Alkimos SeaWater Alliance .

The Alkimos Seawater Desalination Plant is being delivered by the Alkimos Seawater Alliance, led by ACCIONA, to create a new source of drinking water for Perth. Stage 1 is designed to produce about 50 billion litres each year, with future upgrades planned to increase capacity. Works include the main treatment plant, a 33.5-kilometre pipeline to Wanneroo Reservoir, and associated blending and stabilisation facilities.

The marine component requires the excavation of two tunnels beneath the Indian Ocean: a 2.5 km intake tunnel and a 4 km outfall tunnel. Both drives are being undertaken by slurry tunnel boring machines launched from a deep secant-piled wall shaft that doubles as the permanent pumping station. Ground conditions are challenging, with the alignment passing through highly permeable coastal limestone under significant groundwater pressure. Continuous placement of precast segment linings is necessary to maintain stability and manage the risk of seawater inflow

During tunnelling works, the Acciona team encountered a failure of the steel keeper plate seal at the tunnel portal, resulting in a high-volume seawater ingress into the shaft. This required tunnelling operations to be paused until the inflow could be controlled.

Direct Inject was engaged to carry out targeted injection works to stabilise the area. The first objective was to fully seal the annulus of tunnel rings immediately in-bye of the secant shaft wall, allowing effective annulus grouting of the TBM rings close to the shield and restoring confinement conditions. The second objective was to stop the active inflow of seawater into the shaft itself. Both targets were successfully achieved, enabling tunnelling activities to recommence safely.

Site conditions added to the complexity of the works. The relatively small-bore diameter of approximately 4m, combined with the presence of the TBM gantries, restricted access to the tunnel rings that required injection. Despite the confined space and the significant pressure from the client team to have tunnelling operations resume, the injection program was completed within three days, allowing works to continue without further delay.