Blast-induced Fracture Research Articles

Energy control and block performance optimization of bench blasting

In this study, rock-like model blast tests with varying toe burdens are conducted and the evolution of blast-induced fractures is analyzed using the digital image correlation (DIC) method. In addition, post-blast fragmentations are image-processed to quantitatively investigate the blasting performance. A three-dimensional numerical model is developed and validated against the experimental results. The effects of blasthole inclination (70°, 80°, and 90°) and short delays on fragment size distribution (FSD) are numerically investigated. Field trials are conducted to verify the numerical results. The experimental results reveal that the overall fragment size increases significantly in proportion to the toe burden. The explosion energy utilization rates range from 3.56 % to 13.16 %, with a tendency to initially increase and subsequently decrease. The numerical results demonstrate that fragments are more evenly distributed with a blasthole inclination of 70°, resulting in a remarkable improvement in rock fragmentation compared to a vertical blasthole. Compared to the toe burden, the influence of short delays on concrete fragmentation is insignificant. The field test results indicate that a narrower FSD range and stable pit walls are achieved by utilizing inclined blastholes in bench blasting. This study provides theoretical guidance for optimizing bench blasting parameters, which has practical significance for improving explosive energy utilization and production efficiency in open-pit mines.

Cracking behavior of brittle materials under eccentric decoupled charge blasting

The decoupled charge structure is often used in controlled blasting. However, in actual engineering scenarios, boreholes are typically horizontal or inclined. The explosive is close to the borehole wall because of gravity action, resulting in an eccentric charge structure. In this study, concentric and eccentric decoupled charge blasting tests were conducted on 250 mm × 250 mm × 150 mm cuboid concrete specimens. The blast-induced fracture propagation behavior was evaluated using digital image correlation (DIC) technology, and surface fracture networks were extracted by image processing to investigate the fractal characteristics. A three-dimensional finite element model was established and verified using experimental data to reveal the influencing mechanism of the coupling medium and eccentricity coefficient on concrete fractures from the energy perspective. Finally, the borehole wall pressure (BWP) and displacement distribution with different charge structures and coupling media are analyzed theoretically, and the application of an eccentric charge structure in smooth blasting is discussed. The results reveal that nonequivalent BWP and displacement are produced in eccentric charge blasting, with a more evident crushed zone appearing on the coupled side owing to the higher energy utilization efficiency of the explosive. Sand had the largest fracture volume fraction ratio between the coupled and decoupled sides, effectively controlling the damage level on the decoupled side. The proposed eccentric decoupled charge-blasting scheme produced directional damage effects in the field test, maximizing the failure of the excavation rock while reducing the damage level to the reserved rock.

Crack propagation mechanism of smooth blasting holes for tunnel excavation under high in-situ stress

Smooth blasting is an important excavation method in tunnel construction, creating a smooth and even excavation profile during the excavation process. In this study, the influence of in-situ stress, hole spacing, and radial decoupling coefficient on the superposition of blasting stress and the cracking characteristic between holes were investigated by using integrated analytical and numerical analyses (FDEM). Based on those results, the design of smooth blasting parameters considering the influence of in-situ stress were suggested, and verified against the data collected during the blasting excavation of diversion tunnel at Jinping II hydropower station. The results indicated that the propagation of blast-induced fractures are significantly influenced by the insitu stress conditions, tending to extend towards the direction of the maximum principal stress thus the holes should be arranged along the direction of the maximum principal stress to ensure the formation of cracks between holes and smooth excavation of the wall.

Correction: Huang et al. The Effect of Preloaded Compressive Stress and Curvature of Defect on Blast-Induced Fracture Behavior by Caustic and Numerical Models. Mathematics 2023, 11, 4532

In the original publication [...]

Phase-Field Based Peridynamics Implementation to Model Blast-Induced Fracture in Brittle Solids

Phase-Field Based Peridynamics Implementation to Model Blast-Induced Fracture in Brittle Solids

Fracture Mechanics Induced by Drilling and Blasting in Underground Openings: Prevailing Practices and Studies

This article presents the prevailing practices and literature review on the study of fracture mechanics of rock mass, during the construction of underground openings, by drill and blast method (DBM). The paper makes an introductory discussion on basic fracture and failure mechanics on the rock, followed by a review due to the blast-induced failure mechanism. The damage zone in drill and blast openings is discussed which is followed by the conclusions of the literature. DBM is a widely acceptable and broadly applicable approach in the underground construction method. The blasting damaged zones are classified as the borehole expansion zone, crushed zone, nonlinear fracture zone, and radial crack propagation zone. Blasting activities create stress waves in construction work, which fundamentally influence the fracture and failure mechanics of rock with pre-existing discontinuities, in-situ stress, and groundwater conditions. On the other hand, the cumulative action of subsequent explosion gas and stress waves defines the final damage limit and crack pattern map. Thus, it is of utmost importance to investigate the blast-induced fracture and failure mechanism to mitigate the instability in the underground openings.

The Effect of Preloaded Compressive Stress and Curvature of Defect on Blast-Induced Fracture Behavior by Caustic and Numerical Models

In this study, the effects of preloaded uniaxial compressive stress and defect (prefabricated by laser cutting) curvature on blast-induced cracks and stress wave propagation were investigated in polymethyl methacrylate (PMMA) specimens using caustics theory. Based on the mathematical relationship between stress and optics, the fracture behavior (the propagation path, dynamic stress intensity factors (DSIFs), propagation velocity, initiation angle of the main crack, and damage degree between the defect and the blasthole) was calculated and analyzed quantitatively. The results show that the preloaded stress could mainly restrain the main crack propagation in the horizontal direction and reduce the initiation angle and damage degree at the defects. Meanwhile, the crack initiation position of the positive curvature defect was not at the end of the defect. In addition, the curvature of the defect significantly affected the blast-induced fracture compared to preloaded stresses. Comparing the main crack with other curvature defects, the DSIFs, velocity, crack arrest time, crack length, and horizontal offset distance of the main crack with negative curvature defect were the largest. The variation in the full stress field in the specimen under blasting and preloaded stress was simulated by a numerical model. The results show that the curvature of the defect has a significant effect on tensile waves at the end of defects, and the tensile stress from high to low was C-25, C0, and C25, in that order.

Quantifying the Permeability Enhancement from Blast-Induced Microfractures in Porphyry Rocks Using a Cumulant Lattice Boltzmann Method

The permeability of rocks is important in a range of geoscientific applications, including hbox {CO}_{2} sequestration, geothermal energy extraction, and in situ mineral recovery. This work presents an investigation of the change in permeability in porphyry rock samples due to blast-induced fracturing. Two samples were analysed before and after exposure to stress waves induced by the detonation of an explosive charge. Micro-computed tomography was used to image the interior of the samples at a pixel resolution of 10.3,mu m. The images were segmented into void, matrix, and grain to help quantify the differences in the rock samples. Following this, they were binarised as void or solid and the cumulant lattice Boltzmann method (LBM) was applied to simulate the flow of fluid through the connected void space. A correction required with the use of inlet and outlet reservoirs in computational permeability assessment was also proposed. Interrogation of the steady-state flow field allowed the pre- and post-loading permeability to be extracted. Conclusions were then drawn as to the effectiveness of blasting for enhancing fluid accessibility via the generation of microfractures in the rock matrix within the vicinity of a detonated charge. This paper makes contributions in three fundamental areas relating to the numerical assessment of permeability and the enhancement of fluid accessibility in low-porosity rocks. Firstly, a correction factor was proposed to account for the reservoirs commonly imposed on digitised rock samples when investigating sample permeability through numerical methods. Secondly, it validates the benefits of the LBM in handling complex geometries that would be intractable with conventional computational fluid dynamics methods that require body-fitted meshing. This is done with a novel implementation of the cumulant LBM in the open-source TCLB code. Finally, the improvement in fluid accessibility in low-permeability rock samples was shown through the assessment of multiple regions within two blasted samples. It was found that the blast-induced loading can generate extended microfractures that results in multiple orders of magnitude of permeability enhancement if the target rock possesses existing weaknesses and/or mineralisation.

Coupling of material point and continuum discontinuum element methods for simulating blast-induced fractures in rock

A rock blasting simulation method is proposed that couples the material point method (MPM) and continuum discontinuum element method (CDEM). Blast-induced rock fractures are captured by the CDEM using normal and shear springs, and the explosive detonation is simulated by the MPM with a Jones-Wilkins-Lee equation of state. A particle–surface/edge contact method is introduced into the MPM-CDEM to calculate the interaction between the detonation products and rock medium. Three numerical examples are presented to verify the effectiveness of the proposed method. The fracture degree is represented as the ratio of the number of fractured springs to the total number of springs, and is used to analyze the evolution of shear and tension cracks under blasting. The simulation results show that the proposed numerical method well simulates blast-induced rock fractures and considers both progressive rock fracturing and the real explosive detonation. In particular, the expansion of the detonation gas, crushed zone around the blasthole, radial cracks, and effects of pre-existing stress on the blast-induced fractures are all successfully simulated.

Investigating the mechanism of rock fracturing induced by high-pressure gas blasting with a hybrid continuum-discontinuum method

To study the rock-breaking mechanism resulting from high-pressure gas blasting, a hybrid continuum-discontinuum method considering the actual breakage of rocks was presented in this study. The mechanical parameters were calibrated against the uniaxial compression test and Brazilian disc test, and the capabilities of simulating blast-induced fractures were verified by laboratory tests. After that, the differences in rock-breaking mechanism between explosive blasting and high-pressure gas blasting were compared from the perspectives of peak pressure and loading rate. The results indicated that the crack patterns, fracture mode and vibration attenuation induced by high-pressure gas blasting were significantly different from that of explosive blasting: (a) no crushing zone and fewer radial cracks were formed in high-pressure gas blasting and the spalling cracks were absent near the free face in high-pressure gas blasting; (b) the difference of rock fracture mode in high-pressure gas blasting and explosive blasting indicated that the tensile fracturing was the dominant fracture mode in high pressure gas blasting which is different from the catastrophic fragments due to compressive-sheared fracturing surrounding the borehole in explosive blasting; (c) the intensity and attenuation factors of peak particle velocity both increased with the increase of peak pressure or loading rate, which indicated that the peak particle velocity induced by explosive blasting results in more intense and faster attenuated vibration disasters than that of high-pressure gas blasting.

Numerical Study of Fracture Characteristics of Deep Granite Induced by Blast Stress Wave

To study the characteristics of rock fracture in deep underground under blast loads, some numerical models were established in AUTODYN code. Weibull distribution was used to characterize the inhomogeneity of rock, and a linear equation of state was applied to describe the relation of pressure and volume of granite elements. A new stress initialization method based on explicit dynamic calculation was developed to get an accurate stress distribution near the borehole. Two types of in situ stress conditions were considered. The effect of heterogeneous characteristics of material on blast-induced granite fracture was investigated. The difference between 2D models and 3D models was discussed. Based on the numerical results, it can be concluded that the increase of the magnitude of initial pressure can change the mechanism of shear failure near the borehole and suppress radial cracks propagation. When initial lateral pressure is invariable, with initial vertical pressure rising, radial cracks along the acting direction of vertical pressure will be promoted, and radial cracks in other directions will be prevented. Heterogeneous characteristics of material have an obvious influence on the shear failure zones around the borehole.

Numerical investigation of blast-induced fractures in granite: insights from a hybrid LS-DYNA and UDEC grain-based discrete element method

Numerical investigation of blast-induced fractures in granite: insights from a hybrid LS-DYNA and UDEC grain-based discrete element method

Effect of Stress Anisotropy on the Efficiency of Large-Scale Destress Blasting

Large-scale panel destressing is a rockburst control technique that is used to create a stress shadow in the ore pillar to be mined. The technique aims to reduce the pillar burst proneness by deviating the major induced principal stresses away from the concerned zone of interest. The destress panels, situated in the pillar hanging wall, are choke-blasted with high explosive energy density, and the blast-induced damage in the panel is accompanied by stress dissipation and stiffness reduction due to fragmentation in the panel. These two effects are traditionally modeled holistically with stiffness and stress reduction factors α and β, respectively, applied to the destressed zone. This paper focuses on the interpretation of Phase 3 destress blasting results at Copper Cliff Mine (CCM) where a stress increase (rather than decrease) was detected in the ore pillar crown, while a stress decrease was recorded in the ore pillar sill (as expected). It is hypothesized that high mining-induced stress anisotropy in the pillar crown caused blast-induced fractures to propagate in the orientation of the major principal stress, a condition that would hinder the destressing effect in that orientation. To verify the hypothesis, a series of panel anisotropic rock fragmentation and stress dissipation factors are iteratively tested in a 3-dimensional back analysis of the Phase 3 destress blast. The analysis takes into consideration the stope extraction schedule per the mine plan to better replicate the mining-induced stress condition in the panel and the ore pillar. The results show good agreement with stress measurements taken in situ using borehole stress cells installed in the ore pillar prior to destressing. The paper discusses the implications of preferential fracture propagation orientation and how it might affect the efficiency of destress blasting.

Experimental testing and numerical simulations of blast-induced fracture of dolomite rock

In this paper, the Johnson-Holmquist II (JH-2) model with parameters for a dolomite rock was used for simulating rock fragmentation. The numerical simulations were followed by experimental tests. Blast holes were drilled in two different samples of the dolomite, and an emulsion high explosive was inserted. The first sample was used to measure acceleration histories, and the cracking pattern was analyzed to perform a detailed study of the blast-induced fracture to validate the proposed method of modelling and to analyze the capability of the JH-2 model for the dolomite. The second sample was used for further validation by scanning the fragments obtained after blasting. The geometries of the fragments were compared with numerical simulations to further validate the proposed method of modelling and the implemented material model. The outcomes are promising, and further study is planned for simulating and optimizing parallel cut-hole blasting.

Numerical modeling of the fractured zones around a blasthole

The high-energy rate of shock waves, as an important part of explosion energy, plays a significant role in the mechanism of rock fragmentation. This energy rate and the mechanism of rock fracturing are controlled by several parameters related to both the explosive charge and rock mechanical properties. In this study, the mechanism of rock fragmentation due to blast-induced shock waves in a single blasthole was numerically simulated by a two-dimensional discrete element code. The numerical results included the stresses around the blastholes, the number of blast-induced fractures and the density of cracks, which were compared with the corresponding experimental results. This comparison showed that the numerical results were in good agreement with their experimental counterparts. It is concluded that the proposed numerical model could be effectively used for the simulation of the crack propagation process around a blasthole.

Numerical simulation of blasting in confined fractured rocks using an immersed-body fluid-solid interaction model

We model blast-induced fracturing and fragmentation processes in fractured rocks using a fully coupled fluid-solid interaction model. This model links a finite-discrete element solid solver with a control volume-finite element fluid solver through an immersed-body method. The solid simulator can capture the deformation of intact rocks, interaction of matrix blocks, displacement of existing fractures and propagation of new cracks. The fluid simulator can simulate the highly compressible gas flow involved in the blasting and explosion process, which is assumed to follow the John-Wilkins-Lee equation of state. We design numerical experiments as follows. First, we generate a series of 1 m × 1 m discrete fracture networks associated with different fracture density and mean length values to consider various scenarios of distributed pre-existing fractures in rock. We apply isotropic/anisotropic in-situ stresses to the rock such that the system reaches an equilibrium state. Then we release the compressible gas associated with a prescribed high pressure in the borehole to simulate explosion, which engenders stress wave propagation and new crack generation in the system. We observe that the presence of natural fractures has a significant impact on the blast behaviour of fractured rocks such that new cracks tend to be arrested by pre-existing discontinuities which however accommodate wing cracks at their tips linking with other structures. Blast-driven cracks attempt to propagate along the maximum principal stress direction if an anisotropic stress condition is imposed. Our research findings have important implications for the design and assessment of blasting for underground excavation in fractured formations.

Numerical modelling of blast-induced fractures in coal masses under high in-situ stresses

Borehole blasting is a technique widely applied to form fracture networks in coal masses. There are many original cracks in coal masses, such as bedding planes and cleat planes. Coal masses are also subjected to high in-situ stress deep underground. However, the effects of pre-existing cracks and in-situ stress on blast-induced fractures are not well understood. In this paper, the isotropic and kinematic hardening plasticity model considering compression and tensile failure is introduced to numerical models, and the contact interface is used to simulate the effect of jointed planes on coal-mass blasting. The effects of jointed plane, constant in-situ stress and lateral pressure coefficient on blast-induced cracks are explored, and the relative peak displacement (Δup) of jointed planes is considered as the basis for determining jointed plane failure. The results indicate that blast-induced cracks tend to expand along jointed planes in the coal mass; the in-situ stress enhances the compression effect and weakens the tension effect in the radial direction of the borehole; and the jointed plane failure zone in coal masses decreases with increasing in-situ stress. The lateral pressure coefficient also has a distinctive influence on blast-induced crack expansion.

Internal Fractures After Blasting Confined Rock and Mortar Cylinders

Blast-induced fines in rock negatively influence multiple aspects of raw-mineral sustainability. The Austrian Science Fund (FWF) sponsored a project to investigate the cause of the fines by studying blast fragmentation through small-scale blast tests and numerical simulations. The paper covers the experimental part of the project focusing on internal blast-induced fracturing and related mechanisms. The blast tests were done by blast-loading confined granite and mortar cylinders. The blast-driven dynamic cracking at the end face of the cylinder opposite to the initiation point was filmed with a high-speed camera. Following analyses covered internal crack patterns, fracture surfaces, and sieving of the blasted cylinders to quantify the amount of fine material created. The internal crack patterns and fracture surfaces were analysed by means of computer tomography (CT) and scanning-electron microscopy (SEM). The CT scans show that the amount of explosive charge affects the changing of the topological features of the crack patterns along the cylinder. They also depict different deformation zones around the blast-hole wall with respect to the blasted material and the amount of charge. Although fracture surfaces of larger fragments do not clearly differ in measured roughness and curvature, the SEM scans of smaller fragments show clear difference in fracture surfaces with respect to the blasted material and the amount of charge. SEM scans of thin sections extracted from the blasted cylinders show different fracture features that could be related to the branching/merging mechanism.

Numerical study on mechanical and hydraulic behaviour of blast-induced fractured rock

In this research paper, smoothed particle hydrodynamics (SPH) with Johnson Holmquist damage model is adopted for modelling of the blast-induced fractures in Barre granite rock. The permeability of the blast-induced rock is then obtained using the implemented finite volume method code in OpenFOAM. It is found that the calculated permeability depends on the direction of fluid flow and with higher value in radial direction than the axial one. This is mainly due to the higher and larger connected pore network in the radial direction. This research work shows that the adopted SPH method along with finite volume method code can be effectively combined to qualitatively and quantitatively predict the fractured network, to analyse geometry of the fractured network, and to calculate the permeability of blast-induced rock.

Effects of vertical and horizontal reflected blast stress waves on running cracks by caustics method

Wave reflection which is inevitable in the explosion plays a significant role in blast-induced fracture. In this paper, transient effects of vertical (P1P) and horizontal (P2P) reflected blast stress waves with wave fronts perpendicular and parallel to crack surfaces, respectively, on blast-induced cracks in a PMMA plate are investigated in the same explosion by the caustics method with high-speed photography. Reflected P and S waves are identified by bright and dark fringes and asymmetrical caustics patterns, which is verified by theoretical calculations. The emphasis is placed on the geometry of distorted caustics patterns which are not consistent with the classical ones. It is found that K-dominated field in the crack tip is violated with unstable crack propagation under reflected P waves, while it is available with stable crack propagation under reflected S waves. To precisely interpret distorted caustics patterns, a novel and simple model based on geometrical optics is established, agreeing well with experimental results and providing accurate dynamic stress intensity factors (SIFs) which are verified by the KId-v relationship. Through accurate SIFs, it is clear that vertical P1P waves tend to cause crack arrest with lower SIFs and crack velocity, while horizontal P2P waves assist in crack opening with higher SIFs and crack velocity. Finally, problems of SIFs measurements by the caustics method in blast-induced dynamic fracture are discussed.