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Flame jet-assisted mechanical rock drilling is expected to solve the hard-rock crushing challenges in underground engineering such as mining, tunneling, and drilling. Therefore, it is important to investigate mechanical behavior of the rock after flame jet-water cooling treatment. In this paper, granite blocks were subjected to flame jet-water cooling treatment, which means the rapid heating of the granite using a flame with a temperature of 1564 °C, followed by water cooling. Then, physico-mechanical behaviors of samples at different distances (all the distances mentioned later mean the vertical distance between the sample axis and the flame jet-water cooling path) were measured. Results showed physico-mechanical parameters of the sample increased as the distance increased. The V p (P-wave velocity), UCS (uniaxial compressive strength) and BTS (Brazilian tensile strength) of the samples increased from 3171.7 m/s, 84.84 MPa and 5.01 MPa at 0 mm to 3619.3 m/s, 145.86 MPa and 7.30 MPa at 70 mm, respectively. Meanwhile, physico-mechanical parameters of samples at 70 mm all reached more than 95 % of those of untreated samples. As the distance increased, cumulative AE counts at peak stress gradually enhanced, and failure modes of uniaxial samples shifted from shear to tensile splitting failure. The SEM (scanning electron microscope) images illustrated that the closer to the flame jet-water cooling path, the more thermal cracks produced in the sample. Meanwhile, the damage factor can well characterize the thermal damage degree of granite. According to the variation of physico-mechanical parameters and SEM images of samples with distances, it can be assumed that the thermal damage width of granite after flame jet-water cooling was about 140 mm. It was much greater than the 50 mm width of spalling pits, which greatly increased the feasibility of thermal-assisted mechanical rock drilling. The tensile stress induced by the temperature gradient below the spalling pit is the fundamental cause of thermal damage to the rock, which is further aggravated by water cooling. The findings can provide some guidance for flame jet-assisted mechanical rock drilling. [ABSTRACT FROM AUTHOR]

Abstract The theoretical formulas were firstly derived to quantitatively assess the relative velocity around a circular tunnel induced by blasting loads, and the influence of wavelength-to-tunnel-diameter ratio (λ/D) on the amplification effect of vibration velocity was theoretically investigated in detail. A two-dimensional numerical model was then established by the discrete element code PFC2D to further investigate how the λ/D ratio and the in situ stress (including the lateral pressure coefficient and the stress level) influence the failure mode of underground tunnels subjected to blasting loads. The theoretical results reveal that the relative velocity in the incident side of the tunnel decreases with the increase of λ/D ratio, while the relative velocity in the shadow side increases as the λ/D ratio increases. When the λ/D ratio is larger than 20, the amplification effect of vibration velocity becomes negligible, and the loading condition of whole tunnel can be viewed as "quasi-static" loading. The numerical simulation results indicate that the λ/D ratio has a large effect on the failure mode of underground tunnels subjected to blasting loads. For the λ / D ratios smaller than 1, the blasting load mainly induces spalling crack in the incident side of the tunnel. For the λ / D ratios larger than 2, the dynamic failures mainly emerge in the roof and floor of the tunnel. The blasting load with larger amplitude can induce larger damage zone in tunnel surrounding rock. In addition, the spalling crack in the incident side of the tunnel shortens gradually with the increase of lateral pressure coefficient and buried depth of the tunnel, while the damage zone in the roof and floor of the tunnel enlarges with the increase of lateral pressure coefficient and buried depth of the tunnel. [ABSTRACT FROM AUTHOR]

In this study, we determined P-wave tomographic images of the Yongshaba deposit, an underground mining zone in Guizhou Province, China, by inverting arrival-time data of micro-seismic events and blasts recorded by a passive seismic array consisting of 28 sensors during January to April 2014 using an event location technique and a travel-time tomography method that can handle complex seismic discontinuities. The damping parameter used in the damped least-squares method is determined with the help of trade-off curve. To reveal internal P-wave velocity changes of the study area under severe mining activities, a time-lapse data partition scheme is used. To assess the human influence on the underground structure, we introduce a parameter to measure the relative velocity changes in different periods. The resolution of the tomographic images and the robustness of the obtained features are examined by conducting a series of checkerboard resolution tests and a restoring resolution test. Our tomographic results obtained from the entire data set reveal a prominent low-velocity (low-V) zone and many obvious high-velocity (high-V) zones. These features match well with the geological setting and the excavation plan carried out in the mine, according to in-site surveys. The low-V zone may reflect empty volumes, stress releases and rock breaking and cracking caused by mining activities, whereas the high-V zones are probably the consequences of local stress concentrations caused by regional stress redistribution. The time-lapse tomographic images may reveal the process of stress concentrations caused by rock breaking due to the continuing excavations in the entire observation period, indicating that the mining activities influenced the rock property and caused complex changes of the underground structure. [ABSTRACT FROM AUTHOR]

A theoretical formulation was first established to evaluate the dynamic stress concentration factor (DSCF) around a circular opening under conditions of blasting stress wave incidence. A two-dimensional numerical model was then constructed by the particle flow code (PFC) in order to simulate the dynamic responses around an underground tunnel subjected to blasting load. In the simulation, a series of horizontal blasting stress waves were applied to an underground tunnel under various in situ stress states, and then the dynamic responses around the tunnel were analyzed from the viewpoint of the dynamic stress concentration and energy evolution. The results of theoretical analysis indicated that obvious dynamic effects occur at tunnel boundary during blasting stress wave incidence, and the DSCF at the roof and floor of the tunnel is much larger than that at two sidewalls when blasting stress wave was applied to left model boundary. The numerical results showed that high static compressive stress concentration around the underground tunnel results in the accumulation of substantial strain energy at the same location. The roof and floor of the tunnel are more prone to dynamic failures during the blasting loading process. In addition, the analysis of energy dissipation indicated that the strain energy reduction and the residual kinetic energy are positively related to the lateral pressure coefficient and the burial depth of the tunnel, and the residual kinetic energy is much larger than the strain energy reduction under the same condition. Furthermore, for an underground tunnel subjected to high in situ stress, the blasting stress wave with lower amplitude is sufficient to trigger severe dynamic failures. [ABSTRACT FROM AUTHOR]

A multi-step interpolation approach was proposed to model the three-dimensional grade distribution and to demarcate orebody in a thin and anfractuous bauxite seam. A pre-mining demonstration procedure was established to virtually perform fully mechanized longwall mining in the modelled bauxite seam. Meanwhile, a comprehensive evaluation function expressed by the sum of dilution and loss rates of bauxite was utilized to determine the optimal cutting heights (1.71–2.69 m) and a suitable diameter (1.5 m) for the cutting drums mounted on a double-drum shearer to achieve no-waste and high-recovery mining of the demarcated 106,026 m 3 bauxite orebody. In addition, a numerical model of strata containing geological defects (a karst cave and fault), discontinuities (joints and stratifications) and a mountain was established to simulate the discontinuous motion and overburden fracturing disturbed by the longwall mining in the modelled bauxite seam. The results indicated that the overburden presented reliable stability and impermeability, in which no large-scale caving in goaf, no water influx, no cave collapse, no fault slip, and no mountain slope landslide occurred. This was due to arched roofs above the Karst cave and the mining-induced fractured zone, the subcritical condition of the longwall panel in the dip section, and support from the protective pillar between two adjacent panels. [ABSTRACT FROM AUTHOR]

When open-pit bench blasting approaches a site with underground cavities, concerns arise regarding the safe blasting distance to prevent collapse of the cavities because such collapses may induce other secondary problems related to the safety and workability of the site. It is therefore necessary to estimate the stability of underground cavities in the proximity of bench blasting. In this paper, blasting tests of small scale were carried out in a site with cavities and field blasting test data were measured to obtain the site-specific attenuation relations of PPV and principal frequency of stress waves. A numerical model is then developed based on these relations to calculate bench blasting induced stress wave propagation. The accuracy of the numerical prediction is verified by the field measured data. The verified numerical model is used to perform a series of simulations. The results are then used to assess the roof stability of cavities adjacent to upper bench blasting. Based on the numerical results, the response characteristics of cavity roof with respect to different explosive charge mass are also studied. [ABSTRACT FROM AUTHOR]

A theoretical model was first established in order to evaluate the physical possibility of a dynamic response occurring around an existing tunnel under conditions of unloading wave incidence. Based on the steady state solution of the wave expansion approach, transient solutions subjected to unloading waves with different unloading times were obtained. A three-dimensional numerical model was then constructed in order to simulate dynamic responses around an existing tunnel under unloading disturbance forces. In the simulation, dynamic unloading was carried out through a new excavation after stress redistribution had been finalized in the examined tunnel, and parametric studies were then conducted for various unloading times, stress levels and tunnel spacing configurations. The results show that dynamic effects are induced around an existing tunnel under high levels of initial stress and at high unloading rates. The results of the dynamic analysis show that the PPV of unloading-induced microseism can be as high as that of an explosion-induced wave, and damage can be inflicted on existing tunnel walls (especially on the incident side of a tunnel). This dynamic effect diminishes dramatically with an increase in tunnel spacing and unloading times. [ABSTRACT FROM AUTHOR]

In this study, we investigated the failure process and evolution mechanisms in circular tunnels under a complex stress path of deep in-situ stress + excavation unloading + stress adjustment using a simulation test based on loading first and then drilling (LFTD) mode. This test was performed on cubic sandstone specimens using a true-triaxial testing system combined with a self-developed drilling unloading test device. For comparison, another set of sandstone specimens, similar to the original specimens but with Φ 25 mm circular through-holes, were processed to perform another set of simulation tests based on drilling first and then loading (DFTL) mode. Under constant lateral stress, compared with the DFTL test, the initial failure vertical stress was lower, and the surrounding rock was more prone to failure in the LFTD test, indicating that drilling unloading causes damage to the surrounding rock, thereby inducing a strength-weakening effect. Under constant lateral and vertical stresses, sidewall failure in the LFTD test was more severe. The sidewall's initial failure vertical stress difference between the LFTD and DFTL tests increases with increasing lateral stress or initial hydrostatic pressure. The effect of drilling unloading becomes more apparent under a higher initial stress state. Comparing the same types of simulation tests under different stress states, the initial failure vertical stress increases with the lateral stress or initial hydrostatic pressure, inducing a strength-strengthening effect in the surrounding rock. Comparing the strength-strengthening effect caused by increasing the lateral pressure or initial hydrostatic pressure and the strength-weakening effect caused by drilling unloading shows that the strength-strengthening effect is more evident. [ABSTRACT FROM AUTHOR]

To investigate the static mechanical properties and failure characteristics of damaged rocks after impact under realistic in-situ stress, dynamic triaxial compression tests with different strain rates and confining pressures were conducted on diorite by a modified split Hopkinson pressure bar. The damage factor was calculated according to the change of the longitudinal wave velocity of rocks before and after the impact, and the residual strengths of the damaged diorite were obtained by uniaxial compression and Brazilian tensile tests. In addition, three brittleness indexes based on the residual strength were adopted to evaluate the brittleness of damaged specimens, and fractal dimension was introduced to study the fragmentation characteristics. The results show that the dynamic peak stress, strain, elastic modulus, damage factor and plastic deformation of diorite all increase with increasing strain rate, i.e., rate dependence, and the increasing rate decreases with increasing confining pressure. The increment in confining pressure leads to the increase of dynamic peak stress, elastic modulus, static residual strength and brittleness, and the increasing rate increases with increasing strain rate, while other physio-mechanical parameters decrease with confining pressure. The post-peak behavior of dynamic stress-strain curves is divided into two classes according to the strain rate and fracture state of rocks under dynamic loading. Upon the strain rate increases above 87 s−1, the brittleness of damaged specimens generally decreases to moderately brittle or even low brittle, and the corresponding drillability becomes much easier. The fractal dimension of damaged specimens increases with the increase of confining pressure and decreases linearly with the increase of strain rate. [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]

Abstract: Sanshandao gold mine, located at the east coastline of Bohai Sea in the Shandong Province, is the first subsea metal mine in China. Since the mining activities are carried out under sea, it is of vital importance to maintain the stability of the crown pillar and to keep the sea water out from the excavations. In this paper, the minimum required thickness of crown pillar is determined based on 3D numerical modelling and analysis. A realistic geometric subsea gold mine is modelled by integrating the usage of SURPAC and FLAC3D. The numerical analysis is carried out by FLAC3D, in which the influences of sea water pressure as well as mining sequences have been considered. The distributions of the principal stresses, displacements, plastic zones and pore pressures in the crown pillar are obtained by simulating the cut-and-fill stoping method at different excavation levels (above level −115m). The field displacement observation shows that the vertical deformation rate of crown pillar is smaller than 0.023%. It reveals that the reserved safety factor is about 1.43 when using cut-and-fill stoping method from level −165m to −115m in the subsea gold mine. The mining activities may extend to level −95m according to the numerical analysis results. A four-year-field practice shows that the numerical analysis is helpful to determine the minimum crown pillar thickness in the challenging subsea gold mine. [Copyright &y& Elsevier]

Abstract: A new testing technique, which relates the physical stress state of rock subjected to simultaneous coupled static and dynamic stresses, is presented. The method involves modification of a split Hopkinson pressure bar, such that the test specimen is subjected to coupled axial static pre-stress, axial impact loading, and optional confining pressure. Tests on siltstone specimens with different coupling loads showed that the strength of the specimens under coupling loads was higher than their corresponding individual static or dynamic strengths. In the grade size distribution, the percentage of small size particles of fractured specimen increases with higher coupling loads. The strength of rock under coupling loads decreases rapidly when the axial pre-compression stress is greater than 70% of the static strength of rock (with identical impact loading). However, with constant axial pre-compression stress and increasing impact loading, the strength of siltstone increases initially and reaches a maximum constant value. [Copyright &y& Elsevier]

When an underground tunnel is excavated suddenly in the high-stress rock mass, the unloading wave will generate from the tunnel boundary and propagate into the surrounding rock mass, which can induce strong dynamic vibrations and even damage to the adjacent existing tunnel. This study aims to investigate the dynamic response of existing tunnel induced by cylindrical unloading wave resulting from neighboring tunnel excavation. A two-dimensional mathematical physical model was first constructed based on the superposition principle of coupled static and dynamic loads, in order to derive the formulas for calculating the dynamic stress, velocity and displacement around existing tunnel induced by cylindrical unloading wave. A two-dimensional numerical model established by particle flow code (PFC) was then implemented to verify the theoretical analysis, and further to investigate the failure characteristics of existing tunnel under coupled static in situ stress and dynamic unloading wave. A series of parametric studies were conducted in the theoretical and numerical models to investigate the influence of unloading rate, unloading path, in situ stress and excavation radius on the dynamic response of existing tunnel. The results show that the unloading wave induced by tunnel excavation in high-stress rock mass can induce strong dynamic response in the surrounding rock mass of adjacent existing tunnel, and the increase of stress level and unloading rate will aggravate the dynamic effect. The peak value of velocity induced by nonlinear unloading is larger than that induced by linear unloading. In addition, the excavation-induced unloading wave can induce dynamic failures in the surrounding rock mass of existing tunnel under high in situ stress. Therefore, the unloading wave induced by adjacent tunnel excavation should be taken into consideration when evaluating the stability of deep tunnels. [ABSTRACT FROM AUTHOR]

Stress conditions around deep underground mine openings can significantly influence rock fragmentation and stability, and thus the cuttability of the targeted rock. In this study, rock breakage experiments and associated regression analyses indicate nonlinear rock cuttabilities (decreasing followed by increasing) with increases in the differences between biaxial confining stresses and the values of uniaxial confining stresses. Rock breakages were found to be efficient and safe under low and no-stress conditions that require low indentation force and depth, cutting work and specific energy to completely split the rock wtih no rockburst risk. Stress concentration initially impeded rock breakage, although high uniaxial stress improved rock cuttability. Inducing high stress to fracture the rock and produce an excavation damage zone (EDZ) via stress release effectively transformed the stress condition into low confining stress or even the stress-free condition, improving rock cuttability significantly and preventing rockburst. Mining of rock in the EDZ around the pillar could be efficient, cost-effective and safe when using roadheaders, which showed high cutting efficiencies, low pick wear failures, high machine stabilities and no rockbursts. In addition, a binary linear regression model was proposed to determine the thickness variation of the EDZ correlated with the excavation span and a coupled index of rock properties and buried depth of opening. The results indicated that the thickness of the EDZ increases with increases in the buried depth of the opening, which can improve the applicability of non-explosive mechanized mining in deep mines. [ABSTRACT FROM AUTHOR]

Judgement of rock burst proneness of rock materials is one of the basic problems in the field of rock burst research. In this study, a peak-strength strain energy storage index is proposed for estimating and classifying the rock burst proneness of rock materials. The method for determining this index is also introduced in this paper. The peak-strength strain energy storage index is defined as the ratio of the elastic strain energy density to the dissipated strain energy density corresponding to the peak compressive strength of rock specimen. In order to obtain the elastic strain energy density and the dissipated strain energy density at the peak strength, a series of single cyclic loading-unloading uniaxial compression tests on nine rock materials were conducted. The relationships between the total input energy density and the elastic strain energy density at different unloading stress levels were investigated. The results show that, for every rock material, the elastic strain energy density increases linearly with the increase of the total input energy density. Based on this linear storage energy law, the elastic & dissipated strain energy density at the peak strength can be calculated for each specimen, and the peak-strength strain energy storage index can be obtained accordingly. A new criterion for rock burst proneness of rock materials is proposed. The rock burst proneness of nine rock materials estimated with the proposed criterion agreed well with the laboratory test results. [ABSTRACT FROM AUTHOR]

Abstract To investigate the dynamic Mohr-Coulomb and Hoek-Brown strength criteria under the low confining pressures at high strain rates, the dynamic uniaxial and triaxial compression tests of sandstone were conducted with a conventional and modified triaxial SHPB system, respectively. The confining pressures are 5.0 MPa, 7.5 MPa, 10.0 MPa, 12.5 MPa and 15.0 MPa, and the strain rates varied from approximately 40 s−1 to 160 s−1. The results show that the cohesion is a linear positive correlation with the logarithm of strain rate, and the internal friction angle is a linear negative correlation with the logarithm of strain rate. The change of cohesion is more obvious than that of the internal friction angle. The rock parameter m also exhibits a linear positive correlation with the logarithm of strain rate. Furthermore, the relationships between cohesion, internal friction angle and rock parameter m and strain rate were obtained. On this basis, the unified dynamic Mohr-Coulomb and Hoek-Brown strength criteria at high strain rates were established. According to the established dynamic strength criteria, the strength of sandstone under high strain rates and low confining pressures can be estimated theoretically. By comparing the theoretical curve and test data, it can be concluded that established dynamic Mohr-Coulomb and Hoek-Brown strength criteria of sandstone are appropriate for both high strain rates and low confining pressures. [ABSTRACT FROM AUTHOR]

Backfilling inaccessible cavity through raise can provide underground support for the active open-pit operations and the raise must be taken down in a single blast for safety reasons. Conventional raising techniques are not economically and technically feasible, and the maximum advance depth in one blast is limited. In this paper, small-scale crater tests were carried out in a site with cavities and field blasting test data were measured and evaluated to obtain the basic data for raise-scale blast designs. The raising design parameters with 250-mm hole diameter, including charge weight and height for each slice, slice height, hole spacing and delay interval, were derived. A scheme of multiple deck blasting based on vertical crater retreat (VCR) drop-raising method was designed for an abandoned cavity with 30-m cover. The lower 12-m cover was first blasted to investigate the practical blast performance. Charging and timing patterns were adjusted accordingly and the left 18-m cover was successfully opened up. Through raising tests, it is shown that the advantage of two available free surfaces up and down should be maximized, and smaller slice height and alternate initiation sequence are benefit for raising extension. A numerical model is then developed and it is calibrated against the 12-m raising test. By combining the calibrated numerical model with the raising test results, the effects of hole layout and in-slice delay are examined. It is shown that circular pattern and in-slice delay are positive for multiple-deck raising blast. Based on the foregoing analysis, the hole layout, charging and timing patterns are optimized. The optimized scheme was applied to field raising blast and a 32-m raise was successfully excavated in one blast. [ABSTRACT FROM AUTHOR]

Coupled static-dynamic tests were carried out on notched semi-circular bend (NSCB) granitic specimens using a modified split Hopkinson pressure bar (SHPB) device, to investigate the effects of axial pre-force and loading rate on dynamic fracture behavior of rocks. Digital image correlation (DIC) method was adopted to obtain the deformation field and fracturing process. The results indicate that the pre-force (up to 50% static peak force) enhances the fracture parameters overall, including fracture initiation toughness, fracture energy, crack propagation velocity (CPV) and fracture propagation toughness, but the effect of loading rate is more significant than that of the pre-force. Digital images show that the increase in pre-force would cause the dominant crack to symmetrically bifurcate since the pre-force strengthened a fan-shaped area for the specimen near the incident bar. The loading rate larger than 124 GPa·m1/2/s could lead to the secondly fracture of the specimen and thereby more fragments. Compared with previous studies, the present investigation deepens the understanding of fracture characteristics of rocks under coupled static and dynamic loads by further reducing the pre-force interval (0.25 kN). In addition, a new method based on the DIC technique is proposed to calculate crack opening distance (COD) and CPV, which can improve the efficiency and accuracy compared to the strain gauge method. Moreover, fracture propagation toughness is also emphasized apart from the initiation toughness. The findings can provide some enlightenments for rock fracture dynamics and crack propagation of highly stressed rocks. [ABSTRACT FROM AUTHOR]

Understanding the stress distribution, deformation and fracture behavior around a circular hole or inclusion is critical for the stability assessment of rock structures such as tunnels and backfilled openings. In this study, the analytical solutions of stress and displacement distribution around a circular inclusion of different elastic properties were first derived based on the theory of elasticity. Then prismatic sandstones with a circular hole in different filling modes were prepared and tested in uniaxial compression tests. Progressive deformation and fracturing behavior around holes were analyzed by digital image correlation technique. Moreover, the energy evolution characteristics of specimens were studied to reveal its deformation and failure mechanism. The results show that after filling, the radial and tangential stress and displacement of the inclusion increase, while the stress and displacement around the surrounding rock of the hole decreases, thereby improving the strength and deformation properties. In addition, the crack types and initiation positions around the unfilled and filled holes are quite different due to the stress redistribution. Based on the energy evolution characteristics and the major principal strain fields, the stress-strain behavior is divided into five stages. It is found that the inclusion not only has passive supporting and energy absorption effects on the surrounding rock, but also has a similar crack arrest effect on the initiation and propagation of new cracks. Finally, the effects of inclusion elastic properties and lateral stress coefficient on the stress concentration characteristics around the filled hole were discussed. [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]

To explore the variation in rock acoustic emission (AE) characteristics with strain rate, uniaxial compression tests at different loading rates and impact loading tests were conducted on granite using a MTS322 rock mechanical test system and split Hopkinson pressure bar (SHPB) system, respectively. The effect of the strain rate on the AE characteristic parameters, rock fracture properties, and destruction evolution were systematically analyzed. The results demonstrated that with increasing strain rate, the cumulative AE count decreases as a power function, and the variation between the cumulative AE count and strain rate can be fitted log-linearly with a slope of 0.48. The peak frequencies of the AE signals are mostly distributed in the zones of 0–100 kHz, 175–250 kHz, and 400–550 kHz. The signal proportion in the 0–100 kHz zone gradually increases with strain rate, while the signal proportions in the 175–250 kHz and 400–550 kHz zone exhibit decreasing trends. A transition of a sudden increase in the RA -value and decrease in the AF -value occurs when the stress reaches a certain level, and the stress level corresponding to this transition will increase with strain rate. Meanwhile, the RA–AF distribution is mostly concentrated on the abscissa in the low strain rate tests, but gradually concentrates on the longitudinal axis as the strain rate increases. This indicates that tensile cracking becomes the dominant fracture mode with increasing strain rate. The b -value decreases with increasing strain rate in the uniaxial compression tests; however, the b -value in the impact loading tests is higher than that in the uniaxial compression tests. Furthermore, to distinguish the signals generated by stress wave propagation from the signals generated by rock fracturing in the impact loading tests, a four-parameter k -means algorithm is used to conduct a clustering analysis. The results indicate that the signals can be classified into four clusters: tensile fracturing signals (cluster A), mixed stress wave and shear fracture signals (cluster B), mixed stress wave and tensile fracture signals (cluster C), and stress wave signals (cluster D). [ABSTRACT FROM AUTHOR]

Particle gradations significantly affects the dynamic response of water-saturated soft coal. To obtain the dynamic response of soft coal to various particle gradations, static and dynamic mechanical properties of water-saturated soft coal with different particle gradations were tested and compared. During these processes, the enhancement coefficient of the dynamic mechanical parameters to the corresponding static mechanical parameters was set as an index to describe the effect of non-uniformity coefficient Cu and median diameter d 50. The results demonstrate that both the static and dynamic mechanical parameters of the water-saturated soft coal were affected by particle gradations. The enhancement coefficient was affected by both the d 50 and Cu , which was microscopically attributed to effect of the d 50 and Cu on the average dynamic liquid bridge force and scale dynamic rupture energy among the particles based on the microstructure and dynamic fracture evolution processes. In addition, the dimensionless scale rupture energy model among the water-bearing particles varied with the d 50 and Cu were further established and applied to discuss the effect mechanism of particle gradations on the dynamic response of water-saturated soft coal. On this basis, the support measures of roadways in coal seam with different particle gradations were further discussed. [ABSTRACT FROM AUTHOR]

Water content and incident energy are important factors in influencing the mechanical properties and stress field of coal seam, which are closely related to the dynamic disaster intensity. In this study, after doing some static compression tests in different water content, comprehension dynamic compressive tests on pre-stressed coal specimens under different water contents and incident energy levels were conducted using a modified Split Hopkinson Pressure Bar test system. The effects of water contents and incident energy levels on dynamic behavior of the studied coal were evaluated by analysis of dynamic stress-strain curves, inspection of the occurred damage and in-situ test on the mine site. For detail analysis, some indices including peak strength, elastic modulus, dissipation energy, impact energy index, and failure characteristics were employed. The results indicated that dynamic strength and elastic modulus are negatively correlated with the water content and positively correlated with the incident energy. Two types of the dynamic failure behavior including incident energy-dominated failure and incident energy-induced failure were analyzed based on dissipation energy and stress-strain curve types. The coal burst occurrence possibility was proposed according to the dissipation energy density. Finally, based on laboratory test results and field engineering practices, the dynamic disaster mechanisms and their respective prevention concepts were further discussed. [ABSTRACT FROM AUTHOR]

It is essential to quantify the ground motions induced by seismic events with different focal mechanisms in underground mine. To quantitatively calculate the induced ground motions, we expressed the motions as the convolution of the seismic source and the wave propagation path using seismological method. Firstly, we investigated the theoretical ground motion patterns using four idealized focal mechanisms. Results show that the pure compressional failure source and the pure tensional failure source induced the same ground motion patterns. The vibrations they induced attenuate extremely fast. The pure shear failure source shows closely azimuth related pattern with four quadrants around the source. The pure explosive failure source induces vibrations attenuating toward all directions away from the source. But the vibrations are not attenuated gradually with the increasing distance. Then, we investigated the ground motions under complicated focal mechanisms using recorded fault slip events, cavity collapse events, and tensional failure events. The fault slip events show a four quadrants ground motion distribution in the horizontal plane and a two quadrants distribution in the vertical plan. But the cavity collapse events show a two quadrants distribution in the horizontal plane and a four quadrants distribution in the vertical plane. It is concluded that the induced ground motion at a certain location is controlled by the dual effects of the azimuth and the source focal mechanism as well as the distance and the source magnitude. [ABSTRACT FROM AUTHOR]

Abstract To investigate the effects of inclusions filled in holes, including inclusion strength, stiffness and shape, on the mechanical properties and fracture evolution of rock, a series of uniaxial compression tests were conducted on prismatic sandstone containing a prefabricated hole filled with different types of inclusions using a servo-hydraulic machine. Digital image correlation (DIC) and acoustic emission (AE) techniques were applied jointly to record and analyze the deformation and crack process of sandstone. The results indicate that both the inclusion type and shape are important factors affecting the strength and deformation properties of sandstone. The relatively rigid inclusion not only significantly improves the mechanical properties of pre-holed specimens, but also can provide a better support for structural integrity of rock after failure. Observations show that both DIC and AE techniques are capable of revealing the fracture evolution of specimens, and there is a good consistency between the mechanical behavior and surface strain fields and internal AE signals. Based on the strain localization characteristics, the crack sequences and relative stress levels for each group of specimens are summarized, especially the crack development inside the inclusion is first observed. The crack behavior and crack type around inclusions are greatly influenced by the inclusion type and shape due to dramatic changes in stress distribution around inclusions. After rock failure, the crack extent around inclusions can be characterized by the cumulative AE counts, and the final failure modes of specimens under different filling conditions can be categorized as tensile failure, shear failure and mixed tensile/shear failure. [ABSTRACT FROM AUTHOR]

Abstract This paper reports the further study carried out by the authors on the technique of long-hole raising in one blast. In the previous work reporting the results of preliminary study, method of vertical crater retreat (VCR) multiple-deck shots with different and smaller slice heights was used to excavate the raise in one blast. A 32-m raise in the cover of an abandoned cavity was successfully broken through. In this paper, an improved technique is suggested to overcome the overbreak and freezing in a long-hole raising blast. In the raising process, long-hole presplitting is first performed at the periphery of the blasting pattern, and then inner VCR multiple-deck shots with the same slice height are conducted according to a specific initiation sequence without in-slice delays. The previously calibrated numerical model is used to simulate the blasting process. The rock mass behaviours and the raising mechanisms are examined. It is shown that after the introduction of long-hole presplitting, the overbreak and freezing are greatly relieved, and the direction of advance and timing pattern are vitally responsible for the evolution of rock damage. This suggested technique was then applied to field raising blasts, and a 29-m raise and a 24-m raise under special cavity conditions were successfully developed. It demonstrates that, compared to the method of purely VCR multiple-deck shots, the design process can be greatly simplified and the present technique leads to better blasting performances. [ABSTRACT FROM AUTHOR]

Over the past several decades, raise excavation has been widely employed in underground mining, civil engineering and military engineering. One-step raise excavation with burn cuts, where all the boreholes are pre-drilled and detonated at one time and no workers need to be underneath the freshly blasted and dangerous ground, is an important and promising method in raise excavation. Cut parameters, especially the parameters of prime cut which used empty hole as a free surface and swelling space, have significant influence on the effect of raise formed. In this study, two small-scale experimental methods, spiral hole spacing method and observation hole method, are designed to determine the prime cut parameters such as hole spacing ( L ), stemming length ( L s1 , L s2 ) and air deck length ( L a ) which are normally determined by empirical formula. In order to study the feasibility of the two methods, numerical analysis and experimental tests are conducted in V zone of Sandaozhuang molybdenum mine (SMM), in which there are large numbers of underground goafs need to be controlled by filled raise. The Riedel–Hiermaier–Thoma (RHT) material model, which considers compression damage and tension damage effect under blasting loading, is employed in the LS-DYNA software to study the rock damage zone. Meanwhile, the field tests are carried out according to the two small-scale experimental methods. The comparison results show that the damage zone of numerical simulation has a good agreement with the experimental data. Further, the optimal prime cut parameters obtained from experimental tests are applied in one-step filled-raise excavation, and a 23 m raise that meets the design requirements is formed through the proposed technology. The results indicate that these cut parameters determined by the small-scale experiments are suited for one-step raise excavation. This study can provide two simple field experiments to determine the important prime cut parameters of one-step raise excavation. [ABSTRACT FROM AUTHOR]

A numerical model is presented to reproduce the stress induced fracturing process of brittle rocks. Material heterogeneity at the zone level of the model is considered, and distinct initial microcrack lengths and orientations are also included. The rock material is prescribed with a Weibull distribution, and the microcrack lengths and orientations are defined by distinct distribution functions. Stress intensity factor approach is adopted to characterize the stress condition of the microcracks, and is used as a failure criterion for the zones of the model. The mechanical response of rock specimens is investigated on models under different external loading conditions. Obvious nonlinear characteristics before the peak strength can be observed in models under uniaxial compression. Consistency can be found between simulation results and lab test data. The influence of microcrack conditions and heterogeneity levels on the simulation result is also investigated. The macroscopic failure modes of models are studied with different distributions of initial microcracks, and correlations are found between the microcrack conditions and fracturing processes. Especially, scattered zones and obscure fracture patterns are produced in the model defined by exponential and uniform distribution, compared with clear shapes of macroscopic fractures observed in models defined by normal or lognormal distributions. The simulations also show that the differences in the mean value of microcrack orientations obeying certain distribution result in distinct macroscopic fracture patterns, from shear bands at lower mean orientations to tensile fractures at higher mean orientations. Typical fracture patterns observed in laboratory are successfully reproduced in the model, which demonstrates the capability of the model in reproducing faithful rock behaviors through prescribing specific microscopic characteristics. [ABSTRACT FROM AUTHOR]

To discriminate the type of the induced seismicity in the underground mine, full waveform inversion and statistical methods were used to investigate the different microseismic source mechanisms in a more quantitative way. To eliminate possible effects caused by specific configuration of monitoring network, potential error in source location and wave propagation mismodelling, Jackknife resampling was used to obtain the probability density functions of inversion results and the moment tensor uncertainties. Firstly, the expected prior probability functions for mining blast, cavity collapse, fault slip, and tensional failure were defined, which are related to the percentage of the double couple, the compensated linear vector dipole, and the isotropic components. Secondly, the likelihood of a given candidate event to a certain source model was calculated, and the source type was determined according to the moment tensor decomposition of the case with the highest probability density. Tests with synthetic datasets were performed to evaluate the stability of the inversion in case of poor data quality and inaccurate source location. The calculated results with noise levels from 0% to 20% are all clustered closely around the initial source in the source type diagram, which indicates the reliability of the derived focal mechanism. The discrimination method of seismic sources in underground mine using full waveform inversion was verified by 8 events of field investigations in the Yongshaba Phosphate Mine, China. Results show that events 1, 5, 6 and 8 are tagged as fault slip events. Their probability of fault slip events are 0.53, 0.57, 0.37 and 0.60 respectively. The events 2, 3 and 7 are recognized as the cavity collapse events corresponding the probability 0.46, 0.51 and 0.50, respectively. The mechanism of the event 4 is recognized as a tensional failure with the probability 0.57. [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]

In one-step raise excavation, spherical cartridge blasting mode is easy to be implemented due to minimal requirement of the hole-deviation. Nevertheless, it may induce cumulative damage to the country rock. This study firstly uses a validated rock model (Johnson-Holmquist model) to simulate the damage evolution process of a raise by spherical cartridge blasting mode in LS-DYNA software. Then a field test is carried out to examine the numerical results of spherical cartridge blasting mode. Both the numerical and test results indicate that because of highly confined rock mass and restricted free face in deep raise, a large charge is required in spherical cartridge blasting mode which leads to extensive damage on the wall of the raise. In order to solve such a problem, the pre-split blasting technique is developed to optimize spherical cartridge blasting mode. According to the subsequent numerical results, the improved spherical cartridge blasting mode is successfully applied in another filling raise. This study provides an effective solution to the difficulties that are encountered in one-step raise excavation by spherical cartridge blasting mode. [ABSTRACT FROM AUTHOR]