Research on the Evolution Characteristics of Floor Stress and Reasonable Layout of Roadways in Deep Coal Mining

The safe and efficient mining of deep coal resources is severely restricted by the dynamic disasters caused by high gas and high ground stress. Taking a deep mine in China as the research background, a mechanical model of the front supporting stress with the working face was constructed through theoretical calculations. Based on the limit equilibrium theory, the stress distribution in the plastic zone and the elastic zone of the lateral working face was derived; based on semi-infinite plane mechanics model, the floor vertical and horizontal stress distribution was deduced. Then, the roadway surrounding rock stress and displacement field distribution evolution characteristics were revealed through numerical simulation. On this basis, the reasonable floor gas drainage roadway (FGDR) layout was determined: internal staggered layout was used with FGDR, the vertical distance to the working face is 20m, and the horizontal distance to the working face end is 15m; the open-off cut of FGDR was arranged in an external staggered layout, the vertical distance to the working face is 20m, and the horizontal distance to the open-off cut of the working face is 15m. It is an important practical significance for the layout of FGDR, the control of surrounding rocks, and the improvement of gas drainage effects under similar conditions through the research results.


Introduction
Among the main coal mining countries in the world, such as the United States, Australia, Germany, the United Kingdom, Poland, and Russia, other mining industries are relatively developed, while Germany and Russia enter deep mining earlier. There are 30 mines in Russia's Donbass mining area with mining depth of 1200-1350 m [1][2][3][4][5][6]. The average mining depth of a Polish coal mine is 690 m, and the deepest coal mine has reached 1300 m. The coal mining depth in Japan and Britain has reached 1125 m and 1100 m, respectively. China is rich in coal resources and widely distributed, and its coal output ranks first in the world. 76.3% of the total coal resources are buried more than 600 m, and 59.5% are buried more than 1000 m. With the increase of mining intensity, the mining depth of China's coal mines increases at a rate of 8-12 m per year. A large number of coal mines have entered the deep mining stage rapidly. There are 55 coal mines with mining depth of more than 1000 m, mainly distributed in Shandong, Henan, Anhui, Hebei, Heilongjiang, Jilin, and Liaoning in the eastern and northeast regions. The maximum mining depth reaches 1501 m [7][8][9][10][11][12]. The specific distribution is shown in Figure 1. It is estimated that in the next 10 years, most of the existing coal mines will enter the deep mining environment of 1000-1500 m. With the increase of coal mining depth, it is mainly faced with the problems of high gas, high ground stress, and high ground temperature, especially the compound dynamic disaster caused by high gas and high ground stress, which seriously restricts the safe production and becomes the bottleneck restricting the safe and efficient mining of deep coal resources.
The deep rock mass is in the harsh environment of "high ground stress, high temperature, and high karst water pressure and dynamic disturbance of blasting and mechanical excavation." The mine pressure of roadway is strong, and maintenance is difficult. According to incomplete statistics, when the depth of a mine is 1000 m, the repair rate of roadway is about 3-15 times that of 500-600 m under the same conditions, and the serious disrepair rate of some mine roadways is more than 20% [13,14]. Shoushan No. 1 Coal Mine of Henan Pingbao Coal Industry Co., Ltd. has an average buried depth of more than 750 m, stratum dip angle of 8-12°, and well-developed geological structure. It is a high gas outburst mine. Under the condition that there is no mining protective layer to realize regional outburst prevention, only through the construction of FGDR, cross layer drilling was used to implement predrainage outburst prevention technology for coal seam gas. Due to the double effects of driving and mining stress, the surrounding rock of roadway, especially the floor, is continuously deformed. The key parameters such as vertical distance and horizontal distance between FGDR and coal seam roadway directly affect the amount of drilling work, gas drainage amount, and protection effect. Therefore, reasonable roadway layout can not only eliminate the gas disaster caused by roadway excavation and working face mining but also effectively reduce the influence of roadway excavation and working face mining on floor gas roadway, making the roadway in low stress environment for a long time and greatly reducing the maintenance difficulty. Therefore, the location selection of FGDR has important practical significance for its own stability and gas control effect.
The evolution characteristics of floor stress distribution caused by upper coal seam mining have great influence on the location selection and surrounding rock control of FGDR. At present, the research results of the distribution and evolution characteristics of the floor rock layer in the upper coal seam working face are mainly based on the finite element numerical calculation and similar material simulation experiments and lack of theoretical research on the floor rock mass movement. At the same time, the floor stress field monitoring research has the characteristics of difficult testing and high cost, and the research progress is slow [15][16][17][18][19][20]. In addition, a large number of domestic and foreign scholars have made a lot of research results on the influence of floor level and gas layer [21][22][23][24][25][26]. However, in view of the differences of mine hydrogeological conditions and mining technology level, the existing research results still do not have universal applicability. Therefore, for the high gas outburst mine under specific conditions, the selection of reasonable FGDR layout still needs to be combined with the field production conditions for differential research.

The Project Overview
Shoushan No. 1 mine is located in Pingdingshan City, Henan Province, China. The designed annual production capacity of the mine is 0.3 Mt, and the service life is 92 years. At present, No. 15 and No. 16 coal seams are mainly mined, and the coal seam structure is relatively simple. The direct roof of the coal seam is mainly composed of sandy mudstone and mudstone, occasionally with fine-grained sandstone and siltstone. The

Geofluids
floor is mainly composed of mudstone, and part of it is fine-grained sandstone, with occasional false bottom of carbonaceous mudstone. The hydrogeological conditions of the mine are simple, the main coal seam has outburst risk, the gas content is 10.46 m 3 /t, the gas pressure is 1.38 MPa, there is spontaneous combustion tendency, the spontaneous combustion period is 4-6 months, and there is coal dust explosion risk. Based on the safety mining of 12050 working face as the engineering background, the evolution characteristics of floor stress distribution caused by 12050 working face mining were revealed in the paper. On this basis, the layout layer of FGDR is selected, and the relative spatial position relationship between FGDR and upper working face is determined, which creates favorable conditions for gas drainage in 12050 working face. The plan of excavation engineering is shown in Figure 2, and the physical and mechanical properties of rock stratum are shown in Table 1.

Study on Evolution Characteristics of Stress Distribution in Coal Seam Floor
The research on the evolution characteristics of floor stress distribution caused by coal mining is of great practical significance to master the deformation law and failure characteristics of floor rock, to predict water inrush from floor, and to design the position and maintenance technology of floor roadway. The distribution of lateral support stress caused by working face mining is determined by means of theoretical calculation and numerical simulation, and the distribution law of floor rock stress after coal mining is discussed.

Geofluids
3.1. Theoretical Calculation Research. After coal mining, the evolution characteristics of floor stress distribution depend on the propagation law of lateral support stress in floor, as shown in Figure 3. Among them, x 0 is the range of plastic zone in front of working face; σ z1 is the inner supporting stress of plastic zone; x 1 is the scope of elastic zone; σ z2 is the inner supporting stress of elastic zone; l 0 is the total width of elastic zone and plastic zone; Kγh is the peak value of vertical stress, which is located at the interface of elastic-plastic zone; K is the stress concentration factor; γh is the original rock stress of coal seam floor.
When the coal strength is less than the roof strength, the following assumptions are made: (1) the coal is a continuous, homogeneous, and isotropic elastic body; (2) the displacement and deformation of the coal pillar before yielding are small; (3) the shear failure of the coal pillar occurs, and the shear failure surface is parallel to the coal seam; (4) when the coal pillar yields locally, the yield area is regarded as the elastic limit state, and the coal body in the yield area is regarded as a linear elastic body; (5) there is no weak plane in coal pillar. Due to the accumulation of coal gangue at the bottom of goaf and the subsidence of coal pillar under the action of self-weight stress of overlying strata, the lower part of coal pillar can be regarded as in three-dimensional stress state, and its stability is strengthened. Therefore, the stress state of the upper and middle coal pillar is mainly considered in the study.
Because the strength of the strata is greater than that of the coal, the transverse deformation of the coal seam is greater than that of the rock under the self-weight stress of the overlying strata. Therefore, the coal seam has a trend of outward movement relative to the rock stratum, that is, the friction force formed between the coal seam and the roof, that is, the shear stress, as shown in Figure 4. τ xz is the shear stress, σ x is the horizontal stress in the coal pillar, P x is the support strength of the side entry in the goaf, and m is the thickness of the coal seam.
Under the action of self-weight stress of overlying strata, the coal body will have shear failure under the action of compressive stress, and the failure mode conforms to Mohr-Coulomb criterion. Taking a cross section of coal pillar as the research object, its stress is about the neutral plane symmetry, and the stress of coal pillar can be regarded as plane strain problem. Therefore, the differential equation of the plane strain problem is satisfied.
The lateral support stress σ z1 and shear stress τ xz in the limit equilibrium zone x 0 are as follows: According to the equilibrium relation, there is ∑F X = 0, that is, In addition, σ x e = Aσ z e is substituted into equation (3) to obtain the stress distribution in elastic zone: When x = x 0 + x 1 , σ y = γH, the elastic zone width is obtained by substituting equation (4): As shown in Figure 5, take any point M ðx, zÞ on the bottom plate, take any micro segment dξ along the x direction, and regard the force dP = Qdξ as a tiny concentrated force.
Figure 4: Mechanical model of limit equilibrium in coal pillar.

Geofluids
The stress state formed by the tiny concentrated force at point M is as follows: In order to obtain the effect of all distributed loads on point M, the stresses caused by each tiny concentrated force should be superimposed, that is to say, the integral of formula (1) from ξ = −a to ξ = b is obtained: The vertical stress σ z11 and horizontal stress σ x11 produced by the load in the limit equilibrium zone are as follows: Through the superposition of stresses, the stress of the bottom plate is obtained: Among them are the following: It can be seen from Figure 6 that the stress distribution law of coal seam floor caused by working face mining is as follows: (1) In the range of 5 m in front of the working face, the bearing capacity of coal body is sharply reduced and the vertical stress is low; the range of 10-20 m in front of the working face is the stress concentration area; along the depth direction of the coal seam floor, the vertical stress increases first and then decreases; in the range of 20 m behind the working face, the surrounding rock is in the area of vertical stress reduction when the depth of coal seam floor is less than 20 m (2) In the range of 10-20 m in front of the working face, the vertical stress in the coal seam floor reaches the peak at the depth of 7 m and then decreases continuously; when the depth of coal seam floor is greater than 20 m, the vertical stress concentration coefficient is between 1. 6  After the excavation of transport roadway in 12050 working face, the floor stress distribution law is shown in Figure 7. It can be seen from Figure 7 that (1) after roadway excavation, a large range of vertical stress reduction area is formed at the top and floor, the depth of the floor stress reduction area is 25 m, and the vertical stress is less than 10 MPa within the range of 7 m of the floor; (2) with the distance from the roadway floor, the stress gradient becomes more gentle; in the horizontal direction, the vertical stress of the floor first decreases and then increases, and the range of the plane stress reduction area with the floor depth of 14 m is the largest; (3) due to the influence of the soft floor and the close coal seam, a large range of horizontal stress concentration area is formed in the floor, with the floor depth greater than 10 m and the front and rear of the roadway 10 m.
After 12050 working face is mined, the vertical stress distribution law of the floor is shown in Figure 8. It can be seen from Figure 8 that (1) after deep mining, the ground pressure appears violently; within 20 m depth of the floor behind the working face, the stress decreases in a triangular distribution; the maximum stress in the area is only 3 MPa, and the stress concentration coefficient is greater than 2 within the range of 5-14 m in front of the working face; (2) when the depth of the floor exceeds 20 m, the depth of the floor is 15 m in front of the working face to 30 m behind the working face; within above range, the maximum vertical stress is 25 MPa, the minimum is 3 MPa, and the stress concentration coefficient is 0.15-1.25, which is the reasonable layout area of FGDR. In front of the working face, the stress concentration coefficient is more than 1.25, the surrounding rock stress of the roadway is large, and the maintenance is difficult; in the rear of the working face, considering that the length of the gas drilling hole should not be too long, the FGDR should not exceed 15 m behind the working face.
Therefore, considering the distribution law of floor stress field after transport roadway excavation and 12050 working face mining, the most suitable area for FGDR layout is floor depth greater than 20 m, 10-15 m in front of the working face and 10-15 m behind the working face.

Research on Layout of Gas Drainage Roadway in Coal Seam Floor
Considering the factors such as the length of gas drainage borehole which should not be too long, the effect of gas drainage, and the maintenance difficulty of FGDR, two kinds of FGDR layout are proposed: external staggered layout and internal staggered layout. The horizontal distance between FGDR and transport roadway of 12050 working face is

FGDR Excavation.
After the excavation of FGDR, the distribution law of surrounding rock stress field is shown in Figure 10. The following can be seen from Figure 10:

12050 Working Face Transport Roadway Excavation.
After the excavation of transport roadway in 12050 working face, the stress field distribution law of roadway surrounding rock is shown in Figures 11 and 12. (1) The excavation of transport roadway is equivalent to the roof pressure relief of FGDR, and the surrounding rock stress is obviously transferred to the direction of transport roadway. The vertical stress of surrounding rock is 16 MPa at the right side depth of 9 m and 18 MPa at the left side depth of 8 m (2) The horizontal stress and the vertical stress of the left side and the two sides of the FGDR increase slightly after the excavation of the transport roadway, while the vertical stress of the right side decreases. With the approach of the transport roadway, the stress difference before and after excavation gradually increases, with the maximum difference of 1.7 MPa. The FGDR is in a stress environment that is easy to be maintained      Figures 13 and 14.       10 Geofluids influence on the floor, followed by the roof, and the two sides have less influence (3) After mining in 12050 working face, the vertical stress of the roof and floor of FGDR has increased, especially the vertical stress of the floor increases greatly, the maximum increase value is 12.2 MPa, the depth of the floor is 10 m, the increase rate is 78.2%, the maximum increase value of the second roof vertical stress is 2.6 MPa, located at the roof depth of 10 m, and the increase rate is 17.2%. Judging from the distribution of horizontal stress, the mining face has the greatest influence on the floor, followed by the left side, the right side next, and the roof the least To sum up, when the FGDR is staggered outside, the stress concentration degree of the two sides and the floor is greatly increased, and the roof is also increased; the horizontal stress of the two sides is basically unchanged, while the horizontal stress of the roof and floor is greatly increased. In addition, after mining, the maximum roof subsidence is 150.3 mm, the floor heave is 47.8 mm, the left side is 104.3 mm, and the right side is 19.1 mm.

Internal Staggered Layout of FGDR.
The numerical simulation model of internal staggered layout of FGDR is established, as shown in Figure 15. When the working face is excavated to half length, the FGDR is arranged in the sand and mudstone with the floor thickness of 12 m, and the middle roadway is the FGDR. The same as the external staggered layout, the FGDR will also be affected by three times of stress disturbance.

FGDR Excavation.
After the excavation of FGDR, the distribution law of surrounding rock stress field is shown in Figure 16. Compared with the external staggered layout, the stress and displacement distribution characteristics of the surrounding rock of the FGDR have no significant difference, so we will not repeat it.     (1) After 12050 working face mining, the vertical stress of two sides of FGDR changes greatly. The peak value   13 Geofluids of vertical stress of left side is 2 m. The maximum stress difference is 14.6 MPa before and after mining. Compared with that before mining, the difference is 156.7%. When the depth is about 10 m, the vertical stress is basically the same before and after mining. The peak value of vertical stress on the right side is at depth 2 m. The maximum stress difference is 14.7 MPa before and after mining, which is 221.6% less than that before mining, and the depth is about 4 m. The vertical stress difference before and after mining is basically the same (2) After 12050 working face mining, the horizontal stress of two sides of FGDR decreased greatly, and the change law of the two sides was basically the same: as far away from the roadway surface, the horizontal stress gradually increased, but the increase gradually decreased and finally tended to be stable; compared with before mining, the maximum reduction value of the left side horizontal stress was 6.2 MPa, which was at the depth of 6 m. The maximum reduction of horizontal stress on the right side is 6.4 MPa, and the depth is 10 m. With the distance from the end of the working face, the horizontal stress difference of the right side of the roadway increases gradually before and after mining (3) After mining in 12050 working face, the vertical stress of FGDR roof and floor is reduced. With the distance away from the roadway surface, the stress difference before and after mining gradually increases, especially in the roof, the maximum reduction value is 12.9 MPa. After mining, the maximum vertical stress of roof is only 3.84 MPa, and the maximum vertical stress is 5 m deep (4) After mining in 12050 working face, the horizontal stress at the roof of FGDR changes greatly, and the horizontal stress of floor increases. Within the depth of 5-6 m, the horizontal stress of the same position before and after mining is equal; the horizontal stress difference of roof before and after mining changes greatly, the maximum value is 4 m deep, the maximum decrease value is 15.5 MPa, and the decrease rate is 119.2% To sum up, the mining face has a great influence on the vertical stress and horizontal stress of the surrounding rock   14 Geofluids of the FGDR, and the vertical stress of the right side is the biggest. In addition, after mining, the roof subsidence of FGDR is 78.0 mm, the floor heave is 105.5 mm, the left side is 149.7 mm, and the right side is 48.8 mm.

Determination of the Position of Open-Off Cut in FGDR.
The vertical distance between the open-off cut of FGDR and No. 15 coal seam is 20 m. After 12050 working face is mined, the distribution law of surrounding rock stress and displacement is shown in Figure 21.
(1) The outer side of 12050 working face is the stress increasing area, the area with horizontal distance more than 15 m from the working face changes gently, the displacement range is between 50 and 100 mm, and the stress concentration coefficient is To sum up, the open-off cut of the FGDR should be arranged outside the working face and the horizontal distance from the working face is greater than 15 m. Considering the need of gas drainage, the vertical distance between the FGDR and the working face is 20 m, and the horizontal distance is 15 m.

Determination of Position of FGDR.
After the 12050 working face is mined, the deformation of the surrounding rock of the roadway is shown in Figure 22 when the external and internal staggered layouts are carried out. The displacement of the roof and floor and the displacement of two sides are 198.1 mm and 183.5 mm and 123.4 mm and 198.5 mm, respectively, when the external and internal staggered layouts are carried out. Compared with the external staggered layout, the two sides of the roadway move closer, but the roof subsidence is significantly reduced, the safety factor of the roadway is significantly improved, and the vertical stress peak value of the left side of the roadway is more than 40 MPa; the surrounding rock is in a high stress environment for a long time, which is not conducive to the stability of the whole roadway, significantly reducing the gas drainage effect and increasing the safety risks of the working face. Although the floor heave of the roadway is large, the surrounding rock of the working face is in a low stress environment for a long time after mining, which is conducive to the long-term stability of the roadway. Therefore, the layout of FGDR is determined as internal staggered layout.
To sum up, internal staggered layout is the FGDR, with a vertical distance of 20 m from 12050 working face and 15 m horizontal distance from the end of 12050 working face; external staggered layout is adopted by the open-off cut of FGDR, with a vertical distance of 20 m from 12050 working face and 15 m horizontal distance with the open-off cut of 12050 working face. The specific spatial position relationship is shown in Figure 23.

Conclusions
(1) Based on the limit equilibrium theory, the stress distribution in the lateral plastic zone and elastic zone of

Data Availability
The data used to support the findings of this study are available from the corresponding authors upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.