Accessing the water

An environmentally protected escarpment forms one wall of the dam. It presented an engineering challenge as a natural barrier to bringing communications and power services into the dam, and taking water away from the dam. As the escarpment is 50 – 60 metres in height above the level of the pumping station, it also adds difficulty to the task of pumping water from the dam.

Leighton Contractors realised that it was necessary to bore through the escarpment to connect these services, and engaged specialist drilling contractor UEA Pty Ltd to undertake this work as a subcontractor. UEA had recently purchased a Vermeer Navigator® D300x500 horizontal directional drill – the first such unit in Australia.

The scope of work encompassed four straight bores, each of approximately 330 metres, to link with services laid up to the eastern side of the escarpment. The bores were to house a 110 mm diameter communications conduit, two 140 mm diameter electrical conduits and a 900 mm water main, with all conduits to be polyethylene.

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Because of environmental constraints, a geotechnical investigation could only be conducted in the valley areas. The sandstone outcrop surrounding the Shannon Creek area was likened to Hawkesbury sandstone found around the Sydney basin, and its strength was estimated at 40 – 60 MPa (5,800 – 8,700 psi).

Harder than expected

The project seemed ideal for the Vermeer Navigator D300x500 rig, as its rail could be positioned horizontally against the vertical face of the escarpment, facilitating the six per cent upward grade required for the bore. A Digitrak Eclipse Steering Tool would be used to guide the pilot bores through the escarpment to the exit point.

While drilling the pilot bores and installing the small diameter conduits, UEA realised that the ground conditions were much harder than originally thought. UEA decided that the reamer it planned to use for the large diameter bore would be inadequate, and that hole openers would be required. The ground hardness was estimated at up to 120 MPa (17,400 psi) in places, requiring specification of medium formation hole openers. A 1,066 mm diameter hole opener was required for the 900 mm water main.

Tough work made easy

Despite the hardness of the rock UEA director Keith Whittaker said, “The Vermeer handled the 42-inch opener very easily, and it was unusual for it to be off idle and under load for the entire project.”

Because the hole was installed on an upward grade, it was self-cleaning. This eliminated the need for complex drilling muds, and the only mud used on the job was for the pilot bores.

Large volumes of recycled water were used to flush the hole of cuttings at the low exit point, and keep the cutters submerged in fluid. The water flowing through the rods was supplemented with recycled water fed through the already installed electrical conduit, to provide sufficient volume.

Although the water pipe had a 90 tonne dead weight, it was easily installed and pull back force did not exceed 10 tonnes during the operation.

The Vermeer drilling rig handled the unexpectedly severe conditions of the site, and allowed UEA to provide the clients with a solution that overcame these challenges.