The Melbourne 2030 plan identifies the Southeast region of Melbourne as one of the fastest growth areas in Australia. As the population of Pakenham’s designated urban growth area continues to rise, so to does the need to manage an increase in sewage flows.
South East Water will invest more than $A50 million during the next four years and in excess of $A75 million over 25 years in major sewerage works for the Pakenham and Narre Warren area to ensure increased sewage flows are accommodated well into the future.
South East Water’s commitment of more than $A75 million over 25 years makes this the largest capital works project undertaken in the company’s history. The project will comprise five stages and will see the use of cutting edge Trenchless Technology to construct one of the biggest gravity sewer projects in Australia.
Not only will the project accommodate further development and increasing sewage flows in this growth corridor, it will also alleviate operational, environmental and social risks associated with limited sewer capacities.
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Stages 1 to 4 are taking place during 2004-2006 (having started in April) and Stage 5 will take place during 2006-2008. Future works will continue during the following 20 years, ensuring the entire region is well placed for managing increased sewage flows as urban growth continues.
Specific works include:
• the extension of the Hallam Valley Main Sewer (Stages 1 to 4); and
• construction of a system to transfer excess sewage flows from Officer and Pakenham to the Hallam Valley Main Sewer (Stage 5).
During the planning phase of this project, South East Water conducted a full environmental assessment of the construction area. Biodiversity, archaeology, flora, fauna and native land vegetation were reviewed during the process.
Trenchless Technology will be used to construct most of this project, as the pipeline will generally be between 6 and 12 metres deep. Appointed contractors, GWC Group Pty Ltd, have purchased a tunnel-boring machine specifically for this construction – the longest construction of its type in Australia to date.
Trenchless Technology significantly reduces both visual and environmental impacts as an open-cut process would result in at least a sixty-metre wide construction strip and stockpile of excavated material.
There are two main benefits resulting from the construction of this major sewage pipeline, namely;
• Relieving existing sewerage infrastructure (pipelines and pumping stations) that are now reaching capacity as a result of the rapid increase in land development over the last few years. At least five existing pumping stations will no longer be required once the project is completed therefore eliminating risk of pumping station sewage overload; and,
• Providing capacity for future sewage flows from proposed developments in the Officer and Pakenham West areas.
GWC Group Pty Ltd was awarded the contract for the construction of the Pakenham – Narre Warren Sewerage Transfer System (approx value $34 million) for South East Water in September 2003.
The works comprise of the installation by microtunnelling of approximately 9 km of plastilined sewer pipe and construction of associated manholes. This project is one of the longest pipe jacking projects undertaken in Australia to date.
Based upon the expected ground conditions being predominately stiff to very stiff clays at the design depth of the sewer, an EPB1500 microtunnelling machine from Herrenknecht was selected as the machine most suited to the scope of works.
The construction method involves sheet piled drive and receival shafts for microtunnelled drives. The drive shafts are 9.0 m diameter circular while the receival shafts are 5.5 m x 4.5 m rectangular. Shaft depths range from 6 m to 13 m, with the ground water level in areas 2 m to 3 m below existing surface levels.
The main jacking station consists of two two-stage telescopic cylinders with full hydraulic retraction capable of jacking forces up to 700 t and stroke length of 4 m. The pipe used is a 1.5 m diameter plastilined jacking pipe with a length of 3 m. Drive lengths are between 170 m and
485 m using interjack stations as required.
During the microtunnelling operation, excavated material is removed from the tunnel face through the pipe in 1.6 m3 muck skips using a hydraulic winch. This method of operation avoids disposal issues with excavated materials normally associated with slurry machines.
Line and level is controlled using a self levelling drifting laser over the longitudinal and lateral axes with motor driven direction and inclination adjustment. The laser system is unique in the fact it has a split beam, one beam aligned with the target in the TBM and the other aligned with a special reference target mounted on the wall of the shaft. This reference target continuously monitors the position of the laser and automatically corrects laser position for any small movements of the laser unit. It is believed to be the first time this system has been used in Australia.
Pipe lubrication is achieved using an automatic lubrication system which is fully programmable and avoids the need for entry into the pipe as it can be programmed and controlled from the surface. The system can be programmed for various injection port sequences and timing allowing for higher volumes of bentonite or increased frequency of injection at specific “trouble spots.” Specific ports can also be turned off should the need arise. Injection of the bentonite slurry for lubrication is through three injection ports located at one third points around the circumference of the pipe. Every fifth pipe is used for lubrication.
At the time of writing seven of eight shafts (drive and reception) for Section 1 (2,800 m) were completed and three drives had been completed, the longest 383 m and the shortest 273 m. Maximum jacking forces experienced to date is in the vicinity of 350 tonnes although for the most part jacking forces have remained below 200 tonnes.
The expected project completion date is June 2006.
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