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Published: 17 January 2025
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International Journal of Coal Science & Technology Volume 12, article number 4, (2025)
1.
School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo, China
2.
School of Civil Engineering and Communication, North China University of Water Resources and Electric Power, Zhengzhou, China
3.
Institute of Mechanics, Chinese Academy of Sciences, Beijing, China
To reveal the rock burst mechanism, the stress and failure characteristics of coal-rock strata under different advancing speeds of mining working face were explored by theoretical analysis, simulation, and engineering monitoring. The relationship between energy accumulation and release was analyzed, and a reasonable mining speed according to specific projects was recommended. The theoretical analysis shows that as the mining speed increases from 4 to 15 m/d, the rheological coefficient of coal mass ranges from 0.9 to 0.4, and the elastic energy of coal mass accumulation varies from 100 to 900 kJ. Based on the simulation, there is a critical advancing speed, the iteration numbers of simulation are less than 15,000 per mining 10 m coal seam, the overburden structure is obvious, the abutment pressure in coal mass is large, and the accumulated energy is large, which is easy to cause strong rock burst. When the iteration number is greater than 15,000, the static force of coal mass increases slightly, but there is no obvious rock burst. Based on engineering monitoring, the mining speed of a mine is less than 8 m/d, and the periodic weighting distance is about 17 m; as the mining speed is greater than 10 m/d, and the periodic weighting distance is greater than 20 m; as the mining speed is 3–8 m/d, and the range of high stress in surrounding rock is 48 m; as the advancing speed is 8–12 m/d, and the high-stress range in surrounding rock is 80 m. Moreover, as the mining speed is less than 8 cut cycles, the micro seismic energy is less than 10,000 J; as the mining speed is 12 cut cycles, the micro-seismic energy is about 20,000 J. In summary, the advancing speed is positively correlated with the micro seismic event; as the mining speed increases, the accumulated elastic energy of surrounding rock is greater, which is easy to cause rock burst. The comprehensive analysis indicates the daily advance speed of the mine is not more than 12 cut cycles.
In recent years, rock burst has become one of the major hazards facing coal mining in China, gradually expanded from deep mines in the Middle East to northwest regions such as northern Shaanxi, Shenfu, Ningxia, and Xinjiang (Dou et al. 2021, 2022; Qi et al. 2019; Jiang et al. 2022; Pan et al. 2020; Luo et al. 2023). The mining conditions in northwestern China are generally favorable, with abundant reserves and large production capacity. This region contains several coal bases with billions of tons of coal, playing a crucial role in ensuring energy supply and fulfilling important tasks in guaranteeing energy security. As a result, mining operations in northwestern China are characterized by high extraction intensity and fast face advancement. It is necessary to in-depth study the relationship between the mining speed and the rock burst risk, and scientifically determine the working face advancing speed according to the specific conditions.
Rock bursts are caused by the sudden release of elastic deformation energy accumulated in the coal-rock mass of the mining face (National Coal Mine Safety Administration 2013, 2018). Therefore, under the same geological conditions, the faster the mining speed, the more elastic energy released by coal and rock mass per unit time, and the greater the risk of rock burst. By drawing lessons from rock burst accidents, the industry regulatory authorities have even published some mandatory restrictions on mine production and working face advancing speed. For example, the production scale of rock burst mines should not exceed 8 million t/a, furthermore, there should be no increase in production capacity (National Coal Mine Safety Administration, 2019). In the mine with a serious rock burst, ' One mine, two faces, three blades', that is, the number of coal mining faces in the mine with serious rock burst is controlled within 2 per mine, and the advancing speed of coal mining face is strictly controlled within 3 cut/day (General Office of Shandong Provincial People's Government 2019).
The time effect of working face advancing speed is an important factor affecting the elastic energy release of the surrounding rock, and energy release is the root cause of a dynamic phenomenon. Currently, the study of the correlation between mining speed and mining pressure, stress distribution, and energy accumulation and release has received significant attention from many scholars. This research aims to guide for the prevention and control of rock bursts. Xie et al. (2007) taking the Xieqiao Mine as the background, a study was conducted on the surrounding rock stress, displacement, and damage under different advancing speeds of the working face, and found that the range of ahead low-stress zone and the surrounding coal-rock mass damage zone decreases as the advancing speed increases. Wang et al. (2010) conducted numerical simulations and field monitoring to obtain the conclusion that with the advance acceleration of the fully-mechanized mining face, the loading speed of the surrounding rock in the goaf increases, and the strength and brittleness of the coal body also increase, the stress peak position is closer to coal wall. Wang et al. (2012) conducted a study on the influence of high-speed advance on the periodic weighting of Huojitu mining by field measurements and theoretical analysis, and found the length of the continuous periodic weighting significantly increases under high-speed advance conditions (> 10 m/d). Liu et al. (2019) carried out the long-term monitoring and data analysis of the No.10 Mine in Pingdingshan, and found that reducing the mining speed to about 0.5 m/d in the fractured area can ensure the integrity of rock structure, the safety production in the roof separation can be guaranteed when the mining speed is controlled in 0.8 - 1.1 m/d. Xu et al. (2019) analyzed the influence of mining velocity on the overlying strata movement, surrounding rock failure, stress distribution and loading features of Guqiao coal mine, and found that the development range of plastic zone in the overlying strata decreased as the mining speed of working face increased, the stress concentration position of coal wall became shallower, and the stress concentration factor became smaller. Li et al. (2022) studied the effect of the mining rate in Xiaojihan coal mine by the microseismic (MS) monitoring and simulation, and found that as the advancing speed increase, the deformation time of the surrounding rock mass is shortened, the number of MS events and the stress redistribution zones was unstable, and dynamic geological disasters could occur. Wang et al. (2023) analyzed the variation characteristics of microseismic monitoring events under different advance speeds of Kouzidong Mine, and found the high-level microseismic events increase as the propulsion speed, and the high-level events transferring to the goaf in the second square stage.
After coal mining, the roof subsidence, stress redistribution of surrounding rock, strata failure, and goaf compaction all require a certain amount of time to complete. The field monitoring results indicate that the pressure duration of the stope is 0.5 - 3.0 d. Under the fully mechanized mining technology, the interval time between coal caving and roof shifting is the compression deformation time of coal wall and newly exposed roof, and this time difference at different advance speeds is large. The above time interval corresponds exactly to the creep rate of coal-rock material. The strain rate required for stable creep of coal-rock materials is generally 10–10 to 10−6 s-1, and the maximum magnitude of 10−4 s-1 (Cai et al. 2002; Zhang et al. 2008). The creep time of common coal, sandstone, and mudstone materials is generally 0.1 ~ 100.0 h, while the rheological coefficient of coal can reach 0.3 ~ 0.4, and the rheological coefficient of rocks is 0.6 to 0.8 (Chen et al. 2009; Wu et al. 1996; Gao et al. 2007). The rheological characteristics of coal and surrounding rock during mining are the main factors that cause the change of surrounding rock stress, support pressure and strata movement. Yang et al. (2016) analyzed the distribution characteristics of deformation damage and strain energy of surrounding rock, and found with the increase of the advancing speed, the loading rate of coal maximum main stress and the unloading rate of minimum main stress in coal wall front are increased, coal strain energy density increases, which leads to the increase of the probability and degree of dynamic disasters. Zhao et al. (2018) studied the energy accumulation and release law of the roof under the advancing speed effect, and found that the advancing speed and the release energy of the roof increase exponentially, and the safe advancing speed is given according to the specific conditions. Feng et al. (2019) analyzed the influence mechanism of advancing speed on the release energy of hard roof breaking. The larger advancing speed increases the cantilever length and peak stress concentration factor of the roof, and the distance between the peak and coal wall increases, which leads to the increase of bending deformation energy of the roof and the increase of elastic energy release. Furthermore, taking Hujiahe Mine as an example, the critical mining speed of a hard roof is calculated to be 4 m/d. Tan et al. (2019) deduced the calculation method of kinetic energy in coal mass during mining, the relationship between advancing speed and kinetic energy is analyzed, the kinetic energy evaluation index of burst risk is put forward, and the requirement advance speed of safe mining is given according to the mining speed and kinetic energy magnitude.
Due to the complexity of the rock burst mechanism, the positive correlation between mining speed and energy accumulation and release is relatively clear. However, for the specific engineering conditions, how to determine the mining speed influence on the stress and failure characteristics of coal and roof is still unclear by the scientific and reasonable method. For the super-large mines in Shaanxi and Mongolia mining areas, it is necessary to coordinate the double contradiction between high-intensity mining and rock burst control. Because the different mining speeds influence the deformation and failure of coal-rock strata by the rheological effect, which will change the energy accumulation and release of coal-rock mass. Therefore, based on the deformation and failure mechanism of coal-rock mass, this paper attempts to investigate the time effect of elastic energy release of the surrounding rock, and gives a reasonable evaluation of the advancing speed by combining simulation, engineering conditions, and field monitoring data.
Under the mining disturbance, the abutment stress in the working face front is increased, which makes the energy accumulation of coal mass increase, as shown in Fig 1. Different advancing speeds will affect the stress distribution, and then affect the elastic energy accumulation. In this section, the relationship between advancing speed and energy accumulation-release of elastic energy is theoretically deduced.
The stress distribution diagram of coal mass in the working face front
According to the equilibrium conditions of the coal mass of working face, it can be seen that:
where σt is the tangential stress of coal mass, σr is the radial stress of coal mass, φ is the internal friction angle of coal mass, and c is the cohesion of coal mass.
From Eq. (1), when the radial stress is 0, the coal mass is subjected to unidirectional compression, and the maximum value of unidirectional compression stress in coal mass is:
where σc0 is the compressive strength of coal mass.
Due to the rheology characteristics, different advancing speeds affect the mechanical properties of coal mass. The relationship between rheological coefficient and advancing velocity can be expressed by the following Eq. (3):
where \(v\) represents the advancing speed, \(r_{1}\) is the rheological coefficient of coal.
The uniaxial compressive strength of the rheological coal is shown in Eq. (4).
Considering the rheology characteristics, Eq. (1) can be written as:
Based on the Kastner Formulae, the stress distribution law and the plastic zone radius of coal mass are following (Wang & Yu 2021):
where \(\gamma\) is bulk density, \(H\) is the distance from the surface, \(p_{i}\) is the support stress, \(L_{1}\) is the distance between the roadway center and side, and \(L\) is the length from the roadway center.
The energy density at any point away from the roadway center is shown in Eq. (8):
where E is the elastic modulus of the coal.
The coal in front of the working face can be divided into a plastic zone and an elastic zone. Assuming that the elastic energy of plastic zone coal has been completely released, only the elastic energy of elastic zone coal is considered. When the working face advances, the released elastic energy of coal seam is shown in Eq. (9):
where \(h_{1}\) is the thickness of coal seam.
For a specific engineering condition, the rheological coefficient ranges from 0.4 to 0.9, which is corresponding to 4–15 m/d of advance speed. The rheological coefficient of coal mass is as follow:
As the distance between the roadway center and side is \(L_{1} = 3 {\text{m}}\); the overburden pressure is \(\gamma H = 5 {\text{MPa}};\) the cohesive strength of coal mass is \(c = 1.2 {\text{MPa}}\); the internal friction angle of coal mass is \(\varphi = 28^{^\circ }\); the elastic modulus of coal mass is \(E = 1.2 {\text{GPa}}\); the support stress is \(p_{i} = 0\), the coal seam thickness is \(h_{1} =\) 3 m. Combining Eq. (8), the relationship between the advancing speed and the energy density is shown in Fig. 2:
Energy density distribution of coal under different advance speeds
From Eq. (9), under the different advance speeds, the accumulated elastic energy of coal mass can be calculated, as shown in Fig. 3:
The relationship between coal release energy and advance speed
From the above analysis, different advancing speeds correspond to different rheological coefficients of coal mass, and the advancing speed is inversely proportional to the rheological coefficient of coal mass. That is, the faster the advancing speed, the smaller the rheological coefficient of coal mass, the greater the abutment pressure of coal seam, the smaller the plastic failure area of coal mass, the greater the elastic energy of coal mass accumulation, and the greater the possibility of rock burst disaster in working face.
A mine located in the deep mining area of Dongsheng coalfield, and the coal seam of the Yanan Formation in the Middle Jurassic period of the Jurassic System is mined. The coal seam of 21201 working face has a moderate burst tendency, and the mining depth is about 630 m. The cutting-off length of the working face is 300 m, the advancing length is 3900 m, the average dip angle of coal seam is 30, and the geological structure is simple. The working face is arranged with three entries, with one side being goaf and the other side being solid coal. The 21201 working face is arranged by 172 of ZY10000/16/32 hydraulic supports, the support number is 1 # ~ 172 # from the transportation laneway to the return laneway. The resistances of 81 # and 87 # hydraulic supports are monitored, as shown in Fig 4.
Layout diagram of mining working face
Based on the exposure status of the two entries of the working face during the excavation, the coal thickness of the working face is 1.9 - 3.0 m, and the average coal thickness is 2.7 m. The pseudo-roof is a thin layer of sandy mudstone and mudstone with a thickness of 0.2 - 0.5 m, locally sandwiched with siltstone and coal line. The immediate roof is siltstone and argillaceous sandstone with a thickness of 2.53 - 7.80 m, containing a large number of plant fossil fragments. The main roof is fine sandstone and medium sandstone, with a thickness of 13.8 - 23.3 m, mainly composed of quartz with high strength. It is identified that the 2–1 coal seam has a strong burst tendency, and the roof and floor rock formations exhibit a weak burst tendency.
The 21201 working face adopts a fully mechanized mining technology. The shearer drum width is 0.8 m and the diameter is 1.8 m. During three months, the production schedule of the 21201working face and the resistance change of 81 # and 87 # hydraulic supports are shown in Fig 5.
Statistics of production schedule and support resistance during 3 months
From Fig. 5a, the average advance speed of working face is 8.5 m per day. Affected by the production conditions, the daily advancing speed is not uniform, with only 4 to 5 m at the lowest and exceeding 12 m at the fastest. Combined with Figs. 5b and 5c, as the average advancing speed of the working face was 5.7 m/d during June 2 to 15, the periodic weighting distance of this stage was 17.1 m; as the advancing speed was 7.5 m/d during July 11 to 18, the periodic weighting distance was 17.5 m. Moreover, as the average advancing speed was 10.4 m/d during June 17 to 25, the periodic weighting distance was 20.7 m; the average advancing speed is 10.4 m/d during August 2 to 6, the periodic weighting distance was 21.1 m; the average advancing speed was 10.5 m/d during July 4 to 9, the periodic weighting distance was 25.4 m.
Generally, the advancing speed of the working face is larger, and the periodic weighting distance is larger, which causes a greater mine pressure appearance. Concretely, as the average advancing speed is less than 8 m/d, and the dynamic load factor of working face is about 1.06; when the advancing speed is greater than 10 m/d, the dynamic load factor is about 1.09.
To study the stress change of surrounding rock under different advancing speeds of working face, 86 # and 89 # stress meters were installed at 948 m and 1040 m from the open-off position in the small coal pillar side. Combined with the production schedule, the stress changes of the surrounding rock monitored by two stress meters are shown in Fig 6. As the working face was advanced to about 900 m away from the open-off cut on June 5, the surrounding rock stress measured by the 86 # stress meter began to increase significantly. When the working face advanced to about 920 m away from the open-off cut on June 12, the surrounding rock stress measured by the 86 # stress meter increased to the maximum value. As the working face advanced to about 960 m away from the open-off cut on June 17, the surrounding rock stress measured by the 89 # stress meter began to increase obviously. When the working face advanced to about 1030 m away from the open-off cut on June 23, the surrounding rock stress measured by the 89 # stress meter increased to the maximum value.
Relationship between advancing speed and surrounding rock stress
Combined with the advancing speed, as the average daily advancing speed of the working face is 3 to 8 m during June 5 to 12, the surrounding rock stress in the ahead 48 m range of working face is in creased. However, the surrounding rock stress in the 80 m range of the advanced working face is increased as the average daily advancing speed is 8 to 12 m during June 17 to 23. Generally, as the average daily advancing speed of the working face is larger, the area where the surrounding rock experiences abutment stress is also larger, therefore, the elastic energy gathered in the surrounding rock is greater, and the rock burst phenomenon caused is more serious.
To study the correlation between microseismical events and advancing speed, the microseismical monitoring data of three consecutive months in June, July, and August were counted, as shown in Fig 7. From Fig 7a, the number of microseismical events and the number of microseismical events with larger energy increase significantly with the increase of advancing speed. From Fig 7b, there is a significant positive correlation between the advancing speed and the microseismical energy. In June, most days of the advancing speed in the 8 knife below, this stage of microseismical energy is less than 10,000 J; when the daily advancing speed is smaller than 12 cut/day, the microseismical energy is below 20,000 J; however, as the daily advance degree is more than 12 cut/day, the microseismical energy is mostly more than 20,000 J. In July, the microseismical energy is below 20,000 J. As the daily advance speed increases, the microseismical energy shows a relatively slow increase. In August, the advancing speed is below 10 cut/day, and the microseismical energy is less than 20,000 J; however, when the advance speed is greater than 12 cut/day, the microseismical energy is as high as 40,000 J. This shows that the number of advance cut per day is an important factor affecting the microseismical energy.
Statistics of microseismical event
From the above analysis, the microseismical activity is stronger after the daily advance speed exceeds 10 cut/day, but it is within the weak risk range (10,000 ~ 100,000 J). The maximum daily advancing speed of the working face determined in the rock burst protection design is 12 cut/day, which meets the production conditions of the 21201 working face. Therefore, it is suggested that when the advancing speed of the 21201 working face exceeds 12 cut/day, the mine pressure and microseismical data should be analyzed in detail to ensure that the energy index of microseismical activity is within the range of burst hazard index.
Due to the complexity of the rock burst mechanism, simulation is a suitable method to study the influence of mining speed on the rock burst mechanism. Continuous-discontinuous element method (CDEM) is an explicit numerical method coupled with finite element and discrete element. This method has been successfully applied to geotechnical, military, mining, and other projects (Zhang et al. 2019, 2020 and 2023). In this paper, CDEM is used to conducted the numerical analysis on the force, deformation, and failure characteristics of coal and roof under different advancing speeds.
According to the 225-HK25 borehole of 21201 working faces, a numerical model of strata with a length of 300 m is established, as shown in Fig 8.
20201 working face numerical model
From the model bottom to top, the strata respectively is 10m thick sandy mudstone (layer 1), 2.7m thick coal seam (layer 2), 4 m thick sandy mudstone (layer 3), 13 m thick siltstone (layer 4), 27 m thick sandy mudstone (layer 5), 20 m thick fine-grained sandstone (layer 6), 20 m thick siltstone (layer 7) and 21 m thick sandy mudstone (layer 8). In the practical engineering of this paper, the overlying strata thickness above the model is 542.7 m, and the average volume force of the above strata is 27 kN / m3. To improve the computational efficiency, a uniform load of 14.65 MPa is applied to the model upper to simulate the self-weight of the overlying strata with 542.7 m thickness. The four sides and bottom of the model are fixed constraints. In simulation, the mechanical properties of coal-rock layers are shown in Table 1.
Layer number | Rock type | Density (kg/m3) | Modulus of elasticity (GPa) | Poisson ratio μ | Cohesive force (MPa) | Tensile strength (MPa) | Internal friction angle φ (°) |
|---|---|---|---|---|---|---|---|
1 | Sandy mudstone | 2410 | 18.0 | 0.1625 | 5.60 | 1.53 | 32.0 |
2 | Coal seam | 1269 | 3.50 | 0.1950 | 8.10 | 0.92 | 26.0 |
3 | Sandy mudstone | 2410 | 18.0 | 0.1625 | 5.60 | 1.53 | 32.0 |
4 | Siltstone | 2520 | 30.7 | 0.1250 | 8.70 | 3.03 | 32.0 |
5 | Sandy mudstone | 2410 | 18.0 | 0.1625 | 5.60 | 1.53 | 32.0 |
6 | Fine-grained sandstone | 2520 | 30.7 | 0.1250 | 1.01 | 3.03 | 31.2 |
7 | Siltstone | 2300 | 18.0 | 0.1250 | 1.01 | 1.53 | 32.0 |
8 | Sandy mudstone | 2410 | 18.0 | 0.1625 | 5.60 | 1.53 | 31.2 |
To study the influences of the advancing speed on mine pressure behavior, the excavation process of working face advancing 60 m is simulated. CDEM uses a dynamic explicit calculation method, the real time of the mining process can be characterized by the product of the iteration number and critical time step. For the excavation process of each 10 m coal seam, the numerical iterations of 5000, 10,000, 15,000, 20,000, 25,000, 35,000, 45,000, and 60,000 steps are carried out, which can characterize the different advance speeds at some extent. To study more efficiently the influence of advancing speed on rock burst, this paper skips the initial mining stage of working face, and directly carries out the simulation analysis of coal excavating from the model left side.
The simulation results show that when the working face advances to 60 m, the coal seam produces obvious displacement and deformation near the coal wall. In this stage, the displacement, deformation, strain, stress and energy characteristics in coal seam near the mining working face are analyzed to reveal the rock burst mechanism.
At a critical advance speed of mining face, the deformation of coal mass is shown in Fig. 9. From Fig. 9a, when the working face is advanced to 60 m, the vertical compression deformation of coal mass near the coal wall bottom is the largest, and the maximum compression strain of coal mass reaches 0.4007. From Fig. 9b, the coal mass of the coal wall bottom also produces the maximum horizontal strain, and the horizontal expansion strain is 0.4529.
Strain contour of coal seam
To quantitatively characterize the effect of advancing speed on the coal deformation, the maximum strain of coal seam under different mining speeds is counted, as shown in Fig 10. From Fig 10a, with the increase of iteration number in each excavation stage, the maximum compression strain of coal presents a nonlinear growth law of quadratic function, and the fitting coefficient R2=0.9886. From Fig 10b, with the increase of the simulation iteration number in each excavation stage, the maximum horizontal expansion strain of coal also shows the nonlinear growth law of quadratic function, and the fitting coefficient R2 = 0.989.
Maximum strain statistics of coal seam
According to the mechanical characteristics of coal materials, the limit plastic strain of coal mass is smaller than 4% (Song et al. 2023). From Fig. 10, as the step number of numerical iteration is larger than 15,000 during the excavation process of each 10 m coal seam, the largest strain of coal mass is larger than 4%, which indicates that the coal mass has undergone significant deformation and failure, the accumulated elastic energy of coal mass is smaller. On the other hand, as the step number of numerical iteration is smaller than 15,000 during the excavation process of each 10 m coal seam, the advancing speed of working face is faster, and the rock burst phenomenon is prone to occur.
To characterize intuitively the coal seam failure, at a critical advance speed, the coal migration near the working face is shown in Fig. 11. From Fig. 11a, when the working face advances to 60 m, the coal seam top near the coal wall is in the compression state, and the maximum vertical displacement reaches 0.49 m. From Fig. 11b, the coal wall bottom is in a tensile state, and the maximum horizontal displacement reaches 0.61 m.
Displacement contour of coal seam
To quantitatively characterize the influence of advancing speeds on the coal migration, the maximum displacement of coal seam under different mining speeds is counted, as shown in Fig. 12.
Displacement statistics of coal seam
From Fig. 12a, with the increase of simulation iteration steps in each excavation stage, the maximum compressive displacement of coal presents a nonlinear growth law of quadratic function, and the fitting coefficient R2 = 0.977. From Fig. 12b, with the increase of simulation iteration number in each excavation stage, the horizontal expansion displacement of coal also shows the nonlinear growth law of quadratic function, and the fitting coefficient R2 = 0.9528.
To further explore the influence of advance speed on mine pressure, the vertical abutment stress of coal in a certain ahead range of the working face is compared and analyzed, as shown in Fig 13.
Abutment stress contour of coal seam under different advance speeds
From Fig. 13a, when the simulation iteration number of each 10 m coal seam excavation is 5000 steps, the maximum abutment stress inside coal seam is 6.956 × 107 Pa, and the coal position with maximum stress is close to the coal wall. From Fig. 13b, when the simulation iteration number of each 10 m coal seam excavation is 10,000 steps, the maximum abutment stress inside coal seam is 6.504 × 107 Pa, and the maximum stress position is relatively far away from the coal wall. From Fig. 13c, when the simulation iteration number of each 10 m coal seam excavation is 15,000 steps, the two positions of coal seam are in an obviously high stress state, and the maximum abutment stress is 5.665 × 107 Pa. From Fig. 13d, when the simulation iteration number of each 10 m coal seam excavation is 20,000 steps, the range of coal seam in an obviously high stress state is further far away from the coal wall, however, the maximum abutment stress is 5.754 × 107 Pa. Furthermore, from Fig. 13e, when the simulation iteration number of each 10 m coal seam excavation is 60,000 steps, the coal rock mass is fully compacted, the maximum stress position of coal seam is further transferred to the depth, and the maximum abutment stress is 5.817 × 107 Pa.
To quantitatively analyze the influence of the advancing speed on the abutment stress of coal seam, the maximum vertical abutment stresses under 8 kinds of simulated iteration steps are counted and compared, as shown in Fig 14. From Fig 14a, when each 10 m coal seam is mined, the envelope range of the coal abutment stress curve under 5000 iteration steps is obviously larger than that of 10,000 iteration steps is obviously larger those of 15,000 iteration steps. However, the envelope range of the coal bearing stress curve is relatively close under the simulated iteration steps of 15,000, 20,000, 25,000, 35,000, 45,000, and 60,000. More concretely, from Fig 14b, for every 10 m coal seam mined, when the simulated iteration number increases from 5000 to 15,000 steps, the maximum abutment stress of coal seam decreases significantly from 7×107 to 5.7×107 Pa. However, the maximum abutment stress of coal seam increases slightly from 5.7×107 to 5.817×107 Pa when the iteration number increases from 15,000 to 60,000 steps.
Advanced stress statistics of coal seam
Fig. 14 indicates that as the envelope range and peak of the abutment stress curve are larger, the elastic energy gathered inside the coal seam is greater, which is more easily to cause the rock burst phenomenon. Thus, the phenomenon of rock burst is easy to occur when the simulation iteration number is less than 15,000 steps per excavation 10 m coal seam. When the simulation iteration number per excavation 10 m coal seam is greater than 15,000 steps, the rock burst phenomenon may be prevented.
To explore comprehensively the influence of mining speed on rock burst, the migration and damage characteristics of coal-rock structure under different advancing speeds are shown in Fig 15. From Fig 15a, when the iteration steps per 10 m coal seam excavation is 5000, the overlying rock structure in the 60 m goaf is in the overlapping rock beam structure as a whole. When the iteration steps per 10 m coal seam excavation is 10,000, the overlapping rock beam structure of 4 m thick sandy mudstone in the goaf is in the dynamic failure process. When the iteration steps per 10 m coal seam excavation exceeds 15,000, the rock beam structure of 4m thick sandy mudstone is in a stable state again. That is, as the advance speed is larger, the beam structure size of overlapping strata is larger, which is prone to cause larger energy accumulation. From Fig 15b, as the advancing speed of the working face slows down, the area where the overburden structure above the coal seam is dynamically damaged in the current state gradually decreases. Concretely, the iteration steps per 10 m coal seam excavation is larger than 15,000 iteration steps per 10 m, there is no new damage to the overlying rock structure above the coal seam at the current stage. That is, as the advance speed is larger, the current dynamic failure degree of overlapping strata is larger, which is prone to cause the larger possibility of rock burst occurrence.
Migration and damage of overburden strata
To further explore the rock burst mechanism, the kinetic energy of coal-rock structure in the stope is counted through secondary development of simulation. The kinetic energy variation of coal-rock structure under different advancing speeds is shown in Fig 16.
Kinetic energy statistic of overlying strata
From Fig 16, in each stage of 10 m coal seam excavation, the kinetic energy of coal-rock structure decreases rapidly and then slowly with the increase of the iteration number, and the fitting coefficient R2 = 0.9987. As the simulation time of coal mining per 10 m is greater than the iteration step of 15,000, the kinetic energy of the stope system maintains a low value, that is, the possibility of rock burst is small.
In summary, the reference (Chen et al. 2009) shows that coal materials have significant time creep characteristics. As the iteration steps per 10 m coal seam excavation is larger, the mining speed is slower, and coal mass near the working face needs to undergo a long time of mining stress, which has produced significant migration and plastic deformation, as shown in Figs. 10 and 12, the accumulated elastic energy of coal mass is smaller, therefore, the possibility of rock burst is small.
As shown in Figs. 15 and 16, when the advancing speed is smaller than 15,000 iteration steps per 10 m coal excavation, the faster mining speed makes the overburden structure show more integrity, which is in the process of dynamic failure under mining disturbance, the ahead stress of coal seam is larger, and the energy generated in overburden structure is larger, which is easy to cause strong rock burst. When the advancing speed is larger than 15,000 iteration steps per 10 m coal excavation, the slower mining speed makes the overburden structure in a stable stress equilibrium state after full migration in each excavation. The slow and sufficient evolution process of the overburden structure can effectively reduce the occurrence and intensity of rock burst.
From Figs. 13 and 14, when the iteration steps per 10 m coal excavation is less than 15,000, the mining speed is fast, the force fulcrum formed by the large overburden rock structure is close to the coal wall, and the coal mass at this force fulcrum position is subjected to a large mining force, which in turn accumulates a large amount of energy to cause a strong rock burst in the roadway in the working face front. As the iteration steps per 10 m coal excavation is larger than 15,000, the mining speed is slow, and the acting point number of the large overburden rock structure in coal seam increases, that is, the deep coal wall far away from the working face is uniformly loaded, so the intensity degree of rock burst is weakened.
In this section, the research results are further discussed to reveal the influence of advance speed on the rock burst mechanism.
In this study, the theoretical analysis is firstly carried out, and the formula expression of advance speed and coal elastic energy is deduced, which indicates that the advance speed increases from 4m/d to 15 m/d, and the elastic energy of coal mass decreases from 900 kJ to 100 kJ. Theoretical analysis shows that as the advance speed is faster, the rock burst is more prone to occur.
Based on this, the study then carries out on-site monitoring, and the relationships between the advancing speed, pressure appearance and acoustic emission events are analyzed. The monitoring analysis shows that as the advancing speed is faster, the periodic weighting distance is larger, the mine pressure phenomenon becomes more obvious, and the energy released by the acoustic emission monitor is greater. On-site monitoring also shows that the advancing speed is faster, it is more likely to cause rock burst disasters.
Further, this paper carried out the simulation, and the relationships between the advancing speed, coal mass stress, failure characteristics, and energy accumulation are discussed. It is concluded that the advancing speed is faster, the complete structure size of overlying strata is larger, the energy released by the dynamic failure of the strata structure is greater, and it is more likely to cause rock burst disasters.
Generally, theoretical analysis, field monitoring, and numerical simulation all indicate that the advance speed is faster, it is more likely to cause rock burst disasters. Numerical simulation also shows that there is a critical value, as the advance speed is greater than this critical speed, rock burst disasters are easy to occur. However, the final determination of the critical advance speed needs to be combined with specific engineering conditions and on-site monitoring, which can provide a theoretical basis for the formulation of local safety regulations.
To reveal the rock burst mechanism, the stress and failure characteristics of the coal-rock structure under different advancing speeds were investigated by theoretical analysis, engineering monitoring, and simulation in this paper. The following conclusions can be drawn:
(1) Theoretical analysis indicates that the advancing speed is inversely proportional to the rheological coefficient of coal mass, which in turn affects the elastic energy accumulation of coal mass, the larger advancing speed may cause the rock burst disaster. As the advance speed increases from 4 m/day to 15 m/day, the rheological coefficient of coal mass ranges from 0.9 to 0.4, and the elastic energy of coal mass accumulation varies from 100 to 900 kJ.
(2) The result analysis of the project monitoring indicates that as the average daily advancing speed of the working face is less than 8 m/d, and the periodic weighting distance is about 17 m. As the average daily advancing speed is larger than 10 m/d, and the periodic weighting step distance is greater than 20 m. Moreover, there is also a significant positive correlation between the advancing speed and the microseismical energy events in coal seam mining process of the working face.
(3) The simulation results also indicate that as the iteration number of simulation is less than 15000 for every 10 m coal seam mined, the structure characteristic of overlying strata is more obvious, the abutment stress in the coal seam is larger, and the energy accumulation is greater, which is easy to cause strong rock burst disaster. When the simulation iteration number is above 20,000 steps, the kinetic energy of coal-rock mass is smaller, and there is no obvious rock burst phenomenon although the static force of coal mass increases slightly.
(4) Theoretical analysis, numerical simulation, and engineering monitoring all indicate that as the advance speed of working face is faster, the rise area of the advance stress of working face is larger, the elastic energy accumulated in the surrounding rock is greater, so the rock burst phenomenon is more serious. There exists a reasonable range and boundary for the advancing speed of the working face, which can effectively reduce the risk of rock burst during coal mining. Combined with the design specification of burst protection, the maximum daily advancing speed of the working face in the project of this study should not exceed 12 cut.
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14 August 2023
09 November 2023
11 May 2024
November -0001
https://doi.org/10.1007/s40789-024-00743-4
Springer
