Chain instability mechanism and evaluation method of coal pillars-roof system in room and pillar mining goaf

To evaluate the stability of the residual coal pillars-roof system in room and pillar mining goaf, a three-dimensional thin platemechanical model of elastic foundation for the system was established considering the difference in the interaction relationship between coal pillars and roof at different positions. The chain instability mechanism of this system was revealed from the three-dimensional spatial structure perspective, and the influence of coal pillar spalling under time effect was considered. An evaluation method for the chain instability of this system was established. The results show that the load borne by a single coal pillar at different positions and the remaining coal pillars stability and roof deflection response characteristics caused by its instability are different. The deflection of the four-side fixed roof is smaller than that of the four-side simply supported roof under the same coal pillars conditions. After the instability of a certain coal pillar, the load increase of the remaining coal pillars during the four-side fixed roof is greater than that of the four-side simply supported roof. The spalling of coal pillars over time leads to a gradual decrease in their effective bearing size and bearing capacity, indicating the signicance of considering the coal pillar spalling due to time effect. Simplifying the roof into a thin plate and applying the energy variational method can obtain its approximate bending solution, thereby optimizing the roof fracture criterion with smaller errors. The chain instability of coal pillars-roof system is a dynamic process of "overall stability-local instability-instability diffusion-overall instability". By integrating a modified single coal pillar instability criterion and an optimized roof fracture criterion, the developed dynamic evaluation method can effectively evaluate the chain stability of coal pillars-roof system.