Ground pressure control with bottom pillar caving mining method (2)
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 the S-falling prism is sliding, m2;
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/s2;
Β-upper rock caving angle, (°);
C=cA;
C-bonding force, Pa;
A-upper plate collapsed prism cross-sectional area, m2.
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.
Soviet Kryvyi Rih Liebknecht ore mining using the mining sequential wings toward the center from the large mining ore to be a central portion of the pressure, so that segment concrete roadway Roadway and transportation stage, the metal stent Suffered from 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.
A topless Tower Crane, also known as a flat top tower crane, is a type of construction crane that does not have a traditional horizontal jib or boom. Instead, it features a horizontal machinery arm that is used to lift and move loads. The absence of a jib allows for easier maneuverability and greater flexibility in confined spaces.
Topless tower cranes are commonly used in urban construction projects where space is limited. They can be easily erected and dismantled, making them ideal for projects with tight timelines. The lack of a jib also allows for better visibility for the crane operator, enhancing safety on the construction site.
These cranes are typically mounted on a concrete pad or a tower section, and their lifting capacity can range from a few tons to several hundred tons. They are often used for tasks such as lifting and placing heavy materials, equipment, and prefabricated components during the construction process.
Overall, topless tower cranes offer numerous advantages in terms of efficiency, versatility, and safety, making them a popular choice in the construction industry.
Flat Top Crane,FlatTop Tower Crane,Flat Top Type Tower Crane,10T Flat Top Tower Crane,Flat Top Tower Crane Advantages
SHEN YANG BAOQUAN BUSINESS CO., LTD , https://www.syconstructionhoist.com