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The gob-side entry 150–180 m behind the 14201 working face at Majialiang coal mine is severely deformed. We developed a new technique to control the behaviour of the high-located main key stratum (HMKS) to improve mine safety. The failure type and breaking span of the HMKS were determined based on key stratum theory and the voussoir beam model. By monitoring the deformation of the entry and surface subsidence, we found that the main cause of the large deformation of the gob-side entry was sliding of the HMKS. A new technique is proposed to add grout between the separated beds during mining. Physical simulations indicated that this technique is efficient in controlling sliding of the HMKS and avoiding strong dynamic loading, with the peak abutment stress reduced by 59%. Grouting stations were set up at a spacing of 150 m behind the advancing work face based on the principles and key parameters of the technique and the geological conditions at Majialiang coal mine. Before the HMKS began to slide, we injected high-water content materials with a water to cement ratio of 1.5:1 into the bed separation space to prevent breaking of the key strata. [ABSTRACT FROM AUTHOR]

In multiple seam coal mining, the fractures induced by the mining of the lower coal seam may propagate to the goaf of the upper coal seam that is mined earlier. The fracture connection leads to the leakage of hazardous gas. To fully understand the gas leakage due to fracture evolution, the electromagnetic-based measurement system and the borehole televiewer were adopted in the panel No.8100 in Tongxin coal mine, Datong Coalfield, China. The coal seam thickness is 15 m and it was mined by top-coal caving method. The resistivity and fracture pattern in distinct zones of rock stratum were detected, and the correlation between them was explained clearly. We found that the fracture evolves in two stages as the panel working face advances, i.e., fracture propagation and fracture re-closure. The maximum height of fracture propagation is 150 m, in which the height of the caved and the fractured zones are 80 m and 70 m, respectively. These fractures start to be compacted and are re-closed at the distance of 80 m behind the working face. The extraction of the panel No.8100 results in the formation of a gas-bearing zone (GBZ) with separations and shear fractures rather than connecting the goafs of the carboniferous coal seam (CCS) and the jurassic coal seam (JCS). The GBZ is located in a region from 80 m to 150 m above the goaf and 80 m behind the working face. Based on the above findings, effective gas drainage approach is proposed to reduce the concentrations of the hazardous gases consisted of CH4 and CO. After gas drainage, the maximum concentrations of CH 4 and CO drop to 0.5% and 16.63 ppm, with reductions of 53.7% and 33.9% respectively. • Fracture evolves in two stages, i.e., fracture propagation and fracture re-closure. • The CCS goaf did not connect the upper JCS goaf. • The extraction of the longwall panel results in the formation of a GBZ. • EH-4 detection provided important geological information for coal mining. [ABSTRACT FROM AUTHOR]

Coal pillars are formed by excavation and mining activities in an intact coal seam and play a key role in underground coal mines. Most previous investigations of pillars have mainly focused on existing coal pillar failures. However, few scholars have investigated the failure mechanisms of coal pillars during their formation process. This paper focuses on the failure of a 7 m-wide coal pillar that caused the large deformation of a tailgate in the Sijiazhuang coal mine in China. Field tests and numerical modeling were used to study the initiation, propagation, and failure of cracks within this gob-side coal pillar during its formation. Field monitoring revealed that the maximum roof-to-floor and rib-to-rib convergence reached 860 mm and 1460 mm, respectively. The coal pillar became a yield pillar with substantial fractures. A numerical model was built using UDEC Trigon logic and calibrated with laboratory tests and RQD methods. Both the natural roadway deformation and crack distribution in the coal pillar were simulated. A FISH function was used to document the propagation of shear and tensile cracks in pillars with different W/H ratios, and a damage parameter was adopted to evaluate the failure of these pillars. The results suggest that the most appropriate pillar width is 10 m. Field trials prove that a 10 m-wide coal pillar combined with optimized support measures can effectively control deformation around the tailgate. [ABSTRACT FROM AUTHOR]

• Roof collapse accident caused by cable bolt shear rupture. • Horizontal displacement of rock strata leads to cable bolts shear rupture. • Increasing the rock bolts pretension load can lower the shear force of the cable bolts. • Improving compatibility of support fittings is conducive to exerting the pretension load of rock bolts. Roof collapse of mining roadways has always been key in the studies of the surrounding rock control. One of the critical factors lies in the roof separation and horizontal displacement which led to the cable bolt shear rupture, resulting in the roof fall. This study investigates the roof collapse accident that occurred at the No.15028 tailgate of XinJing coal mine in China. In-situ observations, numerical simulation and laboratory test are conducted to analyse the gradual deterioration of the roof strata. The field observation of the fractures, separations, pretension load and rock bolts loading are conducted during the process of separation. The analysis results indicate that owing to the torque value of rock bolts below 150 N·m (pretension load was 29 kN) and incompatibility of reinforcement fittings of rock bolts, the pretension load failed to solidify the surrounding rocks effectively. Obvious separations were identified at rock depths ranging from 0 to 2.6 m. Simulation results revealed that different flexural deflections caused horizontal displacement in rocks, which is positively correlated with the vertical separation. In weak interlayers of rocks, the severe horizontal displacements led to shear fracture of cable bolts, resulting in roof collapse. An increasing rock bolt pretension load can reduce the horizontal displacement of the rock stratum beyond the anchorage zone, which can lower the shear force of cable bolts by 30% and reduce the shear fracture risk of the cable bolts. Mechanical compatibility tests of the surrounding rock support system are conducted to develop a practical and reasonable reinforcement fittings to increase the pretension load of the rock bolts. Based on the research, a series of control methods are developed to enhance the roof stability. The field tests proved that the newly proposed methods can effectively improve the stability of the roof and eliminate roof collapses. [ABSTRACT FROM AUTHOR]