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To explore the dynamic response and damage evolution of coal under multiple high-pressure air blasting (HPAB), simulated coal specimens were used in the HPAB experiments, and the variation laws of stress field, vibration field, damage field, and cumulative fracture failure process in specimens were analyzed from a macro point of view. A scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP) were used to observe the pore distribution near the blasthole of the specimen and analyze the variation law of pore structure parameters under multiple HPAB from the microscopic point of view. The test results show that (1) the stress wave generated by HPAB has a great impact on the near zone. After multiple HPAB, the damage value at the place 50 mm away from the blasthole increases by 3.91 times compared with the one shock from HPAB, and the strain peak and vibration velocity are reduced by 17.86% and 63.05%, respectively. With the increase of distance, the internal damages of the specimens in the middle and far zones are mainly driven by the stress wave and the high-pressure air, and the strain peak, vibration velocity, and damage degree gradually decrease. (2) With the increase of shock times in HPAB tests, the stress wave attenuation index decreases at first and then increases, and the damages degree of the middle and far zones increase slowly in the first few shocks and then increase rapidly. The site coefficient (k) shows an overall decreasing trend, whereas the attenuation coefficient (α) tends to increase. (3) The multiple HPABs have a great impact on the pore structure of the specimens. Compared with unshocked specimen, the cumulative mercury injection and pore volume increased by 152.04% and 135.05%, respectively. The number of connecting pores with large pore diameter is significantly increased. The multiple HPAB can effectively improve the pore and fracture structure in the specimens and form a relatively developed fracture network channel. The study results have certain guiding value for solving practical engineering problems of low extraction efficiency in low permeability coal seams. [ABSTRACT FROM AUTHOR]

High-pressure air blasting (HPAB) is one type of physical blasting technique that enhances the extraction rate of coalbed methane by impacting the coal mass with high-pressure gas to create cracks within it. First, based on the physical and mechanical parameters of the simulated coal rock mass, the RHT constitutive model of the coal rock mass was established, and its parameters were determined. Then, the laws of crack propagation and stress wave decay in coal induced by high-pressure air blasting were revealed by comparing the effect with that of equivalent explosive blasting. Next, the HPAB experiment was simulated to explore the coal crack propagation law under in-situ stress conditions. Finally, the HPAB experiment was carried out and the results of this experiment were compared with the numerical simulation results. The results indicate that the crack propagation induced by high-pressure air blasting is considered as two major stages, i.e., the crack initiation and crack propagation stage induced by the stress wave and the crack stable propagation stage induced by the duration high-pressure gas. In the case of equal energy, the peak stress wave of high-pressure gas is smaller, decays more slowly and has a longer action time, compared to explosive blasting. Therefore, the number of initial random cracks in coal mass induced by high-pressure air blasting is less, and the range of crack propagation induced by high-pressure air blasting is larger. When λ = 0 (λ is the ratio of the horizontal in-situ stress to the vertical in-situ stress), the in-situ stress in the coal seam can promote the propagation of vertical cracks but inhibit the propagation of horizontal cracks. When λ = 0.5 and 1, the in-situ stress inhibits the propagation of both horizontal and vertical cracks. [ABSTRACT FROM AUTHOR]

• We designed a blasting-vibration damage accumulation experiment, during the experiment, the ultrasonic wave velocity was monitored, and the vibration damage value of the specimens were calculated. • We analyzed the change process of the ultrasonic wave velocity and the accumulation process of vibration damage after the samples were subjected to different vibration loads. • The strength and durability tests of the concrete were carried out to analyze the influence of the effect of damage accumulation from blasting vibration on the uniaxial compressive strength and the durability (impermeability, carbonization resistance, and resistance to chloride ion penetration) of young concrete. • Based on the experiment results, we put forward a blasting-vibration safety standard for young concrete based on the damage accumulation effect, in order to ensure the strength and durability of the concrete, the blasting-vibration velocity should be controlled within 1.5 cm/s. In blasting engineering practice, the accumulation of blasting-vibration damage is inevitable. The objective of this study is to put forward a blasting-vibration safety standard for young concrete based on the effects of damage accumulation. The ultrasonic wave velocity change process and the vibration damage accumulation process of the young concrete were investigated based on the blasting-vibration damage accumulation experiment. Furthermore, the influence of the damage accumulation effect on the compressive strength and durability of concrete were studied. Young concrete subjected to multiple vibration loads, when the vibration velocity is less than 1.0 cm/s, the change process of the ultrasonic wave velocity can be divided into three stages, the damage deterioration effect of vibration do not appear, and the vibration can improve the compressive strength and the durability of the concrete. However, when the vibration velocity is greater than 1.5 cm/s, the change process of the ultrasonic wave velocity goes through four stages, the damage deterioration effects of vibration begin to appear in the second stage and appear macroscopically in the third stage. If the vibration velocity reaches 3.5 cm/s, the damage deterioration effects appear macroscopically from the first stage, the compressive strength and durability of concrete decrease, and the degree of reduction increases with the increasing of vibration velocity. The experimental results indicate that the velocity of blasting ground vibration of 1.5 cm/s should be used as the blasting-vibration safety standard for young concrete based on the damage accumulation effect. These findings provide an experimental basis for the evaluation and control of the blasting-vibration for young concrete. [ABSTRACT FROM AUTHOR]

The propagation and attenuation rule of blasting vibration wave parameters is the most important foundation of blasting vibration prediction and control. In this work, we pay more attention to the influence of the damage accumulation effect on the propagation and attenuation rule of vibration wave parameters. A blasting damage accumulation experiment was carried out, the ultrasonic wave velocity of the specimens was measured, and the damage value was calculated during the experiment. The blasting vibration wave was monitored on the surface of the specimens, and its energy was calculated by using the sym8 wavelet basis function. The experimental results showed that with the increase in the number of blasts, the damage continues to increase; however, the vibration velocity and the main frequency decrease continuously, the unfocused vibration wave energy in the zone near to the blasting source is rapidly concentrated in the low-frequency band (frequency bands 1 to 3), and the energy is further concentrated in the low-frequency band in the intermediate zone and zone far from the blasting source. There is a distortion process in which the vibration velocity and the main frequency increase slightly and the energy of the blasting vibration wave converges to the high-frequency band (the 5th band) before the sudden unstable fracture failure of the specimens. The experimental results indicate that the prediction and evaluation of blasting vibration should consider the variation rule of blasting vibration wave parameters synthetically based on the cumulative damage effect, and it is not safe to use only one fixed vibration control standard for the whole blasting operation. [ABSTRACT FROM AUTHOR]

High-pressure gas (HPG) blasting is safe and environmentally friendly, and has replaced traditional explosive blasting in some applications. The characteristics of HPG impact load and the propagation law of the stress wave in layered rock mass (LRM) under HPG blasting were investigated experimentally and numerically. Through experimental tests of hole wall pressure, a segmented exponential model of the hole wall pressure under the impact of HPG was developed; it accurately accounted for the time history characteristics of the hole wall load. Experimental tests of stress wave propagation were conducted; the soft rock–hard rock interface was found to have a significant effect on stress wave propagation in the LRM. Based on the experimental results, a specific strain model for 10 MPa HPG impact was fitted for a single-hole LRM. A polynomial exponential model and a specific strain model (20 MPa HPG impact) were found for the double-hole LRM. The stress wave attenuation laws for single-hole and double-hole LRM were characterised well. The propagation characteristics of stress waves in LRM under the impact of HPG were analysed and a numerical model of stress wave propagation was established. The numerical model adopted the Riedel–Hiermater–Thoma (RHT) material model for the test parameters. The proposed segmented exponential models of hole wall pressure were applied to the lower, middle and upper parts of the blasthole. With the feasibility of the numerical model analysed, the stress wave propagation characteristics were studied by numerical simulation of an underground pipe gallery. This study provides theoretical guidance and practical value for improving rock-breaking theory and optimising HPG blasting in LRM. [ABSTRACT FROM AUTHOR]

In the blasting construction of underground engineering in layered rock mass, the mechanism of cut blasting and the propagation law of blasting vibration waves are very complex. In this paper, a new method for predicting the blasting vibration velocity of layered rock mass under multihole cut blasting is proposed. The key steps include determining the equivalent elastic boundary and load, establishing the multidegree freedom model of blasting vibration and its motion differential equation, and solving the motion differential equation by time-history analysis method. Two multihole cut blasting tests of different schemes were carried out in the construction site of layered rock mass, and the measured results of blasting vibration waves were obtained. By comparing the time-history curves of the predicted and measured blasting vibration velocity, it can be seen that the time-history curves predicted by the proposed method can reflect the characteristics and attenuation law of blasting vibration waves, and the predicted waveforms are similar to the measured waveforms. By using the proposed method, the prediction accuracy for the peak velocity of blasting vibration in the two tests is 93% and 94%, respectively, and the prediction accuracy for the dominant frequency of blasting vibration in the two tests is 86% and 94%, respectively. The prediction accuracy of the main characteristic parameters of blasting vibration waves is high. So it can be proved that the prediction method proposed in this paper is feasible in effectiveness and accuracy, which can provide important theoretical guidance for the optimization of blasting design and the control of blasting vibration in underground engineering in layered rock mass. [ABSTRACT FROM AUTHOR]

Firstly, the experimental test of blasting vibration was carried out to study the propagation characteristics of the peak particle velocity (PPV) and dominant frequency (DF) of the blasting vibration of layered rock mass under high-pressure gas impact. The test results show that the PPV and DF of blasting vibration of layered rock mass decrease gradually with the increase in distance from the explosion source. The PPV and DF of layered rock mass under the same impact pressure of high-pressure gas are lower than those of single rock mass at the same measuring points. With the increase in hole spacing, the PPV and DF of blasting vibration become smaller, and the blasting effect first becomes better and then worse. Next, the relationship models between the PPV and DF and their influencing factors were deduced by the dimensional analysis method, which can be simplified as exponential forms that decay with the scaled distance. In addition, through a numerical simulation test, it was found that the optimal hole spacing recommended in an excavation example of an underground pipe gallery is 100 cm. Finally, the blasting vibration effects under the two excavation methods of high-pressure gas impact and explosive blasting were numerically simulated and compared. The results indicate that high-pressure gas impact can significantly reduce the blasting vibration effect of layered rock mass compared with explosive blasting. This study has important theoretical guiding significance and practical value for revealing the propagation law, forecasting and controlling the harm of blasting vibration effect of layered rock mass caused by high-pressure gas impact. [ABSTRACT FROM AUTHOR]

The borehole pressure of the rock mass under blasting under consideration is essentially the pressure of the transmitted shock wave acting on the borehole wall. Its value directly affects the fragmentation degree and damage range of the rock mass around the borehole. The whole-process action mechanism of the shock wave in different media and interfaces for air-decoupled charge explosion was comprehensively analyzed, discontinuous Galerkin (DG) method was used to simulate and solve parameters of the multiple media flow field formed by the detonation products and air in the borehole. Two theoretical calculation models of the borehole pressure of air-decoupled charge with discontinuous interface conditions of the pressure and particle velocity were proposed and their algorithm flows were presented. Furthermore, the reliability and feasibility of the proposed models were verified by comparing with the numerical simulation results of the same example under five working conditions. It is suggested to use different theoretical models when using different decoupling coefficients. It is shown that there is an optimal value of the decoupling coefficient for air-decoupled charge, which makes the rock blasting obtain a high utilization rate of explosive energy and good blasting effect at the same time. This study can provide a theoretical basis for revealing the blasting rock-breaking mechanism and optimizing blasting designs. [ABSTRACT FROM AUTHOR]

The vibration caused by the tunnel blasting and excavation will harm the surrounding rock and lining structure of the adjacent existing tunnels. This paper takes a two-lane large-span highway tunnel as the research object, conducts on-site monitoring tests on the impact of vibration caused by the blasting and excavation of new tunnels on the existing tunnels under different blasting schemes, and analyses in detail the three-dimension vibration velocity by different excavation footages. From the vibration speed, it is concluded that the influence of the existing tunnel of the newly built tunnel blasting team is affected by various factors, such as distance, free surface, charge, and blasthole distribution. With different blasting schemes, the greater the amount of charge, the greater the vibration caused by blasting. Existing tunnels correspond to the front of the tunnel, and the axial and radial vibration peaks are greater than the vertical. Although the cut segment uses a less amount of explosive and has a less blasthole layout, there is only one free surface. Because of the clamping of the rock, it is compared with the other two segments. The vibration caused is the largest. Although the peripheral holes are filled with a large amount of explosive, the arrangement of the blast holes is relatively scattered and there are many free surfaces. Hence, the vibration caused is the smallest. Corresponding to the back of the tunnel face, since there is no rock clamp, the vibration caused by the cut segment is the smallest, and the vibration caused by the peripheral segment and the floor segment is relatively large. The vibration caused by the front explosion side is significantly greater than the vibration caused by the back explosion side. The vibration velocity caused by the unit charge of 1.5 m footage is greater than that of the 3.0 m footage. The vibration velocity caused by the unit charge of the cut segment is the largest, and the vibration velocity caused by the peripheral segment and the floor segment is smaller. The research results provide a reference for the blasting control of similar engineering construction. [ABSTRACT FROM AUTHOR]

The blasting vibration signals show obvious zoning propagation characteristics. Because of not considering the specific influences of geological and topographical conditions, the existing zoning methods of blasting vibration cannot fully describe the internal details of blasting vibration signals. Therefore, a new zoning method of blasting vibration based on energy proportion was proposed in this paper, in which the energy proportion in low, medium, and high frequency bands after multiresolution wavelet analysis is used as the zoning index to distinguish the different characteristics of blasting vibration signals in different zones. Based on the proposed zoning method, 343 sets of measured blasting vibration signals were used to train and test the SVM classification models with four different types of kernel functions. The testing results demonstrate that the zoning method of blasting vibration based on energy proportion has high feasibility, flexibility, and reliability, and the SVM classification models with RBF have higher accuracy than models with other kernel functions in blasting vibration zoning prediction. [ABSTRACT FROM AUTHOR]