History and StructureRochester Bridge carries the A2 over the River Medway between Rochester and Strood in Kent. The crossing is owned privately by a charitable organisation known as The Rochester Trust. The Trust has maintained a bridge at Rochester since 1399, and has kept extensive archive records relating to the structures.The current crossing consists of three bridges; two highway bridges and a service bridge. The oldest of these, called the "Old Bridge", was first constructed in the 1850s, and then reconstructed in the 1910s. This bridge comprises three bow arch trusses forming the main river spans, a fourth plate girder river span, and a simply supported land span.The current bridge structure dates back to the 1850s, though a crossing has existed at Rochester since Roman timesThe Strood Abutment, sited between the land span and the river's edge, appears as a massive masonry structure. Modern flood walls flank the abutment as part of strategic defences. A small access door near the river edge leads to a chamber spanning the width of the bridge that houses a now redundant service main. A large surface water drain discharges via a flap valve in the river wall directly adjacent to the abutment.Maintenance reveals a surpriseIn 2013, a number of access panels located in the abutment chamber were opened for maintenance. The removal of floor panels, presumed to cover a trench for the service main, revealed a series of pits, some filled with water, and some with slit. The removal of a small panel in the centre of the internal wall of the chamber, obviously not opened for decades, revealed a substantial chamber the length of the abutment. This chamber was also filled with water and silt, and appeared to incorporate arches leading off to the sides of the abutment, possibly to other unknown chambers.Voids were discovered during routine maintenance in 2013The discovery raised two immediate concerns. The abutment had been assumed to be a solid structure. The presence of confirmed voids, and the possibility of further chambers, raised the issue of the capacity for the structure to carry the loads from the carriageway above. The abutment also forms part of the flood defences for Strood. The presence of water in these chambers raised questions over the integrity of the defence line. Specifically there were concerns that the nearby surface water outfall might be linked to the chambers.What could the chambers be for?Initial investigations were not conclusive. Observations revealed that some chambers appeared to flood tidally, whilst some remained at a constant level. Water testing was attempted but due to the small size and location of the access panels, samples from only a limited number of chambers and pits were able to be retrieved.Physical survey methods to determine the structure of the abutment were explored but none were found to be viable. The limited access and variable nature of the water and silt filling the chambers meant that it was not safe for personnel to enter the structure to undertake investigations. Similarly, the risk of losing any remote surveying equipment was too great to consider their use.As the same time, an extensive search of the Trust's archive records was undertaken. Due to the Trust's policy of retaining all construction records, the original drawings for both the Strood Abutment and surface water drain were found. The drainage system was confirmed to be independent from the bridge structures.Solving the mysteryOriginal plans obtained through the Trust showed brickwork chambers and passagesThe original drawings for the Strood abutment showed the structure to be formed of a series of brickwork chambers with cross passages originally extending to the riverbed. The details of the current structure, were found to match the details on the drawings, enabling the record to be verified.The structural capacity and flood volume of the abutment was able to be calculated, and the structural integrity and flood resistance of the structure confirmed as acceptable, thus eliminating the need to undertake costly and hazardous physical investigations.<!--clear div-->About the authorTim Belcher-Whyte is a Senior Bridge Engineer for Arcadis. He is currently responsible for managing asset maintenance and improvement programmes for the historic Rochester Bridge Trust in Kent.