Sublevel caving mining method, mining because of its safe, simple structure, high degree of mechanization, small prospective mining engineering, large mining intensity, high efficiency ore outstanding advantages, has been widely used in metal mining in. The sublevel caving method without bottom column is to carry out mining and ore mining under the condition of loose rock covering. The formation of the covering layer can buffer the large area of ​​the roof, and it is a necessary condition for ore extrusion blasting. . The ground pressure phenomenon in the sublevel caving method without pillars is mainly manifested in the approach and the contact road. The pressure of the recovery tunnel is derived from the collapsed ore and un-falling ore covered by it [1]. As the depth of mining increases, the thickness of the ore-covered rock mass increases on the way. Whether it is a bottomless column or a bottom column, it will withstand the pressure from the waste rock or loose ore filled in the mining site. The high dynamic stress suddenly generated during the mining is always much larger than the corresponding static stress, which has a great influence on the engineering structure of the ore body [2]. The sublevel caving method without pillars is to release ore under the overburden, and is subjected to the original rock stress field before the excavation. The ore is in the original rock stress state, with the excavation of the approach and the mining. A change in the vicinity of the mining face forms a stress reduction zone and a zone of increase, and the stress increase and decrease zones continue to move with the advancement of the mining face [3]. During the recovery, in addition to the static stress, it is also affected by the movement of the inlet support pressure zone and the blasting vibration during the mining and mining. The service period is short, the force is complicated, and the order of recovery is affected, which constitutes the speciality of the stability of the route [4]. In this paper, the PFC2D software is used to establish the submerged caving method for the sublevel caving method. By simulating the two different mining sequences, the variation law of the pressure lowering area and the increasing area of ​​the ore body along the working surface and the side wall of the ore body are obtained. The change of dynamic pressure during ore mining plays a certain reference role in the support of mine mining access. After the model is established, the particles in the model reach equilibrium. Considering that the particles generated by the model are more and the computing power of the computer, the particles and the system are balanced in 7 times. Each time a part of the particles is generated to make the system reach a relative equilibrium and then regenerate. In another part of the particle, the average unbalance force of the system has 7 large fluctuations. When the whole particle is generated, the model system is run to the final equilibrium, and the average imbalance of the system changes step by step. When the model reaches equilibrium, the top of the ore body will also be stabilized. The initial force on the top of the ore body is shown in Figure 2. Scheme II records the force in the Y direction at the top of the ore body. The top force of the ore body with the ore discharge is shown in Fig. 5. Analysis of the data yields: (1) There is a pressure reduction zone at the first step from the ore discharge, which is within 3 steps of the working face. The pressure rise zone is within 6 steps of the pressure reduction zone. Same as the scheme I, but the range of the pressure stable zone is increased. (1) With the increase of the depth of the sublevel caving method, the thickness of the overburden layer also increases, and the dynamic stress of the ore rock under the cover layer will seriously affect the stability of the mining approach.
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1 model establishment
1.1 The ore particle flow model The basic idea of ​​the particle flow model theory comes from molecular dynamics. It is a tool to study the mechanical properties and behavior of the medium from the microstructure and reflect the mechanical behavior of the medium as the mechanical behavior of the particle assembly. In PFC2D "particles" are considered to be rigid, in accordance with Newton's second law of motion. The contact model of particle flow can be divided into contact stiffness model, contact sliding model, bonding model (contact bonding model, parallel bonding model). The contact bonding model is adopted in this paper. The mesoscopic parameters of the particles in PFC are often very different from the macroscopic parameters in the usual sense. Therefore, a large number of numerical experiments are required to obtain the macroscopic parameters of the particles. However, there is no perfect theory based on the microscopic parameters between the particles. Inferring the macroscopic parameters of the medium, the analytical method of repeatedly adjusting the microscopic parameters is usually used to inversely calculate the macroscopic mechanical parameters of the medium. The comparison between the macro parameters obtained by numerical simulation and the actual parameters is shown in Tables 1 and 2.
1.2 Mining model The “wall†is used as the boundary of the model. The width of the model is 100m and the height is 124m (including the height of the waste rock covering layer is 100m). The model "wall" is segmented so that the force on the "wall" is monitored every 50 steps. Control them to achieve step-by-step ore mining, the mining step distance is 4m, a total of 12 steps. The model generates a total of 23,046 particles. According to the above microscopic parameter calibration, the particles are assigned and balanced under the action of gravity acceleration. Finally, the stress, velocity and displacement of the sphere are initialized to obtain the ore-mining model (see Figure 1). The center axis of the ore body is the Y axis, and the mining path is the X axis, 50m on each side. 
1.3 simulated ore plan
In the model, the initial force on the top of the ore body is not evenly distributed, and the overall height is low on both sides. Wei Qun [6] used the particle discrete element method to simulate the pressure at the bottom of the silo. The result is large in the middle and small on both sides. Due to the variability of the formation of the bulk, the position of the peak points is inconsistent, but it is generally consistent with this simulation.
This simulation uses two different ore-producing sequences. Scheme I is for simultaneous mining of the two sides to the middle. Scheme II is forecasting from left to right, and different order mining methods are realized by deleting the corresponding walls and particles.
2 Simulated ore mining and stress analysis After the initial stabilization, the entire model is in a state of stress balance before the mining is started. Once the ore is started, the equilibrium state of the force in the model is broken, and the particles are subjected to the resultant force and moment, and the change of velocity and spatial position begins. Using the commands in the PFC, the particle position is automatically detected. When the particles fall into the interval of the access space, the process of blasting without the bottom sublevel caving is simulated by deleting the particles. According to the commissioning, it is determined that the particles are removed at 15,000 steps per cycle. In order to save space, the first step and the ninth step are recorded respectively in the first step and the ninth step from the beginning of the ore discharge to the end wall pressure and the top pressure of the ore body (see Figure 3 and Figure 4).
Analysis of the data yields:
(1) The maximum value of sidewall pressure during mining is abruptly increasing and decreasing. This is due to the opening of the ore discharge at the time of ore discharge, the pressure arch on the ore discharge, and the formation of pressure arch at different heights during the ore discharge. With the process of disintegration [7], it can be seen that when the pressure is maximum, a pressure arch appears on the side wall.
(2) When the first step is from ore mining, the pressure in the X and Y directions of the sidewall decreases gradually with the ore-extracting. When the ore is finished, the X and Y directions of all the measuring points on the sidewall are generally smaller than the initial stress. 3(a), (b), this is because the amount of loose rock mass on the side wall of the first step is not enough to form a complete pressure arch.
(3) With the progress of mining, the force of the side wall in the dynamic process of the ninth step is repeated, and the sidewall pressure curve is abrupt with the formation and disintegration of the arch. . As the ore-mining progresses, the magnitude of the force in each of the X, Y directions fluctuates and generally increases.
(4) The pressure at the top of the ore body is not evenly distributed, but it is roughly small in the middle and large, showing fluctuations. As the ore is discharged, the top pressure of the ore body gradually transforms into a pressure reduction zone, a pressure rise zone, and a pressure plateau zone. The pressure reduction zone is extended to one side by the ore discharge port, and the pressure zone rises sharply to the maximum value after the pressure zone is lowered. The pressure plateau is after the pressure rise zone. The first step is within 3 steps from the working face of the pressure reduction zone at the time of the ore release, the pressure rise zone is within 6 steps of the pressure reduction zone, and the ninth step of the ore reduction pressure zone is basically disappeared.
(2) The ninth step disappears from the pressure reduction zone when the ore is discharged.
(3) With the progress of the ore-mining, the pressure above the middle of the ore-outway has increased, but the degree of increase is significantly smaller than that of Scheme I.
3 conclusions
In this paper, PFC2D is used to simulate the sidewall pressure and top pressure of the ore body during the whole process of ore-covering when the height of the overburden is 100m, which better reflects the pressure variation of the sidewall and the top during the ore-mining.
(2) In the ore-mining, the pressure of the side wall is one after another, and the force on the side wall of the ore body is a cyclic loading process.
(3) With the progress of ore-mining, the pressure of the side wall of the ore body will increase, and the dynamic load of the sidewall pressure will often lead to the instability and collapse of the high-stress roadway. The mining approach near the work surface should be strengthened.
(4) In scheme I, the first step is from the top of the working surface of the ore body at the top of the pressure reduction zone, and the pressure is increased within the 6 steps. In the ninth step, the pressure drop zone at the top of the ore body disappears, and the top pressure of the ore body is greater than the initial force of the top of the ore body. It can be seen that as the working face advances, the pressure reduction zone gradually decreases until it disappears, and the top of the ore body will gradually increase in the next few steps. In order to prevent the mining road from falling to the top, and to help the secondary mining, the secondary access road should be supported.
(5) By comparing the top force curves of the mines of schemes I and II, the stresses on the top of the ore bodies at the top of the two sides of the scheme will be superimposed, and the maximum pressure will reach twice the pressure in the middle of the mine.
references:
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[3] Wu Shaohua. Analysis of the force-bearing process of the sublevel caving method without pillars [J]. Chemical Mining Technology, 1993: 16-18.
[4] Li Zhaoquan, Gong Qinrong. Research on stability and recovery approach support selected Chengchao iron ore [J]. Metal Mine, 1994 (12): 22-24.
[5] Tian Ruixia, Jiao Hongguang. Application status and analysis of discrete element software PFC in mining engineering [J]. Mining and Metallurgy, 2011, 20 (1): 79-80.
[6] Wei Qun. The basic principle of the discrete element method numerical method and program [M]. Beijing: Science Press, 1991.
[7] Zhu Huanchun. PFC and its application in mine caving mining research [J]. Journal of Rock Mechanics and Engineering, 2006, 25(9): 1928-1931.
[8] Wang Peitao, Yang Tianhong, Liu Xiaobo. Numerical simulation of particle flow in the influence of edge angle on the unloading of sublevel caving method without pillars [J]. Metal Mine, 2010 (3): 12-16.
Article source: Mining Technology; 2017.12(1)
Author: Wang Chenglong, Wang Ying; Inner Mongolia Jin Tao Ltd., Chifeng City, Inner Mongolia 024 327
Teng Fei , Yuan Shuai; Changchun Gold Design Institute, Changchun, Jilin 130000
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