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OPERATING STRATEGIES The borehole's final depth exceeded the original target by 200 ft with less than 1% hole deviation overall. sample recovery rate, straight cased bore- hole and the flexibility to offer geotechnical sampling via the split spoon sampler. Using an SR-121 sonic rig, the crew met the task's challenge one step at a time by tackling the depths in stages. For the first 107 m (350 ft), they drilled a 228-mm (9-in.) borehole while tripping the drill string every 6.1 m to pull a split spoon geo- technical sample for the first 61 m (200 ft). For the second stage, the team drilled to 152 m (500 ft) using a 203-mm (8 in.) bit with casing. Going beyond 228 m (750 ft), they used a 178-mm (7-in.) bit with casing for the third stage. But even after passing the targeted drill depth of 213 m, the drillers still had not reached the bedrock formation. Needing to find the true depth of the waste rock dump, the crew had confidence the sonic rig had the capability and pullback to go deeper. Moving ahead, they drilled to 264 m (867 ft) using a 152-mm (6-in.) bit with casing. However, the last stage couldn't be drilled with casing, as they had to switch to a 102-mm (4-in.) borehole. Leaving only the bit for the final push to 274.3 m (900 ft)—and setting a new Boart Longyear record for sonic drilling—they reached a 28% greater depth than initially targeted. It took 16 12-hour shifts—192 hours total—to reach the new record depth for sonic drilling, and the company lost two of those shifts to rain along the way. Additional accomplishments: The entire depth was achieved by dry drilling, and Samples obtained from the sonic drilling project pro- vide up-to-date, accurate information about the dump's interior composition. provided 100% in-situ core samples at less than 1% hole deviation. Safety, said the company, was a top and constant concern. Lead driller Gabe Caredenas—given authority to halt drilling if he believed hole conditions were unsafe— decided whether or not the crew would pro- ceed to additional depth at each stage. "With the perfect conditions, sonic tech- nology and a well-trained crew, we were able to prove the strengths of sonic technology," said Ronald Cain, project manager for Boart Longyear. "We always knew we had the potential to go deeper with sonic drilling, and with constantly improving technology we'll continue to set new standards." www.e-mj.com NOVEMBER 2012 • E&MJ; 121