## 2 Method  
### Combined Long and Short Hole Cutting  
Based on the conventional tests and construction of oblique and straight hole cutting on site, a combination form of long and short straight hole cutting was determined through theoretical research with the objective of improving both the blasting footage and the blast hole utilization rate. The schematic diagram is shown in Figure 1, which illustrates the distribution of blast holes at the four corners and the center of the quadrilateral. All blast holes were detonated using bottom charging and digital electronic detonators. Among them, holes 1 to 5 were classified as shallow, while holes 6 to 10 were designated as deep, and the appropriate delay time was set between the two categories of holes based on on-site pre-test results. The detonation of holes at the same depth was simultaneous, and all holes were straight holes. In order to prevent the occurrence of collapses between adjacent holes, it was essential to maintain appropriate distances between deep and shallow holes.  
Figure 1: Schematic diagram of long and short eye combination cutting groove: (**a**) hole layout; (**b**) charge placement in profile.  
Upon detonation, shallow holes are the first to explode, thereby creating a more ideal cutting environment for deep hole blasting. Concurrently, the uncharged portion of the deep hole generates novel free surfaces and compensation space for the shallow holes, thereby reducing the minimum resistance line and the clamping effect of the rocks. During deep hole blasting, a shallow blasting funnel and a surrounding rock damage zone have already formed in the cutting area. The minimum resistance line of the rock mass in the blasting area is reduced, and the anti-blasting ability is diminished. The formation of larger groove cavities and crushing areas can be precisely and effectively achieved through the use of electronic detonators. If the rock-breaking ability of explosives stays the same, then a combination of long and short straight holes in the cutting form can effectively reduce blasting vibration and enhance the cutting effect, while addressing the issue of the low utilization rates of deep hole blasting boreholes.  
The combination of long and short straight hole cutting methods offers a number of advantages: (1) the blast hole is fully utilized, as during shallow hole blasting, the empty hole effect in the upper part of a deep hole without charge is fully utilized, which enhances the early blasting effect and is more conducive to improving the utilization rate of the blast hole; (2) the initial blasting funnel formed by shallow hole detonation increases the area of the free surface and the effect of the blasting; (3) in comparison to traditional straight hole cutting, the volume of the groove cavity is enlarged, providing a larger free surface and compensation space for the subsequent blast; (4) in comparison to oblique hole cutting, the combination of short and long straight hole cutting is not subject to the limit of the dimensions of the tunnel face, which can be applied to the construction of small-section tunnels; and (5) the spacing between each group of cutting holes (a and c) is greater than the corresponding dimensions of the straight hole cutting and the oblique hole cutting, which can provide a larger groove cavity, but it is necessary to pay attention to the interval b between the deep hole and shallow hole in each group of holes.