A34 Wolvercote Viaduct Replacement Piling Scheme
A 250m long viaduct carrying high volumes of traffic on the A34 Oxford Western by-pass is on the road to recovery with the aid of Bachy Soletanche Limited. The leading geotechnical specialist is conducting a staged bearing pile contract for main contractor, Costain as part of the A34 Wolvercote Viaduct Replacement Scheme near Oxford which will take place while keeping the route open to traffic.
At first glance, the piling contract is quite straightforward. However, as the £44.4million Highways Agency project will replace the current viaduct – which carries the A34 Trunk Road over the River Thames Flood Plain, the Oxford to Birmingham main line railway, the Oxford Canal and the A40 Oxford to Cheltenham road – Bachy Soletanche is contending with a few obstacles along the way. Contracts Manager, Vijay Pookat explained:
“The project involves working close to live traffic, next to the railway line and by the Oxford canal. In addition, there is limited access for the site’s machinery and equipment; in particular, there was no access to the south side of the canal except by foot. Costain had subsequently planned for this therefore our job was to successfully lift a mini piling rig and accessories across the canal to install 12 240/273 mm diameter temporary foundations using the mini pile technique. These, along with 4 750mm Continuous Flight Auger (CFA) piles on the north side, are for a site access bridge placed over the canal.”
Bachy Soletanche is conducting the piling works in three stages over a two year period. The first of these, for the temporary structure alongside the existing viaduct, took place in mid June and required the installation of 128 750mm CFA piles drilled to a maximum depth of approximately 23m across eight piers (16 piles for each pier). Five of the piers were completed within 4 weeks, but due to site constraints, Bachy Soletanche demobilised and returned to site in September to complete the final three.
The second phase of piling to replace the existing northbound viaduct is scheduled to commence around June 2009 and the third stage, for the southbound viaduct, should begin in March 2010. Again, Bachy Soletanche will utilise the CFA piling process to install 112 750mm diameter piles at each stage.
The conclusion to re-build the deteriorating structure, which was built in the 1960’s, rather than update it came after a ‘Whole Life Costing Analysis’ determined it a more cost effective and least disruptive alternative to the travelling public. With this in mind, a strategic plan of action is in place where a temporary deck will be built to allow diversion of the northbound A34. During this time, the present north viaduct will be demolished and rebuilt. For the second phase, the northbound traffic will be moved back onto the newly built north viaduct and the south viaduct will be demolished and re-built to top of piers. Then the moveable deck (used as part of the southbound diversion) will be transported from the temporary structure and slid onto the south structure piers to form the completed viaduct.