Underpinning the Concord Road bridge under traffic – Westconnex M4 East Project

WestConnex is the largest transport infrastructure project in Australia. It is part of the Australian Federal Government and New South Wales Government’s efforts to ease congestion on Sydney’s roads by widening existing motorways and constructing new tunnels and bridges. M4 East is the section of WestConnex which extends from Haberfield to Homebush and includes the new Concord Road Interchange (currently under construction). The Concord Road Interchange is a complex junction of new bridges, cut-and-cover tunnel portals, retaining walls, widening and altering the alignment of the existing M4 Motorway lanes. Part of the works involves altering the alignment of the eastbound and westbound lanes under the existing Concord Road Bridge. To facilitate these works, the bridge needs to be underpinned with permanent support. The existing bridge abutments are founded on piles which are to be supported on a rock ledge permanently supported by rock bolts and prestressed ground anchors. Further, it is a requirement of the project that the Concord Road Bridge remain open to traffic for the duration of the works. Tight deflection criteria were imposed due to structural requirements at expansion joints. Additionally, the existing Concord Road Bridge was designed and constructed in the 1980s and there was limited information on the ground conditions and as-built founding levels of the bridge abutment piles. These factors, in addition to the requirements for working with low headroom under the bridge, were some of the key challenges during the detailed design and construction support of the works. This paper focuses on the methodology that was adopted to address these challenges during the design and construction phases of the project.