Leach-Welshpool Alliance with MRWA – Perth, Western Australia. Image is courtesy of Main Roads WA.


Perth, Western Australia



Main Roads WA


BG&E Perth
Leach-Welshpool Alliance with MRWA

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 is 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 have a direct impact on economic activity in Western Australia through loss of productivity and increased freight costs.

The interchange upgrade will improve 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. Other benefits of the Leach Highway link to the client and community include:

  • creating 600 local jobs;
  • reduced congestion and delays;
  • improved vehicle, cyclist and pedestrian safety and efficiency;
  • better connections;
  • improved traffic flow;
  • reduced peak travel times;
  • consistent journey times;
  • eased pressure on the local road network;
  • increased capacity for vehicle barrier impacts and derailed train collisions;
  • improved survivability and utilisation of assets assisting with post-disaster recovery;
  • increased structural capacity to accommodate heavier, taller and wider transport loads;
  • reduced carbon emissions; and
  • sustainability advantages.

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

  • upgrade of approximately 2.2 km of Leach Highway between Orrong Road and Albany Highway and the upgrade of approximately 500 m of Welshpool Road between John Street and Ewing Street;
  • modifications to maintain existing standards of affected local roads;
  • a new grade separated interchange which consists 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;
  • a Principal Shared Path (PSP) along the western side of Leach Highway from Sevenoaks Street to Orrong Road;
  • shared paths connecting the PSP to local roads, and replacement or realignment of affected existing paths.
  • traffic noise mitigation measures adjacent to noise sensitive developments;
  • resurfacing of existing roads;
  • boundary fencing and pedestrian and cyclist safety fencing;
  • accommodation works within properties affected by the project;
  • roadside and median safety barriers;
  • drainage including underground drainage, swales and basins;
  • installation and modification of roadway lighting;
  • installation and modification of public space lighting to shared paths, footpaths and stairs;
  • provision of new and modified signage, gantries and pavement markings; and
  • installation and modification of Intelligent Transport Systems along Leach Highway and on key connecting roads.

The BG&E civil designers proposed a major sustainability and congestion saving innovation for this project, changing the client preliminary design from a traditional grade separated interchange into a free-flow grade separated roundabout interchange. The roundabout is oval shaped to allow a compact design, while achieving satisfactory entry characteristics. The ‘long’ sides of the oval are straights rather than larger radius curves to maximise legibility of the change in curvature. The roundabout was designed to accommodate the swept paths of a 27.5 metre B-Double and 12.5 metre single unit truck turning simultaneously.

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 which will now carry the northbound Leach Highway traffic. The superstructure is a complex, haunched, continuous bridge comprising 8 No. 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 m 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.