Search Results (3)

Close-distance coal seams are common in underground mining, and their spacing is short, which produces strong mining disturbance. In instances where the upper seam has been mined and a goaf has formed, a notable issue arises during the lower seam’s mining, characterized by substantial deformation of the roadway along the goaf. Field exploration and three-dimensional geological modeling have revealed that the fourth and sixth working faces and pillar of seam No. 5 are all under seam No. 2’s goaf, with an average distance of 16.70 m. Simultaneously, the double compression effect of the pillar, induced by the linkage rotation of key blocks of the lower and upper seams, is analyzed. The induction mechanism and path of the large deformation are expounded. It is thus proposed that the pillar’s width should be determined by gob-side entry, driving beneath the goaf, with the roof near the pillar being cut off in advance to realize the path of cutting off the compressed pillar. Through the simulation comparison of five kinds of pillar width combined with engineering practice, it has been determined that the best width is 8 m, and the abutment pressure is distributed in a double-peak saddle shape, with the result that the load-bearing ability is notably significant. Through the comparative simulation of roof-cutting, it was found that roof-cutting helps the roof to collapse near the pillar-side and decreases the vertical stress peak to 16.46 MPa, the shear stress peak to 5.93 MPa, and the J₂ peak to 7.23 × 10¹³ Pa, which further alleviates the pressure on the pillar. In the field, the haulage roadway’s roof was cut by two-way shaped-charge blasting, and the sandy mudstone (5.90 m) was successfully cut off. Concurrently, anchor cable reinforcement was implemented on the roof and two ribs of the ventilation roadway in proximity to the pillar, thereby ensuring stabilization and mitigating the mining effect. The engineering research provides a case and scheme reference for the operation of gob-side entry driving beneath close-distance goafs worldwide. Full article

The construction of underground reservoirs in coal goaf is a new technology aimed to realize the sustainable development of coal mining-water storage-surface ecology in arid areas of northwest China. The key to the feasibility of this technology is that underground coal mining cannot affect the near-surface aquifer, and the amount of water entering the underground reservoir must meet the needs of the coal mine. Taking Buertai Coal Mine, one of the largest underground coal mines in the world, as an example, this article used similar simulation, numerical simulation and in-situ test methods to study the height of the water-conducting fracture zone of overlying strata and water inflow of underground reservoirs. The results show that, under the repeated mining of the 22- and 42-coal seams, the maximum height of the water-conducting fracture zone is 178 m, and the distance between the near-surface aquifer and the 42 coal is about 240 m, so the mining has little effect on the near-surface aquifer. During the mining period of the 22-coal seam, the groundwater of the Zhidan and Zhiluo Formations was mainly discharged vertically, while the groundwater of the Yanan Formation was mainly a horizontal flow during the period of the 42-coal mining. In this way, the total water inflow of Buertai Coal Mine reaches 500 m³/h, which not only meets the needs of the mine, but also, the rest of the water can irrigate about 98 hectares of farmland nearby. Underground reservoirs in coal goaf could achieve sustainable development of coal mining, groundwater storage and surface ecology in semi-arid areas. Full article

To explore the control technology on surrounding rock of gob-side entry retaining (GSER) below a goaf in a near distance coal seam (NDCS), research was conducted on the floor ruin range, the floor stress distribution features, the layout of the GSER below near distance goaf, the width of the roadside filling wall (RFW), and the control technology of the GSER surrounding rock below the near distance goaf after upper coal seam (UCS) mining. The results show that (1) the stress of the goaf floor has obvious regional features, being divided into stress high value zone (Zone A), stress extremely low zone (Zone B), stress rebound zone (Zone C), stress transition zone (Zone D), and stress recovery zone (Zone E) according to different stress states. The stress distribution features at different depths below the goaf floor in each zone also have differences. (2) Arranging the roadway in Zone A below a coal pillar, the roadway is at high stress levels, which is not conducive to the stability of the surrounding rock. Arranging the roadway in Zone B below the goaf floor, the bearing capacity of the surrounding rock itself is weak, making it difficult to control the surrounding rock. Arranging the roadway in Zone C, the mechanical properties of the surrounding rock are good, and the difficulty of controlling the surrounding rock is relatively low. Arranging the roadway in Zone D and Zone E, there is a relatively small degree of stress concentration in the roadway rib. (3) When the RFW width is 0.5–1.5 m, stress concentration is more pronounced on the solid coal rib, and the overlying rock pressure is mainly borne by the solid coal rib, with less stress on the RFW. When the RFW width is 2~3 m, the stress on the RFW is enhanced, and the bearing capacity is significantly increased compared to RFW of 0.5–1.5 m width. The RFW contributes to supporting the overlying rock layers. (4) A comprehensive control technology for GSER surrounding rock in lower coal seam (LCS) has been proposed, which includes the grouting modification of coal and rock mass on the GSER roof, establishing a composite anchoring structure formed by utilizing bolts (cables); the strong support roof and control floor by one beam + three columns, reinforcing the RFW utilizing tie rods pre-tightening; and the hydraulic prop protection RFW and bolts (cables) protection roof at roadside. This technology has been successfully applied in field practice. Full article