The longest HDD project undertaken by the company to date was 700 m in Burdikens gap, northern New South Wales. More recently, Blaxland Pacific undertook a large diameter project in Craigieburn, crossing Aitkens Creek. With ground conditions featuring bluestone with fractures, the project consisted of two 160 m pipes spaced only 2 metres apart due to easement constraints.
The first hole was 350 mm diameter with four 63 mm conduits pulled in for communications, and the second hole was 700 mm diameter with nine 110 mm conduits pulled in for electrical. The depth of the project was 14 metres.
Obstacles faced included a 750 mm trunk sewer running perpendicular to the drill path, overhead powerlines along the drill path which created interference with the Digitrak system, and 100 Mpa of bluestone rock with fractures all through the formation.
The key to this difficult project was to successfully drill under the 750 mm trunk sewer and clear it without damage, but given its depth and close distance from the starting point this was a tricky event. This meant that planning of the project would need to be second to none. Blaxland used their Atlas Bore Planner software to create the optimum curvature for both pilot holes to enter, clear the sewer by the required minimum two metres and punch out in the correct position. To complete these pilot holes a mudmotor was used. With the 45 per cent entry and exit angles required, the adjustable sub on the front of the mudmotor was at a sharp 2.35 degrees to achieve the tight bend radius of the designed drill path.
Article continues below…The reaming upsizing took several weeks to complete and was a tough task. Fifty thousand pounds of pullback weight was used on the rock hole openers to penetrate through the bluestone. Because of the great depths a very thick mud was needed to carry the bluestone cuttings out of the hole. Several passes were made to clean all of the rock cuttings from the holes before the pipes could be pulled in. Fifty thousand pounds of pulling force was used during the reaming process to penetrate through the tough bluestone, and being a fractured formation this was an extremely rough, bumpy process.
On the final 700 mm reaming pass a small rock wedged itself against the stabiliser and prevented the hole-opener from penetrating any further. Fortunately this occurred 5 metres from the finish but because of the steep entry angle of the drill path a large excavator was needed to excavate down to the depth of the bore to clear the small nuisance rock. Once this was cleared, the final few metres could be backreamed to complete the job.
