Leach Highway is a strategic freight and regional link in Western Australia connecting Fremantle and the Inner Harbour with the industrial areas, including the Kewdale Intermodal Rail Freight Terminal and Perth Airport. The at-grade intersection of Leach Highway and Welshpool Road was one of Perth’s most congested and dangerous intersections, with at least 50,000 vehicles travelling through the intersection each weekday.

A high percentage of heavy vehicles travel on this route. Given the critical location of this road link in relation to major industrial areas, intermodal freight facilities and the Perth Airport, transport inefficiencies directly impact economic activity in Western Australia through loss of productivity and increased freight costs.

The interchange upgrade improved the most dangerous and second most congested intersection in Western Australia, with 224 crashes recorded between 2015 and 2019, of which 204 were rear-end collisions.

BG&E was the sole civil and structural designer for the project, which included

• A new grade separated interchange consisting of a roundabout with road bridge carrying Leach Highway over Welshpool Road.
• A new duplicate road bridge on Leach Highway adjacent to the existing bridge spanning over Armadale-Perth railway line, Sevenoaks Street and Railway Parade.
• Upgrade of barriers and installation of anti- throw/electrification screens on the existing bridge.
• Strengthening of the existing bridge to accommodate a higher performance level barrier.
• Installation of deflection walls and pier protection walls to bridge piers in the rail reserve.

The bespoke Bridge #1882 is a new duplicate road bridge carrying Leach Highway southbound traffic over Armadale-Perth railway lines, Sevenoaks Street and Railway Parade. It was constructed adjacent to the existing 50- year bridge that now carries the northbound Leach Highway traffic. The superstructure is a complex, haunched, continuous bridge comprising eight precast post-tensioned (PT) T-beams to match the profile of the existing adjacent bridge.

The superstructure type and complexity adopted for the bridge originated from the project scope of works and technical requirements with constraints on the overall depth to fit within the restrictive vertical clearances over the railway and under the high voltage cables, and to be of weight that could be safely lifted, particularly for the central 37 metre span beams over rail. Each beam was precast in three straight post-tensioned lengths with a slight angular change at the construction joints to accommodate the road geometry that is curved in plan and profile.

The central beam was lifted in and temporarily supported by the haunched cantilevered portions of the outer beams. Following coupling of continuity PT ducts, the connections were then made rigid and then the full length of each beam was finally post-tensioned.