1. Introduction
Underground excavations in urban areas are primarily carried out for the construction of road and railway tunnels to expand transportation networks. Methods for excavating rock-based tunnels can be broadly categorized into blasting methods, which use high-power explosives, and mechanical excavation methods, which crush rock using machinery. With the increasing trend of tunnel excavation projects, various excavation methods have been developed and commercialized. However, from the perspectives of constructability and cost-effectiveness, blasting methods maintain a comparative advantage. Nonetheless, blasting excavation has drawbacks, including complaints arising from vibrations caused by the use of high-power explosives and the tendency for overbreak to expand in weak or highly jointed rock formations [1,2,3]. An analysis of the environmental dispute statistics published by the Ministry of Environment in December 2023 shows that complaints related to construction site noise and vibration have been increasing annually from 2019 to 2023, posing significant constraints on blasting operations [4]. As a result, extensive research has been conducted to minimize blasting-induced vibrations and overbreak.
One way to reduce blasting vibrations is by improving the tunnel cut area. Jo et al. (2012) showed that installing large-diameter blast holes in the tunnel cut area zone resulted in reduced blasting vibrations compared to conventional V-cut blasting methods [5,6]. However, a downside is the need for special drilling equipment to be deployed inside the tunnel, additional safety measures, and extra time for work. To minimize overbreak, Jang et al. (2013) proposed overbreak management using artificial neural networks and the Overbreak Resistance Factor [7,8,9,10], but they did not suggest drilling or blasting patterns for different rock types. Mottahdi et al. (2018) conducted a sensitivity analysis using artificial neural networks and concluded that drilling, blasting, and RMR were the most effective factors for overbreak control [11], but they did not propose a blasting method. Liu et al. (2023) optimized tunneling parameters and overbreak prediction modeling for various tunnel cross-sections [12,13] but faced limitations due to numerical analysis. To reduce blasting vibrations and minimize overbreak, Jeong et al. (2007) developed a notch bit for forming notch grooves on the outer sections [14,15], but, due to wear, jamming, and other issues during drilling, there are few cases of its application in practical fieldwork.
The efficiency of blasting during tunnel excavation can vary depending on rock characteristics and drilling conditions, and the areas where vibrations are most intense can also differ. Considering the rock characteristics, which are an unchanging parameter in blasting operations, adjusting the drilling patterns and detonation sequences would be the most effective approach. However, there is a lack of research on methods to minimize vibrations and overbreak during tunnel blasting without the need for additional equipment. Therefore, it is essential to develop technologies that can block and reduce vibration generation.
Full-scale comparative blasting experiments were planned and conducted by the authors at a railway tunnel construction site using a blasting method that combines crack induction holes and presplitting blasting techniques to minimize overbreak and control vibrations alongside the conventional Smooth Blasting method in this study. The ground vibration propagation characteristics and overbreak amounts from the combined blasting method and from the typical Smooth Blasting method were compared and analyzed.
2. Full-Scale Experimental Blasting of the Combined Blasting Method
2.1. Experimental Site and Rock Mass Conditions
The experimental area is located at the Samsung-Dongtan railway tunnel site in Suseo-dong, Seoul, South Korea. The Samsung-Dongtan railway tunnel is a metropolitan express railway connecting Gangnam-gu, Seoul, and Hwaseong-si. Figure 1 shows the satellite image and location map of the blasting experiment area.
The geological condition of the experimental blasting face can be divided into Precambrian gneiss intruded by Mesozoic igneous rocks, both unconformably overlain by Quaternary alluvial deposits. The stratigraphy, based on the ground surface, consists of soil (weathered soil), weathered rock, soft rock, moderately hard rock, and hard rock in sequence.
The RMR classification value of 41, obtained from face mapping at the experimental blasting site, indicates that the rock mass is in a fair condition. Figure 2 shows the geological map and the face condition at the experimental blasting location.
2.2. 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 → drilling → charging → blasting and vibration measurement → muck removal → 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 (Φ 32 mm) and fine explosives (Φ 17 mm), with a non-electric detonator system. All blasting holes were drilled using an Φ 45 mm bit, and marking was applied to the tunnel face to reduce drilling errors, using a jumbo drill dedicated to tunnel excavation.
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. The space between holes in the Smooth Blasting method was 550 mm, with a burden of 700 mm, and the charge used was 17 mm diameter fine explosives, inducing decoupling effects. For the combined blasting method, the spacing of the crack induction holes was 400 mm, with pre-splitting holes placed 100 mm below the crack induction holes, spaced 400 mm apart [17,18]. The charging was identical to that of Smooth Blasting, but the pre-splitting holes were detonated first in the blasting sequence to initiate the pre-splitting. Figure 5 shows the appearance of the experimental blasting.
In this study, to verify the effect of the combined blasting method, vibration velocity meters were installed in a straight line on the surface directly above the tunnel. The overbreak condition was analyzed, and after blasting, the rock fragmentation at the tunnel face was measured using a total station.
2.3. Blasting Vibration Measurement and Analysis Methods
To measure the vibrations in the upper ground caused by blasting, the following steps were planned: (1) selection of an appropriate vibration meter, (2) securing installation locations, (3) fixing the sensors and testing the connection cables, (4) collecting data during the blast, and (5) analyzing the data using dedicated software. To measure ground vibration at a certain distance from the tunnel face, the Minimate Plus (Mfr: Instantel, Country: Canada, City: Ottawa), a dedicated blasting vibration measurement device, was used. The Minimate Plus is a compact blasting vibration meter with built-in sensors, allowing for vibration measurement without disturbing the terrain or interfering with traffic and pedestrian movement. As shown in Figure 6, in this experiment, two vibration meters were installed at 5 m intervals and 34.0 m directly above the tunnel face for each blast. The data acquisition mode was set to Continuous Mode, and the measurement time was set to record up to 10 s after the blast. To ensure accurate data collection when measuring blasting vibrations on the ground surface, pins were installed on the sensors to prevent errors that may arise from variations in contact with the ground. Table 2 shows the detailed specifications of the equipment and installation setup [19].
The data collected from each experimental blasting pattern were analyzed using the dedicated program Blastware 10.0. The measured data consist of three components based on the measurement direction: vertical, longitudinal (in the plane direction towards the measurement point from the vibration source), and transverse (perpendicular to both the vertical and longitudinal directions). The data used in this study are PPV (Peak Particle Velocity), which is based on the USBM’s research findings that link structural damage to PPV [20,21,22].
2.4. Overbreak Condition Analysis Method
The authors used the SOKKIA IX-Series(Mfr: Topcon, Country: Japan, City: Tokyo) laser rangefinder and PowerTunnel Plus specialized software to measure tunnel overbreak. The laser rangefinder is an integrated device combining an Electronic Distance Measurement (EDM) tool and a Theodolite, allowing both distance and angle to be measured simultaneously without additional equipment. The method for measuring tunnel overbreak with the laser rangefinder involves inputting the designed tunnel cross-sectional data, installing the laser rangefinder inside the tunnel after blasting, and directing the laser at the tunnel wall to measure distances and store the coordinates of each point [23]. The collected measurement data were then compared to the designed tunnel cross-sectional data to analyze the overbreak amount.
3. Experimental Results and Analysis
3.1. Ground Vibration Measurement Results and Analysis
The vibration was measured at the surface directly above the tunnel, and the vibration levels at each location were analyzed. The data measured at NO.1 indicated that, for general blasting, the PPV ranged from 0.468 to 0.805 cm/s, while, for combined blasting, it ranged from 0.495 to 0.730 cm/s. Based on the maximum PPV, the blasting vibration speed was approximately 9.3% lower in the combined blasting. At NO.2, the PPV for general blasting ranged from 0.892 to 1.260 cm/s, while, for combined blasting, it ranged from 0.570 to 0.838 cm/s. Based on the maximum PPV, the blasting vibration speed was approximately 33.5% lower in the combined blasting.
The result of lower PPV during combined blasting is considered to be due to the formation of cracks between the pre-splitting holes and voids, which caused the blasting vibrations in the cut and expansion sections to be blocked at the periphery, thereby reducing vibrations.
To analyze the vibration reduction characteristics of similar methods, the relevant literature was reviewed, and correlations were drawn. According to a study that analyzed Smooth and Presplit Blasting through experiments [24], the blast vibration levels at 30 and 50 m showed a reduction of 14% and 40%, respectively, with the Presplit Blasting method. The vibration reduction rate for FEREX excavation, a method similar to Combined Blasting, was between 25% and 39% [25]. FEREX blasting, which uses Pre-Splitting and Empty Holes, follows the same principle of vibration reduction but differs in that it uses Fring units instead of conventional explosives. Nonetheless, the vibration reduction rates of similar methods range from 14% to 40%, which shows a similar correlation to the 33.5% reduction in vibration observed with Combined Blasting.
The reason for the higher vibration measured at NO.2, which is farther from NO.1, is not entirely clear, but it is speculated that the NO.1 measurement point was installed in the opposite direction of the tunnel excavation, causing the blasting vibrations to be higher at NO.2. This result is consistent with the study by Seo Yeong-chun et al. (2001) titled “A Study on the Vibration Propagation Characteristics of Tunnel Blasting,” which reports that, during tunnel blasting, the magnitude of vibration decreases in the order of the tunnel’s advancing direction, perpendicular direction, and opposite direction.
Table 3 shows the vibration data measured for both general blasting and combined blasting methods and Figure 7 is a comparative graph.
3.2. Analysis of Pre-Crack Effect
To analyze the vibration characteristics at each measurement location, a time-lag analysis was conducted. Time-lag analysis is a method used to observe the magnitude of vibrations occurring at different times of detonation, making it useful for comparing and analyzing vibration levels across different regions of the tunnel. The results of the time-lag analysis for each blasting cycle and measurement point are shown in Figure 8.
Analyzing the vibration levels in the cut area, the maximum vibration speed during general blasting was 0.894 cm/s, while, during combined blasting, it was 0.595 cm/s, showing a 33.4% reduction in vibration. Similarly, in the expansion, periphery, and floor sections, the vibration levels reduced from 1.260 cm/s to 0.697 cm/s, resulting in a 44.6% reduction in vibration. This reduction is believed to be due to the formation of cracks between the pre-splitting holes and voids, which blocked the blasting vibrations from propagating to the periphery, thus reducing vibrations.
However, during combined blasting, the vibrations at the periphery were larger than those in the cut, expansion, and floor sections. This is thought to be due to the energy from pre-splitting blasting not forming complete cracks, with some of the energy being converted into elastic waves that contribute to the blasting vibrations [26,27,28,29].
The factors influencing blast vibration propagation can be classified into geological conditions and blasting conditions, as summarized in Table 4 [30,31,32,33]. Blast vibration is most strongly correlated with the amount of explosives used and the distance to the measurement point. Additionally, geological conditions also have a significant impact. These geological conditions are unchangeable parameters [16,34], and factors such as joint spacing, orientation, and fracture zones significantly influence blast vibration propagation [35,36].
To minimize blast vibration, it is necessary to adjust the blasting conditions, which are variable parameters. The most influential factors include the type of explosive, charge per delay, and delay time of detonation. Additionally, drilling angle, burden, and hole spacing also affect vibration levels.
When conducting pre-splitting blasting to minimize vibration, the type of explosive and burden and hole spacing are considered the most critical factors. Low-vibration explosives exhibit a detonation velocity of approximately 60% of that of emulsion explosives, resulting in relatively lower vibration levels [37]. Furthermore, reducing the minimum burden helps decrease vibration levels [38].
3.3. Overbreak Measurement Results and Analysis
Table 5 shows the data measuring the overbreak after blasting. To compare the overbreak amounts, the analysis was conducted based on the sections where combined blasting occurred (measured from 3 to 11). The maximum overbreak amount was measured based on the vertical distance from the over-excavated point to the planned excavation line, and the overbreak cross-sectional area was calculated as a two-dimensional area between the excavation line and the overbreak line.
The maximum overbreak amount during combined blasting was 0.458 m, while the maximum overbreak amount for general blasting was 0.558 m, indicating a reduction of approximately 17.9%. Upon comparing the overbreak cross-sectional areas, the average overbreak cross-sectional area during combined blasting was calculated to be 3.378 m2, while the average overbreak cross-sectional area for general blasting was 4.235 m2, showing a reduction of approximately 20.2%.
Overall, based on Point 7 (Tunnel Center Line), the left side of the tunnel shows less overbreak than the right side, indicating better rock conditions. In general blasting, there is a significant difference in overbreak between the left and right sides of the tunnel, whereas the variation is less pronounced in combined blasting. The overbreak on the left side, where the rock conditions are better, does not show a significant difference between general and combined blasting. On the other hand, the overbreak on the relatively poorer right side is significantly reduced during combined blasting. Therefore, it can be inferred that combined blasting can achieve a smoother excavation surface and reduce the amount of overbreak regardless of rock condition variations.
The small amount of overbreak observed during combined blasting occurred because, when the outer pre-splitting holes were detonated, a tensile stress wave was generated behind the compressive wave from the blast hole wall, leading to the formation of cracks. The concentration of tensile stress around the crack induction holes induced crack propagation, resulting in the formation of a smooth fracture surface.
4. Discussion and Consideration
In tunnel blasting, the location where vibrations are most significant is the cut area, where the confinement is greatest and the charge amount per blast is the highest. Various patented methods have been developed to reduce vibrations in the cut area [39]. However, the results observed in this experiment by the author showed a different outcome.
Analyzing the vibration propagation characteristics through time-lag analysis, as shown in Table 6, the maximum vibration point for general blasting was found in the expansion section. Therefore, to reduce vibrations, it would be more effective to apply controlled blasting in the peripheral areas that can block vibrations in the direction of sensitive structures. In addition, for combined blasting, the largest vibrations were observed at the pre-splitting blast holes. It is believed that the vibrations were higher due to constraints such as the drilling conditions of the crack induction holes and the size of the hole diameter during pre-splitting blasting.
Kim et al. (2011) stated that, in the case of Smooth Blasting with pre-split cracks induced by empty holes, smooth fracture surfaces are formed up to a spacing of 40 cm [28]. Lee et al. (2018) explained that the shape of the guide hole (circular, notched, or diamond-shaped) affects the crack control effect through experiments on crack control in blasting [17]. Therefore, to increase crack formation and suppress significant vibration during the preliminary blasting of peripheral holes, we plan to research the shape, size, and spacing of crack-inducing holes, as well as the explosive charge and firing sequence of pre-split blasting holes. To reduce errors from these various variables, dynamic numerical analysis and scaled model experiments should be conducted, and validation through full-scale field experiments is required.
When the principles of combined blasting are established and applied to underground tunnel blasting, it is expected to reduce overbreak, lower reinforcement costs, and minimize vibrations, thereby preventing damage to adjacent structures. This would ultimately result in more economical blasting operations, both directly and indirectly.
5. Conclusions
To summarize our findings, a comparison of the blasting effects between the combined blasting method using crack induction holes and pre-crack blasting, and the conventional Smooth Blasting method was conducted through full-scale blasting experiments at a railroad tunnel construction site. The propagation characteristics of ground vibrations and overbreak amounts were analyzed for both methods, leading to the following conclusions:
(1). Based on the analysis of the blast vibration reduction rate using the measured PPV for both conventional blasting and combined blasting, the vibration magnitude in combined blasting was found to be reduced by from 9.3% to 33.5% compared to conventional blasting.
(2). Due to the pre-splitting effect during combined blasting, the blast vibration in the core section was reduced by 33.4%, while the vibration in the enlarged section, perimeter, and floor areas was reduced by 44.6%.
(3). The amount of overbreak during combined blasting was reduced by 17.9%, and a comparative analysis of the overbreak cross-sectional area confirmed a reduction of approximately 20.2%.
(4). An analysis of the blasting vibration transmission characteristics through time-differential analysis revealed that, in general blasting, the maximum vibration occurred at the expansion area. To reduce vibrations, it is recommended to apply controlled blasting in the peripheral area during blasting to block vibrations in the direction of sensitive structures.
(5). When the crack induction hole and pre-splitting blasting methods are applied to underground tunnel blasting, it can reduce overbreak, lower reinforcement costs, and minimize vibration generation, thereby preventing damage to adjacent structures. This is expected to result in both direct and indirect economic benefits for the blasting operation.
Data curation, J.-W.Y.; formal analysis, J.-W.Y.; investigation, J.-W.Y.; methodology, J.-W.Y.; project administration, J.-W.Y.; validation, J.-W.Y.; writing—original draft preparation, J.-W.Y.; conceptualization, S.S.L.; project administration, S.S.L.; supervision, S.S.L.; writing—review and editing, S.S.L. All authors have read and agreed to the published version of the manuscript.
Not applicable.
Not applicable.
Data is available upon request unless classied as confidential.
The authors declare no conflicts of interest. The funders had no role in the design of this study, the collection, analyses, or interpretation of data, the writing of the manuscript, or the decision to publish the results.
Footnotes
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Enlarge this image.Figure 2. Rock conditions at the experimental blasting site. (a) Geological map; (b) face condition.
Enlarge this image.Figure 4. Experimental blasting pattern diagram. (a) General blasting pattern; (b) combined blasting pattern; (c) charging details.
Enlarge this image.Figure 4. Experimental blasting pattern diagram. (a) General blasting pattern; (b) combined blasting pattern; (c) charging details.
Enlarge this image.Figure 5. Image of experimental blasting. (a) Cut area; (b) explosives and detonators used; (c) drilling measurement; (d) smooth blasting; (e) combined blasting.
Enlarge this image.Figure 7. Comparison graph. (a) Measurement point NO.1; (b) measurement point NO.2.
Enlarge this image.Figure 8. Blasting vibration analysis by area. (a) General blasting; (b) combined blasting.
Step-by-step test blasting process and key details.
| Sequence | General Blasting | Combined Blasting | Key Points |
|---|---|---|---|
| Marking | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | Reduce drilling error |
| Dilling | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | Drilling length: 2.2 m |
| Charging | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | Main blast: Emulsion |
| Stemming | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | Stemming: Sand |
| Connect | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | Non-electric detonator |
| Basting and | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | Location: Directly above |
| Overbreak measurement | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | Use of dedicated equipment |
Detailed specifications and installation of the blasting vibration meter.
| Classification | Specifications | Instrument Installation View | |
|---|---|---|---|
| Seismic | Range | 254 mm/s | [Image omitted. Please see PDF.] |
| Trigger Levels | 0.127–254 mm/s | ||
| Air Linear | Range | 88–148 dB | |
| A Weight | Range | 50–110 dB | |
| Event Storage | Full Waveform Events | 300 EA | |
| Full Waveform | Fixed Record Mode | Manual, Single Shot, | |
| Battery Life | - | 10 Days | |
Blasting vibration measurement data.
| Sequence | Blasting Method | Vibration Levels at Each Measurement Point (cm/s) | |||
|---|---|---|---|---|---|
| NO.1 | NO.2 | ||||
| PPV | Max PPV | PPV | Max PPV | ||
| 1st | General | 0.468 | 0.805 | 1.260 | 1.260 |
| 2nd | 0.805 | 0.892 | |||
| 3rd | Combined | 0.495 | 0.730 | 0.838 | 0.838 |
| 4th | 0.730 | 0.570 | |||
Group of factors affecting blast-induced vibration.
| Group | Type | Parameter | Degree of Effect | ||
|---|---|---|---|---|---|
| Severe | Moderate | Mild | |||
| Geological conditions | Uncontrollable variables | Distance between blasting point and structures | ○ | ||
| Overburden | ○ | ||||
| Topography and stratum | ○ | ||||
| Rock condition | ○ | ||||
| Atmospheric condition | ○ | ||||
| Blasting conditions | Controllable variables | Type of explosive | ○ | ||
| Charge per delay | ○ | ||||
| Delay time of detonation | ○ | ||||
| Drilling angle | ○ | ||||
| Burden and hole spacing | ○ | ||||
| Stemming conditions | ○ | ||||
| Primer position | ○ | ||||
| Charge weight | ○ | ||||
Overbreak measurement data after blasting.
| Classification | General Blasting | Combined Blasting | ||
|---|---|---|---|---|
| 1st Blasting | 2nd Blasting | 3rd Blasting | 4th Blasting | |
| Overbreak | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] |
| Overbreak | [Image omitted. Please see PDF.] | |||
| Overbreak | [Image omitted. Please see PDF.] | |||
Maximum blasting vibration occurrence area.
| General Blasting | Combined Blasting | Remarks | |
|---|---|---|---|
| [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | First |
| [Image omitted. Please see PDF.] | Second | ||
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