Two undersea tunnels for Western Australia’s planned Alkimos Seawater Desalination Plant have been completed, marking the end of a 15-month tunnelling programme for the A$2.8bn ($2bn) project.
The tunnels comprise a 2.5km intake and a 4km outfall, extending 1.6km and 3km offshore respectively.
The intake tunnel is intended to draw seawater into the plant, while the outfall tunnel will return brine to the ocean. The project said the structures are the longest desalination intake and outfall tunnels in Australia.
Tunnel boring machines Mary and Karli excavated up to 50m per day around 20m below the seabed, according to the update. The works involved installing about 32,000 locally made concrete segments.
The tunnelling was delivered by the Alkimos Seawater Alliance, a partnership formed by the Water Corporation alongside ACCIONA and Jacobs Group to design, construct, and operate the Alkimos Seawater Desalination Plant.
The two machines are now being demobilised ahead of work to connect the tunnels to undersea intake and outfall structures. Those structures were installed by Beverley, described as Australia’s largest registered jack-up barge.
Marine exclusion zones around the offshore work areas have been removed. Some vessel activity will continue, the update said, to support final inspections, surveys and commissioning checks.
The project said the tunnels have been sized to allow for future expansion beyond the plant’s first-stage capacity of 50bn litres of drinking water.
It added that the first stage is expected to help supply more than 2.5 million people in Western Australia and reduce reliance on groundwater sources affected by climate.
Construction activity is now shifting towards fitting out the plant’s buildings and commencing electrical and mechanical works across the 29ha site.
The project said it is supporting more than 1,500 local jobs and remains on track to be fully operational in 2028, with pipeline and site works described as well advanced.
Western Australia Premier Roger Cook said: “The culmination of 15 months’ work, these tunnels are an amazing feat of engineering – two of the longest undersea tunnels in Australia – and proudly built using Made in WA materials.
“They establish the critical connection between the future Alkimos Seawater Desalination Plant and the ocean, bringing in the seawater and safely returning the brine.
“As WA’s next major water source, this vital Cook Labor Government project will help meet the needs of our growing population and reduce the pressure on our precious groundwater, while delivering strong economic benefits for the state.”
