The project involved the installation of two sewage lines; a 300 metre siphon under the Maribyrnong River and 100 metres of gravity pipeline continuing on the downstream side of the Siphon. The new pipeline replaces an out-dated pumping station.
The first line undertaken was 100 metres of 280 mm poly on a two per cent grade; it was a straight forward drive, especially compared to the 300 metre line.
The industry maxim “÷that every individual horizontal drilling project has its own set of characteristics’ proved very true for the second section of the project, said AHD. After spudding in on the north side in an attempt to drill the pilot hole for the river component, the drilling crew members soon realised that the ground conditions consisted of an old tip site full of rubbish, bricks and even steel structures. This presented a challenging problem with the drilling fluid or rather the lack of fluid returns.
Initially AHD’s drillers were going to install casing but decided before doing so they would attempt to drill the pilot hole from the south side upstream end. The pilot hole was drilled with success from the south side, until the half way mark when the mud motor went under the river; the drilling returns were lost, never to be seen again. Thankfully, due to the formation geology there was no drilling fluid to the surface. This was closely monitored, as the mud motor operated on its intended path through a very difficult patch of river rock and cobble stone alluvial ground conditions.
After the mud motor punched out of the ground on target, drilling operations were switched back to the downstream side, ready for the reaming stage to begin. The 6 å_ inch pilot hole was then reamed out to 420 mm diameter with a medium formation tci three way hole opener. The bit was chosen because of the diverse ground conditions that this configured hole opener can handle. AHD Trenchless had an objective to run the hole opener from one to the other without a changeover, which can be a lengthy process without gaining any real progress.
The reaming process proved to be identical to the pilot hole stage, drilling fluid returns were lost approximately half way under the river. This resulted in the need to make several passes until the pipe was ready to be pulled in.
The pipe was pulled in from the south to the north, as this process progressed the hydraulic displacement of the pipe forced all of the drill cuttings into the formation. Combined with the bentonite, this acted almost like a grout formulae to seal the hole and allowed the drilling fluid returns to come back to the rig, enabling the crew to stop pulling pipe and instead pump drilling fluid in a stationary position to flush out any drilling cuttings. An estimated three cubic metres were removed.
AHD said that the pipe was then successfully pulled back to the rig with some crashing and banging as it was pulled through the river rocks and old rubbish dump.
AHD would like to thank City West Water and MFJ Constructions for their professionalism and dedicated project management over the duration of this project.
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