Introduction

The development and progress of human society have long relied on coal as one of the main energy sources1. Coal has always been the most widely stored and distributed pillar energy source on Earth, with superior ease of use and abundant downstream products. As of the end of 20222, the world’s proven coal resource reserves have reached 1.07 trillion tons, with the United States accounting for 23.2%, reaching 248.941 billion tons, and China accounting for 13.3%, reaching 207.886 billion tons3. Although major global energy producers and consumers are continuously adjusting their energy structure, the dependence of countries on coal resources will be difficult to change for a considerable period in the future, as shown in Fig. 1.

Fig. 1
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Global coal consumption trends (2000–2025) and China’s energy supply and demand relationship (2020–2025). [Coal medium-term market report—Details—Trove.].

With the continuous underground mining of underground coal mines4, the solid coal seams that were originally used as support foundations for rock layers were extracted, which disrupted the stress balance of the original rock5. The overlying rock layer, which was originally supported by fixed constraints at the lower boundary, undergoes continuous damage and displacement after losing support constraints6. The degree of surface movement and deformation caused by coal mining activities varies due to factors such as the mining height, coal seam thickness, mining intensity, mining method (completely collapsed or filled mining method), mechanical properties of the overlying rock, and surface structures. The protection of important surface infrastructure, selection of engineering locations, and design of avoidance schemes for water sources are all influenced by the above factors. Inaccurate prediction of the surface movement range and deformation degree can lead to house collapse, water leakage pollution, and even more serious instability disasters. Therefore, studying the mechanism of surface displacement and settlement caused by rock movement during coal mining to predict the range and degree of surface movement and further designing measures to control and weaken surface movement has important practical significance and is in line with the purpose of safe and green mining7.

The study of rock movement mechanisms in underground coal mining has always been a popular and difficult research topic in the fields of mine pressure and rock control. Domestic and foreign experts and scholars have proposed many theoretical models and effectively verified them based on onsite measurements and laboratory experiments, which have been successfully applied in the field. For example, the academician Qian Minggao, Professor Shi Pingwu, and Professor Xu Jialin divided the overburden movement caused by mining into a “collapse zone, fracture zone, and bending zone” and obtained a negative exponential function curve calculation formula related to the displacement and mining process parameters after stable rock movement through deep base point observations8. The academician Song Zhenqi9 proposed a “Dynamic Structural Model of Rock Movement and Support Pressure Distribution in Mining Sites”. Guided by this theoretical model, the structures formed during the mining process of working faces can be divided into two types: internal stress field and internal stress field structural models, which effectively predict and control mine pressure and surface rock subsidence movement by changing structural characteristic parameters10.

The theory and methods for predicting rock strata and surface subsidence caused by underground coal mining can be expressed in the following forms11,12,13,14,15. (1). The method of using dimensionless typical curves to represent the distribution of surface point movement and deformation in the main section of the moving basin is based on monitoring data, namely, the typical curve method. This method requires a large amount of accurate and reliable measured data and can be applied only to rectangular or approximately rectangular excavation spaces. (2). Based on the typical subsidence curve characteristics obtained from actual measurements, an appropriate fitting function is selected, and data processing is used to determine the function or constant in the equation to predict the movement and deformation of the main surface section, namely, the profile function method. This method is also based on actual measurement data for prediction, with high accuracy requiring a relatively small data volume, and is generally suitable for rectangular and approximate rectangular working faces. (3). Based on the principle of superposition, combined with empirical scoring methods and theoretical methods, the influence function method calculates that the subsidence of any point underground is equal to the total subsidence impact of each microelement mining within the mining range. This method does not directly establish an influence function based on actual observation results and is suitable for irregular or geometrically complex extraction spaces; (4). Based on the principle of superposition, for a point on the surface and at a specified depth of the coal seam, only mining activities within a certain range below the surface point have an impact on that point, and mining activities outside the image circle have no impact. This method is expected to be cumbersome, complex, and inaccurate and is rarely used.

Based on these four methods, research on the influencing factors of mining subsidence in underground coal mines includes the following16: (1). Mechanical properties of overlying strata and rock layer combinations; (2). The thickness of the loose layer and the water content of the overlying rock; (3). Coal seam dip angle; (4). Mining thickness and mining depth; (5). Length and width dimensions of the goaf; 6. Repeated mining; (7). Mining methods and roof management methods; (8). Working face advancement speed; (9). Topography and landforms; and (10). Geological structures such as faults and folds.

Brief summary

The ten influencing factors involved in the four traditional methods for calculating the surface movement of rock layers, considering the controllable adjustment factors, are summarized as follows:

  • 1. Whether there is repeated disturbance;

  • 2. Thickness of once mining operation;

  • 3. Mining of overlying strata affects the height.

Control measures for these three influencing factors are proposed, and the displacement and stress of the direct top, basic top, and key layers between different measures are monitored and analyzed.

Calculation methods for surface subsidence and rock deformation

In the process of underground coal mining, with the excavation of coal seams, the overlying rock layers move under the action of their own weight and original rock stress, gradually affecting the surface and causing subsidence17,18,19,20,21,22. A large amount of engineering practice monitoring data shows that the main causes of surface subsidence are the following factors: mining thickness, coal pillar size, distribution of faults and aquifer structures, etc. Minimal research has focused on the influence of different mining methods on the movement of overlying strata and surface subsidence under the same mining thickness conditions23. Therefore, drawing on the assumption of a plane semi-infinite body in elastic mechanics, a mechanical analytical calculation method is provided for the calculation of overlying strata subsidence displacement. The rock movement and stress state are divided into zones, as shown in Fig. 2. A schematic diagram of the simplified mechanical model for the movement and settlement of underground rock layers is shown in Fig. 3.

Fig. 2
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Schematic diagram of the dynamic mechanical state of the overlying rock structure.

Fig. 3
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Mechanical model for estimating surface subsidence. (a) Simplified diagram of stress calculation in rock layers using semi infinite body method in elasticity. (b) Simplified diagram of undetermined coefficient method calculation.

Based on the assumption of an elastic semi-infinite body in elastic mechanics24,25,26,27, the rock layer overlying the key layer up to the surface can be assumed to be a semi-infinite component for internal force calculation. The width of the rock layer movement influence zone under the semi-infinite body is r0, and the logarithmic spiral calculation curves are r1 and r2. The three parameters in Formula (1) determine the range of the rock layer movement influence zone under the semi-infinite body.

$$\left\{ \begin{gathered} r_{0} = \frac{B}{{2\cos (45^{ \circ } - \frac{\phi }{2})}} \hfill \\ r_{1} = \frac{B}{{2\cos (45^{ \circ } - \frac{\phi }{2})}} \cdot e^{{ - \frac{{\pi (45^{ \circ } - \frac{\phi }{2})\tan \phi }}{{{\pi \mathord{\left/ {\vphantom {\pi 2}} \right. \kern-0pt} 2}}}}} \hfill \\ r_{2} = \frac{B}{{2\cos (45^{ \circ } - \frac{\phi }{2})}} \cdot e^{{ - \frac{\pi \tan \phi }{2}}} \hfill \\ \end{gathered} \right.$$
(1)
$$\left\{ \begin{gathered} \varepsilon_{I} = \frac{{2\sum\limits_{i = 0}^{5} {a_{i} h^{i} } }}{{k_{1} \cdot (2H - 2h + D_{f} )}} \hfill \\ \varepsilon_{II} = \frac{{2\sum\limits_{i = 0}^{5} {a_{i} h^{i} } }}{{k_{1} \cdot (2H - 2h + D_{f} )}} \cdot \left( {1 - \frac{{x - L_{1} }}{{L_{2} }}} \right) \hfill \\ \end{gathered} \right.$$
(2)

Formula 2 is the strain calculation formula, where εI and εII are the plane principal strains of Region I and Region II, respectively, where B is the excavation width of the mining site considering the Saint–Venant principle, m. φ is the average internal friction angle of the rock layer. The horizontal axis position, m, is selected for calculation. H is used to select the position of the vertical axis for calculation, m. H is the burial depth of the mining site, m. Df is the length of the line segment om, m28,29,30.

$$\begin{gathered} E_{I} = \frac{v}{{k_{1} }}\left\{ {\left[ {a_{0}^{\prime } \ln (C_{1} - h) + \sum\limits_{i = 1}^{5} {\frac{{a_{i}^{\prime } }}{i}} } \right] \cdot \left( {H - \frac{x}{{k_{1} }}} \right)^{i} } \right\} \hfill \\ E_{II} = \frac{v}{{D_{f} k_{1} }}\left( {H + D_{f} - \frac{x}{{k_{1} }}} \right)\left\{ {[a_{0}^{\prime } \ln (C_{1} - h) + \sum\limits_{i = 1}^{5} {\frac{{a_{i}^{\prime } }}{i}] \cdot (H - \frac{x}{{k_{1} }})^{i} } } \right\} - \frac{v}{{D_{f} k_{1} }}\left[ {a_{0}^{\prime \prime } \ln \left( {H + \frac{{D_{f} }}{2} - h} \right) + \sum\limits_{i = 1}^{6} {\frac{{a_{i}^{\prime \prime } }}{i} \cdot h^{i}_{{\left| {_{h}^{H} } \right.}} } } \right] \hfill \\ E_{III} = \frac{2v}{{k_{1} }}\left\{ {\left[ {a_{0}^{\prime } \ln (C_{1} - h) + \sum\limits_{i = 1}^{5} {\frac{{a_{i}^{\prime } }}{i}} } \right] \cdot \left( {H - \frac{x}{{k_{1} }}} \right)^{i} } \right\} - \frac{2vx}{{k_{1}^{2} }}\left\{ {\frac{{a_{0}^{\prime } }}{{H + D_{f} - h}} + a_{1}^{\prime \prime \prime } \ln (H + D_{f} - h) + \sum\limits_{i = 2}^{5} {\frac{{a_{i}^{\prime \prime \prime } }}{i - 1} \cdot h^{i - 1} \left| {_{h}^{H} } \right.} } \right\} \hfill \\ \end{gathered}$$
(3)
$$E(x) = E_{1} + E_{2} + E_{3}$$
(4)
$$dw = \frac{{\tau_{s} dy \cdot dh}}{E}\frac{1 + v}{{8\pi (1 - v)}}(1 - \frac{h}{{H_{x} }})(1 - \frac{\left| y \right|}{b})[\frac{{(4 - v)h^{2} }}{{\sqrt {(h^{2} + x^{2} + y^{2} )^{3} } }} + \frac{{8 - 16v + 8v^{2} }}{{\sqrt {h^{2} + x^{2} + y^{2} } }}]$$
(5)

To further solve the displacement and subsidence of the strata movement zone affected by underground mining, the subsidence at any point can be solved by E = EI + EII + EIII, where v is the Poisson’s ratio and is dimensionless. C1 = H + 0.5Df.w is the integration formula for any strip element in a vertical coordinate system.

In the process of underground coal mining operations, mining disturbances cause rock fractures, bending, sinking, and movement. The synergistic effect between multiple layers of hard and anti-bending layers maximizes the degree of surface rock subsidence. Each hard bending rock layer can change a large range of stress and displacement fields around it. When two hard bending layers are close enough, the famous “shadow effect” occurs, which maximizes the synergistic effect to prevent surface subsidence. The interlayer spacing of hard bending rock layers, the physical and mechanical properties of interlayer rock layers and the physical and mechanical properties of the hard and bending resistant rock layers themselves are used as influencing factors to control the stress shadow effect. In general, to maximize the control effect of stress shadows on surface subsidence, efforts should be made to use control measures to ensure the integrity of far-field hard and bending resistant rock layers. A schematic diagram of the mechanical structure function is shown in Fig. 4.

Fig. 4
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The schematic diagram of the mechanical structure function.

N. I. Sneddon described the stress distribution around the main interlayer fractured medium based on the Westergaard stress function Z, and the stress component expression is as follows:

$$\begin{gathered} \sigma_{x} = p_{0} (\frac{L}{{\sqrt {L_{1} L_{2} } }}\cos \frac{{2\theta - \theta_{1} - \theta_{2} }}{2} - 1) - p_{0} \frac{L}{c}(\frac{{c^{2} }}{{L_{1} L_{2} }})^{\frac{3}{2}} \sin \theta \sin [\frac{3}{2}(\theta_{1} + \theta_{2} )] \hfill \\ \sigma_{y} = p_{0} (\frac{L}{{\sqrt {L_{1} L_{2} } }}\cos \frac{{2\theta - \theta_{1} - \theta_{2} }}{2} - 1) + p_{0} \frac{L}{c}(\frac{{c^{2} }}{{L_{1} L_{2} }})^{\frac{3}{2}} \sin \theta \sin [\frac{3}{2}(\theta_{1} + \theta_{2} )] \hfill \\ \tau_{xy} = p_{0} \frac{L}{c}(\frac{{c^{2} }}{{L_{1} L_{2} }})^{\frac{3}{2}} \sin \theta \cos [\frac{3}{2}(\theta_{1} + \theta_{2} )] \hfill \\ \end{gathered}$$
(6)
$$dw = \frac{1}{2}\{ erf[\frac{\sqrt \pi }{{\sigma_{x} }}(x + H_{1} \cot \theta )]\} - erf[\frac{\sqrt \pi }{{\sigma_{y} }}(y - (D - s)\cos \alpha + H_{2} \cot \theta )]$$
(7)

Equations (6), (7) can be used to calculate the three-dimensional stress in a semi-infinite body while considering the influence of composite rock layers where θ1 and θ2 are the calculated polar angles in the polar coordinate system.

p0 is the interlayer pressure calculated via Formula (5) and the elastic modulus, MPa. L is the fracture distance of the rock layer, m. L1 and L2 are the fracture distances of adjacent rock layers on the upper and lower surfaces, m, respectively. H1 and H2 are the thicknesses of adjacent rock layers on the upper and lower surfaces, m. c is the cohesive force of the rock layer, MPa; erf is the Gaussian error function. α is the dip angle of the rock layer, °. x is the horizontal position variable selected for calculation, m. D is the rock stiffness coefficient calculated from the elastic modulus and rock size, GPa · m. s is the relative displacement of the upper and lower surfaces of the selected position for rock calculation, m.

Brief summary

The mathematical expressions of Formulas 5–7 show that the stress shadow effect between hard rock layers is affected mainly by: 1. the thickness and collapse distance of the hard rock layers; 2. Rock mechanical strength properties; 3. Mechanical boundary conditions of the rock layers; and 4. Intervals between rock layers.

In response to these four parameters, to weaken the internal stress of the lower roof and ensure safe mining while retaining the upper hard layer structure to weaken surface displacement, the method of reducing stress weakening disturbance through layered mining should be considered in subsequent experiments and compared with the method of full-thickness mining.

Geological and mining process conditions of the working face

A coal mine working face in the Binchang mining area of Shaanxi Province was selected for engineering geological conditions research. The Binchang mining area generally has a deep burial depth, numerous surface villages, water bodies, and structures, as well as protective water sources such as the Jing River and the Hujia River. In this study, coal seam #4, which is an extremely thick coal seam, has a burial depth of approximately 502–643 m and an average coal thickness of 17 m. The overburden of coal seam #4 is thick and hard, and the failure of the thick and hard roof can easily cause coal wall deviation, roof caving, and significant mining pressure phenomena such as face impact and surface subsidence31,32,33,34.

Laboratory physical and mechanical property tests were conducted on coal rock samples collected by grouping the #4 coal seam. The size of the roof and coal seam blocks taken on site should not be less than 15 cm × 15 cm × 15 cm. The content includes visual density, uniaxial compressive strength, tensile strength, elastic modulus, Poisson’s ratio, bonding force, internal friction angle, etc., providing a basis for analyzing the caving behavior of coal seams and determining the reasonable support strength of supports. The average density is 1312.95 kg·m-3, and the average uniaxial compressive strength, elastic modulus, and Poisson’s ratio are 17.185 MPa, 3.359 GPa, and 0.201, respectively. The average density of coal blocks in samples 4–2 is 1,340.30 kg·m-3, and the average uniaxial compressive strength, elastic modulus, and Poisson’s ratio are 13.366 MPa, 2.865 GPa, and 0.179, respectively. The average density of coal blocks in samples 4–3 is 1370.88 kg·m-3, and the average uniaxial compressive strength, elastic modulus, and Poisson’s ratio are 13.760 MPa, 2.924 GPa, and 0.196, respectively. The average density of coal blocks in samples 4 to 4 is 1329.28 kg·m-3, and the average uniaxial compressive strength, elastic modulus, and Poisson’s ratio are 16.507 MPa, 2.814 GPa, and 0.189, respectively.

Numerical simulation calculation of the displacement strain field evolution of overlying rock under different mining processes

The impact of overlying rock fracture movement on surface displacement under the aforementioned background conditions is studied, and a numerical simulation experimental model is established based on the average rock occurrence situation in the drilling inspection report. The width of the working face is 200 m, the section coal pillar is 20 m, the advancing length is 1,200 m, and the 50 m model boundary is retained. The occurrence status of the mining area and the physical and mechanical properties of the surrounding rock are shown in Fig. 5a, and the spatial position relationship of the mining area excavation is shown in Fig. 5b.

Fig. 5
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Schematic diagram of the relationship between the overlying rock column chart and the spatial position set of the mining area.

There are two layers, namely, the 1st and 4th coal seams, with burial depths ranging from 232.41–446.85 m and floor elevations ranging from 240 to 480 m. The average minable thickness is 2.33 m, accounting for 25.5% of the entire area. The No. 1 coal seam is a relatively stable coal seam with a simple structure, little variation in thickness, obvious variation patterns, a single coal type, and a locally exploitable distribution range.

Coal seam 4 is the main mining seam of the Yadian Coal Mine, with 16.01–93.17 m from coal seam 1, a burial depth of 417.85–632.29 m, and an average thickness of 12.07 m.

The thickness of the No. 4 coal seam does not change significantly along the strike direction but changes significantly along the dip direction. Overall, the thickness changes from thick to thin to pointed from north to south, spanning four levels of ultrathick, thick, medium-thick, and thin coal seams. In summary, the No. 4 coal seam has a simple structure, with little variation in thickness along the strike direction and significant variation along the dip direction. However, the change pattern is obvious, and the coal type is single, belonging to a relatively stable coal seam that can be mined in most parts of the distribution range.

The average thickness of the #4 coal seam being mined is 17 m, and the thickness of the direct roof mudstone layer that collapses with mining is 6 m. The basic roof layer thickness of the fine sandstone is 10 m. The key layer is estimated via the classic formula of key layer discrimination, and the key layer is the 15 m thick hard coarse-grained sandstone above the coal seam, which is 39.5 m thick. According to the onsite stress measurement values, an equivalent load is applied to the model boundary. The model has 6 free surfaces, with displacement boundaries: fixed vertical displacement on the upper boundary, fixed normal displacement on all sides, and fixed horizontal vertical displacement at the bottom. The constitutive relationship of the rock mass adopts the Mohr–Coulomb criterion, the Coulomb slip model is used for joint fractures, and the jset joint generator is used to generate regular joints. The physical and mechanical parameters of the coal and rock used in the numerical simulation calculations were obtained mainly from onsite data collection and laboratory experiments.

To control the influence of mining method variables, three different mining methods were selected: 1. Large mining height fully mechanized top coal caving (CMTC), 2. Large mining height full mining height at once (LMHT), and 3. Layered fully mechanized top coal caving (LCMTC) as an excavation method. The evolution law of the stress field of the overlying rock displacement field under different mining method conditions was studied. The measurement points were arranged at different rock layers to monitor the vertical displacement, principal displacement scalar, vertical stress, and maximum principal stress.

The FLAC3D finite element numerical simulation software was used to simulate the evolution relationship between relevant factors and rock migration patterns in different artificial buffer layer technologies. The model size was set to 1500 m × 530 m × 300 m (X × Y × Z), with the upper boundary subjected to measured vertical uniformly distributed loads, the XY-YZ surface subjected to measured horizontal structural stresses, and the bottom boundary being a fixed constraint boundary.

The LMHT method requires the entire height of the coal seam to be extracted in one pass, which increases the disturbance range of the overlying rock. In addition, the size, weight, and working capacity of equipment, such as shearers and hydraulic supports, also increase, resulting in greater initial production investment. However, owing to its high mining height, the recovery rate is high, and only two production tunnels need to be excavated.

The CMTC method is different from the one-time LMHT method. The CMTC method is a top coal caving mining process that results in relatively mild disturbances to the overlying strata, but its coal recovery rate is relatively low.

The LCMTC method divides the entire coal layer into two layers for mining, further reducing the disturbance range. However, this requires the excavation of four mining tunnels, and against the background of the tendency of rock to burst under these geological conditions, disturbance to the surrounding rock of the tunnels is an unfavorable factor35,36,37,38,39,40,41,42.

An analysis of the contour map of the vertical displacement and vertical stress in Fig. 6 reveals that the basic top has a typical mechanical stress state of plate rock, and the displacement and stress fields have an O‒X distribution. The displacement at the center of the plate is the largest, the vertical stress is the smallest, the deflection is the largest, and the main bending moment value is the smallest. When extending toward the boundary of plate mining, the displacement decreases, the vertical stress increases, the deflection decreases, and the main bending moment value reaches its maximum. The distribution law of the displacement and stress field in the key layer clearly reveals that it has mechanical characteristics like those of the basic top layer of fine sandstone and that the vertical stress at the same plane projection position decreases as the vertical displacement decreases. These rules conform to the classical key layer theory and roof subsidence theory, verifying the reliability of the model.

Fig. 6
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Mining technology1: Comprehensive mechanized top coal caving mining technology with large mining height (CMTC).

An analysis of the contour maps of the vertical displacement and vertical stress in Fig. 7 reveals that, compared with the CMTC method and the LMHT mining method, the vertical displacement at the same plane projection position of the basic roof and key layer increases, and the vertical stress decreases. This is because during the one-time extraction of the coal seam, the stress release speed is fast, and the free space for downward movement of the overlying rock increases, resulting in sufficient displacement. During the mining process, the working resistance of the LMHT method increases, but after mining is completed, the displacement increases, and the vertical stress decreases.

Fig. 7
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Mining technology2: Large mining height technology (LMHT).

An analysis of the contour maps of the vertical displacement and vertical stress via the LCMTC method in Fig. 8 reveals that after first mining the upper layer and then mining the lower layer within a certain interval, the displacement at the same plane projection position of the same rock layer is greater than that via the CMTC method and LMHT method, and the stress is further reduced; this is because, after the LCMTC method is used to recover the entire layer of coal resources, the overall time required is significantly longer than that when the other two methods are used, and layered mining methods are more unfavorable for controlling the subsidence of overlying rock layers.

Fig. 8
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Mining technology2: Layered comprehensive mechanized top coal caving (LCMTC).

The 9 displacement stress evolution curves of different rock layers under different mining methods are compared in a coordinate system, as shown in Fig. 9. An analysis of the vertical displacement vector curve reveals that in the monitoring results of the same layer’s measuring line, when the LCMTC method for mining is selected, both the vertical displacement vector and the principal displacement scalar are greater than those when the LMHT method is used, whereas the displacement of the LMHT method is greater than that of the CMTC method. The results of comparing different rock layers via various methods reveal that the displacements of the basic top and the direct top are similar and far greater than the displacement of the key layer. In terms of stress analysis, under the three different mining methods, the monitoring stress results of the same rock layer measurement line are relatively similar, with little difference. However, under the same mining method conditions, the basic top stress is greater than the key layer stress and greater than the direct top stress; this indicates that during the mining process of the three methods, the direct top increases with mining, and the stress is fully released, which is consistent with the onsite monitoring data results.

Fig. 9
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Evolution law of displacement and stress.

As the working face advances forward after cutting, the vertical displacement and principal displacement scalar from the cutting position to the front of the coal wall clearly show a distribution pattern of first increasing and then decreasing. When the LCMTC method is used for mining, the maximum principal displacement scalar and maximum vertical displacement of the direct top and basic top exceed 5.0 m, whereas the maximum principal displacement scalar and maximum vertical displacement of the key layer exceed 3.0 m. There is not much difference in the stress state inside the direct roof strata under the three mining methods, but when the LMHT method is chosen, the maximum principal stress and vertical stress peak of the basic roof strata exceed 50 MPa. Based on the analysis of the evolution patterns in Figs. 9, 10, when the CMTC method and LCMTC method are chosen, the subsidence of the overlying rock is relatively large. However, during the mining process, the degree of stress increase in the overlying rock near the mining site is significantly lower than that in the LMHT method, and the mining pressure in the working face is lower. From the perspective of controlling the subsidence and movement of key strata in the far field, selecting the LMHT method for mining can significantly reduce the subsidence of the overlying strata.

Fig. 10
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Comparison of Evolution Laws of Different Mining Processes.

Brief summary

From the perspective of controlling mining pressure in the working face, the direct top stress of the CMTC method (2.4 MPa) at the same measuring point is 53.2% greater than that of the LMHT method (1.566 MPa) and 51.4% greater than that of the LCMTC method (1.585 MPa). In terms of controlling surface displacement, the vertical displacement of the LCMTC method at the same location (3.32 m) is 10% lower than that of the LMHT method (3.649 m) and 15% lower than that of the CMTC method (3.818 m). Because the three mining methods impose little burden on the mining pressure manifestation and support load of the working face and the LCMTC method has a significant weakening effect on far-field surface rock movement, the artificial buffer layer method is operable in experiments and onsite.

Conclusions

Artificial buffer layer technology has been proposed to alleviate the average subsidence of rock layers above key layers below the surface. Based on the actual engineering background and geological conditions on site, finite element numerical simulation experiments were conducted on the overall state of the surrounding rock and the overlying rock in the far and near fields. The evolution laws of the vertical stress, maximum principal stress, vertical displacement, and principal displacement scalar of the overlying rock in the far and near fields were compared under three different mining methods and process conditions. Based on the research results of previous scholars, the calculation formula for predicting rock movement via the semi-infinite body method under different mining technology conditions has been further improved. The above process has achieved good mechanism interpretation effects through onsite monitoring feedback. The main conclusions of this study are as follows.

  • 1. In some underground coal mines where it is difficult to use backfill mining or ground grouting technology for backfilling, it is necessary to optimize the existing backfilling process to effectively reduce the displacement of surface rock layers and achieve control of surface subsidence due to geological conditions and economic benefits. This can be achieved through a combination of theoretical calculation methods, numerical simulation methods, and onsite monitoring. CMTC and LMHT mining methods should be selected to effectively reduce the subsidence of the near-field surrounding rock, whereas the LCMTC mining method should be prioritized when the subsidence displacement of the far field and up to the surface overlying rock is considered.

  • 2. Considering the similar manifestations of vertical displacement and principal displacement and the large difference between vertical stress and principal stress, the LCMTC method reduces the disturbance of vertical stress on overlying rock layers to a certain extent due to the different directions of principal stress caused by different mining methods.

  • 3. In future research, hydraulic fracturing and physical similarity simulation experiments will be conducted on different methods to verify the reliability of the natural artificial release layer method, and process parameters will be obtained to provide onsite guidance for construction.