# The Efficiency of Large Hole Boring (MSP) Method in the Reduction of Blast-Induced Vibration
## 1 Introduction  
Drill and blast is widely well-known as the most economical and efficient method to excavate rocks for the construction of underground structures; however, this method has consistently caused problems of vibration and noise due to a large amount of explosives in sensitive urban environments [1, 2, 3, 4, 5]. The key factor for tunnel blasting in urban areas is producing maximum blasting efficiency while controlling the blast-induced vibration, and a blasting design was carried out considering the tunnel support pattern and distance from safety facilities [6]. However, there is a limitation in minimizing the blast-induced vibration by reducing the charge per delay and advance per round. Additionally, use of a low-vibration explosive not only delays the construction period but also increases the blasting cost.  
Formation of free faces is the most effective way to reduce blast-induced vibration [7, 8]; however, tunnels generally have only one free face compared to open-pit mines. Cut blasting is generally performed to create an additional free face, and is the most effective and one of the most commonly utilized methods for reducing blast-induced vibration [9]. V-cut and cylinder-cut, which are traditional cut blasting methods, are mainly adopted to achieve this; however, initial blast-induced vibration is maximized due to concentrated charging of explosives on the cut area to create an additional free face [10]. For this reason, it is difficult to control the vibration within an allowable standard near safety facilities, and various tunnel blasting methods have been developed [11, 12, 13].  
Large empty holes over 250 mm in diameter have been used to reduce the blast-induced vibration as a type of cut blasting at a tunnel site in Korea. Furthermore, having a large free face is more effective for improving blasting efficiency [14]; however, there is a limit for boring a large hole because the boring time increases, which leads to an increase of the construction period. A large hole with a 362 mm diameter has been reported to show a 25-30% vibration reduction effect compared to the conventional blasting method [15], and the effect of vibration reduction compared with V-cut was up to 79% for 2 holes [16].  
However, existing outdated boring machines are not usually used because there are many critical drawbacks, such as numerous components, complex preliminary work, long installing/uninstalling and construction times, and smoke problems caused by diesel engines in the tunnel. Recently, an enhaced boring machine which greatly reduces the preparation and construction times was developed with a 382 mm hammer bit for increasing the blasting efficiency. The advantages of this method are that it not only creates a large free face but also secures the geological information beyond the tunnel face using a real-time boring data monitoring system mounted on the boring machine, and a borehole alignment tracking and ground exploration system [17].  
In this study, we assessed the vibration reduction efficiency of the multi-setting smart-investigation of the ground and pre-large hole boring (MSP) method. The characteristics of the traditional blasting method and the MSP method were compared, and the effects of free face formation with respect to size of the drilled holes were analyzed theoretically. In addition, real-scale test blasting was performed at a subway construction site, utilizing various blasting patterns of general blasting and the MSP method along the tunnel. Vibration was monitored and this data was analyzed to compare the vibration reduction efficiencies, and a detailed analysis of the effect of detonation delay time on vibration propagation characteristics was performed as well.