Research on three-field evolution and control effect of pillars structural in shallow buried closely spaced multi-seams mining

In order to explore the control method of pillar pressure-reduction and surface damage-reduction in shallow buried closely spaced multi-seams mining, taking No.1^-2 and No.2^-2 seams mining in Ningtiaota coal mine in Shennan mining area as the background, combining with physical simulation and theoretical analysis, the three-field evolution in shallow buried closely spaced multi-seams mining is obtained, and the relationship between concentrated stress, surface subsidence, mining fractures and different pillars structural is revealed.The classification of pillars structural is proposed, and the pillars structural effect is revealed.The result shows that with the pillar malposition distance increases, the maximum vertical stress of lower pillar shows as “large-small-large”,avoiding the transfer concentrated stress by upper pillar, and also the high pressure zone of upper middle goaf, it is beneficial to pillar pressure-reduction.The surface subsidence fall stages based on coal pillar malposition distance and pillars structural are determined, including subsidence fall maximum stage, subsidence fall decreasing stage and subsidence fall stable stage, the surface fractures are the smallest in the subsidence fall stable stage.The control mechanism of coal pillar malposition distance and pillars structural to overburden and surface fractures is revealed, and the reasonable pillar malposition distance range is determined.Based on three-field evolution, the classification of pillars structure is proposed, it contains superposition pillars structure, staggered pillars structure, separate multi-pillars structure in pressure-reducing zone and in compacted zone, and the pillars structural effect is revealed.When the upper and lower pillars form “separate multi-pillars structure in pressure-reducing zone”,the concentrated stress, surface uneven subsidence and the concentrated fractures can be controlled effectively, and it is the best pillars structure.The pressure-reducing model is established based on the control of pillar concentrated stress, the damage-reducing model is established based on the control of overburden displacement and fracture, and the reasonable pillar malposition distance determination method is put forward to realize a coupling control of pillar pressure-reduction and surface damage-reduction, the research results can provide basis for safe and damage-reduction mining in shallow buried closely spaced multi-seams.