A Case Study – Geotechnical and Geophysical Investigations of an Old Stone Heritage Bridge at Parramatta, Greater Western Sydney NSW, Australia

This paper presents the methods used and results obtained from a combined geotechnical and geophysical investigation of Lennox Bridge located in Church Street Parramatta, NSW. Lennox Bridge is a Heritage-listed, 19th century, stone arch bridge spanning Parramatta River, constructed in 1839 and widened in 1934-35 under the direction of the Department of Main Roads, which included removal of a western footway and erecting a new reinforced concrete structure. Assessing the foundations and properties of a composite bridge constructed in different centuries presents significant challenges. A Borehole Ground Penetrating Radar (BGPR) technique was employed to assess the depth of the abutment foundations of the older sandstone bridge. A downhole magnetometer was used to determine the founding depth of the later concrete bridge by detecting the presence of rebar inside the concrete structure. Next to the sandstone bridge abutments, a set of boreholes were drilled at a distance of 1m and 4m from the edge of the abutment wall. The purpose of the furthest borehole was to provide a benchmark BGPR profile (without abutment) with which to compare the output from the sounding nearest the wall. The BGPR profiles of the two boreholes are compared to differentiate the presence of stone blocks used for the construction of the abutment and foundations embedded in an alluvial soil profile, down to bedrock. Boreholes were also drilled from the top of the bridge through the abutment to further confirm founding depths below the centre of the abutments.