Evolution of surrounding rock distortion energy during closely spaced coal seam mining and innovative coal pillar design method

This article innovatively examines the selection of the optimal width of the upper coal pillar during the mining of closely spaced coal seams geological conditions based on the engineering failure analysis of stress deviator (distortion energy density index). A theoretical calculation formula for the stress deviator of the surrounding rock at the coal pillar floor is derived. Analysis of the theoretical formula reveals that the load magnitude on the coal pillar and the width of the coal pillar (D) are the primary factors influencing the distribution characteristics of the stress deviator at the coal pillar floor. Theoretical and numerical research consistently demonstrates the distribution characteristics of stress deviator in the bottom plate of coal pillar. The area of stress deviator concentration on the coal pillar floor is primarily situated at the boundary of the coal pillar, gradually diminishing as the horizontal distance from the coal pillar and the depth of the surrounding rock increase. The overall distribution of the stress deviator exhibits a symmetrical "Y-shaped" pattern. With a smaller coal pillar width (D), the influence range of the stress deviator decreases, but the concentration is more pronounced. As the coal pillar width increases, the concentration of stress deviator gradually diminishes, with in a significant decrease in the peak value but an increase in the influence range. Based on the research findings regarding the distribution characteristics of stress deviator in the coal pillar floor, this article innovatively determines the optimal width of the upper coal pillar to be 20 m during the mining of closely spaced coal seams in the Malan Mine. To validate the accuracy and reliability of this research, additional simulation experiments and industrial experiments were conducted. The stress and strain fields of the surrounding rock in the simulation experiments, along with the results of on-site investigation and drilling observation, confirm that a coal pillar width of 20 m ensures stability.