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Deep caverns are frequently subjected to transient loading, resulting in different failure characteristics in the surrounding rock compared to those in shallow caverns. Previous research has rarely focused on the transient responses of non-circular caverns. To address this gap, a theoretical solution for the dynamic stress concentration factor around a U-shaped cavern under transient stress waves was derived based on elasto-dynamic theory and conformal mapping. The theoretical results were validated through simulations using the discrete element software PFC2D 7.0 (Particle Flow Code in two dimensions). Additionally, the energy evolution and failure pattern of the surrounding rock under coupled static–dynamic loading were investigated. The results indicated that, when the stress wave was horizontally incident, rockburst failure was more likely to be observed in the cavern floor, while dynamic tensile failure was prone to occur in the incident sidewall. Furthermore, when the incident direction of the stress wave aligned with the maximum principal stress, more violent rockburst occurred. Moreover, when the rising time of the stress wave was greater than 6.0 ms, the peak dynamic stress concentration factor converged to a stable value, and the surrounding rock could be considered to be in a quasi-static loading state. These findings provide insight into the failure mechanisms of deep caverns and could guide the design of cavern supporting structures. [ABSTRACT FROM AUTHOR]

In deep rock engineering, caverns are often disturbed by engineering loads from different directions. To investigate the dynamic response of deep U-shaped caverns under different incident orientations, a theoretical solution of the dynamic stress concentration factor along the cavern boundary was derived based on the wave function expansion and conformal mapping method, and the failure characteristics around the cavern were further investigated by PFC2D (Particle Flow Code in two dimensions). As the incident orientation increases from 0° to 90°, the dynamic compressive stress concentration area transforms from both the roof and the floor to the sidewalls, and the peak dynamic stress concentration factor of the roof decreases from 2.98 to −0.20. The failure of the floor converts from dynamic compression shear failure to dynamic tensile failure. Compared to a stress wave incident from the curved boundary, a stress wave incident from the flat boundary causes severer damage. When the stress wave is incident from the sidewall, the cavern with a larger height-to-width (h/w) ratio exhibits severer damage. Conversely, the cavern with a smaller h/w ratio tends to fail as the stress wave is incident from the floor. This paper provides a basic understanding of dynamic responses of the deep U-shaped cavern. [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]

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]

Micro cracks inside rocks can open or close under variable external forces, which significantly affects the rock modulus and wave velocity. However, conventional simulations typically ignore or homogenize micro cracks primarily because of statistical intricacy and implementation difficulties. A micro-cracked rock model was proposed—characterized by the inclusion of unbonded contacts with an initial gap, based on the flat-joint contact model—to study the mechanisms of micro-cracked rocks and rock masses. The relationship between the static modulus and crack parameters in the proposed model was estimated based on the energy additive theory. Additionally, the macromechanical properties affected by the crack intensity and width were investigated using uniaxial compression, wave velocity measurements, Brazilian splitting, and direct shear simulations. Based on the simulated results, a new calibration procedure was formulated to efficiently calibrate the crack-closure stage under uniaxial compression. Furthermore, the relationships between the P-wave velocity, cracks, and biaxial stresses were investigated. The results indicate that existing micro cracks can transition between open and closed states under variable biaxial-stress conditions, influencing wave propagation and resulting in anisotropy. The performance observed during the transient unloading of a tunnel validated the capability of the proposed method for dynamic analysis in jointed-rock-mass engineering. [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]

Near-infrared (NIR) luminescent materials have unparalleled advantages in certain fields, such as anti-counterfeiting. Nd3+ ion doped Gd 2 O 3 near-infrared phosphor was successfully prepared by co-precipitation method and high-temperature solid-phase reaction. The structure and morphology of Gd 2 O 3 :Nd3+ cubic phase phosphors were studied in detail. The prepared phosphor was optically analyzed by fluorescence spectrometer. The research results show that with the increase of the Nd3+ doping concentration, the luminous intensity of the near-infrared phosphor in the near-infrared region shows a trend of first increasing and then decreasing. When the Nd3+ ion doping concentration is 1.0 mol%, the obtained phosphors have the best luminous intensity in the near-infrared region under the excitation of 808 nm laser. Gd 2 O 3 :Nd3+ phosphor emits near-infrared light due to the intra-4 f transitions of Nd3+ ions. The concentration quenching theory is used to prove that the concentration quenching of Gd 2 O 3 :Nd3+ belongs to the cross relaxation caused by electric multipole. Due to the unique optical properties of this near-infrared phosphor, it is expected to be further developed in the field of fluorescent materials. [ABSTRACT FROM AUTHOR]

• Tangential stress distribution around U-shaped caverns were calculated. • Failure process around the U-shaped cavern was simulated by PFC2D. • Height-to-width ratio and principal stress direction effect failure characteristic. • Failure mechanism of caverns with different height-to-width ratios were discussed. Understanding the stress distribution and failure characteristics around a U-shaped cavern is vital to the stability analysis of rock structures such as tunnels and roadways. In this study, the stress distribution around a U-shaped cavern with different height-to-width (h/w) ratios under biaxial compression was first analyzed based on the complex variable theory. Then, numerical simulations were conducted using PFC2D to study the failure process and energy evolution around a U-shaped cavern with different h/w ratios. Theoretical solution shows that the maximum stress concentration factor occurs at the bottom corner of the U-shaped cavern. With the h/w ratio increasing from 0.75 to 3.0, the stress concentration factor around the sidewall shows a declining trend, while the opposite trend is observed in the vault and floor. Numerical results show that when the maximum principal stress is parallel to the vertical direction, the thin slab buckling occurs in the sidewall of the cavern with low h/w ratios (h/w = 0.75 and 1.0), accompanied by the release of relatively high kinetic energy. Instead, the wedged-shaped slab spalling occurs in the sidewall of the cavern with high h/w ratios (h/w = 1.5, 2.0 and 3.0), which can be considered as a static process. Although the U-shaped cavern with low h/w ratios has higher integrity after failure, more stain energy is accumulated in the remaining surrounding rock, which is likely to cause rockbursts under a higher stress state or dynamic disturbance. When the maximum principal stress is horizontal, the rock slab separates from the cavern floor and presents a reverse trend with the increasing h/w ratio. The above results indicate that the h/w ratio and the direction of maximum principal stress should be considered comprehensively in the design of the support system of U-shaped caverns. [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]

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]

Oil-based drilling fluids are widely used in deep drilling, and their cooling effect on shale reservoirs is critical for wellbore stability. To reveal the sudden cooling effect of oil-based drilling fluid on the dynamic compressive behavior of deep shale reservoirs, oil-based fluid cooling experiments were conducted on heated Longmaxi shale samples (25–200 °C) with five bedding orientations, followed by a series of dynamic compression tests using same incident energy. Anisotropic macroscopic thermal fractures occurred in the samples at 200 °C after oil-based fluid cooling, and the imbibed oil-based fluid entirely covered the bedding surfaces during cooling. The post-peak behavior of the dynamic stress-strain relationships of samples illustrated bedding- and temperature-dependent characteristics. Dynamic strength of the samples decreased with increasing temperature for each bedding orientation, and the dynamic strength versus bedding orientation showed a V-shaped curve at all temperatures. Anisotropic fragment degree of the samples was classified into three groups using debris screening analysis. Further, the dynamic strength reduction responses and the possible nature of dynamic strength anisotropy in shale reservoirs after oil-based drilling fluid cooling were discussed. This study helps to improve the oil-based drilling fluid application in shale reservoirs and understand the behavior of deep reservoirs during drilling. • Oil-based fluid cooling experiments were conducted on heated Longmaxi shale samples at different temperatures. • Flash point of base oil should be essential to determine thermal damage of shale reservoir after oil-based drilling fluid cooling. • Post-peak dynamic stress-strain behavior of samples after oil-based fluid cooling is bedding-dependent and temperature-dependent. • Strength magnitude response should result from multifactor competition and strength anisotropy is a nature of stress wave propagation. [ABSTRACT FROM AUTHOR]

Highlights • Effects of mixing time on the backfill process were investigated. • Mixing time affects the fluidity of backfill slurry. • Pore structure of hardened backfill is influenced by mixing time. • Analysis of the relationship between backfill slurry and hardened structure. • The optimum mixing time results in the highest mechanical strength of backfill. Abstract Mixing time is one of the operational parameters that can affect the rheological and mechanical behaviors of cemented backfill. The objective of this study was to investigate the effect of mixing time on the cemented backfill process. The backfill mixture consisted of a solid waste phosphogypsum (PG), a hydraulic binder, and water. The fresh slurry of backfill was sampled at seven different mixing times (ranging from 5 to 240 min), and its physical properties including apparent viscosity, electrical conductivity, bulk density, air content, bleeding rate, and setting time were measured. The slurry samples were then filled into molds to prepare hardened backfill samples. The pore structure, microstructure, and unconfined compressive strength (UCS) of the hardened backfill samples were determined at different curing ages. The results indicate that mixing time significantly affects the complete process of backfill, from the fresh slurry to the hardened structure. The optimum mixing time was found to be 60 min, under which the backfill slurry exhibited the best properties, including the lowest apparent viscosity, which enabled easier transport. Furthermore, the optimal slurry also showed the lowest air content and the highest bleeding rate, both of which resulted in a low porosity and consequently a high mechanical strength of the hardened backfill structure. On the other hand, outside of the optimum mixing time, either insufficient mixing or overmixing may lower both the fluidity of the slurry and the strength of the hardened backfill. This study therefore demonstrates that the mixing time is an important factor in the design of an easy-delivery and high-strength backfill system. [ABSTRACT FROM AUTHOR]

Excavation unloading is a primary stress condition for engineering rock mass in deep underground. Based on the unloading stress condition during the excavation operation, this paper employed a distinct element method (DEM) to simulate the unloading responses of intact and preflawed rock specimens. The simulation results revealed that the unloading failure strength, unloading damage thresholds, and cracking characteristics were largely dependent on the inclination angle α of the preflaws. With the increase in the flaw inclination angle, the unloading failure strength of a preflawed specimen exhibited a sigmoidal curve increasing trend, and it decreased by 5.5%-20% compared to the unloading failure strength of an intact specimen. Based on the crack accumulation in specimens, three damage thresholds were identified under unloading condition and two damage thresholds σci and σdi were found to be increased with the increase in the flaw inclination angle. Furthermore, when the flaw inclination angle was smaller, cracks were initiated around the preflaws, and there were obvious axial splitting cracks in the failure modes of preflawed specimens, while axial splitting cracks were few in the preflawed specimen with a larger flaw inclination angle and none in the intact specimen. These unloading responses indicate that inducing preflaws can reliably reduce the unloading failure strength and promote the cracking process of hard rock during an excavation unloading process. Moreover, inducing preflaws with a smaller inclination angle (e.g., vertical to the unloading direction) will be more helpful for the unloading failure and rock cracking during an excavation unloading process. [ABSTRACT FROM AUTHOR]

Abstract Thermally stable Li(La, Gd)MgWO 6 :Eu3+ phosphors with high quantum efficiency were obtained by the sol-gel method. The symmetry of this double perovskite was reduced by doping Gd3+ ions in order to enhance the luminescence properties. In addition, phase structure and luminescence properties were characterized. A second-order John-Teller (SOJT) distortion of WO 6 octahedron existed in this double perovskite and was investigated in detail for the first time. The WO 6 octahedron of LiLa 0.95- x Gd x Eu 0.05 MgWO 6 distorted gradually and the SOJT distortion started to disappear while x ≥ 0.7. Intensities of 5D 0 -7F 2 transition were greatly enhanced and reached the maximum at x = 0.5 under the excitation of 465 nm. The LiLa 0.45 Gd 0.5 Eu 0.05 MgWO 6 phosphor showed excellent thermal stability and the thermal quenching mechanism was discussed by the configurational coordinate diagram. The optimized LiGd 0.5 Eu 0.5 MgWO 6 showed high quantum efficiency, which is much higher than commercial Y 2 O 2 S:Eu3+ and Y 2 O 3 :Eu3+ phosphors. The synthesized phosphors emit intense red light under the near ultraviolet and have great potential in light emitting diode. Graphical abstract Image 1 Highlights • Second-order John-Teller distortion in the double perovskite. • Monoclinic double perovskite with double orderings. • Thermally stable phosphors with high quantum efficiency. [ABSTRACT FROM AUTHOR]

Highlights • Simulation of tunnel failure induced by structural plane is used by FDEM approach. • Rock heterogeneity and excavation unloading process was fully considered. • Dip angle, location, frictional coefficient and lateral coefficient were studied. • The crack propagation and failure intensity is related to selected parameters. • Slabbing failure and shear slip failure may interact and promote mutually. Abstract Construction of tunnels is often associated with fault or structural features that could affect tunnel stability during the construction phase and service life. Therefore, failure characteristics of hard rock around circular tunnels induced by pre-existing structural features under high in-situ stresses have been the subject of various studies. In the current study, a combined finite element approach, namely ELFEN, has been used for better reflection of entire failure process (including crack initiation, propagation and coalescence) and intrinsic properties of hard rock mass, thus rock heterogeneity, around circular tunnels during excavation unloading process. Parametric analysis which consider the dip angle, location (exposure or not), frictional coefficient of structural planes and lateral pressure coefficient was conducted in detail to reveal the mechanical responses of circular tunnel induced by structural plane under unloading condition. Numerical results indicate that the failure intensity of rock tunnel is a function of both dip angles and frictional coefficients of structural planes. The most critical dip angles of structural planes leading to failure in tunnel rocks largely depend on the frictional coefficient. Also, the results indicate that the released strain energy for the case of exposed structural plane is higher than those not intersecting the tunnel, leading to more violent rock failure for the former. With the increase of the lateral pressure coefficient, the failure intensity and damage extent around the tunnel is aggravated, especially for the roof and floor of the tunnel. Rock failure can be categorized as slabbing failure near excavation boundary and shear slip failure, controlled by structural plane. Progressive slabbing failure induced by excavation unloading may activate internal structural planes and the extensive release of energy caused by shear and slip failure may in turn further induce the slabbing failure. Rockburst is more prone to be triggered under such condition. [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]

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]

Purpose The purpose of this paper is to examine the effect of application of a heat pipe in an aspect of hydrostatic thrust bearings on thermal balance and deformation and the role of this application in increasing the rotating speed of a workbench.Design/methodology/approach Numerical simulations of oil film temperature field, the temperature field and thermal deformation of the bearing’s workbench and base were performed by finite element analysis (FEA) software for both the traditional hydrostatic thrust bearings and the heat pipe ones.Findings Oil pad and workbench of the hydrostatic thrust bearings are fabricated with a heat pipe cooling structure, which can take away most of the heat generated by shearing of the oil film, control the temperature rise and thermal deformation of the hydrostatic thrust bearing effectively, avoid the dry friction phenomenon and finally improve the processing quality of equipment.Originality/value The heat pipe hydrostatic thrust bearings could control the temperature rise and thermal deformation of the hydrostatic thrust bearing effectively, avoid the dry friction phenomenon and improve the processing quality of equipment. [ABSTRACT FROM AUTHOR]

Abstract The evaluation of rockburst is becoming increasingly important as mining activities reach greater depths below the ground surface. In the literature, rockburst assessment has been tackled by many researchers with various methods. However, there has not been a study that examines and compares different rockburst assessment methods. In this paper, rockburst classification and its varying definitions are briefly summarized. A comprehensive review of the research efforts since 1965 then follows. This includes empirical, numerical, statistical and intelligent classification methods. Of particular significance is that in all the above-mentioned techniques, the review highlights the source of data, timeline of study and the comparative performance of various techniques in terms of their prediction accuracy wherever available. The review also lists current achievements, limitations and some promising directions for future research. [ABSTRACT FROM AUTHOR]

This study aims to determine the influences of confining stresses and cutting parameters on hard rock fragmentation using a conical pick. Using a true triaxial testing apparatus, single/double pick forces and static/coupled static-dynamic pick forces were applied to granite rock specimens to break them confined by biaxial, uniaxial and no lateral stress conditions. The corresponding cuttabilities were estimated and compared by the peak pick force, insertion depth and disturbance duration at rock failure and the associated failure patterns. The results showed that excavation-induced unloading and cracking, which can change the biaxial confining stress conditions into the uniaxial and decrease uniaxial confining stress level, respectively, can significantly improve the hard rock cuttability. The experimental, theoretical and regressive results indicated that the hard rock cuttability initially decreases and then increases as the level of uniaxial confining stress increased. A moderate uniaxial confining stress instead improves the rock cuttability, but a high stress level may induce rock burst triggered by conical pick penetration. Therefore, only the hard rocks under stress-free and low uniaxial confining stress conditions can be easily fragmented with high safety and efficiency, as a complete splitting failure occurs. In addition, the hard rock cuttability can be also improved by the dynamic pick disturbance and the artificial defects such as excavation-induced fractures, pre-slits and boreholes in rock mass. [ABSTRACT FROM AUTHOR]

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

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]

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]

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]

Micro-seismic P-phase arrival picking is an elementary step into seismic event location, source mechanism analysis, and seismic tomography. However, a micro-seismic signal is often mixed with high frequency noises and power frequency noises (50 Hz), which could considerably reduce P-phase picking accuracy. To solve this problem, an Empirical Mode Decomposition (EMD)-cosine function denoising-based Akaike Information Criterion (AIC) picker (ECD-AIC picker) is proposed for picking the P-phase arrival time. Unlike traditional low pass filters which are ineffective when seismic data and noise bandwidths overlap, the EMD adaptively separates the seismic data and the noise into different Intrinsic Mode Functions (IMFs). Furthermore, the EMD-cosine function-based denoising retains the P-phase arrival amplitude and phase spectrum more reliably than any traditional low pass filter. The ECD-AIC picker was tested on 1938 sets of micro-seismic waveforms randomly selected from the Institute of Mine Seismology (IMS) database of the Chinese Yongshaba mine. The results have shown that the EMD-cosine function denoising can effectively estimate high frequency and power frequency noises and can be easily adapted to perform on signals with different shapes and forms. Qualitative and quantitative comparisons show that the combined ECD-AIC picker provides better picking results than both the ED-AIC picker and the AIC picker, and the comparisons also show more reliable source localization results when the ECD-AIC picker is applied, thus showing the potential of this combined P-phase picking technique. [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]

Microseismic sensing taking advantage of sensors can remotely monitor seismic activities and evaluate seismic hazard. Compared with experts' seismic event clusters, clustering algorithms are more objective, and they can handle many seismic events. Many methods have been proposed for seismic event clustering and the K-means clustering technique has become the most famous one. However, K-means can be affected by noise events (large location error events) and initial cluster centers. In this paper, a data field-based K-means clustering methodology is proposed for seismicity analysis. The application of synthetic data and real seismic data have shown its effectiveness in removing noise events as well as finding good initial cluster centers. Furthermore, we introduced the time parameter into the K-means clustering process and applied it to seismic events obtained from the Chinese Yongshaba mine. The results show that the time-event location distance and data field-based K-means clustering can divide seismic events by both space and time, which provides a new insight for seismicity analysis compared with event location distance and data field-based K-means clustering. The Krzanowski-Lai (KL) index obtains a maximum value when the number of clusters is five: the energy index (EI) shows that clusters C1, C3 and C5 have very critical periods. In conclusion, the time-event location distance, and the data field-based K-means clustering can provide an effective methodology for seismicity analysis and hazard assessment. In addition, further study can be done by considering time-event location-magnitude distances. [ABSTRACT FROM AUTHOR]

Eu 3+ -activated BaLaMgNbO 6 red-emitting phosphors were synthesized by a high-temperature solid-state reaction method. Phase analysis and luminescence were characterized by X-ray diffraction (XRD) and photoluminescence excitation and emission spectra. The XRD patterns showed that BaLaMgNbO 6 had a monoclinic structure with space group P 21/ n . The excitation spectra consisted of a broad charge-transfer band and some sharp f - f absorption peaks characteristic of Eu 3+ . The intensity ratio of I 615 /I 590 was used to detect the chemical environment of Eu 3+ . The chromaticity coordinates of BaLa 0.7 Eu 0.3 MgNbO 6 were (0.67, 0.33), indicating that the BaLaMgNbO 6 :Eu 3+ phosphors were excellent red-emitting phosphors. Under excitation by near-ultraviolet (UV) and blue light, the phosphor not only exhibited intense red emission but also showed high color quality. The Ozawa and Dexter energy-transfer theories were employed to calculate the theoretical quenching concentration and determine the concentration quenching mechanism. In addition, the activation energy of BaLa 0.7 Eu 0.3 MgNbO 6 was calculated through the Arrhenius equation. A configurational coordinate diagram was used to explain the thermal quenching mechanism. [ABSTRACT FROM AUTHOR]

The fracture propagation and failure process in rock are subjects of great interest in rock mechanics. In the present study, an approach combining the finite element method (FEM) and the discrete element method (DEM), named ELFEN, is adopted to simulate and investigate the failure process of a typical hard rock (Carrara marble) under static and dynamic splitting ring tests. The FDEM software is firstly validated by simulating the test of a Brazilian disk under static loading. Then, the failure of circular ring specimens is numerically studied under static and dynamic loading, with the ratio of internal to external diameter (represented by λ ) of the specimen groups varying from 0.1 to 0.6, with increments of 0.1. Under static loading, with the increase in internal diameter, the failure mode is transformed from diametrical splitting to four-fan-shaped failure (when λ ≥ 0.4). Under dynamic loading, four-fan-shaped failure occurs from geometric axial symmetry to axial asymmetry, which means that, with increasing internal diameter, the tensile cracks along the horizontal diametrical direction gradually deviate toward the top loading platen. The peak load of the circular ring has a descending trend with the increase in internal diameter under both loading conditions. A rational load value (peak load or crack initiation load) is suggested to calculate the tensile strength of circular rock ring specimens under the splitting ring test. Finally, the numerical results are compared with previous experimental results. The good agreement between the two sets of results, in terms of the failure modes and variation trend of the indirect tensile strength with increasing internal diameters, verify the accuracy and applicability of the FDEM approach. [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]

[Display omitted] • The crack-closure stage was first implemented in discontinuous deformation analysis. • Tangible micro cracks are generated by updating the vertices of the blocks. • The initial modulus decreased with an increasing number of cracks. • The strain in crack-closure stage was approximately equal to the crack aspect ratio. • A function between the crack parameters and crack-closure stress was established. Micro-cracks, which are prevalent in crustal rock masses, can significantly affect the physical and mechanical properties of rocks. The closure of micro-cracks explains the initial non-linear stress–strain curves observed during compression in brittle rocks. In this study, the plasto-elastic deformation was implemented through crack closure in the discontinuous deformation analysis (DDA). Tangible micro-cracks were formed by updating the vertex positions as opposed to setting the virtual gap and width. These micro-cracks were arranged randomly, but still adhered to specific crack parameters (crack intensity, aspect ratio, and dip-angle distribution). The uniaxial compression simulation results verified that these micro-cracks could open and close under external stress and execute the crack-closure stage. The results indicated that the increase in crack intensity primarily decreased the ratio of the initial to elastic moduli, and the crack aspect ratio was approximately equal to the crack-closure strain. An inhomogeneous dip-angle distribution led to anisotropic plasto-elastic deformation owing to the different numbers of cracks perpendicular to the axial stress. According to previous theories and reasonable assumptions, a function between the crack parameters and crack-closure stress was first established. Finally, the experimental results for two specific rocks were reproduced using this method, which can be applied to research on rock properties or rock mass engineering influenced by crack closures. [ABSTRACT FROM AUTHOR]

Mining-induced fractures influence the stability of overburden and may provide pathways for transferring heat and mass in underground environment. A novel approach was proposed to quantify the separation and fracture evolution in the undermined overburden, consisting of (1) theoretical distribution models of the void ratios of fractures (VRFs), based on analytical evaluation of key strata subsidence, and (2) numerical modeling using the universal distinct element code and a sequence of image post-processing procedures. For a longwall mining panel, both theoretical calculations and numerical simulations indicated that VRF first increased rapidly, then gradually decreased, and finally stabilized to a minimum from the surrounding edges of disturbed strata to their centers, and gradually decreased from deep to shallow strata. The fractures exhibited a “fracture-rich arch” distribution type along a vertical section. Numerous fractures occurred around the arch feet near the perimeters of mined-out area and/or arch crown near the center of disturbed strata. The distributions of VRFs show the heights, shapes and damage intensities of fractured zones, and can also be used as porosity parameters to determine the permeabilities of mining-disturbed overburdens. Therefore, the VRF models can be used as a quantitative parameter to assess the extent of possible risk zones, e.g. for water inflow into the mine or escape of hazardous fluids to the surface. [ABSTRACT FROM AUTHOR]

The discrimination of microseismic events and quarry blasts has been examined in this paper. To do so, Principal Component Analysis (PCA) and Artificial Neural Networks (ANN) have been used. The procedure proposed has been tested on 22 seismic parameters of 1600 events. In this work, the PCA has been used to transform the original dataset into a new dataset of uncorrelated variables. The new dataset generated has been used as input for ANN and compared to Logistic Regression (LR), Bayes and Fisher classifiers, which classify microseismic events and quarry blasts. The results have shown that PCA is effective for rating variables and reducing data dimension. Furthermore, the classification result based on PCA has been better than those based Ref. [22] and without PCA methods. Moreover, the ANN classifier has obtained the best classification result. The Matthew's Correlation Coefficient (MCC) results of the PCA, Ref. [22] and without PCA based methods have reached 89.00%, 73.68% and 82.04%, respectively, thus showing the reliability and potential of the PCA based method. [ABSTRACT FROM AUTHOR]

Seismicity partitioning is an important step in geological structure interpretation and seismic hazard assessment. In this paper, seismic event location (X,Y,Z) and Euclidean distance were selected as the K-Means cluster, the Gaussian mixture model (GMM), and the self-organizing maps (SOM) input features and cluster determination measurement, respectively, and 1516 seismic events (M>-1.5) obtained from the Yongshaba mine (China) were chosen for the cluster analysis. In addition, a Silhouette and Krzanowski-Lai- (KL-) combined S-KL index was proposed to obtain the possible optimum cluster number and to compare the cluster methods. Results show that the K-Means cluster obtains the best cluster “quality” with higher S-KL indexes on the whole and meaningful clusters. Furthermore, the optimal number for detailed geological structure interpretation is confirmed as eleven clusters, and we found that two areas probably have faults or caves, and two faults may be falsely inferred by mine geologists. Seismic hazard assessment shows that C5 and C7 (K=11) have a high mean moment magnitude (mM) and C1, C2, C3, and C4 (K=11) have a relatively high mM, where special attention is needed when mining. In addition, C7 (K=11) is the most shear-related area with a mean S-wave to P-wave energy ratio (mEs/Ep) of 41.21. In conclusion, the K-Means cluster provides an effective way for mine seismicity partitioning, geological structure interpretation, and seismic hazard assessment. [ABSTRACT FROM AUTHOR]

The gradual accumulation of phosphogypsum (PG) has attracted worldwide attention because of its occupation of considerable surface land and serious environmental consequences. This work demonstrates the feasibility of PG-based cemented paste backfill (CPB) by considering its physical properties and environmental effects, thus providing an approach for significant disposal of PG in an economical way. Firstly, the mechanical properties of the PG-based CPB specimens were tested. The specimens were prepared with original PG, and the maximum compressive strength reached around 1.5 MPa with the phosphogypsum/cementitious agents ratio of 4:1 and the slurry concentration of 66% at 120 d of curing age. Then, the pollution potential of the PG before and after its inclusion into cementitious agents was investigated. The results showed that when PG was included into backfill samples, the release of P, F and metals dramatically decreased. Additionally, the results of five rounds of leaching tests with backfill samples having different curing ages suggested that the risk of diffusion of pollutants was much less than that from the leachates of PG. Synthetic precipitation leaching procedure (SPLP) results suggested that the metal contents in leachates were within the limits defined in the Chinese standard (DZ/T 0290-2015). Therefore, the PG-based CPB technology could provide an economical alternative for the consumption and reutilization of a significant amount of PG while alleviating its pollution potential. [ABSTRACT FROM AUTHOR]

Purpose The purpose of this study with the rapid development of the heavy/large mechanical equipment, the heavy computer numerical control (CNC) vertical lathe has become the ideal processing equipment for the parts of those mechanical equipments. The main factor which affects the machining quality and efficiency of heavy CNC vertical lathe is the mechanical properties of the hydrostatic thrust bearing.Design/methodology/approach This paper did the research based on the large size sector oil pad’s lubrication performance of the hydrostatic thrust bearing in the heavy/large equipments, establishing the lubrication performance distribution mathematical model of the velocity field, flow field, pressure field and so on, analyzing the bearing behavior of the large size sector oil pad.Findings The results show that the oil flow generated by the plate relative motion will be greater than that generated by the pressure difference in area B, with the rotational speed’s increasing of the hydrostatic thrust bearing, and the direction is opposite. The oil flow generated by the centrifugal force will be greater than that generated by the pressure difference in area C, with the rotational speed’s increasing of the hydrostatic thrust bearing, and the direction is opposite. When the rotational speed of the hydrostatic thrust bearing is too high, the friction heat will be not easy to be sent out. The bearing rotating speed should be lower than the comparatively smaller one of ω1 and ω2, which can help avoid the rise of too high temperature.Originality/value The research provides powerful theoretical foundation for practical application of the large size sector oil pad hydrostatic thrust bearing, its structure design and operating reliability, realizing the lubrication performance prediction of the large size hydrostatic thrust bearing. [ABSTRACT FROM AUTHOR]

The effect of a preexisting crack with different inclination angles and lengths on mechanical properties, fracture initiation, and propagation in a Brazilian disc was investigated in this paper. In the experiment, digital image correlation (DIC) method was employed to record the deformation in the specimen. Different failure patterns, depending on inclination angles and lengths of the preexisting crack, were observed. The fracture initiation position deviates from the tips of the preexisting crack with the inclination angle increasing from 0° to 72° at an interval of 18° per increment. Secondary cracks are more likely to occur in the Brazilian disc with a more inclined and longer preexisting crack. A finite-discrete element combined method ELFEN was used in the numerical investigation to simulate the failure process. This investigation shows that there are good correlations between the experimental and numerical results. Digital image correlation is a good method to obtain the quantitive full-field strain distribution and to observe the strain evolution process in a jointed rock. [ABSTRACT FROM AUTHOR]

The rock-shotcrete interface has become common in underground tunnels following the widespread application of shotcrete as a support element. The static and dynamic tensile properties of the rock-shotcrete interface were measured using Instron 1342 and split Hopkinson pressure bar (SHPB) apparatus. The effect of loading rate and interface geometry on interface tensile behavior was studied using four levels of loading rate and six types of interface geometries. The results demonstrate that the interface has typical loading rate dependency, which means that bearing capacity of the interface increases with an increase in the loading rate. The bearing capacity and failure modes of the interface are sensitive to the interface geometry. The measured peak load exhibits an early increase and later decreasing trend; there is a critical value (1.2 mm and 1.8 mm under static and dynamic load) in geometry parameter “h” as the dominant failure mode changes from adhesion to cohesion. An interface with greater “h” leads to low bearing capacity. Therefore, the rock surface should be a reasonable roughness during rock surface preparation, and an appropriate depth of sawteeth is crucial to obtain reliable support strength in tunneling engineering. [ABSTRACT FROM AUTHOR]

In the present study, a combined finite/element approach is used to analyze failure characteristics of a rectangular hard rock with different sample height-to-width ratios based on the uniaxial compression test. The objective of this paper is to illustrate the transitional mechanisms from shear fracture to extension failure of brittle rocks and the occurrence conditions of surface-parallel slabbing. Numerical results show that failure modes have experienced the shear, tension with shear and surface-parallel slabbing failure with the decrease of sample height-to-width ratios. It is seen from the simulation results that the shear band in larger height-to-width ratios is composed of an amount of extension meso-cracks that are parallel to the loading direction, and the slabbing failure tends to be the result of the accessible propagation of extension meso-cracks due to short sample height, which validate the essence of extension failure in hard rocks. The good agreement between simulation and laboratory testing also shows the practicability and advancement of a combined finite/discrete element approach. [ABSTRACT FROM AUTHOR]

Seismic P phase arrival picking of weak events is a difficult problem in seismology. The algorithm proposed in this research is based on Empirical Mode Decomposition (EMD) and on the Akaike Information Criterion (AIC) picker. It has been called the EMD-AIC picker. The EMD is a self-adaptive signal decomposition method that not only improves Signal to Noise Ratio (SNR) but also retains P phase arrival information. Then, P phase arrival picking has been determined by applying the AIC picker to the selected main Intrinsic Mode Functions (IMFs). The performance of the EMD-AIC picker has been evaluated on the basis of 1938 micro-seismic signals from the Yongshaba mine (China). The P phases identified by this algorithm have been compared with manual pickings. The evaluation results confirm that the EMD-AIC pickings are highly accurate for the majority of the micro-seismograms. Moreover, the pickings are independent of the kind of noise. Finally, the results obtained by this algorithm have been compared to the wavelet based Discrete Wavelet Transform (DWT)-AIC pickings. This comparison has demonstrated that the EMD-AIC picking method has a better picking accuracy than the DWT-AIC picking method, thus showing this method's reliability and potential. [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 testing method of spalling strength at different static pre-confining pressure is proposed in this paper. Using a modified split Hopkinson bar facility, a static pre-confining pressure was loaded before dynamic loading. The pull-back method is used to calculate the spalling strength and the free surface velocities of the specimen were measured by a laser detector system. The experimental results indicate that the spalling strength is related to the static pre-confining pressures. When the impact loading and rate effect are almost the same, the results demonstrated that the spalling strength decreases with an increase in the confining pressure. [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]

Eu 3+ ions activated NaLaMgWO 6 phosphors were successfully synthesized by an improved sol-gel method using citric acid and polyethylene glycol as complexing agents. Crystal structure and doping site were investigated by XRD, Rietveld refinement and Raman spectra. The phosphors had monoclinic double perovskite structure with space group C 2/ m , as well as layered ordering of A-site and rock-salt ordering of B-site. The blueshift of Raman T 2g (1) mode manifested Eu 3+ ions had entered into A-site. Thereafter, luminescence properties, such as excitation and emission spectra, CIE coordinates, concentration quenching and thermal quenching were discussed. The quenching concentration for hypersensitive electric dipole transition of Eu 3+ reached up to 50.0 mol%. The delayed concentration quenching was observed in NaLaMgWO 6 : Eu 3+ phosphor. The theoretical quenching concentration was obtained based on L. Ozawa's theory, and the quenching mechanism on Dexter's theory. Excellent thermal stability of this phosphor shows that it is a potential red phosphor for solid state lighting. [ABSTRACT FROM AUTHOR]