Fracture Evolution Between Blasting Roof Cutting Holes in a Mining Stress Environment

Blasting roof cutting and pressure relief is an effective technical way to solve the problem of thick and hard roof. In order to solve this problem, it is necessary to carry out research on the evolution of cracks between the cut holes of the blasting roof. The univariate comparative analysis method is used to analyze the evolution law of the fissures between the cuts under different factors. Furthermore, it is concluded that the broken zone and fissure zone of the surrounding rock of the single-hole blasting hole wall are symmetrically distributed in the confining pressure environment, and the fissure zone and the surrounding rock fissure zone between the holes show an "X"-shaped continuous ev 1 olution. By analyzing the evolution law of cracks between blasting holes, the critical discriminant equation of penetration between blasting holes under mining stress environment is given, which is used to optimize the technical plan of blasting roof cutting. Engineering practice shows that the blasting roof cutting scheme has achieved a good seam effect, creating good initial conditions for the cutting of thick and hard roofs.


Introduction
As coal mining depth increases, geological conditions in which coal mining face encounters thick and hard roofs become more common. The lateral and rear overhanging range of the working face, which forms during the mining process, is greater under thick and hard roof conditions. Strong mining pressure appears, which has a severe impact on the stability of roadway bearing capacity. According to relevant studies, using presplitting blasting roof cutting technology to achieve roof directional splitting reduces the lateral cantilever length of the roof and the mining pressure on the roadway and has achieved good application effects in many mines (Liang et al.2021;Wei et al.2021). At present, blasting roof cutting has become one of the effective technical ways to solve this engineering problem.
The key to achieving the goal of cutting roof is to evolve and penetrate cracks between blasting roof cutting holes. As a result, thick and hard roofs can be cut off along the crack direction, reducing the suspended roof size.
In recent years, many experts and scholars have conducted a considerable research on the pre-splitting blasting of thick and hard rock formations. Wang et al. (2013) used deep-hole pre-splitting blasting technology to control the roof cutting and collapse of a shallow coal seam face in the Shendong mining area to avoid or reduce large-scale roof pressure . Zong Qi(1994) analyzed the influence of explosion shock waves on the expansion range of a crack and posited a correction formula for the radius of the crack area in a deep-hole blasting surrounding rock. Gao Jinshi and Zhang Jichun(1989) comprehensively considered the influence of stress wave and blasting gas on rock fractures and obtained formulas for calculating the radius of a blasting hole wall surrounding the rock fragmentation zone and fissure zone. Wang Congcong et al.(2018) (Zhu Z M et al.2007; Zhu W C et al.2013;Dehghan et al.2012). Cai Feng et al.(2007) used LS-DYNA to simulate the process of coal blasting and discussed the propagation characteristics of the explosion stress waves and expansion of coal cracks during blasting. Yang Renshu et al.(2016) used LS-DYNA to discuss the effect of the explosion stress waves and explosion gas on the blasting medium during the blasting process. Zhou Shengcai et al.(2013)

Engineering background analysis
The main mining 3-1 coal in the 113103 working face of Bojianghaizi CoalMine in Ordos has an average buried depth of 670 m, an average mining thickness of 4.64 m, and an average inclination of 3°. The roof is managed by the total caving method. The return airway of the working face is 5.2-m wide and 3.8-m high, the width of the coal pillar is 11 m, and the anchor beam provides net support. The columnar distribution of the structure of roof rock strata is shown in Fig.1.
Field investigations have revealed that the roof is composed of mudstone and fine sandstone composite thick and hard roof rock formations, and the length of the suspended roof on the goaf side is large, causing the return airway to be severely affected by mining. The bottom heave of the roadway reaches 1.5-2.0 m, and the roof is anchored. The de-anchoring phenomenon occurs, and the roadway section shrinkage rate near the exit of the working face is more than 50%, as shown in Fig.2. The roadway section is reduced, wind speed is high, dust is flying, and movement of pedestrians and transportation of materials are difficult, which severely affects the production safety of the working face. Therefore, given the above problems, during the mining period, the roof of the roadway is blasted to cut the top and relieve the pressure to ensure safe mining.
Affected by the mining of the previous working face, the top slab of this working face broke into a cantilever beam structure, where Block A was bent and deformed under the action of the overlying rock load. Due to the long cantilever, the mining roadway was under the mining pressure environment. Block A is broken by blasting and topping, reducing its cantilever length and the rotational force acting on the roadway. The key technology when blasting the roof is that each blast hole crack can expand along the line of the blast hole and form inter-hole cracks that evolve and penetrate, causing the entire Block A to be cut off along the strike.    The simulation effect of single-hole blasting is shown in    Confining pressure is uniformly applied to the surrounding nonreflective boundary. The univariate comparative analysis method is used to design the simulation comparison plan, as shown in Table 3.

Influence of aperture on the evolution of cracks between roof cutting blasting holes
The simulation results of the blasting rupture gap penetration with different apertures are shown in Table 4 and blasting effects. According to the analysis, the larger the aperture, the more explosives are required in the blast hole, and the higher is the blasting dynamic load generated after blasting, which in turn will form a larger rupture gap evolution scale.
Only when sufficient detonation energy is achieved can the blast fracture between holes be formed, and the pores can evolve and penetrate. As shown in Table 5   As shown in Table 6

Influence of rock mass strength on the evolution of blasting perforation cracks
As shown in Table 7    After the explosive is detonated, the blasting dynamic load of high temperature and pressure impacts the hole wall on both sides of the hole, and its peak stress is remarkably greater than the compressive strength of the rock mass. The energy of the shock wave decreases rapidly while crushing the rock until the energy is exhausted. Therefore, the explosion impact area can be roughly divided into blasting crushing zone and blasting fissure zone (Guo Deyong et al.2019), that is, the stress and failure distribution of the surrounding rock of the blast hole wall under blasting dynamic load is shown in Fig.11 In the formula, n1 represents the explosion impact pressure coefficient, generally n1 = 8-9; ρ0 represents explosive density; V0 represents explosive detonation velocity; dc and db are roll and blast hole diameters, respectively; and lc and lb are the axial charge and axial chamber lengths, respectively.
In the formula: ε is the loading strain rate. In engineering blasting, the rock loading rate ε is between 10 and 10 5 s -1 (Shan Renliang et al.1997). In the crushing zone, the loading rate is higher, which can be taken as ε=10 2~1 0 4 s -1 ; in the fracture zone, the loading rate is further reduced, which can be taken as ε=10 0~1 0 3 s -1 , β is the lateral stress coefficient, β=μd/(1-μd); σc is the static compressive strength of the rock mass; r0 is the radius of the blasthole; μ is the Poisson's ratio of the rock mass, and μd is the dynamic Poisson's ratio. Under engineering blasting conditions (Dai J 2001), generally μd =0.8μ.
Because the blasting effect is affected by numerous factors and the research on this problem is insufficiently comprehensive, the coefficients λ1 and λ2 are introduced.
According to the relevant research (Guo Deyong et al.2016;Yang Y Q et al.1995), λ1 = 0.7-0.9 is used in the crushing zone.
After introducing λ1, the radius a of the crushing zone can be obtained as follows: According to elastic mechanics, the force at any point in polar coordinates can be expressed as follows (Shan Renliang 1997): The general solution is: In the formula, A, B, C, and D are undetermined coefficients.
Here, formula (4) can be expressed as follows: From the literature (Guo Deyong et al.2019), the dynamic load P′ of blasting at the edge of the crushing zone can be obtained as follows: In the formula, r represents the comparison distance, 0 i /r r r  , ri represents the distance from any point to the blast hole center, r0 represents the blast hole radius, α represents the stress wave attenuation coefficient, and α = 2 ± μd/(1−μd), take positive in the shock wave action area and negative in the compressive stress wave action area. Therefore, the boundary conditions are determined as follows: At boundary a r  , Combined with the displacement condition, B = 0 can be obtained, and A and 2C can be obtained as follows: According to the problem of a circular hole with an infinite boundary, the radial and circumferential stress of the surrounding rock of the blasting hole are as follows: By substituting (14) In the formula, k represents the strength change coefficient of the rock mass under dynamic load, generally taken as 1.3 (Gong Fengqiang 2010). Therefore, the critical criterion [d] for penetrating the blasting roof crack under the mining stress environment is In the formula, [d] represents the critical criterion for the penetration of the blasting head gap and n is the effective safety factor, which is taken as 0.8 in the text.

Engineering practice
According to the site working conditions, Poisson's ratio μ   (1) In a confining pressure environment, the fracture zone of the surrounding rock on the hole wall of single-hole blasting is symmetrically distributed with the fracture zone, and the diameter of the fracture zone is 5-7 times that of the blasting hole, and the diameter of the fracture zone is approximately 5 times that of the fracture zone. During blasting top cutting, the broken area of the surrounding rock between holes and the fracture area show "X" type penetration evolution. The blasting dynamic load between adjacent holes forms a superposition effect in the transmission process to enhance the blasting effect of fracture penetration between holes.
(2) When blasting roof cutting is implemented, the hole diameter has the most obvious impact on the expansion scale of blasting crack. The larger the hole diameter, the stronger is the superposition effect of blasting dynamic load, and the larger is the evolution scale of crack expansion. The hole spacing has a particularly key impact on the effect of top cutting and crack formation. Moreover, the confining pressure and rock mass strength have an inhibitory effect on the development of blasting crack expansion, and the impact effect is small in a certain range.
(3) Through mechanical analysis, the critical criterion of blasting top clearance penetration in the mining stress environment is deduced. Along with the field peeping results, it is concluded that the key to blasting top clearance pressure relief is that through cracks can be formed between blast holes after blasting, and the entire thick and hard roofs can be cut off along the cracks to achieve pressure relief.
Acknowledgements This study was funded by the National Natural Science Foundation of China (Nos.52074008, 52074007), the Anhui Collaborative University Innovation Project (GXXT-2020-056) .
Data Availability: All data generated or analyzed during this study are included in this published article.