Coupling support technique for coal roadway under double gobs in close coal seams

In the process of lower coal roadway support under double gobs in close coal seams, there are problems such as the whole destruction of the lower coal seam and the intermediate strata, the thin roof of the lower coal roadway with the smaller support space, the upper rock of the lower coal roadway without anchor bearing layer, the roof falling, the anchor cable offing, and so on. On the basis of the engineering background of No. 11103 haulage roadway in Fenxi coal mine, the research methods of theoretical modeling analysis, numerical simulation analysis and field engineering test are comprehensively adopted. In this paper, the layout of the roadway with floor insertion in the lower coal seam, the distribution characteristics of surrounding rock deviatoric stress, the control principle of broken surrounding rock and the coupling support method of shed–cable–prop are systematically studied. The study shows that many measures are needed to ensure the stability of the surrounding rock in the strong mining roadway of the lower coal seam. The lower coal seam roadway section was modified from the original rectangular section to adopt an arched section to counteract the broken roof. The staggered position of the lower coal seam roadway layout was modified from the original coal roadway position to a coal–rock roadway, so as to avoid the area of high deviatoric stress in the coal pillar and the area of crushed coal body, and to leave an effective support space for the roof. The support scheme of the lower coal seam roadway was improved to use anchor cable and single prop to support the weak section of U‐type steel shed with local coupling, and realize unequal force coupling by adjusting the size of the force at different coupling points. In the field monitoring the deformation of the roadway is small, and the support effect is good.


| INTRODUCTION
The research on the stability of surrounding rock of the lower coal roadway in close coal seams is a cross-subject involving geology, mining, mechanics, physics, and so on.The premining roadway of the lower coal roadway in close coal seams is affected by the strong mining of the upper coal seam, resulting that the roof structure of the premining area in the lower coal seam is multifissure and unstable, and the stress environment is complicated. 1,2he phenomenon of stress concentration occurred in the section coal pillar left in gob after the mining of the upper coal seam, and the key blocks in the overlying strata structure were rotated and unstable, which caused the punching failure of the floor.Both of them have a strong influence on the layout of the roadway and the stability of the surrounding rock in the lower coal seam.4][5] The double pressure relief during the strong mining of the close coal seam group caused the coal seam roof to be destroyed many times, and because of the failure degree of the rock fissure.The fissure expansion degree and range are larger than those of the single coal seam mining fissure.The related factors of floor failure in close coal seam are coal seam mining thickness, coal seam buried depth, geological structure, coal seam dip angle and rock seam dip angle.
In the numerical simulation study of deviatoric stress in the roadway, Xie et al. 6 analyzed the asymmetric evolution law of the deviatoric stress in the surrounding rock of the back-mining roadway based on the combination of the deep mining, filling mining and gob-side entry retaining each other, and emphasized that the focus of the control of the surrounding rock and the main point of the support is the rock body in the peak of the deviatoric stress zone.Yu et al. 7 study the stability of surrounding rock in various stress conditions and compare the relationship between the distribution of plastic zone and deviatoric stress.The models of positive and angular symmetric instability under various lateral pressure coefficients are obtained.He and Zhang 8 obtained the instability mechanism of high-level stress roadway by studying the distribution of deviatoric stress and plastic zone of roadway surrounding rock under different lateral pressure coefficients.Xu et al. 9 used the numerical simulation method to study the characteristics of the deviatoric stress field in the floor with coal pillars.The results show that with the increase of coal pillar width, the peak value of deviatoric stress of coal pillar floor decreases, which can guide the reasonable layout of coal roadway in a short distance.
1][12][13][14][15][16] However, there is little or no research on the geological conditions of the upper and lower coal seams which are very close to each other.In this paper, the study of the upper and lower coal seam is 1 or 0 m spacing, the mining characteristics of double gobs, this study fills the previous study [17][18][19][20][21][22] of very close coal seams mining.At present, a large number of scholars [10][11][12][13][14][15][16][17][18][19][20][21][22] have focused on two main aspects in the support of lower coal roadway under close coal seams.On the one hand, it concentrates on the location of the lower coal roadway arrangement, and the roadway of the lower coal seam tries to avoid the stress concentration area of the coal pillar as much as possible, and it is applied more on the horizontal misalignment.On the other hand, the support of the lower coal roadway, most of the U-type steel shed, anchor cable for support, but lack of U-type steel shed and anchor cable coupling support research.This thesis comprehensively adopts research methods, such as theoretical modeling analysis, numerical simulation analysis, and on-site engineering tests.The results of this paper can be used for reference and guidance to the layout of roadway in close coal seams, the law of surrounding rock broken and deformation, the control mechanism of surrounding rock stability and the technique of shed-cable-prop coupling support.

| Project overview
For this paper to deeply analyze the geological conditions of the strong mining coal roadway under double gobs in the close coal seam, the mine is Fenxi Coal Mine located in Xiaoyi City, Shanxi Province, China, and its geological environment is shown in Figure 1.The integrated histogram shows that the upper 9# seam (1.6 m) and the lower 10 + 11# (7.8 m) coals are extremely close seam structures, with 1 m thick mudstone as the interbedded rock.The overlying rock stratum of the 9# seam is a double-main roof structure, the lowmain roof is 7 m thick k2 limestone, the high main roof is and 5 m thick k3 limestone, and the intermediate interlayers are 3 m thick mudstone, 4.1 m thick fine-sandstone, and 2.3 m thick sandy-mudstone.The already mined workings 7102 and 7014 in the 9# seam leave a 16 m wide coal pillar in the center.The left side of 10 + 11# coal is already mined working face 7104 with 2 m thickness, and the right side is already mined working face 11101 with 7.8 m thickness, leaving 28 m wide coal pillar in the middle.
Premining No. 11103 working face is arranged in the 10 + 11# coal seam on the left side of the lower part of the seam, the average thickness of the coal seam is 5.8 m, which belongs to the Carboniferous Taiyuan Formation, the average inclination angle of the coal seam is 6.5°, for the near-horizontal coal seam, the coal seam is more complex, and the gangue is multilayer mudstone.The floor of the 10 + 11# coal seam is 6.13 m thick sandymudstone.The studied close lower seam No. 11103 haulage roadway is arranged in the lower part of the already mined working face 7104, with the upper part of the roof being a double gobs structure, as shown in the spatial arrangement of the working face in Figure 1.

| Control difficulties in roadway
The lower coal roadway support under double gobs in close coal seams has the characteristics of double gobs, multiple mining, asymmetric mining and fully mechanized caving mining.The mining difficulties and field observations are shown in Figure 2.  | 2387 F I G U R E 2 Mining engineering drawing and control difficulties of panels.

| Numerical model
On the basis of the engineering geological conditions and the research requirements of the coal mine in Fenxi, a three-dimensional (3D) numerical model of x × y × z = 325 m × 220 m × 105 m (length × width × height) is established as shown in Figure 3.According to the coal seam buried depth of 231 m, the average unit weight of overlying strata is 25 kN/m 3 , the upper applied load is 4.03 MPa, the lateral pressure coefficient is taken as 1.2, the gravitational acceleration is set at 9. 80 m/s 2 , the working face is advanced along the y-direction, the xdirection displacements of the left and right boundaries are fixed, the y-direction displacements of the front and rear boundaries are fixed, and the z-direction displacements of the bottom are fixed.The Mohr-coulomb model is adopted, and the mechanical parameters of overlying strata are shown in Table 1, the data for this table comes from the mine.

| Evolution law of deviatoric stress of surrounding rock in roadway
In this paper, based on the previous basic research, [6][7][8][9] the deviatoric stress is used as the model analysis index.It can better explain the reasonable layout and control of the lower coal seam roadway under double gobs in close coal seams, and understand the mechanical essence of the deformation, and failure of the floor strata and the surrounding rock of the roadway.The deviated stress state is what remains after subtracting the hydrostatic pressure from the stress state, as shown in Figure 4.
s 1 is the maximum deviatoric stress tensor, as the main reference.The expression of s 1 is (1) According to the mining condition of the lower coal roadway under double gobs in close coal seams, the 3D numerical model of sequential mining is shown in Figure 5.The distribution of the deviatoric stress in the four mining stages is as follows: ① The distribution of deviatoric stress at one side of the gob stage is as follows: the peak value of deviatoric stress is located at the edge of the coal wall, showing a single peak value, and the deviatoric stress of floor has become an asymmetrical distribution.
② In the stage of double gobs on one side, the upper coal wall edge is at the peak value of deviatoric stress, the lower coal wall edge is at the area of deviatoric stress reduction, and the difference between the deviatoric stress value of the lower coal wall edge and that of the upper coal wall edge is large.
③ The general trend of deviant stress at the stage of gob on both sides of the upper coal seam and gob one side of the lower coal seam is as follows: the deviatoric stress double peak zone appears in the single coal pillar, the whole appears "Saddle-shaped" distribution, the single coal pillar concentration stress disturbance range increases.
④ The overall distribution trend of deviant stress at the double gobs stage on both sides of the upper and lower coal seams is as follows: the peak value zone of deviatoric stress of coal pillar changes from double peak value zone to single peak value zone, and the | 2389 disturbance range of concentrated stress of doublelayer coal pillar increases.

| Horizontal offset effect of deviatoric stress evolution
The distribution curve of deviatoric stress of surrounding rock in the lower coal roadway under double gobs in close coal seams with different horizontal stagger spacings is shown in Figure 6.For example, point (26, 4.63) means 26 m away from the coal pillar, and its peak deviatoric stress is 4.63 MPa.The analysis of deviatoric stress curve shows that the deviatoric stress peak value appears in the range of 1-4 m of roadway pillar side under double gobs, and the deviatoric stress peak value appears in the range of 5-25 m of pillar floor.The overall distribution trend of deviating stress of coal pillar side presents multipeak value and decreases with the increase of horizontal offset of roadway.When the horizontal offset is 8 and 10 m, the peak value of the first eccentricity stress of the coal pillar is 5.63 and 5.39 MPa, respectively, which is larger than the peak value of eccentricity stress of the coal pillar floor.When the horizontal offset is 12 and 14 m, the first peak of deviation stress of the coal pillar side is smaller than the peak of deviation stress of the coal pillar floor.
The deviatoric stress cloud diagram in Figure 6 shows that with the decrease of horizontal offset, the roadway roof area changes from a low deviatoric stress area to a high deviatoric stress area.When the horizontal offset is less than 12 m, the high eccentricity stress in the roof area and the high eccentricity stress in the coal pillar influence each other and fuse.When the horizontal offset is more than 12 m, the high deviating stress in the roof area does not fuse with the high deviating stress in the coal pillar.The low deviating stress area is formed by the combination of the low deviating stress of the floor and the low deviating stress of the roadway area under the double gobs.

| Vertical offset effect of deviatoric stress evolution
When the roadway is arranged in the floor of the lower coal seam, the roof of the roadway and the two sides of the roadway are arranged in the damaged floor coal seam at the same time, which will lead to poor stability of the surrounding pressure of the roadway.Therefore, the method of vertical stagger is adopted to change the roadway from full coal roadway to half coal and rock roadway and to enhance the stability of both sides of the roadway and the surrounding rock of the floor.The comparative analysis of different vertical offsets (i.e., vertical bottom insertion) roadway wall rock deviatoric stress cloud diagram is as follows: The distribution curve of deviating stress with different vertical offsets is shown in Figure 7.The analysis shows that under the double mined-out area, the peak value of deviatoric stress appears in the range of floor line presents outward expansion and gradually diminishes.

| Control countermeasures
The research methods of the theoretical modeling analysis, numerical simulation analysis and the field engineering trials were used to explore the arrangement of coal roadway, deviatoric stress environment and failure mode of the surrounding rock, control principle and method of fractured surrounding rock.The results show that: 1.The lower coal seam roof was destroyed overall, the arch section was used to confront the broken roof, and the hollow grouting anchor cable was used to improve the stability of the broken surrounding rock.2. The roadway was arranged by the bottom plate to avoid the roof and the two plates in the broken coal seam, and the bottom depth is 1.5 m, so that the two plates were in the bottom rock layer with good integrity, with a high carrying capacity, and can leave support space in the upper roof.3. To establish a mechanical model of coupled support of shed-cable-prop coupling in arched roadway, and to derive the location of the weak section of the U-shaped steel shed under the bias load force.The anchor cable and the single prop are used to locally strengthen the weak section and realize the unequal force coupling by adjusting the coupling point force, so that the U-type steel shed has no weak section.
Accordingly, the shed-cable-prop coupling support structure is proposed.The control measures are shown in Figure 8.

| Analysis of coupling support structure
1][32][33][34][35][36][37] "U-steel + anchor + prop" refers to Ushaped steel shed, hollow grouting anchor cable and single prop.The coupling support of single prop, hollow grouting anchor cable and U-shaped steel frame is used to deal with Coupled bending stress of U-shaped-steel+anchor+prop under uniform load.
the weak surface of the support structure in case of large deformation of the broken surrounding rock.The synchronous passive support and active support are used to achieve the linkage of deep and shallow control of the broken surrounding rock.The continuous destructive mechanical behavior of the broken roof is interrupted at intervals to prevent overload or damage of a single support.Coupled bending stress under uniform load is shown in Figure 8.
The two-hinged arch model of the coupling support structure of hollow grouting anchor cable, single prop and U-shaped steel frame under the conditions of uniform and eccentric load of surrounding rock on the arch crown, arch shoulder and arch wall, respectively, is established to analyze the weak section position of U-shaped steel frame, and study the bending stress and deformation of U-shaped steel frame after the coupling support of hollow grouting anchor cable, single prop and U-shaped steel frame.
Coupled bending stress of shed-cable-prop under uniform load is shown in Figure 9. Set the arch foot as the fixed hinge, the radius of the arch crown as R 2 , that is, ce section, the radius of the arch shoulders as R 1 , that is, bc and ef sections, and the height of the two sides of the arch as H, that is, ab and fg sections.Release the binding force of hinge bearing a and replace it with X i .X 1 , X 2 , and X 4 are, respectively, the forces exerted on the U-shaped steel frame by the hollow grouting anchor cable at different positions, and X 3 is the force exerted on the U-shaped steel frame by the single strut.The force method equation under the action of the decoupled resultant force can be solved by the force method in structural mechanics.
Where δ 1j is the displacement of hinge bearing a in the hinged arch model under the action of X j , m; Δ 1q is the displacement of hinge bearing a in the hinged arch model under the external load of the arch structure, m.where M xj is the bending moment of each segment of the basic structure under the action of X 1 ; M qi is the bending moment of each segment of the basic structure under the action of q; s is the length along the axial direction of the support structure; E is the elastic modulus; I is the moment of inertia of the section to the neutral axis.After calculation, it can be concluded that | 2395 According to the second theorem of the card type, the U-shaped steel frame supporting structure is less affected by the shear force and axial force, which can be simplified and only the axial force and bending moment can be calculated.Due to the symmetry problem, the bending moment equations of each segment of ab, bc, cd, de, ef, and fg are ab fg bc ef cd df If the horizontal displacement of point a is zero, then

+ ( ) + ( ) + ( ) .
x EI When the three-center arch is uniformly loaded as a whole, as shown in Figure 9, that is, q 1 = q 2 = q 3 = q 4 = q 5 = q, the bending moment equation of each section can be obtained.After derivation, the position of weak section and the maximum bending moment can be obtained as follows: The yield load concentration of U-shaped steel frame support under uniform load is The width of three core arch roadway is 4.5 m, height is 3.0 m, h 3 is 1.5 m, h 1 is 1.5 m, R 1 is 1.174 m, R 2 is 3.104 m, U-shaped steel frame model is U29, height is 124 mm, theoretical weight is 29 kg/m, elastic modulus E is 200 GPa, Poisson's ratio is 0.3, moment of inertia I is 612.1 cm 4 , bending section coefficient W x is 92.3 cm 3 , yield strength is 500 MPa, elongation is greater than or equal to 26%, and the above parameters are substituted into Formulas ( 6)-( 9).It can be obtained that the yield load concentration of U-shaped steel frame is 112 kN/m, the weak section position of arch rib is 0.89 m, and the weak section position of arch shoulder is α 1 /2, the weak section position of the vault is the central axis.
Assume that in the two-hinged arch model of the coupling support structure, there are X 1 , X 2 , X 3 , …, X n in the arch rib, arch shoulder, and arch crown, which are, respectively, the forces exerted on the U-shaped steel frame by the hollow anchor cable at different positions and the single strut, and the force method equations of the shed cable coupling support structure can be obtained as follows: where δ ij refers to the displacement of X j point caused by X i acting on the shed cable coupling support structure, i = 1, 2, 3, …, n, j = 1, 2, 3, …, n; Δ ip is the displacement of X i point caused by each segment load acting on the shed cable coupling support structure; K x is the stiffness coefficient of the support.
In formula (10), the mode of δ ij and Δ ip is xi qi (11)   where M xj is the bending moment of each segment when X j acts on the shed cable coupling support structure.
Under the action of X 1 , X 2 , X 3 , and X 4 , the bending moment equation of each segment of arch wall coupling support is Formula (10).The bending moment equation of each segment of arch shoulder coupling support is + cos ( sin − sin ), The bending moment equation of each segment of arch roof coupling support is ab fg bc ef cd df (13)

| Offset load support analysis
On the basis of the above-mentioned deviatoric stress evolution law of the surrounding rock of the coal seam comprehensively released roadway, it can be seen that the coal pillar side and the upper area are in a high-stress environment affected by the concentrated stress of the coal pillar, and there are different bias loading conditions.Therefore, the following two bizarre load models are set up.Model-1 is that the roof stress is greater than the two sidewalls of the roadway, this situation is the force when the roof of the overlying rock layer comes WU ET AL.
| 2397 under pressure.Model-2 is that the roadway surrounding rock stress is affected by the movement of the coal pillar and overburden, both the roof and the coal pillar sidewall stress are larger, and the stress on the sidewall of the solid coal is smaller than the force on the sidewall of the coal pillar.The analysis model of unequal force coupling support of abnormal load ectopic coupling point is as follows: (1) Symmetric partial load Model-1: The vault load q 3 = 1.5q, the load on both sides of the spandrel and the arch side are q 1 = q 2 = q 4 = q 5 = q, that is, the top pressure is greater than the lateral pressure.
(2) Asymmetric partial load Model-2: The load on the left side of the vault, arch shoulder, and the arch rib is q 3 = q 1 = q 2 = q 4 = 1.5q, and the bag on the arch rib is q 5 = q, that is, the pressure of the vault, one side of the arch rib and both sides of the arch shoulder is greater than that of the other side of the arch rib.
Bring the above load coefficients into Formulas ( 9), (12), and ( 13), respectively, the bending moment equations of each section can be obtained, and the bending moment diagram of each unit of shed cableprop coupling support can be drawn.
In the first model, as shown in Figure 10A, the maximum pressure is greater than the lateral pressure (Model-1) under load conditions.The bending moment of each section of the three-center arch is relatively small when the U-shaped steel shed is supported.The order of the maximum bending moment of each unit is arch rib > vault > arch shoulder.Each section's maximum bending moment values are 38, 36, and 23.5 kN m, respectively.The bending moment of each unit is symmetrical along the vertical center line of the roadway.The bending moment of the vault and the two sides is inward, and the bending moment on both sides of the arch shoulder is outward.
Model-2, as shown in Figure 11A-Conditions for asymmetric eccentric loading of the vault, one side of the arch, and both sides of the spandrel are more significant than the other side of the turn (Model-2).The bending moment distribution law of each section under the coupling support of U-shaped steel shed and shed-cable-prop is: 1.When the U-shaped steel shed is supported, as shown in Figure 11B, under partial load, the bending moment of the left arch is toward the inside of the turn, more significant than the bending moment of the right angle, and the maximum bending moment ratio is about 3.1:1.The left arch shoulder bending moment is toward the inside of the turn, less than the right arch shoulder bending moment, and the proper bending moment is toward the outside of the turn.The maximum bending moment value is 53.3 kN m. 2. When the coupling support of shed cable and prop is applied, the same force is applied to the ectopic coupling point; as shown in Figure 11C, the applied force of anchor cable is In summary, the bending moment of each section of model four is relatively tiny.When the coupling effect is good, the applied force of the coupling point is the applied force of the anchor cable (left) X 1 = X 2 = 120 kN, (right) X 4 = 160 kN, and the applied pressure of the single prop X 3 = 200 kN.On the basis of the layout position principle and support control idea of fully mechanized top-coal caving mining roadway in extremely close-distance double-gobs coal seam proposed above, combined with theoretical analysis, numerical simulation, engineering analogy method, and the actual production situation of Fenxi Mining, the support scheme of No. 11103 haulage roadway in Fenxi Mining was determined, as shown in Figure 12.The support parameters of the roadway section are shown in Table 2.
The anchor cable passes through the 29U-steel-shed to form a shed-cable coupling support structure.

| Support effect
To evaluate the reliability of the roadway support scheme of the No. 11103 mining face, and to grasp the bearing performance of the support, the displacement of the roadway surrounding rock, the pressure of the U-steel support, and the pull-out force of the anchor cable were monitored.The observation scheme is shown in Figure 13.① Monitoring the surface displacement of the roadway surrounding rock using the cross-measuring point method.Roadway surrounding rock monitoring includes roof, floor, and two ribs displacement monitoring.② Using a hydraulic pillow to monitor the pressure of the U-steel support.Hydraulic pillows are arranged in the middle of the two ribs and shoulders of the U-steel support, and hydraulic pillows are close to the U-steel support.
The relationship curve of displacement in the mining stage is shown in Figure 13A.At the stage of roadway excavation, the stability time of the surrounding rock was approximately 32 days after the roadway support was completed, and the maximum approach of the roof and floor was 83 mm.The maximum approach of two ribs was 51 mm.In the mining stage of the panel, the influence of mining on the roadway displacement of the roof, floor, and two ribs was concentrated in the range of 35 m.During this period, due to the continuous adjustment and movement of the roadway roof strata, the relative displacement of the roadway surrounding rock changed.The maximum relative displacement of roadway roof and floor and two ribs was 232 and 191 mm, respectively.
The pressure monitoring of the U-steel support in the roadway is shown in Figure 13B.In the panel's mining stage, mining's impact on the U-steel support was concentrated in the range of 0-30 m.The maximum pressure of the coal pillar spandrel, the coal pillar rib, the solid coal spandrel, and the solid coal rib were 63, 56, 46, and 42 kN, respectively.The overall change trend of the pressure value was increasing in the range of 0-15 m, decreasing in the range of 15-30 m, and then tends to be stable.In the stable stage, the maximum pressure of coal pillar spandrel, coal pillar rib, solid coal spandrel and solid coal rib were 43, 40, 30, and 25 kN, respectively.The roadway deformation is small in the field monitoring, and the support effect is good.At the stage of mining back to the working face, the impact of mining action on the roadway anchor cable support is concentrated in the 30 m range, in the 0-15 m range of anchoring force is increasing trend, the peak location is about 15 m, indicating that this range is a stronger impact of mining action, the maximum value of the anchoring force of the solid coal rib arch shoulder anchors, coal pillar rib arch shoulder and arch rib anchors are 303, 278, and 268 kN, respectively, and the maximum value of the anchoring force of the solid coal rib arch shoulder anchors, coal pillar rib arch shoulder and arch rib anchors are 303, 278, and 268 kN, respectively, and the anchoring force is decreasing trend in the 15-30 m range, indicating that this range is a weakened mining impact.In the range of 15-30 m, the anchoring force shows a decreasing trend, indicating that this range is the weakening zone of mining influence.Outside the range of 30 m, the anchoring force values of solid coal rib arch shoulder anchor cable, coal pillar rib arch shoulder and arch rib anchor cable are 220, 180, and 170 kN, respectively.left arch side anchor cable coupling point force is 120 kN, the right arch side anchor cable coupling point force is 160 kN, and the central prop exerts a force of 200 kN, which makes the U-type steel shed have no weak section, and there is no damage to the U-type steel shed in the field monitoring.It conforms to the stress environment of unequal load around the peripheral rock of the roadway.

1 .
The upper coal seam and the lower coal seam are very close, and close coal seams are 1 or 0 m distance.The 11103 working face is affected by strong mining of upper coal seam and adjacent coal seam before excavation, which results in the surrounding rock of the lower coal roadway being in the broken coal seam as a whole, and the local roof falling of the surrounding rock of the roadway occurs during excavation.2. After the double gobs are mined, the double coal pillars are left, the stress of the double coal pillars are superimposed.The surrounding rock of the lower coal roadway is greatly affected by the stress concentration of the side coal pillars, and the roadway is in a highstress area.3. The single bolt and cable cannot adapt to the support of the coal seam roadway under the close coal seam.The distance from the roof of the lower roadway to the upper gob is short, the support space is small, and the upper gob leads to the lower roadway roof without an anchor bearing layer.Under these two conditions, many accidents of bolt unanchoring and roof falling occur in the lower coal roadway, therefore it is extremely difficult to support.4. The deformation and destruction degree of surrounding rock of coal roadway is aggravated by the strong mining of fully mechanized top-coal caving face.It results in large deformation of roadway surrounding rock, deviatoric stress loading of roadway surrounding rock, bending and damage of U-shaped steel shed.

F I G U R E 1
Mining engineering drawing of working face.WU ET AL.

F
I G U R E 5 Visualization of the three-dimensional distribution of deviatoric stress in sequential mining.1-4 m of roadway pillar side, and the peak value of deviatoric stress appears in the range of 5-25 m of roadway pillar side.The peak value of deviating stress of coal pillar side decreases with the increase of vertical bottom insertion depth of roadway.When 0-1 m vertical bottom insertion is arranged in roadway, the peak value of deviation stress of coal pillar side is 3.87 MPa, and the position is 1 m in the shallow surrounding rock of coal pillar side.When the roadway is arranged at 1.5 m vertical bottom insertion, the position of the peak deviatoric stress shifts to the deep surrounding rock of the coal pillar, and the position is 3.5 m in the deep surrounding rock of the coal pillar.The shallow surrounding rock of coal pillar is in the low deviating stress region.According to the partial stress nephogram in Figure 7, the results are as follows: (1) The partial stress peak region is formed in the floor area of irregular double-layer coal pillar, and the maximum partial stress value is more than 4.5 MPa.(2) Under the double mined-out area, the floor and the coal pillar are affected by the concentrated high deviating stress in the certain vertical distance, and the floor far away from the coal pillar is in the low deviating stress area.(3) In the upper part of the lower coal seam floor line, the high deviatoric stress of the surrounding rock appears "irregular ellipse," and the position of the maximum outward expansion axis.The high deviatoric stress of the lower part of the lower coal seam F I G U R E 6 Deviator stress nephogram of surrounding rock of fully mechanized top-coal caving roadway in coal seam under different horizontal offset double gobs (unit: MPa).

F
I G U R E 7 Deviator stress nephogram of surrounding rock of fully mechanized top-coal caving roadway in coal seam under double gobs with different vertical offsets (unit: MPa).

9
Coupled bending stress of U-steel + anchor + prop under uniform load.WU ET AL.
180 kN, the applied force of single prop is X 3 = 200 kN, the bending moment of left side arch and spandrel is to the outside.The maximum bending moment value is 98.4 kN m, more significant than the ultimate bending moment of the left side arch, 65.7 kN m, when there is no coupling support.The bending moment of the right spandrel increases relatively.The maximum bending moment is 66.8 kN m, which indicates that the anchor cable applies the higher force, and the bending moment value of both sides increases reversely.The overall reduction of the staff at the ectopic coupling point is shown in Figure11D.The applied pressure ofF I G U R E10 Bending moment diagram of symmetrical eccentric load U-shaped steel shed (kN m).(A) Model-1 and (B) U-steel support.the anchor cable is X 1 = X 2 = X 4 = 120 kN, and the applied force of the single prop is X 3 = 200 kN.The bending moment of the three-center arch is relatively reduced, but the bending moment of the right arch increases, and the maximum bending moment is 36.4kN m. 3. Different forces are applied at the ectopic coupling point when the cable-prop coupling support is used in the shed.As shown in Figure 11E,F, the force applied by the anchor cable (left) X 1 = X 2 = 120 kN, (right) X 4 = 160 kN, and the pressure applied by the single prop X 3 = 200 kN, the bending moment values of each section of the three-center arch is relatively reduced; When (right) X 4 = 180 kN, the relative arch shoulder bending moment on both sides increases, indicating that the local coupling effect is poor when the same pretightening force is applied to the close ectopic coupling point anchor cable under eccentric load.The ectopic coupling point anchor cable uses different pretightening troops within a reasonable range, which strengthens the coupling support effect of the shed-cable-prop.

F
I G U R E 11 Partial load model (IV) bending moment of cable Prop coupling support of shed (kN m).(A) Model-2, (B) U-steel support, (C) coupling point equal force 180 kN, (D) coupling point equal force 120 kN, (E) unequal force left 120 kN Right 160 kN, and (F) unequal force left 120 kN Right 180 kN.

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I G U R E 12 Coupling support structure of No. 11103 haulage roadway.

1 .
The optimization methods for the section and layout of the roadway under double gobs in close coal seams are to use an arch section to counteract the broken roof, and to use grouting anchor cable to improve the bearing stability of the broken peripheral rock.The staggered position of the lower coal seam roadway layout was modified from the original coal roadway position to a coal-rock roadway, with a horizontal staggered distance of 12 m, and 1.5 m inserted into the rock layer of the coal seam floor, so as to avoid the area of high deviatoric stress in the coal pillar and the area of crushed coal body, and to leave an effective support space for the roof.2. The reasonable judgment criterion for horizontal misalignment of the lower coal seam roadway is that the first deviatoric stress peak value of the coal pillar side is smaller than the deviatoric stress peak value of the coal pillar floor, which indicates that the horizontal misalignment position of the lower coal seam roadway avoids the high deviatoric stress area of the coal pillar floor.The reasonable criterion for vertical misalignment of the lower coal seam roadway is that the peak deviatoric stress is greatly shifted to the deep surrounding rock of the coal pillar side, and the shallow surrounding rock of the coal pillar side is in the low deviatoric stress area.3. On the basis of the uneven load stress environment around the peripheral rock of the roadway under double gobs in close coal seams, it is proposed to adopt the mechanical model of unequal force coupling support of U-shaped-steel + anchor + prop.The unequal force coupling is realized by adjusting different coupling point forces, such as the left arch side anchor cable coupling point force is 120 kN, the F I G U R E 13 (A, B) Surface displacement monitoring curve of No. 11103 haulage roadway.
Rock parameters table.
T A B L E 1 F I G U R E 4 Decomposition of the deviatoric stress state.
T A B L E 2 Roadway section support.