Numerical Model for Fracture Instability Mechanism and Reinforcement Technology of Irregular Coal Pillar Under the Mining Abutment Stress: A Case Study from Da-yang Coal Mine

Irregular coal pillars, with complex structures, are very prone to instability damage under the abutment stress. The irregularity of these irregular coal pillars hinders the application of planar stress model analysis, resulting in a three-dimensional fracture instability problem. Previous studies have employed the ultimate bearing strength of coal pillars and the specific gravity of the plastic zone as criteria for evaluating coal pillar instability, frequently leading to a conservative determination of safety factors. In this paper, a three-dimensional finite element model of irregular coal pillar fracture instability is established to solve the problem of irregular coal pillar fracture instability under the abutment stress. Moreover, the criteria for inducing the instability of the sharp corner of irregular coal pillars were determined, which revealed that the instability mechanism of irregular coal pillars is "crack sprouting in the sharp corner area-expanding to crack penetration-coal pillar fracture and instability." In this way, the sharp corner of irregular coal pillars is classified into fracture instability zone, plastic load-bearing zone, and elastic load-bearing zone. The influence of angle, strength, mining depth, and support strength on the bearing zone of irregular coal pillar is revealed: With the increase in angles, there is a continuous decrease in the proportion of fracture instability zones and an increase in the proportion of plastic bearing zones, while the proportion of elastic bearing zones decreases. Furthermore, as the strength, support strength, and mining depth increase, the proportion of fracture instability zone decreases linearly, while the proportion of plastic bearing zone decreases continuously. Simultaneously, there is a continuous increase in the proportion of the elastic bearing zone. The research focuses on the irregular pillar of 3307-working face in Da-yang Coal Mine. A three-dimensional finite element calculation model is established to analyze the fracture instability of the irregular pillar, determining the region of irregular pillar instability. Additionally, a technology of over-excavation reconstruction and support pressurization bearing strengthening technology for the irregular coal pillar is proposed. The finite element model for the fracture instability of irregular coal pillars has an obvious applicability due to the effect of the support in the field application. The research results can provide a theoretical basis and practical experience for stabilizing irregular coal pillars under similar conditions.