Karori is a western suburb of the urban area of Wellington New Zealand some 4 km from the city centre. In a project for the renewal of wastewater pipes, known as Karori Fast Track Waste Water Renewals, the client Wellington Water, in association with its design consultant GHD required the replacement of some 650 m of mainline and 336 m of lateral pipes due to the ageing infrastructure that had resulted in inflow and infiltration. The existing pipe network was constructed in the 1920s.
Having completed a desk-top study of the area and ground conditions, it was decided that the most effective replacement option was primarily trenchless pipe bursting. A small amount of open cut works was also used where absolutely necessary due to site conditions. Whole catchment areas were analysed based on records and renewals were prioritised. The final selection of the pipes for replacement was left to construction team.
The contract for delivery of the project was awarded to G P Friel Ltd. as main contractor which was to undertake all works including the trenchless portion. The trenchless pipe bursting option selected for the main line pipes was dynamic pipe bursting using pneumatically-powered impact hammers as the bursting equipment. The laterals were burst using a hydraulically-powered static system.
WHY PIPE BURST
There were several reasons for the choice of pipe bursting including:
- Online renewals preserved the space in the corridor and reduced the risks associated with unknown clashes (in other words a proven corridor)
- There were no capacity issues so size for size replacement was sufficient, which well suits a pipe bursting solution.
- Reduced risks and issues associated with significant amounts of open trenching for example:
- > Disruption to users of the corridor/route (such as vehicles and pedestrians)
- > Reduced excavation safety risks
- > Reduced material exports and imports are required to and from the site – which offers the additional advantage of a lower carbon footprint due to reduced vehicle movements
- A dynamic system was preferred for the mains because:
- > The existing pipes were earthenware and could be shattered easily.
- > The dynamic system keeps the loads in the winch low and eliminates the requirement for significant temporary works.
A static system was preferred for the laterals because:
- > Pulling in short lengths meant that the load on the equipment was low
- > The tooling for such a system has a smaller footprint which meant it was more efficient when dealing with short length bursts as there were no long launch pits.
GP Friel utilised its stock of Tracto-manufactured equipment to provide the bursting equipment. The mains pipes were burst and replaced using the Grundocrack Olympus 180G pneumatically-powered pipe bursting tool in combination with a 367 cfm compressor alongside a Grundowinch RW 5000 which, being hydraulically-powered provided a constant tension on the winch cable.
The laterals pipes were burst using a Grundotugger, also from Tracto, which was powered using a PTO from the hydraulics of a mini-excavator.
PROJECT SCOPE
In total the mains and laterals across three streets in the Karori area were to be replaced on this project.
The 650 m of main burst was completed in 10 sections with the existing 150 mm diameter pipe being replaced with 160 mm o.d. PE100 SDR17 pipe. There were also thirteen (13) associated chamber replacements.
The 336 m of laterals being replaced were associated with forty (40) individual properties. The existing 100 mm diameter pipe was replaced with 110 mm o.d. PE100 SDR17 pipe.
The PE100 SDR17 pipe was supplied to GP Friel by Hynds and manufactured by Waters and Farr. The new PE pipe was supplied in straight 12 m long sticks with jointing being completed on site. Jointing was primarily completed utilising butt fusion, using a Georg Fischer CNC400 Butt Welder. Connections and saddles comprised Georg Fischer EF fittings which were welded as necessary on site using a Georg Fischer
MSA 2.1 welder.
