Failure analysis of coal pillars and overburden from underground water reservoir under the mining-water invasion coupling effect.

• A FLAC3D zonal osmotic unsaturated seepage model including water immersion weakening effect based on water-resisting coal pillar was established, and the influence of water seepage distribution on the stability of water-resisting coal pillar under mining and water immersion superposition was studied. • It is proposed that the lateral boundary intersection of the water-conducting fissures on both sides of the overlying strata and the connection between the seepage zone on the waterlogged side and the plastic zone on the mining side inside the water-resisting coal pillar as the critical conditions for the failure of both. • A reasonable retention width determination method based on the distribution of the internal water seepage, the central elastic compression tight water barrier, and the plastic zone of the water-resisting coal pillar were proposed. • Based on the deficiencies of finite element numerical simulation calculations in simulating fractured rock formations, a Fish language based on the stiffness weakening effect of overlying strata was developed. • It is proposed to consider the influence of the water barrier capacity of the overlying strata when the coal pillar with different widths is retained, that is, the stability of the "coal pillar-overburden" synergy water barrier capacity on the safe mining of the working face. The construction of coal mine underground reservoirs utilizing old goaf as storage space plays an important role in the ecological environment and economic development of arid mining areas. The destruction and design of coal pillars have always been the focus of research on the stability of coal mine underground reservoirs. Under the combined effect of mining and water immersion, the water-resisting coal pillar is easy to cause gradual damage and instability. Based on this, a numerical simulation method of fluid–solid coupling considering the zoned osmotic unsaturated seepage characteristics and the water immersion weakening effect of water-resisting coal pillars, and the collaborative water-blocking mechanism of coal pillars and overlying strata were proposed to quantify the stability of coal mine underground reservoirs. In the simulation process, the coupled distribution characteristics of the seepage field, plastic zone, and stress field of the permeable flow of coal pillars with different widths under mining and water immersion were realized, and the water-blocking mechanism of the overlying strata based on the distribution characteristics of the water-conducting fissure lateral boundary was studied. Meanwhile, it is proposed that the lateral boundary intersection of the water-conducting fissures on both sides of the overlying strata and the connection between the seepage zone on the waterlogged side and the plastic zone on the mining side inside the water-resisting coal pillar as the critical conditions for the failure of both. Therefore, a reasonable design method for the width of the water-resisting coal pillar was proposed to prevent failure, which includes "water seepage zone + elastic pressure-dense water-resistance zone + plastic zone" three-zone distribution and considers the distribution characteristics of the overlying strata, which are "water-conducting fissure zone + middle extrusion water-blocking zone". The reasonable width was determined to be 150 m for the water-resisting coal pillar, and on this basis, the dynamic degradation mechanism of the load-bearing capacity of the 150 m coal pillar under multi-field coupling during the entire mining process was quantitatively analyzed. Therefore, an engineering method was proposed to support and reinforce the water-resisting coal pillar and reasonably drain the water in the upper segment. This study is expected to provide theoretical guidance for the stability research and reasonable width determination of the water-resisting coal pillar in the protection of water resources in the water-rich goaf and the safe mining of adjacent working faces. [ABSTRACT FROM AUTHOR]