Search

The development of eco-friendly and efficient alternative binder to conventional Portland cement is a challenging issue that deserves to be paid more attention. The mineral additive-modified magnesium potassium phosphate cement mixture was introduced in sediment solidification recycled as alternative roadbed material. The feasibility of mineral additive-MKPC blend in solidifying sediment and its mechanical performance were probed into deeply through unconfined compressive strength, durability, microstructural and mineralogical tests. The results demonstrated that it existed an optimal mineral additive content where the peak strength was achieved, and an excessive amount caused the strength degradation. The unconfined compressive strength of solidified sediment increased with curing age and exhibited a growing potential. Struvite-k crystals were identified as the major cementing phase in mineral additive-MKPC solidified sediment. C-S-H gels and Ca2P2O7 ċ 2H2O were recognized as secondary phases for fly ash-MKPC blend and MgSiO3 for silica fume-MKPC blend. As the molar ratio of M/P and curing age grew, the total porosity was reduced due to the pore filling of mineral grains and densification of hydrated phases. The strength of solidified sediment after durability tests showed a tendency of first decline and then increase with durability period. Overall, the superior benefits including excellent strength performance, strong resistance to external environmental damage and highly densified microstructure could be expected for mineral additive-MKPC blend in sediment solidification as a promising roadbed filling. [ABSTRACT FROM AUTHOR]

To study the damage and acoustic emission (AE) energy evolution of basalt under high temperature, a series of tests under uniaxial stress are carried out on basalt heated at 25 °C, 200 °C, 400 °C, 600 °C, 800 °C and 1000 °C. Through the comprehensive analysis of AE parameters such as absolute energy and amplitude, the AE characteristics and damage evolution laws of basalt were explored. The results show that high temperature has a significant effect on the characteristics of rock stress–strain curve. With the increase of temperature, the compaction stage of the curve becomes longer, the plasticity increases, and the peak strength of basalt decreases. The higher the heat treatment temperature, the faster the strength decreases, and the strength decreases by 63.91% at 1000 °C. The AE localization points converge from the dispersed state to the aggregated state. With the increase of temperature, the period of convergent nucleation of the AE localization sites is advanced, high amplitude AE signals shifted from a concentrated distribution near the peak stress to a discrete distribution throughout the entire process. The heat treatment of the rock has an additive effect on the burstiness. 600–800 °C can be used as the threshold temperature of basalt to transform from brittle failure to plastic failure. The high temperature effect is responsible for the transition of the AE cumulative absolute energy at 400–600 °C. The influence of high temperature on damage evolution is discussed. The initial damage increases exponentially after 800 °C. With the increase of temperature, the evolution rate of damage variable becomes faster and gradually changes from linear to nonlinear. [ABSTRACT FROM AUTHOR]

Reinforced concrete structures play a pivotal role in island and reef engineering projects. Given the resource constraints typical of island regions, substituting traditional manufactured sand aggregate with reef limestone not only reduces reliance on river sand but also addresses the issue of disposing of waste reef limestone slag generated during excavation. However, the performance characteristics of reef limestone concrete, particularly its bond strength with reinforcing steel, warrant further investigation. This is particularly true for the bond–slip behavior of the reinforcement. This study aims to elucidate the effects of various parameters on the bond performance between steel and reef limestone concrete through central pullout tests. These parameters include the type and diameter of the reinforcement, bond length, and loading rate. The investigation encompasses the analysis of load–slip curves, bond failure modes, and variations in bond stress. Additionally, using the Abaqus software, a numerical simulation was conducted to analyze the mesoscopic stress characteristics, thereby revealing the mechanisms of bond formation and failure modes between steel reinforcement and reef limestone concrete. The results indicate that the bond–slip curve for reef limestone concrete reinforced with ribbed rebars and Glass Fiber-Reinforced Polymer (GFRP) rebars can be broadly categorized into four phases: minor slip, slip, decline, and residual, with the residual phase exhibiting a wave-like pattern. The predominant failure modes in reef limestone concrete are either pulling out or splitting. The bond stress in reef limestone concrete decreases with an increase in rebar diameter and bond length; conversely, it increases with the loading rate, although the ultimate slip decreases. The mesoscopic failure characteristics of reinforced reef limestone concrete, as simulated in Abaqus, are consistent with the experimental outcomes. [ABSTRACT FROM AUTHOR]

Nuclear power is a high-quality clean energy source, but nuclear waste is generated during operation. The waste continuously releases heat during disposal, increasing the adjoining rock temperature and affecting the safety of the disposal site. Basalt is widely considered a commonly used rock type in the repository. This study of basalt's mechanical characteristics and damage evolution after thermal damage, with its far-reaching engineering value, was conducted by combining experimental work and theory. Uniaxial compression tests were conducted on basalt exposed to 25 °C, 500 °C, 700 °C, 900 °C, and 1100 °C conditions, and acoustic emission (AE) equipment was utilized to observe the acoustic emission phenomenon during deformation. This study was carried out to examine the mechanical characteristics, the sound emission features, the progression of damage laws, and the stress–strain framework of basalt after exposure to different types of thermal harm. As the temperature rises, the rock's maximum strength declines steadily, the peak strain rises in tandem, the rock sample's ductility is augmented, the failure mode changes from shear to tensile failure, and cracks in the failure area are observed. At room temperature, the acoustic emission signal is more vigorous than in the initial stage of rock sample loading due to thermal damage; however, after the linear elastic stage is entered, its activity is lessened. In cases where the rock approaches collapse, there is a significant surge in acoustic emission activity, leading to the peak frequency of acoustic emission ringing. The cumulative ring count of acoustic emission serves as the basis for the definition of the damage variable. At room temperature, the damage evolution of rock samples can be broken down into four distinct stages. This defined damage variable is more reflective of the entire failure process. After exposure to high temperatures, the initial damage of the rock sample becomes more extensive, and the damage variable tends to be stable with strain evolution. The stress–strain constitutive model of basalt deformation is derived based on the crack axial strain law and acoustic emission parameters. A powerful relationship between theoretical and experimental curves is evident. [ABSTRACT FROM AUTHOR]

To explore the mechanical properties and damage characteristics of basalt under high temperature and high pressure, triaxial compression tests are conducted on thermal damage rock samples, and the evolution process from progressive damage to macroscopic failure of rock is tracked and quantified by CT image reconstruction and acoustic emission technology. The results show that: (1) the ability of basalt to resist load and deformation changes from slight enhancement to rapid deterioration with the increase of temperature, and 600 ℃ is its threshold. The failure mode of rock gradually changes from brittleness to plasticity with the increase of temperature, and its post-peak stress gradually presents certain plastic flow characteristics. (2) With the increase of temperature, the initial damage variable D0 of basalt presents a stage feature of changing from low-speed development to rapid growth. When the temperature exceeds 400 ℃, the total damage variable D will have obvious precursor characteristic of small increase before the drastic change, and this characteristic will become more obvious with the increase of temperature. (3) The main mineral types of basalt hardly change with the increasing temperature, while the temperature has a certain influence on the proportion of some components. The proportion of anorthite and enstatite shows the changing trend of first decreasing and then increasing, while the evolution of andesine is opposite, and the proportion of other mineral components is basically unchanged. (4) The main reason for the improvement of mechanical properties of basalt within the temperature threshold is that the expansion of mineral particles and the constraint of confining pressure promote the closure of internal primary cracks and increase the compactness. However, when the heat treatment temperature of basalt exceeds the threshold value, the structural deterioration caused by heat treatment is gradually prominent, and the defects generated play a leading role in the mechanical properties of rock. Highlights: The mechanical properties of thermal damage basalt under triaxial loading were clarified and its temperature threshold was obtained The evolution process from progressive damage to macroscopic failure of basalt was tracked and quantified The microscopic failure mechanism of thermal damage basalt was revealed The number of tensile cracks of saturated coal increased with the cyclic loading frequency due to the prominent lag effect of strain behind stress. [ABSTRACT FROM AUTHOR]

To explore the thermal damage deterioration characteristics of basalt, the evolution of physical parameters, mechanical properties and failure modes was investigated. Based on computed tomography image reconstruction techniques, the spatial distribution and morphological characteristics of the pores of basalt were explored. The results indicate that thermal damage leads to the phase transition of basalt mineral grains and uncoordinated expansion and deformation, increasing the thermal deterioration of rock specimens. The temperature of 800 °C is the threshold for rapidly deteriorated physical properties of basalt, which has deformation characterized by the transition from ductility to brittleness. With the increase in temperature, basalt specimens transit from shear failure to tensile-shear combined failure, and then to tensile splitting failure. Meanwhile, irregular block-shaped collapse is transformed to strip-shaped rock fragments spalling. The crack width based on CT technology and three-dimensional (3D) image reconstruction of the crack volume can quantify the structural deterioration characteristics of basalt induced by thermal damage. When the temperature increases: 25 °C → 600 °C → 1000 °C, the corresponding porosity of the rock changes from 6.86% → 7.04% → 18.02%, exhibiting an evolution from low-speed development to high-speed growth. The thermal damage sensitivity of different lithologies at high temperatures differs, among which, the thermal damage sensitivity of marble is the highest, followed by granite and then basalt, the sensitivity of sandstone is the lowest. [ABSTRACT FROM AUTHOR]

Strainburst often occurs in the loading process of tangential stress concentration from surrounding rocks after excavation and unloading of deep rock mass. The concentrated tangential stress is relatively larger on the tunnel walls, which decreases towards the interior of surrounding rocks with a certain gradient. To explore the fracture evolution characteristics of surrounding rocks under different tangential gradient stresses and understand various phenomena in the strainburst process, a large‐scale true‐triaxial gradient and hydraulic‐pneumatic composite loading rockburst test device was adopted to carry out the strainburst tests under four different gradient stresses. Based on the macroscopic failure phenomena, distribution of rockburst debris, macro‐micro morphology of failure cross section, and monitored data of acoustic emission (AE), the fracture evolution of rockburst specimens was analyzed under different stress gradient loadings. The results indicate that the stress gradient has an obvious influence on the fracture characteristics, resulting in different rockburst phenomena. With the increase of stress gradient, the failure stress of rockburst decreases, while the dynamic failure characteristics of rockburst trends to be significant. The increase of stress gradient contributes to higher fragmentation degree of debris, more uniform distribution and better continuity. The total number of cracks in the model decreases with the rise of the stress gradient, and the proportion of shear failure increases, which is the main reason for the enhancement of the rockburst intensity. [ABSTRACT FROM AUTHOR]

Rockburst is a safety problem that needs to be solved urgently in deep rock and mineral exploitation. The stress gradient is an important factor affecting rockburst. To explore the influence mechanism of stress gradient on rockburst, the loading and unloading tests of rockburst physical model under different stress gradients are carried out using the self-developed gas–liquid composite rockburst test device. Based on the strain test results of rockburst model, the spatial distribution characteristics of elastic energy are analyzed. The particle size distribution characteristics and throwing kinetic energy of debris after the rockburst of model are studied. The following conclusions are obtained: (1) the value of the stress gradient coefficient m reflected by the strain within the range of the model loading rockburst gradient 1 affects the temporal and spatial distribution of energy release process; (2) With the increase of the vertical loading stress gradient, the rockburst debris gradually changes from large plate debris to small flake, block and powder debris. The maximum ejection distance of rockburst debris gradually increases, and the dynamic failure phenomenon of rockburst becomes more and more obvious; (3) As the loading stress gradient increases, the throwing kinetic energy of rockburst debris per unit mass shows an increasing trend. The research results are of great significance to tunnel safety construction and rockburst early warning. Highlights: The stress gradient is an important factor affecting rockburst in deep rock mass engineering. The value of the stress gradient coefficient affects the temporal and spatial distribution of energy release process. As the loading stress gradient increases, the rockburst debris gradually changes from large plate debris to small flake, block and powder debris. With the increase of the stress gradient, the maximum ejection distance of debris gradually increases, and the dynamic failure tends to be severe. [ABSTRACT FROM AUTHOR]

To study the effects of different stress gradient on rockburst hazards, a novel experimental technique was proposed and a series of tests under different stress gradient paths were conducted on specimens (400 mm × 600 mm × 1000 mm) by using a true-triaxial gradient and hydraulic-pneumatic combined test device. The damage phenomena, force characteristics and acoustic emission (AE) signals of the specimen during rockbursts were systematically analyzed. The experimental results indicated that different stress gradient distributions not only affect the failure mechanism of the specimen, but also affect the energy conversion characteristics. With the stress gradient increasing from 0 to 6, the index r decreasing from 0.93 to 0.37, and the power law growth characteristics of the energy acceleration tends to be obvious. The energy released after rockburst accounts for 31.1%, 41%, 77.7%, and 87.5% of the total energy, respectively, which is an important reason for the severe damage tendency of rockburst. Rockburst intensity can be judged according to the stress/strength ratio corresponding to the inflection point of the AE energy fitting curve under different stress gradients, which can be used as an important precursor of rockburst. [ABSTRACT FROM AUTHOR]

Understanding the effects of thermal fatigue damage on the failure mechanisms of rocks is a key concern in underground engineering. The effects of high temperature on the physical–mechanical behaviors and the failure mechanism of basalt under uniaxial compression are investigated with a combination of acoustic emission (AE), computed tomography (CT), and scanning electron microscope (SEM). The high temperature heavily affects the physical–mechanical properties of basalt but has no effect on the mineral compositions. The evolution characteristics of inter‐event time function F(τ) and cumulative AE energy can be employed to characterize the fracture process of thermally damaged basalt. The damage mechanisms of thermal fatigue are attributed to the occurrence of intergranular cracks, intragranular cracks, and transgranular cracks and irregular holes within basalt. The failure mechanisms of basalt change from shear fracture to mixed tensile–shear fracture and finally to tensile fracture based on the statistical characteristics of low and high dominant frequencies. Highlights: The thermal fatigue effects on the physical–mechanical behaviors of basalt are studied.The relationship between failure process and AE characteristics is constructed.The damage mechanisms of thermal fatigue for basalt are revealed.The failure mechanisms of basalt under uniaxial compression are revealed. [ABSTRACT FROM AUTHOR]

To study the influence of fracture dip angle on the mechanical properties and fracture evolution mechanism of granite under triaxial stress state, MTS 815 mechanics test system was used to conduct triaxial tests on granite with different fracture dip angles, and PCI–II acoustic emission (AE) system was used to monitor the whole process information. The results show that the brittle characteristics of fractured samples with 30°, 45° and 60° dip angles are obviously weakened, while the plastic characteristics are enhanced. The fractures destroy the structural integrity of rock, resulting in the reduction of rock resistance to load and deformation. With the increase in fracture dip angles from 0° to 90°, the compressive strength and elastic modulus of rock samples show a nearly "U"-shaped changing trend of decreasing first and then increasing, and the deterioration ratio coefficients are 7.8 ~ 43.3% and 7.5 ~ 66.9%, respectively. Due to the difference of fracture dip angles, the failure of granite sample shows two modes: "through-cutting fracture surface failure" and "shear failure along fracture surface." It mainly depends on the angle between fracture surface and maximum principal stress. The fracture dip angle affects the variation law of AE signals during the fracture process. Especially for the fractured rock samples with 30°, 45° and 60° dip angles, the AE ring count and energy show obvious "migration" phenomenon, and the signal concentration distribution area is widened and moved backward. With the increase in dip angle, the active degree of rock fracture decreases at first and then increases. [ABSTRACT FROM AUTHOR]

To investigate the fracture mechanical behavior and failure mechanism of jointed rock mass under compression and shear load. Conventional shear tests and shear tests under normal disturbances were conducted using an electro-hydraulic servo-motor loading system. Meanwhile, the discrete-element program particle flow code was adopted to establish a numerical shear model, and to discuss the microscopic deterioration characteristics and energy dissipation mechanism during shear fracture of rocks with discontinuous joints under joint action of normal static loads and dynamic disturbance. Compared with the conventional shear tests, shear test results under normal disturbances show the following specificities in terms of their macroscopic and microscopic mechanical properties as well as energy evolution: (1) frequent dynamic disturbances accelerate the non-steady fracture process of jointed rock samples and promote occurrence of the weakening effect of shear fracture. (2) The step-like abrupt increase in micro-cracks becomes more obvious before reaching the peak shear stress. (3) The energy-storage capacity and failure resistance of the rocks are weakened. The research results are of great significance for further understanding the dynamic catastrophe effect of deep rock mass. [ABSTRACT FROM AUTHOR]

To investigate the influence of stress gradients in the surrounding rock of tunnels on the frequency features of acoustic emission (AE) waveforms, a true-triaxial gradient, and hydraulic-pneumatic loading apparatus were used to conduct rockburst experiments under four different stress gradients. The AE waveforms were recorded in real time throughout the entire loading process using an AE instrument. A fast Fourier transform was applied to extract feature points from the frequency spectrum of the AE waveforms. The main contribution of this study is the qualitative analysis of the feature points using the proposed variance/mean ratio (VMR) under different stress gradients. It was found that: (1) the cumulative amplitude of the dominant frequency of the feature points decreases with increasing stress gradients, indicating a reduction in crack size in the specimens. (2) The feature points exhibit varying evolutionary patterns with loading under different stress gradients, with low-frequency, high-amplitude feature points tending to concentrate at the moment of rockburst as stress gradients increase. (3) Fluctuations in the VMR curve can be utilized to identify stages of specimen deformation, and regular changes in these stages were observed with increasing stress gradients. (4) As the stress gradient increases, the fluctuation of the VMR curve gradually decreases, indicating that the increase in stress gradient accelerates the alternating development of macroscopic scale cracks and small-scale cracks. The results of the experiments are expected to provide insights into the mechanism of rockburst in underground constructions. [ABSTRACT FROM AUTHOR]

The tangential stress of surrounding rocks is large on tunnel walls. While from tunnel walls to the interior of surrounding rocks, the tangential stress declines and approaches the in situ stress in a gradient manner. To study the influences of stress gradient on the failure mechanism of rockbursts, a mesoscopic model was established based on the discrete element software particle flow code (PFC). The model was used to simulate rockburst disasters in the loading process of gradient stresses and analyze the failure modes and energy evolution process under different gradient stresses. Using the PFC platform, an acoustic emission (AE)–based simulation method at the mesoscopic scale was proposed according to the moment tensor theory to explore features of AE events, including the spatio-temporal distribution and fracture strength of the model during rockbursts. By analyzing the failure process, the increase in the applied stress gradient is found to accelerate the deterioration process of materials and promotes samples to fracture rapidly along dominant main cracks. The number of derivative cracks and the total number of cracks are significantly reduced, and the model shows a change from tensile failure to shear failure. As the applied stress gradient grows, the proportion of elastic energy storage in the model increases before a rockburst, and the rate of release of energy rises accordingly during the rockburst. The AE count at fracture points on the unloading face of the model is normally distributed with changes in the strength M, and the overall AE intensity is enhanced as the gradient increases. [ABSTRACT FROM AUTHOR]

Particle flow numerical simulation was used to reproduce the transient unloading process of a deep-buried single fracture tunnel. The influence of fracture characteristics on the transient unloading effect was analyzed from the aspects of stress state, deformation characteristics, fracture propagation, and energy conversion. The results shows that the surrounding rock stress field of the deep-buried tunnel is divided into four areas: weak stress area I, strong stress area II, stress adjustment area III, and initial stress area IV. The fracture has an important impact on the stress adjustment process of transient unloading of the deep-buried tunnel, and the stress concentration area will be transferred from the bottom corner of the chamber and the vault to the fracture tip. With the increase in the fracture length, the distance from the stress concentration area at the fracture tip to the free surface gradually increases, and the damage area of the surrounding rock gradually migrates to the deep area of the rock mass. At this time, the release amount of strain energy gradually decreases and tends to be stable, while the dissipation energy shows a near 'U' shape change trend of decreasing first and then increasing. Under different fracture angles, the number of mesocracks is significantly different. Among them, the number of mesocracks in the 60° and 30° fractured surrounding rocks is greater followed by the 0° fractured surrounding rock, and the number of mesocracks in the 45° and 90° fractured surrounding rocks is relatively less. In addition, the proportion of compression-shear cracks shows a change trend of increasing first and then decreasing with the increase in the fracture angle, and it reaches the maximum value in the 45° fractured surrounding rock. [ABSTRACT FROM AUTHOR]

Joint is an important factor affecting the stability of engineering rock mass. To quantitatively analyze the local failure characteristics, the point cloud data of joint before and after shear were obtained by 3D white light scanning technology. Firstly, the problem of cloud surface skew of point cloud data was solved by constructing rotation matrix and translation matrix, and the datum of joint was determined. Then, according to the marked points and the undamaged area, the joint point cloud data before and after shear are aligned, and the point cloud data with equal spacing were obtained by interpolation algorithm. Finally, a 3D description method of joint local failure characteristics is proposed. The proposed method can determine the scope of surface failure of joint, quantitatively analyze the locality of its failure characteristics, provide data support for distinguishing the types of shear failure and offer theoretical basis for further research on shear mechanism. [ABSTRACT FROM AUTHOR]

The tailings pond is a dangerous source of man-made debris flow with high potential energy. The oxidative acidification of tailings may cause the instability of the pond and induce serious safety accidents. The influence of oxidation and acidification degree on macro mechanical properties of tailings is discussed from the aspects of mineral composition and microstructure. The results show that as the degree of oxidation and acidification of tailings sand increases, the overall structural performance and load-bearing capacity decrease, and its cohesion (c) and internal friction angle (φ) show a decreasing trend. In fact, the engineering properties of tailings with different oxidation and acidification degrees are dominated by the physicochemical composition and structural characteristics. On the one hand, as the degree of oxidation increases, acidic substance will neutralize with CaCO3 and CaMg(CO3)2, resulting in the loss of cemented substance and the decrease of cementation force between tailing sand particles as well as the gradual destruction of the integrity of tailing sand. On the other hand, the increase of oxidation and acidification degree of tailing sand leads to a gradual reduction of outline (2D) fractal dimension and gray surface (3D) fractal dimension of surface laminated structure as well as the obvious reduction of laminated structure and its roughness of tailings sand. [ABSTRACT FROM AUTHOR]

Rockburst is a common geological disaster in the process of deep rock excavation, and stress gradient is an important factor affecting rockburst. In this paper, the concept of “stress concentration” was proposed. The relevant data of a large number of engineering rockburst cases were statistically analyzed, and a new rockburst criterion of stress–strength ratio considering stress gradient of surrounding rock was established. The research results revealed that the rockburst criterion considering the stress gradient distribution of surrounding rock could effectively overcome the problem of non-uniform division of rockburst criteria in the traditional strength-to-stress ratio. It can fully reflect the influence of tunnel size effect on rockburst. There was a significant correlation between rockburst strength and stress gradient, and different rockburst strengths showed obvious zoning concentration phenomenon on the two-dimensional plane. The established rockburst criterion considering the stress intensity ratio of surrounding rock had strong applicability, and the prediction accuracy could reach more than 90%. The proposed rockburst criterion in this paper has certain theoretical and engineering guiding value for the early warning of underground engineering disasters. [ABSTRACT FROM AUTHOR]

Among the factors affecting rock mass failure, the stress state has the greatest influence on the mechanical behavior of rock. The mechanical behavior of rock depends on the change of stress path, and the stress of rock mass is closely related to deformation and failure. In order to study the influence of surrounding rock stress state on the failure mode of surrounding rock, based on the theory of elastic mechanics and fracture mechanics, this paper analyzed the stress state of surrounding rock element and crack propagation of surrounding rock, and estimated the potential failure mode of surrounding rock in different areas of a phosphate mine in Yichang. The results showed that: the surrounding rock element is affected by the gradient stress, and a larger stress gradient corresponds to a larger principal stress ratio and variation direction of the surrounding rock. The failure mode of surrounding rock is affected by the coupling effect of principal stress ratio and principal stress direction. Under the action of certain crack angle and appropriate confining pressure, different fracture mechanisms may occur in the surrounding rock compression. But the partition of surrounding rock failure mode region can predict the surrounding rock failure mode to a certain extent. [ABSTRACT FROM AUTHOR]

To investigate the mechanical properties and damage characteristics of granite under frequent disturbance loads in the process of underground engineering construction, laboratory uniaxial compression tests were conducted on granite under combined dynamic and static loading conditions. The following conclusions were reached: (1) under a dynamic disturbance, the failure stress of granite grows gradually as the initial stress and disturbance load rise due to the coupling of damage and strain-rate effect; (2) the characteristic stresses of granite specimens grow with the increasing amplitude of disturbance Δσ under the disturbing loads; with the same Δσ, the characteristic stresses show an increase trend with the increasing initial stress σm; (3) the particle size distribution of rock fragments broken under the disturbance load follows the fractal law, and the fractal dimension F gradually enlarges with the growth of Δσ, indicative of an increased degree of fragmentation; and (4) the damage variable grows rapidly at first, then steadily, and, finally, shows a rapid growth trend again under the disturbance loads. The Δσ significantly influences the number of cycles and rate of change of the damage variable during the steady increase. This research has certain theoretical significance and engineering guidance value for dynamic disaster recognition and control. [ABSTRACT FROM AUTHOR]

Strainburst is a kind of safety concern caused by the release of elastic strain energy from surrounding rocks under a tangential stress gradient. In this paper, laboratory rockburst model tests under four different stress gradients were conducted. Based on the acoustic emission (AE) data, the influence of tangential stress gradient on the energy evolution of strainburst was studied. The results indicate that (1) by controlling the tangential stress gradient loading at the top of the specimen, the change process of tangential and radial stress gradient from surrounding rocks caused by excavation disturbance, to a large extent, can be simulated. (2) The failure phenomena and failure stress of the specimen are both related to its tangential stress gradient distribution. As the tangential stress gradient increases, the failure stress is reduced, but dynamic failure phenomena become more evident. (3) With the increase of tangential stress gradient, the accumulation period of elastic strain energy lengthens, whereas the dissipation and release periods shorten during the loading process. As the tangential stress gradient rises, when the specimen rockburst occurs, the more dramatic decrease and faster decreasing rate of AE b-value reveal the increase in the specimen's proportion of shear failure. Also, the critical index (r) of probability density distribution shows a downward trend, which indicates that the AE energy in the low disturbance area decreases. In contrast, the AE energy span in the high disturbance area widens and the energy level becomes higher. [ABSTRACT FROM AUTHOR]

For studying the mechanical properties of sulfur-containing tailings material in the acidification environment, the tailings material was soaked in H2O2 solution with different concentrations to simulate the oxidation and acidification of the tailings with different accumulation history. The change laws and mechanisms of its mechanical properties after oxidation and acidification were discussed, and through the PFC (particle flow code) mesoscopic discrete element analysis method, the connection between mesoscopic particles and its macroscopic properties was established. The experimental results show that the chemical immersion tests on tailings materials with a certain ratio of hydrogen peroxide to concentrated sulfuric acid can well simulate the joint action of oxidation and acidification, so as to simulate the tailings with different accumulation history. Under the action of oxidation and acidification, the physical and mechanical properties of tailings material are mainly affected by the particle size and contact characteristics. The angle of internal friction decreases, influenced by the reduction of particle size, while the change of cohesive force is greatly correlated with both of the two factors. The shear modulus under different porosity and friction coefficients is basically the same, but the increase of porosity or the decrease of mesoscopic particle friction coefficient will greatly weaken its shear capacity, that is, its mesoscopic particle compactness and contact characteristics have an important impact on the macroscopic mechanical properties of tailings. [ABSTRACT FROM AUTHOR]

Dynamic characteristics of rock masses are an important part of the safety and stability analysis in rock mass engineering. The split Hopkinson pressure bar (SHPB) tests on the phosphate rock in Yichang are conducted, combined with the Split-desktop digital image method and mesoscopic discrete element method (DEM). Both macroscopic and mesoscopic characteristics of the phosphate rock have been studied from such perspectives as stress and strain, fractal characteristics of fragment size, and the spatial and temporal evolution of cracks. The research results show that the dynamic strength of the phosphate rock presents obvious strain rate effect, but the dynamic elasticity modulus is not so sensitive to the strain rate. There is a great correlation between the failure degree of rock specimens and the strain rate. The fractal characteristics of specimen fragments can be used to quantitatively evaluate the rock failure degree under impact loading. According to the characteristics of crack propagation, the rock failure process under the impact can be divided into four stages, namely the elastic crack-free stage, the crack initiation stage, the rapid crack growth stage, and the slow crack development stage. Moreover, with the increase of impact loading, micro-cracks are activated in large quantities. The specimens gradually change from the local damage mode to the axial splitting failure mode and to the crushing failure mode, and in this process, the number of tensile cracks takes an absolute advantage compared with the number of shear cracks. [ABSTRACT FROM AUTHOR]

In order to explore the specimen failure characteristics during rock-burst under different gradient stress conditions, in this paper, a novel experimental technique was proposed; a common series of tests under two gradient stress paths were conducted on rock similar material specimens using the true-triaxial gradient and hydraulic-pneumatic combined test apparatus. And plaster was used as the rock similar material. In the experimental process, several rock-burst debris with area sizes of 100 mm2 were collected, and the fractal dimensions of typical detrital section crystal contours were analyzed and calculated using a scanning electron microscopy (SEM) method. The results showed that the specimens' failure characteristics which had been induced by the two gradient stress processes were various. Also, the mesoscopic morphology of the rock-burst detrital section had effectively reflected its macroscopic failure characteristics. It was found that the fractal dimensions of the crystal contours of the specimen's detrital section had fractal characteristics, and the box-counting dimension based on the SEM image could quantitatively describe the rock-burst failure characteristics. Furthermore, under the same magnification, the fractal dimensions of the crystal contours of the splitting failures were found to be relatively smaller than those of the shearing failures. [ABSTRACT FROM AUTHOR]

Background. Parkinson’s disease is a neurodegenerative disease in elder people, pathophysiologic basis of which is the severe deficiency of dopamine in the striatum. The purpose of the present study was to evaluate the neuroprotective effect of low-frequency rTMS on Parkinson’s disease in model mice. Methods. The effects of low-frequency rTMS on the motor function, cortex excitability, neurochemistry, and neurohistopathology of MPTP-induced Parkinson’s disease mice were investigated through behavioral detection, electrophysiologic technique, high performance liquid chromatography-electrochemical detection, immunohistochemical staining, and western blot. Results. Low-frequency rTMS could improve the motor coordination impairment of Parkinson’s disease mice: the resting motor threshold significantly decreased in the Parkinson’s disease mice; the degeneration of nigral dopaminergic neuron and the expression of tyrosine hydroxylase were significantly improved by low-frequency rTMS; moreover, the expressions of brain derived neurotrophic factor and glial cell line derived neurotrophic factor were also improved by low-frequency rTMS. Conclusions. Low-frequency rTMS had a neuroprotective effect on the nigral dopaminergic neuron which might be due to the improved expressions of brain derived neurotrophic factor and glial cell line-derived neurotrophic factor. The present study provided a theoretical basis for the application of low-frequency rTMS in the clinical treatment and recovery of Parkinson’s disease. [ABSTRACT FROM AUTHOR]

To investigate the mechanism of the neuroprotective effect of edaravone in substantia nigra (SN) of the 6-OHDA-induced rat model of Parkinson's disease. Animal model of Parkinson's disease was induced in male Sprague-Dawley rats by injecting 6-OHDA into the left medial forebrain bundle. Subsequently, rats were intraperitoneally injected with 0.3, 1, or 3 mg/kg of edaravone for 14 days or with 3 mg/kg edaravone for 14 days followed by 14 days of no treatment. We evaluated the effect of edaravone on the rotational and normal behavior of the rats, and on the number of tyrosine hydroxylase (TH)-positive cells, the amount of Nissl bodies, and the levels of glutathione (GSH), and malondialdehyde (MDA) in the SN. Edaravone treatment at 3 mg/kg significantly reduced apomorphine-induced rotational behavior ( P < 0.01), improved the spontaneous behavior, prevented the decrease in the levels of TH-positive cells, Nissl bodies and GSH, and inhibited the increase in the levels of MDA ( P < 0.05) in SN of rats with 6-OHDA-induced PD. Edaravone exerted a long-term neuroprotective effects in 6-OHDA-induced PD animal model by attenuating changes in the levels of GSH and MDA in SN, caused by oxidative stress. Edaravone prevented 6-OHDA-induced behavioral changes and de-pigmentation of SN that results from the loss of dopaminergic neurons. [ABSTRACT FROM AUTHOR]