pier classically found in a curved multi-span bridge. Arch thrusts either side intercept through the pier at an angle causing an outward force.
The resulting differential settlement, and a continuing 2mm tilt each year, will certainly lead to collapse. But underpinning with minipiles was ruled out by the silty sandy ground and the location of three water mains immediately alongside which carry half of Leeds' supply.
The solution - devised by consultant Thomason Partnership for owner Yorkshire Water - claimed as a UK first - is to stress the entire structure to 560kN force using four bars placed through its water duct. By passing the bars over steel saddles anchored at each pier, the prestress counteracts the outward thrust on the piers.
The plan meets the client's demand that work on the listed structure is economic, invisible and that no damage is caused to the water mains. Contractor for the pounds118,000 repair is Morrison Construction with stressing specialist Ancon-CCL.