Interflow Awards
The Coogee Diversion Sewer is an important pipeline taking much of the flow from eastern Sydney to the Malabar treatment plant.
Detailed investigation and condition assessment showed that some 50 metres of a 1,830mm diameter section had deteriorated to a stage where normal safety margins had been exceeded. The remaining sections were in slightly better condition but problems could be expected to escalate to a critical stage within the next 5 years.
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The deteriorated section of pipe started some 4 metres below a residential area in Maroubra, a Sydney beachside suburb, passed under a public reserve, then beneath the Anzac Rifle Range before entering a large inlet structure just upstream of the Treatment Plant. Depths below ground level ranged from 3.3 to 7 metres.
The Contract required the installation of a structural liner to restore the structural integrity and flow capacity of the deteriorated sewer. Being a structural liner meant that, in the most badly deteriorated sections of the pipeline, it had to be designed to take all loads from soil and vehicles as if the original pipeline had no remaining strength.
Installation of the liner had to be accomplished while maintaining services to the many thousands of people who relied on the pipeline. Excavation in the residential areas had to be minimised, while in the Rifle Range, because of contaminated ground, was not permitted at all.
Typically on large diameter pipeline rehabilitation projects a “slip liner” would be installed, comprising discrete lengths of pipe pushed up inside the deteriorated pipeline and joined together.
However on the Coogee Diversion Sewer Rehabilitation project such a solution presented difficulties because:
Excavation of large launch pits would have been required. The pipeline would have needed to be taken out of service, possibly disrupting services to householders.
On this project Interflow, in partnership with pipe technology company Rib Loc Australia Pty Ltd offered an innovative solution that optimally met the project requirements.
Rib Loc manufactures a range of steel reinforced polyethylene pipes that are sold as alternatives to reinforced concrete pipe. The manufacturing process for the pipe uses a continuous strip of profiled high density polyethylene, which is smooth on one side, but ribbed on the other. Inside each rib is a continuous strip of steel, which gives the pipe its high stiffness. The edges of the profile are welded together by a winding machine to form a pipe.
For the Coogee Diversion Sewer project Interflow developed a work method for using the winding machine at the base of the access chamber. The profile was fed from above ground to the winding machine which continuously welded the edges together to form a steel reinforced polyethylene “pipe” inside the deteriorated host pipe. The diameter of the winding machine was set to wind the liner slightly smaller than the diameter of the deteriorated pipe.
The newly developed liner was given the name of Ribline.
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Management of sewer flows was a vital part of the project to ensure maintenance of services to householders and the safety of the installation process.
The flow management system developed by Interflow combined working at night when flow was at its lowest with the installation of an extensive bypass system to pump flow around the section of pipeline being lined.
Winding of the liner was always undertaken with about 200mm of water trapped in the bottom of the pipeline. This allowed the liner to float after it was wound and so minimised friction with the pipe wall. Long lengths of liner could be successfully wound using this method.
After installation the gap between the outside of the liner and the inside of the host pipe was filled with cementitious grout. The grout had the functions of holding the liner in position inside the host pipe as well as filling voids in the wall of the deteriorated pipeline.
The Project was completed without excavation and with minimal disturbance to the surrounding community. It was a demonstration of ingenuity using Australian developed technology that provided the optimal solution to a difficult engineering problem.