Bachy drives through A34 Wolvercote Viaduct Replacement Piling Scheme
The high traffic viaduct of 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 staggered 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, but 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 when our team of engineers first arrived on site. As such, there was no access to the south side of the canal except by foot. It was subsequently our job to successfully lift a mini piling rig and accessories across the canal to install 12 no. 240/273 mm diameter temporary foundations using the mini pile technique. These, along with 4no. 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 which took place in mid June 2008 and required the installation of 128 750mm CFA piles drilled to approximately maximum 23m in depth across eight piers (16 piles for each pier). Five of the piers where 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 the piling works commenced with three mini pile rigs working under the east and temporary decks and will be complete by Easter 2009. These works involved installing 36 450mm diameter mini-piles in the order of 20m deep, with reinforcement cages up to 9.7m long, for ten of the new piers supporting the proposed cross heads. The more cost effective CFA piling is then scheduled to commence in April 2009, to pile the remaining four piers, installing 16 750mm dia. piles up to 21m. 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.
The conclusion to re-build the structure rather than update the original, which was built in the 1960’s, 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 require building to divert traffic from the northbound A34. During this time, the present north viaduct will be demolished and rebuilt. For the second phase, the southbound traffic will divert to the newly built north viaduct. Then the moveable carriageway (used as part of the northbound diversion) will be transported from the temporary deck and slid onto the southbound foundations to form its new overpass.