Stability Analysis and Evaluation of Multi-layer Empty Zone in Iron Deposits

The ore mining forms an underground goaf, destroying the initial equilibrium state of the original rock stress, and the resulting secondary stress field redistributes the stress of the surrounding rock to reach a new equilibrium. In this process, if the empty area is not handled in time or improperly, a series of geological disasters will occur, which will seriously affect the safety of people's lives and property, and bring unfavorable factors to the development of the enterprise, with serious consequences.
Taking a mine in Chengde, Hebei Province as the research background, the mine was put into operation in 2006, mainly mining Fe23 ore bodies. 100m at the south side of the surface of the mine construction of railways, highways, mining office, staff quarters, cultural centers and other public facilities, there are gobs of security poses a threat to surface buildings. There are also adjacent mining areas on the east side of the mining area, and some of the roadways are connected, and the two mining influence each other. After years of mining, the mine has formed a certain-scale goaf. At present, there has not been a fall in the empty area. However, as the exposure time of the goaf increases, the stress value of the surrounding rock changes, and the ground pressure activity will become more frequent. Obviously [1], gradually threatening the safety of surface buildings. Therefore, it is necessary to analyze the stability of the existing empty area and provide reasonable suggestions for the treatment of the goaf in the mine to ensure the safety of the life and property of the mine and nearby residents [2].
1 Project Overview
1.1 ore body characteristics
The iron ore is located in the strata of the Baimiao Formation of the Taitazi Group in the Archean Period. It is mostly layered or lenticular. The overall tendency is from north to east, the dip angle is 33°～85°, and the thickness Fe23 is 8.37～10.16m. The shape and occurrence of the ore body are strictly controlled by the direction and occurrence of the gneiss line.
1.2 Status of the goaf
At present, two mines have been built in the mining area, all within the rock moving boundary. 346, 323 and 303m3 middle sections have been mined, and the open field method is used for the roadway top picking stop. The transport roadway is arranged in the vein, and the roadway is taken. The field is about 10m high and the top column is about 10m high. The details of the goaf are shown in Table 1.

2 simulation results analysis
2.1 Model parameter settings
After the actual investigation of the mine goaf and the simplification of some goafs, the model size (length × width) is 340m × 380m, the total number of units is 3134, and the number of nodes is 9633. The X-axis direction of the model is set to horizontal displacement, the model Y-axis is set to vertical displacement from the bottom to the surface direction, and the corresponding vertical load is applied to the upper boundary stress of the model. The model deformation is set to a large deformation. For the convenience of calculation, it is assumed that the model medium is continuous, homogeneous, without initial stress and isotropic elastoplastic material. The mechanical parameters of the selected materials are shown in Table 2.

2.2 Simulation results
The two-dimensional model is used to calculate the stability of the goaf. After the stress parameters and boundary conditions are set for the initial model, the unmined model is calculated first, and then the goaf model is calculated. The calculation results mainly include: the displacement of the goaf section before and after the mining in the X and Y directions; the section of the goaf before and after the mining and the shear stress in the Y direction.
Through the ANSYS program simulation calculation, the stress distribution and displacement distribution around the empty area are obtained. Taking the different sections of Y=25m, Y=80m and Y=100m as an example, the calculation of the empty area result is analyzed.
It can be seen from the simulation diagram that after the ore body is mined, the natural equilibrium state of the original rock stress field is broken. Under the action of the overlying strata, the stresses of the rock mass and the roof surrounding the goaf are redistributed and balanced, and the inner side of the stope The concentrated stress generated by the rock mass is relieved, and the surrounding rock of the stope loses the support of the original ore body in the lower part of the stope, and stress concentration occurs at different positions of the top and bottom of the stope and the surrounding rock. Due to the effect of the underlying strata, the stresses of the rock mass around the goaf are redistributed and balanced, and the concentrated stress generated by the rock mass inside the stope is slowed down. The stress concentration and tensile stress concentration are analyzed separately, and the influence range and displacement are judged by the results.
2.2.1Y=50m
It can be seen from Fig. 1 to Fig. 4 that the stress concentration phenomenon occurs in the goaf in the middle section of different depths, and some places can be regarded as a symmetric distribution. Among them, due to the location of the empty area and the deep burial, the upper part of the vacant area and the two sides have large tensile stresses. The maximum tensile stress in the local area reaches 1.41 MPa. The phenomenon of concentration of compressive stress is exhibited, and the maximum compressive stress value is 3.04 MPa. From the distribution of shear stress contours around the empty area, the maximum shear stress of the 303 m middle section is distributed in two diagonals of the empty area, and the maximum shear stress is 1.24 MPa.

From the longitudinal displacement profile of the Y-axis, it can be seen that the rock mass around the empty area (including the top and bottom of the empty area and the two gangs) have different displacements into the empty area, and the top plate of the 303m middle empty area moves downward. The maximum value is 13.3 cm, which is relatively large.
2.2.2Y=80m
Fig. 5 to Fig. 8 show the distribution trend of the inner space, force and displacement of the longitudinal section of the stope Y=80m after excavation of the ore body.

It can be seen from Fig. 6 that due to the formation of the middle vacant zone of 303, 323, and 343 m, tensile stress concentration is exhibited at the two sides of the top plate of each empty zone, and the maximum tensile stress is 1.83 MPa, while the pressure around the empty zone is expressed. The stress has a maximum value of 3.53 MPa. It can be seen from Fig. 7 that the shear stress in the empty zone is mainly distributed in the apex angle of the empty zone, and the maximum value of the shear stress is 1.06 MPa.
Figure 8 shows the longitudinal displacement of the longitudinal section at the stope Y=80m after the excavation is completed. It can be seen from the figure that the surrounding rock of the stope sinks, the maximum displacement is 66mm, which occurs in the middle of 343m. The surrounding rock of the roof of the area should be observed more.
2.2.3Y=100m
Fig. 9 to Fig. 12 show the distribution trend of the empty area, stress and displacement in the longitudinal section of the stope Y=100m after excavation of the ore body.

It can be seen from Fig. 10 and Fig. 11 that after the ore body is mined, the stress of the rock mass inside the surrounding rock of the goaf is released, and the surrounding rock of the upper part of the stope loses the support of the original ore body in the lower part of the stope. Tensile stress is present everywhere, the maximum value is 1.47 MPa, and the compressive stress is 2.54 MPa at the middle of 303 m.
Figure 12 shows the longitudinal displacement of the longitudinal section at the stope Y=100m after the excavation is completed. It can be seen from the figure that the surrounding rock of the stope sinks, the displacement value is 1.4cm, and the ore body is also surrounded. There are varying degrees of slight displacement.
3 conclusions

(1) Due to the deep burial, in the different section analysis, the top plate and surrounding rock of the goaf in the middle section of 303m have large displacement, and the roof, surrounding rock and pillar will have different degrees of risk of falling and collapse. The surrounding rock is unstable and needs to be strengthened for inspection and safety protection.
(2) After the formation of the goaf, the stress concentration of the top plate and the pillar part of the goaf in the middle section of the 303m section is the most serious, and has a greater impact on the pillar and the roof.
(3) In the middle section of 303, 323, 343m, the stability of the goaf in the middle section of 303m is the worst, and the possibility of collapse is the greatest. It is urgent to strengthen monitoring and treatment.
references
[1] Fu Shigen, Li Quanming, Wang Yunhai, et al. Research on impact assessment method of goaf on surface buildings [J]. Chinese Journal of Safety Science, 2007 (8): 143-147, 2.
[2] Lu Hongjian, Gan Deqing. Comprehensive analysis model for stability of retained goaf in iron deposits [J]. Metal mines, 2013 (3): 62-65.