Search Results (3)

The collapse of thick-hard roofs after coal has been extracted is not a consequential process in all cases. Rather, it happens due to the augmentation of high stress conducted at depth, followed by a wider range of damage as the floor cracks. The extent and spread of the cracks in the floor indicate the intensity of the collapse, and the mine will be submerged by the high-pressure water of the coal ash. Therefore, it is particularly important to study the mechanism of the combined effect of high stress on the roof and confined aquifer on the deformation and failure of the coal seam mining floor. This study analyzes and compares the impact of thick-hard magmatic rocks on the destruction of thin floor rock layers in coal seams. Plastic theory calculations are used to determine the plastic zone yield length of floor destruction under hard roof conditions, and the location and height of the maximum floor destruction depth are solved. An empirical formula and BP neural network are used to establish a prediction model for floor destruction. The results of the model’s prediction of the depth of floor failure were compared with the measured values, with an absolute error of 2.13 m and a residual of 10.3%, which was closer to the true values. The accuracy of the theoretical model and prediction model is verified using numerical simulation and on-site in situ measurements. Based on this, the deformation and destruction forms of the floor under pressure and the water inrush mechanism are summarized for mining under the condition of a thick-hard roof. Thus, the floor is subjected to high vertical stress, accompanied by significant disturbances generated during coal seam mining, resulting in intense working face pressures. The floor near the working face coal wall will experience severe compression and shear deformation and slide towards the goaf. The floor in the goaf is relieved of high vertical stress, and horizontal stress compression will result in shear failure, leading to floor heave and further increasing the height of the floor destruction zone. After the mining of the working face, the goaf will undergo two stages of re-supporting and post-mining compaction. During the re-supporting stage, the floor rock undergoes a transition from high-stress to low-stress conditions, and the instantaneous stress relief will cause plastic deformation and failure in the coal seam floor. The combined action of primary floor fractures and secondary fractures formed during mining can easily create effective water channels. These can connect to the aquifer or water-conducting structures, making them highly dangerous. The main modes of floor water inrush under the condition of a thick-hard roof are as follows: the high-stress mode, inducing a floor destruction zone connected to the water riser zone; the mining damage mode, connecting to water-conducting faults; the mining damage mode, connecting to water collapse columns; and the coupled water inrush mode, between the mining damage zone and the highly pressurized water floor. Full article

Hard and thick magmatic rocks are widely distributed in many mining areas in China. Their fracture migration could cause mechanical effects such as the evolution of overburden structure, the sudden change of surface subsidence, and the transformation of accumulated elastic properties, inducing strong dynamic phenomena and even coupled geo dynamic disasters. In this study, by means of theoretical analysis and similar material simulation testing, the fracture of hard thick magmatic rock and the fracture development characteristics of stope are analyzed, and the following research results are obtained: (1) the mechanical model of an elastic foundation beam is established, and it is found that the bending moment in the middle of hard thick magmatic rock is greater than the bending moment at the end, and the magmatic rock first produces fractures in the middle, and then the initial fracture occurs. (2) The existence of hard thick magmatic rock blocks the development of fractures in the longitudinal direction. The bed separation and fracture undergo three processes of generation, development and closure. When the working face advances 160 m, 200 m and 270 m, the maximum bed separation shape on the strike section experiences triangle trapezoid crescent shape. (3) A ladder type fissure channel is formed above the working face side and the open cut hole side. When the working face is advanced 160 m, the ladder type fissure channel develops to the bottom of the hard thick rock layer, providing a ladder type channel for gas flow. After the hard thick rock layer is broken, a large number of elastic properties are released, which produces a strong impact force on the gas rich area of the bed separation, and also causes a sharp rise in the gas pressure of the bed separation at the bottom of the magmatic rock. A large amount of high-pressure gas in the bed separation space flows reversely to the working face along the ladder type gas channel, which can easily induce a gas outburst, coal and gas outburst or other disasters. (4) The correctness of the similar simulation experiment is verified by the field data. Technical measures for preventing gas outbursts in bed separation by surface drilling and pressure relief gas extraction are put forward. The research results have a certain guiding role for the prevention and control of dynamic disasters and the design of roadway support under the condition of a hard and thick key stratum overlying the working face. Full article

When the hard and thick key strata are located above the working face, the bed separation structure is easy to be formed after mining because of the high strength and integrity of the hard and thick key strata and the initial breaking step is large. After the hard, thick strata are broken, the overburden will be largely collapsed and unstable in a large area and the dynamic disaster is easily induced. In this study, considering the fundamental deformation and failure effect of coal seam, the development law of the bed separation and the fractures under hard and thick magmatic rocks and the mechanism of breaking induced disaster of hard and thick magmatic rocks are studied by similar simulation tests. The results of the study are as follows: (1) The similar material ratio of coal seam is obtained by low-strength orthogonal ratio test of similar materials of coal seam, that is, cement:sand:water:activated carbon:coal = 6:6:7:1.1:79.9. (2) The magmatic rocks play a role in shielding the development of the bed separation, which makes the bed separation beneath the magmatic rock in an unclosed state for a long time, providing space for the accumulation of gas and water. (3) The distribution pattern of the fracture zone shows different shapes as the advancing of working face and the fracture zone width of the rear of working face coal wall is larger than that of the front of the open-off. (4) The breaking of magmatic rocks will press the gas and water accumulated in the bed separation space below to rush towards the working face along the fracture zone at both ends of the goaf. The above results are verified through the drainage borehole gas jet accident in the Yangliu coal mine. The research results are of great significance for revealing the occurrence process of dynamic disasters and adopting scientific and reasonable preventive measures. Full article