Study on Pillar Design for Longwall Mining under Weak Immediate Roof and Floor Strata in Indonesia

More than 99% of coal mining in Indonesia is open-pit mining. Nowadays, the adoption of the underground coal mining method is discussed to extend coal production due to the increasing demand for coal. However, the geological conditions in Indonesia are very weak compared to the U.S., Europe, and Australia where the longwall mining method is often applied. Especially, the mechanical properties of immediate roof/floor in shallow depth are weaker than those of coal. Therefore, the control measures to maintain the stability around the developing area should be discussed for the safe longwall mining operation. This study discusses the design of safety pillar width in longwall mining under weak geological conditions by using FLAC3D. It is found that the conventional equations for the determination of the pillar width, Obert-Duvall, Holland-Gaddy, and Bieniawski equations, can be adopted to maintain the stability of the pillar itself, but are not suitable for the stability of the longwall face because of the influence on the neighboring panel extraction. Additionally, the increase of the pillar width can significantly reduce the fracture zone around the longwall face while increasing the setting load of the shield support can slightly improve the stability. The pillar design needs to be considered not only the pillar strength but also the stability of the longwall face under the weak geological conditions in Indonesia.