Study on time sequence for opposite mining of adjacent working faces under ultra-close coal seam goafs with thick loess layer

In opposite-direction underground mining of two adjacent working faces under the ultra-close goafs with the thick loess layer, significant stress superposition occurs and the coal pillar stability between the working faces is in question, which may trigger strong ground pressure disasters. Taking the opposite-direction mining of the 2206 working face in the Haiwan No. 3 Well and the 2201 working face in the Wangcaihuopan Mine of the Northern Shaanxi Jurassic Coalfield as the engineering background, the overburden stress environment and the stability of the coal pillar between the two working faces before and after the opposite-direction mining were investigated using the numerical simulation, and the stress superposition effect of the surrounding rocks of the two working faces under different mining time sequences and the deformation laws of the surrounding rocks of the roadway were analyzed. The results show that the roof of the room-and-pillar area and the coal pillars are relatively stable after the mining of the 2-2 upper coal seam. The overburden failure of the longwall goaf reduces the stress in the goaf. The vertical stress of the remaining coal pillars between the working faces is asymmetrically distributed, and the peak of vertical stress on the side of the longwall goaf is higher than that on the side of the room-and-pillar area. During the opposite-direction mining of adjacent working faces of the 2-2 coal seam, the superposition effect of the concentrated stress of the remaining upper coal pillars and the mining-induced stress of the working faces affects the stability of the coal pillars and the surrounding rocks of roadway between the working faces. When the opposite-direction mining distance between the two working faces is less than 30 m, the maximum vertical stress of the coal pillar during the mining of the 2206 working face alone is 4.7% and 10.0% lower than that during the mining of the 2201 working face alone and the simultaneous mining of the two working faces, respectively. When only the 2206 working face is mined, the deformation of the surrounding rocks of the 2206 ventilation roadway is the smallest, while the two working faces are mined simultaneously, the deformation of the surrounding rocks of the ventilation roadway is the largest. After the two working faces stagger by −20 m, the stress of coal pillars and the deformation of surrounding rocks of the roadway almost tend to be stable. Considering the actual situation of on-site mining, a reasonable and safe mining time sequence for adjacent working faces was determined, and its rationality was verified through physical simulation. The findings are implemented on site with good performance by avoiding the relocation and shutdown of the working faces and achieving remarkable economic benefits. The research provides a reference for the safe mining of working faces under similar geological and mining conditions.