Study on the deterioration law of mechanical properties and damage characteristics of coal pillar dams with repeated immersion in coal mine underground reservoirs
Authors: Wang Beifang , Zhou Duo , Zhang Jing , Liang Bing 
Published: November 2023

Abstract
The continuous operation of coal mine underground reservoirs exposes the dam body of coal pillars to mining disturbances and prolonged water flooding, resulting in the deterioration of coal pillars' mechanical properties and posing a serious threat to the stability of the dam body. To this end, coal samples from the proposed pillar dam body in the 5 − 2# coal seam of Daliuta Mine in Shendong Mining Area were selected for conducting water absorption tests and triaxial compression tests under conditions of repeated water immersion, to investigate the deterioration law of the mechanical properties and acoustic emission damage characteristics of the coal samples when subjected to repeated water immersion, and to analyze the mechanism behind the deterioration of the coal samples under the influence of water-rock interaction. The results indicated that: (1) The saturated water content of the coal samples exhibited a progressive increase as the number of soaking times increased, but with a diminishing rate of growth. (2) As the number of soakings increased, the compressive strength, cohesive force, and internal friction angle of coal samples gradually decreased. Notably, the deterioration effect was more pronounced in compressive strength and cohesive force, while the decline in internal friction angle was relatively minor, and the total deterioration degree and the stage deterioration degree among the three samples had evident cumulativity and non-uniformity. The progressive rise in water immersion instances led to a gradual attenuation of the deterioration effect. Simultaneously, the confining pressure exhibited a certain inhibitory impact on the deterioration of coal sample strength. (3) Comparison with the dry coal samples, the average AE count rate of coal samples subjected to a single immersion in water exhibited a significant decrease, and subsequent immersions in water for two, three, and four times resulted in a very minor decrease in the average AE count rate. (4) The AE cumulative ringing counts in coal samples exhibited varying degrees of decline as the number of water immersions increased. Specifically, the most significant decrease in AE cumulative ringing counts occurred after the initial water immersion, followed by a gradual decrease thereafter. The energy-releasing capacity of the coal samples decreased, while their plasticity exhibited a gradual increase. (5) A damage model was developed for coal samples based on the frequency of water immersion. The model indicated that the damage to coal samples increased as the number of water immersions increased, and the rate of damage gradually decreased and eventually stabilized. (6) The mechanism of coal deterioration under the interaction between water and coal was analyzed. Through repeated water immersion, the physical, chemical, and mechanical interactions between water and coal induced alterations in the internal microstructure of coal samples, resulting in the deterioration of mechanical properties such as compressive strength, cohesive force, and internal friction angle, which was a cumulative damage process from the microscopic to the macroscopic level.