The fault-slip type of rock burst is a major threat to the safety of coal mining, and effectively recognizing its signals patterns is the foundation for the early warning and prevention. At first, a mechanical model of the fault-slip was established and the mechanism of the rock burst induced by the fault-slip was revealed. Then, the patterns of the electromagnetic radiation, acoustic emission (AE), and microseismic signals in the fault-slip type of rock burst were proposed, in that before the rock burst occurs, the electromagnetic radiation intensity near the sliding surface increases rapidly, the AE energy rises exponentially, and the energy released by microseismic events experiences at least one peak and is close to the next peak. At last, in situ investigations were performed at number 1412 coal face in the Huafeng Mine, China. Results showed that the signals patterns proposed are in good agreement with the process of the fault-slip type of rock burst. The pattern recognition can provide a basis for the early warning and the implementation of relief measures of the fault-slip type of rock burst.
The fault-slip type of rock burst is one of the main types of rock burst in coal mines, and it is a great threat to the mining safety due to its devastation and the large amount of coal extruded [
Many researchers have made contributions to the monitoring and early warning of rock burst and proposed many kinds of monitoring approaches (e.g., electromagnetic radiation, AE, microseismic signals, and stress) [
Although the fault-slip type of rock burst is greatly influenced by the mining abutment pressure, it is different from the strain type of rock burst. In fact, the root of the fault-slip type of rock burst is the relative slipping of fault walls [
Influenced by the geological changes and tectonic activities, there are many geological formations in coal-bearing strata. Usually, large build-up of elastic strain energy occurs near the geological formations. The forces in coal and rock are in an equilibrium state without mining activities. Taking a single normal fault as an example, the mechanical model of the fault-slip is shown in Figure
Mechanical model of the fault-slip.
Uninfluenced by mining activities, the fault walls are in stable state and do not move. According to the force balance of the lower wall, the shear force,
Considering the geometrical relationship
then the shear stress,
The shear strength of the sliding surface can be obtained as [
If the shear stress at the sliding surface is larger than its shear strength, the fault walls will slip relatively, and vice versa. Thus, the slipping criterion of the fault walls is
Without any mining activities, no rock burst occurs for the fault walls do not slip relatively. According to (
For example, when the coal face moves from the upper wall to the normal fault, the abutment pressure distribution is shown in Figure
The cross section of a normal fault and abutment distribution: C1 is the preexisting tectonic pressure curve; C2 is the mining abutment pressure curve; and C3 is the superimposed pressure curve.
Before the fault-slip occurs, the rough sliding surface will be smoothed under high stress, with accompanying failure of coals and rocks. Therefore, whether the fault-slip type of rock burst will occur or not can be determined by recognizing the patterns of stress, energy, and failure degree of coal and rock near the sliding surface.
Patterns of the electromagnetic radiation intensity with the closing of the coal face to the fault.
The coal face is approaching the fault. Hence, the electromagnetic radiation intensity near the sliding surface begins to increase for the rocks are loaded by both the preexisting tectonic pressure and the mining abutment pressure. Since the coal face continues advancing to the fault, if the electromagnetic radiation intensity is maintained at a high level for a long time and then decreases slowly (see the trapezoid type in Figure
Patterns of AE energy with the closing of the coal face to the fault.
The AE energy near the sliding surface rises because the rock flaws extend and merge under the superimposed pressure when the coal face is close to the fault. As the coal face continues advancing to the fault, if the AE energy is maintained at a high level for a long time and then decreases slowly (see the trapezoid type in Figure
Patterns of the energy released by microseismic events with the closing of the coal face to the fault.
The coal face is approaching the fault. Hence, the energy released by microseismic events near the sliding surface increases because both the microseismic energy and frequency increase due to the larger amount of rock ruptures. Since the coal face continues advancing to the fault, if the released energy is maintained at a high level for a long time and then decreases slowly (see the trapezoid type in Figure
The trial work was conducted at number 1412 coal face in the Huafeng Mine, Shandong Province, China. The coal seam is simple in geological formation. The buried depth of the coal seam is from 1083.5 to 1215.8 m, with an average of 1170 m. The thickness of the coal seam ranges from 5.5 to 6.9 m, with an average of 6.2 m. The dip angle is from 30 to 34°. The immediate roof is 5.8 m thick sandy mudstone and 7.9 m thick gritstone. The basic roof is 12.6 m thick fine sandstone and 10.4 m thick siltstone, as shown in Table
Lithology description.
Sequence | Appellation | Thickness (m) | Symbol | Description |
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1 | Siltstone | 10.4 |
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Grey black, medium- and thick-bedded |
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2 | Fine sandstone | 12.6 |
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Grey black, siliceous cement and dense |
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3 | Gritstone | 7.9 |
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Grey, sandy structure and well sorted. |
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4 | Sandy mudstone | 5.8 |
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Hard coal core and sandy evenly distributed. |
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5 | Number 4 seam | 6.2 |
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Semibright with simple structure. |
The strike length and the incline length of the coal face are 2190 and 143 m, respectively. The longwall top-coal caving mining technology is used and the coal face advances 12 m per day. There is a normal fault named F18 crossing the face, about 1052.7 to 1198.6 m away from the open-off, with the dip angle of 62° and the throw of 8.5 m, as shown in Figure
Number 1412 coal face and station layout.
The average compressive strength of coal is approximately 19.58 MPa. The immediate roof is siltstone and sandstone with the average thickness of 19.3 m, and its average compressive strength is of 69.26 MPa. The main roof is siltstone and fine sandstone with the average thickness of 38.2 m, and its average compressive strength is of 102.8 MPa. Both the coal and the roof strata show some burst trend.
The monitoring work was conducted from July to December 2014. An electromagnetic radiation monitoring instrument (KBD5, Xuzhou Fuan Technology Co., Ltd.), an AE monitoring system (KJ623, Uroica Mining Safety Engineering Co., Ltd.), and a microseismic monitoring system (KJ551, Beijing Anke Technology Co., Ltd.) were used as the monitoring devices, as demonstrated by Figure
Monitoring devices.
KBD5 electromagnetic radiation monitoring instrument
KJ623 AE monitoring system
Installation of AE sensor
KJ551 microseismic monitoring system
The monitoring results were shown in Figure
Monitoring results.
The electromagnetic radiation intensity
The AE energy
The energy released by microseismic events
A rock burst of magnitude 1.3 occurred in the coal face on October 18, which causes about 38 tons of coal thrown into the face. Compared with the variations of the electromagnetic radiation intensity, AE energy, and energy released by microseismic events as the coal face advances, the rock burst occurred after a rapidly increasing period of the electromagnetic radiation intensity, after an exponential rising period of the AE energy, and about two days before the energy released by microseismic events reached its second peak.
Effectively recognizing the signals patterns is not only the promise of the early warning for the rock burst but also the basis for the implementation of relief measures. As one of the main types of rock burst, the fault-slip type of rock burst is a great threat to the mining safety, and it is necessary to investigate the recognition approaches of its signals patterns.
As mentioned above, the rough sliding surface is smoothed under high stress before the fault-slip occurs, accompanied by the failure of coals and rocks. Studies show that the strength of the electromagnetic radiation signal is positively related to the stress state of rocks and the signals of AE and microseismic event can reflect the damage and failure degree of rocks [
During the coal face closing to the fault, the fault-slip type of rock burst will happen under some conditions, for example, when the electromagnetic radiation intensity near the sliding surface increases rapidly, or the AE energy rises exponentially, and the energy released by microseismic events experiences at least one peak and is close to the next peak. In situ investigations in number 1412 coal face of the Huafeng Mine showed that the signals patterns are in good agreement with the process of the fault-slip type of rock burst. Therefore, we can give advance warning of the fault-slip type of rock burst by analyzing the variations of the electromagnetic radiation, AE, and microseismic signals, which also provides some basis for the implementation of relief measures.
The stresses near the sliding surface are redistributed due to the mining activities. Once the shear stress at the surface is larger than its shear strength, the fault-slip will occur, which is the root of the fault-slip type of rock burst. Before the fault-slip occurs, the surface will be smoothed under high stress, with accompanying failure of coals and rocks. Before the fault-slip type of rock burst occurs, the electromagnetic radiation intensity near the sliding surface increases rapidly, the AE energy rises exponentially, and the energy released by microseismic events experiences at least one peak and is close to the next peak. In situ investigations showed that the signals patterns are in good agreement with the process of the fault-slip type of rock burst. Based on the effective recognition of the electromagnetic radiation, AE, and microseismic signals, the early warning of the fault-slip type of rock burst can be achieved, so is the determination of the implementation of relief measures.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was supported by National Natural Science Foundation of China (nos. 51274133, 51174137, and 51344009), Shandong Province Natural Science Fund (no. ZR2010EEZ002), Shandong Province “Taishan Scholar” Construction Project Special Fund, Doctoral Scientific Fund Project of the Ministry of Education of China (no. 20123718110013), and Open Project of State Key Laboratory of Mining Disaster Prevention and Control Co-Founded by Shandong Province and the Ministry of Science and Technology (no. MDPC2013KF12).