Effects of Subdrilling on Broken Zone Beneath Bench Floor at Kevitsa Open Pit Mine: A Case Study

Abstract The effect of subdrilling length on the broken zone beneath the bench floor was studied at the Kevitsa open pit mine. Five full-scale blast tests were conducted in the mine where each blast was divided into a test area and a reference area. In all the test areas, a 1.0 m subdrilling length was used, while in all the reference areas, a 1.5 m subdrilling length was employed. Two diameters of blastholes (165 mm and 229 mm) and two detonator initiation methods (the middle-detonator position and double-detonator position) were involved. The rock fragment size from both test and reference areas was measured during the muckpile extraction process. After removing muckpiles, the coordinate data of the bench floor were captured using handheld GPS devices. The broken zone beneath the bench floor was detected while drilling holes on the bench floor for lower level blasts. The test results indicated that implementing the 1.0 m subdrilling effectively reduced the median broken zone by 18.9%-55.9%, compared to the conventional 1.5 m subdrilling. Meanwhile, the short subdrilling length had minimal impact on other blasting outcomes, including the post-blast elevation of bench floor and rock fragmentation. In addition, numerical simulation was conducted using the three-dimensional bench model with a single blasthole. The simulation results revealed that the maximum shear stress concentration zone beneath the bench floor for the 1.0 m subdrilling was located 0.5 m higher than that for the 1.5 m subdrilling, consistent with the broken zone measured from field tests.