Case Study: More Than a 28% Increase Beyond Goal Depth

Waste rock dumps are often placed adjacent to mine pit areas, and as mining expands more room is needed for the waste rock. Certain physical and chemical characteristics of the waste rock dumps are often needed to better understand the geotechnical and geochemical behavior of the large dumps and to evaluate potential mineral value. Sonic drilling enables the collection of highly representative samples and excellent sample recovery. This method of drilling has been used for the re-exploration of dumps, tailings and heap leach pads.

Waste rock dumps consist of mainly unconsolidated material – and it’s often difficult to know how deep they are or what types of material they contain – which can lead to challenges in drilling. Sonic technology provides a solution by being able to produce 100 percent accurate in-situ core samples through varied ground conditions.

Boart Longyear Drilling Services took on the challenge of drilling core samples from the unconsolidated waste rock dump at Kennecott Utah Copper’s Bingham Canyon Mine. The intent of the drilling was to define the contents of the waste rock dump. The waste rock dump consisted of rock blast material ranging in size from 254 to 304.8 millimeters (10 to 12 in) in diameter and was made up mainly of porphyry deposits (granite-like rocks).

With a targeted depth of 213.36 meters (700 ft), the main goal was to provide a detailed continuous sample of the waste rock dump material and confirm bedrock depth. Boart Longyear also needed to install piezometers (water level monitors) and lysimeters (moisture content monitors). Geotechnical samples would also need to be taken every 6.096 meters (20 ft) to confirm stability and moisture content for the first 60.96 meters (200 ft).

Sonic drilling is the perfect method for drilling in unconsolidated material, such as the waste rock dump at the Bingham Canyon Mine, because of its sample recovery rate, straight cased borehole and the flexibility to offer geotechnical sampling via the split spoon sampler.

As drilling commenced, Boart Longyear took on the task one step at a time by tackling the depths in stages. For the first 106.68 meters (350 ft), they drilled a 228.6 millimeter (9 in) borehole while tripping the drill string every 6.096 meters (20 ft) to pull a split spoon geotechnical sample for the first 60.96 meters (200 ft).

For the second stage, the team drilled to 152.4 meters (500 ft) using a 203.2 millimeter (8 in) bit with casing. Moving deeper to 228.6 meters (750 ft), they used a 177.8-millimeter (7 in) bit with casing for the third stage. Surpassing their targeted drill depth of 213.36 meters (700 ft), the drillers still had not reached the bedrock formation.

Needing to find the true depth of the waste rock dump, Boart Longyear felt like the rig still had the capability and pullback to go deeper. Easing forward they drilled to 264.261 meters (867 ft) using a 152.4-millimeter (6 in) bit with casing. The last stage couldn’t be drilled with casing as they needed to move to a 101.6-millimeter (4 in) borehole. Leaving only the bit for the final push to 274.32 meters (900 ft) – and setting a new Boart Longyear record for sonic drilling – they reached a 28 percent greater depth than initially targeted.

It took 16 shifts of 12 hours (192 hours) to accomplish the new record depth for sonic drilling. Boart Longyear lost two of those shifts (24 hours) to rain. Another key accomplishment was the entire depth was reached through dry drilling and achieved 100 percent in-situ core samples at less than 1 percent hole deviation.