松软煤层卸压开采覆岩裂隙演化特征及精细化分区OA

[Objective]During the mining of coal seam groups,substantial gas is prone to rush into the mining face from adjacent coal seams through interconnected mining-induced fractures.This poses a threat to safe coal mining.Under pressure-relief mining of soft coal seams,their low permeability and high stress sensitivity present challenges to the suc-cessive mining of protective and protected coal seams.Specifically,mechanisms behind the overburden fracture evolu-tion remain poorly understood,and it is difficult to identify preferential pathways for gas migration accurately.[Meth-ods]This study investigated coal seam groups in the Jiahe mining area of Hunan Province as the engineering back-ground.Using physical simulations with similar materials and numerical simulations based on the 3DEC software,this study systematically examined overburden fracture evolution and stress distribution patterns during pressure-relief min-ing of the coal seam groups.Accordingly,a fine-scale zoning model tailored to soft coal seams was developed.Using this model,the layout of high-level boreholes for gas extraction was optimized.[Results and Conclusions]After the mining of coal seam Ⅵ as the lower protective layer,the overburden exhibited a caving zone height of 4.2 m and a frac-ture zone height of 23.6 m,providing initial fracture fields for the subsequent pressure-relief mining of protected coal seam V.The overburden fractures of mining face 2152 of coal seam V exhibited distinct multi-stage evolution.The height of cross-layer fractures increased with the advancement of the mining face,tending to stabilize after the advan-cing distance reached 37.6 m.In contrast,delamination fractures kept propagating upward,with an ultimate height of up to approximately 55 m.Ultimately,an O-shaped fracture circle that penetrated the fracture and caving zones was formed in the overburden.Physical simulations with similar materials and numerical simulations yielded consistent results,with the caving zone height of coal seam V estimated at 8.5 m and 7.2 m and the fracture zone height of the coal seam de-termined at 37.6 m and 38.4 m,respectively.Meanwhile,physical and numerical simulations revealed that stress concen-tration zones formed by re-compaction occurred near advancing distances ranging from 35 m to 36 m and from 96 m to 100 m,respectively.The fracture evolution patterns indicate that the fracture zone in the conventional vertical three zones can be further subdivided into a cross-layer fracture zone and a pressure-relief fracture zone.Furthermore,using delamination rates and breaking angles as quantitative boundaries,horizontal four zones were reconstructed:a coal wall disturbance zone,a detachment layer development zone,a voussoir beam structure zone,and a goaf compaction zone.Among these,the voussoir beam structure zone contained preferential pathways for gas migration.Field gas extraction using four sets of high-level boreholes demonstrates that boreholes with final holes arranged in the central part of the cross-layer fracture zone(distance from the roof:25 m)outperformed in gas extraction,with a gas volume fraction of 86.4％and a pure gas flow rate of 2.62 m3/min.These results corroborate that the zoning model developed in this study can provide key parameters for the optimal horizon selection and regional targeted layout of high-level boreholes under pressure-relief mining of soft coal seams.