The project team were required to develop and implement innovative design and construction methods to address various constraints and overcome technical complexities at a number of bridge crossings. However, as common design and construction principles were used for at most of the bridge sites in this section particular attention has been given to the complexity of the bridges constructed at Canning River and Brixton Street.
In both cases different design and construction methodologies were developed to overcome the significant environmental and social constraints and accommodate the strict aesthetic and environmental requirements that were applied at each location through Main Roads Scope of Works and Technical Criteria document.
Main Roads prescribed a number of specific requirements for the bridges at Canning River that included:
The project team developed a design for Canning bridges that would see two 73 metre identical bridges constructed side by side. Each bridge was designed to carry two lanes of one-way traffic, with sufficient deck space to add a third lane in future., and the design and construction solutions developed took into account the bridges' location on the perimeter of one of the main tributaries to the Swan River and its proximity to established bushland and residential developments.
The bridges' location provided technical complexities not only for design, but also construction and operation as the bridge elements needed to ensure the pleasing aesthetics, provide for two directional traffic, provide access to the shared use path and local path network, minimise impacts on adjacent residents (during and after construction) and ensure no impact on Canning River or the surrounding bush area.
Further technical complexity was identified as it was determined that noisy piling work was necessary and efficient access to the site needed to be provided to ensure the project program could be met.
The project team developed an elegant structure design that incorporated the Main Roads stringent and specific location requirements, as well as its general structural aesthetic requirements. The project team sought aesthetic expertise to determine the most appropriate visual fašade for the structure, while the embankments and structure size were minimised to blend with the adjacent river environmental and reduce the potential impacts on adjacent residences.
For construction potential environmental impacts were also controlled through the implementation of an extensive Environmental Management Plan that was developed with the Water and Rivers Commission and Department of Environmental Protection.
To assist with site access permission was obtained to install a temporary causeway crossing in the river immediately adjacent to the bridge area, which would provide construction crews with easy access to the structure. The causeway was designed to minimise the risk of disturbing the river flow and function while the works were completed, and was constructed by placing clean rock and other fill within the river to form a hard surface. Three culverts or corrugated steel pipes were also placed across the span of the causeway to channel water and ensure the flow of the river was maintained at all times.
The implementation of this initiative was considered imperative to the efficient operation of the work site, and enabled the bridge beams to be installed without disturbing the river's function overcoming another issue of technically complexity.
In addition to installing a temporary river crossing, a solution was required for the installation of piles (upright bridge supports) that needed to be driven into the riverbed to provide the completed bridges load support network. It was particularly important that the installation method minimise noise impacts on adjacent residents as piling took some months to complete.
To address this the project team decided to use a system known as 'Frankipiles' to minimise the noise impacts of the work. With this method the piles are driven into the ground by a large mechanically operated hammer, however, noise is minimised by allowing the surrounding ground to absorb the pile driving impact and subsequent noise.
The Brixton Street traffic bridge was constructed over the new road and was not required to provide any connection to the new highway network. Main Roads again set stringent requirements for the bridge to ensure that the significant environment and infrastructure constraints were acknowledged and addressed.
These constraints added significantly to the technical complexity of the bridge design and construction task issuing a challenge to the project team.
Main Roads requirements included:
A further complication was the presence of the CMS high pressure gas pipeline and the BP oil pipeline parallel to the highway at this site. These services could not be relocated, and dictated pier and abutment locations.
The bridge abutments were of mechanically stabilised earth (MSE) in common with three of the other bridges. The MSE system was also adopted for the southern approach, allowing vertical walls of up to eight metres height to be constructed within an extremely narrow corridor
A fauna underpass was constructed through the approach embankment, requiring specially tailored wall panels to accommodate the box culverts used for the underpass.
The six spans of the bridge were each constructed using the precast prestressed T-Roff beam system, with in-situ concrete decks. The deck-edge forms were of precast concrete, the forms becoming the permanent parapet.