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This study investigates wave propagation of layered composite rock mass composed of multiple joints and assorted rock materials. Firstly, the effect of multiple-reflection between joints on wave transmission was explored by modifying the method of characteristics. Secondly, the superposition of direct and indirect transmission induced by multiple-reflection was analyzed. Waveform variations in wave transmission were compared across three different layered composite rock mass. Finally, the effects of wave parameters (frequency), joint parameters (stiffness and number), and rock parameters (wave impedance ratio) on both direct and indirect transmission induced by multiple-reflection were considered. The results indicate that the effect of indirect transmission induced by multiple-reflection on both particle velocity and stress transmission of waves cannot be ignored in many cases. The results further indicate that effect of indirect transmission induced by multiple-reflection on both particle velocity and stress transmission coefficients decreases with increasing incident wave frequency, while it increases with increasing joint numbers. The effect initially increases and subsequently decreases with increasing joint stiffness. Furthermore, the effect of indirect transmission induced by multiple-reflection on the particle velocity transmission coefficient increases while its effect on the stress transmission coefficient decreases with increasing wave impedance ratio. [ABSTRACT FROM AUTHOR]

Understanding fractures creating process during the dynamic indentation of a spherical carbide insert is a key factor to optimize the operation of a percussive drilling system and improve penetration rates and tool life while drilling in hard, abrasive rock formations. This study presents experimental results of dynamic indentation tests with combined action of an impulsive load and a static load of a bit using a single spherical carbide insert on five types of rocks. A dedicated and innovative experimental setup using a gas-gun drop test was developed to reproduce the loading forces and mechanical characteristics of a Down-the-Hole (DTH) hammer drill. A high-speed camera was used to visually determine when the major fractures, such as radial and side cracks, coalesced to create large fragments and to determine the bit displacement curve during the whole indentation process by tracking the end of the bit. To characterize the damage in the rocks, the mass of the fragments was weighed, and the Specific Energy (SE) was calculated. Experimental results showed a strong correlation between the major changes in the slope of the stress signals recorded in the drill bit and the major fragmentation events recorded with the high-speed camera. In the granites, large fragments typically associated to side cracks were generated during the loading phase. In contrast, in the limestone and the sandstone, the fragments were generated during the unloading phase. Finally, it was found that in granites, the earlier the side cracks occur relative to the maximum indentation time, the lower the efficiency of the indentation. [Display omitted] • The fracture behavior of five rocks was studied with the aid of a high-speed camera. • Three distinct rock responses based on major fracture events were established. • Side cracks in igneous rocks occurred before or at the end of the loading phase. • The specific rock energy was correlated to the early or lately detection of side cracks. [ABSTRACT FROM AUTHOR]

Gelatin is frequently used as a reservoir analogue for modeling hydraulic fracture (HF) growth in brittle, organic-rich rocks; its transparency enabling direct, visual observation of the HF growth. However, not many works have assessed the scalability of laboratory experiments on gelatin blocks with-respect-to "real-world" HF stimulation treatments. In this work, lab-scale parameters are compared against typical slickwater and crosslinked-gel stimulation treatment parameters from the Barnett Shale in northeastern Texas, for comprehensive (geometric, kinematic, and dynamic) dimensional-scaling evaluations. Dimensionless groups relating lab-scale physical models that display HF growth to the field-scale parameters are used to quantify the representation of elastic deformation, crack formation energy, treatment time-duration, and stress confinement, honoring the governing principles for fluid-driven fracture propagation in linearly-elastic media (applicable to HF growth in rocks, as well as magmatic intrusions, such as dykes or sills). The analysis demonstrates gelatin as a reservoir analogue to exhibit enhanced scaling against crosslinked gel compared to slickwater stimulation treatments, with the laboratory conditions accentuating toughness over viscous effects in comparison to field conditions, something which if neglected can lead to biased conclusions. Henceforth, a set of laboratory parameters is proposed for gelatin blocks of a practically-attainable powder concentration that yields improved scalability against the prototype parameters, minimizing the discrepancy between the lab-scale and the corresponding field-scale values for each dimensionless group, inside the limits of standard laboratory equipment. Identical dimensional-scaling evaluations are performed on lab-scale parameters from past experimental studies reported in the literature where gelatin samples were used, to the extent that the published data allows, indicating poor scaling of the physical phenomena. Results and conclusions from such inadequately-scaled experiments should be treated with suspicion. [ABSTRACT FROM AUTHOR]

This study theoretically investigates the scattering characteristics of blasting stress waves, treaded as elastic waves, at a linear interface crack in a deep rock mass with an even compressive in-situ stress field. First, the basic equations of the deep rock mass theoretically treaded as a homogeneous isotropic medium with an initial stress field are derived based on the linear elastic, finite deformation, plane strain, isotropic, and small deformation hypotheses, and incremental stress theory. The linear interface crack is theoretically treaded as two displacement-free boundaries, of which the interpolation displacements are described by introducing two dislocation density functions. Next, the mathematical model of the blasting stress wave scattering at the linear interface crack is established through a set of singular integral equations of the first type for these two dislocation density functions with Cauchy kernels. Finally, the dislocating and opening displacements of the linear interface crack and the displacement and effective stress fields of the scattered stress wave are numerically calculated. The principal stress field of the scattered stress wave is compared with the experimental data to qualitatively verify the rationality and correctness of the mathematical model. Theoretically, it is found that even compressive in-situ stress with an amplitude of tens of megapascals significantly affects the displacement and effective stress fields of the scattered stress wave near the linear interface crack. In conclusion, considering even compressive in-situ stress is greatly significant when studying the wave and scattering behaviors of deep rock masses. This mathematical model provides a theoretical guidance for the study of stress wave energy transformation and crack propagation in the process of rock blasting and mineral mining. [ABSTRACT FROM AUTHOR]

Dynamic impact tests were carried out on red sandstone specimens with single bonded planar joint at various angles based on a split Hopkinson pressure bar (SHPB), to investigate the stress wave propagation and fracturing evolution of the specimens comprising two blocks. The results indicate that the bonded joint will reduce the dynamic strength from 3.10% to 32.13% compared with the intact specimen. The larger the joint angle is, the more significant the wave attenuation is. The variation trend of energy absorption rate is similar to the tendency of the dynamic strength, showing a positive correlation between the energy absorption rate and the dynamic strength. Tensile cracks dominate the specimen failure behavior during the dynamic fracturing process by observing the images captured by a high speed camera, leading to the slabbing failure with different degrees of slippage along the joint surface. In addition, it is found that the variation tendency of dynamic strength based on the experimental results is consistent with the theoretical analysis. [ABSTRACT FROM AUTHOR]

Abstract Due to the limited free face in narrow-vein stoping, blasting is usually subjected to excessive confinement effect that often result in difficulties in ore loss and dilution. The aim of this study is to analyze the blasting confinement mechanism in narrow-vein mining, and to quantitatively define the confinement effect. The PFC and LS-DYNA numerical modelling programs are combined to investigate the effect of blasting-induced stress waves and detonation gas effects. A confinement coefficient in narrow-vein mining is introduced and defined as the ratio of the blasting crater volume or area formed in an infinite free face to that in a confined face. The blasting confinement coefficient is used to rate the confinement severity. On this basis, the effects of minable width and burden on the confinement coefficient are analyzed. The numerical simulation results indicate that the confinement coefficient decreases with increasing minable width for a given burden. The relationship between the minable width and the confinement coefficient can be described by a first-order exponential decay function. For a given minable width, the confinement coefficient increases with increasing burden. The relationship between the burden and the confinement coefficient can be expressed by a first-order exponential growth function. Based on the numerical modelling results, a design chart for the blasting confinement effect in narrow-vein mining is introduced as a guide for preliminary design of narrow vein blasting to optimize production by reducing overbreak (dilution) and underbreak (ore loss). Highlights • Narrow vein blasting is usually subjected to excessive confinement effect. • Blasting confinement effect results in underbreak or overbreak. • A blasting confinement coefficient is introduced to rate the confinement severity. • A chart is developed for estimating confinement severity in narrow-vein mining. • The chart can be uses as guide in preliminary blast design in narrow vein mining. [ABSTRACT FROM AUTHOR]

The effect of blasting vibration on the macroscopic failure characteristics of underground tunnels has been investigated, but little attention has been paid to the stress and deformation characteristics of underground tunnels under the combined action of explosion waves and in-situ stresses. Consequently, enhancing the bearing capacity of surrounding rock masses by adopting effective reinforcement measures is an urgent problem that needs to be solved. Due to high in situ stresses at great depths, it is difficult to conduct field tests on various reinforcement methods. In this study, using the deep underground tunnels as a prototype, large-scale physical model tests were adopted to examine the stress and deformation characteristics of the unlined tunnels, the rock bolt reinforced tunnels, and the lined tunnels under the coupling action of in-situ stress and blasting waves induced by different charge quantities, with the purpose of providing the necessary theoretical and technical support for the design and support of deep protective engineering. [ABSTRACT FROM AUTHOR]

The propagation properties of stress waves in rock masses are significantly affected by high temperature. This paper presents an experimental investigation on the frequency-spectrum characteristics of stress waves in high-temperature granites. The pendulum impact test was performed on high-temperature granite bars to generate stress waves. The frequency-spectrum variation of stress waves in high-temperature granites was investigated by adopting the discrete Fourier transform. The effects of temperature on both the spectral attenuation ratio and the average energy dissipation rate of the harmonic wave were discussed. The results show that frequency spectra of stress waves occur attenuation when stress waves propagate in high-temperature granites. The frequency-spectrum attenuation is more obvious in the granite with higher temperature. Moreover, the spectral attenuation ratio and the average energy dissipation rate of the harmonic wave are significantly affected by the high temperature. The spectral attenuation ratio and the average energy dissipation rate of the harmonic wave increase as temperature increases. [ABSTRACT FROM AUTHOR]

In deep hard rock mines, the drilling and blasting method is mainly used for mining, which results in a complex environment of high static stress and frequent dynamic disturbance in some underground engineering surrounding rocks for a long time. In addition, a large number of underground engineering are in the geological tectonic zone with poor stability, which often triggers rock disasters when stress waves propagate to the bedding. Therefore, it is significant to study the failure process and mechanism of underground engineering in bedded rock masses under combined dynamic and static loading. In this study, a combined dynamic and static impact experiment was conducted on circular-hole-containing shale specimens with different bedding angles using a modified split Hopkinson pressure bar (SHPB) device, and the damage process was recorded using Digital Image Correlation (DIC) technique. The experimental results show that the stress-strain curves of shale specimens under combined dynamic and static conditions include three stages elasticity, yielding, and failure. With the increase of the bedding angle, the dynamic strength and Young's modulus of the specimens showed a trend of decreasing and then increasing, with obvious anisotropy. However, with the increase of pre-static load, the dynamic strength and Young's modulus of the specimens showed a tendency to increase first and then decrease. In addition, repeated refraction (transmission) and reflection of the stress wave between the beddings and the stress concentration phenomenon of the circular holes resulted in different failure characteristics of the specimens with different bedding angles. [ABSTRACT FROM AUTHOR]

Methane deflagration fracturing is a new reservoir stimulation method that serves the efficient development of shale gas reservoirs. However, the propagation law of deflagration fractures is still unclear. In this paper, a numerical model considering the effect of stress wave impact and gas drive of deflagration fracturing was established based on the continuum–discontinuum element method (CDEM). The correctness of the numerical model was verified by comparing it with a laboratory experiment, the steady and unsteady analytical solutions of gas flow, and the approximate solution of fracture propagation. Then, numerical simulations of methane deflagration fracturing in vertical wells and horizontal wells under different factors were carried out to analyze the fracture mechanism. The results indicate that deflagration fracturing in vertical wells can break through the stress concentration around the borehole; the initial radial fractures are formed under the action of stress wave impact and then propagate substantially under the driving action of high-pressure gas. The in-situ stress difference affects the deflagration fracture propagation and makes the half-fracture length in the direction of maximum principal stress larger than that in the direction of minimum principal stress. The more significant the stress difference is, the more noticeable this deviation will be. When the deflagration peak pressure is high, the reservoir burst degree is large, which is conducive to enlarging the stimulation range of deflagration fracturing. Staged deflagration fracturing in horizontal wells can form 5–8 obvious fractures perpendicular to the horizontal borehole in each explosion section. A large cluster spacing and explosion section length are conducive to expanding the stimulation scope. Moreover, the propagation of deflagration fractures will be induced by the natural fractures, and the natural fracture with a considerable length or a slight angle between the dip angle and the propagation direction of deflagration fractures is more likely to be activated. • The deflagration fracturing for shale gas reservoir stimulation is proposed. • The numerical model considers the effects of stress wave and gas drive. • The propagation law of deflagration fracture is revealed by numerical simulation. • Deflagration fracturing can break the stress concentration to form complex fracture. [ABSTRACT FROM AUTHOR]

A thermo-poroelastodynamic (TPED) model incorporating the thermo-osmosis and thermal-filtration phenomena is used to analyze the transient response of a borehole subject to a non-hydrostatic stress field. The coupled TPED model is developed on the basis of equations of motion, fluid flow, heat transfer and constitutive equations so as to couple the stress wave, pressure wave and thermal wave in early times. Poroelastodynamics (PED) and thermo-elastodynamics (TED) models correspond two particular cases of the current model. The borehole problem is decomposed into the superposition of an axisymmetric mode and an asymmetric mode, corresponding to axisymmetric and deviatoric loadings, respectively. The semi-analytical solutions in the Laplace transform space are inverted numerically to obtain time-dependent displacement, pore pressure, temperature and stresses. Comparisons are performed among different sub-models to examine the influence of thermal effect on the response of pore pressure and stresses as well as the influence of pore water on the response of stress components. A direct comparison among the dynamic, quasi-static and static models shows that inertial effect is very important in the early times and it contributes to the early-time wave behavior. [ABSTRACT FROM AUTHOR]

During a mine-by experiment performed at the Mont Terri Underground Research Laboratory located at the transition between the sandy and the shaly facies of the Opalinus Clay formation, excavation induced micro-acoustic events were recorded in the so-called Gallery 08 (the EZ-G experiment). A first cluster of events occurred in the vicinity of the eastern sidewall of Gallery 08, and a second cluster was observed ahead of the advancing tunnel face. For each recorded micro-acoustic event (AE), all located in the sandy facies of the Opalinus Clay formation, the total stresses associated with the onset of inelastic deformations were estimated using a three-dimensional numerical model. The numerical analysis is based on the assumption that the rock mass behavior in the vicinity of the excavation is essentially elastic before the stress redistribution causes damage evidenced by the triggered AE activity. For the cluster located at the tunnel sidewall, the source mechanism analysis reveals the predominance of extensional failure (tensile) events. The numerical analysis of each individual micro-acoustic event suggests that the differential stresses at the onset of damage range between 3 and 10 MPa. These values are in reasonable agreement with crack initiation thresholds obtained in the laboratory from samples of the various sub-facies types of the sandy facies in the Opalinus Clay formation, which range between 2 and 18 MPa. For the cluster located ahead of the tunnel face, the source mechanism analysis indicates the predominance of local shear failure events. This is consistent with observed shear dislocations on bedding planes within weak beds in the sandy facies that have similar strength properties as the shaly facies. Thus, the modelled shear and total normal stresses acting across the average bedding plane orientation at each event location were modelled and used to estimate the in situ shear strength along the bedding planes. The model suggests a friction angle of 33.4° and a cohesion of 0.43 MPa. The results are overall consistent with the bedding plane strength obtained through undrained direct shear tests on specimens of the shaly facies, suggesting that the observed weak beds with a strength similar to the shaly facies govern the behavior at the tunnel face. [ABSTRACT FROM AUTHOR]

In large scale rock blasting operations, such as in mining, quarrying and tunneling, there are usually a number of blast holes simultaneously, and the explosive charges are fired in different sequences. This indicates that the later-fired holes may have effect on the propagation of the cracks induced by the first fired blasting. This paper tried to investigate the effect of two empty holes on the behavior of the cracks propagating between them, and blasting experiments were conducted by using circular sandstone and PMMA specimens with a centralized blast hole, a pre-crack and two empty holes. The two-hole spacing was designed in the range from 10 mm to 45 mm, and the outer diameter of the circular specimens was 600 mm for sandstone and 400 mm for PMMA. The central blast hole was charged with a detonator and it was fired to provide dynamic loading which drives the pre-crack to propagate. Meanwhile, by using linear equation of state and the major principal stress failure criterion, numerical models were established by using AUTODYN code, and the effect mechanism of empty holes on crack propagation behavior was studied numerically. The crack propagation paths predicted by the numerical models agree well with the experimental results. The results show that: (a) Empty holes have the arrest function on outgoing cracks, and the arrest function depends on the two hole spacing; the shorter the two hole spacing is, the stronger the arrest function is; (b) As two hole spacing is less than a certain value, outgoing cracks can be completely arrested; (c) The stress perpendicular to the propagating crack plays a key role in arresting cracks. [ABSTRACT FROM AUTHOR]

Understanding wave propagation across rock joints is of great importance in rock engineering, petroleum engineering, and other geophysical applications. Previous studies have shown that wave behaviours across filled rock joints are highly affected by the properties of filling materials. However, how clay minerals and water saturation of the infilling affect wave attributes across filled rock joints is still poorly understood. In this study, ultrasonic tests were conducted on kaolinite- and bentonite-filled rock joints to investigate the roles of clay minerals and the water saturation of infillings on elastic P-wave behaviours across individual filled rock joints. The test results show that the infilling's water saturation and mineralogical compositions have combined effects on P-wave signatures of single rock joints filled with clayey soils. Particularly, for the kaolinite-filled rock joint, P-wave velocity and attenuation first increase and then slightly decline with the increasing water saturation. By contrast, for the bentonite-filled rock joint, an increase in water saturation causes gradual decreases in velocity and attenuation. The integrated interpretation of the experimental results indicates that the difference in heterogeneity saturation due to clay hydration is the first-order reason for the discrepancies in saturation-dependent P-wave attributes between kaolinite- and bentonite-filled rock joints. The findings of this study can help further understand wave propagation and attenuation across rock joints filled with fluid-saturated clayey soils. [ABSTRACT FROM AUTHOR]

High quality of cracking and low depth of damage in sidewall is usually required in a blasting excavation. On the contrary, large spacing of blast holes can efficiently reduce cost. Therefore Timing Sequence Control (TSC) Method can be applied, so as to meet the challenge. In the TSC method, there is a very short delay between two adjacent blast holes, i.e., the Primary borehole (PBH) and the Target borehole (TBH), in sequence. Study of the cracking process around adjacent borehole indicates, larger stress concentration on the hole wall, if the TBH initiates after the stress wave from PBH has past it. The initial crack generated on the TBH is most likely to occur along the direction of borehole attachment. Our study also shows that, keeping the borehole spacing and borehole diameter as constant, a through crack can be formed between the adjacent boreholes when the delay time, spacing of PBH and TBH, and transverse wave velocity obey a certain relationship. Keeping the hole spacing between PBH and TBH, borehole diameter and the delay time for PBH and TBH as constant, borehole through cracks between the adjacent boreholes can be formed only when the spacing of TBH equal or below 25 times of borehole diameter. [ABSTRACT FROM AUTHOR]

The present study investigates the frequency-temperature coupling effects on wave propagation through granite. The pendulum impact test was conducted on the thermally treated long granite bars. The wave propagation coefficients, such as attenuation ratio and wave velocity, were determined for the granite under five groups of high-temperature and six groups of incident waves with different frequencies. A relationship between the dynamic modulus of granite, frequency of stress wave and the high temperature was revealed. The frequency-temperature coupling effects on the dynamic modulus were discussed. The results show that the attenuation ratio, wave velocity and dynamic elastic modulus increase with the increase of frequency. The attenuation ratio increases with the increase of temperature, while the wave velocity and dynamic elastic modulus decrease with the increase of temperature. The proposed relationship between the dynamic modulus, frequency and temperature can efficiently describe the frequency-temperature coupling effects on the granite with acceptable error. The present study has potential in the application of wave propagation through granite in the high-temperature environment. • Frequency-temperature coupling effects on wave propagation were investigated. • Relationship between dynamic modulus, frequency and high temperature was revealed. • Frequency-temperature coupling effects on the dynamic modulus were discussed. [ABSTRACT FROM AUTHOR]

There are still varying opinions on the relative spatio-temporal contributions of blasting stress wave and blasting gas in the theoretical study of rock blasting due to the transient nature of the blasting process and the complexity of the blasting effect. Based on this, the study proposes a cavity charge structure entailing quantitative and differentiated research on the effects of blasting stress wave and blasting gas over time and space. The time and space action characteristics of blasting gas are regulated by changing the charge structure of the borehole and placing a cavity between the charge and borehole wall, and the "double-peak" evolution law of blasting strain is obtained. The first peak is generated by blasting stress wave, and the second peak is generated by blasting gas, with a delay between the first and second peaks. Further analysis shows that the cavity has different effects on blasting stress wave and blasting gas. Finally, the space-time effect of blasting stress wave and blasting gas can be fully utilized to achieve controllable rock fragmentation by adjusting the charge position and the cavity volume in the cavity charge structure. [ABSTRACT FROM AUTHOR]

The analysis of stress wave propagation through a rock mass is an important issue in rock engineering, and the discontinuous deformation analysis (DDA) method provides an effective tool to solve this problem. In this paper, based on the motion equations of block system with the Newmark integration method, the DDA method improved by the viscous boundary and the force input method is employed, and some investigations are made to extend the capability of the DDA method to address the wave propagation problem. First, for providing a quantitative method to control the damping of the DDA method, the computing methods of the damping ratio for both viscous damping and numerical damping are presented, which can effectively simulate the energy dissipation of stress wave propagation. Second, based on the one-dimensional DDA model, a further study on the size choice of a sub block for stress wave propagation is developed, which considers the homogeneous rock and the joint rock mass, and suggested values of block element ratio for both P-wave incidence and S-wave incidence are obtained by comparing with the theoretical solutions. Last, abilities of the DDA method to simulate both the no tension and slipping characteristics of a joint are validated, and the effects of the joint on wave propagation are also investigated. [ABSTRACT FROM AUTHOR]

Shear slip of rock joints under dynamic disturbance is a major factor causing disasters such as landslides and cavern instability. To explore the dynamic shear deformation behaviors of rock joints, an impact-induced direct shear test applicable for jointed rock specimen is conducted. A biaxial Hopkinson pressure bar (BHPB) equipment is employed, which can simulate intensive disturbances encountered during engineering construction in rock masses. Tangential and normal dynamic stresses on rock joint can be measured simultaneously through stress wave acquisition. Dynamic displacements are obtained based on a two-dimensional digital image correlation (2D-DIC) technique. Firstly, the feasibility of the experimental method is verified by trial tests on metallic specimens. The measurement technique for obtaining the dynamic stress and displacement of joint is proposed. Then, a series of dynamic shear experiments are carried out on jointed and intact granite specimens, respectively. The dynamic shear deformation of the specimens is obtained, and the effects and mechanisms of undulating angle and initial normal stress on the dynamic shear deformation are discussed. The test results show that, due to the variable input force and normal stress, the shear rate varies. The dynamic slip of the joint can be divided into three sub-stages: slip initiation, secondary stress accumulation, and post-slip. The shearing process of intact rock includes two stages: shear stress accumulation and post-peak damage. Deformation rebounding is also observed in the jointed and intact granite specimens. The maximum shear displacement is negatively correlated with the normal stress and the undulating angle. Based on the distribution and evolution of displacement field in rock specimen, the effects of undulating angle and normal stress on the shear strength and the maximum shear displacement are revealed. Finally, the damage modes of rock joint under different conditions are discussed. [ABSTRACT FROM AUTHOR]

The correlated lattice bond cell model (CLBC) is used to account for the characteristics of the mesostructure of rock. Each mineral grain is modeled as a discrete unit bond cell. A linear modified Stillinger–Weber potential is adopted to characterize the unit bond cell. By this method, the lattice model can not only represent the variable Poisson ratio, but can also simulate more complex fracture behaviors than the two-body interaction-based lattice model. It can precisely simulate the stress wave propagation in rock with different Poisson ratios. With a simple bond rupture criterion, the spalling fracture induced by stress wave is well simulated. To represent the heterogeneity of rock, the moduli of bond cells are assumed to be random, following a Weibull distribution. The simulation results demonstrate that the distinct spalling fracture is likely to initiate in a hard rock subjected to impact load, but not in a soft rock. The heterogeneity has little effect on the spalling fracture. [ABSTRACT FROM AUTHOR]

Single-hole blast-induced damage in a granitic outcrop has been assessed through both controlled experiments and numerical simulations with a combined finite-discrete element method (FEM–DEM). Damage tomographies from decoupled short and long explosive charges in flooded boreholes were obtained through a high-resolution cross-hole system, by measuring pre- and post-blast seismic velocities around the blastholes. Damage was assessed through crack density, which was calculated by inversion of P-wave velocity measurements through an Effective Medium Theory (EMT) method. The resulting damage was found to be highly asymmetrical around each blast hole, both along the vertical and horizontal planes, despite the apparent isotropy and homogeneity of the granitic rock mass. Experimental results combined with numerical simulations carried out to assess damage from stress waves alone, showed that most damage from experiments was caused by the expansion of gases, while its magnitude and extension were strongly dependent on confining conditions along the blasthole. The difficulty of quantifying the relative contribution of stress waves and gas expansion and the problems inherent in prediction of blast-induced damage are described. [ABSTRACT FROM AUTHOR]

Ground vibration is the most important environmental effect of blasting, and tools for its understanding and control are a prime necessity for the excavation of tunnels and caverns in hard ground. This paper contributes to an improved understanding of the way waves travel in the ground, particularly when there are excavations in the path of propagation, while also enhancing existing numerical models to better simulate that behaviour and thus provide better means to address underground vibration impacts. To this purpose, twenty low intensity blast between neighbour tunnels of an underground complex were performed, followed by a numerical study of the test. The test and simulation showed that, contrary to what is implied in semi-empirical attenuation laws, factors like the propagation path or local amplification can be more important than instant explosive weight and distance to blast. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of the gradient distribution of microdefects on wave propagation through a rock mass. A piecewise three-characteristic lines method is proposed that can consider the gradient distribution of microdefects. A rock mass with a microdefect gradient distribution was explored and compared to wave propagation through a rock mass with a uniform distribution of microdefects. Subsequently, the effects of gradient and frequency on the particle stress transmission coefficient, particle velocity transmission coefficient, and energy transmission coefficient were discussed. The results indicate that a rock mass with a microdefect gradient distribution significantly affects wave propagation. The traditional study of wave propagation in a rock mass with a uniform distribution of microdefects can be considered a special example of the present study. In addition, the particle velocity transmission coefficient and energy transmission coefficient are always less than 1.0. However, when the gradient is large and the frequency is small, the particle stress transmission coefficient is larger than 1.0, which shows that the particle stress amplitude in a rock mass with a microdefect gradient distribution will be amplified. • Wave propagation in a rock mass with a microdefect gradient distribution is studied. • A piecewise three-characteristics method is proposed in the present study. • The piecewise viscoelasticity of microdefects with a gradient distribution in rock is considered. • The effects of gradient properties on stress and velocity transmission are discussed. • The stress amplitude in gradient microdefect distributed rock can be amplified. [ABSTRACT FROM AUTHOR]

The relationship between velocity, acceleration, and dynamic fracture toughness during rapid crack propagation is difficult to effectively capture. In this study, a novel semi-circular holed disc bend is proposed to achieve a long fracture path. Hopkinson pressure rods with crack growth and strain gauges are used to impact the semi-circular holed disc bend specimens and to monitor the cracking process of a mode-I fracture. The experiments show that the crack growth speed increases to a critical value after crack initiation and then oscillates as a result of the reflected stress wave. The dynamic propagation fracture toughness (K IC d) increases with increasing crack velocity for steady crack propagation; however, this does not apply for unsteady propagation. An experimental–numerical–analytical method is developed to determine the dynamic stress intensity factor, which initially increases with time, then decreases, and ultimately increases again. The acceleration (v ˙) can be used to investigate the unsteady crack propagation process. K IC d values for v ˙ < 0 are greater than those for v ˙ > 0. An energy release rate formula related to v ˙ and the crack propagation velocity (v) is proposed and verified by the experiments. A thorough understanding of v , v ˙ , and K IC d is important to analyze high-speed fault rupture processes. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a theoretical study of stress wave propagation across a rock joint with different loading/unloading behavior. Two Bandis–Barton (B–B) models are introduced to describe the stress–closure relationships of a rock joint under loading and unloading situations, respectively. A characteristics method is proposed to investigate the stress wave transmission. Amplitude-dependence and frequency-dependence of unloading effect on the energy transmission are analyzed. The unloading effect on stress waves with different waveforms and stress waves with multiple pluses are evaluated. The results show that the characteristics method can be used to investigate the wave propagation across a rock joint with different loading/unloading behavior. The effect of unloading behavior on energy transmission is amplitude- and frequency-dependent. The stress wave with a rectangular waveform is less influenced by the unloading behavior than the stress waves with other waveforms. Unloading behavior has a significant effect on the energy transmission for the stress wave with both single pulse and multiple pulses. However, unloading behavior has an effect on the wave attenuation provided that the time interval of multiple pulses stress wave is sufficiently small. [Copyright &y& Elsevier]

Abstract: A nonlinear viscoelastic equivalent medium model is proposed for longitudinal stress wave propagation in a rock mass that contains equally spaced parallel joints. The joint deformation is assumed to satisfy the nonlinear Bandis–Barton (B–B) deformational model. By the simplification of the nonlinear B–B model into piecewise linear segments, the material parameters in the equivalent medium model are analytically derived. The nonlinear viscoelastic equivalent medium model is verified by comparison with the results based on the displacement discontinuity method. It is shown that for smaller amplitude stress waves, when the rock joints deform in the linear elastic region, the nonlinear viscoelastic equivalent medium model is reduced to its linear form, whereas for larger amplitude stress waves, the present nonlinear model describes the wave amplitude-dependence of wave velocity. Two important properties, i.e., the wave transmission coefficient and the effective wave propagation velocity, are discussed in detail. [Copyright &y& Elsevier]

Abstract: In order to investigate the transmission and reflection of stress waves across joints, a fractal damage joint model is developed based on fractal damage theory, and the analytical solution for the coefficients of transmission and reflection of stress waves across joints is derived from the fractal damage joint model. The fractal geometrical characteristics of joint surfaces are investigated by using laser profilometer to scan the joint surfaces. The dynamic experiments by using Split Hopkinson Pressure Bar (SHPB) for rock specimens with single joint are conducted to confirm the analytical solution of the coefficients of transmission and reflection. The SHPB experimental results of the coefficients agree well with the analytical solution. [Copyright &y& Elsevier]

Abstract: This paper deals with the unstable rock-slope at Åknes sliding area, located in the county of Møre and Romsdal in western part of Norway. The sliding body has a complex geometry with several sliding planes at different levels, involving unfilled joints, gouge material/brecciated material as well as bridges of intact rock. Stability of the rock slope strongly depends on the shear strength of the sliding plane(s) and this paper discusses the shear strength of the materials present, ranging between the strength of intact rock and crushed, clay containing gouge material. Estimation of the in situ shear strength at the Åknes sliding area is discussed on the basis of triaxial test results and the Barton–Bandis empirical method. Triaxial tests have been carried out on samples of gouge material from a potential sliding plane and intact rock specimens, while the empirical method has been applied for rock joints in the area. Comparison and correlation with experimental results found in literature have also been made. The resultant shear strength range for the zone of sliding has been found to be in the range of 0.5–1.3MPa, depending on the normal stress and the composition of the sliding zone. [Copyright &y& Elsevier]

Abstract: Initiation of failure under three-point bending was observed in sandstone specimens with and without a stress concentrator. Acoustic emission (AE) and electronic speckle pattern interferometry (ESPI) were used to quantify the fracture phenomena through AE locations and ESPI images of an intrinsic damage zone. To investigate mixed mode fracture, beam tests with off-center notches were studied as well. Before about 95% of peak load, the locations of AE were somewhat random for beams with a smooth boundary (no notch). From 96–100% of peak, microcrack hypocenters indicated localization in the form of an intrinsic zone, which was also identified by the high resolution technique of ESPI. The discrete element technique was used to mimic the failure process. A synthetic rock was composed of rigid circular particles that interact through normal and shear springs, and the particles were glued to each other using normal and shear bonds. A microcrack forming in tension or shear was simulated by breakage of the bond between two particles in contact. The synthetic rock was calibrated for elastic properties, unconfined compressive strength, and bending tensile strength. The beam strength with and without notches and the fracture path were studied in the numerical experiments, and the results compared favorably with those obtained from the laboratory experiments. [Copyright &y& Elsevier]

Abstract: Dynamic properties of rock joints are of great interest to mining engineers, seismologists and geoscientists in characterizing the dynamic mechanical behavior of discontinuous rock mass. An experimental study of stress wave propagation across filled rock joints has been carried out using a modified Split Hopkinson Pressure Bar (SHPB) apparatus. Two granitic bars cored from a rock site were used as the incident and transmitter pressure bars in the SHPB tests, while a sand layer of different widths and water contents sandwiched between the two bars was adopted to simulate the filled joint. Each pressure bar was mounted with two strain gauges with a specific spacing. A wave separation method was used to process the test data. The dynamic stress–strain relation of the filled rock joints was derived from the separated strain waves. Finally, the dynamic stress–strain relations from the tests were curve-fitted by using the least square regression method and compared with a traditional joint model. It was found that the wave separation method is very effective for the SHPB tests using short granitic pressure bars. It was also concluded that the joint width and water content had significant effects on the dynamic stress–strain relation of the filled rock joints. Existing joint models were not able to sufficiently describe such dynamic properties of the filled rock joints. [Copyright &y& Elsevier]

Plasma blasting technology (PBT) is a potential alternative to chemical blasting and mechanical cutting methods for fragmentation of natural rocks, concrete, geopolymers, and other rocks-like materials. We present an analytical model of PBT addressing currently inadequate understanding of the dynamics of shock waves generation and propagation versus the electric energy release conditions. The proposed model describes the operation of the electrical discharge circuit, plasma channel initiation and expansion, and the generation and propagation of shock and pressure waves in the destructible solid. The dynamics of the power generator energy conversion into the plasma channel and into the wave of mechanical stresses in the solid are considered and the main factors determining the efficiency of the method, namely the pulse generator circuit parameters, exploding wire length, and shock wave-transmitting media, are evaluated. Solid fracture efficiency is shown to depend on the pressure pulse wave shape which, in turn, is determined by the rate of electrical energy deposition into the plasma channel. Increasing the exploding wire length leads to an earlier formation of the tensile tangential stresses and to their higher magnitude and thus facilitates material's fragmentation. The use of acoustically stiff media for shock wave transfer marginally improves material's fracture efficiency. Preliminary verification of the functionality of the model was carried out using commercial concretes, with good agreement between the analytically derived and experimentally obtained values. The results demonstrate that the proposed model allows to simulate PBT fracture over a wide range of instrumental and process conditions and can therefore be used for PBT process design, thus realising environmental and economic benefits through significant savings in time and experimental confirmation costs. • Electrical energy deposition rate determines plasma blasting technology efficiency. • Electro-fracture induces up to four zones of change of stresses and deformations. • Superposition of direct and reflected shock waves facilitates rocks fracture. • Increasing exploding-wire length increases shock wave energy and improves fracture. • Acoustically stiff media for shock-wave transfer improves fracture marginally. [ABSTRACT FROM AUTHOR]

Ultra-high pressure water jets (UHP-WJ) rock breaking is a promising technology for developing unconventional natural gas reserves. In this study, rock-breaking experiments were conducted using typical reservoir rock samples such as sandstone, shale, and coal. The failure patterns in rocks after impact by UHP-WJ were analyzed. Moreover, the 3D damage field model of the rock was reconstructed based on CT scanning, and the internal crack distribution was visualized and examined. Furthermore, the fracture mechanisms of three types of reservoir rocks under the UHP-WJ impingement were studied. The results indicate that intergranular fractures produced due to shear stress are the dominant type of fracture in sandstones, tensile failure due to stress wave was the main type of fracture observed in shale, and coal exhibited the double breaking characteristics of stress wave effect and water wedge effect. The fragments size distribution of samples shows the largest fragments in shale, followed by coal, and the smallest fragments are observed in sandstone. Finally, the evolution of rock pore structure before and after the impact of UHP-WJ was monitored by nuclear magnetic resonance (NMR) methods. The results reveal that the micropores transform into mesopores and macropores in the sandstone, where the total porosity increased by 14.3%–24.8%. As the jet pressure exceeds 100 MPa, the porosity of shale increases significantly, such that the total porosity of the shale sample increased by 7.6%–37.1%. The mesopores transform into macropores and fractures with the influence of the water wedge effect in coal samples, resulting in the reduction of mesopores and the increase of macropores, where the total porosity of the coal sample increased by 14.6%–40%. The complexity and heterogeneity of pores were reduced under UHP-WJ with the increasing jet pressure. The findings provide a theoretical reference for optimizing the recovery from unconventional natural gas reservoirs. [ABSTRACT FROM AUTHOR]

Accurately predicting the location of a fault in front of a tunnel face is an essential part of deep-buried tunnel excavation. This paper proposes a modified g- λ (0 < λ < 1) model with in situ stress to investigate the propagation and attenuation of a P-wave across jointed rock masses, including a single joint or multiple parallel joints. Based on the modified g- λ model, the differential expressions of the transmitted wave's particle velocity reflecting the interactions between the P-wave and the joints are deduced by using the characteristics method and the displacement discontinuity model. Parametric studies examining the in situ stress, the value of λ , the incident wave frequency and amplitude, and the number of joints are conducted to evaluate the application of the modified g- λ model. This provides support for predicting the location of the fault during the in situ stress tunnel construction when ignoring the shape and width of the fault. The results show that the effective wave velocity of the jointed rock masses increases with increasing in situ stress when a normally incident P-wave impinges upon a single joint. Considering multiple parallel joints, the magnitude and time of the maximum peak of the transmitted wave are decreases and delayed correspondingly with the increase in the number of joints, respectively. Moreover, multisource seismic interferometry is used to predict the location of the fault at different surrounding rock wave velocities and dip angles of the fault, which contribute to the identification of unfavorable geological bodies on the tunnel advanced geological predictions. [ABSTRACT FROM AUTHOR]

High-speed three-dimensional digital image correlation (3D-DIC) technique was utilised to examine the ejection velocity characteristics of rock fragment in dynamic compression tests. The fragment velocity components of fragment were extracted from velocity fields distributed within the fragment area. By identifying the peak resultant velocity, the initial fragment ejection velocity was determined. The kinetic energy of fragments was evaluated with the initial ejection velocity and the mass of the fragments in each size group. Results show that the fragment movement is successively accelerated by stress-wave propagation, crack opening and Poisson's effect which contribute to the axial, circumferential and radial velocity component of fragment, respectively. The variance in circumferential velocity of two neighbouring fragment areas indicates the formation of fragments. The peak value of radial velocity, which is the chief component in ejection velocity reveals the time when ejection taking place. The initial ejection velocity of fragment gradually decreases with increasing size, but increases significantly as strain rate increases. The kinetic energy of fragments ejection comprises 12%∼24% of absorbed energy at strain rate of 69∼100 s-1, and the percentage further increases with increasing strain rates. Ignoring the kinetic energy of fragments will lead to overestimation of dissipated energy consumed by rock fragmentation in dynamic compression. • High-speed 3D-DIC was applied to identify three-dimensional velocity patterns of rock fragments under dynamic compression. • Strain recovery, fragment formation and ejection were revealed by velocity components of fragments, respectively. • Ejection velocity of fragment gradually decreases with increasing size, but increases significantly as strain rate increases. • Kinetic energy of fragments ejection takes more than 20% of absorbed energy with the strain rate above 85 s-1. [ABSTRACT FROM AUTHOR]

Rock joints significantly affect the dynamic response of rock masses and the stability of underground structures. In this study, the dynamic photoelastic test was conducted to investigate the influence of joints on the stress field and wave propagation. The photoelastic equipment as well as the Split Hopkinson Pressure Bar (SHPB) were adopted for the test. The specimen sandwiched between the input and output bars was composed of two polycarbonate plates. One side of the plates was uneven with a few notches, while other sides were smooth and flat. The artificial joints were simulated by the contact of the uneven and flat sides of the two plates. The relation between the stress wave propagation and the stress field around the joint was studied when the joint has different contact area ratios, joint thicknesses, and joint surface distributions. The test results show that compared to the joint thickness, the joint contact area ratio and the joint surface distribution have a significant influence on the wave propagation and stress field near the joint. The transmitted wave is highly correlative with the stress field. This work provides a beneficial attempt to reveal the stress field disturbance and the effect of joint geometrical properties on wave propagation from the photoelastic test. [ABSTRACT FROM AUTHOR]

Abstract: A numerical modeling study of a heated rock was carried out and validated by a laboratory experiment. In the laboratory experiment, a granite cylinder with a diameter of 30cm was fractured by a heater placed in a central borehole. Acoustic emission (AE) monitoring during the experiment showed the development of the damage in the specimen. The numerical simulations were made with a commercially available code PFC2D, using a bonded-particle approach. A total of 15 numerical specimens were built with varying mechanical and thermal properties. During the modeling, AE events were simulated and compared against the laboratory data. The numerical models showed similar failure behavior to that observed in the laboratory experiment. The pre-failure AE events were isolated and randomly located in the sample, while the failure itself appeared as a single large AE event comprising many microcracks aligned along a plane. The results of these unique thermal simulations have been so encouraging that we are proceeding with larger scale thermal problems. [Copyright &y& Elsevier]

Abstract: The paper focuses on the numerical analysis of blast-induced stress wave propagation and related spalling damage in a rock plate or wall. Firstly, a large-scale contact explosion is simplified as a one-dimensional (1-D) strain problem, and a cross-format centered finite-difference scheme is presented. Secondly, the finite-difference code is used to explore the wave propagation in a rock plate, and compared with the commercial software LS-DYNA. Thirdly, a continuum damage constitutive model for rock is introduced and successfully incorporated into the LS-DYNA through its user-defined subroutine. Coupling with the erosion technique, the improved LS-DYNA is used to simulate the blast-induced spalling damage at the back-side of rock plate. Numerical results show that the 1-D finite difference code is convenient and accurate. Also, the user-defined subroutine can well describe the process of spalling damage. [Copyright &y& Elsevier]

Abstract: This study aims to investigate the relation between the Kaiser effect (KE) levels and pre-stresses applied to rock samples in laboratory. For the purpose, a number of test specimens were obtained from different types of rock and a mortar block. Deformation and crack propagation stages were identified for each sample group. Based on the thresholds of these stages, the specimens were subjected to selected pre-stresses under uniaxial and axisymetric triaxial loading under laboratory conditions. Following the laboratory preloadings, acoustic emission (AE) experiments were carried out on the specimens under uniaxial loading conditions to determine the KE levels. The comparisons made between the laboratory pre-stresses and the KE levels suggest that the KE level is not equal to one of the pre-stress values or their differences, and should be related to combined effect of axial and confined pre-stresses, and some rock dependent characteristics. [Copyright &y& Elsevier]

Abstract: To investigate the dynamic fracture mechanism related to blast-induced borehole breakdown and crack propagation, circular rock models containing a single centrally located source of explosive were numerically blasted using the AUTODYN 2D code. According to the material properties and loading conditions, four kinds of equations of state, linear, shock, compaction and ideal gas, are used. A modified principal stress failure criterion is applied to determining material status. The dynamic stresses at the selected target points in a rock sample are computed as a function of time following application of explosive load. It is shown that shear stress (resulting from intense compressive stress) causes a crushed zone near the borehole, the major tensile principal stress causes radial cracks, and the reflected stress wave from free boundary causes circumferential cracks some distance away from the free boundary. The influences of the factors of boundary condition, coupling medium, borehole diameter, decoupling and joint on rock dynamic fracture are discussed. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental investigation of rock salt dilatancy boundary based on combined acoustic emission and triaxial compression tests carried out on the rock salt samples from the Asse Salt Mine, Germany. The experimental results were evaluated to determine the dilatancy boundary under specified stress, stress loading rate and pore pressure. Pore volume changes caused by deviatoric stresses were measured during triaxial compression tests. The dilatancy boundary was then determined from the maximum compression on a stress–strain curve which separates the compression and dilatancy regions. Variations in acoustic emissions that occur during microcrack development under triaxial compression were recorded with ten sensors mounted on the outer surface of the cylindrical samples. A spontaneous increase observed in the cumulative number of acoustic events is consistent with the dilatancy boundary determined from the minimum pore volume. It is confirmed that the dilatancy boundary depends on both stress loading rate and pore pressure. The dilatancy boundary slightly decreases with increasing the loading rate, but increases with increasing the minor normal stress. High pore pressures accelerate the dilatancy. A Biot coefficient of 0.25 for Asse rock salt was determined from the dilatancy boundary. An analysis of two-dimensional (2D) fractal dimensions determined for several samples shows that the samples with smaller dimensions have slightly higher dilatancy boundaries. The dilatancy boundary values of Asse rock salt are lower than those reported for crystalline rocks, although both rock types show similar dilatancy behaviour. [Copyright &y& Elsevier]

Abstract: This paper presents the experimental results obtained with a two-dimensional indentation device controlled by a servo-hydraulic loading system and monitored with the nondestructive techniques of acoustic emission and electronic speckle pattern interferometry. The goals of this research were to evaluate the indentation pressure as well as the size of a damage (plastic) zone, and to study the initiation of tensile fracture at the intact rock-damaged rock boundary, that is, the elasto-plastic interface. The key factors controlling the failure process are (1) the mechanical properties of the rock including the elasticity and strength parameters, (2) the geometric features of the tool such as the wedge angle, and (3) the lateral confinement simulating the far-field stress. A good agreement with regard to indentation pressure and damage-zone radius was found between the experimental and theoretical analyses. Furthermore, the intrinsic crack length, critical in establishing tensile fracture, was estimated and correlated to the grain size. [Copyright &y& Elsevier]

Abstract: When a wave propagates through fractured rock masses, it is greatly attenuated (and slowed) due to the presence of fractures. In the situation where dynamic responses are induced by a far field explosion or vibration, the magnitude of the stress wave is generally too small to mobilize nonlinear deformation of the fractures, so linear deformational behaviour is adopted in studying wave attenuation. Cai and Zhao [Effects of multiple parallel fractures on apparent wave attenuation in rock masses. Int J Rock Mech Min Sci 2000;37(4): 661–82.] used the method of characteristics to study P-wave attenuation across linear deformational fractures by considering interfracture wave reflections. However, the frequency characteristic of the incident wave (a one-cycle sinusoidal wave) was overlooked in their previous study. The present study treats the incident wave as the sum of a series of harmonic waves in the frequency domain. Results and wave phenomena in the previous study are improved and better understood. When the incident wavelength is substantially greater than fracture spacing, an equivalent medium method is developed to explain the wave phenomena in such a situation. In addition, the uncertainty of back analyzing fractured structures and the effects of wave frequency are examined. These results are in accordance with conclusions in the field of geophysics. [Copyright &y& Elsevier]

Abstract: Methods based on acoustic emission and microseismicity (AE/MS) have proven to be valuable for monitoring microcracking at the Underground Research Laboratory (URL) in Canada. The source locations of the seismic events induced by the excavation have helped to map out the extent of the excavation damage/disturbed zone (EDZ) around tunnels. This paper describes methods for advanced processing and interpretation of AE/MS data, and the quantification of changes in material properties using velocity interferometry. Examples of methods that are generally not used routinely in the analysis of AE/MS data are given. These include (1) long-term spatial and temporal source parameter trends and magnitude–frequency studies; (2) source mechanism analysis; and (3) source location refinement by the use of relative location, and clustering techniques. Results from these studies are shown to aid in the understanding of the effect of confining pressure on AE/MS activity around tunnels, and the mechanics of rock failure in the EDZ. Also, the reduction of source location error is shown to be critical for delineating structures that may exist in clusters of AE/MS data. Finally, examples of the use of seismic sources to produce repeat measurements of seismic velocity along raypaths in 3D orientations through a rock mass are given. This is found to be a highly sensitive method for quantifying changes in rock properties such as Young''s modulus, as well as for modelling the change in crack density and saturation occurring in a rock mass with time. [Copyright &y& Elsevier]

Abstract: A technique is presented to simulate seismicity in brittle rock under stress using a distinct element numerical modelling code. Itasca Consulting Group''s particle flow code is used to model rock deformation, damage and the resulting seismicity. With this code, rock is represented by thousands of individual particles bonded together at points of contact. Seismicity results when bonds break under high local stresses and stored strain energy is released as kinetic energy. A full 3D formulation that enables the calculation of seismic event locations, magnitudes and mechanisms (moment tensors) is presented. A simulation of an axial compression test on a granite core sample is used to test the modelling technique. The source parameters of the acoustic emissions produced by the model are considered realistic when compared with similar experiments conducted in the laboratory. The technique is then tested on a field-scale problem by applying the algorithms to the 2D mine-by tunnel excavation simulation (Potyondy and Cundall, Int. J. Rock Mech. Min. Sci., this issue). Seismicity produced by the model is compared to actual seismicity recorded in the field and it is shown that locations, magnitudes and mechanisms match reasonably well. The similarities between the modelled and recorded seismicity provide confidence that the model is behaving in a realistic way. It is then shown how the model could be used to examine the details of the mechanisms behind the recorded seismicity by direct observations of particle forces and motions at the seismic sources. [Copyright &y& Elsevier]

The effect of detonator position on rock fragmentation was studied at the Kevitsa open pit mine. Five full-scale field tests were conducted where the blasts were divided into test and reference areas, and compared with each other. In the test areas, the detonators were placed at or near the middle point of the explosive column, and in the reference areas, corresponding to the conventional blasts used in the Kevitsa mine, the detonators were placed about 1–2 m above the bottom of the blast holes. The rock fragment sizes from both test and reference areas were measured and studied using shovel-mounted machine vision cameras and image analysis. Both theoretical and field studies indicated that the detonator position plays an important role in rock fragmentation, and that the detonator position in the middle of the explosive column yielded significant improvement in rock fragmentation. For example, percentages of fragment sizes x20, x50, and x80 are reduced by 10–27%, 11–30%, and 8–31%, respectively; percentage of large size fragments (>1.0 m) is reduced by 29–99%; percentage of small size fragments (<2.5 cm) is increased by 6–49%. • Detonator location was studied for open pit bench blasting. • The best detonator position is the middle of the explosive charge length. • The middle detonator position significantly improved rock fragmentation. • Fragment size x50 was reduced by 11–24% in the field tests. [ABSTRACT FROM AUTHOR]

The wave propagation through a rock system is affected by the rock microstructure and the pore fluid. By considering the rock microstructure, the upper and lower bounds of P-wave modulus can be refined in the inclusion-based effective medium model. However, the effects of patchy saturation and squirt flow are not included. The patchy saturation due to the inhomogeneous distribution of different fluids and the squirt flow induced by wave propagation can lead to a difference in fluid pressures. Thus, we introduce the fluid pressures to the inclusion-based effective medium model to describe the variation of P-wave modulus during the wave propagation. We assume that the fluid pressures mainly generate the variation of the P-wave modulus and update the elastic tensor. By adjusting the model parameter related to effective fluid modulus, the proposed model can keep good consistency with the known boundary models. Finally, we analyze the effect of water saturation, frequency, and the pressure difference of different fluids on P-wave modulus with the proposed model. [ABSTRACT FROM AUTHOR]

The stress waves have a significant effect on crack propagation during blasting. In this paper, a microsecond controlled blasting case involved with the interactions between blast waves and an opposite propagating crack is studied using dynamic photoelasticity method. The results show that the stress waves influence the local stress field of dynamic crack tip, and alter the crack propagation behavior. Moreover, during crack-wave interactions, both the singular stresses and the nonsingular stresses around the crack tip changed apparently. With the incidence of dilatational wave, the non-singular stresses σ ox and σ oy around the tip of opposite propagating crack increase remarkably, inducing an increase in the crack extension resistance, and decreasing both the crack velocity and the dynamic stress intensity factor K I d. Whereas, when the shear wave impinges onto the opposite propagating crack, the crack tends to increase the crack velocity. In addition, the fractal dimension of the crack velocity decreases during the incidence of dilatational wave, and increases when the shear wave impinges upon the opposite propagating crack. [ABSTRACT FROM AUTHOR]