In recent years, with the sharp reduction of underground mineral resources, underwater mining has gradually gained attention in the industry. Many of the safety problems faced by underwater mining are affected by many complicated factors, which brings great difficulties to the analysis of mining safety and stability [1-2]. The safety and stability analysis methods for underwater mining generally have two types: theoretical calculation [3-5] and numerical simulation calculation [6-9]. Relatively speaking, the numerical simulation calculation method is more widely used. The method analyzes the stope stability by simulating the stope structure parameters [10-12] and the recovery sequence [13-14]. The three-dimensional finite element numerical simulation method has the advantages of economy and high efficiency. It can quantitatively calculate the settlement deformation trend and the stress distribution of surrounding rock during excavation, and reveal the stress variation characteristics of the surrounding rock in the stope in order to achieve reasonable stope stability. Sexual evaluation provides a reliable basis for safe mining.
In this study, to a metal underwater mine, for example, using the ABAQUS finite element software build geological model, simulate the excavation process deposits, sedimentation analysis and displacement caused by the stress surrounding rock To provide a reference for improving the stability of the mining site and ensuring safe mining.
1 numerical model construction
The finite element method fully estimates the stress-strain relationship of rock mass materials, and can analyze the structural stress and displacement. It can effectively simulate the complex geometric conditions, load conditions and rock material properties of underground orebody mining to judge the excavation process. The impact of field stability.
ABAQUS software is a finite element software that calculates extremely fast convergence and is easy to operate. ABAQUS/STANDARD provides powerful elastoplastic static solution function and is a stable and reliable finite element equation solver; ABAQUS/Visualization provides efficient and high-quality post-processing analysis and data display. Since the rock mass belongs to the nonlinear-elastic-plastic medium, this study chooses the software ABAQUS with strong nonlinear solution function as the numerical simulation analysis tool.
The numerical model of this study is along the line of 123#～159# along the ore body, with a length of 450m. It is 600m before and after the ore body is perpendicular to the direction of the ore body, and the water surface in the vertical direction is -400m, which is much larger than the construction excavation size. . The direction along the ore body is the X direction, the direction perpendicular to the ore body is the Y direction, and the vertical direction is the Z direction. The front and back, left and right and lower surfaces of the model are applied with their respective normal constraints, and the upper surface is simulated by free surface; after the earth stress balance is completed, the vertical constraint is applied to the front and back of the model. According to the material characteristics, the model is divided into water layer 5~30m, mud layer 10~30m, bedrock layer, upper plate rock mass, lower plate rock mass, ore body and artificial false top part (Fig. 1). Among them, the artificial false top is embedded in the ore body - 165m position. The physical and mechanical parameters of various materials are shown in Table 1.

2 calculation model and mining order

The load in the calculation model is mainly self-weight stress, the gravity acceleration is 9.8N/kg, and the ground stress balance is applied before the gravity load is applied. The vertical stress gradient value is 25970N/m3, and the static side pressure coefficient is 0.5. During the excavation process, the field variable is changed. When the excavation fills the ore body from the ore body material to the backfill material, the actual gravity of the stope is reduced. Therefore, it is necessary to apply vertical upward force to compensate for excessive gravity. The size is 7938N/m3. The mining scheme is designed to support the inter-column in each middle section. The thickness of the column is 6m and the spacing between the columns is 50m. The model of the inter-column mining scheme is shown in Figure 2.

According to the design, the excavation sequence of the stope is divided into two. The study will be 123#～127#, 127#～131#, 131#～135#, 135#～139#, 139#～143#, 143#～ The lines 147#, 147#～151#, 151#～155# are respectively recorded as 1#, 2#, 3#, 4#, 5#, 6#, 7#, 8#, 9#. The mining excavation sequence is determined as 1#—4#—7#—2#—5#—8#—3#—6#—9# stope. The columns of the 123#, 127#, 131#, 135#, 139#, 143#, 147#, 151#, 155#, and 159# lines are numbered 1#, 2#, 3#, 4#, and 5 respectively. #,6#,7#,8#,9#,10#. In the excavation process, in each excavation process, the ore body material parameters of the excavation part are directly converted into material parameters of the filling body by the field variable function unique to ABAQUS software, and multiple static analysis are established according to the mining sequence. Steps (Table 2).

3 numerical model response analysis
3.1 Settlement analysis
In order to facilitate the observation of the overall settlement, representative observation lines L1, L2, L3 and observation points P1, P2 are arranged on the surface of the bedrock. The L1 line is in the X direction and runs through the maximum position of the bedrock settlement; the L2 line is in the Y direction and above the 123# line; the L3 line is in the Y direction, above the line 143#. The point P1 is the intersection of the L1 and L2 lines, and the point P2 is the intersection of the points L1 and L3 (Fig. 3).

The settlement of the first step of excavation and the L1, L2 and L3 lines after the completion of the excavation is shown in Fig. 4. The sedimentation changes of P1 and P2 points with the excavation step are shown in Fig. 5.

It can be seen from Fig. 4 that during the excavation process, the settlement amount increases continuously, but the increase rate decreases. The maximum settlement of the mining scheme designed in this study is 2.8cm, and the maximum settlement position is above the 123# and 143# lines. . It can be seen from Fig. 5 that in the initial several analysis steps, the increase of the settlement amount of the monitoring point is large, showing an approximately linear increase, and the excavation starts in the 15th analysis step (K5-1 in Table 2, ie, -125m middle section) After that, the increase of the settlement caused by the excavation of the project is significantly reduced, the settlement is slowly increasing nonlinearly, and the settlement at the P1 point is almost no longer increased. It can be seen that when the mining height is above -125m level until the excavation is completed, the mining scheme designed in this study has a good control effect on the settlement.
3.2 bedrock
The bedrock is located between the water layer and the stope. Therefore, it is necessary to ensure that the bedrock layer is intact to prevent the large-scale fissures in the bedrock layer from causing the stope to be filled.
Shear stress in rock mass is an important factor causing shear cracks. Therefore, the shear stress index is used as an index to evaluate the safety of bedrock during mining. Under normal circumstances, the safe mining coefficient is 1.8~2, but the crack caused by shear stress has extremely serious safety hazard. Therefore, this study is based on safety considerations, assuming that the influence of friction angle is neglected, and the bonding force of the ore body is The minimum value is taken as the shear strength of the bedrock, the safety factor is 4, and the shear stress of the bedrock layer is not more than 1.26 MPa / 4 ≈ 300 kPa, and the value is set as the shear stress allowable value.

Figure 6 shows the shear stress distribution of the K7-3 and K8-3 bedrock layers in the analysis steps. The area gray indicates that the shear stress is allowed to exceed the shear stress and may penetrate the bedrock layer to reach the aquifer.

It can be seen from Fig. 6 that the shear stress in the YZ direction has a significant influence on the entire bedrock. When the analysis step K7-3 ends (that is, the end of the excavation of the -95--85 m horizontal ore body), the range exceeding the allowable value of the shear stress penetrates the entire bedrock. It appears on the upper surface of the bedrock, indicating that shear cracks may occur throughout the bedrock layer, which is a dangerous signal for possible flooding. Therefore, the mining design depth of this research program should not exceed -95m.
3.3 artificial false top
The artificial false top is assumed to be a thin plate member, and its bending resistance is mainly considered. The bending moment in the X and Y directions of the artificial false top section at the end of excavation is shown in Fig. 7. It can be seen from Fig. 7 that the position with the largest bending moment of the section is located at the edge of the artificial false top. The shape of the 3# stope is extremely irregular, and the stress concentration is easy to occur. The bending moment of the section is large and the damage is easy; 24#, 5#, The artificial moment of the edge of the 9# stope has a large bending moment, and there is a potential danger of damage. It is necessary to pay attention to the overlap of the artificial false top and the surrounding rock mass to ensure sufficient overlapping strength.

4 discussion
(1) The settlement value increases linearly at the initial stage of excavation, and the increase is larger. With the excavation, the increase of settlement value decreases. The settlement value increases slowly and nonlinearly at the end of excavation. At the end of excavation, the surface The maximum settlement value is 2.8 cm. The vertical shear stress of the vertical ore body in the bedrock is the largest. When the excavation of the -95~-85m horizontal ore body is completed, the shear stress of the whole bedrock layer exceeds the allowable value of the shear stress, and the bedrock has the safety hazard of the through-crack. Therefore, the mining depth should not exceed -95m level to ensure mining safety.
(2) After the excavation, the artificial false top edge is prone to stress concentration, the section bending moment is the largest, and the potential safety hazard of the damage is large. It is recommended to strengthen the lap joint strength of the artificial false roof and the surrounding rock mass.
(3) The simulation analysis of the overall settlement displacement value shows that the settlement value is small after the excavation is completed (-85m level), and the stability of the stope is good, but the simulation analysis of the bedrock layer shows that the shear of the bedrock is excavated to the level of -95m. The stress has exceeded the allowable value of shear stress, and there is a danger that the crack will cause the stope to be filled. Therefore, the mining depth should not exceed -95m, indicating that underwater mining and land mining have different characteristics.

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Article source: "Modern Mining"; 2017.1

Author: Zhao Lijun; plant and dam Gansu Nonferrous Metal Co., Ltd. plant dam lead zinc ore

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