Design of auger highwall mining : a case study at Mae tan coal mine, Thailand

Abstract

Most coal mines that adopted surface mining method have been stopped after reaching the ultimate pit limit. However, in some areas, there is still some significant amount of coal in the highwall that can be extracted using other mining methods. Highwall mining method, which is considered as the semi surface-underground mining operation, was originally developed in the early 1940s in the US. It is suitable to excavate the remaining coal. Technically, there are two types of highwall mining: continuous and auger highwall mining with the rectangular and circular opening respectively. The highwall miners are used to drill the residual to extend the mining life or recovery ratio. In this study, few empirical approaches were compared to the 3D modeling of FLAC3D to design the pillar system in highwall mining special case to Mae Tan Coal Mine. The stress and strength of the pillars were investigated from both types of theories. Several criteria were proposed to estimate one optimum design for this coal mine. The result of stress from the numerical analysis provided almost the same as an empirical theory. Moreover, the strength from numerical modeling also similar to the average empirical strengths. Hence, the average result of empirical equations was suggested to use for Mae Tan coal. The result, at the safety factor of 1.6 for web pillar and 1.0 for barrier pillar, showed that the quantities of coal increased when the penetration length was made deeper, although the percent recovery of coal decreased. According to the pillar design of the three categories, the amount of mined coal was not the same. As the last observation on the stability and coal production, the multiple passes with the auger diameter of 1.5 m, web and barrier pillars of 1.5 m and 2.0 m were suggested. Furthermore, the multiple passes were the combination of double passes at the lower seam and single pass at the upper seam. The double passes at the lower seam were designed by keeping the vertical spacing between two holes of 0.8 m, which is the most stable case. In addition, the safe entry lengths of the upper and lower seam are 55 m and 60 m as the final recommendation for Mae Tan Coal Mine.