Piling to the abutments and outer spans for Great Ouse Railway Bridge- Ely.
Foundation Piling were consulted by Atkins Rail to advise on the provision of suitable piles for a complete bridge replacement over the River Great Ouse in Ely, Cambs. The old cast-iron bridge had been damaged beyond repair, and the lines were currently closed until the bridge could be completely replaced. Atkins - working for Network Rail- had to design a new central bridge span for the river, with side spans to updated standards to allow for expected flood flows on the Great Ouse. Two large RC box-culverts were designed to sit either side of a new replacement central span.
Design for the new structures had to then proceed apace whilst the existing damaged bridge was removed. A large temporary works causeway had to be built by Contractors, Kier Rail, more than 500m across the fields from the nearest feasible access point. The superstructure could then be craned out, and sheet piling put into place to provide cofferdams for the new foundations.
However, from the little records available it was know that the existing bridge had been founded on a forest of timber piles with reinforced iron points. From recent SI, it was assumed that these had been driven through weak alluvium and peats, and through deeper limestone layers and into stiff clay at depths thought to be about 10-14 metres. The timber piles were tightly spaced, and could not be removed so any new piles had to cope with very variable ground and the network of vertical and raked timber in the ground. The new piles needed to be installed quickly and easily, capable of dealing with the timber and other obstructions, and needing quick and easy preparation to allow the bridge substructure works to commence.
Foundation Piling suggested an array of their 170mm Ductile iron piles, driven in grouted 5m lengths to a length of 12-14m in the stiff clay beneath. As an immediate advantage, the ductile piles are handled and driven by an adapted 20t excavator with a Krupp breaker attachment, which could easily work within the tight cofferdams and timber pile heads. The ductile piles would not be damaged or deflected by the timber piles, and in addition could penetrate the limestone shown in ‘shelves’ above the stiff clay. In addition, they could easily be driven vertically, and at rakes of 14 and 27 degrees to vertical to accommodate the high horizontal loads from the bridge. Finally, the pile heads are prepared by cutting off the ductile tube, filling with concrete and fixing a steel rebar and plate to the head- this would simplify the plant needed in the cofferdam, but more importantly was the only piling method to allow the piles to be placed and prepared in the very tight timescale required.
The design was checked using some trial ductile piles driven adjacent to the foundation. These proved that the limestone at 7-8m was more substantial than the SI suggested, and after discussion with Atkins and a static pile test it was clear that the grouted piles driven onto the limestone would indeed be adequate to support the bridge loads. Piling proceeded, first on the East, and then simultaneously on the West abutment, accessed by Kier Rail’s barge ferry provided over the Great Ouse. After a late change to the design of the abutments, grouted ductile piles were also used for the abutment wingwalls to the bridge, bringing the total number of piles on the project up to more than 400.
Despite changing design information, delays in access to and smaller size of the cofferdams, Foundation Piling placed all of the piles on schedule enabling the bridge to stay on programme, and allow the superstructure construction to continue. The Ely project was front-page news in ‘Railway Engineer’ magazine, November 2007, and the completed bridge was open for rail traffic on the target date late in December 2007.