Chetwynd Bridge - River Thame, Alrewas, Staffs CC
When you have a Grade II Star listed cast iron bridge that develops fractures that need to be invisibly repaired,do you dismantle it and send it away or call in the experts to restore it in-situ. That was the predicament thatthe bridge department of Staffordshire County Council found themselves in with the Chetwynd Bridge over theRiver Tame on the A513 at Alrewas.
To dismantle, transport away for repair, repair, return it to site and re-assemble was prohibitive as far as costand time were concerned. Instead Galliford Midlands who had been contracted by the County Council forextensive repairs asked Coventry firm Metalock Engineering to investigate and decide if their Metalock metalstitching repair would be a viable proposition.
The cast iron bridge, constructed in 1824, has three spans with a series of box sections, forming the outstand,supported on the cast iron girder structure. A combination of age, water ingress into some of the box sectionsand vibration caused a series of fractures to develop just above the lower corner joint. These extended nearlyall the way along one side of the bridge and also for a considerable distance along the opposite side. Followingan assessment of the problem by Metalock engineers, it was decided that the metal stitching technique, carriedout in-situ, would provide an effective and long lasting repair without dismantling the bridge.
As the bridge carries a substantial amount of traffic between Alrewas and Tamworth, a Bailey bridge waserected alongside so that the team of Metalock engineers could carry out the fracture repairs without having toworry about traffic.
The Metalock process is accepted as a method of repairing cracked and broken castings across a wide rangeof industries. One of the main advantages of the process is that it is heat free and can be carried out in-situ orone of the company’s workshops. Apertures are jig drilled across the crack to accept multi-dumbell shapedkeys manufactured from a ductile high tensile nickel alloy. The keys are peened into the apertures to becomeintegral with the parent metal. Holes are then drilled and tapped along the line of the fracture and filled withstuds, each one positioned to overlap its neighbour. This combination of keys and studs produces a rigid andpressure tight repair and final peening and hand dressing completes the operation. Once the repair has beendressed, a primer applied and finish painted it is practically undetectable.
For the Chetwynd Bridge, the team of four Metalock engineers spent 3 fairly severe winter months completingnearly 74 metres of fracture repairs. Additionally, the team carried out similar repairs to some strengtheningribs underneath the main deck, where other fractures had developed.
As the method by which the deck is supported has been modified - it is now cantilevered from the three mainpiers - the original cast iron grid no longer has to support the weight and the bridge is now in a condition to givemany more years service.
Before Metalock repaired the 177-year oldChetwynd bridge over the River Thame therewere serious cracks in the box sections alongthe two sides. Metalock cured the problem andthe bridge lives on.