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Groundwater reservoir technology is a vital way to achieve both efficient mining and water conservation in western coalfields in China. The stability of coal pillar dam is an emphasis of reservoir design, which plays an important role in the stability of groundwater reservoirs. Coal pillar dam should ensure the safety of coal mining, as well as the safe operation of groundwater reservoirs. The stability of coal pillar dam is closely related to water weakening and engineering geological conditions, such as buried depth, mining stress, geological structure and so on. In order to study the strength weakening law of coal pillar dam of groundwater reservoirs under water, this paper took Madiliang mine No.5 in western mining area as the research object, carried out a non-destructive water immersion test on coal sample and a uniaxial compression test on coal sample under various water content and repeated soaking. The results show that the water content of coal samples with various water content increased rapidly within 0-20 h; after 20 h, the grew rate slowed down; and in 20-70 h, the water content grew slowly; in 70-140 h, the growth of water content of coal sample was nearly stable. In conclusion, the water content of coal sample increases with the soaking time, but the increase rate gradually decreases, and finally tends to be flat. With the increase of water content( the number of times of soaking), the uniaxial compressive strength of coal sample decreases gradually, but the peak strain increases gradually, and the elastic modulus decreases gradually. Compared with the dry coal sample, the uniaxial compressive strength of the first water-saturation coal sample and the coal sample with the third soaking decreased by 27.1% and 50.0% respectively, and the elastic modulus decreased by 34.6% and 58.5% respectively. Combined with the above results and the elasticplastic mechanics theory, this paper discussed the damage zone and plastic zone width of coal pillar dam under various water content, carried out the research of coal pillar dam width considering overburden rock pressure, water pressure and water weakening, and designed the width of the groundwater reservoir combining with the engineering geological conditions of Madiliang mine. At the same time, the paper further analyzed the influencing factors of the width of coal pillar dam, concluding that the width of coal pillar dam is closely affected by water content, buried depth, and mining thickness. [ABSTRACT FROM AUTHOR]