Abstract
The field monitoring data showed that the fracture of high-position thick and hard roof (THR) was the main reason for the frequent occurrence of high-energy mine earthquake events () in the no. 6 mining district of Dongtan coal mine. In order to study the fracture characteristics of high-level THR and seek a reasonable and effective control method, taking the 63up06 working face as the engineering background, the FLAC model was built to simulate the strata movement law before and after fracturing. The simulation results showed that the THR first breaking span was 340 m, the tensile failure runs through the whole THR, and the subsidence increased rapidly when THR reached the first breaking span. The THR breakage is related to the continuous upward transfer of horizontal compressive stress, which results in the concentration of horizontal compressive stress in THR. After ground fracturing, the first breaking span of THR decreased, and different fracturing positions have a great impact on the THR fracture form. When the fracturing position is close to the center of the first breaking span, there is no large cantilever breaking form in the THR, the THR breaks on both sides of the top and sinks along the hydraulic fracture in the middle, and the collapse degree is relatively uniform. Then, the fracturing test was carried out in the 63up06 working face, the results showed that during the first 400 m of the 63up06 working face mining after fracturing, 702 microseismic events occurred in total, including 12 high-energy vibration events which include 4 mine earthquakes of magnitude 2.0 and 8 mine earthquakes of magnitude 1.0-2.0. Compared with the adjacent working face, the quantity of high-energy mine earthquake events in the 63up06 working face has been significantly controlled.
1. Introduction
The thick and hard roof (THR) above the coal seam is difficult to collapse in time because of its characteristics such as large thickness, high strength, and integrity [1–7]. This part of the rock stratum occurs outside the traditional basic roof, with a thickness of tens of meters or even hundreds of meters. The THR breakage releases a large amount of energy and is easy to induce the rock burst in the stope, which significantly threatens the mining safety production [8–13]. With the gradual transfer of coal mining to the deeper depth, the in situ stress is further increased, the geological conditions are more complex, and the problem of coal mine dynamic disaster will become more prominent [14–18]. Therefore, it is urgent to carry out the relevant research on the occurrence mechanism and prevention of coal mine dynamic disaster induced by the THR breakage.
In the study of fracture mechanisms of the THR, Xu et al. put forward the theoretical failure model of thick and hard rock strata to calculate the rock failure laws in Yangliu Coal Mine, Huaibei Coalfield, China. Then, the reliability of this model was verified by the methods of numerical simulation and surface subsidence measurement [19]. Based on the energy distribution formula of an elastic foundation beam, Liu et al. analyzed the accumulation and release of energy before and after the fracture of the key strata and considered that the nearer the working face, the greater the release energy of the rock beam [20]. Based on the long beam theory, Zhao et al. calculated the first weighting characteristics and extreme fracture step distance and introduced a new technology called deep-hole presplitting blasting (DPB) which could break the THR by drilling holes with different depths and angles at different levels of roof rock and then blasting it [21]. Jia et al. established a thick-hard limestone roof cracking model and discussed the factors that affect roof fracture initiation and extension and found that roof fracture was developed in the stress concentration area below the upper coal face residual coal pillar [22]. Chen et al. developed a roof control scheme of advanced deep hole precracking using 3DEC software for the simulations. It was verified by comparing with the practical application results [23]. Zhang et al. proposed an elastoplastic plate model to study the dynamic elastic-plastic response characteristics of the roof, and the FLAC3D simulation was performed to analyze the failure process of dynamic stress on coal-rock mass [24]. Lan et al. employed an integrated method that included a borehole TV tester, borehole-based monitoring of strata movements, and monitoring of support resistance in the working face to analyze the fracture characteristics of the overlying rock layers and the strata behaviors [25]. The above studies all show that the THR breaking releases a large amount of energy, which threatens the safety production of coal mine. However, there are few studies on the fracture characteristics of high-position THR.
In terms of THR control technology research, Guo et al. proposed the new roadside support method with constant resistance yielding limit supporting under the hard roof, and the feasibility was verified by establishing the theoretical model and field practice [26]. Zhang et al. proposed a solid backfill method for controlling hard roof-induced face bursts and analyzed the interaction between the solid backfill body and the roof under different backfilling ratios [27]. Huang et al. proposed a control technology which involves drilling small aperture drill holes into the hard roof of the initial room with an anchor cable rig and performing hydraulic fracturing to control the hanging roof, based on the stress arch theory and the fracture mechanics [28]. Zhang et al. proposed the “ultradeep hole presplit blasting measure,” the “large-diameter borehole pressure relief measure,” and the “reducing advancing speed measure” to reduce the risk of rock burst, which achieved excellent results [29]. Li et al. developed an advance abutment pressure distribution model to study the distribution law of mining-induced stress under hard roof conditions and proposed a novel method of reckoning intensive factors by monitoring the support pressure [30]. Liu et al. developed a novel method involving hydraulic fracturing to generate vertical cracks to cut off a hard hanging-roof to control the deformation induced by mining the surrounding rock in roadway 5103 of Datong coal mine in China [31]. Zhao et al. first developed a multifield coupling model to investigate the distribution and evolution of stress, permeability, and gas flow field in the conventional and hard roof stopes after the protective seam mining [32]. Sun et al. developed a KZ54-type cutting bit to cut transverse grooves in the hard immediate roof and used directional hydraulic fracturing technology (DHFT) to control roof fracturing and collapse [33].
In summary, the roof control methods mainly include underground support, roof cutting, filling mining, presplitting blasting, and borehole pressure relief, but all have limitations in controlling high-position THR. Yu et al. proposed ground fracturing technology and achieved certain results in Datong Coal Mine [34–36]. Lu et al. used the secondary developed FLAC3D simulation software to determine the target layer of surface fracturing [37]. Gao et al. used the physical model and ABAQUS to research the characteristics of the overburden displacement and strata breaking strength after the vertical crack and horizontal crack existed in the high-level hard roof [38–40]. Based on the self-bearing of bulking rocks, the stability principle of the surrounding rock, and energy dissipation theories, Pan et al. obtained the criteria for determining the fracturing horizon and thickness of the HHR [41]. However, the above studies did not consider the influence of different fracturing positions on the fracture characteristics of high-position THR.
This study theoretically analyzed the mechanism of ground fracturing controlling mine earthquake induced by THR fracture first. Then, taking the 63up06 working face as the background, the FLAC numerical model was built to study the deformation and fracture law of high-position THR, the evolution laws of formation stress, and displacement under different fracturing positions. The fracturing test was carried out in the 63up06 working face, and the large-energy mine earthquake events decreased significantly after fracturing. The research results can provide some theoretical and practical guidance for mine earthquake control in working faces with similar geological conditions like Dongtan coal mine.
2. Engineering Background
2.1. Overview of Coal Mine
Dongtan coal mine is located in the border of the three cities of Zoucheng, Yanzhou, and Qufu in Shandong Province, China. As shown in Figure 1, the no. 6 mining district is located in the south wing of Dongtan minefield, the northern boundary is the security coal pillar of the industrial square, the southern boundary is adjacent to Nantun coal mine, the western boundary is adjacent to the no. 4 mining district, and the eastern boundary is the Yishan fault coal pillar. The no. 6 mining district is about 3.4 km long from east to west and 1.8~2.8 km wide from north to south and covers an area of about 6.9 km2, the ground elevation is +43.52~+55.51 m with an average of +48.43 m, and the underground elevation is -650 m~-750 m. The 63up06 working face is located in the north of the no. 6 mining district, the goaf of 63up05 in the north, and the panel 63up07 (not developed) in the south. The strike length of the 63up06 working face is 1456.3 m, the dip width is 261.0 m, the total area of the working face is 379661.0 m2, and the recoverable reserves are . The lithology of strata above the coal seam is shown in Table 1.
(a)
(b)
2.2. Statistics of Mine Earthquake Events
The 63up04, 63up05, and 63up03 working faces in the mining district have been completed. During the mining period, the mine earthquake events occurred frequently in the mining district. The statistics of high-energy mine earthquake events () occurring 500 m before mining in three panels are carried out, as shown in Figure 2; the number of high-energy mine earthquake events in 63up04, 63up05, and 63up03 working faces is 23, 36, and 16, respectively, and all the large-energy mine earthquake events occurred 100 m~00 m above the goaf behind the working face. As shown in Table 1, there is 263.4 m sandstone stratum group about 100 m above the coal seam. Therefore, it is preliminarily concluded that the occurrence of high-energy mine earthquake events may be related to the fracture of the thick and hard sandstone above the coal seam. The collapse step of this part of the rock stratum is large, and enormous elastic deformation energy is released during strata fracture, which causes the vibration of the rock stratum and further leads to the occurrence of high-energy mine earthquake events.
(a) 63up04 working face
(b) 63up05 working face
(c) 63up03 working face
2.3. Occurrence Mechanism of Mine Earthquake
As shown in Figure 3, the hanging length of the THR gradually increases with the advance of the working face; a large quantity of elastic energy is stored in the THR before breaking. When the bending tensile stress of the THR reaches the tensile strength, the THR will break, and the gravitational potential energy and elastic energy released by breakage will be transformed into impact kinetic energy, which will work on the lower rock stratum to produce impact vibration wave. The impact vibration wave will cause the disturbance of rock mass in the process of propagation in the rock stratum and then induce dynamic response phenomena such as mine earthquake. The relationship of the THR hanging length and the mining length of the coal seam is shown in Figure 4. where is the THR hanging length (m), is the mining length (m), is the vertical distance above the coal seam (m), and is the breaking angle (°).
According to the law of energy conservation, the energy released by the THR breakage mainly includes the gravitational potential energy, the elastic deformation energy, and the kinetic energy [42]. where is the total energy released (J), is the gravitational potential energy (J), is the elastic deformation energy (J), and is the kinetic energy (J). where is the lower separation space height (m), is the total mass of broken strata (kg), is the average load of the upper rock stratum (Pa), is the elastic modulus (Pa), is the thickness of the THR (m), and is the displacement of THR (m).
Then, the following can be obtained when formulas (3), (4), and (5) are substituted into
According to the above formula, it can be found that the energy released by the THR breakage is mainly related to the THR hanging length, upper load, and THR movement velocity. In general, most of the elastic energy is released in the form of thermal energy and acoustic emission, and the seismic efficiency (the ratio of seismic energy to elastic energy released in the fracture process) is only 0.26%~3.6%. At the same time, due to the layered and heterogeneous characteristics of the stratum, the energy attenuation of seismic wave in the stratum medium is a power exponential attenuation function of the propagation path distance, which is approximately expressed as follows: where is the energy after a certain distance of seismic wave propagation (J), is the spatial distance of seismic wave propagation (m), and is the attenuation constant related to formation medium.
2.4. The Mechanism of Controlling Mine Earthquake by Ground Fracturing
Dongtan coal mine is preparing to mine the 63up06 working face. In order to ensure the safe and efficient production of coal mining, it is proposed to conduct ground fracturing experiment before mining to cut off the high-level hard and thick roof to reduce the roof breaking span and weaken the rock strength, as shown in Figure 5, so as to avoid the occurrence of large-energy mine earthquake events caused by the hard roof fracture.
When the vertical fracture occurs in the fractured layer, the stress state of the rock stratum changes [43–45]. The two ends of the stratum are in the state of fixed support stress, and the stratum is cut into blocks A and B by the fracture plane, which is similar to the hinged structure. The stress analysis of the hard roof after fracturing is shown in Figure 6. where is the horizontal thrust on both sides of the rock stratum (kN), is the friction force on the fracture surface of the rock block during rotation (kN), is the internal friction angle (°), and QB are the gravity of the overlying rock stratum (kN), and and are the gravity of the rock stratum itself (kN).
(a)
(b)
Under the influence of the overlying rock stratum and self-weight, stress concentration is easy to occur around the fracture surface after fracturing in the formation, which will cause damage to the rock mass around the fracture surface and weaken the horizontal force between blocks A and B. The friction force on the fracture surface is not enough to maintain the balance between the cutting blocks. The cutting blocks will shear and slip, and the pressure will be released in advance. At the same time, the blocks on both sides of the fracture surface are more likely to break and rotate under the action of vertical force, causing A and B to break at the fixed support ends on both sides, causing the instability of the rock stratum. And the fracturing fluid is retained in the formation, which plays a certain role in hydration of the rock and weakens the strength of the rock. Therefore, under the condition of occurrence of vertical fracture plane, the stress state and occurrence form of rock stratum change, the rock stratum is more likely to rotate along the fracture surface under the action of vertical force, the breaking span of the rock stratum is reduced, and the breaking instability strength will be weakened, which can effectively reduce the energy release and grade of mine earthquake induced by the THR breakage.
3. Numerical Simulation
3.1. Numerical Model
In order to further study the strata fracture characteristics in the presence of high-position THR before and after fracturing, as shown in Figure 7, a simplified strata model was established by using FLAC3D. The model size is (), and it consists of 57000 zones and 64438 grid points. The thickness of the coal seam is 6 m, which is simplified as a horizontal coal seam. The thickness of THR is 100 m, which is 100 m away from coal seam in the vertical direction. In addition, three monitoring lines were arranged at the top, middle, and lower part of the THR to monitor the displacement and stress changes during excavation. The physical and mechanical parameters of each stratum are shown in Table 2.
The upper boundary of the model is a free boundary, and the vertical stress is applied to simulate the gravity of 360 m overlying strata. In the bottom boundary, the vertical and horizontal displacements are fixed. In the two lateral boundaries, horizontal displacement is constrained. Considering that the excavated stratum can be simplified into a plane strain model, the displacement along the -axis direction is fixed. The 100 m coal pillars are reserved on both sides to eliminate the boundary effect. The mining length of the working face is 500 m. The mining method is strike longwall mining, and the natural caving method is used to manage the roof. According to the in situ stress data, the initial in situ stress field is inversed; when the model reaches the initial equilibrium, the stress in the coal seam is , , and , which reflects the real situation of the strata.
3.2. Simulation Scheme Design
In this simulation, it is planned to first study the fracture and collapse law of THR before fracturing during coal seam excavation and then study the impact of single hydraulic fracture at different positions on the rock stratum fracture. The hydraulic fracture is a vertical fracture, the fracture runs through the whole high-position THR, and the fracturing position is located 0 m, 50 m, 100 m, 150 m, 200 m, and 250 m in front of the opening cut; the interface element in FLAC software is used to simulate hydraulic fractures, and the element has friction angle, cohesion, expansion angle, normal stiffness, tangential stiffness, tensile and shear bond strength, and other parameters, which can well simulate the process of sliding, opening, and complete breaking between crack surfaces. And the mechanical parameters at the interface element in this paper are shown in Table 3 [46, 47].
4. Simulation Results
4.1. Movement Law of High-Position THR before Fracturing
4.1.1. Plastic Zone Change
The distribution of the plastic zone can directly reflect the fracture of the upper rock stratum after coal seam excavation. The Mohr-Coulomb constitutive model adopted in this paper is an ideal elastoplastic constitutive model. When the element stress exceeds the yield strength, plastic deformation will occur. After reaching the yield stress, there are two failure modes, tensile failure (tension-) and shear failure (shear-). However, when the stress state of the rock mass changes less than the yield stress, the previous plastic deformation is retained; that is, the area is judged as the previous shear failure (shear-) and previous tension failure (tension-). Figure 8 is the plastic zone distribution map under different mining lengths. Figure 9 is the strata failure height evolution law curve. It can be seen from Figure 8 that the strata above the goaf began to destroy after mining and presented a typical “hump shape” failure mode. The failure mode above the goaf was mainly tensile failure. The stress concentration on both sides of the goaf was obvious, and tensile and shear failure occurred in this area. When the coal seam was mined to 80 m, the failure height reached 8 m and the failure range was small; when the coal seam was mined to 160 m, the failure height increased slightly to 12 m; meanwhile, the high-position THR had not been damaged because of its large thickness and high strength; when the coal seam was mined to 240 m, the failure height reached 130 m, the bottom of the THR was damaged, and a small amount of tensile failure was generated on both sides of the THR top area. Then, the THR failure height increased, and a large number of tensile shear failure areas were generated on both sides of the THR. Until the mining distance reached 340 m, the strata failure height runs through the whole THR. It can be concluded that the THR breaking span is 340 m.
(a) Mining 80 m
(b) Mining 160 m
(c) Mining 240 m
(d) Mining 340 m
As can be seen from Figure 9, the strata fracture during excavation can be roughly divided into two stages with the mining distance of 220 m as the boundary: the slow failure stage before 220 m and the rapid failure stage after 220 m. The growth rate of strata fracture height before mining 220 m was relatively slow, and the maximum failure height only reached 38 m. After mining 220 m, the growth rate of strata failure height had increased significantly. This is because in the early stage, there was the support of high-position THR, the upward propagation speed of strata failure was relatively slow, and the failure developed to the bottom and both sides of the top of the THR in the later stage; in addition, the bearing capacity decreased gradually, so the development of fracture height entered a rapid stage.
4.1.2. Vertical Displacement Change
Figure 10 shows the vertical displacement nephograms under different mining lengths, and Figure 11 is the subsidence curve at different positions of THR. It can be seen from Figure 10 that after mining, the roof above the goaf began to sink, and the maximum subsidence position occurred at the center of the roof above the goaf. With the increase in excavation length, the vertical displacement continued to increase. When the coal seam was excavated to 80 m, the maximum subsidence value reached 0.12 m, and the subsidence was small; when the coal seam was excavated to 160 m, the maximum subsidence value reached nearly 0.24 m, and the growth rate was still slow, only 0.12 m higher than that when the coal seam was excavated to 80 m; when the excavation reached 240 m, the maximum subsidence value reached 0.52 m, and the subsidence value at the bottom of the high-position THR increased. When the coal seam was excavated to 340 m, the maximum subsidence value reached 6.03 m, the calculation was no longer convergent, and the THR reached the first breaking span.
(a) Mining 80 m
(b) Mining 160 m
(c) Mining 240 m
(d) Mining 340 m
(a) Mining 80 m
(b) Mining 160 m
(c) Mining 240 m
(d) Mining 340 m
As shown in Figure 11, in the first 240 m of excavation, the subsidence rate of the THR bottom part preceded the middle and upper part. The subsidence of the rock stratum near the opening cut was higher than that far away from the opening cut; with the increase in the excavation length, the subsidence value of the upper rock stratum was gradually close to that of the lower rock stratum. When the THR reached the first breaking span, the subsidence curves of the upper and lower part near the maximum subsidence position almost overlap. Figure 12 displays the vertical displacement in the THR middle part under different mining distances. It can be seen that when the THR reached the first breaking span, the THR subsidence peak value reached 3.4 m, and the subsidence increased sharply, which once again showed the integrity and instantaneity of high-position THR when it breaks.
4.1.3. Horizontal Stress Change
During the mining process, the deformation and failure of the strata are related to the rock stress state. Therefore, the horizontal stress distribution of the strata under different excavation distances and the horizontal stress distribution at different positions of the THR were selected for analysis. Figure 13 displays the whole strata horizontal stress nephogram during excavation, and Figure 14 is the horizontal stress variation curve at different positions of THR. As can be seen from Figure 13, after mining, the horizontal stress above the goaf decreased and some tensile stress areas appeared, and the horizontal stress concentration occurred on both sides of the goaf, mainly compressive stress. When the coal seam was mined to 80 m, the maximum compressive stress appearing on the coal wall on both sides of the goaf reached 18.1 MPa, and a little tensile stress concentration occurred within 20 m above the goaf; when the coal seam was mined to 160 m, the range of the reduction of horizontal compressive stress continued to increase, and the reduction of compressive stress at the THR bottom part and both sides of the THR top part is obvious; meanwhile, the coal seam excavation had disturbed the THR to a certain extent. When the coal seam was excavated to 240 m, the horizontal compressive stress continuously transferred to the upper part of the stratum, and the maximum compressive stress appearing in the center above the goaf, close to the center of the THR top part, reached 39.6 MPa, which is caused by the loss of stress support under the THR and bearing the weight of the overlying strata. When the coal seam was excavated to 340 m, the THR reached the first breaking span, and a wide range of horizontal tensile stress areas appeared on both sides of the bottom and top of the THR.
(a) Mining 80 m
(b) Mining 160 m
(c) Mining 240 m
(d) Mining 340 m
(a) Mining 80 m
(b) Mining 160 m
(c) Mining 240 m
(d) Mining 340 m
It can be seen from Figure 14 that when the coal seam was excavated to 80 m, the horizontal stress decreased slightly at the THR bottom part, and the horizontal compressive stress increased slightly at the middle and top of the THR. When the coal seam was excavated to 160 m, the above situation became more obvious. When the coal seam was excavated to 240 m, the horizontal tensile stress appeared at the THR bottom part and both sides of the top part, which is caused by the fracture and continuous bending and subsidence at the THR bottom part. When the coal seam was excavated to 340 m, the horizontal compressive stress was further transferred upward, and part of the tensile stress also appeared in the middle of the THR. From the above analysis, it can be seen that the high-position THR fracture caused by coal seam excavation was related to the continuous upward transfer of horizontal compressive stress, resulting in the concentration of horizontal compressive stress in THR. Therefore, overcoming the concentration of horizontal compressive stress can effectively alleviate the dynamic disaster caused by the large-area overall fracture of high-position THR.
4.2. The THR Movement Laws after Fracturing
4.2.1. Vertical Displacement
Figure 15 displays the vertical displacement nephogram when the THR reaches the first breaking span under different fracturing positions, and Figure 16 shows the subsidence curve in the THR middle part under different fracturing positions. It can be seen from Figure 16 that after fracturing, the whole strata had a more sufficient falling degree after excavation than before fracturing, the THR had obvious sliding deformation at the hydraulic fractures, and the first breaking span in all fracturing schemes was reduced as shown in Table 4. It can be concluded that the THR first breaking span is roughly similar under different fracturing schemes, which was greatly reduced compared with that without fracturing, the high-position THR sinks synchronously with the lower weak rock stratum in the process of strata collapse, and there is no long-term roof hanging phenomenon in the process of excavation, so as to avoid the overall instantaneous fracture of the rock stratum, effectively reducing the release of energy.
(a) 0 m away from the opening cut
(b) 50 m away from the opening cut
(c) 100 m away from the opening cut
(d) 150 m away from the opening cut
(e) 200 m away from the opening cut
(f) 250 m away from the opening cut
(a) 0 m away from the opening cut
(b) 50 m away from the opening cut
(c) 100 m away from the opening cut
(d) 150 m away from the opening cut
(e) 200 m away from the opening cut
(f) 250 m away from the opening cut
The selection of different fracturing positions will have an impact on the THR fracture form. Therefore, the vertical displacement in the THR middle part under different fracturing schemes was selected for analysis. As shown in Figure 16, under different fracturing schemes, the THR maximum subsidence values were all close to 6 m, and there were obvious differences on both sides of the hydraulic fracture. When the fracture position was above the opening cut, the maximum subsidence value appeared at 55 m in front of the opening cut. The whole strata broke in the form of cantilever beam, and the overall range of fracture was large. When the fracture position was located 50 m in front of the opening cut, the maximum subsidence value occurred at the fracture position. There was a 50 m cantilever beam on the left side of the fracture. The subsidence value of this part of the rock stratum was small, and there will be a risk of breaking the hanging roof for a long time, which will become a potential safety hazard of dynamic disaster. The strata on the right side of the fracture broke integrally to one side of the goaf in the form of cantilever beam. When the fracture was located in 100 m and 150 m in front of the opening cut, the maximum subsidence value when the THR broke occurred near the hydraulic fracture, and the subsidence of the strata on both sides of the fracture presented an obvious symmetrical distribution form with the fracture as the center line; there was no overall cantilever breaking form, and the THR broke on both sides of the top and sank along the hydraulic crack in the middle. When the fracture was located in 200 m in front of the opening cut, the integral cantilever beam breaking form occurred on the left side of the fracture. When the fracture was located in 250 m in front of the opening cut, the THR maximum subsidence value appeared 100 m to the left of the fracture. To sum up, when the fracturing position is closed to the opening cut, the high-position THR presents an overall cantilever beam breaking form and deviates to one side of the goaf; the overall length of the cantilever beam is large, which may cause some potential safety hazards to the stope due to the influence of the gravity of the rock stratum during breakage. When the fracturing position is close to the center of the first breaking span, there is no overall cantilever breaking form in the THR, the THR breaks on both sides of the top and sinks along the hydraulic fracture in the middle, and the collapse is relatively uniform.
4.2.2. Horizontal Stress
Affected by the article length, this paper only selected the horizontal stress distribution of the THR when the fracture existed 150 m in front of the opening cut for analysis. It can be seen from Figure 17 that the horizontal stress distribution at different positions of the THR showed that the horizontal compressive stress above the goaf decreased, some tensile stress areas appeared, the compressive stress concentration appeared above the coal bodies on both sides, and the compressive stress at the lower part of the THR was significantly greater than that at the middle and upper part, which showed that when the hydraulic crack existed in the THR, the THR no longer bears the gravity of the overlying strata. During the excavation process, the horizontal compressive stress in the stratum does not transfer upward and concentrate. There is a large area of compressive stress reduction in the rock mass on both sides of the hydraulic fracture, so as to avoid the instantaneous release of energy when the THR breaks due to the high concentration of stress, which can play an effective role in the prevention and control of dynamic disasters.
(a) Monitoring line 1
(b) Monitoring line 2
(c) Monitoring line 3
5. Field Fracturing Test
Theoretical analysis and numerical simulation results have proven the feasibility of ground fracturing to control mine earthquake. The field fracturing test was carried out in the 63up06 working face in the no. 6 mining district of Dongtan coal mine. As shown in Figure 18(a), three fracturing wells were arranged along the strike direction of the working face. The microseismic event after fracturing shows that in the direction perpendicular to the formation, the hydraulic fracture has completely cut the THR 100 m above the working face. The expansion direction of the main fracture is shown by the red dotted line in Figure 18(a), which is consistent with the direction of the maximum principal stress. Considering that the THR near the 63up05 goaf above the working face has been fractured, it can be considered that this fracturing test has fully fractured the high-position THR.
(a)
(b)
During the first 400 m of the 63up06 working face mining, 702 microseismic events occurred in total, including 12 high-energy vibration events which include 4 mine earthquakes of magnitude 2.0 and 8 mine earthquakes of magnitude 1.0-2.0, while 35 mine earthquakes of magnitude 2.0 or above occurred in the first 400 m of the 63up05 working face. Although there are still mine earthquake events in the fractured strata after mining, the frequency of high-energy events was significantly reduced compared with that of the adjacent mined working face. Figure 19 is the specific location diagram of the high-energy mine earthquake event in the working face. The mining earthquake events are mainly concentrated in the rear of the working face and near the opening cut, and there was no high-energy mining earthquake event in front of the working face. Therefore, the engineering field experiment shows that the ground fracturing can effectively prevent the occurrence of high-energy vibration events in mines and ensure the efficient and safe production of coal mining enterprises.
6. Conclusion
In this study, the ground fracturing technology was proposed to cut the high-position THR in advance to control and prevent mine earthquake. Taking the 63up06 working face as the engineering background, the movement law of high-position THR before and after fracturing was studied by using FLAC numerical simulation. Then, the fracturing test was carried out in the 63up06 working face. Several conclusions were obtained as follows. (1)According to the FLAC simulation, the tensile failure runs through the whole THR and generated an integral fracture; when THR reaches the first breaking span, the THR bottom subsidence preceded the middle and upper parts, and the THR subsidence value closed to the opening cut was higher than that far away from the opening cut. The THR fracture is related to the continuous upward transfer of horizontal compressive stress, which results in the concentration of horizontal compressive stress in THR. Therefore, overcoming the concentration of horizontal compressive stress can effectively alleviate the dynamic disaster caused by the large-area overall breaking of the thick rock stratum(2)After fracturing, the first breaking span of THR was obviously reduced, and different fracturing positions have a great impact on the THR fracture form. When the fracturing position is closed to the opening cut, the THR presents an overall cantilever beam breaking form and deviates to one side of the goaf, which may cause some potential safety hazards. When the fracturing position is close to the center of the first breaking span, the THR breaks on both sides of the top and sinks along the hydraulic fracture in the middle, and the collapse degree is relatively uniform(3)During the first 400 m of the 63up06 working face mining after fracturing, 702 microseismic events occurred in total, including 12 high-energy vibration events which include 4 mine earthquakes of magnitude 2.0 and 8 mine earthquakes of magnitude 1.0-2.0. Compared with the adjacent working face, the quantity of high-energy mine earthquake events in the 63up06 working face has been significantly controlled. The mining earthquake events are mainly concentrated in the rear of the working face and near the opening cut, and there was no high-energy mining earthquake event in front of the working face
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there is no conflict of interest.
Acknowledgments
The research described in this paper was financially supported by the Key Research and Development Project of Shandong Province in China (2019SDZY02) and the National Natural Science Foundation Project of China (U1762216, 51974353, and 51991362).