Black Rabbit and Peppering, a pair of near identical bridges in West Sussex, are being strengthened as part of a three-year programme of structure maintenance and renewal being undertaken by Railtrack Southern and Laing Management.
The three main girder span underbridges carry two tracks over the River Arun between Arundel and Amberly stations. The strengthening requirement was unusual in that only the centre girder - approximately 40m long - was to be replaced. Access was difficult, complicated further by the tidal nature of the Arun and the presence of a site of special scientific interest.
The contract was awarded to Jackson Rail, which devised a construction method based on an idea from structural engineer Cass Hayward, responsible for the new girder design. Work was carried out during two 52-hour weekend possessions and apart from loading and unloading, no craneage was involved.
The first task was to remove the waybeam ties and place temporary beams in the fourfoot to support the cross girders to within 10mm of their existing level. While this was going on, specially designed portal frame lifting gantries and the main girder were being craned on to rail trolleys at Arundel station.
Before cross girder connections could be cut, the new girder had to be positioned on the bridge with trolleys directly over the two piers, allowing them to take the bulk of the dead load and imposed loading.
Lifting points on the new girder were designed to reflect the pier spacing, so once the beam was positioned, the old cross to main girder connections could be cut.
As with any old structure, condition of the members could not be assessed until they were exposed. Plant and materials were on site to deal with repairs and when, as anticipated, the top flanges of some existing cross girders proved to be badly corroded, new top plates and structural putty were applied to stop further decay.
To allow transport of the old girder back down the track, 6m had to be burnt off each end. Once it was lifted out, work to replace the old bearings started. This completed, the main girder could then be landed. Only two men were needed for the operation, using hand winches on the lifting gantries.
Experience from the first operation led to some programme revisions for the second bridge and these ensured that the project remained on schedule.