From the magazine, HDD

HDD helps upgrade inner city underground power network

The company, Dunmain Pty Ltd, has been using Infrastructure Constructions (IC) for its HDD projects for the past five years. Dunmain are a well respected civil construction company specialising in underground power and have been contracting to Energy Australia for nearly ten years.

The project was required as an upgrade for the Annandale area in Sydney’s inner west. All up approximately 2,070 m of construction was required, with 70 m of that to be bored and the rest to be open trench. Once the new construction works are complete, Energy Australia will pull through new polyethylene insulated copper 400 XLPE cable to upgrade their network.

The project involved installing the four conduits across the intersection of Rose and Johnstone Streets Annandale. This part of the project was designated HDD due to the depth that would be required if it was trenched, as one of the side streets is 6 m lower than the main road. The roads are connected via staircase as there is no road access.

This was not your normal road crossing due to the bore path having to ascend up the 6 m escarpment. Another notable factor in this project was the ground conditions. This area was mostly sandstone so even though it was a relatively short bore, IC still had to make sure they could get enough steer through the rods to ensure the exit pit was reached at the other side.

For this project IC decided to utilise the capabilities of their new Ditch Witch All Terrain 2720 horizontal directional drill. This machine is completely different to any HDD machine IC has had before. The main difference is that the machine runs an inner and outer rod when fitted with its Rock Master drill head. The inners are connected to each other via hex fittings which in turn provide a mechanical drive to the cutting bit during drilling. The outer rods are used to thrust the drill head forward while also giving it steer. When back reaming only the outer rods are being used. At the end of the drill head is a 5 inch tri-cone bit which allows good penetration in rock. The drill head works very similar to that of a mud motor except that the bit is rotated via mechanical force, not water volume. The Rock Master only required between 7 to 15 gpm to run whereas a mud motor can be 80 gpm plus.

It was hoped that this new style Rock Master drill head would have the ability to drill through the rock and at the same time achieve enough steer on the rods to reach the exit point. The machine had been run at a site in Kellyville the previous week but had encountered nothing as tough as on this project. To complete the project IC’s mud recycler was placed on site along with a vacuum tanker to aid with the water requirements of the drill head – which were not really needed – and also during the back reaming stage. Before the drill head was placed in the ground the surface topography and bore path were electronically planned using the Vermeer Atlas Bore Planner. This allowed a very accurate insight into whether the AT2720 could complete the bend radius of the drill rods required for this project. Once this was checked off by the operator then the bore could begin.

While the first rod was being drilled in, hard rock was reached. This was no surprise. What was noted was that the drill head was penetrating at an excellent rate, which worked out to be approximately one rod per 3 minutes. With the Subsite 750 tracking the head, a depth of 3.5 m was reached by the time the drill head had reached the wall. The drill head was then turned to 12 o’clock and carefully steered towards the exit pit. Within a single day the pilot hole was drilled and exited directly in the exit pit.

The drill head was replaced with an 180 mm diameter back reamer. This was slowly rotated and pulled back up the hole. It was noticed that at the exit point where the reamer had been pulled through, the ground was partly broken and had caused a few large boulders to break off and lay in the bottom of the hole. This reamer was followed by an 400 mm diameter reamer. Unfortunately due to the ground conditions and a few broken boulders, three 400 mm diameter reamers were required to complete this project. Once the hole was completed, the reamer was pushed back up the hole and the four conduits were connected via a swivel and then slowly rotated and pulled back down the hole.

In summary, the project was a great success as was the new AT2720. IC purchased this machine to complete specialised rock bores and to date the company believes this will be the ideal machine. There are many more rock jobs on the drawing board for this machine, tougher and longer than that completed already. Time will tell how successful the machine proves to be.

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