To accommodate for the expansion of Perth’s northern suburbs, the Water Corporation of Western Australia initiated the construction of a new wastewater treatment plant (WWTP).
Prior to the construction of the Alkimos WWTP, three major projects had to be undertaken:
Earthworks for the plant including the removal of over three million cubic metres of earth
Construction of the Quinns Main Sewer
Construction of an ocean outfall to take treated waste from the plant
The Alkimos Water Alliance (AWA) was formed to undertake the design and construction works on behalf of the Water Corporation. The alliance partners included Multiplex, Zublin and Macmahon and sub-contractors Worley Parsons, Aurecon and Cardno BSD. Aurecon designed the sewer and the connection from the WWTP to the outfall.
The Quinns Main Sewer has an internal diameter of two metres, and is over five kilometres in length. It is designed for a 100 year life, and is lined with high density polyethylene throughout its length to give it the necessary durability.
The pipe was constructed using two methods, open pit trenching and tunnelling, using a state of the art dual mode tunnel boring machine (TBM) able to operate both in solid limestone and loose sand.
Large concrete lined shafts were designed to lower the TBM to the level of the sewer. These were designed to be temporary structures, yet had to be robust enough to stand the forces from the tunnelling. Smaller permanent access shafts were designed to be fitted inside the large shafts to allow access for maintenance. Temporary roads had to be designed to allow construction vehicles access to sites in undeveloped bushland.
The project called on a wide range of expertise from within Aurecon. Durability experts and tunnels specialists worked with the local team who managed the water, structural and traffic design components of the project.
Issues addressed in the design phase include pipe durability when exposed to sewer gases, ground settlement caused by tunnelling, safety issues in the construction and operation of the sewer and using a combination of caisson and shotcreting construction techniques.
The Alkimos WWTP began operation in 2010 and has a capacity of treating up to 20 million litres of wastewater a day.
This was one of the largest tunnelling projects undertaken in Perth and represents a significant contribution to the State’s infrastructure.