# The Efficiency of Large Hole Boring (MSP) Method in the Reduction of Blast-Induced Vibration
## 5 Discussion
### Analysis of Propagation Characteristics of Blast-Induced Vibration  
In addition, combined CB is located on the tunnel contour line and generates a PS layer along the guide holes before the main blasting to control overbreak and prevent the propagation of blast-induced vibration to the outside of the tunnel [45,46]. According to the research, a vibration reduction effect of approximately 50% has been reported [47], and the vibration is usually reduced to around 30% on the construction site. However, CB-1 has a vibration reduction rate of only approximately 16%, even though combined CB was applied under the same conditions as general blasting, as shown in Table 5.  
As previously stated, the PPV values were extracted in Figure 9. Then, the locations at which these maximum values were measured were overlaid onto the initial design drawings of tunnel cross-section for all blast holes, as shown in Figure 10. Each figure depicts the area showing the first and second maximum value of PPV, and each shaded area represents detonators with the same delay time. According to Figure 10, most areas showing the high-amplitude blast-induced vibration were measured at the stoping, contour, or lifter area rather than the cut area in CB-1, CB-2, and CB-3 patterns, while the CB-4 pattern showed the first maximum value of PPV in the cut area; however, the value of 0.160 cm/s is a significantly lower PPV value compared to other patterns. In particular, patterns CB-1 to CB-3 with combined CB generated maximum PPVs in the contour line despite the use of low-vibration explosives and LD holes. To generate the fracture planes smoothly through the tunnel contour line using PS blasting, appropriate blasting patterns should be designed considering the rock conditions, particularly in jointed rock. In addition, it is important for PS and LD holes to maintain regular spacing in both the longitudinal and lateral directions, and the drilling angle for all blasting holes should also maintain constant directionality, as originally designed.  
As a result of analyzing the drilling condition, the spacing between each hole not only exceeded the design criteria but was not uniformly drilled, indicating that the LD holes failed to act as guide holes to generate the fractured planes. Furthermore, it is assumed that the cracks between the PS holes and LD holes are not smoothly connected owing to the characteristics of the rock mass with a dense distribution of multiple joint sets [48].  
Figure 10: The area showing the first and second maximum peak particle velocity (PPV) values in (**a**) B, (**b**) CB-1, (**c**) CB-2, (**d**) CB-3, and (**e**) CB-4 test blast patterns.  
We estimate that, despite application of the MSP method, relatively high-amplitude blast-induced vibration partially occurred in the contour line owing to detonation without LD holes acting as free faces. Furthermore, the CB-2 pattern demonstrated high-amplitude blast-induced vibration in the contour line, and did not produce fractured layers smoothly, and it failed to prevent propagation of the vibrations generated by the main blasting. Vibration monitoring was conducted under as similar conditions as possible, and it is assumed that the variance in geological conditions between the blasting point and measurement point affected the characteristics of vibration propagation [49]. Therefore, to efficiently apply the low-vibration explosives for combined CB, the advance should be limited to the maximum of 1.5 m, and appropriate patterns should be designed considering rock characteristics. In addition, it is believed that efficient vibration reduction will be possible if drilling errors are minimized by strengthening construction standards.