This paper deals with the theoretical aspects combined with stress analysis over the floor strata of coal seam and the calculation model for the stress on the coal floor. Basically, this research presents the relevant results obtained for the rock burst prevention in the floor of roadway driven along next goaf in the exploitation of thick coal seam with large obliquity in deep well and rock burst tendency. The control mechanism of rock burst in the roadway driven along next goaf is revealed in the present work. That is, the danger of rock burst can be removed by changing the stress environment for the energy accumulation of the floor and by reducing the impact on the roadway floor from the strong dynamic pressure. This result can be profitable being used at the design stage of appropriate position of roadway undergoing rock burst tendency in similar conditions. Based on the analysis regarding the control mechanism, this paper presents a novel approach to the prevention of rock burst in roadway floor under the above conditions. That is, the return airway is placed within the goaf of the upper working face that can prevent the rock burst effectively. And in this way, mining without coal pillar in the thick coal seam with large obliquity and large burial depth (over a thousand meters) is realized. Practice also proves that the rock burst in the floor of roadway driven along next goaf is controlled and solved.
The rock burst in roadway floor is a dynamic disaster during the process of coal mining [
Rock burst in roadway floor is a phenomenon of dynamic instability, the occurrence of which is related not only to the stress environment of the roadway, but also to the mechanical properties of the surrounding coal rocks. In particular, the hard formation underlying the coal seam has a significant influence on the occurrence of rock burst. For example, a rock burst accident occurred at 1410 upper entry of Huafeng Coal Mine on September 9, 2006, with the magnitude of 2.0. Microseismic monitoring revealed that the seismic source was located at 30 m ahead of the driving face and on the limestone (
According to statistics [
The in situ stress is measured [
Composition schematic illustration of rock beam-pillar structure model.
Mechanical model of rock burst in roadway floor caused by the instability of rock beam-pillar.
In the figure,
According to the boundary conditions of the established mechanical model (i.e.,
The total energy stored due to deformation is
Substituting (
If
The strain energy stored by the bending deformation of the floor is calculated as
It can be known from the strength condition that when
Thus, the core of preventing the rock burst in the roadway floor is to get away from the region with increasing stress. The stress concentration on the roadway should also be prevented, so as to reduce the influence of dynamic pressure on the roadway floor and hence the risk of rock burst.
In field applications, the factors such as floor lithology and mining conditions are given. So the position of roadway driven along next goaf is the primary factor influencing the effect of prevention of rock burst in the roadway floor. A case analysis is carried out on 1411 working face of Huafeng Coal Mine to discuss the influence of the position of roadway on the effect of rock burst prevention.
Driving is performed in the 1411 upper entry of Huafeng Coal Mine in 5 segments. For segments 1 to 3, as shown in Figure
Arrangement of roadway driven along next goaf on 1411 working face. (a) Profile of the roadway with small coal pillars. (b) Profile of the roadway without coal pillars.
From segments 4 to 5, as shown in Figure
The location statistics of mine tremor by using microseismic monitoring.
Number | Coordinate/m | Distance from the working face | Horizon | Seismic source | Magnitude | ||
---|---|---|---|---|---|---|---|
|
|
| |||||
1 | 3439 | 4854 | −955 | 11 m behind the working face | 4 m below the floor | 52 m below the upper entry | 1.5 |
2 | 3393 | 4925 | −930 | 45 m in front of the working face | 2 m below the floor | 11 m below the upper entry | 1.3 |
3 | 3402 | 4869 | −936 | 15 m behind the working face | 8 m below the floor | 17 m below the upper entry | 1.3 |
4 | 3382 | 5004 | −921 | 88 m in front of the working face | 3 m below the floor | 3 m below the upper entry | 1.9 |
5 | 3478 | 5337 | −953 | 260 m in front of the working face | 2 m below the floor | 52 m below the upper entry | 1.6 |
6 | 3412 | 5101 | −940 | 48 m behind the working face | 5 m below the floor | 14 m below the upper entry | 1.6 |
The most important reason for the occurrence of rock burst is high stress concentration [
In order to discuss the influence of different roadway arrangement on the effect of floor impact prevention, the geological conditions of 1412 working face of Huafeng Coal Mine and the mining conditions of the nearby working face are considered. The mechanical model of floor under stress is constructed to determine the appropriate position of roadway driven along next goaf by comprehensively considering the stress distribution of the roadway roof and floor.
1411 goaf lies on one side of the 1412 working face and the undrived coal on the other side. According to the literature [
Structural model of lateral strata overlying the roadway driven along next goaf.
It is precisely due to the protection provided by the main roof and the upper immediate roof that the mining roadway on the working face does not have to bear all the weight of the overlying strata. Such structural protection obeys the key strata theory for strata control [
The research on the floor stress distribution is especially important for a reasonable roadway arrangement and the prevention of rock burst. According to elastic mechanics theory, when one concentrated force acts on the plane of semi-infinite body, the stress component of any point in the semi-infinite body can be obtained. Although rock is heterogeneous and discontinuous, the stress distribution in the rock body can be solved by analytical method if the floor of the coal seam is considered as a semi-infinite body. Many scholars have carried out relevant researches and obtained some achievements. For example, Wang et al. [
The analytic method is effective in stress calculation of coal floor. According to the theory of elasticity, the floor of 1412 working face is considered as a semi-infinite body. Based on the distribution of lateral bearing pressure on the working face, the mechanical model of floor of roadway driven along next goaf is built, as shown in Figure
Mechanical model of floor under stress.
Unlike the near-horizontal coal seam, the coal seam with large obliquity has an additional tangential component along the dip direction, the value of which is nonignorable. The lateral bearing pressure of the coal seam is decomposed into the vertical force in parallel with the dip direction of the coal seam and the transverse force vertical to the dip direction of the coal seam. As shown by the theory of elasticity, the tiny transverse force
The vertical force
Among them,
Schematic diagram of stress calculation of coal seam floor with distributed load.
Lateral load
Longitudinal load
Region I is as follows:
Region II is as follows:
Regions III, IV, and V are as follows:
According to the field observation of mine pressure and the literature [
Changes of vertical stress along the dip direction at different depth.
It is known from the figure that after the mining of 1411 working face, the vertical stress at any point in the strata below the floor shows an asymmetrical distribution pattern (increasing with depth) along the dip direction of the coal seam. Among them, the vertical stress at the distance of 0 m from the floor is also 0. The region from the location 10 m ahead of the coal wall to that 15 m behind the coal wall (goaf) is the region of pressure relief for floor rocks under the action of lateral bearing pressure. The pressure relief of the floor becomes more obvious at the place closer to the goaf and to the shallow layer. The minimum stress factors are 0.556, 0.707, and 0.816 at the depth of 5, 10, and 15 m below the floor, respectively. They are located at 2 m and 0 m behind the goaf and 5 m ahead of the coal wall, respectively. At this time, the stress curve has the minimum slope. Moreover, the stress values of the three positions decrease at the shallower layer.
Since the surrounding rock stress is lower in the pressure relief region and the roadway is easier to be maintained, there are 3 possible positions of roadway driven along next goaf: mining without coal pillar on the same vertical surface in 1411 upper entry, mining with small coal pillars at 5 m ahead of the coal wall, and mining with coal pillars 2 m behind the goaf.
The source of rock burst of the floor is the concentrated stress in the elastic region of floor. After the excavation of roadway, the in situ rock stress field undergoes certain variations. The vertical stress begins to transfer to two sides, while the horizontal stress transfers to the roof and floor of coal mine, reaching the maximum in the elastic region of the floor. Therefore, horizontal stress plays an important role in the occurrence of rock burst in the floor. According to the theory of rock mass mechanics, the horizontal stress and vertical stress in the elastic region of the floor satisfy the following relation,
The failure of rock body under triaxial stress satisfies the Mohr-Coulomb criterion:
Strata underlying the coal seam and its physical and mechanical parameters.
Lithology | Thickness/m | Internal friction angle/° | Uniaxial tensile strength/MPa | Uniaxial compressive strength/MPa | Protodyakonov coefficient/f |
---|---|---|---|---|---|
Siltstone | 1.7 | — | — | 50.4 | 3.5 |
Medium-grained sandstone | 4.2 | 35.8 | 10.07 | 113.7 | 5.0 |
Siltstone | 4.8 | 45.8 | 5.06 | 35.6 | 3.5 |
Coarse-grained sandstone | 3.5 | 41.8 | 6.73 | 85.5 | 5.0 |
Siltstone | 4.8 | 51.8 | 6.47 | 62.1 | 5.5 |
Medium-grained sandstone | 5.2 | 47.8 | 8.27 | 128.3 | 5.0 |
The stress components obtained from (
Under different arrangement of roadways driven along next goaf, the stress distribution of the rocks surrounding the roadway is obtained (Figure
Failure curve of floor strata along the dip direction.
As the energy is released abruptly by rock burst, the energy dissipated by the deformation of the surrounding rocks is limited. The roadway is inevitably subject to the impact. If the instantaneity of energy dissipation and release during doing work outward is ignored, then the work done by the impact force generated by the failure of floor strata to the rock body is expressed as
The 1412 fully-mechanized top-coal caving face of Huafeng Coal Mine (Figure
Synthetic plan view of the 1412 fully-mechanized top-coal caving face.
It can be seen from the bar graph of 99-1 borehole near 1412 working face that below the coal seam there are several sets of hard strata with large strength (Table
Based on the prevention mechanism of rock burst in the floor of roadway driven along next goaf, the schemes of mining with small coal pillars and without coal pillars are analyzed synthetically. Meanwhile, the 1412 return airway is placed in the goaf of the 1411 working face (Figures
The specific regions of energy release through the microseismic events during the driving and stopping of the 1412 working face were first determined. The microseismic events were localized to the working face, as shown in Figures
Statistics of microseismic events at an interval of 5 m on the fixed driving face.
Statistics of microseismic events at an interval of 5 m on the fixed stopping face.
It can be seen from Figure
It can be seen from Figure
As shown by the data of microseismic monitoring, the arrangement of the roadway driven along next goaf is favorable to remove the danger of rock burst in the floor. In order to determine the degree of the danger of rock burst, the coal powder monitoring via drilling method was performed. The effect of prevention of rock burst in the roadway driven along next goaf was further evaluated. The coal powder amount reaching the danger threshold was calibrated by reference to the measured coal powder amounts at different depths: 2.91 kg/m at 2-3 m borehole depth, 4.40 kg/m at 4–6 m borehole depth, and 8.40 kg/m at 7–9 m borehole depth. As shown in Table
Coal powder amounts at different borehole depths of 1412 working face.
Borehole depth/m | 2 m | 3 m | 4 m | 5 m | 6 m | 7 m | 8 m | 9 m |
---|---|---|---|---|---|---|---|---|
Coal powder amount/kg | ||||||||
Standard coal power | 1.94 | 2.18 | 2.20 | 2.07 | 2.27 | 2.16 | 2.15 | 2.10 |
Driving period | 2.10 | 2.30 | 2.41 | 2.23 | 2.20 | 2.46 | 3.39 | 2.27 |
Stopping period | 2.35 | 2.43 | 2.58 | 3.14 | 2.47 | 2.34 | 2.45 | 2.76 |
The mechanical model of rock burst in the roadway floor of thick coal seam with large obliquity is built. The occurrence conditions of rock burst and the influence factors are then analyzed using this model. When the stress of floor strata exceeds the limit, the hard floor strata will be damaged, inducing the rock burst. In the meantime, the lower the surrounding rock stress (including the horizontal stress and the normal stress), the smaller the energy stored in the hard floor strata, and hence the lower the impact on the floor during the rock burst. The prevention mechanism of rock burst in the roadway driven along next goaf is revealed in the present work. Thus, the danger of rock burst can be removed by changing the stress environment for the energy accumulation of the floor and by reducing the impact on the roadway floor from the strong dynamic pressure. The mechanical model of floor under stress in coal seam with large obliquity is built. Based on the stress theory and Mohr-Coulomb criterion, the stress distribution curve of coal floor at different depths is plotted. The failure depths of floor at different positions are calculated. Moreover, the relationship of the impact force on the floor with the vertical distance between roadway and the damaged floor strata is provided under different arrangements of roadway driven along next goaf. That is, the larger the depth of plastic failure in the direction vertical to the roadway floor, the smaller the relative resistance on the roadway floor. Based on the analysis of the mechanism of rock burst control, 1412 return airway is placed in the 1411 goaf of the upper working face. Thus, the mining without coal pillars is realized in thick coal seam with large obliquity in deep well. Microseismic monitoring and coal powder monitoring by drilling method were performed to confirm that the roadway floor was free from the danger of rock burst.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors acknowledge National Natural Science Foundation Project (Grant no. 51174002) and Open Foundation of State Key Laboratory of Mine Disaster Prevention and Control Cultivation Base, Shandong University of Science and Technology, Qingdao, Shandong 266590, China (Grant no. MDPC2012KF02).