Revitalising infrastructure with HDD

The new pipeline is approximately 3km in length and will be installed between the Margaret Street Sewage Pump Station and the Ti Tree Bend Sewage Treatment Plant in Invermay, Tasmania. Tamar Estuary River Health Action Plan (TERHAP) combined system improvements project director Andrew Truscott told Trenchless Australasia that the additional pipeline and pumping capacity means that when it rains, it will overflow less from TasWater’s network, and take it all the way through to the treatment plant where it’s treated. 

“The most challenging bit with this pipeline is given how big it is, and how built up the area between the start of the pipeline and the end of the pipeline is, it was really difficult to find a good route,” he said. “We looked at a number of different options, but we decided to go relatively directly but under the Tamar Estuary twice.”

These two sections require tunnelling under the river at a depth of 40m in order for TasWater to find a good route from the pump station and the treatment plant. To do this, the water authority looked to horizontal directional drilling (HDD), as it minimises disruption – a key benefit of trenchless technology. “We were looking at open trenching through busy urban streets, but we found that we were going to have pronounced impact on the community in terms of traffic management and the residential activities within those streets that we go down, in terms of providing access,” Truscott said.

“We also had concerns about ground stability because it is a low-lying area that we’re going through. “Once you start adding all those things in, the cost of trenching the pipeline started to get beyond what our budget allowed for.” Truscott said that in comparison to traditional trenching methods, HDD provided a cost-effective solution. “Once you start getting really deep to avoid other services and those other issues, HDD was our best option, both in terms of community impact, but also cost effectiveness,” he said. The new pipeline project is part of TERHAP, which is delivering new sewerage infrastructure across the city of Launceston, which includes a combined sewerage and stormwater system.

“What that means is, during heavy rainfall periods, the system has a limited capacity. It was designed to overflow during the major rain events so, we have overflows of sewage and stormwater from the network,” Truscott said. A collaborative project between the Australian and Tasmanian Governments, the City of Launceston and TasWater, TERHAP is part of a $129.2 million investment to implement the key actions in the plan. “This project is looking at reducing how often we have those overflows, and reducing how much we overflow when we do,” he said.

“The main way we’re looking to do that is by improving our transfer capacity through upgrading pump stations and building new rising mains, and also including some storage within the network.” So far, the team has completed a section of pipeline, with other aspects ongoing in the project. “We’re in the middle of upgrading an old pump station to increase the pumping capacity and we’ve got this pipeline happening, which is really being done in three parts ¬– the two river crossings and a trench section, which connects those two river crossings,” Truscott said.

“We’re also building a 10 megalitre storage at our water treatment plant. “TasWater is about 30 per cent of the way through the overall project and about 50 per cent complete on the pipeline.” The utility company is targeting project completion by late 2025.

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