CONSULTANT PB Kennedy & Donkin was recently com- missioned by Oxfordshire County Council to assess and design a strengthening scheme for Nell Railway Bridge, south east of Banbury.
The bridge was built in 1848 and carries the B4100 over the Oxford to Birmingham mainline railway, the River Cherwell and a flood relief channel. It is made up of seven spans - three central spans of steel girders with two brick arch approach spans either side. The bridge has been widened since it was built using precast concrete beams and its steel girders replaced with concrete beams.
PB Kennedy & Donkin's assessment showed that the replacement beams were not strong enough for modern traffic loads and the piers and foundations were unable to sustain current derailment impact loads.
Site investigation revealed that the brick piers were supported on stone and mortar pads on top of stone slabs founded in the underlying soft alluvial clay. This overlays a thin layer of terrace gravels below 5m with Jurassic clay and mudrock at depth.
A review of the design of the nearby M40 indicated that future mining in the area could cause subsidence. This precluded using deep piled found- ations, so strength- ening involved infilling original openings in the brick piers with reinforced concrete cast into ring beams immediately above the existing found- ations.
And as sheet piling was needed to support the temp- orary excavations close to the live railway, it was decided to use it in the final design to provide the additional required bearing capacity. Steel sheet and box piles formed a cofferdam that was connected to the ring beams with shear studs. Pile length was selected to give the required improvement in load capacity and to provide a flexible support to act in tandem with the original pad foundations.
The piles were driven into the alluvial clays using a vibratory rig to 0.5m of the required toe level. They were then driven to final toe level with a conventional air hammer.