At present, the research on mining approach is mainly for the shape of roadway section and the optimization of roadway support parameters, but there are few studies on the optimization of mining route section parameters, especially after the excavation of large section soft rock mining road. The stability is poor and the deformation is large, which causes serious deformation and damage of the roadway, high support cost and slow tunneling speed, which greatly affects the mine production efficiency. Li Guichen et al. optimized the section shape of high ground stress roadway, and proposed the concept of “equivalent excavation” and the concept of “invalid reinforcement zone” for roadway support [1]; Meng Qingbin et al. carried out the section shape of deep high stress soft rock roadway. According to the optimization of numerical simulation, the circular and elliptical shapes are the optimal section shape of the deep high-stress roadway according to the magnitude of the lateral pressure coefficient and the principal stress direction [2]. Xing Jun et al. The optimization analysis considers that the reasonable section shape is hexagonal, and the optimal approach size parameter is 3.5m×3.5m×60m (width×height×length) [3]; Wang Wei et al chose different shapes of roadway The section is analyzed and it is found that the difference of section shape has a great influence on the roadway support effect [4]; Feng Wei et al selected six common mining roadway section shapes for numerical simulation analysis, and the results show that the straight wall semi-circular arched roadway The amount of deformation is the smallest, and the effect of reducing the bottom drum is obvious after adding the reverse arch [5]. Relevant experts and scholars have carried out theoretical analysis, similar simulation experiments, numerical simulation calculations and field tests on soft rock roadway support technology, and obtained a series of research results, which solved many theoretical and engineering key technical problems [6-10].
Hemushan iron ore mining approach in a weak formation, its main feature is extremely weak, loose and broken, and the phenomenon of powder ore softening and significantly, such that the mechanical properties of rock and a significant reduction of weakness, in the roadway affected bymiming The deformation and damage in the later period is serious. Based on this, this paper uses FLAC3D numerical simulation to reveal the deformation characteristics of surrounding rock and the distribution of plastic zone after excavation of mining roads under different section sizes, and proposes reasonable support design for large-section roadway support problems. Mine filling mining provides technical support.
1 Numerical simulation analysis of section size optimization of mining approach
1.1 Models and parameters
The section of the mining route is a three-heart arch, and the selected section dimensions (b×h) are 7.0m×7.0m, 6.0m×7.0m, 6.0m×6.5m, 6.0m×6. 0m, 6.0m×5.5m, 6.0m×5.0m, 5.0m×6.5m, 5.5m×6.5m, 6.5m×6.5m, 7.0m×6.5m, 7.5 m × 6.5 m, 8.0 m × 6.5 m, and the model size is 60 m × 60 m × 60 m (length × width × height). Limit the displacement of the left and right bottom surfaces of the model. The top surface is the stress boundary. The uniform load is applied according to the buried depth of the roadway to simulate the self-weight P of the overburden layer. The displacement value is initialized after the balance of the initial geostress field. The calculation model is shown in Figure 1. The physical and mechanical parameters of the surrounding rock mass are shown in Table 1. The Mohr-Coulomb yield criterion is used in this model to reveal the deformation characteristics and plastic zone distribution of the surrounding rock after excavation under different section dimensions to determine the optimal section size.

1.2 Numerical simulation results and analysis
1.2.1 Plastic zone distribution of surrounding rock of roadway
After excavation of the roadway, the stress distribution in a certain range of surrounding rock of the roadway is redistributed, and the stress distribution is uneven. The stress continuously shifts from the periphery of the roadway to the deep part of the surrounding rock, forming a high stress concentration in the two gangs, under different section sizes. The distribution of the plastic zone of the surrounding rock of the mining road is shown in Fig. 2. The maximum damage depth of the plastic zone of the surrounding rock of the mining road under different section dimensions is shown in Fig. 3.

It can be seen from Fig. 2 and Fig. 3 that the top and bottom of the mining road and the two gangs 3 are obviously damaged. Under the condition of a certain section width, the shear failure of the arch begins to gradually deepen with the increase of the height of the roadway. Other parts are extended, and the damage to the floor and gang of the roadway is increased. When the section width is 6.0m, as the roadway height increases from 5.0m to 7.0m, the maximum damage depth of the roof increases from 2.023m to 2.204m, an increase of about 8.95%; the maximum damage depth of the gang is 1.799m. It increased to 2.893m, an increase of about 60.81%; the maximum damage depth of the bottom plate increased from 1.466m to 2.303m, an increase of about 57.10%. Under the condition of a certain section height, with the increase of the width of the roadway, the depth of the plastic floor of the top and bottom of the roadway and the gang have an obvious increasing trend. When the section height is 6.5m, the maximum damage depth of the roof increases from 1.751m to 3.020m with an increase of about 72.47%. The maximum damage depth of the gang increases from 2.368m to 3.111m. About 31.38%; the maximum damage depth of the bottom plate increased from 1.675m to 2.848m, an increase of about 70.03%. The damage range of the surrounding rock gang of the mining road is larger than that of the top floor. The plastic zone of the roof and the floor has the largest increase rate and the damage is gradually increased.
1.2.2 Deformation characteristics of surrounding rock of roadway
After the excavation of the roadway, the stress is released, and the surrounding rock produces different degrees of displacement along the free surface of the excavation. In order to analyze and study the deformation characteristics of surrounding rock after excavation of mining roadway under different section sizes, the maximum vertical and horizontal displacement of surrounding rock of mining road under different section dimensions are arranged in the roof, floor and gang of monitoring roadway. See Table 2, the displacement evolution law of surrounding rock in mining approach under different section dimensions is shown in Fig. 4.

It can be seen from Table 2 and Figure 4 that the maximum vertical displacement of the surrounding rock after mining excavation occurs in the center of the top and bottom plates, and the maximum horizontal displacement occurs in the center of the two gangs. The maximum horizontal displacement of the surrounding rock gang is less than the maximum vertical of the top and bottom plates. The amount of displacement. When the section width is 6.0m, as the roadway height increases from 5.0m to 7.0m, the maximum sinking amount of the roof increases from 269.70mm to 305.55mm, an increase of about 13.29%; the maximum deformation of the gang From 197.90mm to 296.39mm, the increase is about 49.77%; the maximum bottom drum of the bottom plate is basically maintained at about 276.10mm. When the section height is 6.5m, the maximum sinking amount of the roof increases from 242.72mm to 380.25mm with the width increasing from 5.0m to 8.0m, which is about 56.66%. The maximum deformation of the gang is from 246.56mm increased to 278.09mm, an increase of about 12.79%; the maximum bottom drum volume of the bottom plate increased from 248.21mm to 324.46mm, an increase of about 30.72%. In comprehensive comparison, under the conditions that can meet the normal production conditions, the selection of the section with the size of 5.0m×6.5m is more conducive to the self-stability of the surrounding rock of the mining road.
2 large section roadway support design
For large cross section roadway support problems, using high-performance prestressed anchor, anchor mesh metal mesh, shotcrete and steel joists consisting of shotcrete support program. The prestressed anchor can ensure the initial anchoring structure of the shallow surrounding rock and control the overall deformation of the surrounding rock of the roadway. The shotcrete can close the surrounding rock surface in time, and isolate the contact between air and water and surrounding rock, effectively preventing the weather caused by weathering and deliquescence. Rock failure and spalling, reducing the loss of surrounding rock strength; steel mesh can maintain relatively broken rock between anchors, prevent rock falling, improve the overall effect of bolt support, and resist the collapse of broken rock between anchors The pressure increases the support capacity of the supporting structure to the surrounding rock; the joist can connect several anchors together to form a combined effect, and the certain rigidity of the joist can keep the loose rock mass between the anchors intact.
The anchor rod adopts high-performance rebar anchor, the specification is 20mm×1800mm, the spacing between the rows is 700mm×800mm, and the rod body adopts BHRB400 left-handed without longitudinal reinforcement. The diameter of the bolt hole is 28mm. It is extended by a roll of fast 2350 and a roll of medium speed 2350 resin. The anchoring length is not less than 800mm, the anchoring force is not less than 100kN, and the pre-tightening force is not less than 50kN. The bolt preload is selected to be 30% to 50% of the yielding load of the shaft, and the design preload of the 20mm bolt is 50.24kN). The tray is made of arched high-strength tray with a size of 150mm × 150mm × 12mm.

The steel joist beam is welded by 14mm rebar, the size of the vault joist is 2600mm×80mm, the specification of the joist joist is 2200mm×80mm, and the longitudinal reinforcement of each section is welded at the position of the installation bolt, the spacing is 50mm. The two adjacent steel joists are overlapped, and the two steel reinforcement joists are pressed by the anchor.
The steel mesh is welded with 6mm rebar, the mesh size is 2000mm×1000mm, the mesh size is 100mm×100mm, the mesh lap length is 100mm, and the lap joint is double-stranded with 8# wire double-breasted lashing connection. The reinforcing bar and the anchor are pressed.
The shotcrete has a strength grade of C25, a mixing ratio of 1:2:2, a 3% to 5% accelerator, and a spray thickness of 100 mm, covering the anchor tray. The top anchor net spray support structure is shown in Figure 5.

During the implementation of the anchor net spray support site, the convergence deformation of the surrounding rock in the underground mining approach was monitored (Fig. 6). The results show that the deformation of the surrounding rock of the mining road is 32~42mm after the optimized section and the anchor net spray support, which effectively controls the deformation and damage of the surrounding rock of the large section mining road, ensuring the tunneling efficiency and construction safety. It provides a guarantee for efficient mining of mines.

3 conclusions
(1) When the section width is constant, as the height of the roadway increases, the shear failure of the arch foot begins to gradually extend to the deep and other parts, and the damage range of the floor and the gangway of the roadway increases significantly. When the height of the section is constant, as the width of the roadway increases, the depth of the plastic floor of the top and bottom of the roadway increases obviously. Roadway gang damage range than top
The bottom plate is large, the plastic zone of the top plate and the bottom plate has the largest increase rate, and the damage is gradually intensified.

(2) The maximum vertical displacement of surrounding rock occurs after the excavation of the roadway occurs in the center of the top and bottom plates. The maximum horizontal displacement occurs in the center of the two gangs. The maximum horizontal displacement of the surrounding rock gang is less than the maximum vertical displacement of the top and bottom plates. Under the condition that it can meet the normal production, the selection of the section with the size of 5.0m×6.5m is more conducive to the self-stability of the surrounding rock of the mining road.
(3) For the large-section roadway support problem, the high-performance prestressed anchor, metal mesh, steel joist and shotcrete combined support scheme is proposed to effectively control the deformation and damage of the surrounding rock of the large-section mining approach. It provides a guarantee for safe and efficient mining of mines.

references
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