Austral Precast Double Wall provided a time- and cost-effective solution that met the weight and space restrictions of the Exhaust Vent Building for M5 Arncliffe.
The new M5 Arncliffe (MOC3) – a 9-kilometre-long twin tunnel between Kingsgrove and St Peters – has more than doubled the capacity of the M5 East Motorway corridor. Opening to traffic in 2020, the tunnel is tall and wide (in comparison to the existing M5) to reduce emissions, and it has an efficient and modern ventilation system that meets stringent global air quality standards in the tunnels and surrounding suburbs. Austral Precast Double Wall provided a time-and cost-effective solution for the monolithic structure of the exhaust vent building. While not traditionally used in infrastructure projects, Austral Precast Double Wall met the weight and space restrictions of the building, as well as increasing the speed of construction and reducing on-site materials.
The ventilation of the M5 Arncliffe is achieved through an exhaust vent building and supply vent building that work in conjunction with another. The exhaust vent building treats and removes “used air” (vehicle exhaust) from the tunnel, while the supply vent building introduces “new air” into the tunnel. The buildings are directly connected to the tunnel via two 35-metre-diameter underground shafts that descend more than 60 metres from surface level. Several levels of the exhaust vent building below ground support the large mechanical ventilation equipment, while the above-ground portion is 12 metres tall, with four 25-metre-tall stack chimneys that disperse the exhaust air in the ambient environment higher than the nearby roof lines.
The limited space around the exhaust vent building proved challenging for undertaking formwork, and the 12-metre-tall walls would have been difficult to pour full-height in-situ. As an offsite precast product, Austral Precast Double Wall provided a solution to these limitations, as well as benefits for the speed and safety of construction.
“The exhaust vent building is a very tall structure and had restrictions with the amount of space available for efficient construction activity. The project was also under strict construction timeframes and conventional formwork methodology did not solve any of these challenges,” says Austral Precast State Sales Manager, Sanket Das. “The Double Wall provided all the benefits of off-site factory production, and the client gained speed in construction and a greater amount of space on the construction site.”
Austral Precast Double Wall is a structural system of relatively large and lightweight panels that provides an off-form ready-to-paint finish. It consists of two precast reinforced concrete shells connected by a reinforcement-bar lattice girder, which forms a cavity between the two shells. It is an excellent solution for loadbearing walls and will form a solid concrete structure after the cavity is filled with premixed concrete. With less weight than solid precast panels, the Double Wall enables savings in installation costs and time, eliminates the need for scaffolding, and construction activities are mainly performed within the boundary.
The joint venture (JV) design, engineering and construction team – including CPB Contractors, John Holland, Dragados, Aurecon and Jacobs – considered solid precast options before selecting the Double Wall. “Solid precast products would have issues to crane descent in, and the reinforcement requirements and connections would necessitate major design changes,” said a representative of CPB Contractors. “The advantage of the Double Wall to solid walls is the weight, as well as keeping the standard fibre-reinforced polymer (FRP) connection and the same design intent.”
The JV started working with Austral Precast at the end of 2017, discussing and confirming the use of the product during multiple workshops and stakeholder meetings. As the CPB representative described, “These meetings identified the best product to suit the existing design and to simplify the methodology, given the very high specificity of the building and the need for it to be heavily reinforced.” The panels were installed by mid-2019, achieving a more efficient construction time and method than solid precast panels or in-situ formwork.
Quentin describes two main challenges in using the Double Wall for the exhaust vent building. The first was changing the mindset of Roads and Maritime Services, the independent verifier and designers, as the Double Wall is a relatively new product in Australia, especially in major infrastructure projects. The second was ensuring the design intent and engineering requirements could be achieved without impacting the installation timeframe, as the walls are heavily reinforced with deep beams. This required additional reinforcement at the joints, which is not usually necessary for standard shear walls.
However, given the success of the project, WestConnex has since introduced the Double Wall to its independent verifier, designers and the Roads and Maritime Services. The CPB representative explained, “As double walls are still not traditionally used in infrastructure projects, we spent time reassuring the different parties that the quality of the Double Wall is satisfactory and that it is a good alternative on infrastructure jobs, especially where there are weight and space restrictions.”