Drill Shifts Achieve Paste Fill Hole Depth, Decreases by Average of 12.25%

In order to maximize mining underground, it is important to stabilize mined stopes with backfill. This allows for mining to continue safely, because it prevents stope failure from a lack of structural support.

Northern Star Resources Limited awarded Boart Longyear Drilling Services the directional drilling job to develop a paste fill hole at Paulsens gold mine, located in northern Western Australia.

The paste fill hole acts as a delivery system and allows for a paste-like mixture of cement and mine tailings to backfill the underground zones where mining and excavations occurred. This provides additional ground support and pillar recovery, which optimizes further mining of adjacent stopes.

The paste fill hole needed to be drilled to a 310-millimeter (12.25 in) diameter to allow it to be internally lined down its entire length with a 203.2-millimeter (8 in) diameter casing. Because of the unusual requirement for a larger hole, a regular mineral exploration drill rig could not be used.

The traditional method of drilling this type of hole would be to first drill a pilot hole with diamond drills, then enlarge the hole using rotary drilling methods on a second — and potentially even a third — pass. This method could take as many as 45 to 50 shifts to complete the drill.

An additional challenge for the driller was making sure the hole would intersect within the 5-meter (16.4 ft) diameter target in the underground development zone. This created the need for concise and accurate surveying over the duration of the directional drilling. Meaning Boart Longyear would have to work with a third-party specialist, Surtron Technologies, to deliver the paste fill hole on target.

Because of the large hole requirements, Boart Longyear elected to use a water-well-style drill rig with the ability to accommodate large-diameter casing and a deeper drilling capability via a powerful 130,000-pound pullback.

An additional booster compressor was also provided as the entire hole was drilled using conventional DTH hammers. The booster compressor was essential to keep the DTH hammer efficient and to lift and clear the cuttings from the hole.

The Boart Longyear crew set up the rig and was operational within one week after arriving on the jobsite. As drilling commenced, the hole was collared and cased with a 470-millimeter (18½ in) diameter drill bit for the top 6 meters (19.6 ft) before reducing to a 381-millimeter (15 in) diameter. This was progressed to a depth of 56 meters (183.7 ft) before the hole was once again cased. The bottom hole assembly was then reduced to the final diameter of 310 millimeters (12.25 in), and drilling recommenced.

As the hole progressed, multi-shot surveys were regularly undertaken by Surtron Technologies and Boart Longyear adjusted drilling parameters as needed to ensure the hole trajectory remained on target.

“It is great that Northern Star Resources Limited and Boart Longyear can come together on a well-planned and executed program to an outcome using a technique that is unique, but also safe and accurate,” says Bernie Sostak, general manager of business development and technical services at Northern Star Resources Limited. “Both teams are to be commended on a high quality technical success story.”

Actual drilling operations required a total of 39 12-hour shifts to reach the target depth of 656 meters (2,152 ft). Most importantly, the hole successfully intersected its required target at only 2 meters (6.56 ft) from center. The average drilling was 22.6 meters (74.14 ft) per shift, and the single best shift produced 90 meters (295.28 ft) between the depths of 428 and 518 meters (1,404.2 to 1,699.5 ft).

Regular rod pulls were conducted for drill bit inspection/replacement, as well as surveying of the hole. As ground formations became harder, Boart Longyear pulled the rods to switch out drill bits and DTH hammers to provide an optimized combination for the drilling conditions.

“This was one of the most successful surface paste fill delivery holes I have been associated with, and considering it was drilled in one pass, this saved not only time, but, as we all know, money,” added Sostak. “The hole was drilled 656 meters (2,152 ft) from surface and intersected within 2.1 meters (6.9 ft) of target, which was very impressive.”

With the use of a water-well-style rig, Boart Longyear was able to achieve a one-pass shot in 39 shifts, helping the owner to realize approximately $5 million in future gold value per mining level for this particular pillar extraction.