Take an inside look at the design and construction of the extensive WestConnex M4-M5 Link Tunnels project.
The WestConnex M4-M5 Link Tunnels project, a joint venture between Acciona Construction Australia, Samsung C&T and Bouygues Construction Australia, was a mammoth undertaking.
Costing $3.2 billion, it involved construction of two 7.5-kilometre underground tunnels, with the entire project spanning 21,580 metres.
The scope of the job was essentially to fill the “missing link” between the M4 East at Haberfield and the New M5 (now designated M8) at St Peters, said Acciona Project Director Andrew Marsonet.
“It’s the connection between those two tunnels, along with four underground ramps to the Rozelle interchange, to complete the WestConnex tunnel system,” he said.
The project, a 2023 Australian Construction Achievement Award finalist, has both reduced the time between journeys and eased traffic congestion. To get it right, the team were determined to optimise design from the start, and learn from the sub-optimal outcomes of earlier projects.
Three tunnels thinking as one
The M4-M5 Link Tunnels project had a sizable mechanical and electrical engineering (M&E) scope.
“We had two surface substations, four underground substations, two low point sumps and two ventilation structures, ventilation tunnels as well as multiple electrical equipment rooms, other cross passages and in tunnel devices,” Marsonet said. “These all needed to be fitted out and connected to the WestConnex Motorway Control Centre.”
However, the biggest role for M&E was creating the “brain” that linked all three tunnels – an integrated operational management control system (OMCS).
“The OMCS in each tunnel controls its individual systems, such as fire and life safety, traffic management, and deluge systems, and how the tunnel responds in emergency situations,” he said. “Our IOMCS supervised these three individual systems from one operating control room, allowing all the tunnels to operate seamlessly.”
That meant if an incident occurred in the M8, for example, all tunnels would issue emergency warnings or enact different traffic management measures in a pre-programmed manner which can be manually overridden if required.
A rigorous testing and commissioning process ensured the system operated perfectly from day one.
“Our IOMCS ran the three tunnels without any traffic for a six-week period to demonstrate that the software, control systems and physical links worked,” Marsonet said.
“That gave WestConnex and Transport for New South Wales confidence that the tunnel operated successfully and could be opened.”
A collaborative ethos
A key part of the project’s success was how the leadership learned from the various aspects of delivery of other WestConnex and NorthConnex tunnels.
“For example, a theme emerged that because a lot of time was lost in excavation/civil fit-out works, the M&E works were crushed in terms of time,” Marsonet said. “This then required additional unplanned resources to be employed, and the costs just compounded.”
To avoid making the same mistakes, the leadership team decided to start M&E earlier in the program – in parallel with civil fit-out works and tunnel excavation.
The concept of a “tunnel production line” was used from the outset to ensure this process ran as smoothly as possible.
“We’d have a 1 km zone of excavation, including heading and benching at the front, followed by a zone of civil works, such as drainage, paving barriers and waterproofing,” he said. “Then we’d have a zone of M&E installation works [positioned] so you could stand on concrete paving and see excavation works up ahead, then turn around 180 degrees and see M&E fit-out works.”
Working in this fashion meant the tunnel had to be kept squeaky clean, including laying 100 mm of temporary blinding concrete every time the bench was excavated. While adding a cost of several million, this process helped to keep the tunnels free of mud, enabling the ongoing civil and M&E works to progress in a planned and efficient manner.
“We also had to plan all our underground logistics to ensure all fit-out activities could proceed while dump trucks were going past and concrete trucks accessed and egressed the tunnel,” Marsonet said.
Not everyone was convinced this methodology would work, however, meaning leadership had to “onboard the sceptics”.
“The M&E team were unsure if we could manage the air quality, spoil and concrete logistics so they could operate in a clean environment,” he said. “So we developed simulations and videos to explain how we were going to delineate and manage underground logistics.”
Affording the M&E team sufficient time to fulfil their role was a demonstration of their importance to the project, boosting morale.
“Our thinking and planning as leaders, which flowed down to the teams, was all around making sure everyone felt important and listened to, so the problems that arose during the project were solved holistically.”
An innovative shotcrete design
Another example of collaboration that paved the road for optimisation was the shotcrete design, which encompassed both innovative material and structural properties.
While an optimised design was high on the agenda, it also needed to match the specifications provided by Transport for New South Wales, said Leo-Paul Calbrix, a civil engineer at Bouygues who was the design manager on the project.
This included optimising the combination of rock bolts and shotcrete, which provides global and local stability to the tunnels.
“We generalised the use of handlebar plates with studs, so there were points of connection between the shotcrete and the rock bolts,” he said. “That way, any loads on the shotcrete were transferred into the rock bolts.”
This design fulfilled the brief, as the team was asked by WestConnex not to rely on long-term adhesion of the shotcrete to the rock substrate.
“By using rock bolts in combination with shotcrete in the design, we were able to be quite efficient and avoid having a very thick shotcrete arch,” Calbrix said.
The standard shotcrete thickness was reduced from 110 mm to 100 mm, while achieving the same performance. The shotcrete also comprised up to 25 per cent fly ash as a substitute for cement, both enhancing the durability of the shotcrete and reducing environmental impacts.
“If you have less cement in your mix, you have a better carbon footprint,” Calbrix said.
There were numerous advantages to this approach. The use of less materials led to a cost benefit in excess of $10 million, a substantial reduction in the amount of embedded carbon and improved productivity, Marsonet said.
The innovative shotcrete design even garnered international accolades. The American Shotcrete Society recognised the WestConnex M4-M5 Link Tunnels project with the 2022 Outstanding International Shotcrete Project of the Year award.
All these solutions combined ensured theM4-M5 Link Tunnels opened 10 weeks early.
“It wasn’t all perfect but at the end of the day, we opened early, within budget and provided a quality product,” Marsonet said. “The cost of a day on this project was high, so if you save on time, you save on cost.”