Ground pressure control with bottom pillar caving mining method

    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 S ₄ , S ₅ and S ₆ 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 P _m acting on the rock falling in the upper part of the mining stage; the other is the average pressure P _c of the mining ore acting on the bottom column.

The average pressure P _m of the caving rock acting on the upper part of the recovery stage can be calculated as follows:

In the formula:

Γ-disintegrated rock capacity, t/m ³ ;

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 P _m /γH and H/M

The pressure P _c of the ore that acts on the bottom column in the ore block can be calculated as a model with vertical walls. Then the pressure P _c of the ore acting on the bottom column is:

In the formula:

A-factor, A=λƒρ/F;

Λ-side pressure coefficient;

F-block horizontal area, m ² ;

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, ƒ, the height of the ore layer, and the horizontal size (ρ, 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

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:

P _H - pulsating horizontal pressure, 10 ^-2 MPa;

H-mining the height of the ore layer, cm;

D-mineral leakage diameter, cm;

Γ-take ore bulk density, g/cm ³ ;

L _max - 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/m ³ . 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-release, 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/m ² ·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.

    2. Stress changes in ore bodies and surrounding rocks affected by mining

The caving mining method is used to recover thick and extremely thick ore bodies. When the mining depth is increased, under the influence of mining, not only the stress in the upper rock mass changes, but also the impact of the rock mass in the upper plate. The stress will also change. This stress is related to the mass of the prism that collapses in the upper disc and the speed of the downward movement. Applying the impact theory, the stress value can be approximated as:

In the formula:

L-the average sliding distance of the collapsed prism of the upper plate, m;

Contact area when S-falling prism is sliding, m ² ;

Æ‘-coefficient of friction;

Δ-collapse prism impact displacement, m;

M-the mass of the collapsed prism of the upper disc, kg;

G-gravity acceleration, m/s ² ;

Β-upper rock caving angle, (°);

C=cA;

C-bonding force, Pa;

A-upper plate collapses the prism cross-sectional area, m ² .

With the mining work, the stress-strain state of the rock mass within the mining influence will change spatially and temporally.

According to the field measurement and laboratory simulation test (centrifugal model test), the change law is as follows: before the start of the mining work, in the direction along the inclined direction of the ore body, according to the stress value, two upper areas can be divided: the unloading belt a And support the pressure belt b (Figure 7).

Figure 7: The division of stress bands within the influence range of mining

A-unloading belt; b-supporting pressure

In the unloading belt, since the upper part is the extracted area, there is a certain gap in the falling rock, allowing the lower disc sliding body to move upward. Therefore, in this range, the compressive stress in the reverse tilt direction is lowered to cause stretching. In the direction of the vertical direction, the compressive strain increases, and the compressive strain in the strike direction decreases, that is, expansion occurs. This is a condition that causes shear failure and destroys the rock mass.

The stress and strain values ​​in the bearing pressure belt and the unloading belt are different, and the direction of the bearing pressure belt is displaced, pointing to the lower disc and at an angle of 40° to the horizontal plane. As the depth of mining increases, the displacement increases and varies between 40 and 250 mm. The direction of displacement of the unloading belt points to the upper extracted area, which is nearly vertical and is 108° from the horizontal plane. As the depth of mining increases, the displacement changes from 60 to 100 mm.

As the depth of mining increases, the internal stresses of each zone increase in all directions. Uniform compression deformation occurs in the vertical direction, while tensile deformation occurs in the reverse oblique direction, both of which increase proportionally with the depth of exploitation. However, after the mining depth exceeds 600 m, the deformation obviously increases according to the curve relationship (Fig. 8). In the range of impact of mining work, when the mining depth is large, it also causes rheology and stress relaxation.

Figure 8 Relationship between strain and depth in each band

1-support pressure band compression strain; 2-unload belt tensile strain

    Third, the impact of the mining sequence on the ground pressure

The use of caving mining methods, the rational stage of mining, plays an important role in ensuring the safety of mining work and controlling ground pressure activities.

(1) Mining sequence along the strike direction

1. Single wing mining. In the single-wing mining sequence, the secondary stress field formed in the rock mass around the mining space depends on the width of the nugget. When the nugget width is increased from 30m to 150m, the stress concentration around the ore body increases by 0.5 to 3 times (Fig. 9a). When the width of the nugget is 30, 60, 90 m, the stress rise zone has a range of about 50 m. Outside 50 m, the stress returned to normal (Fig. 9b).

Fig. 9 Relationship between the range of stress and stress increase zone around the ore body and the span of the nugget

2. Mining from the two wings to the center. At the beginning of the stage, the ore blocks located in the two wings are far apart from each other, and the secondary stress fields formed around the goaf do not affect each other. The stress field distribution is the same as for single-wing mining. However, when the two ore nuggets are pushed close to each other, the two stress fields are superimposed, especially when several nuggets in the central part are recovered, so that the stress in several nuggets in the central part increases, and the rate of increase is fast.

When the Liebknecht mine in the Krivorogog mining area of ​​the Soviet Union adopts the order of mining from the two wings to the center, the pressure in the central part of the ore body is to be recovered, resulting in the concrete support and the metal bracket of the segmented roadway and the stage transportation roadway. damage.

3. Mining from the center to the two wings. When the stage mining is carried out from the central part of the ore body to the two wings, the stress state of the boundary pillar and the adjacent ore body with the goaf is the same as that of the single wing mining.

The characteristics of ground pressure in many mines at home and abroad indicate that in order to avoid the maximum pressure when the central part of the mining body is avoided, the mining from the center to the two wings is adopted. For example, the Central Mine and the Ingolitz Mine of the Kerry Living Area in the Soviet Union adopted this sort of recovery sequence, and achieved good results, so that the rock on the upper plate fell smoothly with the recovery stage, and there was no stress concentration at the edge of the caving area.

When the original rock stress field is unevenly distributed in the direction of the strike, it is necessary to start mining from the high stress zone first, so as to avoid the superposition of additional stresses in the recovery, resulting in higher stress values ​​in the high stress zone and causing strong pressure activity. For example, when the Yangjiazhangzi Mining Bureau Lingqian Mine Lingbei No. 0 vein VI strip ore body recovered, it had caused ground pressure activities for similar reasons.

The No. 0 vein VI ore body of the Lingqian Mine Ridge in Yangjiazhangzi Mining Bureau is a thick ore body that is inclined to the east, west and north (Fig. 10). During the tectonic movement, the ore body forms a nasal sloping anticline. From the regional structural analysis of the area, the direction of the regional tectonic stress field is north-south, perpendicular to the ore body. The mine uses a sectioned rock drilling stage caving mining method. In the stages above the -195m stage, the order of recovery from the two wings to the center is adopted. Starting from the -55m stage, at each stage, when the mining work advances to the anticline axis, several ore blocks (S4; S5; S6 ~ 13 on the right side of the hatching line) exhibit ground pressure activities. The ramp was destroyed and the blasthole was deformed and broken. It is indicated that there is a large tectonic stress near the anticline portion. After the change of the high-stress zone (shaft part) to the two wings of the mining sequence, the recovery work is relatively smooth.

Fig. 10 Geology plan of the 195m stage of the No. 0 vein VI strip ore body of Yangjiazhangzi Mine

S4, S5, S6-South stop number; N ₁ , N ₂ ... N ₅ - North stop number

The adjacent stope in the upper and lower sections should maintain a certain advanced relationship. Otherwise, when the lower section is to be mined, the adjacent sections of the upper section will be destroyed. The channel 714 grate rake stage 108 and grooves copper ore mine winch rake, the rake 207 tells ore due to subsidence 1.5m (FIG. 11).

Figure 11 Effect of lower section ore mining on the upper section

(2) Mining sequence in the vertical direction

The mining sequence in the direction of the vertical ore body has a great influence on the stress distribution in the surrounding rock of the ore body. According to the production practice and the laboratory similar material simulation test, it is found that the surrounding rock is exposed from the upper plate to the lower disk, and the surrounding rock is exposed at the initial stage of the mining. It may be that the mining rock has not been pushed to the lower plate. As for the movement, the impact stress applied to the lower plate and the un-mining part of the ore body is increased, the mining lane is damaged, and it is difficult to recover the triangular pillar in the lower plate (Fig. 12).

Figure 12 Effect of the mining sequence on the lower plate in the vertical direction

(The arrow in the figure refers to the direction of mining, and the shaded part is the triangular pillar of the lower plate)

When the mining sequence is carried out from the lower plate to the upper disk, the rock body of the upper plate is exposed until the ore body of the triangular pillar of the upper plate is recovered, and it may have to lag back for a certain period of time to collapse.

When mining inclined parallel ore bodies, the upper and lower disc bodies should maintain a reasonable lead relationship. According to some experience in the mining of inclined parallel thick ore bodies using caving mining methods, and simulation tests, it is shown that the lower ore body should be advanced in the upper ore body and maintain a certain advanced height to ensure the mining of the lower ore body. Work is not affected by the pressure on the plate. Its leading height y can be determined according to the following formula (see ΔABC in Fig. 13).

In the formula:

The horizontal distance between L-two ore bodies, m;

A-the inclination of the ore body, (°);

Β-upper plate surrounding rock caving angle, (°).

Figure 13 Schematic diagram of the mining relationship of parallel ore bodies