When studying the pressure-relief effect of hard roof blasting and cutting, the roof-cutting position and angle obviously affect the stability of the rock surrounding the gob-side entry (GSE). In this paper, control of the large deformation of rock surrounding the GSE is evaluated on the basis of the overlying structure and pressure-relief principle caused by roof cutting. Moreover, a mechanics model of a three-hinged arch structure (THAS) and a universal distinct element code (UDEC) numerical model with regard to the overlying rock movement were established to study the relationship among the rotation angle of key blocks in the THAS, the width of the roadway and the wall force beside it, and the optimal cutting position and cutting angle to reveal the pressure-relief effect of roof blasting and cutting and its influence on the support stability of the roadway. The results show that the overlying rock can form a stable THAS after roof blasting and cutting and that the wall stress and the coal-wall displacement are small, which indicates that roof blasting and cutting results in obvious pressure relief. The wall force increases with an increase in the rotation angle of the key block and decreases with an increase in the roadway width. Moreover, the optimal roof-cutting position (5 m) and angle (15°) are obtained with the specific mining conditions. Finally, on-site monitoring of the anchor-cable force and support force in panel 5312 of the Jining no. 3 coal mine is used to verify the pressure-relief effect after roof blasting and cutting. The study results can provide a theoretical basis for reasonable technical means and optimization of supporting parameters in field observation and have important application value for roof cutting and pressure relief for GSE retaining (GSER) technology.
Based on green, scientific, and efficient mining of coal resources, gob-side entry retaining (GSER) technology has been widely applied to underground coal mines. This technology essentially realizes continuous mining without a coal pillar, reduces the roadway excavation rate, eases the difference between mining and excavation, and optimizes the mining layout. However, this technology is restricted by several shortcomings in its development and application such as the diversity of roof position, difficulty in controlling large deformation of the surrounding rock, and mismatch between the characteristics of the wall support and the surrounding rock [
For that reason, in-depth studies on the key technologies of GSER such as the surrounding rock activity, wall support, and roadway support have been conducted [
When a hard, thick, main roof occurs over a coal seam where no immediate roof is present or the immediate roof is thinner, the wall beside the roadway may have difficulty in supporting the hard roof. Therefore, it is necessary to presplit the hard roof and to change the main roof structure, thus resulting in effective pressure alleviation of the GSE [
Hence, it is particularly important to select the best cutting position (hole position) and angle (hole angle) when presplit blasting the roof to achieve better pressure-relief effects. In this paper, to control the large deformation of rock surrounding the GSE, the overlying structure and pressure-relief principle caused by roof cutting was first analyzed. A THAS mechanics model and a UDEC numerical model with regard to the overlying rock movement were established to study the relationship among the rotation angle of key blocks in THAS, the width of the roadway and the wall force beside it, and the optimal cutting position and cutting angle to reveal the pressure-relief effect of roof blasting and cutting and its influence on the support stability of the roadway. The study results can provide a theoretical basis for reasonable technical means and optimized supporting parameters in field observation. Moreover, they have important application value for roof cutting and pressure relief using GSER technology.
When a hard, thick, main roof occurs directly above a coal seam, it is easily suspended at a large scale owing to its high stiffness and strength, which results in highly concentrated stress in the surrounding rock of the roadway [
The overburden structure after roof blasting was applied is shown in Figure
Roof structure of GSE affected by roof cutting.
According to the aforementioned analysis, the overlying structure after roof cutting is very important for the stability of the surrounding rock in GSER technology. The basic parameters determining the overlying structure are the position and the angle of roof cutting.
According to the Voussoir beam theory [
The possibility of slipping instability of the structure increases with an increase in the cutting angle. However, the cutting angle should not be too large, which may cause the drill-hole depth to increase, thus increasing the difficulty of construction.
If the B2 block slips and loses stability after roof cutting, the relationship among the rotation angle, cutting angle, and cutting position is described by drawing a method according to the geometric relationship of the blocks, as shown in Figure
Relationship among rotation angle, cutting angle, and cutting position. (a) Geometric relationship between blocks; (b) relationship between the rotation angle and cutting position.
According to the above analysis, the THAS easily forms with a large cutting position or small cutting angle. However, a longer cantilever relates to high support stress of the roadway with a larger cutting position; thus, the B2 block may slip more easily with a large cutting angle according to equation (
In order to better understand the pressure-relief effect of THAS after roof cutting, the short-arm beam structure was set as the matched group, the mechanical model of the key block was established, and the engineering example was analyzed.
Figure
Mechanical model of the THAS.
Figure
Mechanical model of the cantilever beam structure.
Among them, the wall force
In order to study the problem intuitively, panel 5312 of the Jining No. 3 coal mine was used as the mining background. The depth of panel 5312 is 581 m, the mining height is 3 m, the coal seam dip is 3–6°, and the length and the strike length are 150 m and 626 m, respectively. The excavation size of the GSE is 4.5 m × 3 m, and the width of the wall is 2.7 m. Moreover, the roof is composed mainly of interbedded sandstone and mudstone. The physical and mechanical parameters of the coal and the roof are listed in Table
Physical and mechanical parameters of coal and roof.
Lithology | Thickness (m) | Density (kN·m−3) | Modulus of elasticity (GPa) | Poisson’s ratio | Tensile strength (MPa) | Internal friction angle (°) | Note |
---|---|---|---|---|---|---|---|
Coal seam | 3 | 13.5 | 2.2 | 0.43 | 0.41 | 14.8 | |
Siltstone | 9.07 | 24.1 | 22 | 0.27 | 4.2 | 32 | Main roof |
Sand shale interbed | 8.93 | 25.7 | 15.6 | 0.29 | 3.7 | 31 | Compensated load |
The length
Considering the effect of the elastic foundation, the horizontal distance between the break line of the roof and the roadway can be obtained as [
According to the geological conditions of panel 5312, the parameters
Relationship between the roof cutting and the wall force of different overburden structure.
As can be seen from Figure
According to equation (
Figure
Relationship among (a) rotation angle of the key block and the wall force and (b) roadway width and wall force.
In order to further reveal the influence of roof presplitting and cutting technology on the stress and deformation of the surrounding rock for GSER, UDEC numerical simulation was used to study the pressure-relief effects of the roof cutting angle and cutting position. As a result, the best roof-cutting angle and cutting position were obtained. Hence, a total of 32 different combination schemes were designed by setting different cutting angles such as 0°, 10°, 15°, and 25° and different cutting positions such as 1 m, 3 m, 5 m, 7 m, 9 m, 11 m, 13 m, and 15 m. The sizes of the simulation model and the roadway were 200 m (length) × 81 m (height) and 4.5 m (width) × 3 m (height), respectively, and the wall width was 2.7 m. Moreover, the boundary conditions of the bottom and both sides of this model were full-displacement constraints and horizontal-displacement constraints, respectively, and the model top applied 12.5 MPa vertical stress to compensate for the failed simulation strata. The Mohr–Coulomb model was adopted for the coal and rock mass, and the strain-hardening model was adopted for the wall. An overview of the simulation model and its parameters are shown in Figure
Numerical model.
The simulation results of the 32 aforementioned schemes revealed that the roof structure changes similarly with the cutting-angle variation at different cutting positions. Hence, the cutting position of 5 m was chosen, and the structure variations with different cutting angles as shown in Figure
Variations of the roof structure with different cutting angles: (a) 0°, (b) 10°, (c) 15°, and (d) 25°.
The different cutting angles had different effects on the movement of key blocks after roof cutting. It was easier for the B2 block to be fully cut down to the horizontal state with an increase in the cutting angle. When the cutting angle was less than 15°, as shown in Figures
Accordingly, in order to study the position effect of roof cutting for pressure relief, the cutting angle of 15° was chosen, and the structure variations with different cutting positions as shown in Figure
Variations of roof structure with different cutting positions: (a) 1 m, (b) 3 m, (c) 5 m, and (d) 7 m.
When the cutting angle was 15°, the blocks with different cutting positions can be squeezed and bitten; however, the pressure-relief effect for the main roof is different. When the cantilever length is less than 5 m (Figures
The THAS of the main roof is beneficial to the stability of rock surrounding the GSE; however, THAS formation is closely related to the cutting angle and the cutting position. Through the study and analysis of 32 schemes, it was found that the critical values for THAS formation are a cutting angle of 10°, cutting position of 13 m, angle of 15°, and position of 5 m, as shown in Figure
Stress distribution of surrounding rock before and after the THAS formation. (a) Cutting angle 10° and position 11 m, (b) cutting angle 10° and position 13 m, (c) cutting angle 15° and position 3 m, and (d) cutting angle 15° and position 5 m.
Displacement variation of coal side of GSER before and after the THAS formation.
As shown in Figures
The wall beside the roadway can provide effective support for the roadway and can share part of the load for the solid coal body of the roadway. Hence, the stress concentration in the wall is obviously reduced after the THAS formation. As shown in Figures
The vertical stress of the wall in Figure
According to the aforementioned optimal scheme of roof cutting (cutting angle of 15° and cutting position of 5 m), the vertical stress distribution and its variations of roof cutting and non-roof-cutting were studied by setting up two monitoring lines 0.5 m above the roadway, as shown in Figure
Stress distribution and its variations of roof cutting and non-roof-cutting.
Because of the roadway excavation and face mining, a stress concentration zone occurs in the solid coal and in the wall of the roadway with and without roof cutting. However, the roadway is in the stress relaxation zone between the two zones of stress concentration. The vertical stress of the wall without roof cutting was 17.9 MPa, whereas that of the wall with roof cutting was 11.9 MPa, showing a 33% decrease. Moreover, compared with that without roof cutting, the stress concentration zone moved forward after roof cutting, and the influence range decreased. Hence, the pressure-relief effect with roof cutting is obvious.
Figure
Displacement variations of surrounding rocks of the roadway with roof cutting or non-roof-cutting.
Based on the geological conditions of panel 5312 in the Jining No. 3 coal mine, the optimal cutting position and angle of presplitting blasting for pressure relief were obtained. In order to further explain the effects of roof cutting and pressure relief, this section discusses on-site monitoring performed during the mining process of panel 5312 and analyzes the force of the roof anchor cable and the variation of the support stress.
According to the objectives of this study, a total of seven anchor-cable force sensors marked as A1–A7 were installed in the target roadway. The distances between the sensors and the open-off cut were 30 m, 150 m, 215 m, 230 m, 350 m, 450 m, and 650 m for A1 to A7, respectively, and the anchor-cable force was monitored by using a remote online-monitoring system. Figure
Monitoring curve of anchor-cable force.
The force variations of the roof anchor cables in the three monitoring positions were similar, and the influence range of mining dynamic pressure was essentially stable at 30–35 m. The force of the anchor cable began to increase about 10 m between the monitoring point and the working face. The growth rate gradually intensified. When the distance between the face and the point was about 8 m, the force of anchor cable reached its peak value and then rapidly decreased. When the working face pushed through the monitoring point for 30 m, the force essentially stabilized at 150 kN, indicating that the roof of the retaining roadway had been cut down along the presplitting face to successfully relieve the pressure.
During the mining process of panel 5312, a total of 100 hydraulic supports of type ZY7200-18.5/34 were selected. The rated working resistance of the support was 7200 kN (40 MPa), and the maximum support height was 3400 mm. The stress monitoring points of the supports were arranged as shown in Figure
Monitoring points of support stress.
According to the aforementioned monitoring scheme, the stope can be divided into three areas: the roof-cutting-affected zone, the unaffected area in the middle, and the zone not affected by roof cutting. Three hydraulic supports, Nos. 3, 51, and 91, were selected to monitor and analyze the rock pressure. Among them, support No. 3 was located in the zone not affected by roof cutting; No. 51 was located in the middle unaffected zone, and No. 91 was located in the roof-cutting-affected zone.
Figure
Variation curve of support load.
Statistics of weighting and support stress.
No. | First weighting (m) | Periodic weighting (m) | Support stress (MPa) |
---|---|---|---|
3# | 50 | 31 | 17.3 |
51# | 40 | 19 | 25.9 |
91# | 44 | 26 | 21.2 |
Regarding the pressure-relief effects of hard roof blasting and cutting, the factors selected for the roof cutting position and its angle obviously affects the surrounding rock stability of the GSE. In this study, which focused on controlling the large deformation of this rock, the following results were obtained: Based on the analysis of the overlying structure and pressure-relief principle caused by roof cutting, a mechanical model of a THAS is established. It was determined that the overlying rock can form a stable THAS after roof blasting and cutting. In addition, the wall stress and the coal-wall displacement were small, which indicates that roof blasting and cutting has obvious effects of pressure relief. Taking panel 5312 of the Jining No. 3 coal mine as the engineering background, the relationship among the rotation angle of the key block, the width of roadway, and the wall force beside the roadway was studied. The wall force was found to increase with an increase in the rotation angle of the key block but decreased with an increase in the roadway width. The effects of roof-cutting position and angle were studied, with optimal results found to be 5 m and 15°, respectively. Finally, on-site monitoring of the anchor-cable force and the support force in panel 5312 of the Jining No. 3 coal mine was used to verify the pressure relief effect after roof blasting and cutting.
These study results can provide a theoretical basis for reasonable technical means and optimization of supporting parameters in field observation. Moreover, they have important application value for roof cutting and pressure relief in GSER technology.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest.
This study was funded by the National Natural Science Foundation of China (nos. 51574155 and 51804182), Science and Technology Development Plan of Tai’an (no. 2018GX0045), Shandong Provincial Natural Science Foundation (no. ZR2019BEE065), Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (no. 2015RCJJ057), and Shandong Provincial Key R&D Plan (Public Welfare Special Program) of China (no. 2017GGX20125).