The Dudley–Charlestown wastewater system currently transports waste products from the Dudley–Charlestown catchment via the Kahibah 1 pump station, which pumps flow over the hills in the Glenrock State Recreation Area (SRA), and discharges into a gravity carrier that flows to Burwood Beach Treatment Works.

The construction of the new 500 mm diameter polyethylene (PE) sewer main will increase the system’s capacity and enable the removal of the Kahibah 1 pump station, reducing the chance of overflows after heavy rainstorms.

Horizontal directional drilling (HDD) was used to install the pipeline. The boring of the 510 metre tunnel was made difficult by the site constraints in the SRA, with very limited access to the exit pit and bore line, and a steep grade tolerance of two per cent with no zero or positive grade allowable. The varying geology – comprising conglomerate sandstone, siltstone, mudstone and a fractured 2.5 metre thick coal seam intersecting the bore path for 65 metres – contributed to the challenge.

Design and site preparation Hunter Water, UEA and Bowdens Group worked together to submit design options, a draft proposal and then a final detailed design confirming grade and pipe diameter suitability.

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Once the physical constraints of the bore had been decided, works began to secure approval from the New South Wales National Parks and Wildlife Service to allow for land clearing in preparation for the rig setup and excavation of the 4 x 5 metre deep exit pit.

Site preparation works began in early March 2009, with the clearing of bush land for UEA’s Vermeer D300x500 drill rig, DFE 700-gallons-per-minute cleaning system and other support equipment.

Detailed survey works of the bore line had been undertaken to enable accurate tracking using the Vector Magnetics steering tool. Steering was undertaken using a combination of beacon tracking and Paratrak 2 wire track systems through the dense bush land surrounding the bore line.

Boring through the bush land

The critical grade was maintained with the grade and depth of the pilot checked every two metres along the bore. Ground conditions and cuttings returned were monitored closely on the pilot bore and crosschecked against the supplied geotechnical information in order to gauge when the coal seam would be intersected.

The coal seam was successfully negotiated with no adverse effect on the grade, and the pit was completed in 14 days, on line and only 200 mm higher than programmed.

Due to the fractured coal seam and concerns over fluid loss, UEA decided to backream the bore straight to size. For the bore to be backreamed, a 600 metre PE return line was welded alongside the pedestrian access track, with drill fluid pumped back to the cleaning system for re-use.

The backreaming was completed using PDC reamers, which steadily progressed through the varying ground conditions, with pure black coal chips witnessed coming off of the shaker screens – a sight not usually seen in HDD.

Installing the pipes

With the bore hole completed and conditioned, pipe installation began. Due to the depth of – and limited access to – the exit pit, close proximity to Flaggy Creek, and being within a well – used national park, pipe installation needed to be undertaken in a weld and push operation, with

24 metre pipe lengths welded up and joined to the pipe in the bore hole. The pipe was internally de-beaded before being pushed in with an excavator. Welding progressed smoothly and the pipe entered the exit pit without incident eight days later.

Once installed, the pipe was pressure tested and inspected by CCTV from both ends to confirm there was no pooling in the pipeline, maintaining a constant downstream grade. The Dudley–Charlestown sewer carrier main was commissioned after the access chambers had been constructed, and is now successfully operational.