In March, microtunnelling expert Rob Carr was awarded a contract to deliver the Quinns Main Sewer Extension for Water Corporation in Western Australia. In terms of scale, the project is one of the biggest the company has undertaken in the state.
Water Corporation of Western Australia awarded Rob Carr a design and construct contract to extend the Quinns Main Sewer, which allows conveyance of wastewater flows from Ellenbrook, running through Clarkson and Neerabup National Park to the Alkimos wastewater treatment plant.
“The existing infrastructure can’t handle the expected calculated future flows in the area,” explains Rob Carr Western Australia General Manager Barry Crowley.
Mr Crowley has worked with the company for 20 years, and, under his leadership, the team has installed hundreds of kilometres of main sewers throughout Western Australia.
This project involves 2 km of 1,800 mm diameter sewer main. The extension adds on to Stage 1 of the Quinns Main Sewer project, which was completed more than ten years ago.
“The purpose of Stage 2 is to extend across that area through the national park so development upstream of that area can occur, be it residential, commercial or industrial,” says Mr Crowley.
The project will use a combination of microtunnelling and open trench techniques, with Rob Carr successful in making some innovative changes to the brief for the future delivery of works.
“The preliminary design had a requirement for 700 m open trench and 1.1 km of tunnelling,” says Senior Project Manager Paul Lee.
“We’ve changed that through the design to 1.4 km of tunnelling and 600 m of open trench.”
This innovation combined with Rob Carr’s extensive capability in successfully delivering large diameter microtunnelling projects in challenging ground conditions were two of the reasons Rob Carr was awarded the contract.
“During the bid process and design evolution, we’ve managed to get more tunnelling on the project and less opening trenching works, which results in less impact for surrounding resident street access. From a design aspect it made more sense to do it that way,” says Mr Crowley.
“It reduces the volume of spoil, eliminates reinstatement works, and it’s friendlier to the overall community.”
Much of the eliminated open trench work was to take place in residential areas, and Rob Carr wanted to minimise the potential impact to the community during the design phase and construction.
The company was also successful in its suggestion to include local content.
“When we put in our bid to Water Corporation, their preliminary design was based around the use of glass-reinforced plastic (GRP) jacking pipe and open trench installation,” says Mr Crowley.
“We put in alternative bid for the use of reinforced concrete jacking pipe, which is sourced locally from Humes. That was something the corporation recognised and acknowledged as the preference for the construction and design element.
“That way we achieved our recognised local content and supported local business.”
In terms of scale, the Quinns project is substantially greater than previous projects in the region.
“In diameter, it is significantly larger than what has predominantly been required in Western Australia. The last time there has been an 1,800 mm diameter pipe installed here was 11 years ago,” says Mr Crowley.
“We don’t have the population, therefore we don’t require the diameters the east coast would. Up to DN1200 is usually the diameter used here; so, it is designed for future growth.”
Currently, the project is still in the design phase with limited activity onsite. Rob Carr expects design to be completed in October, with onsite construction likely to begin in February 2022 and running for 12 months.
The depth of the new sewer ranges between 3.5 and 27 m below ground and, while the team has no grave concerns for the future of the project, it will still be an intricate process.
“The construction of the 27 m deep shaft and converting that into a permanent vortex structure is quite intricate and complex,” says Mr Crowley.
“That will be one of the more challenging parts of the construction of the works.”
The Quinns Main Sewer project will also require the installation of six other sewer manholes. The team at Rob Carr said it is looking forward to working closely with design partner SMEC and client Water Corporation WA as the project progresses to its construction phase.
For more information visit the Rob Carr website.