Introduction

Coal occupies an important position in China’s economic development. It is the main structural component of China’s primary energy. Although China is paying more and more attention to the environment, it has so far been unable to change the status of coal in China’s primary energy structure1. In 2020 the annual report on the development of the coal industry clearly states that by the end of 2025, China’s coal consumption will be more than four billion tons, which shows that coal will still be in a dominant position during the “14th Five-Year Plan” period2.

With the increase in coal resource consumption and the continuous mining of coal resources, the advantages and shallow buried deep coal seams have been basically exhausted. Coal enterprises have to mine in depth. Deep mining faces high ground stress, high gas, and high working surface ambient temperature. The problem of high impact ground pressure tendency and high permeability3, especially the problem of high gas, has greatly increased the safety factor of the coal mining working face. The gas migration channel in deep mining is not smooth, and a large amount of gas is compressed and stored in the coal seam. In the gaps, gas is suddenly released when tunneling, causing coal and gas outburst accidents4.

Protective layer mining is the most effective and economical means to prevent coal and gas outburst accidents5. When the protective layer is mined, a mining space will be formed, and a series of changes will occur in the rock formation pressure and gas pressure within the mining influence range. The protected layer will be affected by mining, and the protected layer will be deformed due to pressure relief6, resulting in a large number of cracks in the protected layer, the gas is fully released, which greatly reduces the occurrence of coal and gas outburst accidents. The movement and damage characteristics of the protected layer are the key technology for evaluating the mining effect of the protected layer7.

Many scholars at home and abroad have conducted a lot of research on the damage caused by the movement of the protective layer during mining. Guo Jianxing8 used theoretical and numerical simulation methods to study the stress distribution characteristics of the protected layer. The research results show that after mining of the working face, it has had an impact on the protected layer, and the pressure relief rate has reached 92%. The gas and pressure of the coal seam have been greatly reduced, ensuring the safe production of the working face. Huang Yong9 et al. used numerical simulation technology to study the dynamic pressure relief characteristics of the protected layer during mining of the protected layer. Gas migration has been studied. The research shows that the pressure relief range of the protected layer in protective layer mining gradually increases with the continuous advancement of the working face. The gas in the protected layer gradually flows to the goaf area, and the gas pressure is greatly reduced. The gas drainage effect significantly improved, achieving the purpose of rapid elimination of outbursts; Wang Jing10 et al. used field tests to study the development of cracks in the protected layer after mining of the protective layer. The research results showed that the permeability of the protected layer after mining of the protective layer was lower than that before the mining of the protective layer. Seven times, the pressure relief amplitude is reduced by 50%, and the mining of the protective layer achieves the effect of pressure relief and permeability; Jiao Anjun11 et al. used numerical simulation technology to study the roof stress characteristics and coal seam permeability. The results show that as the protected layer continues to be mined, the permeability of the coal seam is greatly increased, and the gas extraction volume of the protected layer is greatly increased, which has the effect of eliminating coal seam outbursts; Li Li12 et al. used the change characteristics of field data to analyze the relationship between the changes in the surrounding rock and the migration characteristics of the pressure relief gas. The research results it shows that the gas pressure of the protected layer is greatly reduced after the protective layer is mined, and safe mining of the working face can be ensured while maintaining a certain wind speed.

The above scholars have mainly studied the pressure relief characteristics of the protected layer through theoretical analysis, numerical simulation, and field testing, but they have not done much research on the evolution and movement characteristics of cracks. In order to understand the cracks and damage characteristics of the overlying rock layer during mining of the protective layer, this paper uses The protective layer mining working face of a mine in Shanxi was used as the test object. Deep base point technology, underground drilling segmented water injection technology, and underground drilling peep technology were used to conduct research on the evolution of cracks in the overlying rock layer and the protected layer and the damage characteristics of the overlying rock layer during the mining of the protective layer. Conducted research and conducted numerical simulation research on the plastic damage, pressure relief characteristics, and expansion characteristics of the protected layer during the mining process of the protective layer.

Project overview

A mine in Shanxi is located in the Jincheng Mining Area and is affiliated to Jinneng Holding Shanxi Coal Mining Co., Ltd. with a production capacity of 2.4 Mt/a. There are three mineable coal seams within the mine field, namely the 3#, 9#, and 15# coal seams. , among which the 3# coal seam is the main coal seam, the thickness of the coal seam is about 6.0 m, and some areas are high gas areas. It is difficult to achieve the effect of gas control under conventional gas drainage operations. 28 m below the 3# coal seam is the 9# coal seam. The 9# coal seam has lower gas pressure and lower gas content than the 3# coal seam. In order to improve the gas permeability of the 3# coal seam, the 9# coal seam is used as a protective layer for mining to prevent 3 #A gas accident occurred in the coal seam. In order to verify the effect of the protective layer during mining of the protected layer, the 2305 working face was used as a test working face for observation. According to the actual conditions of the working face, three observation boreholes were arranged on the 2305 working face for observation. The borehole layout is as follows as shown in Fig. 1.

Figure 1
figure 1

Working face drilling layout diagram.

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The roof is composite sandy mudstone, siltstone hardroof. Top features: direct top is dark gray sandy mudstone, about 5.12 m thick, occasionally thin coal seam, the basic top is gray white fine sandstone and gray siltstone, about 10.78 m thick, with staggered bedding.

Development and evolution characteristics of mining fissures

Test equipment

Downhole drilling segmented water injection device

The downhole drilling segmented water injection device adopts the two-end plugging and middle water injection test device independently developed by Shandong University of Science and Technology. The device mainly consists of three structures: a testing part, an operating part, and a high-pressure plugging part. The testing principle is that before the protective layer is mined, the protected layer is in its original state, and the cracks in the coal seam are mainly primary fissures. After the protective layer is mined, the protected layer is affected by the mining of the protective layer, and the original equilibrium state is broken13. The original cracks in the coal seam the impact changes, and new cracks will be formed in the protected layer due to mining14. The development of cracks has undergone a major change, which is completely different from before the protective layer was mined. The two change characteristics of the water injection in the test section are used to evaluate the protective layer after mining. The impact on the protected layer15. Its structure is shown in Fig. 2.

Figure 2
figure 2

Structure diagram of water injection test device.

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Underground drilling peep device

The underground drilling peep device adopts the explosion-proof peep device of Wuhan Goode Technology Co., Ltd. This equipment can conduct underground testing16. The structure of the equipment includes a probe testing part, a depth monitoring part, and a central processing part. The structure and imaging are shown in Figs. 3, 4 shown.

Figure 3
figure 3

Drilling TV structure diagram.

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Figure 4
figure 4

Hole wall imaging mechanism.

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The testing principle of the borehole peep is to use the cone reflection and CCD sensor in the imaging system to collect the image in the borehole, and display it in the form of a 360° picture through the processing system. The depth sounding system monitors the probe position and image data in real time and bathymetry data continuously enter the processing system. The data is processed and displayed in a two-dimensional or three-dimensional form. Imaging and depth are continuously collected and processed to present a continuous picture form17.

For the drilling scope, the essence is that when the drilling scope probe is drilling, the picture captured by the probe camera during the whole process is saved as a video, so each picture in the video contains the hole wall information that can be captured by the location of the drilling probe at that time. Through the collection deformation of the pictures, the picture of the viewing angle of the peep probe can be transformed into the picture of the same direction of the drilling hole wall, and then the transformed picture is spliced to get the panoramic picture of the drilling hole contained in the drilling video18.

The crack width is calculated in the expanded image. The fissures on the in-situ hole wall were reflected by the conical mirror through optical transformation to form a panoramic image, reconstructed into the hole wall image, and then cut along the North Pole to the expanded image. The fissures are standard planar fissures that appear sinusoidal in the expanded plot and have no width. For the standard plane fracture, the tendency of the fracture can be calculated from any three points on the fracture, as long as they are not in a line, and the tendency calculated from different points is the same.

Characteristics of fissure development

An underground borehole segmented water injection device and an underground borehole peeping device were used to study the development and evolution characteristics of cracks in the protected layer under the influence of protective layer mining. In order to accurately grasp the development characteristics of cracks in the protected layer, downhole borehole peeping was first performed during the field test test, and then perform a segmented water injection test.

Downhole drilling segmented water injection

During the downhole drilling segmented water injection test, the test section was set to 1 m. In order to accurately grasp the evolution of cracks in the protected layer after the protective layer was mined, three tests were conducted for each test section, and the average value was taken as the value of the test section. Each test section was conducted for 5 min. During the test, the water injection pressure was set to 0.5 MPa and the rubber sealing end was set to 2.0 MPa. The water injection volume of the borehole in the protected layer was tested before and after the mining of the protective layer, and the results were recomposed, as shown in Fig. 5.

Figure 5
figure 5

Water injection curve of different drilling depth.

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It can be seen from the segmented water injection volume curve of the underground borehole in Fig. 5 that the water injection volume of the protected layer borehole fissures before the protective layer is mined is basically maintained at 40 L/5 min. After the protective layer is mined, the protected layer is affected by mining, resulting in a large amount of in cracks, the maximum water injection volume of the protected layer can reach 118 L/5 min, which is nearly three times that before the mining of the protective layer. The mining of the protective layer has a severe mining impact on the protected layer, and the degree of crack development is greatly improved. The mining of the protective layer has a significant impact on the mining of the protected layer. The development of cracks in the protected layer has a very good promoting effect.

Underground drilling peep device

An underground borehole peeking device was used to detect the development characteristics of cracks in the protected layer under the influence of mining before and after mining. The detection results were sorted and analyzed, and the cracks in the protective layer before and after mining were obtained. The changing characteristics are shown in Fig. 6.

Figure 6
figure 6

Fracture evolution image before and after protective layer mining.

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From Fig. 6, the crack evolution images before and after the protective layer is mined, it can be seen that before the protective layer is mined, the protected layer is basically in its original state. There are two cracks in the detection area, and the width of the cracks is generally small. When the protective layer is mined, the number of cracks is obvious. Increase, there are nine fissures in the detection area, and the width of the fissures is larger than before the protective layer was mined, indicating that the impact of mining can play an important role in promoting the development of fissures. In order to further analyze the impact of the protective layer mining on the protected layer, a quantitative analysis was conducted on the changes in cracks under the influence of mining before and after the protective layer was mined. Taking the width of the cracks as the abscissa, the number and proportion of cracks were Make the ordinates to establish coordinate systems respectively, and obtain the relationship curves as shown in Figs. 7, 8 respectively.

Figure 7
figure 7

Relationship curve between crack width and number.

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Figure 8
figure 8

Relationship curve between crack width and proportion.

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The width of crack development in the protected layer can only reflect the degree of promotion of crack development by mining. The larger the crack width, the more severe the impact and the better the effect. It can be seen from the width relationship curve that before the mining of the protective layer, the cracks in the protected layer were mainly small width, and cracks less than 10 mm accounted for 75% of all cracks. Before the mining of the protective layer, the cracks were basically original cracks, and no cracks were found. There are large cracks, and the overall development of the cracks is low; when the protective layer is mined, the number of cracks increases significantly and the width of the cracks increases significantly under the action of mining. The number of cracks increases from 8 before mining to 18 after mining. It has increased by 125%, and the width has mainly changed from less than 10 mm before mining to 10–20 mm, accounting for 61.11% of all cracks. There are also cracks with a width greater than 25 mm, indicating that mining plays a role in the development of cracks. With the promotion effect, after mining, the degree of crack development is greatly increased.

Mining mobile damage characteristics

The intensity of the key layer movement failure of overlying strata affects the pressure relief effect of the protected layer, and the change characteristics can be obtained by studying its movement failure law. The deep base point technology is used to analyze the movement failure law, and the numerical simulation of the failure in the plastic zone is carried out.

Deep base point principle

The deep base point method is an important means of observing rock movement. The main observation principle is to construct a hole in the rock layer, install the base point in different layers of the rock layer, lead the base point to the observation tunnel through the lead line, and reflect the movement of the rock layer where the base point is located by observing the change of the lead line. In the daily observation process, the length of the wire rope from the steel beam to the heavy hammer is measured to analyze the change law of the overburden where the base point is located. The change value is only the change value of the rock layer where the base point is located relative to the position of the steel beam. It is required to take the absolute deformation value of each rock layer, and it is also necessary to observe the settlement of the tunnel and the settlement of the steel beam relative to the tunnel. Then, the actual settlement law of each rock layer can be obtained after the observation data are composited. The movement of the heavy hammer is a composite movement, which mainly includes three forms of movement, namely, the movement of the deep base point relative to the steel beam; the displacement of the top plate of the tunnel where the steel beam is located; and the settlement movement of the bottom plate of the tunnel. During the mining process at the working face, the rock strata affected by mining will sink, delaminate or even break, and the deformation value will be reflected through the base point and the wire rope lead, that is, the base point change value is obtained by directly measuring the value, which is the movement of the rock stratum where the base point is located relative to the steel beam, the principle is shown in Fig. 9.

Figure 9
figure 9

Deep base point technology schematic diagram.

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Deep base point data and analysis

In the process of testing, the change characteristics of the distance between the steel beam and the heavy weight are measured to analyze the change characteristics of the overlying rock. In the measurement process, the absolute value is taken each time as the value of this measurement. The data conforms to the comprehensive analysis of the actual movement and failure characteristics of the overlying rock. The variation characteristics of the depth base point of borehole no. 4 are analyzed, and its settlement curve is shown in Fig. 10.

Figure 10
figure 10

The settlement change curve of deep base point of no. 4 borehole.

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The characteristics of basic roof changes play an important role in the pressure relief effect of the protected layer. As can be seen from Fig. 10, the settlement changes of each rock layer present phased and nonlinear changes, which are mainly divided into three stages: initial deformation, severe deformation and deformation decay.

The initial deformation is located in the range from 45 m in front of the working face to 6 m behind the working face. In this stage, the mining influence of the protected layer is small, and there is a small deformation. The severe deformation is located in the range of 6–35 m after mining on the working face, the rock layer changes obviously, and a large settlement suddenly appears. After analyzing the mining of the protective layer, with the collapse of the lower rock layer, the settlement speed of the weak rock layer is greater than that of the hard rock layer, resulting in the occurrence of the phenomenon of separation, and the separation value is larger, up to 162 mm. Subsidence decay is located at the position 35 m after the stopping of the working face. With the continuous advance of the working face, the subsidence change tends to moderate, and the subsidence speed becomes smaller and smaller. When the subsidence reaches 63 m, it is almost in a stable state. In the vertical direction, the direct collapse area of the rock layer can be divided into the rock layer of the collapse zone. From the basic top to the hard rock, because of the obvious separation phenomenon, this part is divided into fracture zone. The coordinated movement of the hard rock to the upper part of the failure area, because it does not produce cracks, this part is divided into curved subsidence zone.

Numerical simulation of pressure relief of protected layer

According to the field test, the mining of 9# coal seam as a protective layer has an impact on the overlying strata, but the degree of impact and the effect of pressure relief and reflection enhancement need to be further studied. Meanwhile, numerical simulation technology is an effective means to study the failure of the plastic zone, which can greatly make up for the deficiencies in the field test. This paper establishes a geological model based on the geological conditions of the 2305 working face. Using FLAC3D numerical simulation software, the characteristics of plastic zone change, pressure relief and expansion rate of the protected layer were obtained.

Failure characteristics of plastic zone

Through numerical simulation, the variation characteristics of the plastic zone of overlying rock in the direction of strike and dip are shown in Fig. 11. It can be seen from Fig. 11 that in the direction of strike, the plastic failure zone as a whole presents a saddle-shaped distribution, and the overlying rock fracture as a whole presents a saddle-shaped distribution, and the plastic zone develops the highest near the cutting hole and stop-mining line. According to analysis, due to the supporting effect of coal wall, the stress is concentrated, resulting in a large range of plastic zones. In the trend direction, the rock strata are subjected to both shear and tensile failure, and the upper strata of overlying rock are mainly shear failure, which causes the plastic zone to show a changing trend of high on both sides and low in the middle19.

Figure 11
figure 11

Variation characteristics of plastic failure zone.

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Protective layer mining pressure relief characteristics of the protected layer

After the mining of the protective layer, the stress of the protected layer will inevitably change under the influence of mining. The protected layer can effectively relieve pressure, and the permeability of coal seam can be greatly promoted after the relief of the protected layer. The study of its relief characteristics can understand the influence of the protected layer after mining20. Usually, the coal seam pressure relief rate is used to evaluate the effect of the protective layer on the protected layer after mining. The calculation formula of pressure relief rate is

$$\eta_{s} = \sigma^{\prime}_{z} /\sigma_{z}$$

where, \(\eta_{s}\) is the pressure relief rate of coal seam after mining of protective layer; \(\sigma^{\prime}_{z}\) is the vertical pressure of relief after mining of protective layer, \(MPa\) ; \(\sigma_{z}\) is the vertical stress under unmining, \(MPa\).

According to the numerical simulation results, the formula was used to calculate the pressure relief rate of the protected layer, and the pressure relief rate curve was obtained from the pressure relief rate, as shown in Fig. 12. As can be seen from the curves of pressure relief rate of the protected layer under different mining distances in Fig. 12, pressure relief occurs when the working face reaches 150 m, and the pressure relief rate reaches the maximum value when the working face returns to 25 m and 135 m, which are 1.63 and 1.65 respectively. The overall pressure relief curve presents a “saddle” distribution, and the overall pressure relief rate decreases by 60%. It shows that the mining movement has a good pressure relief effect on the protected layer after mining.

Figure 12
figure 12

The pressure relief rate curve of the protected layer under different mining distances.

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Protective layer mining expansion characteristics of the protected layer

After the mining of the protective layer, the original equilibrium state is broken and the goaf is formed21. Under the caving action of the rock layer above the goaf, the protected layer is bound to move and destroy. The deformation of the top and bottom layer is monitored22, and the expansion rate is calculated by the formula, and the anti-reflection effect is judged by the expansion rate:

$$\xi = u/M$$

where, \(\xi\) is the expansion rate of the protected layer under the influence of mining; \(u\) is the displacement difference of the top and bottom of the protected layer under the influence of mining, \(m\); \(M\) is the thickness of the protected layer above the protective layer, \(m\).

Numerical simulation software was used to monitor the change characteristics of the top and bottom of the protected layer under the influence of mining23, record its change value, and calculate its expansion rate by using formula (2). The expansion rate curve obtained from the expansion rate data is shown in Fig. 13.

Figure 13
figure 13

The expansion rate curve of the protected layer under different mining distances.

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As can be seen from the graph of the expansion rate of the protected layer at different mining distances in Fig. 13, the top and bottom of the protected layer move and break during the mining process24, and the expansion rate curve presents a “saddle-type” distribution. Compared with the critical value of 3% in the rules for the prevention and control of coal and gas outburst, the working face meets this requirement. Therefore, the mining technology of the protected layer can realize the safe mining of the working face. The protective layer mining technology is applicable to other working faces of the coal mine25.

Conclusion

  • (1) Before the exploitation of the protective layer, the fracture development of the protected layer is mainly characterized by small width and small quantity. After the exploitation of the protective layer, the fracture development degree of the protected layer is greatly improved under the influence of mining, and the fracture width and quantity are obviously increased; Under the influence of mining, a large number of cracks are produced in the protected layer, and the water injection in the protected layer is nearly three times that before mining.

  • (2) The settlement changes of each rock layer in the basic top show phased and nonlinear changes, which are divided into three stages: initial deformation, severe deformation and deformation decay. Small deformation occurs in the initial deformation stage, the separation phenomenon occurs in the severe deformation stage, and the settlement is in a stable state in the settlement decay stage.

  • (3) After mining, the plastic failure zone, pressure relief curve and expansion curve of the protective layer show a “saddle type” distribution, and the shear failure of the upper layer of the overburden rock is mainly. After the protective layer is mined, the pressure relief rate of the protected layer decreases by 60% under the influence of mining. It meets the requirement of expansion rate 3% critical value in the “rules for preventing and controlling coal and gas outburst”, and the protective layer mining technology is applicable to other working faces of the coal mine.