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Mining site pressure
The mining site pressure is a general term for the stress field acting on the ore body, surrounding rock and pillar of the mining face and its interaction with the support system in the stope. During the mining process, the roadway and goaf formed in the underground destroy the stress balance of the original rock mass, and the stresses in the stope and surrounding rock and ore body are redistributed to form a secondary stress field, so that the pillar, the working face and the surrounding The displacement and deformation of the rock, and even the destruction, such as the roof falling, the collapse or collapse of the pillar, the cracking of the surrounding rock and the patchwork are called the pressure of the mining site. The whole process of development of ground pressure is called mining site pressure activity. In order to ensure normal recovery, the use of ground pressure and technical measures to reduce or avoid the pressure of ground pressure are called mining site pressure control.
Due to the large mining area, the shape of the mining space is extremely complicated. As the mining work progresses, its scale and shape are constantly changing. The rock mass is repeatedly disturbed and presents an extremely complicated stress state. Consequently, ground pressure is different from mine shaft mining, utilities, railways, defense and other underground works of the press, with the intense pressure behavior, spread to a large range of secondary stress field complex, a short time is difficult to know their activity patterns and other prominent Features.
The occurrence conditions of ore bodies and the physical and mechanical properties of ore rocks vary greatly, and the mining methods used are also various. The research and application of the site pressure display law, so as to utilize and control the ground pressure, for safety and economic benefits. Mining underground mineral resources has great practical significance.
Mining in the ore body destroys the balance of the initial stress (original rock stress) in the rock mass, and forms a secondary stress field around the stope (Fig. 1, Fig. 2) where the stress reduction zone occurs in the roof and is caused by the minimum principal stress. In the tensile stress zone, there are stress concentration zones in the two gangs and the pillars.
Figure 1 Stress distribution diagram when mining horizontal ore bodies
Figure 2 Stress distribution diagram when mining inclined ore bodies
The maximum principal stress and maximum shear stress in the inter-column decrease as the dip angle of the ore body increases, while the minimum principal stress begins to decrease and then rises (Figure 3).
Figure 3 Relationship between stress and ore body inclination in ore body
   1. Stress concentration during horizontal and gentle inclined ore body mining
After the ore body is mined, the phenomenon of stress rise in the two gangs and the pillars is called stress concentration (also called support pressure), and the range of stress rise is called stress concentration zone. The stress concentration is usually expressed by the stress concentration factor K, that is, the ratio of the secondary stress to the original rock stress, K>1 is the stress increase, and K<1 is the stress reduction.
The stress concentration factor and the width of the stress concentration zone are related to the mining depth, the nature of the rock, the rock mass structure, the span of the stope and their relationship, and are generally obtained by laboratory tests and on-site measurement methods. When the width of the pillar is larger than the stress concentration zone l 1 +l 2 of the adjacent two mines, the stress distribution is the same as that of the single mine (Fig. 4a); when the pillar width is less than l 1 +l 2 , the pillar stress occurs. The superposition of stress concentrations (Fig. 4b) is a problem that often occurs in underground mining.
Figure 4 Stress distribution in the pillars at different intervals between two mines
The width of a-column is greater than l 1 +l 2 ; the width of b-column is less than l 1 +l 2
   2. Stress concentration during mining of steeply inclined ore bodies
The stress distribution of the steeply inclined veined ore body is shown in Fig. 5, and the concentrated part is between the stages of the pillar (top and bottom columns). According to the numerical calculation of the Pangushan tungsten mine in Jiangxi, the top and bottom columns are the maximum stress concentration areas without stress, and the stress concentration factor is up to 4; the second is the bottom and bottom corners of the stope. The concentration factor is 1 to 3. When there is a structural surface, it causes a large stress concentration in the vicinity thereof, and its stress concentration factor is greater than 5-6. The mining disturbance range is 25m according to the light stress meter and the numerical calculation result is 50m.
Figure 5 Stress distribution after mining of steeply inclined veins
A-single vein photoelastic simulation; b-two vein photoelastic simulation
According to the on-site stress relief test data of the Dajishan tungsten mine, stress concentration occurs in the surrounding rock of the goaf and its surrounding rock. The stress concentration decreases with the distance from the aerial bottom and tends to the original rock stress (Fig. 6). On-site observation found that the 21, 22' veins in the 517m stage were seriously damaged along the vein roadway.
Fig.6 Stress distribution of the surrounding rock at the bottom of the southern group of the 517m stage of the Dajishan tungsten mine
15～22-The vein number; F 9 - fault number; σx, σy, σz-stress component
The field test data of the mine also showed that stress concentration occurred in the surrounding rock at the top of the goaf. The fringe series of the light stress meter of the surrounding rock in the 567m stage gradually decreased from 4 (1/3) of the 8' hole to 1 (2/3) of the 14' hole (Fig. 7).
Fig.7 Stress distribution of the surrounding rock on the top of the 517m stage of the Dajishan tungsten mine