Chenlin Wang

Under multiseam mining, the caved strata of upper gob are affected by the lower coal seam mining, and the caved strata of lower gob are affected by the gravity action of the caved strata in the upper gob, resulting in the morphology of caved strata that is more complex than single coal seam mining. In this study, a similar simulation experiment device is used to analyse the morphology, fracture development degree, and subsidence of caved strata in an abandoned gob under multiseam. The physical model of caved strata in an abandoned gob under multiseam is established. Results show that the morphology of caved strata in the abandoned gob is a trapezoid. The caved strata on two sides of the abandoned gob are mainly horizontal fractures, and the middle part is a compacted area. Large number of voids and fractures exist near the start and end mining positions, and they are connected with each other. Because of the influence of the lower coal seam mining, the fracture development degree and subsidence of caved strata in the upper gob increase. The average increase rate of the maximum subsidence and the peak abscission rate is 24·61% and 16·69%, respectively.

The gob is formed by large-scale voids left after the completion of underground coal mining. A large amount of desorbed gas still remains in the gob. Surface drilling is used to extract gas from abandoned coal mines. The location of surface drilling in abandoned gobs is the key to ensure efficient gas extraction. In this paper, an abandoned gob is divided into the start mining area, the middle area, and the end mining area based on the result of physical similarity simulation experiment. The distribution characteristics of methane concentration in the abandoned gob under the surface drilling extraction are calculated by using the numerical simulation software Ansys. The gas extraction effect in the start mining area, the middle area, and the end mining area is compared. Results show that the methane control range by extraction pipe presents different characteristics along the X-, Y-, and Z-axes of the abandoned gob. When Z = 55 m, the methane control range by extraction pipe reaches the maximum. When Y = 40 m and 60 m, the methane control range by extraction pipe reaches the maximum. In the start and end mining areas, the methane control range by extraction pipe reaches the maximum when X = 70 m and 30 m, respectively. In the middle area, the methane control range by extraction pipe reaches the maximum when X = 100 m and 150 m. The gas extraction effect is the best in the end mining area, followed by the start mining area and finally the middle area. The preferred location of surface drilling is the end mining area of the abandoned gob. The research results provide theoretical basis for gas extraction in abandoned coal mines.

The large-scale collapse of overlying strata in the gob directly affect the safe production of coal mines; they are also the major causes of geological disasters, such as ground cracks, surface subsidence, and ground collapse. In this paper, the movement and caved characteristics of overlying strata during coal seam excavation are studied by conducting a physical model experiment. Results show that overlying strata have different movement and caved laws during the initial, intermediate, and later mining stages. During the initial mining stage, overlying strata do not collapse, and the subsidence is extremely small. During the intermediate mining stage, overlying strata cave along the vertical direction, and caved height gradually increases. Large numbers of cavities, abscission layers, and fractures exist between caved strata. The fractured area gradually increases upward, and the subsidence increases considerably. During the later mining stage, overlying strata cave along the horizontal direction. The abscission layers between the caved strata of the central are compacted. The compacted area is surrounded by a fractured area. The compacted and fractured areas increase along the horizontal direction. The subsidence curves exhibit a horizontal variation. Overlying strata evolve from the self-equilibrium stage to the vertical collapse stage, and finally, the horizontal collapse stage. The fractured area changes from a no fractured area to a fractured area, increases vertically, and finally, increases horizontally. The subsidence curve changes from extremely small to large, and finally, changes horizontally.

A lot of gas resources remain in the abandoned gob. The overlying strata of the abandoned gob are the main places for gas storage and flow. The fracture distribution characteristics of the overlying strata have a significant impact on the gas migration. The mining similarity simulation test device of a plane stress was used to study the deformation and failure characteristics of overlying strata in an abandoned gob. The caved strata of the abandoned gob formed a trapezoidal distribution, and the caving range decreased gradually with an increase in distance from the coal seam. The strata collapsed in the caved zone, whereas the strata collapsed mainly on the bending subsidence in fractured zone. The subsidence curves of caved strata showed a lower concave shape, and the maximum subsidence existed in the middle of the abandoned gob. The caved strata subsidence decreased with an increase in distance from the coal seam. The horizontal fractures were dominant in the fractured zone. The abscission rate of the end mining position was greater than that of the start mining position. Large numbers of vertical fractures existed in the caved zone. The development degree of vertical fractures near the end mining position were larger than that of the start mining position, and the width of the gas-conducting fracture was more than three times that of the start mining position. The development degree, quantity and connectivity of the fracture in the end mining position were better than those in the start mining position.