Surface filling during the mining of steeply inclined thick coal seams is an efficient method for restraining disasters caused by the cascading movement of overburden rocks. This study aims to control rock damage during the mining of thick coal seams steeply inclined at typically more than 45° in fully mechanized coal caving work surfaces with high section heights. Based on the green mining concept, we analyzed the movement of roof strata after filling using multiple methods, including field investigation, theoretical analysis, numerical calculation, and field monitoring. Results show that, in dynamic mine disasters caused mainly by complex coal conditions and strong disturbances in fully mechanized coal caving in large sections, the strength of the filling material is dependent on the features of the surrounding rock and burial depth. Also, the mining-induced peak stress shows a linear increase after filling, with the goafs in stress-free conditions, and failure zones occur in the roof and floor strata after mining. The stability of the rock pillars and overburden strata are better, and there are no large-scale tensile fissures in the ground surface. We adopted an intelligent underground radar detection technique that can reflect the rock-failure characteristics through the propagation characteristics of the electromagnetic spectrum. The detection results show that the coal goafs were filled properly as they were matched with the caving roof, which will collapse along with the release of the top coal, with the filling body able to move downward along with the discharge of top coal. The use of surface filling can restrain the dynamic disaster induced by a fully mechanized coal caving surface with a large section when mining steeply inclined thick coal seams, thereby ensuring safety and promoting the use of green mining practices.
The coal column undergoes three types of force evolution from the formation to the end of mining. This paper takes large mining height working face at No.2 Coal Mine as example to study the ways to avoid dynamic instability of the coal column triggered by the deep mining. By means of geological survey, theoretical analysis, numerical calculation and field verification, the load processes under the three stress stage are proposed, and the evolution law of the coal column is analyzed. The study shows that the depth, large mining height working face, coal pillar force and size altogether determine the damage characteristics of the coal pillar. With the combination of Flac3D and 3DEC, it can be analyzed that the plastic failure and displacement characteristics of the 35m coal column under the action of secondary dynamic load coincide. The perturbation stress distribution is stable, which finally determines the reasonable width of the 35m coal column. Field measurements show that the top and gang of the 35m coal column undergo three kinds of displacement characteristics. The lower part is more stable. The top plate of the upper and lower corner completely collapsed in the emptying area, which can play a good supporting role.
Static fracturing technology uses chemical expansion agents to fracture roofs. With the aim of fracturing corner roofs on deep working faces, in this study, the static fracturing technology was investigated through theoretical analysis, laboratory experiments, numerical calculations, and field practice. The theoretical analysis and experiments demonstrated that the swelling force increased with a decrease in the fracturing hole spacing, and the optimal water-cement ratio was 0.33. Twelve groups of FLAC3D models were designed using SPSSAU. The results revealed that the optimal fracturing effect was achieved when the hole diameter was 60 mm, hole spacing was 40 cm, and hole depth was 6 m. The fracturing effect of hard corner roofs was monitored by peering into the borehole and evaluating the support resistance. Thus, it can be concluded that within the fracturing range, internal fissures in the rock stratum are developed and linked to each other. The support pressure was the highest, 7 h after grouting, with a value of approximately 26.1 MPa, and then decreased gradually to 17.58 MPa, indicating that the static fracturing technology attained the expected results.
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