Australia
Melbourne
2019
Sewer
660 m
Beneath the busy city streets of Melbourne, a vital piece of the city’s infrastructure underwent a quiet but critical transformation. As part of the $6.7 billion West Gate Tunnel Project (WGTP), a 130-year-old sewer line has been successfully diverted to make way for twin tunnels that will ease traffic congestion and improve connectivity across Victoria’s capital. This was possible thanks to an ambitious engineering solution and the use of high-performance GRP pipe technology.
The North Yarra Main Sewer (NYMS), a 2.6-meter diameter brick-lined pipeline, was originally built in the 1890s and carries nearly 20% of Melbourne’s sewage. Located 13 to 15 meters underground, the sewer’s position overlapped with the route of the new tunnels. The challenge? Safely moving the live sewer, without disrupting service and traffic flow, beneath one of Melbourne’s busiest arterial corridors.
To meet this challenge, Amiblu provided 660 meters of DN 3000 Hobas centrifugally cast GRP jacking pipes – making it the largest-diameter GRP pipe-jacked tunnel in Australia to date. The material was chosen for its strength, corrosion resistance, and ability to be installed via trenchless methods without compromising hydraulic capacity. GRP also offered the precision needed to match the old sewer’s internal diameter and maintain flow continuity.
Construction was executed in two drives: 212 meters upstream and 424 meters downstream. A mini tunnel boring machine called Mollie carved its way through the earth at a pace of roughly 12 meters per day, with the Amiblu Hobas GRP pipes immediately installed to line the new tunnel.
To maintain sewage flow through the brick sewer during the installation of the diversion, engineers implemented a mechanized telescopic flume system to avoid risky bypass pumping. The flume was suspended from overhead beams and could be hydraulically retracted once the new connection was ready. Eventually, the wall of the old sewer was cut through to seamlessly divert sewage flow into the new alignment.
Beyond the complexity of tunneling under a live sewer, nature added its own set of challenges. The route’s proximity to the Maribyrnong River meant engineers had to handle the hydrostatic pressure of a high water table. With jacking depths reaching 20 meters, Amiblu had to ensure the pipe couplings and grout ports could endure external pressures of up to 2 bar – a demand that was met with confidence.
Today, the NYMS diversion is fully operational, preserving service continuity for Melbourne’s north and west and futureproofing the sewer network for decades to come. This complex project demonstrates how modern engineering and materials can work hand-in-hand with historic infrastructure. And it’s a reminder that some of the most important infrastructure projects happen out of sight – but never out of impact.
© Amiblu Holding GmbH, 2025
