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This paper reports the results of thermal spallation experiments on specially dried shale samples with 3 bedding orientations (0°, 45° and 90°) under 3 flame temperatures (899 °C, 1243 °C and 1559 °C) and the 3D thermal elastic finite element modelling. Under open flame heating, continuous spallation is observed with ejection of various spalls and popping sounds. After a period of spallation, tensile fractures are formed in the samples and grow perpendicular to the heating surface, except for a sample with bedding orientation of 90° under the low temperature. Increasing flame temperature promotes ejection, popping sounds and spallation rate, but reduces the spallation starting time, spallation duration, depression diameter and depression depth. The model shows that heating induces compressive stress in the surface layer and tensile stresses beneath it. The tensile stress is found to be sufficient to generate large tensile fractures. The ratio between the induced compressive and tensile stresses increases with increasing spallation depth but little affected by the flame temperature. The spallation compressive stress increases with temperature from 29 to 51% of the uniaxial compressive strength. This stress is shown to be sufficient to cause buckling of thin layers separated from the bulk of the rock. The size of the buckling layer is smaller than the size of the spallation zone leading to a mosaic pattern seen on the surface after spallation. The results are important for further understanding of the mechanism of thermal spallation of rocks as well as large scale spallation-like processes in the Earth's crust. [ABSTRACT FROM AUTHOR]

Combined dynamic and static loading tests with equal impact energy (about 150 J) were conducted on granite after high temperature by an improved split Hopkinson pressure bar (SHPB) test device. Effect of axial compression ratios (ACR) on the strength, dynamic elastic modulus (Ed), failure behavior, dissipated energy, and fractal dimension of heat-treated samples was investigated. Results indicated that post-peak strain of samples before 300 °C began to rebound with increasing ACR, belonging to type II. After 300 °C, the ACR would not change the curves’ type, belonging to type I. The sensitivity of dynamic uniaxial compression strength (DUCS) and combined strength to temperature was consistent, but their sensitivity to ACR was different. Empirical equations for variations of strength with ACR and temperature indicated that same strength corresponded to multiple combinations of ACR and temperature, but samples’ crushing degree existed differences. As the ACR or temperature increased, the Ed climbed up and then declined. As the temperature rose or the ACR declined, the percentage of dissipated energy during sample failure increased. When the ACR was 0.8 and temperature was within 300 °C, the dissipated energy was negative, i.e., the sample released elastic strain energy for debris ejection instead of absorbing energy. When the temperature or ACR increased, mass of small-sized sample debris increased, and the crushing degree enhanced. Fractal dimension D well characterized the crushing degree of samples. Scanning electron microscope (SEM) images explained the diminishing mechanical properties of samples with increasing temperature. Results provided a reference for the construction of deep high-stress and high-temperature rock engineering. [ABSTRACT FROM AUTHOR]

Rockburst is one of the common geological disasters in deep underground areas with high stress. Rockburst prediction is an important measure to know in advance the risk of rockburst hazards to take a scientific approach to the response. In view of the fuzziness and uncertainty between quantitative indexes and qualitative grade assessments in prediction, this study proposes the use of a normal cloud model to optimize the theory of unascertained measures (NC-UM). The uniaxial compressive strength (σc), stress coefficient (σθ/σc), elastic deformation energy index (Wet), and brittleness index of rock (σc/σt) are selected as the index of prediction. After data screening, 249 groups of rockburst case data are selected as the original data set. To reduce the influence of subjective and objective factors of index weight on the prediction results, the game theory is used to synthesize the three weighting methods of Criteria Importance Through Intercriteria Correlation (CRITIC), Entropy Weight (EW), and Analytic Hierarchy Process (AHP) to obtain the comprehensive weight of the index. After validating the model with example data, the results showed that the model was 93.3% accurate with no more than one level of prediction deviation. Compared with the traditional unascertained measure (UM) rockburst prediction model, the accuracy is 15–20% higher than that of the traditional model. It shows that the model is valid and applicable in predicting the rockburst propensity level. [ABSTRACT FROM AUTHOR]

Acoustic emission/microseismic (AE/MS) source location plays a vital role in the safety monitoring of circular hole-contained structures (e.g., tunnels, pipelines, and pressure vessels). However, traditional methods may obtain a bad P-wave travel path and poor source location accuracy. To handle this, an analytical solution-based shortest P-wave travel path and a grid search-based AE/MS event source location method are proposed for circular hole-contained structures. Moreover, the objective function of AE/MS source location considers the P-wave arrival time system error. Both synthetic tests and pencil-lead break (PLB) application tests in 2D and 3D circular hole-contained structures show the outperformance of the proposed location methods. First, the analytical method obtains an equal or smaller P-wave travel distance than that of the Dijkstra and A* algorithms, which means the analytical method generates better theoretical data. Then, the average location errors of PLB events for the 2D circle-contained rectangle, 3D full circular hole-contained cuboid, and 3D part circular hole-contained cuboid are 0.57 cm, 1.87 cm, and 2.25 cm, respectively. These location results are better than results based on the shortest P-wave paths of the straight line, Dijkstra, and A* algorithms. Moreover, the proposed method solves that former analytical solutions do not consider the thickness of cylindrical shell structures. Highlights: An analytical solution-based shortest P-wave travel path has been proposed for circular hole-contained structures. The acoustic emission/microseismic source location objective function considers the P-wave arrival time system error. The average location errors of pencil-lead break events for circular hole-contained structures are within 2.25 cm. The analytical solution-based location method obtains better location results than methods based on the straight line, Dijkstra and A* algorithms. [ABSTRACT FROM AUTHOR]

Previous studies on thermal induced damage of rocks concentrated mainly on the analysis of macro-mechanical properties. This paper documents physical and microstructural characteristics of sandstone after high-temperature treatment by using several testing methods at the micro scale. Results indicate that physical including mechanical parameters of tested sandstone show some strengthening effect up to a temperature of about 100 °C, which is related to the evaporation of free water and the thermal expansion of mineral particles. Beyond 100 °C, the physico-mechanical properties of sandstone are weakened significantly. Mass loss and porosity show a continuously growing with increasing temperature, while P-wave velocity, splitting tensile strength, compression strength, and elastic modulus are decreasing. Nuclear magnetic resonance (NMR) imaging tests were carried out to capture the variations in porosity and pore size distribution characteristics of sandstone during the heat treatment. With increasing temperature, the number of macropores and microcracks increase and coalesce. There is a linear relationship between rock porosity and tensile as well as compressive strength. Differential thermal analysis (DTA) shows that the α-β phase transition of quartz minerals occurs at a temperature of 573.4 °C. Additionally, between 400 and 600 °C, more transgranular microcracks are observed detected by optical microscope and scanning electron microscopy (SEM) observations, which proves that quartz phase transition is an important factor to induce microcracks. Thermal treatment also has a significant influence on the variation of the brittle index of sandstone, which makes thermal-induced plastic deformations of the sandstone more obvious. [ABSTRACT FROM AUTHOR]

Microwave-assisted hard rock breakage is a potential solution for exploitation of deeper resources. A series of ultrasonic non-destructive tests and NMR tests were conducted to quantitatively characterize the thermal damage of granite. The effects of temperature, microwave power, and loading rate on the dynamic tensile behaviors of granite were investigated by the SHPB tests. Results indicate that, when the temperature is higher than 400 °C, the ultrasonic velocity of microwave-treated granite samples significantly decreases, and the dynamic elastic modulus is more sensitive to temperature than microwave power. The microwave heating promotes the simplification of pore structure of granite, which manifests as the conversion of the pore size distribution from bimodal to trimodal and the decreases of fractal dimension of macropores. The dynamic tensile strength increases as the increasing loading rate and decreases as the increasing temperature and power. Under dynamic tensile stress, the microwave treatments weaken the brittleness of granite and the dependence of the dynamic tensile strength on the loading rate. The investigation of rock damage and dynamic tensile strength in this study provides guidance for optimizing microwave-assisted rock breakage technology. [ABSTRACT FROM AUTHOR]

In this study, mode I fracture tests on cracked straight-through Brazilian disc (CSTBD) and notched semi-circular bend (NSCB) shale specimens with different sizes were conducted to investigate the difference between maximum tangential stress fracture criterion and the size effect law (SEL) model in predicting apparent fracture toughness (Ka) of shale. In addition, the effects of specimen size and geometry on the Ka and the selection of fracture criterion on the prediction of the inherent fracture toughness (KIc) were also studied. The results show that the Ka increases with the increase of specimen size, and the difference between KIc of shale specimens with different sizes predicted by the fracture process zone length determined by the further improved maximum tangential stress (FIMTS) criterion is the smallest. For the prediction of Ka of NSCB specimen, the results predicted by the FIMTS criterion are the closest to the tested fracture toughness. However, the effect of SEL model applied to the prediction of Ka of NSCB specimens is poor. The effective establishment of SEL model requires high accuracy for test data, especially for the configuration with large variation of the dimensionless stress intensity factor (Y*) with normalized crack length (α). Highlights: Fracture toughness is closely related to the size and configuration of the specimen, which is ultimately attributed to the inconsistent tangential stress distribution at the crack tip. The further improved maximum tangential stress criterion has the best effect on apparent fracture toughness prediction, and higher-order terms of the Williams expansion should be considered in apparent fracture toughness prediction of small-size specimens. The fracture criterion for determining the fracture process zone length should be consistent with the prediction criterion of apparent fracture toughness. The effective establishment of size effect law requires high accuracy tested data, especially for the configuration with large variation of the dimensionless stress intensity factor with normalized crack length. [ABSTRACT FROM AUTHOR]

Uniaxial compression tests and acoustic emission (AE) monitoring of shale specimens with horizontal and perpendicular bedding planes after being subjected to high temperatures were conducted to investigate the effect of a high-temperature environment on shale failure characteristics. The results reveal that specimens in the two bedding directions experienced severe rockburst failure. The rockburst processes of all the tested specimens were similar and can be divided into two main stages according to the specific failure characteristics, spalling stage, and ejection stage, and the increasing temperature had a significant effect on the rockburst characteristics of specimens in the two bedding directions at each stage. The fractal dimension of specimens in the two bedding directions increased with increasing temperature and increased much more for specimens with perpendicular bedding planes. The high-energy AE signals induced by the rockburst failure of specimens became more significant with increasing temperature, and the step-jump frequency of cumulative AE energy for specimens with perpendicular bedding planes increased gradually. The sudden drop in the amplitude of the AE b value resulting from spalling, ejection, and monolithic failure became larger for specimens in the two bedding directions with increasing temperature. Before the peak stress, the AE b value of specimens with horizontal bedding planes showed a stable period that was unaffected by increasing temperature, while the AE b value of specimens with perpendicular bedding planes also showed a plateau period in the prepeak stage when the temperature was lower than 100 °C and disappeared when the temperature exceeded 100 °C. [ABSTRACT FROM AUTHOR]

Elastic modulus is an important parameter in rock mechanics and engineering geology that determines the mechanical properties of rocks. To analyze the effect of temperature and dynamic loading conditions on the elastic modulus of typical rocks, data and results from international publications are classified, analyzed, discussed and summarized. The findings show that the elastic modulus of different rocks is correlated linearly and negatively with temperatures, which can be attributed to dehydration, phase transition, thermal expansion and thermal decomposition of minerals. The elastic modulus is significantly more sensitive to temperature than to strain rate. In addition, the elastic modulus of sedimentary rocks shows a significant strain rate effect, especially in sandstones, yet it is not observed in igneous and metamorphic rocks. The threshold temperature for peak stress degradation of limestone, sandstone, granite and marble is consistent with that of strain rate effect of elastic modulus. Thermal activation mechanism, Stefan effect, inertia effect and dislocations in crystals are the main contributing factors to the strain rate effect of the elastic modulus in rock materials. [ABSTRACT FROM AUTHOR]

This study aims to investigate the rockburst characteristics of hard rock during the successive excavation unloading of twin circular tunnels subjected to high active stresses. The entire evolution process of the rockburst phenomena around the tunnels is reproduced. The numerical results indicate that the unloading rates, burial depths, and presence of structural planes between the twin tunnels play important roles in the occurrence and damage degrees of rockbursts. The failure intensity and dynamic responses are aggregated with the increase of the unloading rate of the subsequent adjacent tunnel. The rockburst damage degree is exacerbated with increasing buried depth, and the rock response of the twin tunnels becomes more sensitive to the dynamic disturbance (as compared to a single tunnel at a great depth). The presence of a structural plane between the twin tunnels has both favourable and unfavourable effects on the stability of the surrounding rock. When the structural plane is parallel to the maximum tangential stress, the dynamic disturbance from the adjacent tunnel can be attenuated by the structural plane or rock joints via reflection and scattering, thus reducing the dynamic response between the twin tunnels. However, for those structural planes oblique to the maximum tangential stress, a violent rockburst is more prone to be induced, owing to the integrated response to shearing and sliding along the structural plane, and slabbing from the excavation unloading process. It is also found that the effect of the structural plane on the rockburst response is largely dependent on the burial depth. [ABSTRACT FROM AUTHOR]

The effect of infilling on mechanical response and crack behavior of pre-stressed rock with double rectangular holes under dynamic load was investigated through a series of coupled static–dynamic loading tests on diorite specimens in three different filling states, i.e., no infilling, single infilling and double infilling, using a modified split Hopkinson pressure bar. The deformation, damage and fracture process of specimens were recorded and analyzed by low-speed and high-speed cameras with digital image correlation method. Test results reveal that under the same dynamic load, the strength of specimens increases first and then decreases with the increase of axial static pre-stress, and reaches the maximum under 25% UCS due to crack closure. The strengthening effect of double infilling on strength is much more significant than that of single infilling and increases with the pre-stress. Observations show that with the increase of pre-stress, the degrees of damage and strain localization of specimens increase, and the superficial damage on the free surface of sidewalls are more severe. The effect of infilling is significant on the crack initiation and propagation, especially on the inhibition of sidewall spalling, rock ejection and the axial displacement failure of rock septum between holes. With the increase of pre-stress, the failure pattern and the crack coalescence mode change from obvious tensile to shear, and the coalescence position changes from the inside to outside of the septum area. However, the change of filling state has little effect on the coalescence mode, only on the coalescence position. [ABSTRACT FROM AUTHOR]

The D-shaped cross section is a commonly used tunnel cross section in underground engineering. To simulate the failure process of a D-shaped hole under deep three-dimensional (3D) high-stress conditions, true-triaxial tests were conducted on cubic granite specimens with a through D-shaped hole, and the failure process of the hole sidewall was recorded in real time. Results show that the spalling failure process of the D-shaped hole sidewall can be divided into four periods: calm, fine particle ejection, crack generation and propagation, and rock slab gradually buckling and spalling. Afterwards, symmetrical V-shaped notches were formed on both sidewalls between the corner and arch springing. The spalling failure shows tensile failure characteristics. Under high vertical stress and constant horizontal axial stress, increasing the lateral stress reduces the severity of the spalling failure and the depth of the V-shaped notch. The initial failure vertical stress of the D-shaped hole sidewall is higher than that of circular hole sidewall, and the failure of D-shaped hole sidewall is mainly characterized by static failure. The failure of the circular hole sidewall is a more severe dynamic failure. When the vertical applied stress is the maximum principal stress, the position of the V-shaped notch tip is 0.20–0.25 h (h is the height of the D-shaped tunnel) from the tunnel floor, whereas that in the circular tunnel is 0.5 d (d is the diameter of the circular tunnel) from the tunnel floor. Specific support schemes should therefore be designed for tunnels with different cross sections according to the damage location, depth of failure zone, and severity of failure. [ABSTRACT FROM AUTHOR]

A physical model test is carried out to simulate the blasting disturbance on the underground tunnel. The test including four blasting events with different blasting locations, where two blasting angles ( α ) and two blasting distances (D) are designed. The surface deformation characteristics of the physical model are observed by a DIC system. The deformation responses of four strain components during blasting are obtained. The influences of the blasting location on the surface deformation and failure characteristics are analyzed. The distributions of strain components εx, εy, εxy, and ε1 are obtained for different blasting locations. The different occurrences of failure are compared for cases where the blasthole is at varying locations from the tunnel. The results show that when the blasthole is far from the tunnel, the failure mainly occurs in the vicinity of the blasthole and the failure pattern of the physical model is conical. When the blasthole is close enough to the tunnel and is near the vertical wall, the surrounding rock fail in a V-shape. According to the failure patterns, the anti-disturbance ability of the arch is obviously stronger than that of the vertical wall. [ABSTRACT FROM AUTHOR]

Under certain extreme conditions in rock engineering works, fast change in temperature in the load-bearing rocks can happen. Known as thermal shock (TS), such process involves rapid temperature rise or drop, which causes fracturing in the rock material and thus can pose as a threat to the stability of the rock structures. To investigate the influence of thermal shock caused by fast cooling on the mechanical property of rock, laboratory tests are performed on heated granite which are cooled with different methods, with the highest cooling rate reaching 167.4 °C/min. The dynamic loading tests are performed on the heated granite specimens utilizing the split Hopkinson pressure bar (SHPB) system. The test results show that the dynamic compressive strength drops with the increase in heating level or cooling rate. This pattern is explained by the nuclear magnetic resonance (NMR) test data that the pores inside the heated granite increase both in size and quantity as heating level or cooling rate rises. Damage patterns of the tested granite specimen fragments are analyzed based on the observation with scanning electron microscope (SEM), and the mechanisms of thermal shock in granite are also discussed. [ABSTRACT FROM AUTHOR]

Keywords: Preset angle shear; Direct shear; Triaxial compression; Cohesion; Internal friction angle; Shear strength parameters EN Preset angle shear Direct shear Triaxial compression Cohesion Internal friction angle Shear strength parameters 2505 2519 15 05/08/20 20200501 NES 200501 Introduction Estimation of shear strength parameters, which are generally known as the cohesion and internal friction angle ( I c i and I i ) in the Mohr-Coulomb (M-C) strength criterion, plays an essential role in rock mechanics and rock engineering (Zhao [34]; Kahraman et al. [14]; Alejano and Carranzatorres [1]; Gong et al. [7]). 6 Shear strength parameters of a marble and b red sandstone obtained by three different testing methods Comparison of Testing Methods for Rock Shear Strength Parameters Hereinbefore, data deviations were observed for marble and red sandstone under the three testing conditions. [Extracted from the article]

To deeply understand the rock failure characteristics under actual engineering condition, in which static geo-stress and dynamic disturbance usually act simultaneously, impact tests were conducted on sandstone subjected to axial static pre-stresses varying from 0 to 75 MPa by a modified split Hopkinson pressure bar. The fracturing process of specimens was recorded by a high speed camera. Dynamic parameters of sandstone, such as strain rate, dynamic strength and energy partition were acquired. Fracture mechanisms of pulverized specimens were identified by the method combining the displacement trend line and digital image correlation technique. Moreover, fragments of failed specimens were sieved to obtain the fragment size distribution. Test results revealed that, under the same incident energy, the dynamic compressive strength increases first, then decreases slowly and at last drops rapidly with the increase of pre-stress, and reaches the maximum under 24.4% of uniaxial compressive strength due to the closure of initial defects. Four final patterns were observed, namely intact, axial split, rock burst, and pulverization. The rock burst only occurs when the pre-stress lies in the elastic deformation stage or initial stable crack growth stage and the incident energy is intermediate. For pulverized specimens, the fracture mechanism is transformed into shear/tensile equivalent from tensile-dominated mixed mode as the pre-stress increases. Specimens with 75 MPa pre-stress release strain energy during failure process, contrary to specimens with lower pre-stresses absorbing energy from outside. The crushing degree of pulverized specimens exhibits a positive correlation with the pre-stress as a consequence of higher damage development in rock. [ABSTRACT FROM AUTHOR]

Rock mass is frequently subjected to rapid cooling in many temperature-dependent engineering applications. In this paper, to understand the effects of water and liquid nitrogen cooling treatments on heated sandstone, P-wave velocity and uniaxial compression tests, Brazilian test, and fracture toughness tests were carried out to evaluate variations in physical and mechanical properties. The microscopic morphology of the thermally treated sandstone was also obtained by scanning electron microscopy (SEM). Test results reveal that the P-wave velocity, uniaxial compressive strength, Young's modulus, tensile strength, and fracture toughness of the sandstone after heating and cooling treatment decrease continuously with increasing temperature. Compared to the water cooling treatment, the liquid nitrogen cooling treatment caused much more damage to the heated sandstone. The ductility of the thermally treated sandstone increased with increasing heating temperature, which was evidenced by the stress–strain response in uniaxial compression tests. The transition from shear-dominant fracturing to complex fracturing of the failure mode demonstrated that damage of sandstone induced by heating and cooling treatment increased with increasing temperature. Based on the SEM observation, the increase in heating temperature and cooling rate can weaken particle cementation and promote the generation and propagation of the micro-cracks in sandstone, and the generation of inter-granular cracks caused the reduction of tensile strength and fracture toughness of sandstone. In addition, the tensile stress caused by the thermal shock during rapid cooling treatment is the main reason of cracks growth in sandstone specimens, and the tensile stress is positively correlated with the temperature difference and the heat transfer coefficient. [ABSTRACT FROM AUTHOR]

To investigate the energy evolution characteristics of rock materials under uniaxial compression, the single-cyclic loading–unloading uniaxial compression tests of four rock materials (Qingshan granite, Yellow sandstone, Longdong limestone and Black sandstone) were conducted under five unloading stress levels. The stress–strain curves and failure characteristics of rock specimens under the single-cyclic loading–unloading uniaxial compression tests basically corresponded with those of under uniaxial compression, which indicates that single-cyclic loading–unloading has minimal effects on the variations in the loading–deformation response of rocks. The input energy density, elastic energy density and dissipated energy density of four rocks under five unloading stress levels were calculated using the graphical integration method, and variation characteristics of those three energy density parameters with different unloading stress levels were explored. The results show that all three energy density parameters above increased nonlinearly with increasing unloading stress level as quadratic polynomial functions. Meanwhile, both the elastic and dissipated energy density increased linearly when the input energy density increased, and the linear energy storage and dissipation laws for rock materials were observed. Furthermore, a linear relationship between the dissipated and elastic energy density was also proposed. Using the linear energy storage or dissipation law, the elastic and dissipated energy density at any stress levels can be calculated, and the internal elastic (or dissipated) energy density at peak compressive strength (the peak elastic and dissipated energy density for short) can be obtained. The ratio of the elastic energy density to dissipated energy density with increasing input energy density was investigated using a new method, and the results show that this ratio tends to be constant at the peak compressive strength of rock specimens. [ABSTRACT FROM AUTHOR]

Fracturing processes of rock are simulated using a micro- and fracture mechanical-based numerical modeling approach. The numerical model considers material heterogeneity and initial microcrack distributions. The gradual formation of macroscopic fractures by coalesced microcracks is successfully reproduced. Distinct failure modes are observed in the model under different loadings. Agreement is shown between the numerical results and laboratory observations. The influence of microcrack orientations on the fracture patterns is quantified by the numerical models. Possible mechanisms describing the splitting failure of hard brittle rocks under uniaxial compression are proposed and discussed. [ABSTRACT FROM AUTHOR]

A series of double perovskite (Gd1−xTbx)2MgTiO6 (x = 0.01, 0.05, 0.1, 0.15, 0.2, 0.3 and 0.4) phosphors were successfully synthesized by solid-state reaction method. The A-site in the host is occupied by two Gd3+ ions, which facilitates the doping of Tb3+ ions. The crystal structure, morphology and luminescence performance were evaluated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, photoluminescence and calculation of CIE color coordinates. A weak emission peak (5D4–7F6, 490 nm) and a strong emission peak (5D4–7F5, 545 nm) existed in the emission spectra under 313 nm excitation. The quenching concentration of Tb3+ reached up to 20.0 mol% was observed in Gd1.8Tb0.2MgTiO6 phosphors. In order to explore the potential application of Gd2−2xMgTiO6:xTb3+ phosphors in white LED solid-state lighting, the fluorescence decay curves and thermal stability were carried out. [ABSTRACT FROM AUTHOR]

It is essential to investigate focal mechanisms of induced seismicity for understanding the rock fracturing, the failure mode, and the hazard evolution in underground mines. But the conventional methods using empiricism to infer the source mechanisms usually lead to ambiguous results for individual events. An optimized moment tensor inversion method using full waveforms was employed to quantitatively determine the rock fracturing orientation and the type of rupture process. Source parameters including the scalar moment, the moment magnitude, the full moment tensor, and the fault plane solutions were resolved of a seismic sequence in fault zones. Results show that the shear failure events in the fault F1 vicinities have similar focal mechanisms and suggest that the fault F1 is a reverse fault. The resolved strikes and dips are basically constant with the orientation of the fault. The events in the fault F2 vicinities are mainly dominated by shear-tensional failure. But the shear events experienced shear rupture and crack opening simultaneously, resulting in slippages not along the actual fault plane. There are more events in the fault F3 area which are characterized by complicated focal mechanisms. Three of the non-shear events are dominated by compressional failure and related to rock collapse, while the other non-shear events are dominated by tensional failure and related to crack opening. The shear dominated events experienced dual effects of shear failure and compression failure. The resolved fault plane solutions cannot reflect the actual geometry of the fault. It is proved that the moment tensor inversion is able to quantitatively analyze the focal mechanism of mining-induced seismicity in fault zones and it provides beneficial understandings of mining-induced fault slips. [ABSTRACT FROM AUTHOR]

Dynamic impact tests were conducted on rectangular marble specimens with flaws to investigate the effect of the flaw angles and ligament angles on the rock dynamic mechanical properties, fracturing behavior, and energy evolution characteristics. A 75-mm diameter split Hopkinson pressure bar (SHPB) device was used in the tests. The experimental results show that the flaw geometries have a strong effect on the dynamic mechanical properties and fracturing behavior of rocks. In general, the effects of the flaw angle and flaw number on the dynamic strength are more significant than the effect of the ligament angle. A high-speed camera was used to monitor and record the crack initiation and coalescence and the failure processes of the marble specimens in real-time. The crack propagation can be divided into two major stages, i.e., the formation of white patches and the generation of macrocracks. The white patches usually appear prior to the peak stress of the specimen. The shear cracks usually initiate at or near the flaw tips and propagate to form X or half-X shear belts that dominate the macroscopic failure under impact loading. Nine types of crack coalescence behaviors are identified for the marble specimens with double flaws, and the macro failure modes of the specimens is usually combined by some of them. The energy absorption properties of the marble specimen are also significantly affected by the pre-existing flaws. The energy absorption ratio can be maximized by setting a certain number of pre-existing flaws with an appropriate inclination angle and a proper ligament angle to effectively improve the fragmentation efficiency of rocks. [ABSTRACT FROM AUTHOR]

To evaluate tunnel ground vibrations induced by fault-slip in underground mine, it is crucial to resolve the peak ground velocity (PGV) and the peak ground acceleration (PGA). To quantify PGV and PGA in three-dimensional space, the forces acting on the fault and the seismic wave radiation pattern were investigated. A qualitative method for ground vibration of tunnels induced by fault-slip in underground mine was proposed and applied to the Yongshaba Phosphate Mine, China. Firstly, the moment tensor of the significant fault-slip event was obtained using a full waveform source inversion method based on the recorded seismograms. It describes the forces acting on the fault and the seismic wave radiation pattern. Then, the inverted source was used as a reference to evaluate the ground vibrations under possible larger seismic magnitudes and probable slip models. The vibrations along the tunnels resulted from different slip models were calculated using the representation theorem by the convolution of the Green's function and the corresponding moment tensor. The velocity series and the acceleration series were obtained. Results show that the spatial distribution of the ground motions are closely related to sensor-source distance, the sensor-source azimuth, as well as the strike, dip, and rake configuration of the slip model. The seismic magnitude has an exponential effect on the distribution of the PGVs and PGAs. The PGV and PGA have similar spatial distribution patterns. The PGV values of the north and vertical components show nearly 5 times higher than the east component. The vertical vibrations affect the widest area. The ground support principles were suggested considering the parameters including the PGVs, the PGAs, the rock mass rating, and the dynamic loads. [ABSTRACT FROM AUTHOR]

The Earth's deep rock mass is subjected to various complex temperature and stress perturbations. To study the effect of temperature and dynamic disturbance on the damage mechanical properties of these rocks, a series of laboratory tests was carried out by means of a modified split Hopkinson pressure bar system. Specimens were heat treated from room temperature up to a maximum of 800 °C and then cooled to room temperature. During repeated loading tests, the dynamic incident energy was kept constant in each cycle. For samples under the same treated temperature, the dynamic strength and deformation capacity degraded gradually with increasing impact number. Furthermore, rock strength decreased with increasing treated temperature, with the number of impacts before failure reduced accordingly. Since damage can be initiated by thermal treatment and then aggravated by dynamic disturbance, according to the observed decrease in rock strength and increase in strain rate under the different temperature conditions, a temperature of 400 °C was found be a significant failure threshold. The maximum strain was employed to describe damage evolution during cyclic impact loading, indicating that fatigue damage is gradually accumulated and maintains a three-segment growth with an increase in repeated impacts. Under the coupled effect of temperature and cyclic impact loading, specimens exhibited two different failure modes: split tensile failure and unloading failure. The micro-properties of fracture morphology in granite after different temperature and repeated impact were also discussed in detail. [ABSTRACT FROM AUTHOR]

Induced seismicity in underground mining zones reflects inner changes of the rock structure caused by human activities, thus providing potential information for hazard assessment. Precise hypocenter parameters and magnitudes of the induced micro-earthquakes as well as their spatial distributions are essential for understanding the seismicity. We first accurately relocate 834 micro-earthquakes recorded by a 28-sensor micro-seismic monitoring system from an underground phosphate mine, using an iterative pseudo-bending ray-tracing method and a 3-D velocity structure obtained by seismic tomography. We then determine the local magnitudes of the events by a well-recalibrated formula, and finally analyze the seismicity for different parts of the mine divided by the fuzzy c-means clustering method according to the spatial distribution of the micro-earthquakes. The bimodal distribution of the recalibrated local magnitudes indicates two fundamentally different processes of rock failure may exist in the mine. They are possibly fracture-dominated rupture and friction-dominated slip, i.e., tensile failure and shear failure of the surrounding rock. The clustering result shows that mining practices cause very different seismic features in different parts of the mine. Furthermore, we find that most of the large events are located in low-V zones, and that events occurred in the lower velocity zones tend to have a correspondingly larger magnitude, as revealed by the seismicity analysis and seismic tomography. [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]

High geostresses and stress gradients are the predominant stress conditions in deep excavation-disturbed rock masses. The aim of this study is to determine the triaxial compressive strength properties of hollow cylindrical granite specimens under a radially non-uniform confining stress field with different radial stress gradients determined by coupled external and internal confining stresses. Triaxial compression testing of hollow cylindrical rock specimens was performed to investigate the influence of the radial stress gradient, external confining stress and specimen length-to-diameter (L/D) ratio on the triaxial compressive strength. The experimental results and regressed failure criteria indicate that the triaxial compressive strengths of the hollow cylindrical granite specimens increase with the external confining stresses, but decrease with an increase in the radial stress gradients. The calculated goodness of fit (R2) and root-mean-squared error suggest that the nonlinear failure criterion based on the Hoek-Brown model is more accurate than the linear failure criterion based on the Mohr-Coulomb model for determining the influences of the external confining stress and radial stress gradient on the triaxial compressive strength. In addition, the triaxial compressive strength increases with a decreasing L/D ratio due to the strengthening end effect of the hollow cylindrical granite specimens and the change in the failure pattern of these specimens from shear to slabbing. [ABSTRACT FROM AUTHOR]

Automatic microseismic P-phase arrival picking is paramount for microseismic event identification, event location and source mechanism analysis. The commonly used STA/LTA picker, PAI-K picker, AIC picker and three proposed pickers have been applied to determine the P-phase arrivals of 580 microseismic signals (the sampling frequency is 6000 Hz). These have been obtained from the Institute of Mine Seismology (IMS) acquisition system of the Yongshaba mine in China. Then, the six above-mentioned pickers have been compared in their picking accuracy, typical waveforms, signal-to-noise ratio (SNR) adaptabilities and quantitative evaluation. The results have shown that: (1) the triggered STA/LTA picker has a good picking stability but a low picking accuracy. While the PAI-K and the AIC pickers have a higher picking accuracy but a poorer picking stability compared with the triggered STA/LTA picker. Moreover, the AIC picker usually has a better picking result than the PAI-K picker; (2) the S/L-K-A picker significantly improves the STA/LTA, the PAI-K and the S/L + PAI-K pickers. Moreover, it obviously improves the AIC and the S/L + AIC pickers’ large picking error (> 30 ms) signals; (3) the picking error ratios of the S/L-K-A picker within 10, 20 and 30 ms achieve 92.76, 95.86 and 97.41%, respectively. The S/L-K-A picker enhances the picking adaptability to different waveforms and SNRs. In conclusion, the S/L-K-A picker provides a new method for automatic microseismic P-phase arrival picking with a high accuracy and a good stability. [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]

Based on the boundary surfaces of bauxite deposit modelled by the two-dimensional biharmonic spline interpolation, it was ascertained that the fully mechanized longwall mining of bauxite is feasible in terms of lithological characteristics, orebed occurrence, physical-mechanical properties of ore-rock, faults, Karst caves, permeable fissures, etc. Furthermore, a comprehensive target function consisting of waste-rock interfused ratio and ore loss ratio was used to obtain the optimal cutting height of shearer by a virtual mining procedure that was proposed to numerically perform the fully mechanized longwall mining. For the 1102 longwall mining panel in Wachangping Bauxite Mine, the optimal cutting height of shearer ranges from 1.16 to 2.32 m supplied from two 1.16-m-diameter cutting drums, which meets the requirement of low-waste and high-recovery mining to achieve high-grade and low-loss exploitations of underground bauxite. These efforts can improve the application and popularization of fully mechanized longwall mining in non-coal underground mines. [ABSTRACT FROM AUTHOR]

To experimentally investigate the stability of underground excavations under high in situ stress conditions, several rock samples with a mini-tunnel were prepared and subjected to monotonic axial and coupled static–dynamic loading until failure. Mini-tunnels were generated by drilling circular or cubic cavities in the centre of granite rock blocks. Strain gauges were used to monitor the deformation of the mini-tunnels at different locations, and a high-speed camera system was used to capture the cracking and failure process. We found that the dynamic crack initiation stress, failure mode and dynamic crack velocity of the specimen all depend on the pre-stress level when the sample is under otherwise similar dynamic disturbance conditions. The crack initiation stress threshold first increased slightly and then decreased dramatically with the increase in the pre-stress value. The specimens were mainly fractured by tensile cracks parallel to the compression line under lower pre-stress, while they were severely damaged with additional shear cracks under higher pre-stress. Furthermore, the propagation velocity of the primary crack was significantly larger than that of the subsequent cracks. The effect of applying different amounts of static pre-stresses on the velocity of the primary tensile crack was similar to that observed for the crack initiation stress threshold; however, it did not affect the velocity of the secondary and subsequent tensile cracks. [ABSTRACT FROM AUTHOR]

P phase arrival picking of weak signals is still challenging in seismology. A wavelet denoising is proposed to enhance seismic P phase arrival picking, and the kurtosis picker is applied on the wavelet-denoised signal to identify P phase arrival. It has been called the WD-K picker. The WD-K picker, which is different from those traditional wavelet-based pickers on the basis of a single wavelet component or certain main wavelet components, takes full advantage of the reconstruction of main detail wavelet components and the approximate wavelet component. The proposed WD-K picker considers more wavelet components and presents a better P phase arrival feature. The WD-K picker has been evaluated on 500 micro-seismic signals recorded in the Chinese Yongshaba mine. The comparison between the WD-K pickings and manual pickings shows the good picking accuracy of the WD-K picker. Furthermore, the WD-K picking performance has been compared with the main detail wavelet component combining-based kurtosis (WDC-K) picker, the single wavelet component-based kurtosis (SW-K) picker, and certain main wavelet component-based maximum kurtosis (MMW-K) picker. The comparison has demonstrated that the WD-K picker has better picking accuracy than the other three-wavelet and kurtosis-based pickers, thus showing the enhanced ability of wavelet denoising. [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]

A three-dimensional coupled interpolation method with three steps was proposed to establish a smooth and precise grade distribution model of bauxite seam. Compared with and supported by the field grade data measured by some exploratory boreholes, the stability of interpolation model established by natural neighbor method is the best to depict a thin bedded deposit to be more physically realistic in data interpolation. Based on this model, the total volume of bauxite seam in the 1102 longwall mining panel is $$189{,}270 \, \hbox {m}^{3}$$ , in which the average values of mass fraction of $$\hbox {Al}_{2} \hbox {O}_{3}$$ and mass ratio of $$\hbox {Al}_{2} \hbox {O}_{3}$$ to $$\hbox {SiO}_{2}$$ are 55.09% and 17.72, respectively. Demarcated by industrial standards, the volume, the average mass fraction of $$\hbox {Al}_{2} \hbox {O}_{3}$$ and the average mass ratio of $$\hbox {Al}_{2} \hbox {O}_{3}$$ to $$\hbox {SiO}_{2}$$ of bauxite orebody are $$96,654 \, \hbox {m}^{3}$$ , 61.58% and 27.10, respectively. These mean that the bauxite orebody has abundant high-grade ore. [ABSTRACT FROM AUTHOR]

Eu ions activated BaLaMgSbO phosphors were successfully synthesized by the solid-state reaction method using NHCl as the flux. The BaCl was selected to substitute BaCO as the starting material to attest that NHCl not only reacted with BaCO but also reacted with other starting materials. The analysis of TG/DSC was used to probe the role of NHCl flux in the process of synthesizing BaLaMgSbO powders. The crystal structure was investigated by XRD and Rietveld refinement, and the morphology by SEM. The phosphors had monoclinic double perovskite structure with the space group P21, as well as the rock-salt ordering of B-site. The luminescence properties, including the photoluminescence excitation and emission spectra, and the color coordinates were investigated. Also, the relationship between the crystalline size and the emission intensity were discussed. With increasing the calcination temperature, both the crystalline size and the emission intensity monotonously increased. The relative intensity ratio of red/orange emission was used to measure the degree of distortion from the inversion symmetry of the local environment of the Eu ions in BaLaMgSbO. [ABSTRACT FROM AUTHOR]

Eu ions activated BaLaMgSbO phosphors were successfully synthesized by the solid-state reaction method using NHCl as the flux. The influence of NHCl, structure, morphology and photoluminescent properties of as-prepared powders were investigated in detail. Long-range ordering of B-site cations existed in this double perovskite. With increasing the amount of NHCl, the morphology of phosphors changed from regular polyhedron shape to globoid shape. Broad and strong A mode existed in the Raman spectrum. The absorption intensity of charge transfer band was higher than the intensity of characteristic f- f absorption of Eu. The luminescence intensity of D-F (electric dipole transition) was much higher than that of D-F (magnetic dipole transition). The optimum adding amount of NHCl was 10.0 wt% while excited by the CTB and the phosphor emitted intense red light. The CIE chromaticity coordinates indicated that the phosphors showed good color purity. Therefore the synthesized phosphor is a potential light conversion red material for light emitting diode. [ABSTRACT FROM AUTHOR]

Dynamic impact tests are performed on prismatic marble specimens containing a single flaw using a modified split-Hopkinson pressure bar device. The effects of pre-existing flaws with different flaw angles and lengths on the dynamic mechanical properties are analyzed. The results demonstrate that the dynamic strength of marble is influenced by the flaw geometry. The dynamic fracturing process of flawed specimens is monitored and characterized with the aid of a high-speed camera. Cracking of marble specimens with a single pre-existing flaw under impact loading is analyzed based on experimental investigations. Cracking involves two major stages: formation of white patches and development of macrocracks. Six typical crack types are identified on the basis of their trajectories and initiation mechanisms. The presence of an artificial flaw may change the failure mode of marble from splitting-dominated for an intact specimen to shear-dominated for a flawed specimen under dynamic loading. Nevertheless, the geometry of the flaws appears to have a slight influence on the failure modes of flawed specimens under impact loading. [ABSTRACT FROM AUTHOR]

1064 nm laser stealth absorbent SmOS was prepared by solid state flux method. The effects of doping Er and Tm on the powder's phase compositions and reflecting property were investigated. The phase composition, morphology and reflectivity of powders were characterized by X-ray diffraction, scanning electron microscopy and ultraviolet visible spectrophotometer (UV-3600PC). The results exhibited that doping Er and Tm cause no significant changes on crystal structure. However, the morphology of powders changed a lot after doping Er/Tm, which influenced on the reflecting property. Because of the characteristic absorption of Er and Tm, the reflectivity of SmOS powders decreased in different degrees. The doping amount of Er was 15 mol%, the lowest reflectivity 0.7450 % was obtained. The optimized doping amount of Tm was 10 mol% and the reflectivity was 0.7403 %. [ABSTRACT FROM AUTHOR]

The article presents an investigation on the impact of hole diameter of the ring-shaped specimens including sandstone on the dynamic mechanical properties of the specimens using the dynamic Brazilian test. It is determined that the modified split Hopkinson pressure bar (SHPB) is used to apply dynamic loading and the deformation behaviour of loaded specimen is investigated upon the digital image correlation (DIC) technique based on the image recorded using the high-speed (HS) camera.

To understand the effects of increasing temperature and loading rate on the flexural tensile strength of Laurentian granite, dynamic flexural tensile strength experiments were carried out by means of a semi-circular bend specimen with a modified split Hopkinson pressure bar system. The tests were performed at different loading rates, specimens were treated from room temperature up to 850 °C, and a high-speed camera was utilized to monitor the failure process of the specimen. For samples in the same temperature group, a loading rate dependence of the flexural tensile strength was observed; it increased consistently with the increase of loading rate. Temperature effects on rock mechanical properties were investigated from the microscopic viewpoint, and the dynamic flexural tensile strength decreased with the treatment temperature. A formula relating dynamic flexural tensile strength to loading rate and temperature is presented to quantify the results. It was found that the change regulation of the dynamic flexural tensile strength of rock is very similar to that of its crack growth along with the increase of loading rate, which indicates that the essence of rock failure is the initiation and propagation of the internal cracks. Compared with our earlier work on dynamic tensile tests using the Brazilian test, it was observed that the flexural tensile strength is higher than the tensile strength. Non-local failure theory can be adopted to explain this discrepancy at low temperature conditions, but it is no longer effective at high temperatures. Under high loading rates, rock failure is initiated at the centre of the half circular disc, and finally it is separated completely into two equal parts. [ABSTRACT FROM AUTHOR]

Water content has a pronounced influence on the properties of rock materials, which is responsible for many rock engineering hazards, such as landslides and karst collapse. Meanwhile, water injection is also used for the prevention of some engineering disasters like rock-bursts. To comprehensively investigate the effect of water content on mechanical properties of rocks, laboratory tests were carried out on sandstone specimens with different water contents in both saturation and drying processes. The Nuclear Magnetic Resonance technique was applied to study the water distribution in specimens with variation of water contents. The servo-controlled rock mechanics testing machine and Split Hopkinson Pressure Bar technique were used to conduct both compressive and tensile tests on sandstone specimens with different water contents. From the laboratory tests, reductions of the compressive and tensile strength of sandstone under static and dynamic states in different saturation processes were observed. In the drying process, all of the saturated specimens could basically regain their mechanical properties and recover its strength as in the dry state. However, for partially saturated specimens in the saturation and drying processes, the tensile strength of specimens with the same water content was different, which could be related to different water distributions in specimens. [ABSTRACT FROM AUTHOR]

Micro heat pipes (MHPs) with excellent heat transfer performance have been the ideal radiating components to meet increasingly higher requirements posed by high heat-flux products. Based on MHPs' working principle, this study deduced capillary limit of a novel MHP with compound structure of sintered wick on grooved substrate, and probed into its forming mechanism: first, high-speed oil-filled spinning was applied to fabricating micro grooves, with optimal spinning and drawing speeds determined; then a mini-type vibration machine was used to help fill copper powders fast and uniformly, with appropriate sintering temperature and time fixed; the manufacturing method that integrates vacuum-pumping-cold-welding with secondary-degassing-cold-welding to increase vacuumizing efficiency. The results of experiments on its heat transfer performance show that the MHPs with sintered-wick-on-grooved-substrate structure fabricated through the proposed forming method can not only acquire much better heat transfer performance, but have advantages such as higher productivity and lower cost. [ABSTRACT FROM AUTHOR]

Voids in overburden disturbed by coal mining provide channels for transfer of heat, methane, groundwater, oxygen, exhaust gas and fire-fighting materials in the underground environment. Based on expressions of strata and ground subsidence, a group of void fraction distribution models are proposed, including porosity in the caved zone, the fracture ratio in the bed separation zone and the fissure ratio in the ground subsidence zone. Combined with a case study of the Antaibao coal mine, China, theoretical calculation and numerical simulation indicate that porosity distribution in the caved zone is U-type and the transverse void fractions in the bed separation zone and ground subsidence zone have M-type distributions. Additionally, the longitudinal void fraction distribution changes from ∩-type to M-type along the strike and dip of the coalbed after the complete subsidence of ground. The distribution of fractures in the disturbed overburden of the goaf presents a 'fractured arch', and fracture density gradually decreases from arch foot to crown. Fire-prone zones, propagating paths of coalbed fires, areas prone to water influx and outlets of boreholes used for fire-fighting all usually appear around the perimeter of disturbed overburden. The transverse void fractions distribution in the bed separation zone changes from ∩-type to M-type with an increase in the size of the goaf. It is suggested that the inlets of gob vent boreholes should be arranged near the center of the bed separation zone in the early stages of longwall mining, and around the perimeter in middle to late stages of mining. [ABSTRACT FROM AUTHOR]

Seismic events and blasts generate seismic waveforms that have different characteristics. The challenge to confidently differentiate these two signatures is complex and requires the integration of physical and statistical techniques. In this paper, the different characteristics of blasts and seismic events were investigated by comparing probability density distributions of different parameters. Five typical parameters of blasts and events and the probability density functions of blast time, as well as probability density functions of origin time difference for neighbouring blasts were extracted as discriminant indicators. The Fisher classifier, naive Bayesian classifier and logistic regression were used to establish discriminators. Databases from three Australian and Canadian mines were established for training, calibrating and testing the discriminant models. The classification performances and discriminant precision of the three statistical techniques were discussed and compared. The proposed discriminators have explicit and simple functions which can be easily used by workers in mines or researchers. Back-test, applied results, cross-validated results and analysis of receiver operating characteristic curves in different mines have shown that the discriminator for one of the mines has a reasonably good discriminating performance. [ABSTRACT FROM AUTHOR]

True triaxial tests have been carried out on granite, sandstone and cement mortar using cubic specimens with the process of unloading the minor principal stress. The strengths and failure modes of the three rock materials are studied in the processes of unloading σ and loading σ by the newly developed true triaxial test system under different σ, aiming to study the mechanical responses of the rock in underground excavation at depth. It shows that the rock strength increases with the raising of the intermediate principal stress σ when σ is unloaded to zero. The true triaxial strength criterion by the power-law relationship can be used to fit the testing data. The 'best-fitting' material parameters A and n ( A > 1.4 and n < 1.0) are almost located in the same range as expected by Al-Ajmi and Zimmerman (Int J Rock Mech Min Sci 563 42(3):431-439, 2005). It indicates that the end effect caused by the height-to-width ratio of the cubic specimens will not significantly affect the testing results under true triaxial tests. Both the strength and failure modes of cubic rock specimens under true triaxial unloading condition are affected by the intermediate principal stress. When σ increases to a critical value for the strong and hard rocks (R4, R5 and R6), the rock failure mode may change from shear to slabbing. However, for medium strong and weak rocks (R3 and R2), even with a relatively high intermediate principal stress, they tend to fail in shear after a large amount of plastic deformation. The maximum extension strain criterion Stacey (Int J Rock Mech Min Sci Geomech Abstr 651 18(6):469-474, 1981) can be used to explain the change of failure mode from shear to slabbing for strong and hard rocks under true triaxial unloading test condition. [ABSTRACT FROM AUTHOR]

The effect of thermal treatment on several physical properties and the tensile strength of Laurentian granite (LG) are measured in this study. Brazilian disc LG specimens are treated at temperatures of up to 850 °C. The physical properties such as grain density, relative volume change per degree, and P-wave velocity are investigated under the effect of heat treatment. The results indicate that both the density and the P-wave velocity decrease with the increase in heating temperature. However, the relative volume change per degree is not sensitive below 450 °C, while a remarkable increase appears from 450 to 850 °C. All cases are explained by the increase in both number and width of the thermally induced microcracks with the heating temperature. Brazilian tests are carried out statically with an MTS hydraulic servo-control testing system and dynamically with a modified split Hopkinson pressure bar (SHPB) system to measure both static and dynamic tensile strength of LG. The relationship between the tensile strength and treatment temperatures shows that static tensile strength decreases with temperature while the dynamic tensile strength first increases and then decreases with a linear increase in the loading rate. However, the increase in dynamic tensile strength with treatment temperatures from 25 to 100 °C is due to slight dilation of the grain boundaries as the initial thermal action, which leads to compaction of rock. When the treatment temperature rises above 450 °C, the quartz phase transition results in increased size of microcracks due to the differential expansion between the quartz grains and other minerals, which is the main cause of the sharp reduction in tensile strength. [ABSTRACT FROM AUTHOR]

The prediction of pillar stability (PS) in hard rock mines is a crucial task for which many techniques and methods have been proposed in the literature including machine learning classification. In order to make the best use of the large variety of statistical and machine learning classification methods available, it is necessary to assess their performance before selecting a classifier and suggesting improvement. The objective of this paper is to compare different classification techniques for PS detection in hard rock mines. The data of this study consist of six features, namely pillar width, pillar height, the ratio of pillar width to its height, uniaxial compressive strength of the rock, pillar strength, and pillar stress. A total of 251 pillar cases between 1972 and 2011 are analyzed. Six supervised learning algorithms, including linear discriminant analysis, multinomial logistic regression, multilayer perceptron neural networks, support vector machine (SVM), random forest (RF), and gradient boosting machine, are evaluated for their ability to learn for PS based on different input parameter combinations. In this study, the available data set is randomly split into two parts: training set (70 %) and test set (30 %). A repeated tenfold cross-validation procedure (ten repeats) is applied to determine the optimal parameter values during modeling, and an external testing set is employed to validate the prediction performance of models. Two performance measures, namely classification accuracy rate and Cohen's kappa, are employed. The analysis of the accuracy together with kappa for the PS data set demonstrates that SVM and RF achieve comparable median classification accuracy rate and Cohen's kappa values. All models are fitted by 'R' programs with the libraries and functions described in this study. [ABSTRACT FROM AUTHOR]

Open joints have significantly different effects on stress wave propagation across them as compared to closed and filled joints. In the present study, a theoretical model is developed based on the analysis of interaction process between stress waves and a single open joint. The analytical solutions to the energy transmission coefficient are mathematically derived for stress waves across an open joint. Parametric studies are conducted to evaluate the effects of various parameters on the energy transmission coefficient. It is found that the energy transmission coefficient follows a similar trend for all types of stress waves, but stress waves with different waveforms have different values. The energy transmission coefficient increases with the increase in the wave amplitude and duration, but decreases with the gap width. It also increases at the outset, and then decreases gradually with the increase in the incident angle for rectangle, symmetric triangle and ascending triangle waves, but it decreases with the incident angle for sine and descending triangle waves. Furthermore, different-shaped stress waves have different critical gap widths for wave transmission. The optimal incident angle and the critical gap width increase as the wave duration increases, but the critical stress amplitude decreases. [ABSTRACT FROM AUTHOR]

The objective of this study is to characterize the physicochemical properties of groundwater in the Sanshandao gold deposit, analyze the laws of motion and evolution of the groundwater system, and provide evidence for the design of undersea mining and safety warnings, using combined fuzzy c-means (FCM) and principal component analysis (PCA) methods. Thirteen physicochemical indicators and two isotopic indicators were surveyed from 100 fissure water samples and 13 typical water samples from Xinli, a deposit of the Sanshandao gold mine in China. First, PCA was used to extract four PCs (with explained variances 48, 25, 10 and 7 %) and the characteristics of the hydrochemistry indicators of each PC were examined. The four PCs are identified as the processes of concentration and dilution, sericite rock orientation and carbonatization, potash feldspathization, and dissolution of carbon dioxide. Next, using the four PCs developed from PCA, 100 fissure water samples were grouped into four spatially continuous clusters using the FCM cluster method. With the aid of graphic technology, the four fuzzy clusters were determined as deep saline water, shallow saline water, bedrock brine and mixed wastewater, following careful analysis of the hydrochemical characteristics of each cluster combined with a related hydrogeological report. Finally, further analysis of the PCA and FCM results determined factors constituting the groundwater system, including sources of water, flow path, mixing zone, local fissure zone, and impact of mining on the groundwater system. This study shows that the combination of PCA and FCM is important for understanding hydrochemical characteristics of a groundwater system. [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]