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Excavation damaged zone (EDZ) widely exists in the surrounding rock of underground roadway due to blasting excavation, which can further change the surrounding rock property and deteriorate the stability of the roadway. In the presented research, a damaged roadway model was developed to investigate the dynamic stress concentration factor (DSCF) distribution around the damaged roadway under a transient plane P wave. The influencing factors on DSCF distribution were discussed, including the shear modulus (μe/μd) of the intact surrounding rock to that of the damaged surrounding rock, the thickness of EDZ (Rd/R0), and the wavelength of the incident stress wave. Results indicated that the damaged surrounding rock around the roadway belongs to stress‐relaxed zones, and the existence of EDZ can reduce the overall DSCF of surrounding rock. As the ratio of μe/μd is increasing, both the maximum compressive and tensile stress concentrations in the damaged zones decrease gradually, which is opposite to that in the intact surrounding rock. A larger EDZ can induce a higher DSCF in the damaged surrounding rock and a smaller DSCF in the intact surrounding rock. As the incident stress wave's wavelength is increasing, the peak value of DSCF grows first before declining, finally followed by a constant value. Furthermore, the maximum compressive and tensile stress concentrations are always found near the surrounding rock that is perpendicular to and along the incoming direction of stress waves, respectively. The stress concentration in both intact and damaged surrounding rock near the incoming side of stress waves is higher than that at the transmitted side. [ABSTRACT FROM AUTHOR]

Stress conditions and preflaws are prominent conditions influencing the cuttability of deep hard rock. This study aims to investigate the cuttability of intact, prefractured, and drilled rocks under biaxial confining stress, uniaxial confining stress, and stress-free conditions using a conical cutter in rock fragmentation tests on true triaxial loading apparatus. Peak cutter force and penetration depth at rock initial fracture and final failure of the rock specimen were used to reflect rock cuttability. The results show that the rock cuttability increases with stress in one direction decreasing under biaxial confining stresses, and presents a decreasing trend followed by an initial increase as uniaxial confining stress increases. The prefractures and boreholes in rock can further improve the rock cuttability, compared with intact rock. A series of cuttability improvement measures were proposed to provide a suitable condition for the application of nonexplosive mechanized mining in hard rock. Finally, comparative field tests were performed on a single-face entryway and a peninsula-shaped pillar, in which the mean value of cutting efficiency increased from 32.6 to 107.7 t/h, and the dust production and cutter wearout failure reduced significantly. [ABSTRACT FROM AUTHOR]

To investigate the mesodamage evolution of rock specimens containing a pre-existing opening under integrated static and dynamic loading, the nuclear magnetic resonance technique was utilised in this study. The changes in the transverse surface relaxation time (T2) distribution, peak area, and porosity of the specimens prior to and due to varying pre-stresses under an identical dynamic loading were systematically analysed. The experimental results show that as the pre-stress level increased the size and the amount of the pores, the peak area and the porosity increase, indicating that more damage is induced under a high pre-stress. Under identical loading conditions, the specimens having 30° and 60° square openings are more susceptible to damage than those having a 0° square opening. However, compared with the specimens containing a circular opening, the specimens with a square opening are more seriously damaged. The hoop stress concentration surrounding the opening induced by the pre-stress and subsequent dynamic loading was further demonstrated. The result indicates that the initiation of rock fracture occurs in the maximum hoop stress concentration area. [ABSTRACT FROM AUTHOR]

The failure modes and peak unloading strength of a typical hard rock, Miluo granite, with particular attention to the sample height-to-width ratio (between 2 and 0.5), and the intermediate principal stress was investigated using a true-triaxial test system. The experimental results indicate that both sample height-to-width ratios and intermediate principal stress have an impact on the failure modes, peak strength and severity of rockburst in hard rock under true-triaxial unloading conditions. For longer rectangular specimens, the transition of failure mode from shear to slabbing requires higher intermediate principal stress. With the decrease in sample height-to-width ratios, slabbing failure is more likely to occur under the condition of lower intermediate principal stress. For same intermediate principal stress, the peak unloading strength monotonically increases with the decrease in sample height-to-width. However, the peak unloading strength as functions of intermediate principal stress for different types of rock samples (with sample height-to-width ratio of 2, 1 and 0.5) all present the pattern of initial increase, followed by a subsequent decrease. The curves fitted to octahedral shear stress as a function of mean effective stress also validate the applicability of the Mogi-Coulomb failure criterion for all considered rock sizes under true-triaxial unloading conditions, and the corresponding cohesion C and internal friction angle φ are calculated. The severity of strainburst of granite depends on the sample height-to-width ratios and intermediate principal stress. Therefore, different supporting strategies are recommended in deep tunneling projects and mining activities. Moreover, the comparison of test results of different σ2/σ3 also reveals the little influence of minimum principal stress on failure characteristics of granite during the true-triaxial unloading process. [ABSTRACT FROM AUTHOR]

High geostress is a prominent condition in deep excavations and affects the cuttability of deep hard rock. This study aims to determine the influence of confining stress on hard rock fragmentation as applied by a conical pick. Using a true triaxial test apparatus, static and coupled static and dynamic loadings from pick forces were applied to end faces of cubic rock specimens to break them under biaxial, uniaxial and stress-free confining stress conditions. The cuttability indices (peak pick force, insertion depth and disturbance duration), failure patterns and fragment sizes were measured and compared to estimate the effects of confining stress. The results show that the rock cuttabilities decreased in order from rock breakages under stress-free conditions to uniaxial confining stress and then to biaxial confining stress. Under biaxial confining stress, only flake-shaped fragments were stripped from the rock surfaces under the requirements of large pick forces or disturbance durations. As the level of uniaxial confining stress increased, the peak pick force and the insertion depth initially increased and then decreased, and the failure patterns varied from splitting to partial splitting and then to rock bursts with decreasing average fragment sizes. Rock bursts will occur under elastic compression via ultra-high uniaxial confining stresses. There are two critical uniaxial confining stress levels, namely stress values at which peak pick forces begin to decrease and improve rock cuttability, and those at which rock bursts initially occur and cutting safety decreases. In particular, hard rock is easiest to split safely and efficiently under stress-free conditions. Moreover, coupled static preloading and dynamic disturbance can increase the efficiency of rock fragmentation with increasing preloading levels and disturbance amplitudes. The concluding remarks confirm hard rock cuttability using conical pick, which can improve the applicability of mechanical excavation in deep hard rock masses. [ABSTRACT FROM AUTHOR]

The paper presents experimental and numerical studies on the feasibility and validity of using prismatic rock specimens in split Hopkinson pressure bar (SHPB) test. Firstly, the experimental tests are conducted to evaluate the stress and strain uniformity in the prismatic specimens during impact loading. The stress analysis at the ends of the specimen shows that stress equilibrium can be achieved after about three wave reflections in the specimen, and the balance can be well maintained for a certain time after peak stress. The strain analysis reveals that the prismatic specimen deforms uniformly during the dynamic loading period. Secondly, numerical simulation is performed to further verify the stress and strain uniformity in the prismatic specimen in SHPB test. It indicates that the stress equilibrium can be achieved in prismatic specimen despite a certain degree of stress concentration at the corners. The comparative experiments demonstrate that the change of specimen shape has no significant effect on dynamic responses and failure patterns of the specimen. Finally, a dynamic crack propagation test is presented to show the application of the present work in studying fracturing mechanisms under dynamic loading. [ABSTRACT FROM AUTHOR]

Dynamic disturbance is sometimes a non-ignorable factor to induce rock failure in underground excavation, especially for high initial geo-stressed locations. In this study, a mathematical physics model was used to characterize the failure mode of brittle rock under static geo-stress and dynamic loading. An implicit to explicit sequential solution method (IESSM) was performed to examine the dynamic fracturing behavior of an underground opening due to dynamic disturbance. In the numerical simulation, the characteristics of strain energy density (SED) and fracturing zone were investigated under various lateral pressure coefficients. The results indicated that different failures were induced around the opening subjected to dynamic. With lower lateral pressure coefficient, spalling is induced at roof and spandrel with a long period for releasing strain energy. With the increase of the lateral pressure coefficient, violent strain rockbursting is observed in highly stressed locations, with instantaneous release of massive strain energy. Moreover, when the geo-stress of lateral pressure coefficient is high enough, the occurrence location and fracturing zone of rockbursting will not be greatly influenced by the dynamic disturbance, including its amplitude and action direction. [ABSTRACT FROM AUTHOR]

A split Hopkinson pressure bar (SHPB) system with a special shape striker has been suggested as the test method by the International Society for Rock Mechanics (ISRM) to determine the dynamic characteristics of rock materials. In order to further verify this testing technique and microscopically reveal the dynamic responses of specimens in SHPB tests, a numerical SHPB test system was established based on particle flow code (PFC). Numerical dynamic tests under different impact velocities were conducted. Investigation of the stresses at the ends of a specimen showed that the specimen could reach stress equilibrium after several wave reverberations, and this balance could be maintained well for a certain time period after the peak stress. In addition, analyses of the reflected waves showed that there was a clear relationship between the variation of the reflected wave and the stress equilibrium state in the specimen, and the turning point of the reflected wave corresponded well with the peak stress in the specimen. Furthermore, the reflected waves can be classified into three types according to their patterns. Under certain impact velocities, the specimen deforms at a constant strain rate during the whole loading process. Finally, the influence of the micro-strength ratio ( $${{\tau_{\text{c}} } \mathord{\left/ {\vphantom {{\tau_{\text{c}} } {\sigma_{\text{c}} }}} \right. \kern-0pt} {\sigma_{\text{c}} }}$$ ) and distribution pattern on the dynamic increase factor (DIF) of the strength DIF were studied, and the lateral inertia confinement and heterogeneity were found to be two important factors causing the strain rate effect for rock materials. [ABSTRACT FROM AUTHOR]

The measurement of in-situ stress is a critical prerequisite for the mining design and the stability analysis of surrounding rock in deep mining engineering. In this study, a novel re-oriented core acoustic emission (RCAE) method incorporating the non-oriented core ground re-orientation and AE techniques was proposed for in-situ stress measurement. First, the principle of non-oriented core ground re-orientation is established according to the spatial coupling relationship between the drilling trajectory and the core surface characteristics. Then, an innovative apparatus was designed for recovering the original spatial orientation of non-oriented cores. Furthermore, the measurement procedures of RCAE method are described in detail, involving suitability analysis, outdoor operations, core re-orientation, specimen preparation, AE test and stress calculation. Using this method, a case study for in-situ stress measurement was performed in an ultra-deep borehole (2360 m) of the Sanshandao gold mine, China. Meanwhile, the verifications of measurement accuracy were conducted based on the tectonic stress inversion and the underground overcoring method. The results show that the measurement accuracy of RCAE method is acceptable, especially the horizontal maximum principal stress and its azimuth measured are well matched with those measured by other methods. In addition, the field applications indicate that the in-borehole procedure of RCAE method only requires drilling trajectory measurement without excessive complex operations, and the adequate measuring depth of this method in the case area is up to 3841 ± 521 m according to the damage stress threshold analysis. Therefore, the RCAE method could break through the challenges caused by the restricted measuring space and large measuring depth when no deep underground access exists. Moreover, the non-oriented cores can be rapidly re-oriented on the ground, which could expand the application value of ordinary geological boreholes in the field of deep geostress and tectonic measurement. [ABSTRACT FROM AUTHOR]

Highlights: [•] A theoretical model was applied to characterise unloading responses under different paths. [•] The propagation of unloading perturbation was represented in 3D contour maps. [•] Numerical simulation was performed for the 2D unloading responses under different paths. [•] Results of numerical simulation were consistent with the mathematical physical model. [•] The effects of unloading rate and path were studied on excavation unloading responses. [ABSTRACT FROM AUTHOR]

Rocks in underground projects at great depth, which are under high static stresses, may be subjected to dynamic disturbance at the same time. In our previous work (Li et al. Int J Rock Mech Min Sci 45(5):739-748, ), the dynamic compressive behaviour of pre-stressed rocks was investigated using coupled-load equipment. The current work is devoted to the investigation of the dynamic tensile behaviour of granite rocks under coupled loads using the Brazilian disc (BD) method with the aid of a high-speed camera. Through wave analyses, stress measurements and crack photography, the fundamental problems of BD tests, such as stress equilibrium and crack initiation, were investigated by the consideration of different loading stresses with abruptly or slowly rising stress waves. The specially shaped striker method was used for the coupled-load test; this generates a slowly rising stress wave, which allows gradual stress accumulation in the specimen, whilst maintaining the load at both ends of the specimen in an equilibrium state. The test results showed that the tensile strength of the granite under coupled loads decreases with increases in the static pre-stresses, which might lead to modifications of the blasting design or support design in deep underground projects. Furthermore, the failure patterns of specimens under coupled loads have been investigated. [ABSTRACT FROM AUTHOR]

Abstract: Three dimensional numerical modelling was used to examine the fracture responses around cavities in rock masses experiencing the stress of excavation. In addition to the primary fracture zone in the near-field, numerical modelling generated a second fracture zone in the far-field and an elastic non-fracture zone between the two fields, i.e., fracture and non-fracture zones occurred alternately around a deep cavity. Further research illustrated that the dynamic load and static stress gradient are two necessary precursors for a far-field fracture in the excavation process. Neither quasi-static loading nor homogeneous stress conditions could induce a far-field fracture. A simple theory is introduced, suggesting that multiple fracture zones occur during excavation due to both the initial stress gradient and the dynamic load. This finding indicates that it may be possible to induce continuous rock fractures in deep underground rock masses by employing optimal excavation methods to generate multiple contiguous fracture zones. [Copyright &y& Elsevier]

Highlights: [•] A numerical method of initialization-dynamic unloading process is presented. [•] The SED and SED rate dominated the unloading process of rock under 3D stress state. [•] The equivalent SED rate was introduced to quantitatively describe the unloading process. [Copyright &y& Elsevier]

Abstract: The unloading process of rocks under high initial stress is complex, and verifying the mechanism of the unloading process in the field or in a laboratory is not straightforward. In this study, the unloading process of rocks under high initial stress was characterised by a mathematical physics model, which was then implemented in the finite element program LS-DYNA for analysis. In particular, the implicit and explicit methods were performed in sequence in the finite element simulation of rocks with initial stresses. In the numerical simulation, the characteristics of the dynamic unloading process of rocks were investigated for various peak initial stresses, initial stress release paths and initial stress release rates (ISRRs). The numerical results indicated that the rock failure could be induced by the release of the initial stress; furthermore, there is a relationship between the magnitude of the unloading failure and the peak initial stresses, the initial stress release paths and the ISRRs. When the initial stresses were at the same level, the equivalent initial stress release rate (EISRR) was introduced to quantitatively describe the characteristics of the unloading process. Using the numerical results, the unloading failure process was characterised, and a method for the static stress initialisation-dynamic unloading of rock was developed. [Copyright &y& Elsevier]

large number of tests have recently been conducted with the Split Hopkinson pressure bar (SHPB) method to determine the characteristics of rock dynamics. However, it is still impossible to get test results at a perfect constant strain rate from this set-up owing to the rate dependency of rock materials. For instance in most cases, dynamic behavior of rock can only be described with an average strain rate. The results from these methods, including rich strain rate information, frequently tend to be inexplicable or self-contradictory. The obtained stress-strain curves can then never be directly treated as constitutive curves as in static tests. In this paper, the reasons behind the controversial stress-strain results with current methods are analyzed. In addition, the requirement for the rock specimen to deform at a constant strain rate is demonstrated after theoretical analysis of correlations among specimen, deforming stress, incident stress, reflected stress and transmitted stress. With test results from SHPB by pulse shaper and special shape striker methods, the requirement is verified. Finally, the method of 3D scattergram considering stress-strain-strain rate simultaneously is brought up to get constitutive relationships of rate-dependent rock. The new method gives reasonable predictions for constitutive relationships of rock at different strain rates. At the same time, the new method has fewer requirements and has a wider application scope for SHPB tests. [ABSTRACT FROM AUTHOR]

The blast pressure acting on the borehole wall is the basis of the dynamic analysis of rock blasting. The history of blast-induced borehole wall pressure (BWP) is difficult to measure owing to the complex interaction between the blasting load and the borehole wall, and the limitations of the general test system. In this work, the complete history of BWP under air-deck charge was measured for the first time, and the stress-response process was analyzed. PVDF (polyvinylidene fluoride) gages and a charge mode for the test circuit were employed. To improve the testing accuracy, the matching relationships among the parallel capacitor, the equivalent capacitance of PVDF gages, and the parasitic capacitance of the coaxial cable were effectively addressed. The experimental data within the span of 0–0.65 m from the explosive cartridge center show that there are two obvious peaks in the complete history of BWP, the first is a μs-level air shockwave load, followed by a ms-level explosion gas load. The peak of BWP is dominated by the peak of explosion gas (Pg) within the scope of about 0–0.35 m from the explosive cartridge center, while is mainly determined by the peak of air shockwave (Ps) in a farther range. The specific impulse distribution suggests that the explosion energy is mainly consumed in the area within the range of about 0–0.25 m from the explosive cartridge center. The strain rate in the borehole wall induced by air shockwave demonstrates a dynamic response (range of 131.9 ~ 2922.6 s−1), while indicating a quasi-dynamic loading (range of 0.1 ~ 2.1 s−1) when induced by the explosion gas. The findings contribute to expanding the understanding of the stress characteristics of the air shock wave and explosion gas and provide accurate initial loads for numerical simulation related to rock blasting of non-fluid–solid coupling. [ABSTRACT FROM AUTHOR]

• A numerical fracture model of granite under indentation conditions is established. • Indentation energy of treated granite is linearly correlated with brittleness index. • Specific energy is more sensitive to temperature as increasing confining stress. • Poly function of specific energy, temperature and confining stress is established. Microwave-assisted rock breakage is a promising solution for drilling and crushing hard-rock formations. The fundamental mechanical and thermophysical properties of microwave-treated granite were obtained, and the factors affecting the fracture behaviour under indentation were investigated using experimental and numerical methods. The results show that the uniaxial compressive strength, tensile strength, brittleness index, thermal conductivity, and thermal diffusivity of microwave-heated granite are negatively correlated with temperature, whereas the specific heat capacity of treated granite is positively correlated with temperature. The fracture process of microwave-heated granite under indentation can be divided into three stages: elastic deformation, fluctuating penetration, and post-peak. The indentation damage zone near the indenter consists mainly of tensile cracks. Meanwhile, regression analysis shows that the peak indentation force, penetration depth, and consumed energy of the microwave-heated sample are linearly correlated with the brittleness index. In addition, microwave treatment can weaken the inhibitory effect of a high confining stress on the damage zone and is conducive to reducing the indentation force and penetration depth of rock fragmentation. The specific energy is more sensitive to temperature under high confining stress conditions, and the relationship between the specific energy, temperature, and confining stress can be described by a 2D poly function. [ABSTRACT FROM AUTHOR]

In this study, the Hoek Brown criteria is primarily employed to calculate the distribution of excavation damaged zone (EDZ) around the cavity buried at 500 m, 1000 m, and 2000 m based on the variation of the in-situ stress and the mechanical properties of the rockmass. Then, the simulation scheme Spectral Element Method (SEM) is applied to investigate the dynamic responses of the excavation-damaged cavity that subjected to the dynamic disturbance, including peak particle velocity (PPV), particle vibration frequency (PVF), particle displacement, and dynamic stress concentration factors (DSCF). Three two-dimensional (2D) models embedding a circular hole with different shapes of EDZ are prepared to numerically study the dynamic responses of the underground cavity, finding the influencing factors, such as the shapes of EDZ, the incident direction of the dynamic disturbance, and the dynamic source frequency. The cavity that is surrounded by EDZ has resembled as the one with a series of low-velocity areas. The results indicate that the underground cavity and EDZ become a scatter to the incoming stress waves that correspondingly produce high amplitude scattered waves. The influencing factors to the dynamic response of the cavity are summarized to be the shape of EDZ, the angle of stress disturbances, and the wavelength of incoming stress waves. We also find that the amplitude of stress disturbance around the cavity is proportional to the areas of EDZ and the frequency of the incident waves. In particular, only the tensile stress concentrates around the cavity that is surrounded by EDZ. [ABSTRACT FROM AUTHOR]

Scientists have attempted to investigate deep underground rock failure. A key challenge to study underground rock failure is the difficulty to survey its process. Therefore, a large number of important and insightful laboratory investigations of underground rock failure experiments are conducted. In this paper, an experimental method is proposed to explore dynamic failure process of pre-stressed rock specimen with a circular hole. The failure process of a rock specimen under different initial static stress coupled with dynamic loading is clearly illustrated by a high speed camera. The experimental results indicated that high static pre-stress coupled with dynamic loading induces rock debris to be ejected at the surrounding circular hole; however, lower static pre-stress coupled dynamic loading cannot induce rock failure. The dynamic stress concentration surrounding the circular hole by transient wave incidence was further demonstrated at the condition of half-sine wave loading. In this condition, the results demonstrated that combined action of static and dynamic stress concentrations induces the primary fractures of rock specimen. [ABSTRACT FROM AUTHOR]

With an increase in mining depth, rockburst is becoming an important problem in roadways of Linglong gold mine, where the overburden is up to 1000 m. Field observations indicate that rock failures are often triggered by blasting at the adjacent workface. In order to understand the rockburst mechanism, numerical investigation associated with an energy index, strain energy density (SED), is conducted to simulate the energy accumulation and dissipation characteristics of the failure process. The results show that rock failure follows different mechanisms under dynamic loading as a result of unloading and loading zones generated around the cavity. Numerical modelling demonstrates that strain burst occurs at the corner and the floor with the release of massive energy, while dynamic spalling occurs at the lower sidewall due to the reflected tensile stress wave at the free surface. The failure types and damage extend from simulated results agree well with field observations. The findings presented in this study will contribute to effective support design strategies in deep underground openings. [ABSTRACT FROM AUTHOR]