#### Abstract

Longwall mechanized top coal caving mining (LMTCCM) in extra-thick coal seams has its own characteristics. The law of mining pressure and overlying strata failure height in extra-thick coal seams are much larger than those of medium-thick and thick coal seams. The key stratum structure morphology also has an important influence on the law of overlying strata movement and stability of surrounding rock. Based on the engineering geological conditions, this paper used the method of theoretical analysis and numerical simulation to study the key stratum structure morphology of LMTCCM in extra-thick coal seams. The results show that under the condition of LMTCCM in extra-thick coal seams, the key stratum forms the structure of low cantilever beam and high hinged rock beam. With the increase of coal seam thickness, the breaking position of cantilever beam is closer to the coal wall. Through theoretical calculation, it is obtained that the breaking length of cantilever beam is 31.5 m and the breaking position of cantilever beam is 15.4 m away from coal wall. With the increase of cycle, key strata will undergo the evolution law from the generation of longitudinal cracks to the hinged structure and then to the cantilever beam structure. The breakage of key strata will cause the expansion of longitudinal cracks and the overall synchronous movement of overlying strata. With the increase of coal seam thickness, the distribution of longitudinal cracks will gradually transfer from the upper part of goaf to the deep part of coal body in space and increase in quantity. This research is of great significance for improving the stability of overlying strata and ensuring the safe and efficient mining of extra-thick coal seams.

#### 1. Introduction

The conventional mining height has three types, including thin coal seam (thickness ≤ 1.3 m), medium-thick coal seam (1.3 m < thickness ≤ 3.5 m), and thick coal seam (thickness > 3.5 m). However, with the construction of large-scale modern coal mine, extra-thick coal seam (thickness ≥ 8 m) appears gradually. China has abundant coal reserves, with thick and extra-thick coal seams accounting for over 40% of the total coal reserves [1]. With the construction of high production and efficient mines, LMTCCM has become the main mining method of extra-thick coal seams [2, 3]. LMTCCM in extra-thick coal seams has its own characteristics, such as enormous coal output per panel, supper high efficiency, remarkable benefits, and so on, which plays an important leading role in the development of China’s coal industry [4, 5]. However, the increase of coal seam thickness and huge mining space make it difficult for the key strata in the overlying strata to form a stable structure [6]. The key strata are generally relatively thick and hard strata, which play a major role in controlling the activities of overlying strata in stope [7, 8]. Violent movement of key strata may cause a series of mining damage problems, such as surface subsidence, support damage, rock burst, and water and gas disaster [9–11]. Therefore, it is of great significance to study the structure morphology of key strata under conditions of LMTCCM in extra-thick coal seams.

In recent years, some research has been done on LMTCCM in extra-thick coal seams. Based on the mechanical calculation, Yan et al. [12] established a mathematical model and put forward a method to judge the roof separation position above roadway, which was successfully verified in the field. By analyzing the variation law of stress, displacement, and plastic zone of surrounding rock, Fei and Jiang [13] discussed the deformation mechanism of retracement roadway in extra-thick coal seams and put forward optimal scheme to control the large deformation of roadway. Zhang [14] presented the relationship between the stability of coal pillar and the reasonable position of split-level longwall gob-side entry in extra-thick coal seams. Wang et al. [15] studied the relationship between horizontal section mining and rock burst under conditions of steeply inclined and extra-thick coal seam and put forward reasonable measures to prevent rock burst. Based on the specific geological conditions, Wang et al. [16] explored the distribution and emission law of gas in extra-thick coal seams. Wang et al. [17] investigated key mining technologies and equipment of LMTCCM in extra-thick coal seams and successfully applied to Tashan coal mine. By building numerical model and field measurement, Si et al. [18] and Fan et al. [19] determined the gas dynamics characteristics in multilevel longwall top coal caving of extra-thick coal seams.

In addition, many scholars also did exploratory research on key strata theory and overlying strata movement. Based on the physical similarity simulation and field surveys, Zhou et al. [20] and Yao et al. [21] determined the distribution law of dynamic ground cracks under the influence of overlying strata structure. Li et al. [22] indicated that spatial relationships between key strata have influence on the height of mining-induced fracture zone. Liang et al. [23] discussed the influence of different key strata structure types on strata behavior with large mining height. Based on the theoretical deduction, Jiang et al. [24] deeply analyzed the instability and fracture mechanism of key strata and dynamic response characteristics. Li et al. [25] demonstrated the dynamic response process of strata movement and working face mine pressure after compound breakage of key strata. Wang et al. [26] analyzed the breakage and instability mechanism of hard-thick sandstone roof and its controlling effect on gas emission. Guo et al. [27] determined roof strata characteristics and support resistance of LMTCCM in extra-thick coal seams through physical similarity simulation and field monitoring. Based on elastic mechanics theories, Li et al. [28] established mechanical model to describe the rotation speed of hinged rock beam structure formed by key strata. Based on the theory of waterproof key strata, Sun and Miao [29] developed a model of an inclined coal seam floor with linearly increasing water pressure. Wang et al. [30] and Jiang et al. [31] studied the relationship between the mining height of coal seam and overlying rock movement under the filling mining face. In view of this, Zhang et al. [32] proposed the method of adopting the short-wall block backfill mining to protection surface water resources. At the same time, the thickness of coal seam is closely related to the law of strata movement. The greater the thickness of coal seam, the more serious the strata pressure behavior [33, 34].

It can be seen from the above that many scholars mainly focus on the roadway, coal pillar, rock burst, and gas control in extra-thick coal seams, strata movement characteristics, and mine pressure behavior. However, there are relatively few studies on the structure morphology of key strata under conditions of LMTCCM in extra-thick coal seams, especially the geometric configuration, breaking position, and influence factors of cantilever beam structure and distribution characteristics of longitudinal cracks in space and quantity. Therefore, based on the engineering background of 8211 working face, theoretical analysis and numerical simulation are used to study the structure morphology of key strata. This research has great significance in realizing safe and efficient mining of extra-thick coal seams and improving the stability of overlying strata.

#### 2. Engineering Background

##### 2.1. Mining and Geological Conditions

The coal mine, located in Datong city, Shanxi Province, China (Figure 1), covers a mining area of 40 km^{2}. The minefield is 8 km long and 5 km wide. The coal bearing strata belong to the Taiyuan Formation, upper carboniferous. The main minable coal seam is coal seam 5, with an average thickness of 15 m. This research involves a representative area: the 8211 working face with a striking length of 780 m and an inclination length of 220 m. Figure 2 shows the generalised stratigraphy column. The false roof is mudstone with an average thickness of 1.7 m. The immediate roof is siltstone with an average thickness of 3.8 m. The key strata control the movement of overlying strata to the surface, which have great influence on the mine pressure behavior [35, 36]. Based on the criterion conditions and calculation, the key strata are determined. The key stratum 1 is medium sandstone with an average thickness of 13.5 m. The interlayer is sandy mudstone and fine sandstone with an average thickness of 39.6 m. The key stratum 2 is also medium sandstone with an average thickness of 13.7 m. The immediate floor is carbonaceous mudstone with an average thickness of 5 m. The main floor is fine sandstone with an average thickness of 10 m.

##### 2.2. Experiments on Rock Mechanical Properties

The breakage and instability of key strata are closely related to the mechanical properties. In order to provide mechanical parameters for theoretical analysis and numerical simulation, uniaxial compression test, splitting testm and shear strength test are carried out on rock samples [37–39]. The uniaxial compression strength, elastic modulus, and Poisson’s ratio are measured by the uniaxial compression test on a total of 6 rock samples. The uniaxial tensile strength is measured by the splitting test on a total of 8 rock samples. The cohesion and internal friction angle are measured by the shear strength test on a total of 7 rock samples (Figure 3).

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#### 3. Theoretical Analysis

##### 3.1. Formation Conditions of Cantilever Beam Structure

Due to huge mining space, overlying strata move violently in LMTCCM of extra-thick coal seams. The key stratum with hinged rock beam structure can be formed at conventional mining height. Cantilever beam structure may be formed under condition of extra-thick coal seam because of large rotary deformation and instability of strata. But, the higher key stratum still can form hinged rock beam structure. Therefore, the key stratum structure morphology is closely related to the location of key stratum and the thickness of coal seam. As shown in Figure 4, when the possible rotating amount of key block is larger than the maximum rotating amount that can ensure the formation of stable structure, cantilever beam structure will be formed. The conditions of forming cantilever beam structure are as follows:where ∆ refers to the possible rotating amount of key block and ∆_{max} is the maximum rotating amount that can ensure the formation of stable structure.

The distance between the collapsed immediate roof and the key stratum is the possible rotating amount. It can be expressed as follows [40]:where *M* is the mining thickness of coal seam; *η* is the loss rate of coal caving; and *K*_{p} is the fragmentation coefficient of collapsed strata; *∑h*_{i} is the thickness of immediate roof and false roof.

According to the theory of mine pressure and strata control, the maximum rotating amount is as follows:where *h* is the thickness of key stratum; *q* is the load generated by overlying strata (*q* = *γH*); *L*_{0} is the periodic weighting interval of working face; *k* is dimensionless coefficient (*k* = 0.1 h); *σ*_{c} is the compressive strength of key strata; *γ* is average bulk density of overlying strata (*γ* = 25 kN/m^{3}); and *H* is burial depth of coal seam.

By substituting formula (2) and formula (3) into formula (1), the conditions of forming cantilever beam structure are obtained as follows [41]:

According to the engineering geological conditions of 8211 working face, the mining thickness of coal seam is 15 m; the loss rate of coal caving is 20%; the fragmentation coefficient of collapsed strata is 1.3; the thickness of immediate roof and false roof is 5.5 m; the thickness of key stratum is 13.5 m; the load generated by overlying strata is 10.25 MPa; the periodic weighting interval of working face is 28.6 m; and the compressive strength of key strata is 71 MPa. The corresponding value is substituted into formula (4). The possible rotating amount of key block is 10.35 m. The maximum rotating amount that can ensure the formation of stable structure is 7.02 m. The calculation results are in agreement with formula (4). It is concluded that the key stratum of LMTCCM in extra-thick coal seam forms the structure of low cantilever beam and high hinged rock beam, as shown in Figure 5.

##### 3.2. Geometric Characteristics of Cantilever Beam Structure

Cantilever beam structure plays an important role in the stability of stope and roadway. Sometimes, it will cause severe coal mine accidents and damage of support equipment in stope, as shown in Figure 6. Therefore, it is necessary to determine the geometric characteristics of cantilever beam structure, including its thickness, breaking length, and position on the coal wall. According to the geological conditions of 8211 working face, the thickness of cantilever beam is 13.5 m, and the breaking length can be solved by the following formula [42]:where *L* is the breaking length of cantilever beam and *S* is the length of working face.

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According to the geological conditions of 8211 working face, the length of working face is 220 m and the periodic weighting interval of working face is 28.6 m. The corresponding value is substituted into formula (5). Finally, the breaking length of cantilever beam is 31.5 m.

The breaking position of cantilever beam on the coal wall can be solved by internal and external stress field theory. Figure 7 shows that cantilever beam is broken, and the pressure transferred from overlying strata to coal is divided into two parts. The internal stress field *S*_{1} is within the breaking line and bears the weight of the strata controlled by the structure of low cantilever beam and high hinged rock beam. The external stress field *S*_{2} is outside the breaking line and bears the weight of the upper strata and additional stress transferred from the internal stress field. It can be seen that the range of internal stress field is the breaking position of cantilever beam.

According to material mechanics, the vertical stress on the coal which is *x* m away from coal wall within the range of internal stress field can be expressed as follows [43]:where *σ*_{y} is the vertical stress on the coal *x* m away from coal wall; *G*_{x} is the stiffness modulus of the coal *x* m away from coal wall; and *y*_{x} is the amount of compression of the coal *x* m away from coal wall.

From the edge of coal wall to the depth, coal will change from two-dimensional stress state to three-dimensional stress state. The stiffness modulus of coal will increase and the vertical compression of coal will decrease. The compression deformation of coal will reach maximum at the coal wall. In order to simplify calculation, it is considered that the compression and stiffness modulus of coal vary linearly within the range of internal stress field. The equation is as follows:where *y*_{0} is the amount of compression of the coal at the edge of coal wall; *x*_{0} is the range of internal stress field; and *G*_{0} is the maximum stiffness modulus of coal in the range of internal stress field.

The vertical stress in the internal stress field can be integrated as follows:

According to formula (7) and formula (8), the following equation can be obtained:

The vertical stress of coal in stope during the first weighting is equal to the weight of rock strata controlled by the structure of low cantilever beam and high hinged rock beam. The following equation can be obtained:where *C*_{0} is the first weighting interval of working face and is the thickness of rock strata controlled by the structure of low cantilever beam and high hinged rock beam.

As shown in Figure 7, the geometric relations of *y*_{0} and *x*_{0} are as follows:where *∆h* is the maximum subsidence of key stratum.

According to the theory of mine pressure and strata control, the following equation can be obtained:

According to formula (11) and formula (12), the following result can be obtained:

The stiffness modulus of coal can be expressed as follows:where *E* is the elastic modulus of coal; *u* is Poisson’s ratio of coal; and *ξ* is the integrity coefficient.

According to formula (10), formula (13), and formula (14), the range of internal stress field can be obtained as follows:

Based on geological data, the thickness of rock strata controlled by the structure of low cantilever beam and high hinged rock beam is 90.6 m; the first weighting interval of working face is 35 m; integrity coefficient is 0.8; Poisson’s ratio of coal is 0.36; the elastic modulus of coal is 2.06 GPa. The corresponding value is substituted into formula (15). The range of internal stress field is 15.4 m. Therefore, the breaking position of cantilever beam is 15.4 m away from coal wall. The geometric configuration of cantilever beam is shown in Figure 7.

##### 3.3. Influence Factors of Cantilever Beam Breaking Position

According to formula (15), it can be seen that there are three factors, geometric parameters, engineering parameters, and mechanical parameters, which affect the breaking position of cantilever beam. Geometric parameters of rock strata include the thickness of coal seam and immediate roof. Engineering parameters include the length of working face, the first weighting interval, and periodic weighting interval. Mechanical parameters of coal and rock mass include elastic modulus, integrity coefficient, and fragmentation coefficient.

The single factor analysis method is used to obtain main influence factors on the breaking position of cantilever beam, as shown in Figure 8. The distance between the cantilever beam breaking line and coal wall decreases with the increase of coal seam thickness. This is because the greater the coal seam thickness, the larger the rotation space of cantilever beam, and the closer the breaking line is to the coal wall. On the contrary, with the increase of immediate roof thickness, the strata thickness controlled by overburden structure, the length of working face, and the first and periodic weighting interval, the cantilever beam breaking position is gradually transferred to the deep part of coal body.

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#### 4. Numerical Simulation

##### 4.1. Block Constitutive Model

UDEC is two-dimensional discrete element software for discontinuous medium, which is represented by discrete blocks. Discontinuous surfaces are the contact surfaces between blocks. Blocks can move and rotate along discontinuous surfaces. It is often used to simulate the fracture characteristics and movement laws of overlying strata. In this research, JSET command is used to generate vertical and horizontal joint groups, and CHANGE command is used to change the material properties of blocks and joints [44–47]. Because the tensile strength of rock is significantly lower than the compressive strength, the Mohr-Coulomb elastoplastic model is chosen as the block constitutive model. The failure envelope of the model conforms to Mohr-Coulomb criterion (shear yield function) with tension cutoff (tension yield function). The failure criterion could be expressed in the plane (*σ*_{1}, *σ*_{3}), as shown in Figure 9.

The failure envelope is defined from point A to point B by the Mohr-Coulomb yield function [48]:where *φ* is the friction angle, *c* is the cohesion, and and from B to C by tension yield function of the form:where *σ*_{t} is the tensile strength.

##### 4.2. Contact Constitutive Model

The contact constitutive model is used to simulate the sliding or opening of the contact. It is very proper to show the failure process of overlying strata after excavation of working face [49, 50]. Figure 10 shows the yielding process of the contact. In the normal direction, the linear relationship between contact stress and displacement is as follows:where Δ*σ*_{n} is the normal stress increment; Δ*u*_{n} is the normal displacement increment; and *k*_{n} is the normal stiffness.

If the normal stress is larger than the tensile strength, tensile failure will occur in the contact.

In the shear direction, the response is controlled by the shear stiffness (*k*_{s}) and the shear strength (*τ*_{max}). The relationship between stress and displacement includes two parts [51].

If , thenor else, if , thenwhere is the elastic shear displacement increment and Δ*u*_{s} is the total shear displacement increment.

##### 4.3. Numerical Model Establishment

In order to further judge the key stratum structure morphology of LMTCCM in extra-thick coal seam, the numerical calculation model of UDEC is established, as shown in Figure 11. The width and height of the model are 200 m and 95 m, respectively. The *X*-axis direction is coal seam inclination, and the *Y*-axis direction is vertical direction. In order to reduce the influence of boundary effect, the excavation length of simulated working face is 100 m. The velocity at the bottom of the model is 0 in the vertical direction. The velocity on both sides of the model is 0 in the horizontal direction. According to the burial depth of working face, vertical stress is applied to the upper boundary to simulate the weight of overburden. The Mohr-Coulomb elastoplastic model is used for blocks, and the Coulomb sliding model is adopted for joints. Based on the previous experiments and X-ray diffraction spectrum of rock samples, the mechanical parameters of blocks and joints are shown in Table 1.

##### 4.4. Structure Failure Characteristics of Key Stratum in Different Cycles

Compared with conventional working face mining, LMTCCM in extra-thick coal seams has its own characteristics. It has large coal seam thickness, mining space, and mining influence, resulting in violent movement of overlying strata and key stratum structure failure. Therefore, in the numerical simulation analysis, the movement characteristics of overlying strata should be reflected as reliably and truthfully as possible, so as to explore the structure characteristics changes of key stratum in different cycles (Figure 12).

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When the model cycles 10000, there is no obvious movement of overlying strata and no deformation of coal body. This is because the key strata are not broken and can bear the weight of overlying strata. When the model cycles 50000, the key strata and immediate roof will have a small bending and deformation. Then, the coal body is extruded, resulting in deformation and destruction. When the model cycles 80000, the immediate roof sinks significantly. The key stratum 1 is broken, and the breakage degree of the key stratum 1 is much greater than that of the key stratum 2. The key stratum 1 forms stable hinged rock beam structure. When the model cycles 120000, the immediate roof collapses completely. Rotation and subsidence occur in the lower key stratum, which forms cantilever beam structure. The upper key stratum forms hinged structure to support higher strata. The failure of coal body in working face reaches the maximum. Figure 12 shows that, with the increase of cycles, key strata will undergo the evolution law from the generation of longitudinal cracks to the hinged structure and then to the cantilever beam structure. The fracture of the key strata will cause the expansion of longitudinal cracks and the whole synchronous movement of the overlying strata. Under conditions of LMTCCM in extra-thick coal seams, cantilever beam structure is formed in the key stratum 1 and hinged rock beam is formed in the key stratum 2. Numerical simulation results are in agreement with the theoretical calculation.

##### 4.5. Structure Failure Characteristics of Key Stratum in Different Coal Seam Thicknesses

When the coal seam of working face is mined, the movement of overlying strata is gradually stable, and the caving zone, fracture zone, and bending subsidence zone are formed in turn. At this time, the coal body is mainly subjected to static pressure. Coal seam thickness is one of the important factors affecting the breaking state of key stratum. So, UDEC is used to simulate the structure failure characteristics of key stratum in different coal seam thicknesses (Figure 13).

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When coal seam thickness is 5 m, overlying strata failure height is relatively small. There is a longitudinal crack in the key stratum 1. The key stratum 2 and the higher overburden encounter slight synchronous subsidence. When coal seam thickness is 10 m, the key stratum 1 forms stable hinged rock beam structure. There are three longitudinal cracks in the key strata. The rotary force of key strata are transmitted to coal body, which results in large deformation of the coal body. When coal seam thickness is 15 m, the key strata form the low cantilever beam and high hinged rock beam structure. There are three longitudinal cracks in the deep part of coal body. When coal seam thickness is 20 m, the key strata still form the low cantilever beam and high hinged rock beam structure. However, there are four longitudinal cracks in the deep part of coal body. According to the above discussions, it can be seen that, with the increase of coal seam thickness, the distribution of longitudinal cracks will gradually transfer from the upper part of goaf to the deep part of coal body in space and increase in quantity.

#### 5. Conclusions

Based on the engineering geological conditions of 8211 working face, theoretical analysis and numerical simulation are used to study the key strata structure morphology of LMTCCM in extra-thick coal seams. The conclusions are as follows:(1)Under conditions of LMTCCM in extra-thick coal seams, the key strata in the overburden strata form the structure of low cantilever beam and high hinged rock beam. With the increase of coal seam thickness, the breaking position of cantilever beam is closer to the coal wall.(2)Through theoretical calculation, it is obtained that the breaking length of cantilever beam is 31.5 m and the breaking position of cantilever beam is 15.4 m away from coal wall.(3)The single factor analysis method shows that, with the increase of immediate roof thickness, the strata thickness controlled by overburden structure, the length of working face, and the first and periodic weighting interval, the cantilever beam breaking position is gradually transferred to the deep part of coal body.(4)With the increase of cycle, key strata will undergo the evolution law from the generation of longitudinal cracks to the hinged structure and then to the cantilever beam structure. The expansion of longitudinal cracks and the whole synchronous movement of overlying strata are beneficial to the prediction of key strata breakage.

#### Data Availability

The data used to support the findings of this research are included within the paper.

#### Conflicts of Interest

The authors declare that they have no conflicts of interest.

#### Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51974317), the Yue Qi Distinguished Scholar Project (800015Z1138), China University of Mining and Technology, Beijing, and the Fundamental Research Funds for the Central Universities (800015J6).