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The aim of this study was to investigate the optimization of cut-blasting methods for deep rock masses under high in situ stress. Several plane fluid–structure interaction (FSI) models were established, and the blasting effects using common cut-blasting methods were analyzed. Considering the damage range of the rock mass and the blast-induced vibration after blasting, the cut-blasting method was selected as suitable for deep rock masses under high in situ stress. Subsequently, the cut-blasting parameters, including blasthole spacing, blasthole diameter, and the distance between the empty hole and the first cut-blasting hole, were optimized through the crack connectedness between the blastholes and the fractal dimension of the damaged rock mass. The results showed that the pentagonal cut-blasting method is more suitable for deep rock masses compared with other methods and the blasthole spacing should be reduced to resist any inhibitory effects from the increasing in situ stress. For in situ nonhydrostatic stress conditions, it is reasonable to choose a wider blasthole spacing in the direction of the major principal stress and a narrower one in the direction of the minor principal stress. Under high in situ stress, the joint optimization of blasthole spacing and blasthole diameter is recommended in order to avoid the poor cutting effect caused by too-narrow spacing. In addition, the formula for calculating the location of empty holes in shallow rock-mass blasting is also applicable to deep rock masses. The partially optimized results were preliminarily verified through comparison with existing field tests. These findings offer a new approach for enhancing the blasting effect on deep rock masses and may provide valuable guidance for the design and construction of cut blasting in deep rock masses. [ABSTRACT FROM AUTHOR]

During blasting in deep mining and excavation, the rock masses usually suffer from high in situ stress. The initial seismic energy generated in deep rock blasting and its attenuation with distance is first theoretically analyzed in this study. Numerical modeling of the multiple-hole blasting in a circular tunnel excavation under varied in situ stress conditions is then conducted to investigate the influences of in situ stress levels and anisotropy on the blasting seismic energy generation and attenuation. The case study of the deep rock blasting in the China Jinping Underground Laboratory (CJPL) is finally presented to demonstrate the seismic energy attenuation laws under varied in situ stress levels. The results show that with the increase in the in situ stress level, the explosive energy consumed in the rock fracture is reduced, and more explosive energy is converted into seismic energy. The increasing in situ stress causes the seismic Q of the rock mass medium to first increase and then decrease, and consequently, the seismic energy attenuation rate first decreases and then increases. Compared to the condition without in situ stress, the blasting seismic energy decays more slowly with distance under in situ stress. Then the seismic waves generated in deep rock blasting are more likely to reach and exceed the peak particle velocity (PPV) limits stipulated in the blasting vibration standards. Under non-hydrostatic in situ stress, the generation and attenuation of the blasting seismic energy are anisotropic. The highest seismic energy density is generated in the rock mass in the minimum principal stress orientation. Its attenuation is dependent upon the in situ stress aligning the wave propagation orientation. [ABSTRACT FROM AUTHOR]

Tunnel blast-induced vibration probably causes damage to the rock mass surrounding the tunnel surface and also to the rock mass of the slope at the tunnel entrance. It is important to simultaneously monitor the vibration on the tunnel surface and on the tunnel entrance slope face, especially when the blasting work face is close to tunnel entrance. During blasting excavation of the traffic tunnel at Baihetan hydropower station, vibration monitors were installed both on the tunnel surface and on the tunnel entrance slope face. Based on the monitoring data, a comparative study is conducted on the peak particle velocity (PPV) and frequency characteristics of the vibrations at these two locations. A three-dimensional FEM simulation of the tunnel blast is then performed to verify the field test results. The field monitoring and the numerical simulation show that there is significant difference between the vibration on the tunnel surface and that on the tunnel entrance slope face. The vibration on the tunnel surface has greater PPV and faster attenuation, while the tunnel entrance slope face has higher frequency and faster reduction rate in the center frequency. These differences are attributed to the different wave types and wave propagation paths. The tunnel surface is mainly surface waves transmitted through the damaged rock mass around the tunnel profile, while the tunnel entrance slope face originates mainly from the body waves transmitted via the undamaged rock mass inside the mountain. The comparisons of the monitored vibrations with the velocity limits specified in the Chinese standard show that the most dangerous vibration that may exceed the limit occurs on the tunnel surface. Therefore, the maximum charge weight used in the tunnel blast is determined by the vibration on the tunnel surface. Under different control standards, the allowable maximum charge weight per delay is further discussed. [ABSTRACT FROM AUTHOR]

This study investigates the propagation and attenuation characteristics of shock waves produced by drilling and blasting. The focus is on a novel method for calculating the attenuation of the shock wave pressure. Based on the propagation of shock waves in a rod with a variable section and a Maxwell model for rock, theoretical solutions for the attenuation of the blast-induced shock wave pressure were derived and the main factors influencing the attenuation speed were analyzed. The attenuation law of shock waves for siltstone, limestone, and granite, with common borehole diameters of 76–150 mm and initial shock pressures of 1–6 GPa, was analyzed through numerical simulations. The results show that the shock wave falls off as the square root of the relative distance. The attenuation speed of the shock wave is mainly related to the borehole size, rock properties, and shock wave intensity. The attenuation speed increases with increasing initial shock pressure on the borehole wall, but decreases with increasing borehole radius. Compared with soft rock, the shock wave pressure attenuates faster in hard rock. Combining theoretical analysis and numerical simulations, an exponential attenuation coefficient was introduced to the attenuation formula. Statistical analysis shows that the exponential attenuation coefficient is linearly proportional to the initial shock pressure. Furthermore, a new method for calculating the attenuation of shock wave pressure was proposed, reflecting the comprehensive influence of rock characteristics and shock wave intensity. The new method is an improvement over existing methods, which only consider the effect of rock properties on shock wave attenuation. The reliability of the new method was verified against the results of field tests. The findings provide a theoretical basis for the optimization of blasting parameters and assessments of blast-resistance for structures. [ABSTRACT FROM AUTHOR]

In tunnel blast applications, vibration monitors are typically placed on tunnel surfaces to measure the responses of rock to explosion loads for statutory compliance. Few publications in the open literature have focused on the differences between the blasting vibration on tunnel surfaces and inside the surrounding rock. During blasting excavation of the experimental tunnels in the China Jinping Underground Laboratory, velocity sensors were arranged both on the tunnel walls and inside the surrounding rock to monitor far-field vibrations. In this paper, the vibration characteristics on the tunnel surfaces and inside the surrounding rock are presented based on the recorded field data. Corresponding empirical formulae for peak particle velocity (PPV) attenuation and dominant frequency variation are derived from these data. A three-dimensional dynamic finite-element model is used to verify the site survey results. The field and numerical studies show that compared with the inside vibration, the tunnel surface vibration has a higher, more readily attenuated PPV and a lower frequency with a slower rate of decline in the dominant frequency. The mechanisms that cause the differences between the surface and inside vibration are discussed in detail. The tunnel surface vibration in the far field is dominated by surface waves, which occupy more than 75% of the total vibrational energy. For this reason, the results presented in this paper do not apply to vibration in the near field, where body waves are expected to be dominant. [ABSTRACT FROM AUTHOR]

During blasting excavation in deep-buried tunnels and mines characterized by high in situ stress, the rock vibration is attributed not only to blast loading, but also to dynamic unloading caused by transient release of the in situ stress on excavation faces in the process of rock fragmentation by blasting. Understanding the vibration frequency characteristics under these two excitation sources is of important signification to determine appropriate vibration threshold limits for structure damage in deep-buried opening excavations. With a theoretical model developed for a deep-buried circular tunnel excavation by the millisecond delay blasting sequence, frequency characteristics and their influence factors are investigated and discussed for the vibrations induced by the blast loading, the dynamic unloading and the combined effects, respectively. The results show that the rising time of blast loading, the duration of dynamic unloading and the dimension of excavation boundaries are the main factors that affect the vibration frequency of blasting excavation in highly stressed rock masses. It is found that, the blast loading with a much shorter rising time accentuates higher vibration frequency than the dynamic unloading with a long duration, and it causes the blast loading vibration to be more readily attenuated as the propagation distance increases. Thus, the unloading vibration may become the main vibration component at far distances where its low-frequency vibration may exceed the vibration limits. The vibration induced by the combined effects has two distinctly dominant frequency bands corresponding to the two vibration excitation sources. The frequency analyses of the vibration records from two underground projects excavated by blasting are presented to demonstrate this finding. The findings of this study also clearly reveal that, reducing the dimension of excavation boundaries is one of the most effective means to prevent the vibrational damage to structures as it increases the vibration frequency and meanwhile reduces the peak particle velocity. [ABSTRACT FROM AUTHOR]

During blasting excavation of jointed rock slopes, the rock masses of the slopes often suffer an abrupt change in displacement, which is called displacement mutation. This kind of displacement mutation seriously threatens the stability of jointed rock slopes. Taking the left-bank spandrel groove slope in the Baihetan hydropower station as an example, the displacement behaviour of the jointed rock slope under excavation was surveyed based on the field monitoring. It is found that the displacement mutation of the jointed rock slope is closely related to the dynamic disturbances of blasting excavation. Then the numerical simulations based on the FLAC3D program were conducted to study the mechanism and influencing factors of the displacement mutation caused by blasting excavation. The results show that the displacement mutation is attributed to the combined actions of blast loading and transient in-situ stress release occurring on blast-created free surfaces if high in-situ stress exists. Under the compression of blasting pressure and in-situ stress, strain energy is accumulated and stored in the rock masses of slopes. As the blasting pressure decays, the accumulated rock strain energy due to blast loading is rapidly released. Meanwhile, the stored rock strain energy under in-situ stress is also quickly released as blasts create a free surface and the in-situ stress initially exerted on this face is suddenly released over a very short period. It is the rapid release of the rock strain energy that causes joint opening and displacement mutations in jointed rock slopes. Magnitude of the joint opening displacement is positively related to the strain energy release rate. The blasting load at higher peak pressure, longer rise time and shorter fall time and the transient in-situ stress release with a higher initial stress and a shorter period correspond to greater strain energy release rates, and thus will result in larger joint opening displacement. • Displacement mutations are observed during blasting excavation of a jointed rock slope. • Rock strain energy is accumulated and stored under blasting pressure and in-situ stress. • Rapid release of accumulated and stored rock strain energy causes displacement mutation. • Magnitude of displacement mutation is positively related to strain energy release rate. • Mutation displacement contains joint opening displacement and strain displacement. [ABSTRACT FROM AUTHOR]

This paper focuses on the comparison of damage induced by smooth blasting and presplit blasting based on the excavation of high rock slope. The whole damage process of the smooth blasting and presplit blasting excavation method is studied by using a cumulative blasting damage numerical simulation technology based on the secondary development of the dynamic finite element code LS-DYNA. The results demonstrate that, in the case of contour blasting with the method of smooth blasting, the total damage of rock slope is a result of cumulated damage induced by the production hole, buffering hole, and smooth hole. Among the total damage, the blasting of the production hole is the main resource, followed by the smooth and buffering holes. For the presplit blasting, the final damage of rock slope is mainly induced by presplit blasting itself. The spatial distribution characteristics of the final damage zone of two methods are compared. Two classes of damage zone could be found in smooth blasting excavation; one of them is the columnar high-degree damage zone around the slope surface and the other is the low-degree damage zone located in the middle of the slope. But in the case of presplit blasting, there is only the columnar high-degree damage zone around the slope surface. Finally, a damage control suggestion for two blasting excavation methods is proposed and verified based on the excavation of the temporary shiplock slopes of the Three Gorges Project in China. [ABSTRACT FROM AUTHOR]

Abstract: During the excavation of rock mass with the method of drill and blast, the release of in-situ stress, also known as excavation load on boundary, is traditionally assumed as a quasi-static process, so the inertia and all other dynamic responses induced by the release of in-situ stress can be ignored. After analyzing the process of release of in-situ stress induced by rock mass excavation with the method of drill and blast and determining the duration of the release of in-situ stress, it is found that the release of in-situ stress induced strain rate can reach a magnitude of 10−1–101/s or higher if the initial in-situ stress has a level of 20–50MPa, it implies that the release of in-situ stress is a transient process, and the transient character of the release of in-situ stress and correspondingly induced dynamic response of surrounding rock should be taken into account together with excavation. Problems such as the determination of redistributed in-situ stress corresponding to initiation sequences of millisecond delays, the way and path of the transient release of in-situ stress, transient release of in-situ stress induced vibration and comparison with blasting induced vibration are also discussed in this paper. Finally, a case of in-situ stress release induced vibration in Pubugou hydropower projects are presented as verifications. [Copyright &y& Elsevier]

Abstract: Due to the complicated blasting load, the diversified medium models and various constitutive relations of the rock mass, and a huge job for simulating blasting of multiple holes, it is very difficult and costly to simulate the blasting vibration accurately in numerical computation. This paper presents an equivalent simulation method so as to transform this complex dynamic problem into an approximate initial-boundary problem. The equivalent elastic boundary applied by the blasting load was developed for multiple holes according to the spatial distribution of rock damage around each blasthole. The equivalent mechanics process of the complex blasting load was performed through analysis of the expansion of the borehole volume, the growth of cracks, the movement of stemming and the outburst of detonation gases. In combination with the blasting excavation of the tailrace tunnel in the Pubugou Hydropower Station, particle vibration velocities in the surrounding rock at different distances from the explosion source were simulated by applying this equivalent method based on the dynamic finite element method. The comparison with field monitoring data indicates that this equivalent simulation method is applicable to predicting the far-field dynamic response of the ground subjected to blasting load, and the selection of rock mass properties near the equivalent elastic boundary has a significant impact on simulation results. [Copyright &y& Elsevier]

When slopes are excavated in rock masses with the blasting method, blasting and stress relaxation will produce an excavation damage zone (EDZ) in the remaining rock mass. This study presents an estimation of the rock mass properties in the EDZ based on the generalized Hoek-Brown failure criterion and the varying disturbance factor. In this estimation, the disturbance factor is quantified by using the acoustic P-wave velocities tested in undamaged and damaged rock masses, instead of a single constant value chosen from the descriptive guidelines. Statistical analysis of the acoustic test data shows that with an increase in depth behind the slope face, the disturbance factor declines from 1.0 to 0 almost in a linear manner. For blasting excavation of slopes in weaker rock masses, the zone in the immediate vicinity of the slope face is fully damaged and thus over this range the disturbance factor remains the constant value of 1.0. As the disturbance factor linearly decreases within the EDZ, the deformation modulus, uniaxial compressive strength and equivalent cohesive strength of the damaged rock mass linearly increase to the undamaged values. By assigning the varying parameters to the rock mass in the EDZ, a numerical calculation based on the FLAC program is conducted to assess the slope stability. It is found that using the varying rock mass parameters within the EDZ will yield completely different failure surfaces and factors of safety from that uses the constant parameters. Therefore, it is significant to develop such an approach to quickly estimate the varying rock mass properties in the EDZ particularly when in-situ tests are not available. [ABSTRACT FROM AUTHOR]

Most of the existing prediction equation of the dominant frequency of blasting vibration refer to the particle peak velocity to attribute the influence of blasting source factors to charge weight in general. In fact, the influence factors of dominant frequency are complicated. Although charge weight is the most convenient fitting parameter, it also has limitations in some cases. This study contributes to the understanding of the relationship between cracked-zone radii and the dominant frequency of vibrations during tunnel blasting. This relationship was studied using theoretical, numerical, and in-situ measurement approaches. It was found that the dominant frequency induced by blast vibrations is closely related to the cracked-zone radius, which is a more comprehensive parameter than the charge weight, as it also considers the type of structure, number and layout of blastholes, mechanical properties of rock, and conditions of the free surface. A prediction equation for the dominant frequency in tunnel blasting was derived from a combination of dimensional analysis and the theory of elastic–spherical wave propagation, whereafter validation is carried out for the proposed equation based on the measured dominant frequency from Pubugou hydropower station. The results showed that a favorable fitting correlation has been obtained in both zero-crossing dominant frequency and mean frequency. The charge weight and contour diameter of each delay were compared as fitting parameters for the dominant frequency. The latter performed better than the charge weight during tunnel blasting. The study is helpful to understand the attenuation characteristics of the dominant frequency induced by tunnel blasting, and has certain reference value for blasting vibration control and blasting design optimization. [ABSTRACT FROM AUTHOR]