Introduction

China’s primary energy source is coal, and the country’s coal sector is the backbone of the high-quality growth of its economy and society1,2,3. In the future, coal will continue to play the role of “ballast stone” for national energy security for a long time4. After a long period of large-scale coal mining in China, the shallow coal resources are almost exhausted and are extending to the deep at the rate of 8–12 m per year5,6,7. It is found that hard and thick strata exist in the overburden of about 1/3 deep mines in China. The fracture development law and rock stress distribution have significant characteristics8,9. Many scholars have demonstrated that movement damage and stress distribution pattern of the upper seam after being mined are directly related to the lithology, structure, and mechanical properties of the hard rock seam above10. As a special coal-bearing stratigraphic structure, the high and thick key stratum are prone to form a large overhang and long step break after being mined11, resulting in abnormal and violent mine pressure at the working face, and even causing different degrees of mine seismic and power disaster accidents. It is a great threat to the safe production of the mine. As a result, it’s essential to manage and research rock movement mined12,13,14.

Researchers have made certain substantial improvements in the study of deformation and fracture rules of high and huge thick rock formations in the stope. In terms of theoretical research, because of the theory of key stratum in the activity of overlying strata in mining areas15. Studied the deformation and fracture laws of ordinary key stratum16, super thick key stratum17, extremely thick magmatic rock key stratum18, thick and hard key stratum19, hard key stratum20, composite key stratums21, and other key strata under different mining conditions of working faces. The fracture modes, fracture distances, and ground pressure manifestation law of different key strata were summarized and analyzed, revealing the mechanical mechanism of key stratum fracture. In terms of laboratory research, physical similarity materials were used to simulate experiments, and various monitoring methods such as acoustic emission technology22, fiber optic sensing monitoring technology23, pressure sensing monitoring, digital image correlation24, peeping instrument, and displacement meter were used. It was found that the overlying rock layer can be divided into different types of key stratum structures based on different lithology and thickness. The “O-X” type structure formed by the initial fracture of these structures can be divided into main and sub types25. Large scale overlying rock structures release enormous energy during the process of fracture, forming mining earthquakes, which are important inducing factors for rock burst. This also lays the foundation for the study of surface subsidence laws and impact dynamic disaster analysis and prevention under the direction of the main stratum26. Regarding the field of study on numerical simulation calculations, the mechanisms causing a rock explosion and the movement law of the major overburden layer were examined27,28,29, and the characteristics of changes in indicators such as fracture development height, peak abutment pressure and maximum range of stress concentration before and after the key stratum breaking were obtained with the functioning face continues to advance30,31,32, which revealed the breaking law of key stratum of overburden and the formation mechanism of structure33,34. At present, due to the characteristics of large thickness, significant spacing with coal seam and long caving step, the traditional elastic theory has a large deviation from the actual engineering situation when studying its breaking law. In addition, in the physical similarity simulation experiment, due to the lack of a real-time monitoring system, it is difficult to effectively explore the evolution law of the movement and deformation of high-level and extremely thick key layers. However, the dynamic impact analysis induced by the fracture of the key stratum of the extremely thick conglomerate is a key issue focusing on the impact of the fracture on the mine safety.

In this paper, by constructing a large-scale three-dimensional model, distributed fiber optic sensing technology, pressure sensors, internal displacement testing devices, and other means are used to study the migration and evolution laws of high and thick key stratum. In order to determine the dynamic law of high-level rock strata movement during mining, numerical simulation is combined with the moving deformation data of overburden 1 and 2 working faces are compared and analyzed. It is revealed that the relationship between the movement of the high and thick key stratum and the dynamic appearance of mine pressure. This offers the prediction’s theoretical foundation of the breakage of the mega-extremely thick key stratum in the stope and the prediction of mine dynamic disaster.

Physically similar material simulation test

Project profile

Geological and mining conditions

A certain coal mine has a production capacity of 2.1 million t/a. The main coal seam being mined is Coal 2, with an average thickness of about 24.0 m, an inclination angle of 3 ° to 11 °, and an average burial depth of 800 m. The structure of the No.2 coal is complex, including multi-layer gangue, poor integrity, and has a certain impact tendency. The overlying strata of the stope are composed of three layers of rock to form a composite key stratum. The physical and mechanical properties of the main coal rocks of Qianqiu Coal Mine are shown in Table 1.

Table 1 Coal rock parameters.
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Geological and mining conditions

A certain coal mine has a production ca

Model design

According to the engineering geological conditions and test purposes, a three-dimensional physical model with length × width × height of 3600 mm × 2000 mm × 2200 mm is designed and built. The model uses river sand, pulverized coal, gypsum and calcium carbonate as raw materials, and paves the rock layer according to the specific proportion of similar materials35. The geometric similarity ratio is 1:400. The unit weight similarity ratio is 1:1.6. The stress similarity ratio is 1:640. In order to facilitate the coal seam mining, 60 mm × 40 mm rectangular galvanized square tubes are designed to replace the coal seam, with a total of 60. In order to observe the movement and deformation law of overlying rock during the excavation of the physical model, the front of the model is sealed with a transparent acrylic plate, and the back is fixed with a steel plate. Since the physical model is restricted by height conditions, in order to better meet the actual simulation requirements, three layers of Cenozoic strata with a thickness of about 85 m are equivalent converted. After conversion, the top load is about 2.5 t, and the loading method is that sandbags are orderly stacked on the top of the model. The model designs two coal mining faces with an inclined length of 800 mm and a strike length of 2400 mm. 600 mm wide coal pillars are reserved at both ends of the model in the strike direction, and 400 mm wide coal pillars are reserved at both ends of the model in the dip direction as the boundary conditions of the model. See Fig. 1 for the paved model. During the excavation of the physical model, a square tube is pulled out each time, which represents that the coal seam is excavated once in the actual project. After the overlying strata of the coal seam have experienced movement and deformation and tend to be stable, the quantitative data of all monitoring equipment are systematically collected at the same time, and the integrity and accuracy of the data are ensured, indicating the completion of the excavation process. Then, the second coal seam will be excavated, and the working faces 1 and 2 will be excavated in turn, a total of 60 times, so that the mining of the working face is completed.

Fig. 1
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Mining design of three-dimensional model.

Experimental test system

The internal displacement measuring device consists of a measuring tube, fishing line (measuring line), displacement measuring point, reading dial, pulley, bracket, etc. The structure is shown in Fig. 2 The principle is that the displacement measurement points embedded in the rock layer are deformed synchronously with the rock layer. When the measurement point’s displacement varies, the fishing line will be pulled to drive the reading dial to rotate, and the angle of the dial will change. The following relationship exists.

$$\omega =\frac{{\pi \varphi r}}{{180}}$$
(1)

In the equation, ω is the measurement point’s displacement, mm; φ is the rotation angle of the dial, °; r is the pulley radius, mm.

Fig. 2
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Device structure for measuring internal displacement.

In the three-dimensional model, three sets of measuring devices were arranged along the strike in the layers that cover working faces 1 and 2 (600 mm, 1200 mm, and 1800 mm from the cutting eye respectively). The three measurement points on each measurement set were located midway through the sub-key stratum I fine sandstone, the middle of the sub-key stratum II lower thick conglomerate, and midway through the primary key stratum high thick conglomerate (460 mm, 860 mm, 1380 mm in height, respectively). The measuring points in working face 1 are named 1-A1/A2/A3, 1-B1/B2/B3, 1-C1/C2/C3, and the measuring points in working face 1 are named 2-A1/A2/A3, 2-B1/B2/B3, 2-C1/C2/C3. A total of 18 measuring points are set up, and the specific distribution position is shown in Fig. 3.

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Distribution diagram of internal displacement measurement points.

The optical fiber sensors were arranged along the middle of the fine sandstone of the sub-key stratum above the 1 # and 2 # working faces, the middle of the lower conglomerate of the sub-key stratum, and the middle of the high conglomerate of the main key stratum. The dimensions of the arrangement are 460 mm, 860 mm, and 1380 mm in height, respectively, and 3600 mm in length. The distance between the inclination direction and the boundary is 400 mm and 800 mm, respectively, named F11, F12, F13 and F21, F22, F23. Total 6. The optical fiber monitoring system consists of NBX-6055 optical nanometer, computer, and Φ 900 μm common single-mode tight-sleeve fiber, as shown in Fig. 4.

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Test monitoring system.

Distributed optical fiber test system

In the BOTDA system based on Brillouin scattering, the frequencies of the two lasers are continuously adjusted. By monitoring the power of the continuous light coupled from one end of the fiber, the corresponding frequency difference when the energy transfer in each small section of the fiber reaches the maximum can be determined. When the external environment of a part of the optical fiber changes, the strain of the optical fiber will change, resulting in the Brillouin frequency of the optical fiber. Due to the linear relationship between Brillouin frequency shift, strain and temperature, the strain information is obtained. When the optical fiber sensor is embedded in the model, the overlying strata will be deformed under the influence of mining, and the embedded sensing optical fiber in the strata will also be axially stretched, so as to sense the strain distribution in the strata.

In order to facilitate the mechanical analysis, the following assumptions are made for the mechanical structure of rock fiber: (1) the rock layer is a uniform and isotropic elastomer; (2) The mechanical properties of the core and cladding are the same, and the slip between the core and cladding is ignored; (3) Fiber rock deformation isotropy.

The interaction between rock mass and optical fiber is illustrated by the following mechanical analysis. Take the optical fiber volume element with the length of DX. At this time, the shear stress τ of the optical fiber rock interface can be expressed as follows.

$$\tau =\frac{D}{4}\frac{{d\sigma }}{{dx}}$$

The interface friction f between optical fiber and rock mass can be expressed as follows.

$$f=\mu {\sigma _N}$$

Where, D is the diameter of the optical fiber; σ is the axial stress on the optical fiber; σ n is the radial stress of the overlying rock mass on the optical fiber; µ is the friction coefficient of the fiber rock interface.

If the shear stress τ of the optical fiber rock mass interface is less than the friction F of the optical fiber rock mass interface, it indicates that the coupling of the optical fiber rock mass is good. On the contrary, there is slip, and the reliability of the monitoring data cannot be guaranteed.

When a small deformation occurs in the rock layer, the rock layer optical fiber mechanical structure is regarded as a simply supported beam structure, the force on the optical fiber is regarded as the continuous load imposed by the rock layer deformation on the optical fiber, and the maximum bending moment occurs in the middle of the optical fiber. When the rock stratum has large deformation, there are longitudinal cracks in the rock stratum, and the exposed optical fiber at the crack is separated from the rock stratum. The opening of the crack produces tensile force τσ on the optical fiber, resulting in stress mutation, that is, the maximum bending moment occurs at the fracture position of the rock stratum. As shown in Fig. 5.

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Stress state of optical fibers.

Analysis of experimental results

Optical fiber sensing composite key stratum deformation and failure

The F11 sensing fiber is implanted in the fine sandstone of the sub-key layer. The deformation pattern of rock layers is shown in Fig. 6a). The face was not affected by mining at the initial stage of mining until it was advanced to 800 mm. There was a noticeable shift in the optical fiber’s frequency shift, indicating a single peak shape. As the working face continued to be advanced, the fiber frequency shift value increased, until the advance to 920 mm, the fiber frequency shift reached the maximum value of 438.98 MHz, and the impact range was about 418 m. The frequency shift curve changed from a single-peak to a double-peak shape, indicating that the first breaking of the sub-key stratum occurred at this time, and the initial breakage distance was 418 m. Subsequently, the face continued to be moved forward, and the fiber frequency shift changed periodically. The distance range of working face advance was 920 to 2400 mm, during which the fiber frequency shift had three peaks. The position was 1360 mm, 1840 mm, and 2320 mm, and the size was 340.95 MHz, 341.47 MHz, and 346.09 MHz, respectively. It indicated that there were three periodic breaks in the sub-key stratum I, the breakage distance was 192 m, and the peak value was significantly smaller than that of the first breaking. The F12 sensing fiber was implanted in the lower thick conglomerate II, the deformation pattern of rock layers is shown in Fig. 6b When the face was advanced to 1840 mm, the fiber frequency shift value reached the maximum value of 165.94 MHz. Subsequently, the face continued to be advanced, and the peak value of the optical fiber frequency shift became smaller as a whole, indicating that the initial breakage of the lower thick conglomerate occurred at this time, and the breakage distance was 736 m. The fiber 2 frequency shift curve as a whole is still single-peaked. The F13 sensing fiber is implanted in the high thick conglomerate above the main key layer. The deformation pattern of rock layers is shown in Fig. 6c. When the advancing distance of the face was 800 to 1360 mm, the frequency shift of the optical fiber was positive, indicating that the optical fiber was in a tensile stress state. The frequency shift curve was single-peaked as a whole. The maximum frequency shift of the fiber was 66.93 MHz. Compared with the maximum frequency shift of F12 of 165.94 MHz, the maximum value was reduced by nearly 99 MHz, about 1.48 times.

The sensing optical fiber F21 is located in a fine sandstone layer above the coal seam 2 of the working face. The deformation pattern of rock layers is shown in Fig. 6d There were three peaks with the peak at 175.91 MHz, which was smaller than the initial incoming pressure peak. The peak position on the right side was constantly moving forward, and the influence range of the optical fiber frequency shift was increasing, indicating that after the first break, there were three periodic breaks in the first sub-key stratum. Figure 6e displays the frequency shift change of detecting fiber F22. The fiber’s frequency shift curve was a single peak as a whole, and the frequency shift values were all positive, demonstrating the tensile condition of the fiber. As the face was advanced, the peak value of the fiber frequency shift continued to increase. Until the mining was completed, the working face kept getting better. The average frequency shift peak was about 165.68 MHz, which was about 36 MHz lower than the maximum peak, indicating that there was no periodic fracture in the second key stratum of the whole face. The frequency shift change of sensing fiber F23 is shown in Fig. 6f. In the process of the face advancing from the cutting eye, the fiber frequency shift was positive and in a tensile state. The maximum frequency shift peak was − 40.56 MHz, indicating that the upper extremely thick conglomerate was affected by mining, resulting in bending deformation, and the bending deformation became larger as the advancing distance increased.

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Frequency shift test curve of horizontal optical fiber in working face.

The results demonstrated that the deformation law of overlying strata in working face 2 was basically consistent with the trend of face 1, but the breakage distance of the key stratum in face 1 is larger than that in working face 2. The changes in the value, position, and shape of the peak fiber frequency shift reflect the dynamic evolution of bending deformation, breaking, and rotational deformation of the key stratum of overlying strata. The frequency shift curve of the sensing fiber in the sub-key stratum I fine sandstone was basically a “saddle” type with two convex ends and a concave middle, that is, double-peaked. The two peaks were characterized by high left and low right. The fiber optic frequency shift curve of the sub-key stratum II lower thick conglomerate was single-peaked and had a single maximum frequency shift peak. There was a maximum frequency shift peak, that is, it was judged that only the initial fracture of the rock layer occurred. The fiber optic test curve of the high thick conglomerate of the main key stratum also showed a single peak, but no maximum frequency shift peak. The overall frequency shift peak was much smaller than the frequency shift peak of the lower group of the extremely thick conglomerate test, indicating that the rock movement and deformation were small and slow, only bending deformation or local cracks in the horizontal layer occurred.

Comparison of key layer displacement and frequency shift

The peak displacement inside the composite key layer and the peak frequency shift of the optical fiber are shown in Table 2. The subsidence of sub critical layer 1 in working face 1 is 13.7 mm to 8.9 mm, the subsidence of sub critical layer 2 is 4.5 mm to 4.2 mm, and the subsidence of main critical layer is 0.95 mm. Comparing the peak frequency shift of the two faces, it can be seen that in working face 1, the peak values of fiber F11 frequency shift of sub-key I fine sandstone were 438.9 MHz, 340.5 MHz, 301.4 MHz and 356.2 MHz, and the breakage distances were 920 mm, 440 mm, 480 mm and 480 mm respectively. In working face 1, the peak values of fiber F11 frequency shift were 256.7 MHz, 194.9 MHz, 154.5 MHz, and 182.1 MHz, and the breakage distances were 800 mm, 320 mm, 320 mm, 400 and 360 mm respectively. The frequency shift peak in working face 1 was obviously larger than that in working face 2, which was about 1.7 times that in face 2. The peak displacement inside the composite key layer is basically consistent with the peak frequency shift of the fiber, and the response between displacement and frequency shift is between 27 MHz/mm to 40 MHz/mm.

Table 2 Peak displacement and frequency shift in compound key layer.
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Characteristics of internal displacement of overburdened rock

During the excavation of the model, the displacement curve of the measuring point inside the overburden is shown in Fig. 7. The vertical coordinate is the sinking amount of the displacement measurement points, and the horizontal coordinate is the excavation distance of the face. Figure 7a shows the sinking curves of measurement points 1-A1/A2/A3, 1-B1/B2/B3, 1-C1/-C2/C3 during the advancing of face 1. Among them, the 1-A1, 1-A2, 1-A3 measuring points were located in the sub-key stratum I fine sandstone. The subsidence curves of the three measuring points basically show a step change and the subsidence of the measuring points has three mutations respectively. The measuring points 1-A1 were mutated at 600 mm, 960 mm, and 1360 mm respectively, and the corresponding subsidence was 2.2 mm, 8.8 mm, and 13.3 mm respectively. The measuring points 1-A2 were mutated at 1200 mm, 1360 mm, and 1840 mm respectively, and the corresponding subsidence was 3.8 mm, 7.5 mm, and 13.7 mm respectively. The measuring points 1-A3 were mutated at 1800 mm, 1960 mm, and 2280 mm respectively, and the corresponding subsidence was 3.3 mm, 5.4 mm, and 8.9 mm respectively. The peak of maximum subsidence is 13.7 mm, and the sudden subsidence positions of 960 mm, 1360 mm, 1840 mm, and 2280 mm also coincide with the fracture position of fine sandstone. The 1-B1, 1-B2, 1-B3 measuring points are located in the thick conglomerate of the second lower of the sub-key stratum. The subsidence trend of the three measuring points remains unchanged in the early stage and gradually increases to the maximum value in the later stage. The peak subsidence values of the three measuring points are 4.5 mm, 5.4 mm, and 4.2 mm respectively, and the mining positions corresponding to the peak subsidence values are 1840 mm, 1880 mm, and 2320 mm. The 1-C1, 1-C2, 1-C3 measuring points are located in the high thick conglomerate of the primary key layer. The subsidence of the three measuring points changes little, and the highest point of sinking is 0.95 mm, indicating that the upper conglomerate is less deformed by mining disturbance.

Figure 7b shows the sinking curve of measurement points 2-A1/A2/A3, 2-B1/B2/B3, 2-C1/C2/C3 when working face 2 advances. The overall change trend is similar to that of face 1. The sudden change position of subsidence of the class A measuring point in face 2 is earlier than that in face 1, and the peak value of subsidence is basically the same. Among the B-type measuring points, the influence of mining on face 2 is earlier than that of working face 1, and the peak page of subsidence in face 2 is larger than that in face 1. Among the Class C measurement points, there was basically no significant change.

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Internal displacement measurement curve of mining overburden. (a) 1# working face. (b) 2# working face.

UDEC numerical calculation

Establishment of numerical model

In order to verify the feasibility of the results of the physical model, the fracture law in the composite key layer is further analyzed, and the essential deformation of the whole process of the key layer movement is revealed more profoundly in time and space. The UDEC discrete element numerical calculation method is used to establish the numerical model (strike length 1840 m, dip length 800 m, height 880 m). The physical and mechanical parameters of rock mass are consistent with the physical model test. Set the lower left corner of the model as the origin of the coordinate system, the right of the X axis as the positive direction, and the up of the Y axis as the positive direction. The boundaries on both sides of the model limit the displacement in the X direction, the lower boundary limits the displacement in the Y direction, and the upper boundary is the free boundary. The Mohr Coulomb model is used for the constitutive relationship of overburden rock, and the area contact Coulomb slip model is used for the constitutive relationship of joints.

Deformation law of mining overlying strata

The Mohr-Coulomb model is used for the constitutive relation of overburdened rock. The distribution of overlying rock displacement and plastic zone under different excavation distances is shown in Fig. 8. When the excavation reaches 288 m, the thick conglomerate undergoes significant deformation without obvious crack development, and plastic breakdown of the rock layer above is mainly located within the 50 m range of the roof. When the excavation reached 480 m. There was a significant separation layer between the conglomerate and mudstone, and the thick conglomerate’s subsurface rock layer cracked and caved. The plastic failure of the overlying rock developed to a range of 210 m from the roof, and the underlying thick conglomerate began to show small tensile cracks, but did not completely crack. The deformation form was bending and sinking. When the excavation reached 720 m, the underlying thick conglomerate broke, with a first collapse step of 560 m and a maximum vertical displacement of 5.0 m. The thick conglomerate’s lowest separation space progressively closed, but in the uppermost portion of the dense conglomerate, the highest vertical displacement was just 0.8 m, and obvious tensile cracks appeared. There was also a large separation space inside the thick conglomerate. When the excavation reaches 960 m. The upper thick conglomerate’s maximum displacement rises to 1.4 m, while the maximum vertical displacement of the bottom thick conglomerate climbs to 5.8 m. The second lower thick conglomerate and underlying rock layer of the sub key layer both see increased development of the plastic zone and joints of the overlaying rock, while the main key layer’s bending deformation continues to rise. The upper thick conglomerate is the main key layer that controls the entire overlying strata. As the mining range gradually increases and reaches its ultimate bearing strength, it breaks, triggering the occurrence of mine rock burst.

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Distribution map of displacement and plastic zone development of overlying rock.

Analysis on breaking law of key strata

Deformation state of super thick conglomerate in the second lower formation of sub key layer

When the working face is excavated for 240 m, the maximum vertical displacement of the lower group of extremely thick conglomerate is 0.10 m, but the whole rock layer is not damaged; When the working face is excavated 480 m, the maximum vertical displacement is 4.49 m; When the working face is excavated 720 m, the maximum vertical displacement is 5.0 m; When the working face is excavated at 960 m, the maximum vertical displacement is 10.85 m, the lower giant thick conglomerate has large fracture deformation, and the lower giant thick conglomerate and the upper giant thick conglomerate have large separation space. As shown in Fig. 9a.

Deformation state of super thick conglomerate in upper formation of main key stratum

When the working face is excavated for 240 m, the maximum vertical displacement of the super thick conglomerate in the upper group is 0.075 m, and there is no plastic zone, indicating that the super thick conglomerate in the upper group is basically not deformed at this time; When the working face is excavated for 480 m, the maximum vertical displacement of the rock layer is 0.309 M; When the working face is excavated 720 m, the maximum vertical displacement is 0.53 M. When the working face is excavated 960 m, the maximum vertical displacement reaches 1.40 M. The upper group of extremely thick gravel formation forms the range of fracture zone under the influence of mining, and the soft rock above the extremely thick gravel formation forms a bending subsidence zone. As shown in Fig. 9b.

Based on the fracture form of the huge thick gravel layer, the lower group of huge thick gravel layer is formed by forming a number of vertical non through fractures close to the side of the goaf, and the overall downward rotation deformation. The upper group of huge thick gravel layer is formed by forming non-through horizontal fractures in the horizontal layer in the middle of the goaf, and the bending and subsidence deformation in the layer. The different fracture modes of the upper and lower groups of huge thick gravel layer are affected by two aspects. On the one hand, the propagation effect of mining influence on the height of the overlying rock, and on the other hand, the mechanical properties of the huge thick gravel layer with large strength and thickness.

Fig. 9
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Deformation state of extremely thick conglomerate. (a) Extremely thick conglomerate of lower formation, (b) thick conglomerate of upper formation.

Deformation law of composite key stratum

The experimental phenomenon indicates that the deformation and failure of the thick gravel layer are the result of the development of longitudinal cracks in the rock layer under the influence of mining. The upper part of the thick conglomerate undergoes significant bending deformation, with obvious development of longitudinal cracks in local areas, and transmits its own load to the lower rock layer, increasing the pressure manifestation of the face roof; The lower part of the thick conglomerate undergoes large-span fracture deformation, which to some extent slows down the phenomenon of advanced support pressure on the working face roof; On the one hand, this is the impact caused by the expansion of the deformation and movement range of the overlying rock, and on the other hand. Research has shown that there are two stage characteristics in the fracture of high level thick conglomerate, which are respectively present in the lower level thick conglomerate of the sub critical layer and the upper level thick conglomerate of the main critical layer. That is, the impact of mining results in two types of fracture characteristics of the overlying high level thick conglomerate: the lower-level thick conglomerate of the sub critical layer forms multiple vertical non-penetrating fractures that are completely fractured, and the upper-level thick conglomerate of the main critical layer forms large non-penetrating fractures and bending deformation within the horizontal layer, as shown in Fig. 10.

Fig. 10
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Schematic diagram of deformation of thick composite key stratum.

According to the field observation, the movement law of the huge thick gravel layer is basically the same as the test results. The upper strata of the extremely thick conglomerate are completely consistent with its movement, and the lower strata have all collapsed36. As the main bearing structure, this layer carries all the loads of the upper strata, and as the composite key layer in the overlying strata of the stope, it controls the ground pressure characteristics and surface subsidence of the working face. Its movement and fracture have a vital impact on the whole stope. When plastic slip or strong shear failure of roof and floor occurs in extra thick coal seam under extremely thick conglomerate, dynamic phenomena such as sudden springing of roadway floor, instantaneous large-area sliding of roadway side and its bottom corner, sudden large-scale subsidence of roadway roof and sudden contraction of roadway integrity occur on site. Especially after the fracture of the thick gravel layer, the huge energy contained in it will be suddenly released, thus forming a strong impact on the mining space, leading to the occurrence of rock burst.

Conclusion

  1. (1)

    The movement and deformation law of the thick composite key layer in the mining area, with the first and third periodic fractures occurring in the sub key layer one (fine sandstone); The second subcritical layer (the thick conglomerate in the lower group) only experienced initial fracture, but the structure did not become unstable; The main key layer (upper group of thick conglomerate) only undergoes bending deformation.

  2. (2)

    The breakage distance of the sub-key stratum I fine sandstone in working face 1 is all larger than that in working face 2. The deformation scale of the sub-key stratum II lower thick conglomerate is smaller in working face 1 than in working face 2, but the fracture distance is larger than in working face 2. The deformation scale of the high thick conglomerate of the main key layer is smaller in working face 1 than in working face 2, and none of them are broken.

  3. (3)

    A distributed fiber optic dynamic response relationship was established for the evolution of the key layer structure, and the trend of the peak displacement of the key layer and the peak frequency shift of the fiber optic were basically consistent. The relationship between displacement and frequency shift was about 24.62–40.63 MHz/mm, revealing the fiber optic frequency shift relationship between rock deformation and structural evolution.