Although the basic mechanisms of coal pillars are well understood, gaps exist in some critical areas, particularly in relation to long-term stability. This study investigates the long-term stability of coal pillars by considering three-pillar system failure modes: (i) pillar failures due to pillar spalling (i.e., reduced pillar width), (ii) pillar failures due to continuous roof failures (i.e., increased mining height), and (iii) pillar failures due to weakened floor. The mechanisms of pillar failure caused by the three failure modes are discussed. The concept of ‘geometrical limits’ is implemented to calculate the critical spalling depth for pillar failure caused by rib spalling, and the critical mining height for pillar failure due to roof falling in long-term. In addition, an analytical technique has been developed to quantitatively assess the bearing capacity of soft floor. This analytical model shows that the bearing capacity decreases linearly with decreasing cohesion of the floor. The assessment approach proposed in this study can help to quantify the long-term stability of coal pillars.

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