1. Introduction
In tunnel blasting and urban underground engineering, smooth blasting and pre-split blasting are usually used as control blasting techniques. However, to reduce the damage of blasting to rock mass, reduce the support cost, and ensure the quality of the tunnel forming, the methods such as the directional fracture blasting technology of the cutting seam cartridge and the uncoupled charging [
1,
2] are adopted to ensure the accurate control the direction of the explosion crack growth, effectively reduce the over-excavation and under-excavation, and improve the forming quality around the tunnel [
3,
4].
The essence of the cutting seam cartridge is to make use of the hard pipe with the axial slotted to make the detonation product act on the specific position of the blasthole wall first and precisely form the initial guiding crack, to meet the requirements of the flatness of blasting profile in directional blasting [
5,
6]. Since its proposal, it has been widely used in slope excavation, blasting around shafts, tunnel excavation, and other projects [
7,
8]. To avoid the excessive fragmentation of the local rock mass caused by the traditional coupled charge form [
9], it will affect the directional blasting effect. The cutting seam cartridge adopts an uncoupled charge structure, which can change the loading rate of the stress wave on the blasthole wall, prolong the action time of the stress wave and gas, increase the energy utilization rate of the explosive and improve the fragmentation distribution of blasting, which is more suitable for fracture forming controlled blasting technology such as directional fracture controlled blasting [
10,
11].
Regarding the cutting seam cartridge blasting technology, a large number of experts and scholars have studied the influence of the material of the cutting seam pipe [
12], the shape of the groove [
13], the way of grooving [
14], the charge dose [
15] and the uncoupled coefficient [
16,
17] on the blasting mechanism and crack propagation law. Cho et al. [
18] used analysis software to simulate the crack growth law under different cutting seam cartridge blasting conditions. Yue, Z.W. et al. [
19,
20] studied the dynamic response effect of air-deck charge with cutting seam cartridge in cement mortar model test by using the super dynamic strain test system and concluded that the blasting effect is relatively ideal when the axial uncoupled coefficient is 1.5~2.0. Wang [
21] combined dynamic caustics experiment and numerical simulation analysis to compare and study the blasting effect of the cutting seam cartridge when the air medium and the plasticine medium are filled between the explosive and the blasthole wall respectively. Wei, C.H. et al. [
22] carried out a simulation study on the evolution of cracks generated by the cutting seam cartridge blasting under different ground stress conditions and analyzed the influence of different ground stress conditions on the blasting effect of the cutting seam cartridge. Yang, G.L. et al. [
23], Yang, R.S. et al. [
24] analyzed the dynamic fracture effect of detonation crack in the cutting seam cartridge blasting and concluded that the blasting effect was the most ideal when the uncoupled coefficient of the cutting seam cartridge blasting is 1.67. Wang, Y.B. et al. [
25] studied the change of the stress intensity factor at the crack tip when the two blastholes are simultaneously initiated under different cutting seam methods.
Most of the above studies on the blasting effect and mechanism of the cutting seam cartridge blasting are conducted by model test [
26], theoretical analysis [
27,
28], and other methods. However, the model experiment is usually in an extreme environment of high temperature and high pressure, so the whole process of the blasting, cannot be observed intuitively. The digital laser dynamic caustics test can only observe the transmission process of the stress wave, and it’s hard to obtain the actual dynamic information [
29]. The theoretical analysis needs to adopt approximate and simplified methods, resulting in differences between the analysis results and actual results. Considering the great difficulty and economic cost of the experiment, it is very important to adopt numerical simulation. Based on the LS-DYNA
® explicit dynamic analysis platform, the dynamic information of the detonation product can be obtained by using the numerical simulation method, and the crack propagation process can be visually displayed [
30]. Scholars such as Yang, R.S. et al. [
31] and Shen, T. et al. [
32] have used numerical simulation to study the cutting seam cartridge, and the results show that the numerical simulation results are consistent with the measured data. However, most of the existing studies assume that the center of the blasthole and the cartridge are coincidental, that is, the concentric uncoupled cartridge structure. In actual engineering, the explosive will be close to one side of the blasthole due to gravity or misplacement, resulting in an eccentric uncoupled cartridge structure. When this structure explodes, the explosion energy does not uniformly act on the rocks around the blasthole and will produce local stress concentration effects around the blasthole wall. Especially for the cutting seam cartridge, due to the existence of cutting seam pipe, there is a large local concentrated load when the detonation products form the initial guided crack [
33]. If the eccentric arrangement, the coupled side, and the uncoupled side will have a quiet blasting difference, which will affect the forming quality of the directional blasting and increase the damage to the surrounding rock [
34]. At present, for the eccentric uncoupled structure of the cutting seam cartridge, the blasting dynamics and crack propagation law are still rarely studied. Therefore, it is necessary to study the eccentric uncoupled blasting effect of the cutting seam cartridge.
In this paper, based on the LS-DYNA® explicit dynamic analysis platform, a quasi-two-dimensional numerical model with five kinds of eccentric uncoupled coefficients is established by using the fluor-solid coupling algorithm. Based on the pressure cloud diagram and pressure-time interval curve output by the post-processing software, the dynamic response behavior characteristics of the explosive, air, the cutting seam pipe, and the rock mass in the eccentric and uncoupled case of the cutting seam cartridge are analyzed. And use the failure element method to simulate the crack propagation effect under completely eccentric conditions. Under the influence of different eccentric uncoupled coefficients, the distribution characteristics of the rock mass explosion stress field and the crack propagation law of the cutting seam cartridge blasting technology are studied to provide a theoretical basis for reducing the possible damage effects. It has important theoretical and practical significance to explore the role of stress waves in the vicinity of the blasting near district when the eccentric and uncoupled arrangement of the cutting seam cartridge and to deeply understand engineering phenomena, which can provide guidance and reference for future construction.