# Investigation on Cutting Blasting Efficiency of Hard Rock Tunnels under Different Charge Diameters
## 1 Introduction
Furthermore, they have verified that the auxiliary blasting of central holes was beneficial to the formation of the cutting cavity conforming to the design cutting depth. [PERSON] et al. [28] and [PERSON] et al. [29] have revealed the cavity forming principle of the double wedge cutting mode through a theoretical analysis and finite element simulation, respectively, and have achieved good application effects in rock tunnel blasting construction. Moreover, [PERSON] et al. [30] have added several central pre-splitting holes in the double wedge cutting mode, to reduce the difficulties of deep hole cutting blasting in hard rock tunnels. Nevertheless, although these modified wedge cutting modes are very effective in improving the cutting blasting effect, they could seize additional detonator segments, which would severely restrict their application and popularization  
Figure 1: Blasting driving process of rock tunnels. (**a**) Cutting blasting. (**b**) Stopping blasting. (**c**) Contour blasting. in the large section rock tunnel with limited detonator segments. According to the above review of the related literature, there is presently no detailed study concerning the effect of charge diameter on cutting blasting in hard rock underground tunnels. Generally, a uniform charge diameter is adopted for all types of holes in the blasting driving of rock tunnels in coal mines. The most commonly used is \(\Phi\) 32 mm blasting holes with \(\Phi\) 27 mm cartridges, and some have used \(\Phi\) 42 mm blast holes with \(\Phi\) 35 mm cartridges [31]. However, the blasting destruction range is limited in the hard rock tunnels with a hardness coefficient greater than 7. If the above small charge diameters are still adopted in the cutting holes, the rock in the cutting cavity is difficult to be effectively broken into rock fragments. At this time, the rock in the cutting cavity is still subjected to the strong clamping force from the surrounding rock and cannot be completely ejected out. Understandably, the poor cutting blasting effect is bound to lead to a small blasting cycle advance, thus resulting in the imbalance between coal energy mining and rock tunnel driving [32; 33]. In recent years, with the fast development and progress of drilling machines, drilling speed has been greatly increased, which makes it possible to apply large diameter holes in the rock tunnels of coal mines. Therefore, in order to raise the cutting blasting efficiency in hard rock tunnels, it was proposed to adopt large diameter charges in the cutting holes. That is, by increasing the charge diameter of the cutting holes, the blasting destruction range of the cutting holes would be further expanded, and hence the hard rock in the cutting cavity would be effectively broken into small rock fragments that were easy to be discarded. In order to investigate the influence of charge diameter on hard rock cutting blasting, three working conditions, namely, \(\Phi\) 32 mm (\(\Phi\) presents diameter) holes matching \(\Phi\) 27 mm cartridges, \(\Phi\) 42 mm holes matching \(\Phi\) 35 mm cartridges, and \(\Phi\) 50 mm holes matching \(\Phi\) 45 mm cartridges, were taken as the study objects in the present study. Initially, the blasting destruction ranges of single holes under the three different charge diameters were calculated theoretically. Secondly, numerical simulations of the cutting blasting under the three different charge diameters were performed to visually display the dynamic propagation processes of the stress waves, and reveal the distribution characteristics of the blasting stress field in the cutting cavity. Finally, to compare the cutting blasting results under the three different charge diameters, a hard rock vertical slope was used as the heading face to carry out the cutting blasting experiments.