Underground reservoir water storage technology has become one of the important approaches to achieve efficient coal mining and water resources protection in mining areas in Western China. As the coal pillars in the underground water reservoir is the main bearing structure of the coal mine, the stability of coal pillars in the underground water reservoir play a vital role in the stable operation of underground water reservoir projects. Based on the characteristics of the coal pillar dam and residual coal pillars on the site of underground reservoir, this paper explores the stress distribution of coal samples under different cyclic loading and unloading conditions and the influence of different water immersion cycles on the strength of coal samples after cyclic loading and unloading through experiment; it analyzes the progressive damage characteristics of coal pillar dam and residual coal pillars during mining influence and water storage by using FLAC^3D numerical simulation. The results show that during the same water immersion cycle, the strength of coal samples showed a trend of sharp decrease followed by steady decrease with different cyclic loading and unloading times, and the strength of damaged coal samples under a single condition decreased. Given the same cyclic loading and unloading times, the strength of coal samples showed a trend of gradual decrease during different water immersion cycles. After multiple stress damages, with the extension of water immersion cycles, the coal samples showed load fluctuations in the pre-peak area and residual strain delay in the post-peak area. Compared with 3 cyclic loading and unloading times during 7 days and 14 days of water immersion, the load fluctuation and strain extension in the post-peak area reached the maximum after 3 cyclic loading and unloading times during 21 days of water immersion. The numerical simulation shows that the coal column suffered tensile damage on the top and shear damage in the middle and at the bottom near the goaf side during the mining period. After the coal pillars were weakened by water logging, with the increase of the weakening times of the coal pillar dam, the plastic zone of the coal pillars gradually extended to the inside of the coal pillars. The plastic zone of the coal pillar dam increased from 6m to 11m after mining, and the residual coal pillars were gradually destroyed, mainly presented as shear damage. The pore pressure inside the coal pillars of the underground water reservoir gradually transferred to the inside, until the coal pillar dam ultimately stabilized. The pore pressure inside the coal pillars gradually decreased from near the goaf side to the inside, and the diffusion is positively correlated with the damage degree of the coal pillars.
 

Underground reservoir;,Coal pillar dam,Water Immersion,Progressive damage