City West Water is a retail water company in Melbourne that is responsible for providing water, trade waste and sewerage services to the Melbourne CBD, Inner and Western suburbs. During regular maintenance inspections, it was revealed that one of City West Water’s 600 mm branch sewers, located in a busy industrial precinct in Tottenham, was in poor structural condition.
Inspections found that the sewer had several partial collapses and multiple longitudinal and circumferential fractures along the 1.1 km length. To complicate matters further, the sewer crossed the busy Westgate Freeway, between 8 to 22 m deep and the ground conditions were solid basalt.
Initial attempts to clear the sewer line using conventional cleaning and clearing technology were unsuccessful and had to be limited due to poor structural condition. City West Water was concerned that repeated efforts to clear the line would cause a full line collapse resulting in an upstream spill. Due to the depth of the sewer, digout repairs were not feasible and a suitable resolution was required. City West Water determined that rehabilitation of the sewer was the best solution.
Subsequently, the rehabilitation works were tendered and awarded to pipe cracking/rehabilitation contractor Harris Civil. Harris was engaged to undertake the sliplining/pipe cracking of 1.1 km of 600 Vitrified Clay pipe with 560 mm Polyethelene (PE) product and manage sewerage flows during the works.
Harris specialises in trenchless rehabilitation by adopting the pneumatic pipe bursting method. After establishing itself in 1968 as a civil contractor, Harris has been involved in bridge, road, gas, water, and sewer main construction. In 1994, Harris re-pioneered pipe bursting into Victoria and now owns and maintains one of the largest range of pipe bursting tools in Australia.
The sewer could have been repaired using several different options, however these posed significant challenges:
* The sewer is at a depth of approximately 16 m in solid basalt, making excavation tedious and expensive;
* The sewer is situated in heavily used areas (a container park) and excavation at these depths would greatly inconvenience the property owners;
* Silt, rubble and debris extended some 200 m upstream of the collapses, and clearing of the sewer using water jet cleaning could cause further damage;
* Using a mechanical robot to repair the pipe was high risk and could result in the expensive equipment being stuck in the sewer; and,
* The location of the collapses was at least 50 m downstream on a 150 m length of sewer between manholes, so hand tunnelling to the collapses was not feasible.
Planning and preparation was required to overcome the challenges presented by this project. Silted lines could not be cleaned. The host pipe maintained a depth range from 8 to 22 m in basalt, making it difficult to access the sewer when required and amplifying the risks related with the required works.
Sliplining was utilised, adopting the pipe bursting technique and using 560 mm PE, with the TTG Mini-Gigant and Bagela RW 10 CT winch from TT Asia Pacific.
Bypass of the line was required in order to isolate the main, enabling work and physical access to maintenance shafts and sewer. Also, as space was scarce, the 560 mm PE pipe had to be butt welded above ground into runs of approximately 150 m.
The entry shaft had to be dug at an appropriate position to avoid inconvenience to property owner and at a depth that facilitated mechanical excavation. This resulted in a shaft being 400 m from the upstream manhole and 700 m from the downstream manhole. The sewer depth at this point was 13.5 m.
A Bagela winch was set up at an upstream manhole first, installing the
400 m in one day, utilising the entry shaft, inserting 150 m at a time. It was essential to use the pneumatic pipe bursting tool to displace 100 to 150 mm of silt and rubble in the host pipe to facilitate new PE pipe. Later the process was repeated in the opposite direction, a distance of 700 m. This was more challenging as the burster was required to clear two partial collapses and extensive silt and rubble was prevalent for over 200 m in the host sewer. Furthermore, this section crossed under the Westgate Freeway (a distance of 80 m) and several large power towers. The installation was carried out over two days, with the TT Pneumatic Burster performing flawlessly.
Post-installation, CCTV inspection revealed new pipe to be on grade and level.
The project was deemed to be a great success, with all stakeholders and property owners greatly satisfied. This further supports the use of Trenchless Technology and the need for the pipe bursting technique when old and damaged sewers/drains (root infested, cracked, collapsed and displaced), cannot withstand high pressure water jet cleaning.
The rehabilitation of such mains can be achieved easily and efficiently using the pipe bursting technique. Although the rehabilitated pipe was smaller than the host pipe in this instance, pipe bursting also provides the opportunity of size for size replacement and upsizing increasing the capacity of the existing drain.