
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39227701
71373
10.1038/s41598-024-71373-8
Article
Study on overburden rock structure characteristics and surrounding rock control technology of island working face
Haoyu Zhu 22203077040@stu.xust.edu.cn

1
Xingping Lai 12
Hao Qiao 1
Pengfei Shan 12
Wenlin Wang 3
Wenhua Yang 1
Longquan Wu 1
Chong Jia 1
1 https://ror.org/046fkpt18 grid.440720.5 0000 0004 1759 0801 College of Energy Engineering, Xi’an University of Science and Technology, Xi’an, 710054 China
2 https://ror.org/01zzmf129 grid.440733.7 0000 0000 8854 4301 Xi’an University of Science and TechnologyMinistry of Education of the Western Mining and Mine Disaster Preventionand Control of Key Laboratory, Xi’an, 710054 China
3 Shaanxi Huabin Coal Industry Co., Ltd, Xian’yang, 712000 China
3 9 2024
3 9 2024
2024
14 2052230 5 2024
27 8 2024
© The Author(s) 2024
2024
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Based on the background of ZF2822 island working face in Xiagou Coal Mine, the characteristics of overburden fracture structure and surrounding rock control technology of island working face are explored by means of theoretical analysis, numerical simulation and field measurement. The results show that the ten morphological structures of the main roof fracture can be divided into four typical symmetrical structures and six asymmetric structures. According to the stress calculation formula of the key block B to the coal pillar, the stress characteristics of the 10 structures are classified and the risk classification is carried out. Four typical structures (a), (b), (c) and (d) were constructed to simulate the stress and strain characteristics of surrounding rock. The overlying strata subsidence displacement values of (a), (b), (c) and (d) at the roadway are 1.75 m, 1.5 m, 0.8 m and 0.6 m respectively, and the maximum stresses are 26.6 MPa, 19.5 MPa, 15.5 MPa and 10.0 MPa respectively, (a) > (b) > (c) > (d). Therefore, the surrounding rock control technology of gob-side roadway in island working face is put forward, the fracture structure of basic roof is determined and the reasonable applicable type of gob-side roadway is selected. The results after pressure relief show that the average daily microseismic energy of ZF2822 working face after pressure relief is 0.87 × 104 J, which is about 42.38% lower than that of April. The average stress is 1.13 MPa lower than that of the mining before the pressure relief area, and the decrease is 22.42%. It shows that the pressure relief has played a very good effect and provides a reference for the same type of mine.

Keyword

Island working face; Basic roof fracture mode; Surrounding rock control technology; Pressure relief measures
Subject terms

Fossil fuels
Coal
the National Natural Science Foundation of China52274138 Pengfei Shan issue-copyright-statement© Springer Nature Limited 2024
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pmcPreface

The isolated island working face is an inevitable problem in the process of coal mining layout design. Its safe and efficient mining is a scientific problem in the mining industry. With the increasing depth of coal seam, the safety problems faced by island working face mining are more prominent1,2. Due to the adjacent characteristics of the island working face, the mine pressure is extremely severe. The roadway of working face is difficult to maintain, and the mining conditions are extremely complex. In the case of hard roof, it is easy to lead to a large area of empty roof and induce the safety of equipment and personnel with strong mine pressure3,4. Therefore, it is urgent to master the law of overlying strata movement and fracture and the stable structure formed is the premise and foundation to realize accurate blasting and efficient pressure relief.

Many scholars have carried out fruitful research on the structure formed by the overlying strata on the island working face. Li5 analyzed the influence of the fracture position of the main roof on the stability of the roof and coal pillar of the gob-side entry. The different fracture characteristics of the main roof under different coal pillar widths are given. Wang6 studied that the long-term stability characteristics of the narrow side of the gob-side entry will directly affect the overall stability of the surrounding rock structure of the gob-side entry. Its effective control is also the key to the successful development of the technology. Four fracture forms of the main roof are proposed. Huo et al.7–10 take the extra-thick coal seam with multi-layer hard roof in Tongxin Coal Mine as the research object. The mechanical mathematical model of "arch shell" equilibrium large structure of hard overburden in large space stope is established by using elastic plate theory, and the mechanism of strong mine pressure in working face is well explained. Many targeted prevention and control measures are put forward for the fracture structure of the basic roof of different overlying strata. Zhu et al.11–13 proposed that pre-splitting blasting is a relatively mature method that can effectively alleviate or reduce the risk of induced burst for the thick and hard roof of (isolated island) working face. On this basis, a series of pressure relief measures are put forward.

In order to solve the technical problems of safe and efficient mining under the complex conditions of strong mine pressure and multiple disasters in ZF2822 island working face of Xiagou Coal Mine14–16. Some scholars have studied the fracture physical properties of rocks and the prediction before disasterse17–19. This paper will combine the basic roof fracture structure of overlying strata and the surrounding rock control technology system of gob-side entry to carry out relevant exploration20,21.

Engineering background

General situation of working face

Xiagou Coal Mine is located in Binzhou City, Shaanxi Province. The mine is located in the southeast of Longdong Loess Plateau. The south is high in the south and low in the north. The south is the gully-cut tableland landform, and the surface is the Jinghe River beach and platform. The dip angle is 0 ~ 3°, and the structure is simple. The main coal seam of the mine is No.4 coal seam, and the inclined shaft is developed by single level up and down. The ZF2822 working face is an isolated island working face currently mined in the mine. Its working face is located at the second level of 401 west of the mine. It is a north-dipping monoclinic structure and is located in the southwest of the mine. The specific location of the underground mine : the east is the 401 downhill protection coal pillar, the west is the 404 downhill protection coal pillar, the south is the ZF1816 working face goaf, and the north is the ZF2815-ZF2820 goaf. The layout of the working face is shown in Fig. 1. The elevation of ZF2822 island working face is between 473 ~ 487 m, the ground elevation is between 840 ~ 860 m, and the buried depth is about 370 m. The upper surface is the plateau beam landform cut by the valley, and the mining of the working face has no effect on the ground facilities. The total length of the coal mining face is 1016 m, and the average thickness of the coal seam is 11.5 m. The longwall comprehensive mechanized top coal caving mining method is adopted.Fig. 1 ZF2822 Layout drawing of the recovery working face.

Mine pressure characteristics

The No.4 coal seam of Xiagou Coal Mine is weak impact tendency, and the joints and cracks in the coal body are obviously developed. The coal around the chamber has low cohesion, low strength and poor bearing capacity. The soft and broken degree of surrounding rock in coal roadway is high, which shows the characteristics of loose, soft and broken in deep coal body. According to the actual situation, the overlying sandstone structure of ZF2822 working face is complete, it is not easy to collapse during mining, and it is easy to form an arc triangle plate near the coal pillar. This increases the load above the coal pillar, resulting in serious plastic damage to the small coal pillar along the goaf and large deformation of the coal wall of the working face. At the same time, ZF2822 working face is a concentrated roadway coal pillar recovery working face arranged in the west second level of 401 mining area of Xiagou Coal Mine, and the stress of roadway surrounding rock is concentrated. Under the influence of mining pressure, the mine pressure of the roadway is obvious, and the side drum and floor drum of the roadway are serious, which reduces the size of the roadway and affects the normal safety production. The design of surrounding rock support scheme of roadway is mainly based on field practical experience, and lacks theoretical basis support. The damage of roadway surrounding rock is more serious, which is mainly manifested in the phenomenon of mesh pocket or roof leakage in the broken roof of roadway, and the coal body on both sides of roadway bulges outward obviously, and the surrounding rock of roadway has the characteristics of large deformation and failure, as shown in Fig. 2.Fig. 2 The pressure of roadway surrounding rock is characteristic.

The joint fissures of No.2 coal seam are developed and show obvious characteristics of deep soft broken coal body. In order to maintain the normal use of the working face roadway, active support must be carried out. However, the surrounding rock of the roadway has always been squeezed out in a large range and floor heave, and the support cost remains high. Therefore, it is urgent to study the fracture structure of overlying strata on ZF2822 working face, so as to carry out efficient pressure relief measures and support schemes22–24.

Spatial structure classification of overburden rock

Main roof fracture structure form of island working face

In the mining process of the working face, whether the overlying strata of two adjacent goafs can form an interactive spatial structure is mainly determined by the width of the coal pillar between the two. The larger coal pillar can effectively isolate the overlying rock fissures between the goafs. Therefore, when the coal pillar width of the working face is less than a certain threshold, the overlying strata between the two working faces may produce synergistic movement and form an interactive spatial structure. With the advance of the working face, the immediate roof may fall and sink irregularly or regularly. This may lead to the separation from the basic roof above, and the fracture, rotation and sinking of the basic roof after the direct roof collapse. This paper mainly considers the fracture structure of the basic roof of the overlying strata. The isolated island working face is defined as the empty mining area on both sides of the adjacent and above areas, and the width of the coal pillar does not reach the minimum width required to isolate the goaf. This kind of working face shows high stress concentration and violent overburden movement. Compared with the non-island working face, the mine pressure is more intense, and it is easy to cause dynamic disasters such as rock burst. The overlying strata around the island working face have been broken, which aggravates the dynamic interaction between the overlying strata and the roof strata after mining. This leads to high peak value, wide disturbance range and rapid change of abutment pressure field in island working face. With the advancement of the working face, irregular or regular collapse will occur at the direct top, which will lead to the separation from the upper basic roof. After the direct roof collapses, the main roof will break, rotate and sink. When the vertical section is taken in the advancing direction of the working face, a ‘masonry beam’ structure will eventually be formed. The formation of this fracture structure is not only related to the thickness and mechanical properties of the main roof, immediate roof and coal seam, but also affected by mining depth, original rock stress state and mining height. Due to the combined effect of these factors, the basic roof fracture structure of the overlying strata of the roadway in the island working face will be more complicated. It is very important to divide the fracture structure of the basic roof of the overlying strata for simplifying the roof structure of the island working face. To clarify the characteristics of each structure, targeted measures can be taken to prevent and control the mine pressure of different types of island working faces. Therefore, the following is a detailed analysis of the main roof fracture structure.

Since the island working face exists in the form of solid coal when it is not mined, the island working face can be regarded as a giant coal pillar to a certain extent. For example, the ZF2822 working face of Xiagou Coal Mine is about 140 m long. Therefore, to a certain extent, the two sides of the island working face can be regarded as two different structures to superimpose. Based on the four fracture structure forms of the basic roof of the overlying strata above the gob-side roadway, the symmetry of the abutment pressure is considered when the left and right sides are alternately the same. The fracture structure of the basic roof of the overlying strata in the roadway of the island working face can be simplified to show the ten morphological structures shown in Fig. 3. Among them, A and B represent the rock blocks formed after the basic roof fracture.Fig. 3 Basic roof fracture structure form. (a) The fracture line of the main roof on both sides is located directly above the roadway, (b) The position of the main roof fracture line on both sides is located inside the solid coal wall, (c) The fracture line of the main roof on both sides is located outside the coal pillar, (d) Both sides of the basic roof can not form hinged structure, (e)The fracture line of one side of the main roof is located directly above the roadway, and the other side is located inside the solid coal wall, (f) The fracture line of the main roof on one side is located directly above the roadway, and the other side is located outside the coal pillar, (g) The fracture line of the main roof on one side is located inside the solid coal wall, and the other side is located outside the coal pillar, (h) The fracture line of one side of the main roof is located directly above the roadway, and the other side cannot form a hinged structure, (i) The fracture line of the main roof on one side is located in the inner side of the solid coal wall, and the other side cannot form an articulated structure, (j) The fracture line of one side of the main roof is located outside the coal pillar, and the other side cannot form a hinged structure.

The following is a mechanical analysis of the four fracture structure forms of the basic roof of the overlying strata of the foundation. If considering the characteristics of the large width of the solid coal, one side of the four symmetrical structures of a, b, c and d is taken for analysis, so as to explore the mechanical characteristics of the ten morphological structures of the basic roof fracture of the roadway in the island working face. When the upper working face advances and the suspended roof area of the goaf is too large, the main roof will break ‘O-X’. The ‘S-R’ sliding instability occurred in the roof rock block of the roadway along the goaf of the coal pillar, thus forming a ‘step rock beam’ structure on one side. Therefore, the immediate roof will not only bear the load of the overlying strata, but also be affected by the dynamic impact caused by the breaking of the basic roof. The stability of immediate roof is one of the key indexes to evaluate the stability of surrounding rock in gob-side entry of coal pillar. Taking the immediate roof as the research object, according to the stress relationship between the immediate roof, coal wall, coal pillar and main roof, the mechanical model of the immediate roof is constructed as shown in Fig. 4.Fig. 4 Mechanical model of immediate roof.

The model is a cantilever beam structure under non-uniform load. qA, qB(x), qC and qD(x) represent the load of rock block A, rock block B, coal wall and coal pillar on immediate roof respectively. The deformation of rock block B and coal pillar is different at different positions. According to Hooke 's law, the loads applied to rock block B and coal pillar are non-uniform, which are expressed as functions of horizontal axis x, respectively. lA, lB, lC and lD represent the length of rock block A, rock block B, coal wall and coal pillar respectively. l represents the width of the gob-side entry, L represents the length of the immediate roof studied, FOy and MO represent the vertical force borne by the coal wall, respectively. Through the static equilibrium condition of the immediate roof, the shear force of the immediate roof is calculated, so as to judge the stability of the immediate roof of the roadway along the small coal pillar. According to the force analysis :1 ∑Fx=0

2 ∑Fy=0

3 ∑Ms=0

The static conditions of the immediate roof in the equilibrium state are:4 FOx-∫0hqydy=0

5 FOy+qClC+∫l+lCl+lC+lDqDxdx-qAlA-∫lAlqBxdx=0

6 12qAlA2+∫lAlxqBxdx-12qClC2-∫l+lCl+lC+lDxqDxdx-MO=0

According to reference25, the pressure of key block B on small coal pillar can be approximately considered as follows :7 QB=4h1-3L1sinθ22h1-L1sinθP1+PB

8 P1=L1γ1H1

9 PB=L1γ2h1

In the formula : P1 is the load applied by the overlying strata to the key block B ; QB is the pressure of key block B on small coal pillar ; L1 is the length of the key block B, m ; θ is the rotation angle of the key block B ; h1 is the thickness of the block rock beam ; γ1 is the average bulk density of the upper load rock layer of the key block B ; γ2 is the average bulk density of key block B ; H1 is the thickness of the upper load rock layer of the key block ; pB is the force of key block B itself.10 QB=4h1-3L1sinθ22h1-L1sinθL1γ1H1+L1γ2h1

The force of the overlying strata on the side of the coal body can be regarded as :11 QA=kγH

In the formula : k is the stress concentration factor ; γ is the average bulk density of overlying strata of coal seam ; h is the buried depth of coal seam ; QA is the force of overlying strata on the side of coal body.

Because the pressure QB of key block B to small coal pillar is non-uniform load and presents stress concentration at the fracture point, qB(x) can be regarded as triangular load, then the maximum pressure Qmax of key block B to small coal pillar is :12 Qmax=4h1-3L1sinθ2h1-L1sinθL-lAL1γ1H1+L1γ2h1

For a, b, c, the length of L-lA is b > a > c. Since other parameters can be regarded as constants. Therefore, the pressure of key block B on small coal pillar QB, c > a > b. At the same time, according to the stress distribution characteristics of triangular load, the stress characteristics of several structures are analyzed as follows:(1) The position of the main roof fracture line is located directly above the roadway (a) : At this time, the high stress is concentrated on the side of the roadway and the coal pillar, and the protection of the roadway and the coal pillar should be strengthened. When the fracture line of the main roof is located directly above the roadway, because the fracture line of the main roof is located directly above the roadway, the fracture of the main roof rotates and sinks, and the coal pillar bears the weight of the direct roof and the main roof and the dynamic load generated by the fracture rotation of the main roof. The support resistance of the coal pillar is large, and the deformation of the surrounding rock of the gob-side entry is large and the maintenance is difficult. The stability maintenance of this kind of structure roadway and coal pillar is the most difficult.

(2) The fracture line of the main roof is located in the inner side of the solid coal wall (b) : At this time, the high stress is concentrated on the side of the solid coal, and the pressure should be prevented from being too large when the working face is mined. Because the main roof fracture line is located inside the coal wall, the solid coal wall bears part of the roof load, and the support resistance of the coal pillar is large. However, the given deformation of the main roof is large, and the coal pillar must adopt a ‘soft + hard’ structure to meet the requirements of the main roof rotation. At the same time, it also meets the needs of supporting resistance of supporting roof. This structure is also a great test for the stability of roadway and coal pillar.

(3) The fracture line of the basic roof on both sides is located outside the coal pillar (c) : At this time, the high stress is concentrated on the side of the coal pillar near the goaf, which is less dangerous to the stability of the roadway and the coal pillar itself. And the dynamic load generated by the fracture rotation of the main roof is mainly concentrated on the side of the goaf, and the impact load on the solid coal, coal pillar and roadway is small. However, it should be noted that the coal pillars on the side of the goaf are too broken, and grouting reinforcement can be adopted if necessary.

(4) The main roof can not form a hinged structure (d) : At this time, the structure is relatively stable, and it will not produce the instantaneous large impact force such as the dynamic load generated by the fracture rotation of the main roof. The force of the coal pillar is mainly the weight of the direct roof and the main roof. The stability of the roadway is good, and the daily maintenance of the roadway and the coal pillar should be done well.

(5) The location of the fracture line of the main roof is inconsistent on both sides : the high stress concentration characteristics on both sides of the island working face are different, and the basic stress conditions on both sides should refer to the analysis of the above four mechanisms. In particular, it should be noted that the structures are formed on both sides of (e), (f) and (g). At this time, it is necessary to prevent the superposition of dynamic load caused by the fracture and rotation of the main roof. In this case, the threat to the safety and stability of roadway and coal pillar is relatively large, and measures need to be taken in advance to prevent the superposition of dynamic load. The three structures of (h), (i) and (j) do not form an articulated structure on one side, and will not produce instantaneous large impact force such as dynamic load generated by the fracture and rotation of the basic roof. The roadway and coal pillar on one side can be treated well.

Therefore, the proposed ten basic roof fracture structures can be divided into four typical symmetrical structures and six asymmetric structures. When the position of the main roof fracture line is located directly above the roadway, the main roof fracture rotates and sinks. The coal pillar bears the weight of the direct roof and the main roof and the dynamic load generated by the fracture rotation of the main roof, and the stress is the largest. The fracture line of the main roof is located in the inner side of the solid coal wall, followed by the force of the roadway and the coal pillar, and the high stress is concentrated at both ends of the solid coal side. The main roof fracture line is located outside the coal pillar, and the high stress is concentrated on the side of the coal pillar near the goaf, which is less dangerous to the stability of the roadway and the coal pillar itself. The structure is relatively stable when the main roof cannot form a hinged structure. When the two sides of the main roof fracture line are inconsistent, (e), (f) and (g) need to prevent the superposition of dynamic load caused by the fracture and rotation of the main roof. In this case, the safety and stability of the roadway and coal pillar are relatively large, and measures need to be taken in advance to prevent the superposition of dynamic load. The three structures of (h), (i) and (j) do not form hinged structure on one side, and will not produce instantaneous large impact force such as dynamic load generated by the fracture and rotation of the basic roof. The roadway and coal pillar on one side can be treated well. The classification characteristics of the ten structures of the main roof fracture in the island working face are shown in Table 1. When the risk level is grade I, the threat is the greatest. Relevant measures must be taken to ensure the stability of roadways and coal pillars. The higher the risk level, the more attention should be paid. When the risk level is grade IV, the fracture structure of the main roof is relatively stable.Table 1 Classification characteristics of ten kinds of structure of main roof fracture in island working face.

Structure name	Structural characteristic	Risk grade	
(a)	The high stress is concentrated on the side of roadway and coal pillar, and the stress is the largest	I	
(b)	The high stress is concentrated on the side of the solid coal, and the force of the roadway and the coal pillar is larger	II	
(c)	The high stress is mainly concentrated on the side of goaf, and the impact load on solid coal, coal pillar and roadway is small	III	
(d)	The structure is relatively stable and will not produce instantaneous large impact force such as dynamic load	IV	
(e)、 (f)、 (g)	At this time, it is necessary to prevent the superposition of dynamic load caused by the fracture and rotation of the main roof, which poses a great threat to the safety and stability of roadway and coal pillar	I	
(h)、 (i)、 (j)	The hinged structure is not formed on one side, which is mainly the high stress concentration on the other side	II	

Determination method of main roof fracture structure form

The fracture structure of the main roof is related to the thickness and mechanical properties of the working face length, the main roof, the direct roof and the coal seam. At the same time, it is also related to mining depth, original rock stress state and mining height. There are two main methods for the fracture structure of the main roof : one is theoretical calculation, and the other is through on-site detection. The effective combination of the two can accurately determine the fracture structure of the main roof of the island working face.

Theoretical calculation

According to the theory of mine pressure, the fracture of the basic roof strata occurs in the coal wall. If the width of the coal pillar is too small, the fracture line of the overlying strata of the adjacent goaf roof will overlap, resulting in the direct connection of the overlying strata. Using the elastic foundation beam model, the distance between the main roof fracture line and the coal wall can be calculated.13 Ld=tan-1β(2αM0s+rQ0)r2M0+αrQ0β

14 β=k/(EI)2+N4EI1/2

15 α=k/(EI)2-N4EI1/2

16 s=N/(EI)

17 r=k/(EI)

For simplicity, the formula of M0 is :18 M0=(Rs+N/bh1)bh126

19 Q′=L(h1r+q)

20 Q0=qL′+Q′

21 N=LQ′2(h1-ΔS)

22 ΔS=h16

In the formula : M0, Q0, N are the bending moment, shear force and axial force of the section corresponding to the position of the coal wall of the working face respectively; N’ is the transverse force formed by the rotation of broken rock ; L is the length of the fractured rock mass ; L’ is the length of the overhanging part of the fractured rock mass; Q’ is the weight and load of the fractured rock mass; ΔS is the difference of vertical displacement at both ends of the fractured rock mass; q is the load concentration of the fractured rock mass in the separation zone; Rs is the tensile strength of the basic roof strata, taking 6.5 MPa ; h1 is the thickness of the main roof strata, taking 11 m ; h2 is the thickness of the overlying strata of the main roof, which is 4.5 m. γ is the bulk density of the main roof strata, 25KN/m3 ; k is Winkler foundation coefficient, 400 MPa ; E is the elastic modulus of the basic roof strata, taking 1.5 GPa;

After calculation, the ZF2822 working face of Xiagou Coal Mine is roughly judged to belong to structure (a) at this time.

On-site detection

The structure of rock strata can be observed by drilling peep, and the cracks filled by drilling cuttings can be found by detecting, measuring and recording cracks and fractures. Through the comparison of multiple measured images, the separation, dislocation and crushing changes of roadway surrounding rock can be described. The collected data are compared after laboratory processing, and the fracture structure of the overlying main roof can be obtained. The schematic diagram of drilling arrangement is shown in Fig. 5.Fig. 5 Drilling layout diagram.

The caving zone height mz is calculated according to the following formula :23 mz=hm-Δkp-1

24 Δ=hm×(1-c)×km

In the formula: hm is the thickness of coal seam; Δ is the filling thickness caused by coal loss; c is the total recovery rate; kp is the expansion coefficient of coal rock; km is the bulking coefficient of caving top coal strata. The calculation can predict that the approximate height of the caving zone of ZF2822 working face in Xiagou Coal Mine is about 4.5 m, which is used as a reference for drilling depth. According to the imaging law of divergent drilling, the ZF2822 island working face in Xiagou Coal Mine belongs to structure (a).

Analysis of stress and strain characteristics of surrounding rock in island working face

Construction of numerical model

In order to explore the plasticizing migration and deformation and failure characteristics of surrounding rock in island working face, aiming at the different fracture structure characteristics of the basic roof of overlying rock in island working face, the discrete element numerical simulation method is used to study the stress and strain characteristics. Based on the above analysis, the numerical analysis of four typical structures (a), (b), (c) and (d) is carried out. Based on the geological drilling data of the mine, the numerical simulation software 3DEC is used to simulate the symmetry of the two sides of the fracture line of the main roof of the island working face. The numerical calculation model of overlying strata is constructed. The size of the model is 900 m (X) × 5 m (Y) × 380 m (Z). The numerical model is shown in Fig. 6. In the simulation process, the Mohr–Coulomb constitutive model is used to monitor the unbalanced force and vertical displacement of the model by hist command.Fig. 6 Numerical calculation model.

Numerical simulation analysis

In the process of coal seam mining, the original stress state of surrounding rock changes. As shown in Fig. 7, the figures (a), (b), (c) and (d) respectively simulate the migration and evolution results of overlying strata that the position of the main roof fracture line is located above the roadway, the position of the main roof fracture line is located inside the solid coal wall, the position of the main roof fracture line is located outside the coal pillar, and the main roof cannot form an articulated structure. Through the comparison of the four structures, it can be concluded that the subsidence displacement value of the coal pillar near the goaf is large, the surrounding rock still has certain integrity, and the coal pillar as a whole has good bearing capacity. The subsidence displacement values of the overlying strata of the four structures at the roadway are 1.75 m, 1.5 m, 0.8 m, and 0.6 m, respectively. The sinking displacement value of the roadway is : (a) > (b) > (c) > (d). When the fracture line of the main roof is located directly above the roadway, the maximum subsidence displacement of the roadway reaches 1.75 m. It shows that when the fracture line of the main roof is located directly above the roadway, the dynamic load generated by the fracture rotation of the main roof is large, and the deformation of the surrounding rock of the gob-side entry is large and difficult to maintain. This is the same as the maximum force when the fracture line is located above the roadway obtained by the previous mechanical analysis.Fig. 7 Evolution distribution characteristics of overburden rock displacement field, (a) The fracture line is located above the roadway, (b) The fracture line is located inside the solid coal wall, (c) The fracture line is located in the goaf, (d) Can not form a hinged structure.

In the process of coal seam mining, the original stress state of surrounding rock changes, and the stress concentration phenomenon will be formed on the solid coal side of coal pillar and roadway, as shown in Fig. 8. When the fracture line of the main roof is located above the roadway, the high stress at this time is concentrated on the roadway and the coal pillar side, and the maximum stress reaches 26.6 MPa, indicating that the dynamic load generated by the rotation of the main roof is large. The fracture line of the main roof is located in the inner side of the solid coal wall. The maximum stress of the coal pillar side reaches 19.5 MPa, which is slightly smaller than that of the structure a, but the stress of the solid coal side near the roadway is larger. When the fracture line of the main roof is located outside the coal pillar, the high stress is concentrated on the side of the coal pillar near the goaf, and the maximum stress on the side of the coal pillar is 15.5 MPa, which is less dangerous to the stability of the roadway and the coal pillar itself. And the dynamic load generated by the fracture rotation of the main roof is mainly concentrated on the side of the goaf, and the impact load on the solid coal, coal pillar and roadway is small. When the main roof can not form an articulated structure, the stress value is significantly reduced, basically below 10 MPa, indicating that the stability of the surrounding rock under the structure is high. Therefore, the peak stress at the coal pillar and roadway is (a) > (b) > (c) > (d).Fig. 8 Distribution characteristics of overburden stress field evolution, (a) The fracture line is located above the roadway, (b) The fracture line is located inside the solid coal wall, (c) The fracture line is located in the goaf, (d) Can not form a hinged structure.

By comparing the stress and strain characteristics of four typical structures, it can be obtained that when the main roof fracture line is located above the roadway, the stress of the surrounding rock of the roadway is the largest, and the deformation of the surrounding rock is large. At this time, the stability maintenance of the roadway and coal pillar is the most difficult. The position of the main roof fracture line is located in the inner side of the solid coal wall. At this time, the stress of the surrounding rock of the roadway is second, but the stress on the solid coal side is large. The support resistance at both ends of the working face is the largest during mining, and the support needs to be strengthened. When the fracture line of the main roof is located outside the coal pillar, the stress value of the roadway is small, and the dynamic load generated by the fracture rotation of the coal pillar near the goaf side is large. When the main roof cannot form a hinged structure, the deformation of the surrounding rock is small, and the stress of the surrounding rock of the roadway is the smallest.

Surrounding rock control technology of gob-side entry in island working face

Surrounding rock control technology system of gob-side roadway

The main roof fracture structure, that is, the location of the main roof fracture line, is closely related to the maintenance status of the gob-side entry and the stability of the surrounding rock. Under different basic roof fracture structures, the activity law of overlying key rock block B is different, and the force difference of its action on coal pillar, roadway and working face is also large. Therefore, there is a big difference between stress and deformation law. At the same time, the overall stability of the gob-side entry will also affect the stability of the main roof structure. The gob-side entry shows obvious stage deformation characteristics during the whole service period. There are obvious differences in roof migration law, surrounding rock stress field evolution law and deformation characteristics in each stage. Based on the above research, the surrounding rock control technology system of gob-side roadway is proposed as shown in Fig. 9. Therefore, the surrounding rock control of gob-side entry should be based on the mastery of the basic roof fracture structure, fully consider the characteristics of the main roof fracture rotation and subsidence under different basic roof fracture structures, the stress and deformation law of the surrounding rock of the gob-side entry in different stages, and take effective control technology to improve the overall stability of the gob-side entry. Before mining, the main roof fracture structure can be determined by physical similarity simulation, numerical simulation and field detection, and a reasonable type of gob-side entry can be selected. During the mining period, roadway support and coal pillar reinforcement should be strengthened. In particular, it can improve the anchorability of surrounding rock and improve the pre-tightening force of bolt. At the same time, when the pressure is too large, the surrounding rock pressure of the roadway can be reduced by roof cutting, pressure relief and blasting. After mining, it is necessary to do a good job in the monitoring technology of the surrounding rock of the roadway along the goaf. Microseismic monitoring, borehole peeping and other methods can be used.Fig. 9 Surrounding rock control technology system of gob-side roadway.

Engineering practice

According to the above analysis of the basic roof fracture structure of ZF2822 working face in Xiagou Coal Mine, the targeted measures are put forward to ensure the safe mining of the working face. According to the actual situation of the site, ZF2822 working face is easy to form a structure above the roadway and coal pillar, which increases the load above the coal pillar, resulting in serious plastic damage to the small coal pillar along the goaf and large deformation of the coal wall of the working face. It is necessary to implement pre-splitting blasting roof cutting and pressure relief to the roof in this range to block the influence of the roof near the goaf on the deformation of the roadway. According to the existing theory, prevention and control practice and successful cases of existing projects, the transportation roadway and return air roadway of ZF2822 working face are protected along the protective coal pillar, and the roof cutting and pressure relief of energy-gathering blasting are adopted. The length of the roof cutting is from the end of the advance support of the current working face to the end of 10 m from the stop line. Considering the limitation of the site environment of the transportation roadway, the construction needs to be 2-3 m away from the coal pillar side. In order to achieve the purpose of disturbing the hard sandstone of the target, the distribution range of the blasthole is limited (76°-81°). The final hole position of the blasting hole in the transportation roadway is the intersection point of the vertical height of the side of the coal pillar goaf and the high level of the hard sandstone. Finally, the depth of the blast hole is calculated to be 37 m, in which the charge position is 15 m, covering the target rock layer of the required disturbance. The length of the sealing hole is 13 m, and the hole has been sealed to the top coal. The range of blasting charge ensures that it is inside the coal pillar and ensures the safety of construction. The angle of the blast hole designed for the air return roadway is 88 degrees along the horizontal direction of the roadway. The charging position is 15 m, which covers the target rock layer of the required disturbance. The sealing length is 13 m, which has been sealed to the top coal. The blasthole parameters of the transport roadway and the return air roadway are detailed as shown in Fig. 10.Fig. 10 Blasthole layout scheme of transportation roadway and return air roadway.

Due to the obvious development of cracks in the surrounding rock of the roadway, in view of the fact that the surrounding rock of the roadway in the blasting area is relatively broken and the actual situation of the recent excavation, it is necessary to further optimize and adjust the support strength to ensure the safety of roadway excavation. Carry out measures such as advanced grouting reinforcement and energy-gathering blasting of roadways. After taking the above pressure relief measures, as shown in Fig. 11, compared with the working face mining in April, there is a significant downward trend in the microseismic energy-frequency of the working face mining after the pressure relief control of the working face roof in May. In the mining process before the blasting area of the ZF2822 working face, the average energy of the single-day microseismic is 0.87 × 104 J, which is about 42.38% lower than the average energy of the single-day mining in April, indicating that the pressure relief measures have played a good effect.Fig. 11 Microseismic energy-frequency variation characteristics of mining in May.

The variation trend of deep hole stress in return airway and transportation roadway of ZF2822 working face is shown in Fig. 12. Before pressure relief, the deep hole stress of ZF2822 working face roadway shows obvious fluctuation characteristics. On the 20th day of mining, there is an obvious peak stress, and the peak stress is 6.22 MPa. During the mining process of the ZF2822 working face in front of the pressure relief area, the average stress of the transportation roadway is about 5.04 MPa, and the average stress of the return air roadway is about 4.82 MPa. The stress change in the mining process of the working face in the pressure relief area shows a significant decreasing trend. The average stress is 1.13 MPa lower than that of the mining before the pressure relief area, and the decrease is 22.42%. During the mining process of the working face in the pressure relief area, the average stress is reduced by 0.49 MPa compared with the mining before the pressure relief area, and the decrease reaches 9.02%, indicating that the pressure relief has played a very good effect.Fig. 12 Stress change trend of ZF2822 working face.

The good effect shows that the targeted pressure relief measures have played a positive role. Ten kinds of fracture structure forms of the basic roof of the overlying strata on the island working face have played a good role in the prevention and control of mine pressure in Xiagou Coal Mine. It has reference for the same type of mine.

Conclusion

(1) The fracture structure of the basic roof of the overlying strata on the working face of the island is divided into ten morphological structures. Four kinds of symmetric structures and six kinds of asymmetric structures were constructed, and the direct roof mechanical models of four typical symmetric structures were constructed. The stress characteristics classification and risk classification of 10 kinds of structures were carried out. The results show that (a) the coal pillar bears the weight of the immediate roof and the basic roof and the dynamic load caused by the fracture and rotation of the basic roof, and the force is the largest. And put forward the judgment method of two kinds of basic roof fracture structure forms of field detection and theoretical calculation. The ZF2822 working face of Xiagou Coal Mine is roughly (a) structure.

(2) Four typical structures (a), (b), (c) and (d) were constructed to analyze the stress and strain characteristics of surrounding rock by numerical simulation. The results show that the subsidence displacement values of the overlying strata of the four structures at the roadway are 1.75 m, 1.5 m, 0.8 m and 0.6 m, respectively. The maximum stress of the four typical structures is 26.6 MPa, 19.5 MPa, 15.5 MPa, 10.0 MPa, (a) > (b) > (c) > (d). (a) The stress of surrounding rock of structural roadway is the largest, and the deformation of surrounding rock is large. At this time, the stability maintenance of roadway and coal pillar is the most difficult. (b) The stress of the structure on the solid coal side is large, and the support resistance at both ends of the working face is the largest during mining, so it is necessary to strengthen the support. (c) The stress value of the structural roadway is small, and the instantaneous load generated by the dynamic load generated by the fracture rotation of the coal pillar near the goaf is large. (d) The deformation of the surrounding rock of the structure is small, and the stress of the surrounding rock of the roadway is the smallest.

(3) During the mining period, roadway support and coal pillar reinforcement should be strengthened. After mining, the surrounding rock monitoring technology of gob-side entry should be done well. According to the fracture structure characteristics of the main roof of ZF2822 working face in Xiagou Coal Mine, the pressure relief measures are carried out. The results show that the average daily microseismic energy of ZF2822 working face in the mining process before the blasting area is 0.87 × 104 J, which is about 42.38% lower than the average daily energy of mining in April. The average stress is 1.13 MPa lower than that of the mining before the pressure relief area, and the decrease is 22.42%. During the mining process of the working face in the pressure relief area, the average stress is 0.49 MPa lower than that before the pressure relief area, and the decrease is 9.02%. It shows that the pressure relief has played a very good effect and provides a reference for the same type of mine.

Acknowledgements

Thank you Professor Lai for your guidance on this article. We also need valuable feedback and suggestions from Engineer Wang Wenlin on improving the manuscript.

Author contributions

Wang Wenlin engineer has provided great help for our field test and data analysis, which is very important for the field application of the paper. Dr. Zhu Haoyu mainly completed the writing of the thesis. Professor Lai Xingping guided the design and conduct of the experiment; Professor Shan Pengfei and Professor Yang Wenhua provided assistance in collecting on-site data and improving the manuscript; Dr. Zhu Haoyu, Dr. Qiao Hao, Dr. Jia Chong, and Dr. Wu Longquan built a numerical simulation and conducted experiments to analyze the data.

Funding

The National Natural Science Foundation of China,52274138.

Data availability

The datasets generated during the current study are not publicly available. Because these data are part of our research project, which is currently under way. We must wait until this research project is completed before we can make all the data public. But the data in this paper are available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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