Regional Victorian water corporation Gippsland Water services more than 61,000 domestic customers as well as major facilities in the power, dairy and paper industries. The utility maintains more than 2000km of water and 1700km of sewer pipelines, but has been consistently faced with the challenge of maintaining reliable supply to scattered towns and centres.

The microtunnelling boring machine in place in the 7.5m deep launch pit.

Gippsland Water has committed to establishing a water grid in its growth areas. A water grid overcomes the historical problems of limited stand alone water systems serving individual towns by providing multiple options to supply each community, ensuring reliability of high-quality water for residents and industry. This professional approach to water resource planning has required a commitment to long-term infrastructure investment.

A major project under this scheme is the construction of the Moe to Warragul interconnect pipeline. The total project is to interconnect the existing supply under the Tarago water supply system with the Moe system. This involves a new 375mm diameter pipeline over the 14.5km from Yarragon to Warragul. New booster pump stations are being installed at Darnum and Trafalgar West.

The first stage of the project, from Yarragon to Darnum, was completed in 2012 at a cost of $5 million. The critical second stage, covering the 8.3km from Darnum to Warragul, is currently being undertaken by Gippsland Water’s principal contractor, Jaydo, and is nearing completion. Capacity will be up to 8ML daily and the cost of stage two is $10 million.

Warragul and nearby Drouin are rapidly growing communities approximately 100km east of Melbourne. The local Tarago system was not designed for the current population growth and places the communities at risk of seasonal water restrictions. An interconnection between Moe and Tarago will allow water transfer between the systems, improving the reliability of supply for both major regional centres as well as several smaller communities between the towns.

Using trenchless technologies to reduce subsidence risks

Construction of the second stage of the pipeline project required crossing a major dual-carriage freeway, the main Gippsland rail line and three rivers. Open trench construction was not possible for these crossings. Investigations by the Gippsland Water assets team led to the decision to use microtunnelling to go under the railway and freeway, and horizontal directional drilling (HDD) to go under the rivers.

“A major concern was the possibility of subsidence under the railway,” said Wayne Ward, Senior Engineer, Project Delivery at Gippsland Water.

“We needed a system that meant that there was always a fully supported bore as we went under the line. The railway was adjacent to the freeway, so it made sense to microtunnel under both at the same time.

“Microtunnelling, boring and placing a sleeve pipe in the same action, meaning that the surrounding earth is always fully supported. There is no requirement for compaction and no risk of subsidence.

“But a different solution was required for the river crossings. Unlike the flat line required to go under the railway and freeway, the river crossings were in comparatively steep gullies.  We needed a flexible system that could go down and up these slopes.

“It also provided the least disturbance to sensitive environmental and cultural areas.”

As well as the rivers, the route crossed areas of Aboriginal cultural heritage and habitat for native flora and fauna found only in Gippsland, including the Giant Gippsland Earthworm and Warragul Burrowing Crayfish.

“HDD provided the easiest and most appropriate solution. The flexibility this allowed in boring made it ideal for the hilly country of west Gippsland,” Mr Ward said.

HDD involves steering a small diameter drill head to bore horizontally underground. Depth and direction can be easily controlled. Once this process is complete, a large diameter ream is pulled back through the borehole, enlarging it to the required pipe diameter. A flexible water pipe is then pulled into place through the length of the borehole. High-density polyethylene (HDPE) pipe was installed as the product pipe in this case.

A break-away swivel connector between the reaming head and the pipe ensures that if the pulling force is greater than the maximum allowed for the new pipe, the break-away swivel will disconnect before damaging the pipe.

Mr Ward said HDD was very successful for placing the pipe under the rivers.

“In total, four HDD drives were required for the crossings. These ranged in length from 100m to 180m. Importantly, all work was done from the surface.

“In comparison, the microtunnelling required shafts 7.5m deep to be constructed before the tunnelling could commence.

“You need to choose the best method for each situation, but HDD has a lot of benefits if the option is there.”

Stage two construction is nearing completion with testing and commissioning to continue throughout the year.

“We are on track to have the full Yarragon to Warragul interconnector operational in 2019, ensuring local communities will have more surety of a reliable high-quality water supply,” Mr Ward said. 

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