Blasting Stress Wave Research Articles

Fracture mechanism of rock around a tunnel-shaped cavity with interconnected cracks under blasting stress waves

The existence of cracks in the surrounding rock significantly affects tunnel stability under dynamic disturbances. To study the fracture mechanism of tunnels in fractured rock masses under blasting stress waves, a new specimen, i.e., the specimen with a tunnel-shaped cavity and interconnected cracks (TCIC), was proposed in this paper. Using these specimens, blasting tests were conducted. The crack propagation data were monitored by crack propagation gauges (CPGs) and strain gauges (SGs). The Riedel-Hiermaier-Thoma (RHT) model was used to perform equal-scale numerical simulations on TCIC specimens. The stress distribution and fracture modes around the tunnel in a fractured rock mass under blasting stress waves were characterised. The results show that the interconnected cracks have an important influence on the dynamic response of the tunnel under blasting stress waves. The stress distribution around the tunnel remarkably changes due to the existence of the interconnected cracks, resulting in the initiation and propagation of these cracks, but there is no fracture on the tunnel boundary. The dynamic crack propagation behaviours are influenced by the incident direction of the blasting stress wave. The crack propagation velocity in the front blasting side of the tunnel is higher than that in the back blasting side. The research results are significant to the analysis of tunnel stability and the support design.

Effect of deformation of a flat plate specimen under in-plane blasting loading on 2D and 3D digital image correlation measurement.

In combination with high-speed photography, a 3D digital image correlation (DIC) experimental system suitable for an in-plane blasting experiment is established to effectively explore the mechanism of blasting. The dynamic response of the specimen under in-plane blasting loading is analyzed on the basis of the model experiment, and the effect of out-of-plane deformation on the 2D-DIC measurement under a blasting stress wave is studied. First, the out-of-plane displacement and strain evolution law of the specimen under blasting load are studied by using a 3D-DIC system. Second, an analytical model of the effect of out-of-plane curved displacement on 2D-DIC measurement under the blasting load is proposed on the basis of pinhole imaging theory. Finally, the specific error value and error percentage of out-of-plane displacement to the 2D-DIC measurement results are quantitatively analyzed on the basis of the established theoretical method. Results show that the theoretical error of out-of-plane curved displacement to 2D-DIC accounts for 10.8% of the measured vertical strain. In the polycarbonate plate specimen, the effect of out-of-plane displacement caused by the blasting experiment on 2D-DIC measurements cannot be ignored. This novel, to the best of our knowledge, experimental system provides a wealth of experimental data, including in-plane full-field strain evolution and out-of-plane displacement changes.

Analysis of the expansion process of cracks caused by blasting stress waves in a PMMA medium with filled defects

The occurrence of defects in a medium has a significant role in the transmission of blasting stress waves and the expansion of blast-caused cracks. In this study, an experiment is first conducted on the crack expansion in a polymethyl methacrylate (PMMA) medium with vertically filled defects under a blasting load by using the digital laser dynamic caustics (DLDC) testing system. The origin, expansion, and termination of wing cracks are analyzed, and the variation of the dynamic stress intensity factor and the crack expansion velocity on the crack tip are studied. An effective inversion to the stress wave transmission and the crack expansion is achieved via a test conducted using the distinct lattice spring model (DLSM), along with a comparison between the stress variation at the center points on the wave heading side, back side, and endpoints of the defect. The characteristics of the impact due to the stress wave are also analyzed. Lastly, the expansion velocity/acceleration difference between the wing cracks are analyzed and compared with various defect characteristics (filled, open, closed, bedding).

Design and application of a comprehensive experimental system for real imaging–virtual imaging of dynamic caustics

On the basis of the principle of caustics, a comprehensive experimental system for real imaging-virtual imaging was designed and established and then applied to dynamic fracture mechanics experiments. An explosion load experimental system was used to investigate the interaction between blast stress waves and prefabricated cracks. The virtual image information of the caustic spot of the crack tip under the radial compressive stress of the explosive load was captured and quantified. Combined with the experimental data obtained by the real imaging system, the entire interaction process of the blast stress wave and prefabricated cracks was quantitatively characterized. The impact load test system was used to carry out three-point bending impact testing on a unilateral offset crack specimen. In the experiment, the load applied to the upper boundary and the stress intensity factor of the crack tip were simultaneously obtained. The comprehensive experimental system had a simple light path and could simultaneously obtain two sets of experimental information for the real and virtual images, respectively. Thereby, abundant and reliable data were obtained for experimental research. In summary, the experimental systems were effectively used in the investigation of impact and explosion dynamics.

Bearing capacity behaviour of prestressed anchor cable under slope blasting excavation

The stability of high steep rock slopes directly influences the success or failure of whole engineering construction processes. Slope excavation and reinforcement with prestressed anchor cable can effectively improve the stability of the slope. However, the blasting stress waves have a great impact on the anchor cables that have been installed or are under construction. In this paper, a new simulating method of prestressed anchor cables is proposed for high steep rock slopes in consideration of the construction process and the reinforcement mechanism. And a simple simulating method of slope blasting excavation load is developed. The bearing capacity of prestressed anchor cables under slope blasting excavation is analysed via an ideal generalized slope model based on the left bank slope of the Jinping I hydropower station. The results show that the degree by which the distance from the blasting zone, the damping ratio, the maximum single-shot dose and the grade of the prestressed anchor cable impact the bearing capacity behaviours of the prestressed anchor cable decrease over time. Anchor cables are most prone to vibration failure within 5 m from the outer anchor head. The increase in tensile force is the largest on the anchor head at the slope surface. The impact of the maximum single-shot dose and damping ratio on the small-tonnage anchor cable is greater than that on the large-tonnage anchor cable. The anti-explosion performance of the large-tonnage anchor cable is better than that of the small-tonnage anchor cable. The foundations and experience needed to optimize the parameters of blasting excavation and anchor cable design for similar high-slope projects in China or other regions are presented.

Study on the Coal Damage and Fracture Mechanism under Multiple Actions of Blasting Stress Wave

Controlled blasting technology is an effective way to enhance coalbed methane recovery, but the theoretical research has fallen behind engineering practice seriously. Therefore, in this work, based on the damage and fracture mechanics theory, the coal damage process and mechanism under the multiple actions of the blasting stress wave (BSW) were analyzed firstly. Then, the simulation experiment of coal damage accumulation was designed and carried out; during the experiment, the ultrasonic velocity of the samples was measured and the damage values were calculated. Under the multiple actions of BSW, the initial radial cracks around the blast-hole wall or in the area near the explosion source are formed under the action of tangential tension of BSW and the internal pressure of the blast-hole, and under the combined actions of stress wave, gas, and in situ stress, the primary cracks in the middle and far area of the explosion source expand and form the mesoscopic cracks because of the damage accumulation at the crack tip. And then the mesoscopic cracks grow and intersect to form a macroscopic crack. Finally, the macroscopic cracks expand and intersect with the initial radial cracks, the samples break suddenly under the load far below their fracture toughness because of the damage accumulation effects. Hence, the research results have guiding significance for better understanding and application of controlled blasting technology to enhance the CBM recovery.

Assessing and controlling of boulder deep-hole blasting-induced vibrations to minimize impacts to a neighboring metro shaft

When a metro is constructed by the shield method in the strata containing boulder group, it is prone to some engineering problems such as serious cutter wear, deformation of the cutter holder head, damage of the cutter spindle, which seriously affects the construction efficiency and bring enormous problems to the shield construction. This paper proposes the vibration reduction measures for the boulder deep-hole blasting to meet the construction safety. To achieve this purpose, the field test was applied to investigate the vibration response of adjacent building under five blasting areas. Then, three-dimensional numerical model was employed to discuss the vibration characteristics of the boulder and adjacent buildings under vibration reduction measures. The results show that increasing the damping hole and the isolation hole and changing the charging layout of the blasting areas can blast the boulder under the condition of ensuring effective and safe construction. Damping hole and isolation hole have a positive role in the control and attenuation of blasting stress wave. All of them have good vibration reduction effect, and the vibration reduction efficiency is more than 90%. The double row damping holes with 12.7 cm diameter and 20 cm spacing have good vibration reduction effect and economic benefit. When double row damping holes are used, the maximum particle velocity of the shaft structure under the influence of blasting stress wave is 1.47 cm/s, which is within the reasonable safety range.

Experimental Investigation of Obliquely Incident Blast Wave Effect on Deflection of Running Cracks

Abstract Dynamic caustics in conjunction with high-speed photography were used in experiments to investigate the interaction of obliquely incident blast stress waves with running cracks. The dynamic photoelastic experiment method was used to supplement and improve the effects of different blast stress wave properties on the running crack. This indicated that the crack-tip mode II stress field affected the crack deflection direction. When the moving crack-tip mode II stress intensity factor KII was positive, the crack deflected clockwise. When the stress intensity factor KII was negative, the crack deflected in the counterclockwise direction. The incident direction of the blast stress wave affected the deflection of the crack. When the direction of the blast stress wave was the same as the direction of the moving crack propagation, the moving crack deflected away from the blast wave source. When the blast stress wave propagation direction was opposite to the crack propagation direction, the crack deflected toward the blast wave source. During the interaction of an obliquely incident blast stress wave with a moving crack, the P wave caused the deflection of the crack. The tails of the P wave and S wave interacted with the kinked crack to increase the mode I stress intensity factor KI and the crack propagation velocity.

Modified mixed-mode caustics interpretation to study a running crack subjected to obliquely incident blast stress waves

Crack-wave interaction is common in blast, producing mixed-mode cracks when stress waves are oblique to cracks. How to measure mixed-mode crack-tip stress under blast stress waves is difficult. To this end, optical caustics method was employed to study a running crack in a PMMA plate subjected to obliquely incident blast stress waves, by which stress problem was transformed into optical problem. Incident P and S waves, as well as reflected waves, were visualized as fringe patterns, and mixed-mode crack-tip stress was represented by a caustics pattern (shadow spot). However, mixed-mode caustics patterns under P waves were not consistent with the classical interpretation, hence a modified mixed-mode caustics interpretation was proposed and then verified by P-wave compression and tension loading cases. In modified interpretation, an iteration procedure was proposed to obtain modified stress intensity factors, i.e. KI and KII, and crack-tip positions which were more precise than those by the classical interpretation. P-wave compression phase decreased KI and crack velocity sharply but produced the largest KII, while the following P-wave tension phase had an opposite effect. S waves and reflected waves produced higher KI and crack velocity with lower KII. KII and crack velocity histories explained curved crack path and microscopic fracture surface. Finally, the reason for the success of the modified interpretation was discussed, and it is indicated that transient stress superposition and delayed redistribution in the crack tip are the mechanism underlying the modified interpretation.

The Influence Law of Multi-point Cumulative Blasting in Slope

In order to study the cumulative blasting stress wave produced by the vibration of soft rock slope, the deformation regularity of the influence of blasting, caused by the horizontal and axial acceleration along with other additional stress waves were analyzed. The vibration displacement formula along with the vibration source stress wave superimposed model were established. The research also deduced the deformation rule for the gangue slope of Lijiahao Coal Mines as a vibration signal recorder was used to collect the blast vibration wave and the simulated analysis of displacement monitoring profile of the X and Y displacement rule under the influence of blast vibration were obtained. The results indicate that with the increase of the shot point distance, the displacement in the Y direction shows a logarithmic decline rule while the change in displacement of the X direction is small. The result also shows that the greater the initiation of multipoint shot firing, the greater the slope deformation and displacement of the rock mass and as a result multipoint nonsimultaneous blasting and control detonation time difference, can effectively reduce the effects of vibration on the slope deformation.

Stress wave superposition effect and crack initiation mechanism between two adjacent boreholes

In rock blasting, the effect that stress wave superposition between boreholes has on rock fragmentation remains controversial. Here, we experimentally study the stress wave superposition effect between two adjacent boreholes by combining the model experiment method and the high-speed digital image correlation experimental system. Numerical simulations based on the continuous-discontinuous element method are also used to analyze the crack initiation mechanism between two adjacent boreholes. The model experiment results show that, in the stress wave superposition area between boreholes, the stress component in the direction of the connection line of the two boreholes is mainly compressive, while that in the direction perpendicular to the connection line is mainly tensile. Superposition of the blasting stress waves between the boreholes strengthens both stress components, but the strengthening effect on the stress component in the direction perpendicular to the connection line (mainly tensile) is significantly less than that in the direction of the connection line (mainly compressive). Moreover, these stress enhancements are not sufficient to induce crack initiation in the stress wave superposition area between boreholes. Numerical simulation analysis further shows that crack initiation in the stress wave superposition area has relatively stringent requirements on the explosive parameters, rock properties and borehole spacing, which are difficult to observe in engineering practice and model experiments.

Experimental Study on Blasting Energy Distribution and Utilization Efficiency Using Water Jet Test

The blasting stress wave and blasting gas generated by explosive blasting are the two main motive powers of rock fragmentation. An experimental method based on water jet test is used to study the energy distribution ratio of blasting stress wave and blasting gas; the utilization efficiency of blasting energy under different borehole constraint conditions is also analyzed. It proves that the blasting stress wave does not cause the water jet, and the blasting gas is the only power of the water column jet. The results show that the energy of the blasting gas and blasting stress wave respectively account for about 64% and 36% of the total energy generated by the explosion of lead azide. The utilization efficiency of the blasting gas energy under strong, medium, and weak constraint conditions is 100%, 80%, and 52%, respectively. The borehole constraint condition is crucial for the effective utilization of blasting energy.

Experimental and Theoretical Analysis of Stress Superposition in Double-Hole Blasts

Abstract Using an experimental system of digital image correlation based on a high-speed camera which can be applied for blast-induced loading, model experiments of single-hole and double-hole blasts were conducted in polycarbonate specimens, and the evolutionary processes of the full-field strain were studied during blast loading. The results showed that the strain peak at the midpoint between two blastholes in double-hole blasts was 2.4–2.7 times as large as that in single-hole blasts. The stress superposition effect of double-hole blasts was much larger than 2, which is predicted by elastic wave theory. In addition, the superposition of double-hole blast stresses was analyzed by elastic and shock wave theories. These results indicated that when examining the superposition of blast stress waves, the shock wave theory is more in line with the experimental results. Based on the experimental results, there were discussions of the definition of the elastic vibration zone in the traditional blast theory.

Deriving formulas of loading distribution on underground arch structure surface under close-in explosion

Arch structure is one of the basic structural form used in underground engineering. Its load distribution under blasting load directly affects/determines its failure mode and damage grade. However, the experimental studies on this issue are currently limited, owing to the complexity and difficulty of the blasting experiments of underground structures. Therefore, in order to explore the propagation and distribution of blasting stress waves in concrete, comparative analyses, which are based on model test results of concrete target implosion, in view of fluid–solid coupling algorithm were firstly adopted. The reliability of numerical simulation algorithms was also verified. Then, the close-in blasting simulation calculations of the underground arch structure were performed using the verified numerical algorithm. The distribution law of the surface load of the underground arch structure was analyzed, and the time-history curves of overpressure and impulse at various points on the surface were obtained. Through the data normalization and fitting method, the load distribution function of structure surface is obtained, and a simplified prediction formula for predicting the vault point blast load parameters was established. Utilizing these formulas, a general method for determining the blast load at any point on the structure surface is proposed.

Numerical simulation on the breaking of coal and rock with different fillers

By setting different stemming in the coal and rock at the bottom of the borehole, the effects on the propagation of blasting stress waves were studied. The ANSYS-LS/DYNA with damage model was used to simulate the propagation of blasting cracks in the coal and rock. The results show that the speed of propagation is significantly accelerated when the stress wave entering the interior of the rock. Different stemming has few effects on the crushing of coal. When water is used as filler, the obstruction on the stress wave is slightly greater than that of the coal itself. When the soil is used as filler, it has the greatest hindrance to stress waves and the smallest fragmentation of rock mass.

Model Test Study on Cylindrical Blasting Stress Wave Propagation across Jointed Rock Mass with Different Initial Stresses

In the drilling and blasting excavation of underground rock mass, the stress wave produced by the blasting holes usually propagates in the form of cylindrical wave, while the rock mass surrounding the underground engineering is initially subjected to the in situ stress. To explore the propagation and attenuation law of cylindrical stress wave in the in situ stressed rock mass, a model test of cylindrical blasting stress wave propagation across the intact and jointed rock mass under different initial stresses was carried out. First, the attenuation law of the cylindrical stress wave in the intact rock mass under different confining pressures is analysed, and then the influence of the confining pressure scales, the angle, and the number of joints on the propagation law of the cylindrical blast wave in the jointed rock mass is studied. The experimental results show that the physical attenuation of the cylindrical wave in the intact rock mass decreases and then increases as the confining pressure increases from zero. Under zero confining pressure, the transmission coefficient of the cylindrical wave in the jointed rock mass decreases with the increase of joint angle, and the transmission coefficient increases with the increase of the joint angle under confining pressure. As the confining pressure increases from zero, the transmission coefficient shows a trend of increasing firstly and then decreasing.

Experimental study on a Mach cone and trailing Rayleigh waves in a stress wave chasing running crack problem

This paper focuses on a blast-induced Mach cone followed by trailing Rayleigh waves during blast stress waves chasing a running crack in an epoxy plate, using the photoelasticity method and high-speed photography. A Mach cone and trailing Rayleigh waves were visualized and interpreted based on isochromatic fringes, agreeing well with theoretical wave paths. The Mach cone was the coalescence of shear wavelets generated by P waves on crack surfaces, which was verified by the half-cone angle, equal to arcsin (CS/CP). Effects of the Mach cone and trailing Rayleigh waves on the running crack were evaluated by measuring crack velocity and dynamic stress intensity factors (SIFs) of the crack tip. It was found that the Mach cone decreased both crack velocity and SIFs, while trailing Rayleigh waves had the opposite effect. Finally, because of similarities, the findings in our experiments were discussed with those in the supershear rupture. In particular, effects of the Mach cone and trailing Rayleigh waves on the running crack were consistent with particle motion near the fault in the supershear rupture.

Numerical simulation of column charge explosive in rock masses with particle flow code

The detonation of a column charge and the damage process in a rock mass are simulated in this study by the particle flow code method. The expansion loading method and the dynamic boundary treatment method are established according to the discrete element mechanism. Mechanical parameters are calibrated on the basis of the Starfield superposition principle and the dynamic compensation method. Dynamic stiffness and microscopic parameters are used for numerical simulations. Numerical test results are consistent with lab experimental test results. The velocity attenuation law is compared with that of theoretical results, and the blasting crater action index is compared with that of field test results, to verify the rationality of the numerical explosion model of the column charge. The mechanism of the rock breaking process under column charge explosion is analyzed. Stress wave propagation, detonation wave propagation, and crack extension are investigated. The stress wave gradient law is obtained, and the effects of blasting rock breaking, crack extension, and explosion effect outside the blasting crater area are determined with different initiation modes. The research method can explain the blasting stress wave propagation law and describe the dynamical failure process in rock masses. This study can provide a reliable numerical analysis method for follow-up blasting research and offer practical guidance on engineering blasting.

Experimental Study on The Effect of Charge Structure of Blasting on Damage to Dirt Band in Thin Coal Seam

Based on the coalmines with thin coal seam containing hard dirt band in Shandong Province, aiming at the problem of difficult mining caused by hard dirt band in the process of thin coal seam mining, the damage to hard dirt band by blasting in different charge structures was studied. According to the propagation law of blasting stress wave in coal and dirt band in different charge structures, an experimental platform was set up, and the damage characteristics of dirt band by blasting stress wave under the action of ordinary charge blasting and shaped charge blasting were studied through similar simulated comparative tests. The results show that in the ordinary blasting, the cracks spread out disorderly along the radial direction of the blasting hole and the damage of the dirt band is not so good; however, in shaped charge blasting, the peak stress increases 1.32 times in the direction of energy accumulation, while decreases 22% in the direction of non-energy accumulation, making the explosive energy more concentrated in the dirt band and forming a long and wide directional main crack in the dirt band.

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.