Sympathetic Treatment for Victorian Bridge
When Network Rail bridge engineers discovered signs of cracking on one of the piers of Rockshaw Bridge No.8, carrying road traffic over the London to Brighton railway line, they were faced with a challenging problem.
Built in the 1870's, the 36 metre long brick structure, at Merstham in Surrey, has three arches supported by two 12.5 metre tall, slender piers, each 1.5 wide by 7 metres deep. The 'down side' pier had developed cracks and was showing signs of distress and movement. Consultants, Crouch Waterfall & Partners, were appointed to identify the full extent of the problem and, with the assistance of Professor Bill Harvey (Bill Harvey Associates), recommended what remedial action should be taken.
The first task was to strap the pier with a steel corset then relieve much of the down load from the bridge deck. Following consultation with the local authority, the road was closed and main contractors BCM Construction removed over 100 tonnes of asphalt and fill.
A borescope inspection revealed that the pier comprised a 9 inch skin construction, with a voided infill of layers of bricks which were not tied to the skin. Settlement of the abutment put excessive stress on the outer skin of the pier causing it to crush. It was imperative that any void filling material used should help create a dense homogenous support, but should not exceed the compressive strength of the original Victorian lime mortar.
Heritage Grout 7, a medium strength lime-based grout from specialist grout manufacturers CMS Pozament was selected. Contractors, Barcol Limited, pumped CMS Pozament's HPRG High Performance Anchor Grout into the base of the pier to form a plug, preventing any seepage of the Heritage Grout onto the track. Holes were drilled in stages, starting at the bottom of the pier, and more than 8 tonnes of flowable Heritage Grout was pumped into the pier voids. To further strengthen the pier, a series of permanent 16mm stainless steel ties and plates were installed that criss-cross through the structure.
The project was a resounding success, and data loggers which monitor the bridge 24 hours a day, confirm that there is now no detectable movement.
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