The coal roof of the lower seam of a bifurcated coal seam is always broken and is caused by the mining of the upper seam, which results in difficult support for the coal roof. This seriously affects the safe and efficient mining of the lower seam. Pre-grouting technology is often used to reinforce the regenerated roof in advance to improve its integrity of the roof. In this study, to provide a scientific basis for the layout of surface pre-grouting boreholes, the bifurcated coal seam at the Xutuan coal mine in the Huaibei mining area in China was analyzed. The height and depth of the damage in the coal roof and floor caused by upper seam mining were calculated using theoretical analysis and numerical simulation. The porosity and permeability of the goaf in different positions were calculated by numerical simulation, and the spatial distribution of the porosity and permeability and the slurry diffusion radius of different porosities were determined. Finally, the grouting section and layout of the pre-grouting boreholes were determined. Based on the design scheme, the drilling of the surface boreholes and the addition of the grouting were carried out. The accuracy of the theoretical analysis was verified by field drilling and grouting activity. The research results can provide a basis for the layout of surface pre-grouting boreholes and the selection of the grouting zone in areas of close-distance coal seam mining.
In recent years, the structural evolution characteristics of the regenerated roof of the lower coal seam have become a research hotspot when the bifurcation coal seam is mined downward. In this paper, taking the bifurcation coal seam of Xutuan Coal Mine in China as an example, the structural evolution characteristics of regenerated roof under the influence of mining in bifurcation coal seam are comprehensively studied by theoretical analysis, field measurement, and indoor similar simulation experiment. The stress transfer law in the floor after mining in the upper coal seam is also analyzed. The results show the overburden structure and stress field change caused by upper coal seam mining. The caving and fracture zones are formed in the roof, the average height of the caving zone is 8.28 m, and the one of the fracture zone is 34.91 m. The results of the field test verify the accuracy of theoretical analysis and similar simulation test results. According to the relative size of the depth of the strong failure zone of the coal seam floor and the coal seam spacing, the rock mass structure of the regenerated roof of lower coal seam is divided into three types: fractured rock mass + scattered rock mass (I), fractured rock mass + scattered rock mass + fractured rock mass (II), and fractured rock mass + bulk rock mass + fractured rock mass + layered rock mass (III), and the stability of the three types of regenerated roof structure is evaluated: III > II > I. The research in this paper can provide a theoretical basis for determining the target area of broken roof control under the mining conditions of bifurcation coal seam and provide guidance for the selection of the location and parameters of the grouting borehole for roof reinforcement.
During downward mining of a bifurcated coal seam, the roof of the lower coal seam is relatively broken and difficult to control due to the mining influence of the upper coal seam. Roof accidents occur frequently during mining of the lower coal seam, reducing mining efficiency. How to ensure safe and efficient mining of the lower coal seam is a significant issue. In this paper, overlying strata migration and fracture characteristics of the lower coal seam, the structure and stability of the regenerated roof, and porosity and permeability characteristics of the overlying strata under the mining influence of the upper coal seam are studied by using similar simulation tests. Results show that the overburden structure of the lower coal seam is altered due to the mining influence of the upper coal seam, and the regenerated roof of the lower coal seam is divided into three structural types from top to bottom, namely: intact rock mass+block fracture rock mass+loose rock mass (type I structure); intact rock mass+block fracture rock mass+loose rock mass+cataclastic rock mass (type II structure); and intact rock mass+block fracture rock mass+loose rock mass+cataclastic rock mass+slab-rent rock mass (type III structure). The stability of each type of rock mass structure is evaluated, and the stability of three types of rock mass structures is III > II > I. The overburden porosity and slurry permeability coefficient are relatively large at the cutting hole and stopping line. The porosity of the caving zone within 70 m of the cut hole and stopping line is greater than 5%, and the permeability coefficient is greater than 0.1 m/s. Based on differences in the surrounding rock porosity and permeability characteristics, the grouting difficulty of overburden is divided into three types of areas: extremely easy grouting areas, easy grouting areas, and difficult grouting areas. The results of this paper can provide reference for the stability evaluation of the regenerated roof and the selection of grouting treatment parameters for the broken roof under similar conditions.
In recent years, bed separation water inrush, as a new type of water disaster, has posed serious threat to mining safety in China’s many coal mines. This study focused on periodic water inrush accidents in the No. 7335 working face owned by Xuzhuang Coal Mine, Jiangsu Province, China. Based on the theory of key strata, the formation positions of the separation layer in the overlying strata were first identified; next, the trapezoid platform model of the fracture of the overlying strata was introduced for illustrating the evolutional process of the bed separation space and accurately calculating the position and the geometrical morphology of the bed separation space. On that basis, the relation between the fracture of the overlying stratum and the inrush of bed separation water was revealed with the use of the plate theory. Moreover, the prevention and control measures of the bed separation water were formulated. Research show that the arrangement of the diversion drilling holes can successfully eliminate the threat of the bed separation water to the lower working face.
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