The pipe jacking operation was part of a suite of works by the Alkimos Water Alliance to install new wastewater services in Perth’s northern suburbs. The Alliance consists of the Water Corporation, Brookfield Multiplex, Macmahon Contractors and Zublin.
Alliance participant Zublin was responsible for the pipe jacking operation, which involved laying the 2,000 mm internal diameter plastic-lined concrete reinforced sewer pipes.
Alkimos Water Alliance representative and Tunnelling Package Manager Josef Kofler said on 24 November 2008 that his team had successfully installed 84 metres of pipe within a 24-hour period using a “÷dual mode’ tunnel boring machine.
“This was an Australia-wide pipe jacking record,” Mr Kofler said.
Pipe jacking in Perth
Construction of the pipe jacking works began on 16 February 2008 and was completed in early March 2009, three months ahead of schedule.
Pipe jacking will start again in April 2009 in a separate area and will involve making three tunnels of a total approximate length of 1,500 metres. This work is in addition to the original work package awarded in 2008.
The original works included the construction of ten temporary jacking and/or receiving shafts along the sewer alignment at varying depths between 8 and 20 metres. At each temporary shaft location, a permanent access chamber was installed for future access for inspections and maintenance. Depending on topography, construction and operational constraints, distances between the shafts varied from between 270 metres and 606 metres. The temporary shaft construction in the sandy strata was undertaken using concrete caisson segments with an internal diameter of 8.8 metres. Where the limestone formation was encountered, the walls of the temporary shafts were lined with reinforcing mesh and shotcrete.
Zublin also designed and manufactured the 3,000 mm long jacking pipes used in the sewer. The vertically wet-cast pipes have an internal diameter of 2,000 mm, a wall thickness of 225 mm and are designed for jacking forces of 8,100 kN. Corrosion protection was provided for the pipes by the addition of a 2.5 mm thick, 360 degree HDPE liner, which was cast into the concrete pipes. A stainless steel collar at each pipe acts as a guide during installation and together with a rubber seal seated on a recess, creates a watertight seal at each pipe joint.
Ground conditions
In general, the geology of the Perth metropolitan region consists of sandy overburden, derived from the underlying layer of limestone bedrock of the Tamala Limestone. Ground conditions along the 5 kilometres of sewer were highly variable, ranging from full face medium dense sand to a full face high strength calcarenite/siliceous calcarenite. In the sandy strata, limestone pinnacles of varying strength and diameter, typical of the Tamala Limestone formation, were encountered by the tunnel boring machine. However, this did not provide any problems for the operation. The rock strength was also highly variable and ranged between 1 MPa to 80 MPa. Dry fine sand was also found to overlay the limestone formation.
Alkimos Wastewater Treatment Plant
The Alkimos Wastewater Treatment Plant will provide a much needed service to Perth’s expanding northwestern corridor and will ultimately serve approximately 750,000 people. The plant will be owned and operated by the Water Corporation of Western Australia and will be built in a staged approach, with an ultimate capacity of 160 megalitres (ML) per day.
The contract to design, construct and commission the first stage of the plant has been awarded to the Multiplex MacMahon Zublin Joint Venture. This first stage will accommodate 20 ML per day, with the second stage required in 20 years.
The first stage will include a series of permanent and temporary structures to be decommissioned progressively during the second and third stages of construction. Works will include a permanent flow reception chamber, temporary screening facilities and a potable water system comprising bores, storage tanks, disinfection and a booster pump station. The plant will have a permanent incoming HV switch room, module one switch room and associated roads, drainage, connecting pipe work, landscaping, security and fire system.
Construction of the plant is scheduled to start in April 2009, with the plant to become operational towards the end of 2010.