# Investigation on Cutting Blasting Efficiency of Hard Rock Tunnels under Different Charge Diameters
## 1 Introduction  
The blasting technique has always been extensively used in various rock excavation projects, especially in the driving of rock tunnels in coal energy mining engineering, because it has the obvious strengths of a higher work efficiency, a more flexible feasibility, and a lower economic input compared with the mechanical driving technique [1; 2; 3; 4]. In the blasting driving process of rock tunnels, there are three steps from inside to outside, namely cutting blasting, stopping blasting, and contour blasting, as shown in Figure 1. Among them, cutting blasting is the first step of full section blasting and the critical link to control the rock breaking effect and the blasting cycle advance. A rational cutting blasting mode is conducive to inducing a large extra free surface and an adequate compensation volume for the subsequent blasting steps [5; 6; 7; 8]. According to the angle relationship between the cutting holes and the heading face, the common cutting blasting modes can be distinguished into inclined hole and parallel hole cutting modes, and the most typical inclined hole cutting mode is wedge cutting blasting [9; 10]. The outstanding weaknesses of straight hole cutting blasting are the many cutting holes, a low energy utilization, and a small cavity space,while wedge cutting blasting is obviously characterized by a small hole amount, a high energy utilization, and a large cavity space [11]. Consequently, wedge cutting blasting has been the most extensively adopted cutting mode in the blasting driving of rock tunnels in coal mines. Differing from the conventional bench blasting, which has two free surfaces, cutting blasting is quite difficult because only the heading face can be used as a free surface [12; 13; 14; 15]. Therefore, improving the cutting blasting efficiency through a rational cutting mode has always been a research focus in the field of rock tunnel blasting driving. In terms of wedge cutting blasting, many researchers and engineers have conducted an enormous amount of research, and have acquired some fascinating and useful research results. For instance, based on the field blasting data statistics, [PERSON] [16] and [PERSON] et al. [17] have compared the blasting results of different cutting modes, and concluded that wedge cutting blasting was the most effective cutting mode for rock tunnel driving. [PERSON] and [PERSON] [18] have presented an approach to computing the parameters of wedge cutting blasting after fully considering the major factors, such as the explosives and rock properties. [PERSON] et al. [19] and [PERSON] et al. [20] have calculated the correlation degree and global priority of the diverse impacting factors of wedge cutting blasting by using gray system theory and an analytic hierarchy approach, respectively. [PERSON] et al. [21] have studied the effect of hole angle symmetry on cutting blasting results, and believed that the symmetrical layout of the cutting holes could contribute to strengthening the stress superposition effect and maximizing the energy utilization. [PERSON] et al. [22] have carried out cutting blasting model experiments by taking the hole angle as a single variable, and then they have determined a reasonable range of the hole angle after analyzing the fragmentation distribution and blasting vibration. [PERSON] et al. [23] have constructed a wedge cutting blasting model by employing the finite element method, and acquired the cavity forming process by defining the rock element failure algorithm. Based on a user-defined rock damage model, [PERSON] et al. [24] have simulated the rock damage distribution in the cutting cavity under different initiation points. In addition to the above studies on conventional wedge cutting modes, there have been many recent investigations involving various modified wedge cutting modes. For instance, [PERSON] et al. [25], [PERSON] et al. [26], and [PERSON] et al. [27] have elaborated on the effects of the auxiliary blasting of central parallel holes on wedge cutting blasting results, using many research methods.