Mechanism of short-wall block backfill water-preserved mining based on water-conducting fractures development-heavy metal ions migration

Short-wall block backfill mining (SBBM) technology can effectively solve the problems of coal resources waste, water resources loss and gangue waste accumulation. However, after the long-term effect of mine water, the internal heavy metal ions in the gangue backfill materials may be leached, which have a certain potential impact on the groundwater environment in the mining area. Therefore, according to the technical characteristics of SBBM technology, comprehensive prevention of water resources loss and water resources pollution during SBBM was studied systematically. First, prevention and control of water resources loss in the mining area. Based on the combination relationship of “coal pillar, backfill body-barrier layer-aquifuge”, The development of water-conducting fractures in overlying strata under SBBM were analysed. The control mechanism of water resources protection under the evolution of overlying strata structure induced during SBBM was revealed. Secondly, prevention and control of water resources pollution in mining areas. The migration model of heavy metal ions in the gob was established. Then, the pollution mechanism of gangue backfill materials to water resources was revealed, the migration law of heavy metal ions in gangue backfill materials was analyzed, the comprehensive prevention and control technology of water resources destruction in mining area was summarized, the comprehensive prevention and control technology of water resources loss-pollution in mining area was summarized, and the design method of filling ratio based on water resources loss-pollution prevention was put forward. The results show that the gangue materials were filled into the gob as the backfill body, the gangue materials would be the permanent bearing body, which bore the load of the overlying strata accompanied with the protective coal pillar among the blocks, which could greatly prevents the collapse of the low-level stratum, restrict the water-conducting fractures to penetrate the barrier layer and protect the integrity of the high-level stratum. In addition, the heavy metal ions in gangue backfill materials were subjected to the coupled action of seepage, concentration, and stress and then driven by water head pressure and gravitational potential energy to migrate along the side of the coal pillar and below the floor stratum, during which mine water served as the carrier. The migration distance of heavy metal ions increased with increasing of the permeability of floor strata, leaching concentration of heavy metal ions/pollution source intensity, depth of floor breaks and water level heights, and decreases with the increase of gangue particle size and surrounding rock stress. Based on this, the comprehensive prevention and control technology of water resources loss-pollution in mining area was summarized, it included mining parameter control technology, backfill parameter control technology, pollution source control technology, transmission route control technology and in-situ water control technology. and the design method of filling ratio based on water resource loss-pollution prevention and control was put forward, which can realize the comprehensive prevention and control of water resource loss-pollution in mining area. This study will provide scientific theoretical basis and guidance for the comprehensive prevention and control of the water resources destruction problems caused by coal mining.