Phosphate mining to open pit mining mainly based, with the deepening of open-pit mining depth, open pit stripping ratio and the difficulty gradually increases, high and steep slope problem has become increasingly prominent, from open pit to underground mining is inevitable Row. In this paper, the deep inclined gently thickened phosphate rock in the 6# pit of Jinning Phosphate Mine is used as the engineering background. According to the similarity principle, the indoor plane strain similarity model test [6] is used to test the upward stratified filling method of the gently inclined medium-thickness phosphate deposit. During the process of opening, the stress and deformation laws and looseness range of surrounding rock and slope of underground stope are analyzed and studied. The numerical simulation method is used to analyze the change law of slope safety factor in underground mining process. Mining provides guidance. Where Cσ is the stress similarity ratio; CE is the elastic modulus similarity ratio; Cc is the cohesion force similarity ratio; CL is the geometric similarity ratio; Cγ is the heavy similarity ratio; Cu is the displacement similarity ratio; Cε is the strain similarity ratio; Cμ Is the Poisson's ratio similarity; Cφ is the internal friction angle similarity ratio; Cf is the friction factor similarity ratio. 1.3 Mining plan and measurement system First, the bottom two layers of ore are taken back, the layer height is 4.3m, and then a layer is filled to form a bottom working space of 4.3m height. Then, a layer is taken back and a layer is filled until the middle section. During the fourth step of mining, during the stress adjustment process, the load above the goaf is transferred to the stable rock mass on both sides of the goaf to form the supporting pressure zone. The measuring point S4-1 is located in the supporting pressure zone and is squeezed. The stress rises. After the end of the fifth step of mining, the stress at the measuring point S4-1 drops sharply. Comparing the curve of the stress of the roof, it can be seen that the looseness caused by the mining is smaller than that of the roof, and it is smaller than the roof with the expansion of the mining project. Using the Geoslope slope analysis software, the finite element stress method was used to analyze the change of the safety factor of the slope caused by mining. Due to the limitation of the limit equilibrium method, the effect of underground mining on the stability of the slope is basically zero. Therefore, the finite element stress method is used for calculation. The constitutive model in numerical analysis uses the Mohr Coulomb model. Then, according to the boundary conditions of the prototype, the gravity load is applied to the model and the top and side edges are applied with constraints and pressures. Nantong Gympro Sports Co.,Ltd , https://www.masportsgympro.com
Jinning phosphate rock No. 6 hole geological structure is not developed, small-scale, less impact on mining; ore body top and bottom slate stone hard, are not developed joint fissure, rock solid and good, off the block less prone to spalling, Geological disasters such as the bottom drum, the surrounding rock is stable, belonging to the hard-semi-hard engineering geological rock group. After the open-air to underground, the upper layered waste rock filling mining method is used for mining (see Figure 1). Open-pit mining of shallow ore bodies between exploration lines 130-138 has ended (the lowest mining elevation in the north is 2200m and the lowest mining elevation in the south is 2190m) and is now used as an internal dump. In order to ensure the safety of underground mining, the design considers to reserve 20m isolation top column above the pulse return airway. According to the occurrence conditions of the ore body, the mining should be gradually advanced from the shallow part to the deep part. The mining range is between 130 and 138 exploration lines and between 2180 and 2100 m. 
1 model test
1.1 Section selection The selection of the simulated section should be representative. After considering the geological conditions, mining methods and mining locations, this representative selects the representative section No. 134 of the No. 6 pit in the south to carry out this similar simulation test. . The simplified section of the section is shown in Fig. 2, wherein the phosphate rock layer has a vertical thickness of 20 m (10 cm in the model), a vertical thickness of 16.06 m (8.03 cm in the model), and an inclination of 36.6°.
1.2 Test material selection In the model test, the ratio of the same physical quantity of the prototype and the model is called the similarity ratio. According to the similarity principle, the similarity judgment between the underground mining and the slope model test is obtained by combining the elastoplastic mechanical equation or the dimensional analysis method. According to:
According to the range of existing simulations and the size of the test frame, as well as other conditions, the set similarity ratio CL=200 and the similarity ratio Cγ=1 are selected. From the similarity criterion, Cσ=CE=Cc=200, Cμ= Cφ=Cf=Cε=1.
According to the conditions of the stratum rock mass, the rock mass quality level is Grade IV, and the qualitative characteristics of the rock mass are: harder and the rock mass is broken. In combination with Tongji University's research on similar materials, through a large number of proportioning tests, it was finally determined that a mixture of barite powder, sand, gypsum , laundry liquid and water was mixed to form a similar material.
According to the determined mining plan, in the test, the model is simplified due to the small size of the model test, which is divided into 5 steps and 5 steps (see Figure 3).
The stress measurement in the model uses a micro-earth pressure sensor. During the model making process, the micro-earth pressure sensor is buried to a predetermined position, and the lead wire is led out and connected to the data acquisition instrument. The arrangement of measuring points is shown in Figure 4. The six horizontal stress measuring points arranged on the left side slope are S1 series, the nine horizontal stress measuring points arranged on the right side slope are S2 series, and the ten radial stress measuring points arranged on the top plate are In the S3 series, the five radial stress points of the bottom plate arrangement are the S4 series.
Displacement observation mainly uses displacement sensor technology. In order to ensure the reliability of the displacement sensor measurement data, the magnetic table of the displacement sensor must be stably and reliably adsorbed on the rigid frame. The arrangement of the displacement sensor is shown in Figure 5.
2 test data analysis
2.1 Displacement analysis As can be seen from Fig. 6, the displacements of U-6, U-7 and U-8 have different degrees to the outside of the slope during the mining process, and are right during the mining 4 and mining 5 The horizontal displacement of the slope of the side slope was greatly affected. The displacements of U-6, U-7 and U-8 were changed by 0.38, 0.92 and 0.67 mm, respectively. Comparing U-6, U-7 and U-8, it can be seen that during the mining process, the slope body is squeezed in the direction of the slope, which results in the maximum horizontal displacement in the middle of the slope. The position near the foot is laterally constrained by the soil. The displacement is minimal and the horizontal displacement of the slope near the top of the slope is between the two.
It can be seen from Fig. 7 that the horizontal displacement of the slope is small, and the left side slope is less affected during the underground mining process.
2.2 Stress analysis
2.2.1 Top plate stress variation curve In order to more intuitively compare the stress changes at various points, the relative value of surrounding rock stress was analyzed.
Figure 8 shows the variation of the radial stress increment of the roof at a distance of 10 cm (the actual distance of 20 m) from the goaf, and Figure 9 shows the incremental change of the radial stress of the roof at a distance of 5 cm (the actual distance of 10 m) from the goaf.
It can be seen from Fig. 8 to Fig. 9 that due to the formation of the goaf, the surrounding rock changes from a three-way state to a two-way state, causing the original stress equilibrium state to be destroyed, resulting in redistribution of stress in the rock mass and tending to a new equilibrium. Forming a loose zone above the dynamic goaf and a bearing pressure zone along the front of the goaf that is continuously advanced as the mining advances. The radial stress of the rock mass above the goaf shows a decreasing trend. As the mining progresses, the radial stress changes first and then decreases.
2.2.2 Floor stress variation As can be seen from Figure 10, after the first, second and third steps of mining, the goaf is located above the measuring points S4-5, S4-4 and S4-3, and the pressure release leads to the point. The stress value decreases sharply, while the measuring points S4-2 and S4-1 are not located in the stress relief zone, and the stress does not change much.
According to the analysis of the test results, the stress of the surrounding rock in underground mining is divided into four stages according to the rate of change: a slow change phase, a sharp change phase (short time), a slow change phase (longer time), and stability. With the expansion of the goaf, the loosening range will also increase, and the goaf will point to the interior of the surrounding rock, and the surrounding rock will be in the loose circle →
Pressure arch → original rock stress state.
3 numerical analysis
It is known from Fig. 11 that in the initial stage of underground mining, the stability coefficient of the right side slope is slowly decreasing, while when the mining is close to the surface, the stability coefficient of the right side slope is greatly reduced, and throughout the process. The stability coefficient of the left side slope is small, and it can be considered that the stability of the left side slope is basically not affected by underground mining.
4 Conclusions (1) For the displacement change of the slope, the displacement of the right side slope gradually increases with the progress of mining, and the direction is mainly toward the direction of the goaf, forming a displacement trend that appears obliquely below. The displacement of the slope near the goaf is large, and the displacement of the slope is smaller at a position farther from the goaf, while the displacement of the left slope is less changed.
(2) Using the finite element stress method to analyze the stability of the slope under underground mining, the analysis results show that in the initial stage of underground mining, the stability coefficient of the right side slope is slowly decreasing, while mining is close to the surface. At the time, the stability coefficient of the upper side slope of the goaf decreased greatly, and the stability coefficient of the left side slope changed little during the whole process.
(3) According to the results of model test and numerical analysis, it is shown that the slope affected above the goaf is the most affected by underground mining. During the mining process, the displacement and stress of the slope above the goaf have changed greatly, and the main direction of the displacement is toward the goaf. The horizontal displacement in the middle of the upper side of the goaf is the largest, and the top of the slope will be settled due to mining, and the horizontal pressure will decrease in the horizontal direction.
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Article source: Mining Technology; 2017.12(1)
Author: Li Zhe; Tongji University, Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Shanghai 200092, China
Wang Menglai ; Yunnan Phosphate Group Co., Ltd., Kunming 650600 , China
Xu Qianwei; School of Transportation Engineering, Tongji University, Shanghai 200092 , China
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