Search

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]

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]

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 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]

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]

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]

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]

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]

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]