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).
Abstract: Mae Moh mine is one of the deepest open pit mines in Southeast Asia, currently 300 m below the ground surface. According to its master mine plan, the mine depth will be approximately 500 m by year 2028. It is located in Lampang, 650 km north of Bangkok. Recently, it produced approximately 15 million tons of lignite and over 90 million m3 of waste a year to serve ten lignite-fired power plants, with total capacity of 2,400 MW. The mine and power plants are operated by a prominent Thai state enterprise, Electricity Generating Authority of Thailand (EGAT). To remove such a huge amount of dense Overburden and Interburden claystone, drill and blasting techniques have been utilized. The overburden and interburden are fractured and loosen by blasting suitable for the production and also to reduce fuel consumption and wear rate of excavators as well. There is, however, a major unwanted consequence of the blasting technique which is induced seismic force. The induced seismic force would potentially cause environmental impacts. To increase safety of the working environment and contribute to the social responsibility, the mine has lately replaced the conventional blasting technique with air deck blasting. With less amount of explosive usage, the air deck blasting results in less ground vibration with satisfied mine production. The measured peak particle velocity, related to the induced seismic force and the ground vibration, was reduced by 32-69 %. Surprisingly, the air deck length recommended at 20% of the explosive charge can reduce the induced seismic force while increasing fragmentation and higher productivity.
To study the mechanism of crater blast formation two blast experiments were planned, one with conventional blast with stemming column and another with air decking. The simulated results were compared with the field observation. Air deck demonstrate better utilization of explosive energy into rock breakage whereas, no proper breakage was visible from conventional crater blast observations.
An explicit numerical model (1&2) of two crater blast hole demonstrated the behavior of stress wave in the air deck and full length of conventional stemming column conditions. Movement of stemming column is allowed in model 2 to see the formation pressure rings and its effect the on rock damage. Numerical results show the improvement in explosive energy utilization through pressure rings behavior in air deck, whereas, wastage of explosive energy through ejection of stemming column are found in conventional stemming model. It is clear that working time of pressure rings increase in the model 1. Stresses and pressures are measured from various monitoring points. Damage contour marked on crushed zone around the blast hole which is equal in both models and free ground surfaces in 1m interval from blast hole. A Full Damage zone was clearly found up to 4m from blast hole in model 1, whereas, less damage zone up to 2m observed in the model 2. Stemming ejection velocity is found much higher in the model 2 as compared the model 1.