Melbourne Water’s M40 watermain, which was laid in the 1920s, is one of the main water supply mains for the inner suburbs and CBD of Melbourne. The section of main being renewed runs alongside the tram lines in St. Georges Road, between Arthurton and Beavers Roads in Northcote, approximately 7 km from Melbourne’s CBD.
The M40 had been experiencing perforation leakage and was taken off-line during the low usage demand period of winter. Kembla was contracted to rehabilitate 220 m of the M40 watermain using a sliplining renewal method.
Environmental concerns and scope of work
The project was subject to a number of significant constraints, including:
- Tight access between tram tracks, council vegetation and a bike path;
- The works could not impact the vegetation along the bike path;
- The works could not impact tram traffic;
- The works could not affect bike traffic;
- Road traffic could not be interrupted during peak periods.
One of the major objectives of the project was to return the main to service prior to the higher summer demand period. The existing main, or host pipe, was approximately 1,350 mm in diameter, consisting of a mild steel locking bar main that had suffered several leaks in recent years.
Kembla decided to rehabilitate the existing main with a 1,100 mm glass reinforced plastic (GRP) pressure main, or sliplining pipe. The new main was inserted into the host pipe using a modified sliplining method, which required two pits to provide access to the host pipe and allow for the new main to be installed.
Pipes were transported to site from an offsite storage area approximately 5 km away to eliminate the need for them to be unloaded in close proximity to the power lines supplying power to the trams and to avoid affecting traffic on St Georges Road.
Project Team
The project was completed by an integrated team made up of the principal Melbourne Water, contractor Kembla Watertech, designer Aurecon, material supplier Fibrelogic and specialist sub-contractor Metro Pipe Services.
Design Considerations
The design of the rehabilitation solution was required to achieve a PN16 pressure rating, minimise the reduction in internal diameter, avoid the potential for future leakage and comply with location constraints.
The adopted design involved using a PN16, DN1,100 mm GRP pipe with external slip couplings finishing with flanged ends supplied by Fibrelogic in South Australia.
Excavation of launch and retrieve shafts
Prior to excavation of the upstream and downstream shafts, the entire site was fenced off from public access, advisory signs notifying road users, trams and cyclists were installed and notification letters sent to affected residents.
Additionally, the immediate area around each excavation was fenced with 1.8 m high temporary fences.
At the downstream end, the excavation was directly opposite the tram stop and in close proximity to stopping trams. The depth of the main was between 500-700 mm below the surface.
At the upstream end, the main depth was approximately 2 m from the surface to the top of the pipe, resulting in a large deep excavation. The edge of this excavation was within 1 m of the tram tracks requiring careful excavation and ground support.
Cutting and capping of host pipe
Once excavations at each end were complete and made safe, the main was ready to be cut and capped.
Sliplining
Sliplining can take many forms – the new pipe can either be pushed into place or pulled into place. Traditionally, it is common to pull a welded pipe string into place.
In this case a welded pipe string could not be used due to limitations in space and materials. A GRP pipe in 6 m lengths was used and each pipe section was joined together in the shaft using mechanical slip couplings prior to being pulled into place.
The sliplining technique was innovative in that it involved pulling the pipe string from the end of the string rather than the front, which is normal practice. This method was used to ensure each pipe joint remained in compression during the installation to eliminate the possibility of damaging or dislodging the subsequent joints of the pipe string.
Due to the space limitations it was not possible to push the pipe string into the host pipe from the upstream end, similar to a pipe jacking operation.
Equipment
The equipment used to pull the pipe string was the Grundoburst 800G pipe bursting rig and rods. With up to 80 tonnes of pulling force available, this compact equipment was more than capable of pulling 220 m of 1,100 mm diameter pipe.
To eliminate the possibility of damaging the slip couplings by dragging through the pipe, it was necessary to support the pipe on specially made steel bands with rollers.
The rollers served a number of purposes: firstly they suspended the pipe joints and coupling above the invert of the host pipe, secondly they reduced the pulling force required and thirdly they centered the pipe in the host pipe to ensure no contact between the new pipe and host pipe occurred during the installation.
Each pipe was then carefully lowered into the launch shaft at the upstream end, joined to the previous pipe and pulled into position. This was then repeated until the full length was sliplined.
Installation of tapers
Tapers were fabricated for each end using measurements taken when the host pipe was initially cut. As per the design, each taper was welded to the host pipe and bolted to the GRP pipe via a flanged connection.
The flanged connections were then wrapped to protect all exposed metal surfaces.
Grouting of the annulus
The annulus was grouted using a flowable low-strength cement based grout. This was undertaken from a number of locations along the alignment.
Each end was backfilled with cement stabilised sand to completely encase the connections and provide support to the shaft edge closest to the tram line.
Commissioning and reinstatement
At the completion of backfill, the site was reinstated and fully top soiled and reseeded. Commissioning of the watermain was undertaken by Melbourne Water’s operations team and closely monitored for any leaks. At the completion of the commissioning process, the main was returned to service prior to the higher summer demand period. It has continued to perform without issue throughout the summer and will continue to supply Melbourne Water’s customers for many years to come.
Melbourne Water Senior Project Manager Bill Welsford said that a trenchless method was chosen due to restrictions on room on-site and a tight timeframe.
“The repair was only over a short length (200 m) and is considered temporary so cost was also a major consideration,” said Mr Welsford.
“Kembla were consulted during the project development, and due to time constraints and Kembla’s demonstrated expertise, Kembla were appointed to carry out the work – successfully completed before the summer peak demand.”