Study on underground mining technology and disaster prevention measures of gypsum mine

Gypsum is a large output, versatile, non-metallic mineral products. China's gypsum resources are extremely rich, and proven reserves rank first in the world. 70% of China's gypsum mines are underground mining, and the depth of gypsum mining is usually shallow. However, in recent years, there have been many accidents in gypsum mines. There are still many problems in China's gypsum mines: 1. Most of the small gypsum mountains have not been formally designed, resulting in indiscriminate mining; 2. The number of mines is large and the scale is small. There are more than 500 mines in China, and the average annual output of each mine is only 50,000 tons. Such a situation with a lot of distribution and small output in China will threaten the ecological environment of the ground, occupy more land, and bury hidden dangers to the ground collapse; 3. The gypsum ore recovery rate is less than 30%, and the waste of resources is serious; There is no systematic research on underground mining of gypsum mines in China, and there is no uniform technical regulation in the fields of mining technology and mining methods.

I. Research status of gypsum mining technology and measures

Many scholars started from the case analysis of gypsum mine accidents in China, and carried out research work on prevention and control measures of ground pressure disasters. Liu Jianping conducted a systematic rock mechanics parameter test on the surrounding rock and ore of Huolian Gypsum Mine. The mechanical parameters of the gypsum ore and surrounding rock were obtained, and the test error of the test results was analyzed. Wei Juncai analyzed the occurrence and development of surface deformation and damage caused by underground mining of gypsum mine by analyzing the disaster prevention measures of the gypsum mine in Shaodong County. It is not only affected by the mining mode, but also directly controlled by the geological conditions of the ore deposit, especially the roof. The influence of protective layer thickness, geological structure, groundwater dynamic conditions and irregular mining is the most prominent. At the same time, the estimation method of goaf stability is given. Zheng Huaichang et al. analyzed the physical turbulence of the roof of the gypsum mine's goaf, and pointed out that the large area of ​​the goaf is the most serious roof accident in the gypsum mine. The control method should include closed treatment and topping treatment. Most of the ore bodies are multi-layer mines, and the ore body is continuous. Therefore, the current technically feasible and economically reasonable hazard control method is the topping treatment of the goaf.

Most of the above research results are underground mining technology and disaster prevention measures for a gypsum mine. The research needs to be deepened to obtain mature gypsum mining technology and disaster prevention measures that can be promoted.

Second, gypsum mining technology research

(1) Mining methods

For precious gypsum ore bodies such as fiber gypsum with high grade and thick ore body, special mining methods should be studied to improve resource recovery rate. Under the conditions of the conditions, it is advisable to use the longwall method to fill the mining method or the room column method. When adopting the longwall sorting filling method, the following provisions shall be observed: mining with upper stratified filling method shall be carried out, and the filling well and its connecting road shall be prefabricated, and then the bottom structure and the bottoming tunnel shall be constructed to create a good Ventilation conditions; after each layer is finished, it should be filled in time. After the last layer of the upward filling method, the top layer should be closely connected. The lower filling method should be topped tightly; the filling should be non-toxic. Harmless; filling should not react chemically with gypsum components.

The gypsum ore body with an impact pressure hazard is mined, and no groundwater exists in the overburden rock. When the caving condition is satisfied, the caving mining method can be adopted. The use of landslide mining method shall comply with the following provisions: surface subsidence shall not cause harm; suspension roof, roof control, roofing distance and roofing method shall be tested and tested safely and reliably; before placing the roof, the force of the pillar shall be checked. Ensure that the retreat is unblocked, and it is forbidden for personnel to stay near the topping area; after the top is placed, the pouring area should be closed in time to prohibit personnel from entering.

When gypsum mines do not have caving conditions or cannot be collapsed to deal with goafs from an economic point of view, most of them use the open field method (house-column method and comprehensive method) to open ore. The mining design of the gypsum mine shall also propose a treatment plan for the goaf. The size of the pillars and mines specified in the design shall not be arbitrarily changed during mining; the retained pillars shall not be recovered or destroyed during the retention period.

The span of the mine and the size of the pillar should be comprehensively determined according to the geological conditions, the depth of the ore body, the hardness of the ore and surrounding rock, and the exposed area of ​​the empty area, so as to avoid the occurrence of large-area goaf fall and impact ground pressure disasters. The form of the pillar has the form of continuous pillar (ie strip column) and spacer pillar (point pillar). The size of the pillar is generally φ=4~6m or 4×4～6×6m. Since the surrounding rock of the gypsum ore body is often rock mass such as top rock and mudstone, the compressive strength and shear strength are low. Therefore, it is necessary to leave a gypsum-protected top layer of sufficient thickness in the mining room to avoid collapse of the top surrounding rock.

On November 6, 2005, the collapse of the Kangli gypsum mine in Shangwangzhuang, Huining Town, Xingtai County, caused great damage. One of the main causes of major casualties is that there are no security pillars left between the mines, or even through, causing the roof to yield damage. Therefore, when underground gypsum mining is carried out, adjacent mines (wells) should be clearly defined. There should be no less than 40m security pillars between the mines (wells). The security pillars must not be mined and destroyed within the specified retention period. . Mines with two mining rights are not allowed to pass through the tunnel. In addition, when the area is continued to be mined around the old airspace area, an isolated pillar of not less than 20m wide shall be left around the old airspace area, and the roadway connecting with the old airspace area shall be strengthened.

(II) Mathematical model for roof stability evaluation of room and pillar mining

It is generally believed that the Mohr-Coulomb yield criterion judges the compression and shear failure of rock mass:

(1)

In the formula (1), σ1 and σ3 are the maximum and minimum principal stresses, respectively, and C and φ are the adhesion force and the friction angle, respectively. When fs>0, the material will undergo shear failure. However, the Mohr-Coulomb yield criterion is difficult to reflect the tensile and tensile shear failure modes of rock mass under the condition of tension. In this paper, the Mohr-Coulomb yield criterion is extended to the tension section, and it is proposed to reflect the shearing, shearing, and The strength criterion of various failure modes such as stretching is used to establish a mathematical model for the evaluation of the roof stability of the gypsum mining room.

Based on the systematic study of the tangential stress distribution of elliptical cracks under two-dimensional stress state, Griffith proposed a Griffith strength criterion which can simulate the tensile and tensile shear failure of linear elastic materials. Figure 1 shows the elliptical crack and its two-dimensional stress state in an orthogonal Cartesian coordinate system.

Figure 1 Elliptical crack in a Cartesian coordinate system

By deriving the variation of the tangential stress σt along the elliptical surface, the Griffith strength criterion can be obtained as:

(σ ₁ -σ ₃ ) ² -8T ₀ (σ ₁ +σ ₃ )=0, ifσ ₁ +3σ ₃ >0

σ ₃ =-T ₀ , ifσ ₁ +3σ ₃ <0 (2)

Where T ₀ is a uniaxial tensile strength.

To define the intersection of the Mohr-Coulomb strength criterion and the Griffith strength criterion as σ _B , then σ _{B is} expressed as follows:

σ _B =2c/[tg(45°-φ/2)] (3)

The mathematical model for roof stability evaluation of gypsum mining room method can be expressed as follows:

σ ₃ =σ ₁ tg ² (45°-φ/2)-2ctg(45°-φ/2),

Ifσ ₃ >0(σ ₁ -σ ₃ ) ² -σ _B (σ ₁ +σ ₃ )=0

Ifσ ₁ +3σ ₃ >0 and σ ₃ <0σ ₃ =-σt,

Ifσ ₁ +3σ ₃ <0 and σ ₃ <0 (4)

Where σt=T. The strength criterion is shown in Figure 2 in a Cartesian coordinate system.

Fig. 2 Mathematical model of roof stability evaluation of room and pillar mining

The model combines the Mohr-Coulomb yield criterion with the Griffith strength criterion to study the stability of the roof during gypsum mining.

Third, gypsum mine ground pressure disaster prevention and control measures

(1) Strengthen roof safety management and disaster forecasting

Specialized agencies or full-time personnel should be set up to be responsible for ground pressure management. Observers should regularly observe whether the floor of the roadway is bottom, whether the roof is sinking or cracking, whether the pillar is cracked or torn, whether the sound is in the air zone, etc. Good detailed observations. Omen before the performance of impact pressure: Influence by mining, rock movement direction increasing the gob area, roof slate layer snaps as increased support pillar cracks or tear, compression destruction phenomenon. When the full-time staff finds the above symptoms, they should immediately withdraw all the underground personnel.

(2) Strengthening roadway and diverticulum support

The main roadway and diverticulum should be placed in the stable rock stratum of the roof. If it is necessary to pass through the weak rock mass, the support should be strengthened, the soft rock mass should be excavated, and the permanent support should be carried out between the excavation working faces. Temporary support should be erected or Special support. In the hard and stable rock formation of the gypsum mine, the tunnel is to be built. If the temporary support is not needed, the roof and the two gangs can be ensured. The corresponding safety measures and regular inspection system should be established. The roof and the two gangs of the working place should be carefully inspected. Problems and handling.

(3) Strengthening treatment measures in the goaf

Corresponding measures for ground pressure activities shall be formulated according to the specific conditions of the surface and the stratum. When the stratum is water-free and the surface is allowed to collapse, measures such as natural caving or forced caving may be adopted; when there is water damage in the stratum or the surface is not allowed to collapse, measures such as barrier bricks, brick walls, and large security pillars may be used. Strengthen permanent support; if the surface subsidence may cause huge disasters, the filling area should be treated by filling.

Fourth, the conclusion

On the basis of summarizing the existing research results in the field of gypsum mine underground mining in China, this paper studies the underground mining method of gypsum mine, and analyzes the mining technology and safety management methods of longwall sorting filling method, caving mining method and empty field method. Aiming at the mining method-house-column method, which is mostly used in China's gypsum mines, the mathematical model of roof stability evaluation during the room-column mining is proposed. This paper also analyzes the countermeasures for the prevention and control of geological disasters in gypsum mines.