Study on large deformation mechanism and surrounding rock control of entry in "three soft coal seam" of deep mine

To address the problem of large deformation challenges encountered in the two entries of the working face during the extraction of "three-soft coal seam" in deep mines, taking the No. 1509 isolated working face in Shanyang Coal Mine as the engineering background, this study comprehensively employs laboratory experiments, field measurements, theoretical calculations, and FLAC3D numerical simulations, it analyzes the lithological causes of original support failure, reveals the deformation and failure laws of roadway surrounding rock before and after mining influence, obtains the relationship between the distribution characteristics of the loose circle and the support range of bolts and cables, and uncovers the large deformation mechanism of the two gate roads in the No. 1509 working face.The theory of "ultimate self-stabilizing equilibrium circle" of soft rock roadway is proposed, it is pointed out that in the treatment of soft rock roadway, the ' floor-two ribs-roof ' of the roadway should be regarded as a whole, and the principle of "controlling the ribs first rather than the roof, and controlling the floor first rather than the ribs, shoulder angles and rib feet as key zones" should be adhered to. This is to enhance the strength and stability of the overall "floor-ribs-roof" support and achieve the effect of "strengthening the floor to reinforce the ribs, and strengthening the ribs (shoulder) to reinforce the roof". According to this theory, the reinforcement support scheme of two entries in 1509 working face is designed and implemented on site. The research indicates that the surrounding rock of the roadway exhibits low strength, with the roof and floor strata characterized by water-induced softening, weathering, and strong swelling properties. The uniaxial axis compressive strength of the surrounding rock ranges from 6.45 ~ 17.04 MPa, the water softening coefficient is 0.39 ~ 0.86, the clay mineral content in the roof and floor is 53.58-72.03%, and the rock sample brokes into flaky rocks after air drying for 2.5 hours. When not affected by mining activities, the roadway roof undergoes weathering and fragmentation, the ribs bulge out, and the floor softens and swells upon contact with water. After being influenced by mining, the original support fails rapidly. Within 50 meters ahead of the working face, the deformation speed of the roadway surrounding rock is the highest, causing comprehensive large-scale deformation in all directions, the deformation speed is slower between 50 and 120 meters ahead of the working face.Field measurements show that before the influence of mining, the loose circle development depth in the roadway sides and shoulders is relatively large, especially the maximum depth in the shoulder area, which exceeds the support range of bolts but does not reach the support range of cables. Under the original support conditions, both the bolts in the ribs and shoulders are in a failed state, requiring the advance support of long anchor cables to be supplemented.Based on the theory of "ultimate self-stabilizing equilibrium circle", the length of the reinforcement anchor cables for the two ribs and shoulders is determined to be 5.5m, and the depth of the floor arch and the length of bolts are 0.7m and 2.4 m respectively. According to the FLAC3D numerical simulation results, after adopting the optimized support scheme, the range of the plastic zone in the roadway roof is reduced by 62% and in the two ribs is reduced by 57%, and the deep range of the plastic zone in the floor is reduced by 88%. After the construction of the test section, the deformation control effect of the roadway surrounding rock is remarkable. This study not only improves the safety and production efficiency of Shanyang Coal Mine, but also has important reference value for the support of soft rock roadway under similar engineering geological conditions.