# Analysis of Blasting Efficiency Using Crack-Inducing Holes and Pre-Splitting Method in Blasting
## 2 Full-Scale Experimental Blasting of the Combined Blasting Method
### Full-Scale Experimental Blasting  
At the Samsung-Dongtan railway tunnel site, general blasting and combined blasting were each conducted twice. The test blasting cycle was carried out in the following sequence: marking \(\rightarrow\) drilling \(\rightarrow\) charging \(\rightarrow\) blasting and vibration measurement \(\rightarrow\) muck removal \(\rightarrow\) overbreak measurement. The first and second blasts were performed using general blasting, followed by the third and fourth blasts using combined blasting. Figure 3 shows the test blasting plan and Table 1 summarizes the step-by-step execution process and key details of the test blasting. To analyze the effect of the peripheral combined blasting, the drilling patterns and charge amounts for the cut, production, and invert areas were applied equally. For each blasting, the excavation advance was 2.0 m, and the cut-hole method used was Cylinder-cut. The explosives used were emulsion explosives (\(\Phi\) 32 mm) and fine explosives (\(\Phi\) 17 mm), with a non-electric detonator system. All blasting holes were drilled using an \(\Phi\) 45 mm bit, and marking was applied to the tunnel face to reduce drilling errors, using a jumbo drill dedicated to tunnel excavation. Figure 3: Full-scale test blasting plan. Figure 2: Rock conditions at the experimental blasting site. (**a**) Geological map; (**b**) face condition. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sequence** & **General Blasting** & **Combined Blasting** & **Key Points** \\ \hline Marking & & & Reduce drilling error \\ \hline Dilling & & & Drilling length: 2.2 m \\ Dilling & & Diameter of drilling: \(\Phi\) 45 mm \\  & & & Equipment: Jumbo drill \\  & & (Eqpt: Rocket boomer 353, & \\  & & Mfr: Atlas Copco AB, & \\  & & Country: Sweden, & \\  & & City: Stockholm) \\ \hline Charging & & & Main blast: Emulsion \\  & & Contour blast: Fine explosive \\ \hline Stemming & & Stemming: Sand \\  & & & Compacting \\ \hline  & & & Non-electric detonator \\  & & MS: Mili Second \\  & & LP: Long Period \\  & & Bunch Connector \\ \hline  & & & Location: Directly above \\  & & Points: 2 locations \\ \hline Overbreak & & & Use of dedicated equipment \\  & & Measuring vertical distance from \\  & & & planned excavation line \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Step-by-step test blasting process and key details. The blasting method applied to the peripheral controlled blasting was analyzed and compared: for general blasting, Smooth Blasting was used, and, for combined blasting, crack induction holes and pre-splitting were applied. Smooth Blasting was developed in Sweden during the 1950s and 1960s. It is a contour-controlled blasting method that uses the decoupling effect to prevent excessive rock breakage during blasting and to create a smooth fracture surface along the excavation line. The decoupling effect occurs when the diameter of the explosive is smaller than the borehole diameter, resulting in a lower maximum pressure inside the blast hole compared to tight loading, which helps to control the rock breakage. The ratio of spacing (S) to burden (B) should be 0.8 or less, and the burden must be greater than the spacing [16]. Combined Blasting is a blasting method that combines crack-inducing holes with the pre-splitting method [16], maximizing the advantages and compensating for the disadvantages of the pre-splitting method. Pre-splitting holes are installed along the tunnel excavation line, with empty holes placed in between to induce cracks and maximize the pre-splitting effect. Explosives with a smaller diameter than the borehole are used, applying the same decoupling effect as in Smooth Blasting. In this method, detonation is initiated first to create artificial cracks along the excavation line. Crack-inducing holes are employed to compensate for insufficient crack formation due to the decoupling effect. As a result, this blasting technique reduces both vibration and overbreak. Figure 4 shows the blasting patterns used in the experiment.