With an installation rate capability of over 500 metres per day and a total installation cost 24 per cent lower than conventional construction, it’s no surprise SA Water was impressed by the Swiftpipe technology used by MPI Pipe & Cable.

SA Water provides water and wastewater services to over 1.4 million South Australians. SA Water’s asset portfolio includes approximately 25,500 km of water and 8,400 km of sewer mains across the metropolitan and rural regions of South Australia.

In order to actively manage these extensive water and wastewater mains, SA Water’s Asset Management Group has renewals programs for each of these pipe systems to allow for a prioritised and sustainable rehabilitation program. The programs of water and wastewater main renewals forms part of SA Water’s Asset Management Plan (AMP).

The network renewal strategy includes a long-term (25 year) planning expenditure envelope, based on assumed asset lives. It is referred to as the ‘Nessie’ model and is applied to all of SA Water’s renewal programs. The ‘Nessie’ model is reviewed periodically to update the profile based on the age profile of SA Water’s infrastructure and any changes to assumed asset lives that are made in that time. In addition, for the water main network, predictive failure analysis has been performed by the CSIRO to investigate the impact on burst rates under various expenditure envelopes. It can therefore be said that the water main network renewal program is very well defined.

Water main renewals

Using a combination of the ‘Nessie’ model and CSIRO expenditure envelopes as a basis for forward planning, a prioritisation model developed in-house allows the planned expenditure to achieve ‘bang for buck’ in terms of reducing burst rates (the number of bursts/km/year) and improving customer service standards (the customers affected by more than three unplanned interruptions per year). The integration between the Geographical Information System (GIS) and Works Management System (MAXIMO) allows for those mains failing most frequently to be identified. The pre-determined expenditure envelope is then used to identify which main replacements will achieve the greatest reduction in burst rates and customer interruptions within the planned expenditure envelope.

To determine the specific water main relays to be targeted within the AMP, a unit rate value between SA Water’s asset valuation unit rates (lower limit) and historical tender costs (upper limit) have typically been applied. However, both of these have been based on conventional open trenching – until now.

Trenchless works in the SA network

SA Water (formerly the Engineering & Water Supply Department) originally trialled Trenchless Technology during the early 1990s, trialling both pipe bursting and slip lining. The outcomes of these trials indicated that the benefits associated with economics and logistics at the time were not significant enough to result in a shift away from conventional open trench installations.

In 2005, SA Water undertook a trial laying a new water main located north of Maitland on Yorke Peninsula to improve the operational flexibility of the water supply network. The reasons for conducting the trial were to assess the effectiveness of the plough-in (Swiftpipe) technique and determine the costs associated with its use for possible application in other parts of the SA Water network. Part of the new main involved the installation of 2.7 km of 125 mm PN16 HDPE pipe. The supply and lay cost of the ploughed-in length of the new pipeline was up to 30 per cent lower than PVC laid conventionally.

In 2006, SA Water combined five water main relays in SA Water’s outer metro north operational region into a single contract. MPI Pipe & Cable were awarded the tendered contract to complete these relays applying Swiftpipe technology as used in the Maitland trial. A summary of the costs and details of the five relays undertaken in the 06/07 financial year employing the Swiftpipe technology is shown in Table 1.

The Swiftpipe technology resulted in significant cost savings, and in turn, a lower unit rate than initially estimated. The technique therefore has the potential to increase the length of mains replaced each financial year. The result will be an even higher reduction in burst rates and improvement in customer service standards beyond that which would otherwise be achieved using conventional open trenching methods within the pre-determined expenditure envelope.

The Swiftpipe technology

The Swiftpipe technology utilises a modified vibratory plough to ‘plough’ new polyethylene pipe into the ground. A vibratory head is attached to a dozer that pulls the new polyethylene pipe into the ground. Unlike pipe bursting, which requires the host pipe to be taken offline while the new pipe is installed, the Swiftpipe technology is offset from the existing pipeline, meaning that customer disruption is minimised.

Advantages of the Swiftpipe technology are as follows:

* Avoids the need for exportation of trench material and importation of embedment material; * The adjustable head on the MPI Pipe & Cable machinery allows a range of depths and offsets to be achieved; * A traceable warning tape can be attached to the vibratory head to allow for an exact alignment to be detected at a later date and avoid costly damages that could otherwise occur by subsequent excavations along the alignment of the new main; * Very few joints are required, increasing the rate of installation and reducing cost; * The PE pipe installed can be provided on rolls between 250 and 500 m in length; * Minimal surface disruption (~35 mm wide strip), making it ideal for water mains aligned along road verges and easements, and allowing the alignment to remain clear of native vegetation; * Installation rates in excess of 500 m per day including site establishment, valve and hydrant installation, road reinstatement (where required), testing, commissioning and cleanup; * Minimises disruption to customers with the only downtime being the direction to transfer the water services from the old water main to the new water main; and, * Very economic water main installation technique.

Other service utilities along the alignment of any new relay are located and carefully excavated locally before the relay commences. Once the relay approaches the service crossing, an extension chain is used to pull the new PE main above or below the existing services after which the standard ploughing method is continued.

The initial concerns over possible ‘scoring’ of the pipe wall as it is pulled through were cast aside during the Maitland trial. The vibratory head is sized slightly larger than the new pipe to be installed and therefore temporarily compacts and displaces the surrounding soil in a similar manner to pipe bursting. This reduced the shear forces along the length of pipeline being ploughed into the ground allowing the rolls of PE pipe to be easily pulled into the ground.

To date relays up to 125 mm diameter have been achieved, and SA Water is currently in discussions with MPI to determine whether 200 mm relays will be achievable as well as determining the benefits and possibilities for installations within bituminised areas to minimise surface disruption.

The economic benefits of the Swiftpipe technology are clear. In addition, SA Water has every expectation that the PE pipe installed using the Swiftpipe method will perform just as well as conventionally laid PE. SA Water will continue to consider the application of Trenchless Technology techniques where economic, social and environmental factors are favourable in comparison to conventional open trenching.