Groundbreaking trenchless solutions overcame geological challenges to protect a precious Tasmanian waterway.
As one of the largest projects of its kind in the Southern Hemisphere, TasWater’s New Sewer Pipeline Project was designed to significantly reduce the frequency and volume of overflow events into the Kanamaluka / Tamar Estuary.
Pipeline Drillers Group (PDG) executed the horizontal directional drilling (HDD) works, achieving a result that stands as a significant accomplishment in the trenchless industry.
PDG installed two 900mm high-density polyethylene sewer pipelines under the river, spanning 660m and 1100m at depths reaching up to 45m. However, the team was met with a variety of difficult geological conditions, including large cobbles and stiff, sticky clay. These troublesome ground conditions required creative engineering solutions to ensure the successful completion of both bores.
To overcome these challenges, PDG used steep, non‑standard entry and exit angles – up to 18 degrees – to avoid adverse soil conditions, such as boulder-rich layers. The bore paths also incorporated compounded vertical and horizontal bend radii to avoid deep channels with poor underlying ground conditions.
The sticky clay presented its own set of problems. Traditional drilling fluid struggled to break down the material, so the team trialled different mud formulations to improve efficiency. The tooling design was also adjusted to better handle the clay and keep annular pressure below critical levels, avoiding potential issues with bore stability.
Breaking new ground
One of the project’s standout features was the use of the large-diameter 1400mm casing at both ends of the bores to overcome the adverse ground conditions. The team installed four lengths of casing via a pneumatic pipe ramming hammer up to 116m. The casings facilitated the HDD process by stabilising the extremely challenging upper ground conditions, ensuring the integrity of the borehole for pipe installation. In addition, custom-made augers were deployed to clean out the casing and remove boulders up to 450mm in diameter. To overcome the challenges posed by the length and size of the installation, PDG employed a unique dual rig intersect methodology, which eliminates the need for traditional drill fluid return lines. This approach involved drilling two pilot holes from opposite ends of the bore and having them meet at a predetermined point under the river. Given the complexity of this technique, particularly with the river crossing, a Gyro steering tool was essential in achieving the necessary accuracy. Despite the logistical challenges of mobilising the equipment and tooling from Brisbane to Tasmania, PDG’s meticulous planning ensured the project stayed on schedule.
Additionally, noise barrier walls were installed to minimise disruptions to the local community, while weather-related delays were mitigated by high-quality access pads built at the outset.
Rising to the challenge
The New Sewer Pipeline Project was completed on time and with no environmental or safety incidents. The installation not only improved local infrastructure but also contributed to the long-term health of Kanamaluka / Tamar Estuary.
PDG’s innovative use of HDD allowed for minimal disruption to the community while providing a sustainable solution to a pressing environmental issue – a prime example of how trenchless technology can provide effective, low-impact solutions to complex infrastructure challenges.
For more information, visit pdgroup.com.au
