Coevolution mechanism and branch of pillar‐overburden fissures in shallow coal seam mining

The large‐scale mining of coal resources promotes the formation of section coal pillar groups between working faces. The overlying strata are supported by coal pillars, which leads to the formation of a unique fracture‐bearing structure in the key stratum, resulting in the evolution of fissures (cracks) and a generally high load on the supports. Studying the fracture‐bearing structure formed by coal pillars and overlying strata (fissures) is the basis for realizing rational mining of working faces. Here, the evolution characteristics of coal pillar‐overburden rock (fissures) in shallow coal seam mining and the working resistance of the support with stable fracture bearing structure of section coal pillar and overburden rock were studied in the No. 1‐2 coal mining of Longhua Coal Mine in Shenmu City, Northern Shaanxi. The fissure evolution characteristics on both sides of the coal pillar and the fracture‐bearing structure of the rock stratum were obtained by physical similarity simulation experiment. The mechanical calculation model of the coal pillar‐overburden rock (fissure) structure was established by theoretical derivation and calculation, and the working resistance of advanced support with a stable mechanical structure was studied. The research shows that the fracture evolution and rock fracture characteristics of a shallow coal seam mining face can be divided into three zones. Through the reanalysis of the mechanical structure of coal pillar‐overburden rock (fissure), the combined mechanical structure of the subkey stratum consisting of a lagging broken step rock beam structure and the main key stratum consisting of a hinged rock beam structure was obtained. Combined with the failure characteristics of the section coal pillar, a mechanical structure calculation method suitable for the bearing characteristics of section coal pillar support in shallow coal buried layers was obtained, and the field measurement results were verified.


| INTRODUCTION
As one of the leading mining areas of shallow coal seam mining in China, the Yushenfu area has numerous mining problems due to high-intensity mining and is a leading area for shallow coal seam mining research. [1][2][3] After the coal seam is mined, a large number of section coal pillars are set between the working faces to protect the roadways on both sides of the coal pillars [4][5][6] and the stability of the overlying strata. 7 This leads to an increase in the concentration stress of the coal pillars and the difficulty of the advanced support of the working face. 8,9 The cause is a variety of complex scientific problems formed by the interaction of coal pillars and overlying strata. Therefore, the study of the coevolution of coal pillar-overburden rock fissures should be solved.
Many studies have been carried out on the reasonable width and stagger distance of coal pillars. Many scholars have studied coal pillar stress characteristics and instability under different geological conditions. They mainly analyzed the plastic failure and creep of coal pillars caused by advanced stress and lateral stress concentration in working face mining, which provided some theoretical guidance for retaining section coal pillars. [10][11][12] For the unique geological conditions of the shallow coal seam, the stability of the bearing structure formed by the overlying strata above the coal pillar is insufficient. The coal pillar can easily destabilize, resulting in secondary activation of the bearing structure of the overlying strata, which leads to an increase in the degree of overburden fissures evolution. [13][14][15] Usually, coal pillar stability determines overlying strata stability, which has a time-dependent effect on the evolution of overburden fissures. At the same time, the generation of overburden fractures leads to an increase in the activation rate of the overburden-bearing structure and a change in the initial stable load, which reacts on the coal pillar to form a cyclic space-time evolution system. 16,17 A shallow coal seam typically has thin bearing strata and a thick load layer, which promotes the formation of permanent fissure channels on both sides of the working face between coal pillars at a certain angle and penetrating the surface; this results in hidden disasters such as air leakage, water inrush, and sand inrush in the working face. 18,19 Due to the supporting role of coal pillars, uneven surface settlement often results, and the stepped subsidence of the surface is caused. Concurrently, O-X breakage and collapse of the working face with the coal pillar as the boundary lead to the stress concentration phenomenon of a triangular area of the working face. [20][21][22] In addition to the influence of coal pillar support, surface fissures (cracks) are mainly controlled by the fracture structure of the upper key stratum of the coal pillar. 23,24 When a step rock beam structure is formed in the fractured rock strata, the surface fissures are mainly step dislocation accompanied by a small number of horizontal dislocation cracks. 25,26 When a hinged rock beam structure is formed in the broken rock stratum on the upper part of the coal pillar, surface fissures (cracks) are mainly horizontal dislocation cracks, and a small amount of step dislocation fissures form at the edge of the fissures. 27 Fissure morphology is similar to the evolution characteristics of the mining-induced cracks in the middle of the shallow coal seam working face. A method of reasonable staggering distance can be adopted in the coal pillar group of the shallow coal seam to realize the closure of fissures. 28,29 Research on the overlying strata and surface fissures (cracks) in the upper coal pillar should include the shape of the overlying strata fracture structure. 30 The coal pillar-overburden rock fissure system should be studied by analyzing the overlying strata fracture structure formation, instability, and stability.
Based on the research premise of shallow coal seam mining, this paper uses similar simulations to study the overlying rock structure and fissure (crack) evolution characteristics after the setting of the coal pillar. Based on the simulation experiment of the coal pillar-overburden rock fissure, the evolution characteristics of the upper strata of the coal pillar and surface fissure are given, and the mechanical calculation model of coal pillar-overburden rock (fissure) is established. This work provides a theoretical basis for calculating surface fissure, overburden fracture-bearing structure, and support load in shallow coal seam mining. For the above research content, the research flow of this paper is shown in Figure 1

| Similar constants and model size selection
According to the mechanical characteristics of coal and the geological conditions of the study area, the geometric similarity constant is: K l = 1/100; time similarity constant: K t = 1/10; the bulk density similarity constant: K c = 1.56; stress similarity constant: K σ = 156 test bench. The experimental model length × width × height = 300 cm × 20 cm × 155.5 cm. The total height of the model is 71.5 cm. The mechanical parameters of the rock layers in the study area are shown in Table 1.

| Experimental material selection
The materials selected for the physical simulation experiment included river sand, fly ash, gypsum, and white powder as the main experimental material, and mica as the model and layered material. A similar simulation test bench for shallow coal seam mining was created using these materials and included a dial indicator, camera, total station, and so forth as experimental recording tools, as shown in  main key stratum. At this time, the load on the key stratum-breaking structure acts on the stress support point of the coal pillar and caves the rock stratum of the working face. The stress support point is in the pressure relief area, and the support point is the stress concentration area.

| Goaf overburden trapezoidal fracture zone
After coal mining, complete rock fracture structure and fissure evolution characteristics formed along the working face ( Figure 4). The rock strata on both sides of the working face are broken and extend to the surface in an approximately symmetrical form, forming a trapezoidal broken rock stratum structure and crack distribution area. Affected by the boundary coal pillars on both sides of the working face, the mechanical support structure formed with the coal pillars and the caving rock blocks in the goaf as the support points. Difficult-to-close longitudinal fissures are produced with the support points and the rock-breaking angles. In this range, the rock strata produce a specific rotation and subsidence. Due to the different degrees of rock strata subsidence, many horizontal fissures form, promoting the development of regional fracture density and opening degree, with the coal pillars and goafcaving rock strata as supporting points.

| Section coal pillar overburden "inverted trapezoid" broken rock pillar area
After the coal seam is mined, several section coal pillars are set between the working faces to protect the mining face. The fracture of the mining strata in the working face promotes the formation of an inverted trapezoid rock pillar support area ( Figure 5). This supports the upper rock (soil) layer, resulting in a noticeable coal pillar load and stress concentration. Both sides of the section coal pillar are the working faces that are being mined or have been mined, resulting in the formation of a rock-breaking structure with the section coal pillar as the center and the overlying strata breaking to both sides. Because the section coal pillar is the mining boundary of the left and right working faces, it can be regarded as two inverted right trapezoidal rock pillar fracture distributions combined. As the working face openoff cut side rock fracture collapse is the process of synchronous deflection fracture, the fractured rock can easily produce a hinged rock beam structure. On the coal wall side of the working face, the rock stratum is supported T A B L E 1 Mechanical parameters of the rock layers in the study area.

Number Lithology
Thickness ( by the broken rock stratum before breaking. At this time, an approximately symmetrical step rock beam structure in the subkey stratum is formed on both sides of the coal pillar. When the main key stratum is broken, the lower caving space decreases. The rotary space of the broken rock stratum is now insufficient, thus forming a hinged rock beam structure. At this time, the subkey stratum has the mechanical structure characteristics of the step rock beam structure and the main key stratum hinged rock beam structure and forms a permanent fissure zone and a stress relief zone between the support points on both sides of the coal pillar. Note that this paper only studies the case where both sides of the coal pillar are advancing. When the physical similarity simulation experiment was left to rest for some time, the inverted trapezoid broken rock pillar area on the upper part of the section coal pillar had refailure and rotation at the location where the rock beam structure formed in the key layer. From the above analysis, the upper load of the coal pillar is affected by the load of the inverted trapezoidal broken rock pillar and the load of the broken structure of rock strata on both sides of the coal pillar. It is necessary to ensure the stability of the fracture structure of the key stratum by inhibiting the development and evolution of rock strata and surface fissures on both sides of the coal pillar. At this time, the mechanical model of coal pillaroverburden rock (fissure) can be regarded as the mechanical calculation model of coal pillar-overburden rock (step rock beam-hinged rock beam) structure.
According to the physical similarity simulation results, the immediate upper roof of the section coal pillar first breaks and collapses after coal mining. As the length of the working face increases during advancement, the upper subkey stratum begins to break and form a step rock beam structure to bear the upper load. The mining space of the working face increases, and the main key F I G U R E 2 Planar physical simulation experiment frame and equipment.
F I G U R E 3 Boundary inverted right angle trapezoid rock pillar area and rock stratum fracture structure diagram.
F I G U R E 4 Overburden rock "trapezoidal" fracture zone in goaf.
F I G U R E 5 Section coal pillar "inverted trapezoidal" broken rock pillar area and overburden fracture structure diagram. stratum breaks. Due to rock fragmentation, the upper rock mass sinking rotary space is reduced, forming a hinged rock beam structure. During mining, the bearing structure of the key stratum tends to be stable and gradually compacted. The inverted trapezoid rock pillar and the section coal pillar support the bearing structure on both sides of the coal pillar. The bearing structure plays a role in supporting the overlying load as well. The experimental results show that when the bearing structure forms by breaking the key stratum, the length of the upper rock pillar is large, and the length of the lower rock pillar is small due to the inverted trapezoid rock pillar on the upper part of the coal pillar. This results in the key stratum breaking. The bearing structure and the inner side of the coal pillar have a lagging failure zone, which still applies force on the key stratum after the damage. The fractured structure of the rock stratum can be approximately regarded as the morphological characteristics shown in Figure 6.
Only the right side of the coal pillar is selected for mechanical analysis because the force and mechanical structures of the section coal pillar are approximately symmetrically distributed. The elements with less effect on the mechanical structure are simplified to make the mechanical structure model more intuitive. At this time, the mechanical calculation model of coal pillaroverburden rock (hinged rock beam structure-lagging broken step rock beam structure) is shown in Figure 7.
In Figure 7, h is the mining height of coal seam, m; h 1 is the thickness of the immediate roof, m; h 2 is the thickness of the subkey stratum, m; h 3 is the thickness of the interlayer rock, m; h 4 is the thickness of the main key stratum, m; h 5 is the thickness of the loess loose layer, m; P 1 is the self-weight load of the control cantilever immediate coal roof, kN; P 2 is the interlayer rock load, kN; P 3 is the load transferred from the loose layer to the main key stratum, kN; P 4 is the load transferred from the lagging failure zone of the interlayer rock stratum to the subkey stratum, kN; P T is the self-weight load of key block E of the subkey stratum step rock beam structure, kN; P J is the main key stratum hinged rock beam structure B key block weight load, kN; L 1 is the breaking length of key block B of main key stratum, m; L 2 is the breaking length of the key block E in subkey stratum, m; L 3 is the breaking length of the key block F in subkey stratum, m; θ J is the main key stratum hinged rock beam structure B key block rotation angle,°; θ T is the subkey stratum step rock beam structure E key block rotation angle,°; β J is the main key stratum hinged rock beam structure rock breaking angle,°; β T is the subkey layer step rock beam structure rock fracture angle,°.  inevitably causes the overburdened rock or soil body to fracture. In this paper, the upper part of the main key stratum is mainly the loose body of the loess layer, which cannot bear a load. It can be considered a uniform load. At this time, the load of the lagging failure zone can be simplified as the load of the loose body. The mechanical structure is shown in Figure 8.

|
When a hinged rock beam structure forms in the main key stratum, due to the load in the lagging failure zone, the load P 3 of the loose body in the upper loess layer can be expressed as where γ 5 is the average bulk density of the loose layer, kN/m 3 ; K G is the load transfer coefficient. The force of the main key stratum breaking block B is The hinged rock beam structural force R J formed by the main key stratum breaking is

| Structural force of subkey stratum lagging broken step rock beam
The structural force R T of the subkey stratum lagging broken step rock beam includes the loess layer loose body load P 3 ; the main key stratum hinged rock beam structural load R J ; the force load P 2 of the interlayer rock layer acting on key block E of the subkey stratum; subkey stratum lagging fracture block load P 4 θ cos T1 ; self-load P T of key block E of subkey stratum lagging broken step rock beam structure is shown in Figure 9 R W P θ P = + cos + , where W 2 is the main key-stratum breaking structure force, kN. Interlayer rock force P 2 is expressed as The mechanical structure of this calculation still conforms to the mechanical calculation model of a masonry beam. Because the rock beam structure is at the boundary of the working face, the fracture distance of the subcritical rock beam does not fully meet the assumption of L 2 = L 3 of a masonry beam. According to the rockbreaking characteristics and experimental phenomena, the rock-breaking length of the key blocks E and F meets L 2 = (1-1.3)L 3 . The uniform load is approximated according to the moment equilibrium condition as the concentrated force acting on the middle of the key rock block. Taking ∑M O = 0, ∑M F2 = 0, ∑M y = 0, it can be obtained as where Q A and Q B are the shear forces of the key blocks on O and F 3 contact hinges, kN/m; b is the sinking F I G U R E 8 Structural mechanics calculation model of main key stratum hinged rock beam.
F I G U R E 9 Structural mechanics calculation model of subkey stratum "lagging broken step rock beam." rotation of key block E of the lagging broken step rock beam structure, m; d is the sinking rotation of key block F of the lagging broken step rock beam structure, m. According to the "step rock beam" subsidence calculation method From Formula (10) Since key block F is basically stable, it can be considered that Q B ≈ 0, and T A can be obtained by combining Formulas (7)-(10) as and ∑M y = 0 can be expressed as The S-R stability theory states that the mechanical conditions for preventing the sliding instability of the subkey stratum lagging broken step rock beam structure are where ϕ tan is the friction coefficient of the rock block is 0.5.
Because the section coal pillar plays a major supporting role in the upper load, from Formulas (12) and (14), the supporting force R T required to ensure the structural stability of the subkey stratum lag break is where b m is the inclined length of the coal pillaroverburden rock (fissure) mechanical model acting on the coal pillar, m.
Force P 4 of the lagging fracture zone of the subkey stratum of the interlayer rock stratum is In the formula, γ 3 is the average bulk density of interlayer rock, kN/m 3 ; b Z is the length of the subkey stratum lag breaking zone, m.
The self-weight load P T of key block E of the delayed breaking step rock beam structure of the subkey stratum is

| Immediate roof force
Self-weight load P 1 of the direct roof strata is expressed as where γ 1 is the average unit weight of the immediate roof, kN/m 3 ; b h is roadway width, m.
To ensure the stability of the coal pillar when the section coal pillar is mined in the working face, the support resistance R of the support near the section coal pillar can be calculated by 1 1 Z where W 1 is the structural force of the rock beam of the subkey stratum, kN; K Z is the dynamic load coefficient of the support.

| Stress distribution of the section coal pillar
Numerous studies have shown that 31,32 plastic zones form on both sides of the section coal pillar during mining because of the mining load, including the fracture zone (the contact zone b m between the section coal pillar and the lagging broken step rock beam structure analyzed in the upper section) and the crack zone (the section coal pillar is affected by the overlying lagging failure zone b Z ). This results in a decrease in the bearing characteristics of the coal pillar.
Here, the load of the upper strata of the coal pillar is simplified, and the stress characteristics of the coal pillar are analyzed. The cantilever rock beam on the upper part of the plastic zone of the coal pillar can be regarded as a combined cantilever rock beam structure, and the rock stratum on the upper part of the elastic zone of the coal pillar can be regarded as an inverted trapezoidal rock pillar group structure, as shown in Figure 11. L m is the width of the section coal pillar, m; b m is the width of the fracture zone in the plastic zone, m; b Z is the width of the crack zone in the plastic zone, m; q is the uniform load on the upper part of the subkey stratum, kN; P is the support load of the section coal pillar, kN.

| Plastic zone width of section coal pillar
According to the balanced relationship of mechanical calculation, the stress balance calculation is carried out on the support load of the coal pillar and the structure of the "combined cantilever rock beam," the structure of the " inverted trapezoid" rock pillar group and the upper load of the subkey stratum. Because the upper load of the section coal pillar is rock load, to simplify the calculation process, it is considered that γ 1 =γ 2 , so it can be obtained as When the section coal pillar is not disturbed by mining, the pillar supporting load σ is where γ is the average bulk density of the overlying rock (or soil) layer on the section coal pillar, kN/m 3 ; H is the mining depth, m.
After the section coal pillar is disturbed by mining, the working faces on both sides of the coal pillar form stress concentrations. The load on the coal pillar can be expressed as where k is the stress concentration factor of the section coal pillar after mining disturbance. The combined cantilever rock beam structure inevitably produces rotation due to mining, forming a fracture zone in the coal wall of the section coal pillar. According to the stress balance calculation equation, b m can be expressed as  where λ is the lateral pressure coefficient of section coal pillar; c is coal cohesion, MPa; ϕ m is the internal friction angle of coal,°; P Z is the support strength of section coal pillar, MPa.
According to the basic mechanical parameters of the coal body in the Longhua Coal Mine and relevant laboratory data, the lateral pressure coefficient λ = 0.7; coal stress concentration factor k = 2.8; coal cohesion c = 1.18 MPa; internal friction angle of coal ϕ m = 37°; section coal pillar supporting strength P Z = 0.08 MPa.
The width b m of the fracture zone of the section coal pillar can be obtained by substituting each parameter into Formula (23)   (24) According to the mechanical calculation model, the lagging breaking step rock beam structure of the subkey stratum and the hinged rock beam structure in the main key stratum are mainly carried by the section coal pillar fracture zone b m. The load in this area can therefore be simplified. The calculation expressions of the crack zone of the section coal pillar can be obtained by substituting Formulas (22) and (24) into Formulas (20)

| Width of the elastic zone of section coal pillar
According to the geometric relationship of Figure 10, the elastic zone width b t of section coal pillar can be expressed as From the above analysis, the support resistance R required for the mechanical structure of the coal pillaroverburden rock (or fissure) to maintain stability during the mining process of the working face can be calculated simultaneously through Equations (19), (24), and (25). However, due to the cumbersome calculation, there is little traceability here. . The load transfer coefficient is K G = 0.65 and 0.5a = 0 due to the small extrusion surface of the key stratum structure.
(1) Structural force of the hinged rock beam of the main key stratum (a) Effective load P 3 of the loose loess layer is  Because the lower part of the subkey stratum is only the immediate roof, W 1 = R T at this time. According to the field measurement, the dynamic load coefficient K Z of the support is 1.4. The supporting load required for the mining support of the mine is found by R W P K = ( + ) = (8369.72 + 2090.77) × 1.4 = 14644.69kN. 1 1 Z The calculation results indicated that the rated working resistance of the support required to ensure the stability of the mechanical structure of the section coal pillar-overburden rock (or fissure) should be greater than 14,644.69 kN. The working resistance of the advanced support used in the field should be 16,000 kN, which meets the production needs.

| Field load characteristics monitoring analysis
The accuracy of the mechanical structure model of the section coal pillar-overburden rock (or fissure) and the mechanical structure conforming to the actual support load characteristics of the mine was verified. Monitoring of the mine pressure data of the 50-360 m advance support arranged in the return air roadway of the 10103 working face of Longhua Coal Mine ( Figure 12) shows that there are only 4 times in the 17 times of pressure. The working resistance value of the advance support is greater than the theoretical calculation value, accounting for about 23.6% of the total pressure times. The maximum value is only 1.01 times the theoretical calculation value, which does not exceed the advanced support's rated working resistance. When the load of the advanced support in the working face is greater than the theoretical value, it is generally accompanied by a larger load after a lower load. This shows that when the support load is low, the sub-key stratum and the main key stratum do not produce synchronous fractures. When the load of the support increases significantly, synchronous breaking occurs. According to this study, field measurements are similar to the theoretical calculation and analysis results, which verifies the scientific rationality of this study.

| CONCLUSION
(1) The physical similarity simulation found that the fissure evolution degree on both sides of the coal pillar in shallow coal seam mining areas was high. Permanent cracks easily form, which leads to the hidden disaster of air leakage, water inrush, and sand inrush in the working face. (2) Based on the fracture characteristics of the key stratum, the mechanical structure model of coal pillar-overburden rock (or fissure) was established. The load of the interlayer lagging broken rock layer affects the subkey stratum, and the mechanical structure model of a subkey stratum lagging broken step rock beam is suitable for the load calculation of the section coal pillar. (3) By analyzing the load characteristics of the coal column in the section, the combined cantilever rock beam and inverted trapezoid rock column group structure of the coal column in the section and its upper rock layer were established. The calculation formula of the elastic-plastic zone of the coal column in the section was obtained. (4) According to the mechanical structure model, combined with geological conditions, it was calculated that the support load required to maintain the stability of the coal pillar support area in Longhua Coal Mine should be greater than 14,644.69 kN. The scientific rationality of the research was verified by field monitoring.