The project comes after a main suffered a major burst in 2009 near the Magistrate’s Court Complex, with condition assessment confirming the existing pipelines have a high likelihood of failure.
The project involves the replacement of two water mains: a 600 mm water main along William Street and Howard Street between Queensbridge Street and Queensberry Street; and a 300 mm water main along William Street between Flinders Street and Victoria Street. The works will see water mains replaced in stages along the entire length of William Street from Flinders Street to Howard Street near the Queen Victoria Market.
While the need for replacement was evident, there had to be a method to replace the mains with minimal disruption to the community. Up to 1,400 vehicles per hour travel through the William Street and Flinders Street intersection between peak weekday hours. Additionally, William Street is also a tram route, so minimising environmental impact would be no easy task.
Why trenchless?Article continues below…
Project proponent City West Water decided to employ Trenchless Technology for the construction of the water mains across tram tracks and major intersections like Flinders Street, Collins Street and Latrobe Street. This allowed for faster construction and less disruption to traffic and local residents. It also meant shutting down tram services could be avoided. Further benefits included minimisation of excavation and reinstatement of pavements; and enabling the removal of their old wrought iron main for recycling.
City West Water contracted Scarriff Pipelines to undertake the work, with trenchless works sub-contracted to Harris Civil. The Trenchless Technologies considered included: pipe cracking, which was deemed not suitable for installing the MSCL pipes required; sliplining, which would have reduced the size and capacity of the pipe; and laser boring and directional drilling, both of which would require a new alignment and posed a higher risk of damaging other utilities during boring and drilling.
Pipe ramming was chosen as the preferred method because it enabled construction along the existing pipe alignment and fit the other necessary requirements.
The equipment used for the pipe ramming works for the 600 mm water main, located in the middle of the northbound carriage way, included: a 60 mm diameter 3 tonne pneumatic hammer, a compressor (1600 cubic feet per minute at 120 psi) and a 20 tonne excavator to establish launch and retrieval pits.
Launch and retrieval pits were established on either side of the tram lines and sections of the existing pipe were removed. An 819 mm diameter steel sleeve was rammed over the existing 600 mm wrought pipeline under the tram line. The existing pipe was then removed and the new MSCL pipe jacked into position.
Pipe ramming is also proposed for the 300 mm water main, located under the kerb and channel, which is to be constructed on a new alignment. City West Water said the demonstrated success of the pipe ramming will enable this technology to be used at the remaining four intersections for this project, and potentially for other future pipeline projects.
Project success to date
So far, the first tram line crossing on Latrobe Street has been completed, with works conducted over two eight hour weekend shifts. The need to excavate and reinstate trenches for the construction of the water main at key locations has now been eliminated, and the impact on traffic and tram services has been reduced.