Underground mining has different degrees of influence on the surface. With the expansion of underground mining, uneven settlement on the surface is caused, and even the surface collapses. In the vicinity of the mining area, mining activities such as collapse pits and ground fissures often occur. Large deformation of the surface [1-4]. If the mining site is close to the surface structure (structure), the stability of the foundation of the building (structure) will be affected, resulting in cracking and destruction of the surface house. To ensure an iron ore expansion in the industry during excavation, the normal operation of residential houses near the mine safety as well as enterprises and institutions, the need for and extent of surface deformation due to mining of qualitative and quantitative analysis.
1 engineering background
An iron mine has been mined for more than 50 years, and the current mining elevation is +250~-200m. The mining area is 1.0484km2. The mining method is the downward approach cementation filling method with a production scale of 450,000t/a. After the expansion, the mining area has increased in the mining area, and the mining depth has been extended from -200m to -350m. The distribution of the ore body is complex, the shape of the ore body is irregular, and scattered ore bodies cannot be mined. However, in general, the ore body is located in the contact zone of marble and diorite , the roof is mostly marble, and the bottom plate is Leica. Rock and altered diorite. The down-track cement-filling mining method adopted by the iron ore can effectively limit the surface deformation caused by mining activities and play a positive role in curbing the development of surface deformation. The method is layered back-up and layered filling from top to bottom, and is extracted by the roadway approach under the artificial false bottom protection of the upper layer. Each layer of mining is after layering the layered contact road to layer Full-height vertical or along the strike to divide the approach, intermittent or continuous mining, after the first batch of roads are completed, the artificial false bottom is laid and cemented to fill the roof. Road, after the entire layer of mining and laying artificial false bottom, filling work, and then proceed to the next layer. The main advantage of this method is that the recovery is safe, the ore recovery rate is high, and the depletion rate is low. The direction, position and quantity of the approach can be adjusted according to the geological conditions of the mine, and the flexibility is strong.
2 numerical simulation
2.1 Model Construction
The iron ore mining area is large, and the ore body is widely distributed on the -1#~53# line. The vertical distribution of the original ore body is mainly from the vicinity of the surface to -200m, and the vertical area of ​​the expansion is mainly -200～-350m, but The ore bodies in the expansion area are relatively thin, mainly distributed on the 19#~23# line, and the thickness of the veins in some areas is small and cannot be mined. The surface of the mining area has a thin Quaternary surface soil. Most of the surface is directly in contact with the rock. The main rock below the surface is marble, which is mixed with a small amount of gravel and limestone. This part can be simplified to the Dali rock formation, and the actual stratum is In order to simplify the numerical simulation process, the part of the stratum is simplified to a horizontal stratum, and the marble stratum is underneath the diorite layer without affecting the calculation results. According to the research scope of the mine, a three-dimensional numerical calculation model was established. The final model size is 2500m×1300m×630m (length×width×height). The calculation model is divided into two geological strata and three calculation groups (large marble, Diorite and ore body), the model has a total of 267,534 three-dimensional calculation units, 48,685 grid nodes. The three-dimensional simulation model and ore body shape are shown in Figure 1.

The boundary condition of the calculation model is that the side is restricted to move horizontally, the bottom surface is fixed, and the upper surface of the model is a free boundary. The models are all simulated by solid elements, and the failure characteristics of the rock and soil materials are in accordance with the Mohr-Coulomb strength criterion. The values ​​of the mechanical parameters required for the model calculation are shown in Table 1.

The numerical simulation operation process of the model is as follows: (1) Calculate the initial stress of the model under the given mechanical conditions and boundary constraints to achieve the initial stress equilibrium state; 2 Calculate the settlement deformation of the surface during the actual step-by-step mining backfilling process, and obtain the surface movement The boundary is used to analyze the safety of the surface structures (structures) during the mining process.
2.2 Analysis of numerical simulation results
According to the actual mining situation of the ore body (0m-above mining method is used, 0~-200m is used for filling method, and the expansion part is also mined by filling method), and the surface deformation displacement cloud map is obtained, as shown in Fig. 2. It can be seen from Fig. 2 that there are mainly two settlement basins on the surface of the mining area. The maximum settlement of the left basin is about 25 mm and the right side is about 75 mm. The two basins are concentrated distribution areas of the ore body. Both ore bodies are thick and large. The amount of earth and stone and ore is large, so the surface settlement value is large, which is consistent with the actual situation of the formation of collapse pits at two locations. Because the mine was exploited by the caving method in the early stage, it failed to fill in time to cause the formation of the collapse pit. The tailings have been filled and filled in the old goaf, and the surface collapse pit has also been landfilled. The surface subsidence has been basically becoming steady. Taking 15mm settlement as the moving boundary (black thick solid line in Figure 2), it can be seen that the main buildings (structures) and houses in the mining area are located outside the moving boundary, and the underground mining of the iron ore affects the surface structures (structures). Smaller. The accumulation of the maximum settlement on the surface of the mining area was counted. The results are shown in Figure 3.

It can be seen from Fig. 3 that the maximum settlement of the surface is about 75 mm, and the surface settlement growth rate is faster in the mining process before the expansion, and the surface settlement tends to be stable after the expansion. This is because: 1 the top plate is a marble roof, which is relatively complete, with good geological conditions, high strength and strong resistance to deformation; 2 with the increase of mining depth, although the influence range on the surface is increased, the impact on the surface The degree is gradually reduced, and the distance from the surface when expanding the mining is more than 300m, so the deformation caused by the surface is negligible; 3 due to the filling of the old goaf caused by the caving mining, the subsequent mining activities are caused by the cement filling method. The subsidence caused by mining activities is limited, and the strength of the rock and soil body is protected. Therefore, the impact caused by mining the lower ore body is correspondingly reduced, and the surface settlement caused by the expansion of mining is less changed [5-7].

Under the influence of underground mining work, the surface will form two settlement centers. With the increase of the depth of expansion, the impact of mining on the surface is gradually reduced. After the expansion and filling of the stope, the surface is affected by the mining, and the resulting settlement deformation tends to be smooth and stable. The surface subsidence center is located inside the mining area. Due to the concentrated spatial position of the ore body, the surface subsidence cloud map is in a closed state elliptical shape, and the maximum settlement center coincides with the existing collapse pit position. The main building (structure) around the mining area is located outside the boundary of the surface movement, and the impact of the expansion of production is small.
3 Conclusion

The FLAC3D software is used to simulate the influence of underground mining on the surface deformation of an iron mine. The results show that the main buildings (structures) of the mine are located outside the mining moving boundary, and the mining process has less impact on it. The results of the research and analysis have certain reference value for ensuring the safe production of the mine.
references
[1] He Guoqing, Yang Lun, Jia Fengcai. Mining mining subsidence [M]. Xuzhou: China University of Mining and Technology Press, 1991.
[2] Hu Lizhen, Li Yunan, Lei Yin , et al. Numerical simulation of upward stratified filling mining [J]. Metal mines, 2014 (1): 5-8.
[3] Zhou Xiaochao, Zhou Ming, Li Xiaowu. Ground settlement simulation of gently inclined ore body mining [J]. Metal Mine, 2015(1): 16-19.
[4] Ma Haitao, Liu Ningwu, Wang Yunhai, et al. Summary of research on disaster prevention technology in goaf of metal mines [J]. China Safety Production Science and Technology, 2014 (10): 75-80.
[5] Xue Yuzhong. Practice of full tailings filling and sealing engineering in mine extra-large goaf [J]. China Mining Engineering, 2006, 35(2): 11-13.
[6] Shu Jisen. The relationship between mining engineering and surface displacement [J]. Journal of China University of Mining and Technology, 1997(4): 30-33.
[7] Yang Buhua. Progress of Study on the stability of underground mined-out area of non-coal mine [J]. Minerals and Geology, 2008, 22(5): 473-479.
Author: Xu Xiangxi, Wu Bin; Shandong Jinling Mining Co., Ltd;
Article source: "Modern Mining"; 2016.8;
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