In a bold move that reflects ACCIONA'S commitment to sustainable infrastructure, the Western Harbour Tunnel (WHT) project is pioneering the installation of two underground slurry treatment plants. It’s an initiative that stands out for both its environmental and engineering ingenuity.

Traditionally, slurry treatment plants are located above ground, often requiring a large surface footprint and extensive piping networks to transport slurry from tunnel boring machines (TBMs) to treatment facilities.

However, in a world-first, following extensive client consultation, the WHT team has opted to place the plants within two ventilation caverns deep underground.

“These caverns, which will eventually house the tunnels’ permanent ventilation fans, are being repurposed during construction to host the treatment plants – one for each TBM – making efficient use of space and reducing surface-level disruption,” explained Sam Harrison, Construction Manager Western Harbour Tunnel.

The slurry itself plays a critical role in TBM operations, particularly under Sydney Harbour’s soft marine sediments. It helps maintain face pressure at the TBM cutterhead, supporting soft and or unstable and inconsistent ground conditions, thereby ensuring safe tunnelling.

As the TBM cuts through the ground, the slurry becomes laden with spoil (rock, sand, clay and sediment) which must be removed to allow it to be reused to maintain the machine’s performance.

The underground plant filters and reconditions the slurry before sending it back to the TBM, minimizing water consumption, wear and tear, and reducing downtime.
Multiple sustainability benefits are being realised from this underground setup.

They include the elimination of kilometres of steel piping that would otherwise need to be laid, a reduced number of slurry pumps saving energy, and fewer heavy vehicle movements on local roads.

Additionally, being underground provides a natural barrier to the noise and light pollution that a slurry treatment plant operating 24/7 produces, reducing disruption to residents and businesses in the area.

The plants’ design also incorporates rubber-lined and ceramic-lined pipes in hard-to-access areas to reduce wear and reduce maintenance needs. With a capacity to process up to 6,000 cubic metres of slurry per hour across two TBMs, the system is built with high redundancy to ensure continuous tunnelling operation.

Emily Kahn, who is a Mechanical Engineer working as part of the assembly team, said she’s proud to be involved in the initiative.

“It’s great to be involved in implementing the first underground slurry treatment plant of its kind in the world. I think the project’s unique setup – working below ground with limited overhead access – is a driver of innovation and problem-solving,” said Ms Kahn.

This initiative is one of several sustainability-focused stories emerging from the WHT project, alongside spoil reuse and cable redundancy reduction.

Together, they reflect a culture of innovation fostered by the project’s incentivised cost target contract, encouraging teams to find long-term, cost-effective, and environmentally responsible solutions.

As the TBMs prepare for launch and the slurry plant nears operation, the Western Harbour Tunnel continues to set new benchmarks for sustainable infrastructure delivery.