Drill and blast is the excavation method adopted to remove overburden material at the open pit coal mine of PT Buma Job Site Lati. Recently, the company applied deep hole drilling for blasting with double rods to reach the depth of 10-18 m. The main explosive was an emulsion explosive with target explosive consumption of 0.23 kg/m3. The blast hole was not fully charged but vertically decoupled using air decks at the middle and bottom of each blasting hole. Blasted rocks were then measured by digital photograph, and the size of P80 was found to range from 200 to 800 mm. The evaluation results indicate there is close relationship between the explosive consumption and the fragment size as well as the digging time. The air deck technique adopted in this study has been giving good results in terms of fragmentation size and explosive consumption.
In mining operations, drilling and blasting with a deep hole is a preferred method to reduce lost production time caused by the delays in blasting, to increase blasting inventory, and to minimize the number of drill pads so the drilling deviation can be minimized as well. Recently, PT BUMA Job Site Lati (called BUMA from now on) applied drilling for blasting operations with double rods to reach the depth of 10-18 m. Consequently, explosive consumption was high in the lower part of the blast hole, then an air deck was used to distribute the explosive along the blast hole. Another benefit of the air deck was that it cut the waiting time of the on-site sensitized explosive expansion to accommodate gassing.
In this study, the performance of blasting operation using the air deck is evaluated in terms of fragmentation. This study aims to check the effectiveness of using an air deck with the productivity target at the particular mine and to develop a blasting-fragmentation model that can be used to predict the size of the fragmented rock.
2. Literature Review
The air deck method is well-known in blasting operations to improve the quality of the blasting results. In the early 1940s, Russian scientists first came up with the idea of using an air gap between explosive columns. This method reduced explosive consumption in blasting activity. Melnikov et al. (1979) mentioned that an air deck can act as an energy accumulator. Marchenko (1982) found that pressure in an air deck would expand micro fractures that were previously generated by the main shock wave during blasting. Pompanna and Chikkareddy (1993) concluded that the presence of an air gap in the blast hole can reduce ground vibration and back break at the Kudrremukh iron mines. Jhanwar et al. (1996) revealed that the mechanism of air deck can reduce 25-30% of explosive consumption. Chiapppeta (2004) conducted experiments in the field and found that the air deck technique could remove the sub-drill which in turn reduced the explosive consumption by 16-25%, decreased vibration due to blasting by 33%, and improved fragmentation by 25%.
Utilization of an air deck will increase the fracture network due to the secondary shock waves formed as the result of wave reflection in the air gap. The fracture degree increases as a result of secondary shock waves as the duration of the shock wave effect on the rock mass around the blast hole becomes longer. The pressure reflections from the upper and lower explosive columns will collide in the middle of the air deck and is expected to interact with the surrounding rock mass to form additional radial fractures (Moxon et al., 1993; Zhang, 2016; see Fig.1). Air deck methods have been used in some open pit mines to reduce the consumption of explosive and to improve fragmentation (Chiapetta, 2004).