First, the pressure on the mine roadway in the bottom column 
The bottom-column caving mining method uses the ore to be released through a mineway provided in the bottom column. Therefore, the stability of the mine roadway in the bottom column directly affects the safety and economic benefits of the mining work. During the mining process, the bottom of the electric column was damaged due to excessive pressure, so that the ore could not be released. For example, the Yangjiazhangzi Mining Bureau Lingqian Mine North-150m stage, the S4, S5, S6 ore blocks of the No. 0 vein VI ore body were damaged due to the ramp, and the ore could not be recovered. Therefore, the ground pressure control problem of this mining method is mainly to maintain the stability of the mining roadway.
The pressure on the bottom column is different at different stages of the recovery.
The first stage: after the mining block is applied, the upper part of the electric roadway is still solid, although there is pressure on the bottom of the ore block at this time, because the unmined ore body has a certain bearing capacity. Therefore, the pressure on the bottom column is small.
The second stage: after the ore falls, the loose ore collected is accumulated in the upper part of the bottom column. At this time, the bottom column not only has to withstand the pressure of the ore's own weight, but also bears the pressure of the overlying collapsed rock to transfer the ore. Therefore, the pressure is significantly increased compared with the first stage.
The pressure acting on the bottom column consists of two parts: one is the average pressure Pm of the caving rock acting on the upper part of the recovery stage; the other is the average pressure Pc of the mining ore acting on the bottom column.
The average pressure Pm of the caving rock acting on the upper part of the recovery stage can be calculated as follows:
In the formula:
Γ-falling rock capacity, t/m3;
M-orbital horizontal thickness, m;
K-reflects the physical and mechanical properties of the caving rock and its interaction coefficient with the boundary wall conditions, which is determined experimentally. Soviet Kerry live Rogge mining area K = 0.6;
B-geometric parameter, B = tga · tgβ / tga + tgβ (a is the inclination of the ore body; β is the angle of the upper plate collapse);
H- mining depth, m.
When the mining depth is not large, the calculation results according to the above formula are not much different from the data determined by laboratory tests (Fig. 1). When the mining depth is increased, the average pressure Pm of the caving rock acting on the mining stage is smaller than the weight γH of the caving rock column.
Figure 1 Relationship between Pm/γH and H/M
The pressure Pc of the ore that acts on the bottom column in the ore block can be calculated as a model with vertical walls. The pressure Pc that the ore is applied to the bottom column is then:
In the formula:
A-factor, A=λƒÏ/F;
Λ-side pressure coefficient;
F-level size of the nugget, m2;
H-stage height, m;
Ρ- ore block perimeter, m;
Æ’ - Take down the coefficient of friction between the ore and the side wall.
The average pressure P acting on the bottom column is the sum of the above two partial pressures, ie
The symbol in the formula is the same as before.
When the ore is released from the ore, the ore is rubbed between the ore and the unmined ore wall, so that the pressure distribution on the bottom column is uneven, and the pressure at the edge of the stope is low, and the center pressure is high (Fig. 2 ).
Figure 2 Pressure distribution on the bottom column
It can be seen from the formula that the average pressure P acting on the bottom column is related to the physical and mechanical properties of the ore, γ, Æ’, the height of the ore layer, and the horizontal size (Ï, F) of the ore. According to the production practice and laboratory tests, the pressure is large when the horizontal area of ​​the nugget is square, which is unfavorable to the stability of the mining roadway. The nuggets with a side length ratio of 1:3 to 1:4 have greater stability.
For example, when the section height is 35-40 m and there are 2 to 3 ramps in one ore block, the pressure values ​​acting on the bottom column under different ore body thickness and mining depth are listed in Table 1.
Table 1 Relationship between pressure on the bottom column and mining depth and ore body thickness
Mining depth (m) | γH (MPa) | Ore body horizontal thickness (m) | |||||
25 | 50 | 100 | |||||
Pressure on the bottom column P (MPa) | |||||||
P | Pc | P | Pc | P | Pc | ||
100 200 400 600 800 1000 | 3 6 12 18 twenty four 30 | 1.40 2.25 3.40 4.20 4.60 4.80 | 0.67 1.0 1.9 2.2 2.5 2.6 | 1.90 3.0 4.7 5.9 6.9 7.75 | 1.5 2.2 3.4 4.4 4.8 5.5 | 2.1 3.4 6.0 7.75 9.20 10.3 | 1.7 2.5 4.5 5.7 7.0 7.7 |
Theoretical analysis shows that after excavating the bottom space in the compressive stress field, the vertical stress is arched in the upper and lower R regions, and the stress is formed near the end of the bottom space U, that is, the arch C. Concentration zone (Figure 3, Figure 4).
Figure 3 Mechanical effects after the bottom of the nugget
(The dotted line in the figure is the range of the pull-down effect, and its height is 1/8 of the width of the bottom.)
Figure 4 Increased stress in front of the bottom space
1-measured pressure curve; 2-calculated pressure curve; 3-pull space
Jinshan store iron ore stress concentration factor of 2 front undercut. This has a certain influence on the stability of the mine roadway in the bottom column, and should be considered when calculating the roadway support strength.
The third phase. As the ore is discharged, the pressure acting on the bottom column is lowered. This is because, with the ore discharge, the ore in the upper part of the funnel is loosened twice, and a loose, ellipsoidal shape is formed in the upper part of each of the ore hoppers. Loose body. The ore inside the ellipsoid is loosened and is free from the load transmitted from the upper part, forming a pressure-free arch on the upper part (Fig. 5). The load of the loose ore in the upper part of the arch is transmitted to the nearby funnel, and the pressure in the upper part is raised. In the range of loose ellipsoids, the pressure in the upper part of the concentrating funnel is lowered and a pressure-reducing zone appears.
Figure 5 Pressure change in the upper part of the funnel during ore discharge
1-release body; 2- loose body; 3-stress transfer direction;
A-stress reduction zone; b-stress rise zone
The ore discharged from the funnel is intermittent, so the pressure-free arch is pulsating at the same time interval in the flow belt, causing the horizontal support pressure of the pulsation. According to laboratory simulation studies, the pulsation level pressure has the following relationship with the height of the collapsed ore layer, the diameter of the funnel, the bulk density of the ore, and the ore size of the mined:
In the formula:
PH-pulsation level pressure, 10-2MPa;
H-mining the height of the ore layer, cm;
D-mineral leakage diameter, cm;
Γ-take ore bulk density, g/cm3;
Lmax - The maximum edge size of the ore block, cm.
In order to ensure the stability of the outflow roadway of the bottom column, in addition to selecting the appropriate type of support according to the mechanical properties of the rock mass of the bottom column, it is necessary to determine the size of the ore block reasonably. According to the experience of the Soviet Union in the use of caving mining methods, properly reducing the size of the nuggets can significantly reduce the pressure on the bottom column. For example, when the Dzerzhinsky mine reduces the size of the nugget to 40 m, the pressure acting on the bottom column during the ore mining process is equivalent to the weight of the collapsed rock column of 3 to 4 times the width of the nugget, with an average of 2.9 to 3.8 MPa. , is 56% to 73% of the weight of the collapsed rock column.
In addition, the method of increasing the strength of the ore can be used to reduce the pressure acting on the bottom column. Figure 6 depicts the relationship between the pressure and the ore-extraction strength of the Soviet Dzerzhinsky mine column. Increasing the ore-removing intensity can reduce the pressure on the bottom column, reduce the existence time of the mine roadway, and maintain the stability of the bottom column. After the mining site begins to mine, the mining operation should not be interrupted at will. Stopping the ore mining will increase the pressure on the adjacent stope and make the mineway roadway in the bottom column collapse. For example, the Derzhinski Mine No. 1 ramp stopped the mine. At the beginning, the pressure on the adjacent No. 10 ramp was reduced by half, but then the pressure was increased to 5.5 MPa, which is equivalent to the weight of the overburden (210m). high).
Figure 6 Relationship between pressure on the bottom column and the strength of the ore
1-mining intensity; 2-pressure
The three stages noted above can be confirmed from measured data on the pressure changes experienced by the bottom column of a mine using the caving mining method in the Soviet Union.
The alpine mine in the Ural region of the Soviet Union is a skarn type iron deposit with an average thickness of 25-40 m and a dip angle of 40-48°. The upper plate is skarn, the uniaxial compressive strength is 86MPa, the lower plate is diorite , and the uniaxial compressive strength is 120MPa. The ore has a compressive strength of 130 MPa and a bulk density of 40 t/m3. The mining depth is 140m. The stage forced collapse method is adopted. A load cell is embedded in the midsole column and measured by the load cell:
1. After the ore collapses, the pressure of the bottom column is 3.5-3.8 MPa;
2. In the initial stage of ore-leading, the pressure on the bottom column is 2.6 MPa, and the pressure is reduced by 35% (the ore-concentration intensity is 2 to 2.5 t/m2·d);
3. After 45 days of mine release, the pressure of the bottom column is 2.75 MPa.
The pressure (γH) was calculated to be 4 MPa according to the height of the collapsed ore layer. It is larger than the measured value.
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