The progressive failure mechanism for coal pillars under the coupling of mining stress and water immersion in underground reservoirs

The underground reservoir in the coal mine provides a new way for the storage and utilization of mine water resources in mining areas in western China. Combined with the characteristics of coal pillar (CP) in underground reservoirs, the cumulative damage law of the samples under cyclic loading and unloading (CLU) and different immersion times was explored by laboratory experiments. The progressive damage characteristics of underground reservoir CP during mining and impoundment are studied based on the laboratory tests. At the same CLU time, a gradual decrease in the coal samples strength was observed for different cycles of water immersion. Compared with 3 CLU times during 7 days and 14 days of water immersion, the load fluctuation and strain extension in the post-peak area reached the maximum during 21 days of water immersion. The CP exhibits the “damage – seepage – cumulative damage” effect in the water immersion-stress coupling environment. After mining, the plastic zone of the coal pillar dam (CPD) increased from 6 to 10.5 m. The residual coal pillars (RCP) were gradually destroyed, mainly showing shear damage. Underground reservoirs have a gradual shift of pore pressure to the interior of the CPDs until it is finally stabilized. Inside the CPs, the pore pressure gradually decreases from the side near the coalfield to the inside, and the diffusion is positively correlated with the damage degree of the CPs.