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A.G. Coombs is currently involved in some of Australia’s largest transport infrastructure projects, including the delivery of mechanical services for the new Town Hall Station, as part of Melbourne’s Metro Tunnel Project.
The design and construction of the Metro Tunnel’s main tunneling works, five underground stations and station fitout was awarded to the Cross Yarra Partnership (CYP) consortium. This consortium comprises Lendlease Melbourne Metro, John Holland, Bouygues Construction, John Laing and Capella Capital, operating in a Public Private Partnership (PPP).
“A.G. Coombs has proven to be a valuable specialist technical partner on the Metro Tunnel Town Hall Station, with our technical experience in underground rail stations and prefabricated solutions bringing advantages to the construction site and overall program,” says Michael Roberts, Metro South Project Manager for A.G. Coombs.
One of the Victorian Government’s Big Build infrastructure projects, the Metro Tunnel is set to untangle Melbourne’s underground City Loop by creating a new end-to-end rail line from Sunbury in the west to Pakenham in the south east.
The project includes twin 9km long rail tunnels and five new underground stations, including Town Hall in the centre of Melbourne’s CBD.
Located beneath Swanston Street between Flinders and Collins, Town Hall Station will provide a passenger connection to nearby Flinders Street Station, allowing commuters to conveniently interchange between Metro Tunnel and City Loop services. It will also improve passenger and public access to City Square, Federation Square and other notable landmarks in the CBD.
The station is located at a depth of 27m below street level and features a trinocular cavern with three overlapping tunnels that integrate the concourse with train platforms on one single level.
This design has resulted in 19m wide platforms – some of the widest metro platforms in the world – as well as a concourse featuring a cathedral-like vaulted ceiling.
In late 2021, A.G. Coombs was awarded a construct-only contract to deliver the mechanical services for Town Hall Station by managing contractor, CYP Design & Construction Joint Venture.
The concourse, electrical and communications rooms are conditioned by a water-cooled chiller plant located within an Over-Station Development (OSD) nearby. This plant serves chilled water fan coil units (FCUs) throughout, while a small boiler plant provides heating to retail spaces and back-of-house.
Due to rail-specific requirements, all plant has been designed and installed in a duty/standby configuration, allowing A.G. Coombs to prefabricate duty/standby fans and coil arrangements with motorised dampers and valve sets making site installation significantly more straightforward.
Pipework modules that connect services from the Flinders OSD plantroom to the concourse were also prefabricated. Given their installation some 9m above the concourse floor, this method removed the significant risk associated with personnel working at heights.
Considerable detail has gone into the design lifecycle of plant and equipment that will operate in a high humidity environment, with consideration given to specialist equipment design life and corrosion prevention.
As well as the construction of mechanical services and ventilation systems serving the station concourse, A.G. Coombs has been responsible for the installation of the Over-Track Exhaust (OTE) system, and specialist rail tunnel fan units capable of exhausting 200,000l/s of air when operational.
“We went through a steep learning curve in regard to understanding this specialist rail tunnel ventilation systems and critical systems,” says Sasheeka Amunugama, Project Engineer at A.G. Coombs. “This was assisted significantly by A.G. Coombs’ considerable experience in the new Sydney underground metro stations.”
“The heavy-weight specification of 6mm plate steel superstructure associated with these systems, including transition pieces and connections to attenuators, required specialist steel prefabrication techniques. These pieces were so large and heavy, they were designed and constructed into four quadrants before being lifted and joined onsite.”
Construction sequencing associated with working underground also presented numerous challenges and required close coordination with other trades.
Incredibly, the rail tunnel ventilation system at Town Hall Station operates in unison with the ventilation systems within adjacent underground stations to either supply or exhaust air through the expanse of the connected rail tunnel.
And unlike the 45-year old City Loop, the Metro Tunnel rail tunnels are separated from each station concourse by doors, effectively sealing the tunnel.
In the event of a concourse fire, the Environmental Control System (ECS) plenum that conditions the platform via supply air in normal operation reverses to become a smoke exhaust system.
“Because there are so many critical spaces in an underground station, the fire mode is quite complex and resulted in us workshopping with CYP and the contractor working on the other CBD underground station, to develop a holistic solution that was both achievable and consistent across both sites,” says Sasheeka.
A.G. Coombs personnel continue to work on site, with Town Hall Station expected to reach completion and open with the Metro Tunnel in 2025.
This article originally appeared in BE – Building Efficiency 2023-2024 magazine
