工程科学与技术 (Jan 2024)
Characteristics and Evolution Mechanism of Uneven Subsidence of Overburden during Multi-layer Mining of Metal Mines
Abstract
ObjectiveThe multi-layer mining of metal mines can produce multiple superimposed disturbances on the overburden, causing deformation, damage, settlement and even collapse of the roof surrounding rock of the goaf, which poses a threat to the mine construction and underground production. At the same time, this mining method provides conditions for the interaction of goafs at different levels, making the behavior of overburden pressure extremely complex, as well as the law of overburden settlement. Therefore, it is a long-term challenge to describe the response mechanism of overburden during the multi-layer mining. To contribute to this filed of research, this study explores the migration law and the evolution mechanism of the overburden during multi-layer mining of metal mines.MethodsIn this study, the multi-layer mining of thick and large orebody in Baixiangshan Iron Mine was taken as the research background. Firstly, the large-scale similarity model test and the digital photographic deformation measurement technology were used to reveal the characteristics of stress change and the movement law of the overburden during multi-layer mining. Then, the 3D geomechanical model of the thick and large orebody in Baixiangshan Iron Mine was established. Based on the discrete element software 3DEC, the movement of overburden and the evolution of fracture zones during multi-layer mining were further simulated. And according to the characteristics of uneven settlement of overburden and the disturbed degree, the surrounding rock was divided into three different regions. Finally, based on the damage theory, the evolution mechanism of uneven settlement of the overburden during multi-layer mining was revealed.Results and Discussions In the model test of multi-layer mining, the displacement of overburden showed a gradually increasing trend on the whole with the deepening of mining. Since the mining sequence was from the right to the left side, the compression of the right filling body was more significant than that of the left. As a result, the subsidence of overburden on the right side was significantly greater than that on the left side, showing the characteristics of uneven subsidence. When the first-layer mining was completed, the Fault F2 was initially activated, and the overburden on the right side of the fault subsided as a whole (the maximum subsidence was about 0.33 cm). The mining of the second-layer was carried out on the basis of the first-layer mining, so the migration of the overburden was obviously further intensified, and the maximum settlement center also shifted from right to left. After the second-layer mining, the maximum settlement center transferred from the right side of the mining area to the center of the second layer goaf, and the maximum displacement increased to about 1.33 cm. Since the third-layer mining was carried out on the basis of the disturbance of the first two layers, the superposition effect of the three-layer mining made the response of the rock mass more intense. With the collapse of the overburden near the Fault F2, the maximum settlement area completely moved to the vicinity of Fault F2, and the maximum settlement reached about 1.74 cm. At the same time, the entire overburden on the right side of the fault undergone overall dislocation settlement along the Fault F2, and the fault was also completely activated. The numerical simulation results showed that the vertical displacement field of the overburden developed into an arch shape at the beginning of mining. Influenced by the Fault F2 on the left, the arch displacement field was slightly tilted to the right. With the continuous multi-layer mining, the vertical displacement of the roof surrounding rock gradually increased. At the same time, the original arch displacement field was gradually extended to the surface and changed from arch to quadrilateral. However, the surrounding rock near the goaf still maintained an arched displacement field and continued to propagate in the rock mass above the goaf, resulting in the continuous expansion of the overburden subsidence. In the early stages of mining, some fractured units appeared on both sides of the mining area. With the continuous mining, the number of fractured units on both sides of the mining area was further expanded, but the fractured units were mainly concentrated on the right side of the Fault F2 and increased upward along the fault. In the later stage of mining, however, the vertical fracture units increased and developed horizontally in an arch-shaped distribution. With the continuous mining, the number and scope of fractured units further increased, and a large number of connected fractured units appeared on both sides of the mining area, which finally accumulated to form the fracture zones. From the above results, it could be seen that in the process of multi-layer mining, the roof surrounding rock of the mining area was the repeated disturbance area, and it was also the area with the largest settlement. The displacement of the overburden outside the disturbance area was very small, so there must be a transition area, in which the difference between the displacements on the two sides was so large that the rock migration fractures were generated. As a result, the overburden above the mining area exhibits the characteristics of uneven settlement.ConclusionsIn the similarity model test of multi-layer mining, the displacement of the overburden during the first-layer mining was small, but when mining the second and third layers, the displacement of the overburden increased significantly under the superimposed mining disturbance, ending with the large-scale collapse near the Fault F2. Although the multi-layer mining caused the activation of Fault F2, as well as the overall subsidence of the overburden on the right side of the fault, the Fault F2 also blocked the left propagation of stress and deformation to a certain extent. The numerical simulation results showed that the subsidence of the overburden near the middle of the goaf was the largest, which gradually increased with the continuous multi-layer mining. Since the damage of the surrounding rock above the mining areas was continuously superimposed during the multi-layer mining, the overburden was unevenly settled due to the different damage degrees in different areas, which led to the fracture zones on both sides of the goaf.
