Technology of Coal Seam Long Borehole Blasting and Comprehensive Evaluation Method of Pressure Relief Effect in High Rockburst Proneness Longwall Panel

On August 2, 2019, a catastrophic rockburst disaster occurred in Tangshan mine, causing death of 7 miners. After the investigation, the coal mine is facing reproduction. Taking the 0291 panel as the engineering background, this paper studies the coal seam blasting pressure relief and ground stress monitoring technology in working face retreat. During the roadway development and working face excavation, coal seam blasting was adopted to transfer the high ground stress of coal seam to the deep ground of the coal body. The blasting operation is presented in detail in this paper. In the working face retreat stage, drilling powder method, hydraulic shield resistance monitoring, roof displacement, and vibration monitoring methods are implemented. The results show that the pressure relief range of coal seam is 4–12 m in the coal mass after blasting. The shield working resistance is stable at 20–30 MPa. The range of relative displacement of the roof is about −1.0 to 2.5 mm, and the maximum vertical vibration velocity is in the range of 7–11 cm/s, up to 12 cm/s. The measured parameters are acceptable, so it is concluded that 0291 panel can be safely mined. This study provides a reference for the coal seam blasting design for rockburst coal mine and provides a technical means for the analysis of pressure release effect and dynamic pressure monitoring during working face retreating.


Introduction
Rockburst is a dynamic phenomenon of violent destruction of coal and rock mass in roadway or working face in the process of coal mining, which is often accompanied by coal and rock mass throwing, loud noise, and air wave. It is one of the major disasters in coal mine. With the depletion of shallow resources, coal mining extends to the deep underground. e geo-structure and stress environment of underground rock mass are becoming complicated, resulting in more intensity and high frequency disasters in deep mining compared with shallow engineering. According to statistics, by the end of 2020, there are 138 rockburst mines in China [1]. On August 2nd, 2019, a serious rockburst accident occurred at the Tangshan coal mine, causing 7 deaths. e essential method to prevent rockburst is to reduce the ground stress of surrounding rock or migrate the high stress to the far ground [2][3][4]. Coal seam blasting, water injection, and borehole drilling are pressure relief manner to reduce the risk of rockburst. Among them, coal seam blasting can effectively release the elastic deformation energy accumulated in coal and rock mass, change the structure of the surrounding rock, and reduce the stress concentration, so it can be used to prevent rockburst disaster.
In view of the mechanism of coal seam blasting on the prevention of rockburst, Zhang et al. [5,6] developed strength weakening and impact reduction theory. e peak stress area of the roadway rib is moved to the deep coal and rock mass via blasting, and the stress concentration near the roadway surface is reduced. Qi et al. [7][8][9] put forward the stress control theory, which believes that seam blasting causes stress redistribution of coal and rock mass. It can effectively reduce the ground stress level and move the peak stress far away from the rib to reduce impact risk. Xue et al. [10][11][12] put forward a hydromechanical coupling model and analyzed the damage characteristics of coal seams during gas fracturing and put forward the fracture criterion of type-I fracture under the action of multiple stress states in the seams fracturing process.
In terms of influencing factors and application of seam blasting, Pan et al. [13][14][15] proposed hole blasting technology based on impact start-up theory and established a "double peak" model of blasting pressure relief and impact prevention.
e field test of blasting conducted by Sun [16][17][18] in Gushan coal mine shows that the prevention of rockburst by blasting is a process of modification-stress reduction-energy dissipation. It leads to surrounding rock support deterioration, structural instability, and roadway large deformation. Ouyang [19] proposed multistage blasting technology for roof, floor, and coal seam under the dual influence of extremely thick and hard roof and complex geological conditions. is paper uses the reproduction of Tangshan coal mine after 82 rockburst disaster as engineering background. Coal seam blasting was implemented to a panel before and after working face retreating. Blasting design and operation are presented in detail. e technics to evaluate the pressure relief effect after blasting are manipulated.
is paper provides a case study on the coal seam blasting design, operation, and pressure relief effect monitoring technology of high rockburst proneness coal mine, which provides a reference for similar engineering projects.

Study on Blasting Parameters
During the explosion, a cavity is formed at the explosion point. Compression fracture zone, fracture zone, and elastic vibration zone are successively formed from the center to outside [20]. e blasting technology can also be applied to prevent coal and gas outburst, rock softening, high ground stress chamber, and large deformation [21][22][23][24].
In terms of blasting parameters, Wang [25] used numerical simulation to analyze five key parameters affecting the pressure relief effect, namely, the distance from starting point of the pressure relief area to the roadway surface, the distance from the end of the pressure relief area to the roadway surface, the radial range of the pressure relief area, the depression angle of the pressure relief hole, and the spacing of the pressure relief area.
Liu et al. [26] selected 6 influencing factors of blasting and 3 indexes on pressure relief blasting parameters to conduct numerical calculation and orthogonal test. It is found that the optimal parameters are 3# coal, 8 kg charge, 90 mm hole diameter, 15 m hole depth, 3 m hole spacing, and decoupling coefficient is 4. Wei et al. [27] used FLAC3D to simulate the stress change and migration in the roadway before and after pressure relief under different blasting parameters. It shows that the larger the charge, the more the transfer of the stress peak area to the deep ground. However, the peak stress magnitude does not decrease significantly. e greater the thickness of coal seam, the more the decrease in stress peak, but the transfer distance of stress peak area is about 12 m. e greater the mining depth, the smaller the transfer distance from the stress peak area to the depth.
After blasting, a pressure relief zone is formed, which transfers the high ground stress to the deep ground. It is necessary to calculate the radius of compression fracture zone and fracture zone formed by blasting. Under the condition of uncoupled charge in the infinite medium, the pressure of transmitted shock wave generated after cylindrical charge explosion is as follows [28,29]: where p is the pressure of shock wave transmitted into coal, MPa; p 0 is the explosive density, kg/m 3 ; V d is the detonation velocity of explosive, m/s; K is the radial decoupling coefficient of charge, K � r b /r c , r b and r c represent the radius of explosion hole and charge radius, respectively, mm; and l c is the axial coefficient of charge. When there is no air column in the axial direction, l c � 1; c is the expansion adiabatic index of explosion products, generally taken as 3; n is the stress increase coefficient during expansion, normally taken as 10. e explosion stress in the coal body can be solved as a plane strain problem, and the 3D stress field in the coal body can be expressed as follows: where σ r , σ θ , and σ z represent radial, tangential, and axial stresses, respectively; r is the contrast distance, r � r i /r b , r i is the distance from the calculation point to the charging center and r b is the radius of the blasting hole; α is the pressure attenuation coefficient, in compression crushing ; the stress analysis of blasting pressure relief is shown in Figure 1. e expression stress intensity (equivalent stress) in coal and rock is as follows: Substituting equations (3) and (4) into (5), we get )σ r . According to Mises's criterion, the criterion of blasting compression fracture zone and fracture zone is as follows: Shock and Vibration where σ i is the stress at any point in coal and rock mass, MPa; σ c d is the dynamic compressive strength of coal and rock mass, MPa; and σ t d is the dynamic tensile strength of coal and rock mass, MPa. Substitute equations (1), (2), and (6) into (7) and (8), respectively, and the estimation formulas of radius R c of compression fracture zone and radius R p of fracture zone are as follows: where R c is the radius of compression crushing area after blasting, mm; R p is the radius of fracture area after blasting, mm; and r b is the radius of blasting hole, mm.

Engineering Background.
e studied area is 0291 panel of Tangshan coal mine. e buried depth of 0291 panel is about 800 m. e coal seam thickness is 8.6-11.7 m, with an average thickness of 10.5 m. It is semidark briquette. e inclination of the coal seam is 5°-39°, with an average of 22°. Absolute gas emission is 16.51 m 3 /min, and the absolute carbon dioxide emission is 1.82 m 3 /min. Coal dust is explosible, and the spontaneous combustion period of coal seam is 10 months. e coal seam, roof, and floor of 0291 panel have impact tendency. Microearthquake events appear continuously in the excavation and mining operations. In the roadway development, through the monitoring and analysis of the drilling cuttings method, the area with burst risk above the medium in the 0291 panel was identified, as shown in Figure 2.

Blasting Layout in Roadway Development.
e principle of "pressure relief first" was adopted in the roadway development in medium burst risk segment. One single borehole blasting scheme is adopted around the heading machine, as shown in Figure 3.
One Φ 42 mm borehole is arranged for head-on loosening blasting along the axis of the roadway. e rib blasting is arranged within 30 m from the heading face. e rib boreholes are 0.5-1.5 m away from the floor, and their interval is less than 10 m. e length of all blasting borehole is of 12 m. If the width of coal pillar is less than 5 m, no blasting hole is drilled.
#3 emulsion explosive specific for coal mining was used in all blasting. One single column charge is 300 mm in length and 300 g in weight. e charge quantity of each borehole is 10-15 columns. Two identical electric detonators were used in each hole. e sealing length was less than 1/3 of the borehole length. e blasting method is forward charging, parallel connection in holes, series connection between holes, and simultaneous initiation of 5 holes. Pressure releases first, and then excavation was operated in a circular manner; it ensures that there is always more than 5 m blasting section in the front of the heading face.

Blasting Layout in Working
Retreat. During the face retreat of 0291 panel, the burst risk area is determined through the drilling powder method. In the risk area and 10-15 m outside of two ends, coal seam blasting is adopted, as shown in Figure 4. e blasting began from 10-15 m from the burst risk area and ended 10-15 m away from the burst risk area. Blasting hole is Φ42 mm, 12 m in length, and interval less than 5 m, and the drilling is 0.5-1.5 m away from the floor. e explosive and blasting method are the same as those during development. Blasting started on September 21, 2019, and ended by January 9, 2021, as shown in Table 1 and Figure 5. e blasting location included open cut, main gate, and tailgate.
A total of 309 blasting holes were drilled during working face retreat; among them, 16 were in open cut, 171 were in main gate, and 122 were in tailgate.

Monitoring Technology for Pressure Release Effect.
In terms of the evaluation of pressure relief effect, several technics can be used combined or lonely, such as conventional electromagnetic radiation, microseismic, seismic CT, and drilling powder [30][31][32][33]. e following literature is noticeable. Cong et al. [34] proposed the test method of blasting pressure relief effect by using seam wave velocity   In this study, the monitoring technology of drilling powder, hydraulic shield pressure, roof separation, and vibration monitoring were used to evaluate the effect of the pressure relief measure.

Drilling Powder.
e length of the blasting borehole is 12 m, and the distance between the blasting point to the roadway rib is 9-12 m. e drilling powder was measured before and after blasting to evaluate the pressure relief. e  Shock and Vibration 5 monitoring hole is Φ42 mm with a depth of 12 m. e results are shown in Figure 6. Compared with the coal powder generated before and after blasting, it shows that the pressure relief range is 6-12 m from the rib. e amount of pulverized coal in the explosion section decreases noticeably, which reflects the reduction of coal stress in this section. e amount of pulverized coal from 4-9 m decreased to some extent, indicating that the blasting influencing area is from 4-12 m.

Hydraulic Shield
Resistance. e working face retreat began from October 2019. Before mining, the coal in front of the open cut has been blasted for pressure relief. erefore, the monitoring data of the hydraulic shield working resistance are selected from March 2020, as shown in Figure 7. Among them, 77-98 shields are the upper support of the mining face, 28-71 shields are the middle support, and 7-21 shields are the lower support.
e results show that the support resistance is in the range of 20-30 MPa, which is normal. After blasting, the working resistance of all shield supports is stable, and there is no severe pressure on the working face in the face retreating.

Roof Movement in Front of the Working Face.
e roof movement was monitored during the working retreat. e monitoring duration is 3 days, while the working face retreat is 5 m. e orifice is taken as the zero point, the position of each anchor point is calculated, and the data of the same anchor point are combined at different times to obtain the overall change of roof movement. e calculation method of anchor point position in the monitoring hole is (10). e L1 sensor data are taken as an example; the calculation result is shown in Figure 8.
where S i represents the depth of each anchor point of the measuring hole, mm; a i is the measured value of each anchor point in the measuring hole, mm; A represents the total length of the measured line of the Ionometer, mm, A � 10 m; and A 0 represents the hole depth of each measuring hole, mm. e first anchor of each measuring point is selected as a fixed point. e displacement between other anchor points and the fixed anchor point can be calculated. e displacement in different dates is compared, and the movement of the roof at the measuring point can be determined. e calculation method of the displacement between each anchor point and the fixed anchor point is equation (11), and the relative displacement can be obtained using equation (12).
where L i is the distance between the ith anchor point and the first anchor point (i � 2, 3, ..., n), mm; b i is the measured value of the ith anchor point, mm; b 1 is the measured value of the first anchor point, mm; n is the number of anchor points; and L i ′ is the spacing value in different time periods, mm.
e relationship between the relative spacing of measuring points and the distance between measuring points and working face is shown in Figure 9.
Result shows that the relative displacement is in the range of −1.0-+2.5 mm, which suggests that the displacement of the roof is acceptable under dynamic pressure. It is worthwhile to mention that the relative displacement has negative value, which indicated a vibration movement of the roof due to the disturbance of the mining activities, such as cutter movement or hydraulic shield advance.

Roof Vibration Monitoring.
e TC-4850 vibrometer is used to monitor the roof vibration in front of the working face. e equipment was installed in the tailgate on June 26, 2020, and the monitoring period is 2 days. e vibrometer was installed at the tailgate 7 m away from the working face, as shown in Figure 10. A total of 4   Shock and Vibration vibrometers were installed, namely, Z1-Z4, respectively. e drilling position is 1.0 m away from the mining rib, and the installation hole is 6-7 m length in vertical direction. e probe is a cube with a dimension of 66 mm. A steel cable is anchored in the hole. At the exposed part of the cable, the probe is glued to the steel plate using gypsum. e collector is fixed on the metal mesh with rope. e trigger level of the measurement is 0.2 cm/s, the acquisition rate is 8000 SPS (sample points/s), and the recording time is 5 s. e waveform curve of each vibration can be obtained via interpretation of collected data, as shown in Figure 11. Since this monitoring is only aimed at the vertical movement of the roof, the data are screened accordingly. e data related to roof vertical movement are selected, and the  Figure 12 shows Z1 monitoring data as an example.
Result shows that along with the advances of the working face, the vibration of the roof in front of the working face increases. e maximum vertical velocity of the roof 7 m in front of the working face is about 6 cm/s, and it increases to 15 cm/s while the working face advances 3 m. However, the overall vibration intensity is still normal based on engineering experience. It suggests that the roof vibration is most likely attributed to the mining activity, such as coal cutting or hydraulic movement, rather than the dissipation of the energy release accumulated in the coal mass.
From monitoring data, it can be concluded that the panel can be mined safely after coal seam pressure release via seam blasting.

Conclusion
Taking the reproduction of Tangshan coal mine 0291 panel as the engineering background, this paper adopts the coal seam blasting pressure relief method during roadway development and panel excavation to reduce the burst risk. e pressure relief effect and the influence of mining disturbance on the surrounding rock around the working face are     Shock and Vibration monitored and analyzed by the jointed method in the mining stage. e following conclusions can be drawn: (1) Coal seam blasting can transfer high ground stress of coal seam to the deep ground of coal mass, leading pressure relief of coal body near the roadway rib. e results of drilling powder measurement show that the pressure relief range of coal seam blasting is 4-12 m. For a rockburst panel, the intensive blasting parameters should be adopted. At present, the mining of 0291 panel has been underway, and the monitoring and analysis results have been verified.
is study provides a reference for the blasting pressure relief design of deep rockburst mine and provides a technical means for the analysis of pressure release effect and dynamic pressure monitoring during mining.

Data Availability
e data used to support this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.