Alkimos Seawater Desalination Plant Perth, WA

The Alkimos Seawater Desalination Plant (ASDP) – Stage 1 is a major infrastructure initiative aimed at delivering 50 gigalitres per annum of desalinated seawater to Perth’s Integrated Water Supply Scheme, with provision for future expansion to 100 GL per annum. The project also includes treatment of approximately 4.9 GL/year of groundwater and is being delivered through an Alliance contract, incorporating marine tunnels, deep shaft excavations, process facilities, and associated utilities.

A critical component of the civil works is the excavation and dewatering of deep shafts that willaccommodate the seawater intake and pumping station infrastructure. The excavation consists of a twin-round, peanut-shaped shaft arrangement, with each circular section having a radius of 26 m.

These are centrally joined to form a continuous footprint. The existing ground level is approximately 10.8 m AHD, and the excavation will extend to -11.0 m AHD, requiring a groundwater drawdown of approximately 13 metres, as the groundwater level is around 1.0 m AHD.

The excavation is supported by secant pile walls extending to depths of approximately -25.0 m AHD, providing both structural support and hydraulic cut-off. Subsurface conditions primarily comprise saturated sandy formations, including the Ascot and Osbourne Formations—both exhibiting moderate to high permeability and contributing significantly to groundwater inflow during shaft construction.

To address these challenging hydrogeological conditions, NPE was engaged to deliver a complete dewatering solution—comprising design, supply, installation, and operation—rated for up to 200 L/s capacity. The system includes internal submersible pumps and extraction wells strategically positioned to achieve depressurisation and lower groundwater levels beneath the excavation base, ensuring stability and preventing base heave.

In addition to dewatering, NPE implemented an on-site water treatment system to ensure extracted groundwater met environmental discharge criteria. The treated water was then reinjected into the aquifer through a series of dedicated reinjection wells, enabling 100% of the treated water to be sustainably returned to the groundwater system. This approach minimised environmental impact and supported compliance with regulatory requirements.

NPE successfully implemented the dewatering and water management system, achieving the target drawdown and enabling safe, dry, and uninterrupted construction of the twin-shaft structure. The system is engineered for operational flexibility and sustained performance, supporting staged excavation and seamless integration with broader construction activities across the ASDP site.