The Water Corporation of Western Australia has aimed to provide a safer and more efficient wastewater system in Leederville by upgrading Perth’s 80-year old main sewer pipeline as part of a five year upgrade program. The existing sewer was constructed in the 1930s using unlined concrete pipes and is now exhibiting significant corrosion due to sulphuric acid attack.
The project involved the replacement of existing underground sewerage pipes across variable installation conditions, including major roads and rail crossings. Work commenced on the project in January 2007 and is expected to be complete by January 2008.
Preliminary planning activities investigated replacement of the existing sewer using conventional open-trench installation. However, after quantifying the projected major impacts to both traffic and the local community, Trenchless Technology presented the most practical and cost-effective solution.
The products
The J-Series jacking pipe range is a world-class product suited for use with modern, closed-faced microtunnelling equipment. The J-Series is an Australian first and is manufactured domestically throughout Humes’ national network of manufacturing sites. It complies with the specification requirement – AS4058-2007 – and features a watertight joint, wide jacking faces and a secure steel collar, which are all essential for longer jacking runs and for use in unstable ground conditions.
The J-Series pipes are manufactured using the spun process, which provides an off-form finish to jacking faces of the pipe and consistently high concrete quality, density of compaction, and hence durability throughout. Steel collars are fabricated to tight tolerances (å±1.5 mm In diameter) and held firmly in place, and in-round, during casting. The pipe spigot is formed using a machined former fixed to the mould. One of the many benefits of using Humes’ J-Series Jacking Pipes comes from this advanced jointing system.
All prefabricated components for the project were manufactured at the Humes Welshpool factory in Perth. Each pipe weighing approximately 5.5 tonnes, incorporated swift-lift anchors for safe and efficient lifting and were transported to site utilising steel bolsters for effective site logistics.
To exceed the 80-year service life of the existing pipeline, Plastiline, a plasticised PVC Corrosion Protection Lining (CPL), was cast with the pipes and pipe-socket collars manufactured from Grade 316 stainless steel. Plastiline is a 1.5 mm thick CPL, and was installed to provide a 350 degree lining central about the pipe obvert – allowing a 10 degree invert drainage gap. As is typical with jacking-pipe lining, continuity was achieved by welding a cover strip across each pipe joint.
Plastiline, which has a proven Australian history of 46 years for providing concrete protection in harsh environments against H2S attack, was incorporated in accordance with WSAA Standard Specification WSA 113-2002.
Installation
DJ & MB MacCormick employed their remote-controlled, laser-guided, Herrenknecht closed-face microtunnelling machine to undertake the works. General sandy soil conditions and construction processes permitted a drive performance of approximately 30 m per day, with a longest drive-length from starting to target shaft of 280 m.
An Aus-Gel Xtra Bentonite drilling medium was pumped to the excavation face via slurry discharge pipes, with the closed-face machine passing excavated material back through a cone-crusher to return pipes. The slurry pipes, machine hydraulics and power were supported on specially designed frames so as not to damage the Plastiline CPL during jacking and slurry removal.
The following construction times were indicative for critical-path tasks resulting in an overall average drill-rate of 9 m per day.
Vertical shafts
For this project, 400 mm thick insitu caissons – 5.5 m x 4.5 m x 5.6 m deep – were specified as starting and target shafts. These were constructed of 30-40 MPa reinforced concrete, and upon completion the caissons were broken down to approximately 1.5 m in height and mass filled around the pipe to manhole connections.
The caissons were poured insitu in nominal lifts of approximately 1.5 m above ground, cured and then lowered by excavating with a clam-shell bucket. A concrete “÷plug’ cast as the base-slab of the shaft provided both stability and a watertight structure.
Reinforcement was excluded from the shaft wall at the pipe exit and entry locations. A thrust block was also cast at the back wall to provide the necessary reaction to the 850 tonne jacks. A single rubber ring gasket was then used to prevent infiltration of ground water and soil ingress into the shaft during jacking.
Tom Kavanagh of DJ & MB MacCormick recommends the use of precast caissons shafts instead of insitu caissons. “[The use of] precast caisson shafts could have reduced the construction program for each drive length – increasing overall average drilling rate, and further reducing disruption/congestion in the city.”
Drilling
As is standard practice, an annulus – in this case 10 mm – was formed between the tunnel excavation and the outer-diameter of the pipe due to the over-sized machine bore. This process serves to minimise frictional drag and maximise jacking lengths.
Low-strength grouting of this anulus is sometimes specified with the intent of minimising point or local loads on the pipeline system, however there is seemingly little “÷science’ attributed to this concept.
For this stage of the Perth sewer, four grouting ports were specified for each fifth pipe. With the excavation predominately through the sand lenses of Perth, DJ & MB MacCormick allowed for the annulus volume plus 50 per cent in estimating the grout volume requirements. As part of the pipe manufacturing process, Humes installs either normal ferrous or stainless steel grouting ports into the J-Series jacking pipe.
The Perth Main Sewer installation demonstrated that Humes’ J-Series jacking pipe is a world-class trenchless product in an industry where benchmarks have been traditionally set abroad.