Abstract
The large-scale collapse of overlying strata in the gob directly affect the safe production of coal mines; they are also the major causes of geological disasters, such as ground cracks, surface subsidence, and ground collapse. In this paper, the movement and caved characteristics of overlying strata during coal seam excavation are studied by conducting a physical model experiment. Results show that overlying strata have different movement and caved laws during the initial, intermediate, and later mining stages. During the initial mining stage, overlying strata do not collapse, and the subsidence is extremely small. During the intermediate mining stage, overlying strata cave along the vertical direction, and caved height gradually increases. Large numbers of cavities, abscission layers, and fractures exist between caved strata. The fractured area gradually increases upward, and the subsidence increases considerably. During the later mining stage, overlying strata cave along the horizontal direction. The abscission layers between the caved strata of the central are compacted. The compacted area is surrounded by a fractured area. The compacted and fractured areas increase along the horizontal direction. The subsidence curves exhibit a horizontal variation. Overlying strata evolve from the self-equilibrium stage to the vertical collapse stage, and finally, the horizontal collapse stage. The fractured area changes from a no fractured area to a fractured area, increases vertically, and finally, increases horizontally. The subsidence curve changes from extremely small to large, and finally, changes horizontally.
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Introduction
Longwall mining is widely used in underground coal mining1,2,3. This automatic mining method exhibits the advantages of large coal production and high efficiency4,5,6. In China, about 95% of underground coal mines use the longwall mining method, which provides technical guarantee to the development of China’s coal resources7. The coal production of China has become the largest in the world8,9. Coal accounted for 55.3% of China’s total energy consumption in 2023. At present, coal is still the main energy source in China.
After coal is extracted from the underground, the original equilibrium state of the strata is changed10,11,12,13,14. The stress is redistributed, and overlying strata of coal seam are deformed and destroyed15,16,17,18. Overlying strata are affected by coal seam mining in different degrees, leading to varying levels of deformation and failure19,20,21. The caved, fractured, and continuous deformation zones are formed from bottom to top22,23,24. When a sufficiently wide longwall panel is mined, overlying strata can form the caved, fractured, continuous deformation, and soil zones25.
The caved zone is dominated by broken rock, without continuity and bedding plane. A large number of voids and fractures exist between caved strata26,27. The fractured zone is at the upper part of the caved zone. The caved strata of the fractured zone are broken and discontinuous, and the bedding plane is separated. The fractures of caved strata are developed, and vertical fractures run through several strata28,29. The continuous deformation zone is at the upper part of the fractured zone, and the rock strata bend and sink. The original bedding plane is maintained between rock strata, and only a few fractures exist. The degree of deformation and failure of soil zone largely depends on the geological conditions.
The caved and fractured zones have developed fractures, which are the flow channels and storage spaces for gas and wate30,31,32,33. During the coal mining, accumulated gas in the gob flows into the mining panel, resulting in the risk of gas over-limit34,35,36. If mining-induced fractures run through the aquifer, then a large amount of groundwater floods into the mining panel and roadway, causing mine water inrush37,38,39,40. The damage range extends to the ground surface, resulting in geological disasters, such as surface subsidence, ground cracks, and ground collapse41. Mine gas leaks to the ground surface through mining-induced fractures, causing the greenhouse effect42,43. In the previous studies, scholars have not divided the evolution process of overlying strata in the gob during coal mining into stages, and there is no clear understanding of the movement law of overlying strata in different mining stages. To realize efficient, safe, and green mining, studying the movement and caved laws of overlying strata during coal mining is urgent.
In the current study, the movement and caved laws of overlying strata during panel mining are investigated by conducting a physical model experiment. First, the caved law of overlying strata is analyzed, and the caved characteristics of overlying strata during the initial, intermediate, and later mining stages are determined. Second, the subsidence law of overlying strata is analyzed, and the movement characteristics of overlying strata during the initial, intermediate, and later mining stages are determined. Finally, the evolution model of movement and caving of overlying strata is established.
Experimental scheme
Research object
The research object is panel 3305 in coal mine no.5, Hebi, Henan Province, China. The average buried depth of Panel 3305 is 527.25 m. The strike length and dip length of panel are 442.05 m and 102.8 m, respectively. The typical geological log profile is shown in Fig. 1. The minable coal seam is the No. II1 coal seam, and its average thickness is 8.26 m. The fully mechanized longwall mining method is used in this panel. The movement and caved laws of overlying strata under single coal seam mining are studied.
In addition, the movement and caved laws of overlying strata under multi-seam mining are studied. The number of coal seams is increased in the experimental model. The No. II1 coal seam is set as 4 m, and a coal seam with a thickness of 4 m is arranged at a position of 25 m below it.
Similarity coefficient
In the physical similarity simulation experiment, the experiment model and prototype must follow the similarity criterion. The similarity ratio is mainly considered from geometric parameters, mechanical parameters and time parameters. The geometric, stress and time similarity ratio of experiment model and prototype is 1:100, 1:173 and 1:10, respectively. The similarity coefficient can determine the size of experimental model.
Typical geological log profile of Panel.
Similar materials
Sand, calcium carbonate, borax and gypsum are used as similar materials in the experiment model. The amount of sand is the largest, followed by gypsum and calcium carbonate. Borax is used to prevent similar materials from condensing into blocks. The rock strength can determine the ratio number of materials. The amount of materials can be calculated in accordance with the thickness of rock strata, ratio number, and size of the experiment model. The length, height, and width of the experiment model are 2500, 1300, and 200 mm, respectively. The proportion of similar materials in the experimental model is listed in Table 1.
Laying of experiment Model
In the physical similarity simulation experiment, the rock strata are laid from bottom to top. The front and rear of the model are fixed with steel plates during the experiment. Proportionally configured similar materials are poured into the model frame. The similar materials are compacted by the iron blocks. The mica sheets are sprinkled evenly on the top of compacted similar materials. After the laying of experimental model is completed, it is dried for 7 days. Finally, the steel plates of the experimental model are removed.
To monitor the subsidence, the displacement monitoring points are arranged on the model surface, as shown in Fig. 2. In the experiment model of single coal seam, the displacement monitoring points are laid seven layers, and the distance between monitoring points is 10 cm. In the experiment model of multi-seam, the displacement monitoring points are laid nine layers, and the distance between monitoring points is also 10 cm.
The displacement monitoring points of experimental model: (a) single coal seam, and (b) multi-seam.
Coal seam excavation
The left, right and lower boundaries of the physical model are displacement constraints. The width of the protective coal pillar of a panel is 30 cm. The coal seam is excavated from the left side of the model, and the excavation step is 10 cm. The time interval of each excavation is 30 min to ensure that the overlying strata do not move. In the experiment model of multi-seam, the shallow coal seam is excavated first, followed by the deep coal seam.
The subsidence is monitored with an XTDP photogrammetric system. After each excavation, a camera is used to record the coordinates of displacement monitoring points. A tape is used to measure the range of caved strata. The data analysis software is used to read the captured photos, and calculates the subsidence.
Results and discussion
This section describes the subsidence and caved laws of overlying strata during the initial, intermediate, and later mining stages. The height, length, and area of caved strata at different excavation distances are analyzed. The caved law of overlying strata at three different stages is expounded. The morphology and fracture types of caved strata are determined.
Initial mining stage
The movement law of overlying strata in the two experimental schemes is shown in Figs. 3 and 4. In the model experiment of single coal seam, overlying strata do not collapse before 50 cm excavation, as shown in Fig. 3. Overlying strata maintain their integrity and are in the self-equilibrium state. In the model experiment of multi-seam, overlying strata of the upper and lower gobs do not collapse before 50 cm and 40 cm excavation respectively, as shown in Fig. 4. During the initial mining stage, overlying strata do not collapse and subsidence is extremely small.
The initial mining stage of single coal seam: (a) excavation of 10 cm, and (b) excavation of 50 cm.
The initial mining stage of multi-seam: (a) excavation of 10 cm in the upper gob, (b) excavation of 50 cm in the upper gob, (c) excavation of 10 cm in the lower gob, and (d) excavation of 40 cm in the lower gob.
The first stage is the initial mining stage. Overlying strata do not collapse, and no evident abscission layers and fractures occur. The subsidence is extremely small. Coal seam mining exerts minimal influence on overlying strata. This stage can be called the self-equilibrium stage of overlying strata.
Intermediate mining stage
This section presents the movement and caved laws of overlying strata during the intermediate mining stage. The height of caved strata at different excavation distances is calculated. The variation characteristics of the subsidence curve are analyzed.
Single coal seam
(1) Caved law
The caved law of overlying strata in the model experiment of single coal seam is illustrated in Fig. 5. When coal seam is excavated to 60 cm, the immediate roof collapses in the form of bending subsidence. The height of caved strata is 8.71 cm. When coal seam is excavated to 90 cm, the caved range extends to the lower part of the D2 survey line. The increased height of caved strata is 13.16 cm. When coal seam is excavated to 100 cm, the caved range extends to the lower part of the D4 survey line. The increased height of caved strata is 27.32 cm. When coal seam is excavated to 110 cm, the caved range extends to the lower part of the D6 survey line. The increased height of caved strata is 22.62 cm. When coal seam is excavated to 140 cm, the caved range extends to the lower part of the D7 survey line. The increased height of caved strata is 6.34 cm. The caved strata changes along the vertical direction.
Caved law-single coal seam: (a) excavation of 60 cm, (b) excavation of 90 cm, (c) excavation of 100 cm, (d) excavation of 110 cm, and (e) excavation of 140 cm.
Subsidence curve-single coal seam: (a) excavation of 60 cm, and (b) excavation of 140 cm.
(2) Movement law
The subsidence of overlying strata in the model experiment of single coal seam is shown in Fig. 6. When coal seam is excavated to 60 cm, the survey lines do not collapse, and their subsidence changes slightly. When coal seam is excavated to 140 cm, rock strata at the D6 survey line collapses and subsidence increases significantly. The lower part of the D7 survey line has an evident concave abscission layer, resulting in the small subsidence of the D7 survey line. The subsidence curves of caved strata change in the vertical direction.
Upper gob in multi-seam mining
(1) Caved law
The caved law of overlying strata is illustrated in Fig. 7. When coal seam is excavated to 60 cm, rock strata at the FD1 survey line collapses in the form of bending subsidence. The height of caved strata is 8.69 cm. When coal seam is excavated to 80 cm, the caved range extends to the lower part of the FD2 survey line. The increased height of caved strata is 2.65 cm. When coal seam is excavated to 90 cm, the caved range extends to the lower part of the FD4 survey line. The increased height of caved strata is 17.62 cm. When coal seam is excavated to 110 cm, the caved range extends to the lower part of the FD5 survey line. The increased height of caved strata is 11.68 cm. When coal seam is excavated to 140 cm, the caved range extends to the lower part of the FD6 survey line. The increased height of caved strata is 10.36 cm. The caved law is consistent with that of the single coal seam mining.
Caved law-upper gob: (a) excavation of 60 cm, (b) excavation of 80 cm, (c) excavation of 90 cm, (d) excavation of 110 cm, and (e) excavation of 140 cm.
Subsidence curve-upper gob: (a) excavation of 60 cm, and (b) excavation of 140 cm.
(2) Movement law
The subsidence curve is illustrated in Fig. 8. When coal seam is excavated to 60 cm, rock strata at the FD1 survey line collapse and the subsidence increases significantly. The subsidence curve exhibits a concave shape. When coal seam is excavated to 140 cm, rock strata at the FD5 survey line collapse and subsidence increases significantly. The lower part of the FD6 survey line has an evident concave abscission layer, resulting in the small subsidence of the FD6 survey line.
Lower gob in multi-seam mining
(1) Caved law
The caved law is illustrated in Fig. 9. When coal seam is excavated to 50 cm, the lower strata of the SD1 survey line collapses. The height of the caved strata is 8.79 cm. When coal seam is excavated to 60 cm, the caved range extends to the lower part of the SD2 survey line. The increased height of caved strata is 6.21 cm. When coal seam is excavated to 90 cm, the caved range extends to the lower part of the SD3 survey line. The increased height of caved strata is 16.29 cm. The caved law is consistent with that of the single coal seam mining.
(2) Movement law
The subsidence curve is illustrated in Fig. 10. When coal seam is excavated to 50 cm, the lower strata of the SD1 survey line occurs collapse, resulting in the small subsidence of the SD1, SD2, and SD3 survey lines. When coal seam is excavated to 90 cm, rock strata at the SD2 survey line collapse and subsidence increases significantly. The lower part of the SD3 survey line has an evident concave abscission layer, resulting in the small subsidence of the SD3 survey line.
Caved law-lower gob: (a) excavation of 50 cm, (b) excavation of 60 cm, and (c) excavation of 90 cm.
Subsidence curve-lower gob: (a) excavation of 50 cm, and (b) excavation of 90 cm.
Caved and Movement models during the Intermediate Mining Stage
The second stage is the intermediate mining stage. Overlying strata collapse along the vertical direction. The height of caved strata increases gradually with the advance of the panel, as shown in Fig. 11. This stage can be called the vertical collapse stage. Large numbers of cavities, abscission layers, and fractures exist between caved strata. The fractured area is trapezoidal distribution. The subsidence curve changes in the vertical direction as shown in Fig. 12. The variation characteristics of subsidence correspond to the caved characteristics of overlying strata.
Caved model-Stage II.
Movement model-Stage II.
Later mining stage
This section presents the movement and caved laws of overlying strata during the later mining stage. The length of caved strata is calculated at different excavation distances. The variation characteristics of the subsidence curve are analyzed.
Single coal seam
(1) Caved law
During the later mining stage, the caved law of overlying strata is depicted in Fig. 13. When coal seam is excavated to 150 cm, the abscission layers between the caved strata of the central are compacted, forming a compacted area. The upper and lower lengths of the compacted area are 33.20 cm and 58.91 cm, respectively. The upper and lower lengths of the fractured area are 83.88 cm and 150 cm, respectively. When coal seam is excavated to 170 cm, caved strata increase along the horizontal direction. The upper and lower lengths of the compacted area are 40.01 cm and 68.35 cm, respectively. The upper and lower lengths of the fractured area are 99.83 cm and 170 cm, respectively. When coal seam is excavated to 190 cm, caved strata increase further along the horizontal direction. The upper and lower lengths of the compacted area are 68.35 cm and 102.12 cm, respectively. The upper and lower lengths of the fractured area are 126.09 cm and 190 cm, respectively. The caved strata changes along the horizontal direction.
Caved law-single coal seam: (a) excavation of 150 cm, (b) excavation of 170 cm, and (c) excavation of 190 cm.
(2) Movement law
During the later mining stage, the subsidence curve is illustrated in Fig. 14. When coal seam is excavated to 150 cm, the caved distance of the D1–D7 survey lines in the horizontal direction is 139.55, 120.13, 106.32, 99.53, 93.86, 87.49, and 68.36 cm, respectively. When coal seam is excavated to 190 cm, the caved distance of the D1–D7 survey lines in the horizontal direction is 185.25, 168.53, 156.85, 143.77, 140.37, 128.36, and 110.33 cm, respectively. The subsidence curve changes along the horizontal direction.
Subsidence curve-single coal seam: (a) excavation of 150 cm, and (b) excavation of 190 cm.
Upper gob in multi-seam mining
(1) Caved law
During the later mining stage, the caved law is illustrated in Fig. 15. When coal seam is excavated to 150 cm, the abscission layers between the caved strata of the central are compacted, forming a compacted area. The upper and lower lengths of the compacted area are 43.49 cm and 68.79 cm, respectively. The upper and lower lengths of the fractured area are 92.57 cm and 148.83 cm, respectively. When coal seam is excavated to 170 cm, caved strata increase along the horizontal direction. The upper and lower lengths of the compacted area are 59.87 cm and 91.72 cm, respectively. The upper and lower lengths of the fractured area are 112.49 cm and 170 cm, respectively. When coal seam is excavated to 190 cm, caved strata increase further along the horizontal direction. The upper and lower lengths of the compacted area are 62.55 cm and 95.78 cm, respectively. The upper and lower lengths of the fractured area are 119.53 cm and 183.52 cm, respectively. The caved law is consistent with that of the later excavation stage of single coal seam.
(2) Movement law
During the later mining stage, the subsidence curve is illustrated in Fig. 16. When coal seam is excavated to 150 cm, the caved distance of the FD1, FD2, FD3, FD4, FD5, and FD6 survey lines in the horizontal direction is 132.16, 122.19, 113.52, 108.29, 92.37, and 104.31 cm, respectively. When coal seam is excavated to 190 cm, the caved distance of the FD1, FD2, FD3, FD4, FD5, and FD6 survey lines in the horizontal direction is 181.76, 165.52, 151.63, 132.34, 120.67, and 112.77 cm, respectively. The opening of the subsidence curve gradually increases with an increase in excavation distance. The variation law is consistent with that of the later mining stage of single coal seam.
Caved law-upper gob: (a) excavation of 150 cm, (b) excavation of 170 cm, and (c) excavation of 190 cm.
Subsidence curve-upper gob: (a) excavation of 150 cm, and (b) excavation of 190 cm.
Lower gob in multi-seam mining
(1) Caved law
During the later mining stage, the caved law is illustrated in Fig. 17. When coal seam is excavated to 100 cm, the abscission layers between the caved strata of the central are compacted, forming a compacted area. The upper boundary of the compacted area is connected to the upper gob. The upper and lower lengths of the compacted area are 18.59 cm and 40.25 cm, respectively. The upper and lower lengths of the fractured area are 46.43 cm and 100 cm, respectively. When coal seam is excavated to 150 cm, caved strata increase along the horizontal direction. The upper and lower lengths of the compacted area are 53.39 cm and 74.59 cm, respectively. The upper and lower lengths of the fractured area are 68.26 cm and 150 cm, respectively. When coal seam is excavated to 190 cm, caved strata increase further along the horizontal direction. The upper and lower lengths of the compacted area are 116.17 cm and 132.73 cm, respectively. The upper and lower lengths of the fractured area are 45.61 cm and 190 cm, respectively. The caved law is consistent with that of the later mining stage of single coal seam.
During the later mining stage, the lower gob is connected to the upper gob. The lower gob is affected by the gravity of the upper gob. The compacted area of the lower gob appears when coal seam is excavated to 100 cm. The compacted area of the upper gob appears when coal seam is excavated to 150 cm. The compacted area of the lower gob is earlier than that of the upper gob. After the upper gob is affected by the mining, the fracture development degree increases on two sides.
In addition, the compacted area in the upper gob is also change. When coal seam is excavated to 100 cm, the upper and lower lengths of the compacted area in the upper gob are 58.87 cm and 97.85 cm, respectively. When coal seam is excavated to 150 cm, the upper and lower lengths of the compacted area in the upper gob are 80.74 cm and 120.13 cm, respectively. When coal seam is excavated to 190 cm, the upper and lower lengths of the compacted area in the upper gob are 83.39 cm and 122.76 cm, respectively. The compacted area of the upper gob increases gradually.
Caved law-lower gob: (a) excavation of 100 cm, (b) excavation of 150 cm, and (c) excavation of 190 cm.
(2) Movement law
During the later mining stage, the subsidence curve is shown in Fig. 18. When coal seam is excavated to 100 cm, the caved distance of the SD1, SD2, and SD3 survey lines in the horizontal direction is 92.31, 78.67, and 65.02 cm, respectively. When coal seam is excavated to 190 cm, the caved distance of the SD1, SD2, and SD3 survey lines in the horizontal direction is 185.27, 175.28, and 161.78 cm, respectively. This subsidence curve changes along the horizontal direction. The variation law is consistent with that of the later mining stage of single coal seams.
The subsidence changes again after the upper gob is affected by the mining, as shown in Fig. 19. When coal seam is excavated to 150 cm, the maximum subsidence of the FD1, FD2, FD3, FD4, FD5, and FD6 survey lines increases by 4.69, 4.40, 5.44, 5.38, 5.32, and 5.26 mm, respectively. When coal seam is excavated to 190 cm, the maximum subsidence of the FD1, FD2, FD3, FD4, FD5, and FD6 survey lines increases by 4.50, 4.10, 3.60, 3.56, 3.52, and 3.48 mm, respectively.
Subsidence curve-lower gob: (a) excavation of 100 cm, and (b) excavation of 190 cm.
Subsidence curve of the upper gob affected by the mining: (a) excavation of 100 cm, and (b) excavation of 190 cm.
Caved and movement models during the later mining stage
The third stage is the later mining stage. The overlying strata collapse along the horizontal direction. The length of caved strata increases gradually with the advance of panel, as shown in Fig. 20. This stage can be called the horizontal collapse stage. The abscission layers between the caved strata of the central are compacted and exhibit a trapezoidal distribution. The compacted area is surrounded by a fractured area. The fractured and compacted areas increase gradually along the horizontal direction. The subsidence curve changes in the horizontal direction, as shown in Fig. 21. The variation characteristics of subsidence correspond to the caved characteristics of overlying strata.
In multi-seam mining, the upper and lower gobs interact with each other during Stage III. The lower gob is connected to the upper gob. The caved strata of the upper gob continue to change under the influence of mining. The caved strata of the upper gob move downwards, as shown in Fig. 22. The subsidence curve changes in the vertical direction, as shown in Fig. 23.
Caved model-Stage III.
Movement model-Stage III.
Caved model-multi-seam mining.
Movement model-upper gob affected by the mining.
Evolution models of caved strata during coal mining
During panel mining, overlying strata undergo a dynamic evolution process. As shown in Fig. 24, the overlying strata experience the self-equilibrium stage, followed by the vertical collapse stage, and finally, the horizontal collapse stage. As shown in Fig. 25, the subsidence of overlying strata exhibits a stage of extremely small subsidence, followed by a large subsidence, and then a horizontal change occurs. The fractured area presents a stage of no fractured area, followed by the vertical, and then, horizontal increase of the fractured area. During Stage III, a compacted area appears in the middle of caved strata, and increases along the horizontal direction. In addition, the lower gob is connected to the upper gob in multi-seam mining. Caved strata of the upper gob move downward again.
Evolution model of caved strata.
Evolution model of subsidence.
Conclusions
In this work, a physical similarity simulation experiment is conducted to study the movement and caved laws of overlying strata during coal seam mining. The movement characteristics of caved strata during the initial, intermediate, and later mining stages were determined. The evolution model of movement and caving of overlying strata is established. The major conclusions drawn are as follows.
(1) During the self-equilibrium stage, overlying strata in the gob do not collapse. No evident abscissions layers and fractures exist between overlying strata. The subsidence of overlying strata is extremely small. Overlying strata in the gob can realize self-equilibrium.
(2) During the vertical collapse stage, overlying strata collapse along the vertical direction, and the height of caved strata increases gradually. Large numbers of cavities, abscissions layers, and fractures exist between caved strata. The fractured area is trapezoidal distribution and gradually increases upward. The subsidence curve changes in the vertical direction.
(3) During the horizontal collapse stage, overlying strata collapse along the horizontal direction. The abscission layers between the caved strata of the central are compacted and presented a trapezoidal distribution. The compacted area is surrounded by a fractured area. The compacted and fractured areas increase along the horizontal direction. The subsidence curves change in the horizontal direction. In multi-seam mining, the caved strata of the upper gob continue to move downwards under the influence of mining of lower coal seam.
(4) During coal mining, overlying strata evolve from the self-equilibrium stage to the vertical collapse stage, and finally, the horizontal collapse stage. The fractured area of overlying strata changes from a no fractured area to a fractured area, increases vertically, and finally, increases horizontally. The subsidence curve of overlying strata changes from an extremely small subsidence to a large subsidence, and finally, changes horizontally.
Data availability
Data are available from the corresponding author on reasonable request.
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Funding
This work was supported by the Doctoral special fund of Hebei University of Engineering (No. SJ2401002055), the key research and development project of Xingtai City (Grant No. 2022ZC064).
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Wang wrote the main manuscript text. Sun and Shen reviewed the manuscript.
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Wang, C., Sun, L. & Shen, H. An aftermath analysis of caving characteristics and movement of overlying strata in fully mechanized longwall gob. Sci Rep 14, 28095 (2024). https://doi.org/10.1038/s41598-024-79968-x
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DOI: https://doi.org/10.1038/s41598-024-79968-x
