Effect of Sequential Free-Face Formation on Specific Charge, Advance Rate, and Vibration in Rock Blasting Excavation = 암반 발파 굴착에서 순차 자유면 형성이 비장약량, 굴진율, 발파진동에 미치는 영향

국문 초록 (Abstract)

최근 터널 및 지하공간 건설이 심부화 및 대단면화됨에 따라 높은 굴착 효율성이 요구되는 한편, 도심 및 환경 민감 지역의 공사 증가로 엄격한 발파진동 규제 준수가 동시에 요구된다. 암반 파쇄와 진동 전파에 결정적 영향을 미치는 자유면(free face)은 중요한 설계 변수로 알려져 있으나, 그 정량적 효과에 대한 체계적 비교 연구와 근거자료는 상대적으로 부족하다. 특히 실제 현장 조건을 반영하는 순차발파(sequential blasting)에서 지연 기폭에 따라 단계적으로 형성되는 자유면의 복합적인 영향에 대해서는 관련 연구가 미흡하여, 이에 대한 역학적 분석이 요구된다. 이에 본 연구는 암반 발파에서 자유면의 수와 순차발파에 따른 자유면 형성이 굴진율, 비장약량, 발파진동에 미치는 영향을 정량적으로 규명하고, 이를 통해 효율과 안정성을 동시에 향상시킬 수 있는 발파 설계의 공학적 기반을 마련하는 데 연구의 목적이 있다.
본 연구는 수치해석으로 다양한 변수의 정량적 영향을 분석하고, 축소 모형실험으로 그 경향성과 실제 파괴 거동을 교차 검증하는 통합적 연구 방법을 통해, 자유면 수와 순차발파에 따른 자유면의 발파 효과를 분석하였다. 수치해석은 문헌의 누두공 발파실험(crater blasting test) 결과와의 비교를 통해 모델의 신뢰도를 우선 검증하였으며, 검증된 모델을 이용하여 이상적인 2, 3, 4자유면 조건에서의 단일공 발파와 선행 발파에 의한 암반의 예비손상효과를 고려한 순차발파 시뮬레이션을 수행하였다. 이후, 수치해석과 동일한 조건으로 제작된 대형 콘크리트 시험체에서 실제 발파 실험을 수행하여 해석 결과의 경향성을 검증하고 주요 발파 거동을 분석하였다.
연구 결과, 자유면의 수가 2개에서 4개로 증가함에 따라 굴진율은 점진적으로 향상되고 최대입자속도는 일관되게 감소하는 경향을 보여, 자유면 확보가 발파 성능 향상의 핵심 요인임을 확인하였다. 특히 본 연구를 통해 순차발파로 생성된 자유면(sequential free face)이 3, 4자유면 단일공 발파보다 우수한 성능을 나타냄을 규명하였다. 실제 실험에서 2자유면 단일공 발파 대비 순차적 3자유면(3SF) 조건의 굴진율은 13.7% 향상되었으며, 이는 이상적인 4자유면 단일공 발파 시의 굴진율 향상 13.1%보다 높은 수치이다. 진동 저감 효과 또한 뚜렷하게 나타나, 95% 신뢰수준 상한치 기준으로 3SF 조건의 PPV는 2자유면 단일공 발파 대비 58.0% 감소하였으며, 이는 4자유면 단일공 발파 조건의 진동 저감률 47.5%보다 높은 저감 효과이다. 또한, 비장약량에 대한 수치해석 결과, 목표 굴진율 및 파괴각도를 유지하는 조건에서 3자유면과 4자유면 단일공 발파는 2자유면 대비 약 30%의 장약량 절감이 가능하였다. 순차발파 조건에서는 동일한 성능 기준을 만족하면서 후속공의 장약량을 최대 40%까지 절감할 수 있는 것으로 나타나, 이는 진동 제어의 추가적인 수단이 될 수 있음을 확인하였다.
순차발파의 이러한 효율 향상은 선행 발파가 유발하는 암반의 예비손상 효과와 유효 저항선 감소로 설명할 수 있다. 예비손상 효과는 미세균열망을 형성하여 후속 발파에너지의 방사형 전파를 차단 및 산란시키고, 동시에 파쇄 방향으로의 에너지 집중을 유도한다. 또한, 선행 발파로 형성된 파쇄면은 후속공의 유효 저항선을 감소시켜 인장파의 반사 효율을 높인다. 이러한 복합 메커니즘은 굴착 효율을 극대화하고 지반 진동은 최소화하는 역할을 한다. 이러한 연구 결과를 바탕으로, 본 연구에서는 각 발파 라운드에서 자유면 구성에 따라 발파면을 구역화하고, 2자유면(2F) 구역을 최소화하고 순차적 3자유면(3SF) 구역을 최대화하도록 기폭순서를 설계하는 자유면 구역화 발파 패턴(Free-Face-Zoned blasting pattern)을 제안하였다. 본 연구의 결과는 향후 터널 발파 설계 시 기존의 경험적 접근을 보완하고, 자유면 효과를 극대화하는 설계를 통해 굴착 효율 증대, 발파진동 저감으로 인한 안정성 향상 및 장약량 절감에 따른 경제성 확보에 기여할 것으로 기대된다.

다국어 초록 (Multilingual Abstract)

As tunneling and underground construction trend toward longer tunnel alignments, larger cross-sections, and greater depths, high excavation efficiency is required. In parallel, the expansion of construction in urban and environmentally sensitive areas has led to increasingly stringent regulations regarding blast-induced vibrations, making strict control imperative. A free face is defined as an exposed, traction-free rock surface. At a free face, expansion space for gases and fragmented material is provided, and incident compressive waves are reflected as tensile waves. Consequently, fracture initiation near the surface, the preferred direction of crack growth, fragmentation, and vibration propagation are strongly influenced by the presence and geometry of free faces. Despite this central role in blasting, systematic comparative studies and empirical data that quantify these effects remain limited, particularly regarding the influence of the number of free faces and their sequential formation. Therefore, the objective of this study is to quantitatively evaluate how the number of free faces and their sequential formation in sequential blasting affect advance rate, specific charge, and blast-induced vibration, thereby establishing an engineering basis for blast designs that simultaneously enhance efficiency and stability.
In this study, the two-, three-, and four-free-face configurations are denoted 2F, 3F, and 4F, and the corresponding sequentially formed three- and four-free-face configurations are denoted 3SF and 4SF, respectively. To investigate these effects, an integrated numerical and experimental methodology was employed, combining three-dimensional dynamic finite element analysis (FEM) with scaled model tests to cross-validate trends and actual fracture behavior. The numerical model was first validated against crater blasting test results from the literature. The validated model was then used to simulate single-hole blasts under idealized 2F, 3F, and 4F configurations, as well as sequential blasting simulations that accounted for preconditioning. Here, preconditioning refers to the alteration of rock properties within a damage zone resulting from preceding blasthole detonations in the same round. Subsequently, blasting experiments were conducted on large-scale concrete specimens fabricated under the same conditions as those used in the numerical analysis to verify the simulation trends and analyze the primary blasting behavior.
The results confirmed that, as the number of free faces increased from two to four, the advance rate progressively improved. At the same time, the peak particle velocity (PPV) consistently decreased. This verifies that increasing free-face availability is a key factor in enhancing blasting performance. A principal finding of this study is that sequentially generated free faces outperform 3F and 4F single-hole blasts. Specifically, in the experiments, the sequential three-free-face (3SF) configuration achieved a 13.3% increase in advance rate over the 2F single-hole baseline. This exceeded the 12.8% increase from the idealized 4F single-hole blast. Regarding vibration reduction, the 3SF configuration reduced PPV by 58.0% at the upper 95% confidence limit compared with the 2F baseline. This was greater than the 47.5% reduction observed under the 4F single-hole configuration. Furthermore, numerical analysis revealed that, while maintaining the target advance rate and breakout angle, 3F and 4F single-hole blasts enabled a charge reduction of approximately 30% compared to the 2F baseline. Under sequential blasting, charges for subsequent holes could be reduced by up to 40% while maintaining the same performance criteria. This confirms sequential blasting as an additional means of vibration control.
This enhanced efficiency in sequential blasting is attributed to preconditioning from preceding detonations and a reduction in effective burden. Preconditioning creates fracture networks that promote wave scattering, attenuate the vibration source, and partition explosive energy preferentially toward fragmentation and rock movement. Concurrently, the additional free faces generated by preceding detonations become the new nearest free face, reducing the effective burden for subsequent charges. The smaller effective burden strengthens tensile-wave reflection at the free face, enhancing breakage. Based on these findings, a Free-Face-Zoned blasting pattern was proposed that partitions the round into zones by free-face configuration and designs a delay sequence to minimize 2F zones and maximize 3SF zones. The results of this study are expected to supplement conventional empirical approaches in future tunnel blast design, contributing to improved excavation efficiency, enhanced stability through vibration reduction, and increased economic feasibility via charge reduction by utilizing designs that maximize free-face effects.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• Chapter 2. Literature review 5
• 2.1 Fracture mechanisms induced by free faces 5
• 2.2 Influence of free faces on fragmentation 8
• 2.3 Influence of free faces on specific charge 12
• 2.4 Influence of free faces on advance rate 18
• 2.5 Influence of free faces on blast-induced vibration 26
• 2.6 Synthesis of prior work and research gaps 32
• Chapter 3. Numerical analysis of effects of free faces on specific charge, advance rate, and blast-induced vibration 36
• 3.1 Modeling approach 36
• 3.1.1 Explicit dynamic finite-element analysis (LS-DYNA) 36
• 3.1.2 RHT constitutive model 41
• 3.1.3 Explosive equation of state (JWL) 51
• 3.1.4 Arbitrary LagrangianEulerian (ALE) coupling 54
• 3.1.5 Model parameters and input values 56
• 3.1.6 Mesh sensitivity and configuration 63
• 3.2 Model validation against small-scale crater tests 69
• 3.2.1 Experimental reference and test description 69
• 3.2.2 Validation against experiments 73
• 3.3 Parametric analysis of specific charge and advance rate across free-face configurations 79
• 3.3.1 Case definitions and model configuration 79
• 3.3.2 One-free-face configuration 86
• 3.3.3 Two-free-face configuration 87
• 3.3.4 Three-free-face configuration 99
• 3.3.5 Four-free-face configuration 109
• 3.3.6 Results and discussion on the effect of the number of free faces on specific charge and advance rate 119
• 3.4 Analysis of specific charge and advance rate under sequential blasting with progressive free-face expansion 128
• 3.5 Parametric analysis of blast-induced vibration across free-face configurations 139
• 3.5.1 Case definitions and model configurations 139
• 3.5.2 Results and discussion on the effect of the number of free faces on blast-induced vibration 141
• 3.6 Analysis of blast-induced vibration under sequential blasting with progressive free-face expansion 152
• 3.6.1 Case definition and model configuration 152
• 3.6.2 Results and discussion on the effect of blast-generated free face on blast-induced vibration 153
• Chapter 4. Experimental validation of numerical predictions for advance rate and blast-induced vibration under varying free face configurations 160
• 4.1 Experimental approach for single-hole blasting 161
• 4.2 Parametric analysis of excavated volume and advance rate across free-face configurations 167
• 4.3 Parametric analysis of blast-induced vibration across free-face configurations 173
• 4.4 Experimental approach for sequential blasting 180
• 4.5 Analysis of advance rate under sequential blasting with progressive free-face expansion 184
• 4.6 Analysis of blast-induced vibration under sequential blasting with progressive free-face expansion 189
• Chapter 5. Discussion 199
• 5.1 Synthesis of key findings on the dual benefit of strategic free-face utilization 199
• 5.2 Design implications and a conceptual free-face-zoned blasting pattern 202
• 5.2.1 Concept of Free-Face-Zoned blasting pattern 202
• 5.2.2 Systematic design guideline for proposed pattern 207
• 5.2.3 Comparative design analysis of conventional, proposed, and electronic blasting patterns 211
• Chapter 6. Conclusions 218
• References 223
• 초록(Abstract in Korean) 237