To deeply understand the rock failure characteristics under actual engineering condition, in which static geo-stress and dynamic disturbance usually act simultaneously, impact tests were conducted on ...sandstone subjected to axial static pre-stresses varying from 0 to 75 MPa by a modified split Hopkinson pressure bar. The fracturing process of specimens was recorded by a high speed camera. Dynamic parameters of sandstone, such as strain rate, dynamic strength and energy partition were acquired. Fracture mechanisms of pulverized specimens were identified by the method combining the displacement trend line and digital image correlation technique. Moreover, fragments of failed specimens were sieved to obtain the fragment size distribution. Test results revealed that, under the same incident energy, the dynamic compressive strength increases first, then decreases slowly and at last drops rapidly with the increase of pre-stress, and reaches the maximum under 24.4% of uniaxial compressive strength due to the closure of initial defects. Four final patterns were observed, namely intact, axial split, rock burst, and pulverization. The rock burst only occurs when the pre-stress lies in the elastic deformation stage or initial stable crack growth stage and the incident energy is intermediate. For pulverized specimens, the fracture mechanism is transformed into shear/tensile equivalent from tensile-dominated mixed mode as the pre-stress increases. Specimens with 75 MPa pre-stress release strain energy during failure process, contrary to specimens with lower pre-stresses absorbing energy from outside. The crushing degree of pulverized specimens exhibits a positive correlation with the pre-stress as a consequence of higher damage development in rock.
The desire for high-energy-density batteries calls for the revival of the Li metal anode. However, its application is hindered by enormous challenges associated with Li deposition/desolvation ...behaviors, such as side reactions, volume change, and dendrite formation. To overcome these challenges, Li deposition must be controlled to remain below the separator. Further, to enable longer cycle life, Li deposition should be constrained below the solid electrolyte interphase (SEI). To achieve these goals, it is critical to have a deep theoretical understanding and corresponding strategies. This paper examines Li plating/stripping in terms of behaviors, mechanisms, and influencing factors, and it proposes general strategies to control Li deposition. Comprehensive design strategies for the electrode, electrolyte, and their interface are essential. Three dimensional (3D) anodes are recommended to store most of the Li deposited below the surface of the anode. Artificial interface engineering can reduce the risk of Li deposition outside of the 3D anode, while electrolyte engineering favors Li transport, regulates Li deposition, and suppresses dendrites, serving as the final barrier to uncontrolled Li deposition. This paper reviews systemic theories and solutions to control Li deposition below the interface, paving the way for a promising route to build safer lithium metal batteries.
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•Li deposition should be controlled below the SEI (separator) for cyclability (safety).•Summary of theories on Li deposition behavior is made.•Systemic strategies from the aspects of electrode，interface，electrolyte，and cell are reviewed.•Key parameters for practical composite Li metal anode are recommended.
Scientists have attempted to investigate deep underground rock failure. A key challenge to study underground rock failure is the difficulty to survey its process. Therefore, a large number of ...important and insightful laboratory investigations of underground rock failure experiments are conducted. In this paper, an experimental method is proposed to explore dynamic failure process of pre-stressed rock specimen with a circular hole. The failure process of a rock specimen under different initial static stress coupled with dynamic loading is clearly illustrated by a high speed camera. The experimental results indicated that high static pre-stress coupled with dynamic loading induces rock debris to be ejected at the surrounding circular hole; however, lower static pre-stress coupled dynamic loading cannot induce rock failure. The dynamic stress concentration surrounding the circular hole by transient wave incidence was further demonstrated at the condition of half-sine wave loading. In this condition, the results demonstrated that combined action of static and dynamic stress concentrations induces the primary fractures of rock specimen.
•A theoretical model was applied to characterise unloading responses under different paths.•The propagation of unloading perturbation was represented in 3D contour maps.•Numerical simulation was ...performed for the 2D unloading responses under different paths.•Results of numerical simulation were consistent with the mathematical physical model.•The effects of unloading rate and path were studied on excavation unloading responses.
The unloading process of rock mass is critical to the research of excavation disturbances of tunnels in deep mines, and the dynamic effects induced by the release of in situ stress cannot be ignored. In this study, a mathematical physics model was applied to characterise the unloading mechanisms of brittle rock under different stress paths in two dimensions using the universal discrete element code PFC2D for numerical simulations. The excavation relaxation method was employed to control forces applied to the tunnel internal surface to investigate the influence of various in situ stresses, the unloading rate and path on the dynamic effects. Longer unloading time can mitigate the dynamic effects within a certain time range. Nonlinear unloading paths prevail over the linear path in releasing kinetic energy. Furthermore, the exponential path that represents “slow followed by fast” unloading induces the most peripheral displacement, while the cosine path that represents “fast followed by slow” unloading yields the most cracks around the tunnel. The results also indicated that increasing the ratio of horizontal and vertical in situ stresses can exacerbate the dynamic effects. The proposed model agreed well with the theoretical solution and provided a basis for understanding the evolution of the unloading response around the tunnel.
Water content has a pronounced influence on the properties of rock materials, which is responsible for many rock engineering hazards, such as landslides and karst collapse. Meanwhile, water injection ...is also used for the prevention of some engineering disasters like rock-bursts. To comprehensively investigate the effect of water content on mechanical properties of rocks, laboratory tests were carried out on sandstone specimens with different water contents in both saturation and drying processes. The Nuclear Magnetic Resonance technique was applied to study the water distribution in specimens with variation of water contents. The servo-controlled rock mechanics testing machine and Split Hopkinson Pressure Bar technique were used to conduct both compressive and tensile tests on sandstone specimens with different water contents. From the laboratory tests, reductions of the compressive and tensile strength of sandstone under static and dynamic states in different saturation processes were observed. In the drying process, all of the saturated specimens could basically regain their mechanical properties and recover its strength as in the dry state. However, for partially saturated specimens in the saturation and drying processes, the tensile strength of specimens with the same water content was different, which could be related to different water distributions in specimens.
•A fuzzy comprehensive evaluation model was developed for the forecasting of rock bursts.•Laboratory AE measurements were performed to select MS indices used as input to the fuzzy model.•The ...application of this model has been successfully demonstrated in a coal mine in China.
Rock bursts have become one of the most severe risks in underground coal mining and its forecasting is an important component in the safety management. Subsurface microseismic (MS) monitoring is considered potentially as a powerful tool for rock burst forecasting. In this study, a methodology for rock burst forecasting involving the use of a fuzzy comprehensive evaluation model was developed, which allows for a more quantitative evaluation of the likelihood for the occurrence of a rock burst incident. In the fuzzy model, the membership function was built using Gaussian shape combined with the exponential distribution function from the reliability theory. The weight of each index was determined utilising the performance metric F score from the confusion matrix. The comprehensive forecasting result was obtained by integrating the maximum membership degree principle (MMDP) and the variable fuzzy pattern recognition (VFPR). This methodology has been applied to a coal mine in China to forecast rock bursts. To select MS indices for rock burst forecasting using the fuzzy evaluation model, laboratory acoustic emission (AE) measurements of coal samples collected from the mine were performed. The model parameters were first calibrated using historical MS data over a period of four months, during which six rock burst incidents were observed. This calibrated model was able to forecast the occurrence of a subsequent rock burst incident in the mine.
Coal is the major energy resource in China, but the recovery of coal is constantly under the threat of rock bursts, which has been impeding safe and highly efficient coal production for decades. A ...better understanding of the rock burst mechanism and its monitoring and management strategies is critical to improving coal extraction efficiency with an improved health and safety environment. In this paper, rock burst mechanism was further investigated based on the hypothesis of coupled static and dynamic stresses, in which mining-induced abutment stresses were considered as the major component of the static stresses while the dynamic stresses were categorised into two types (seismic-derived and impact-derived dynamic stresses). Subsequently, on-site monitoring strategies, including microseismic (MS) monitoring, hydraulic support pressure (HSP) monitoring and seismic velocity tomography (SVT), were implemented to understand the responses of mining-induced static and dynamic stresses. To be specific, four estimation methodologies were proposed for quantitatively assessing velocity-derived static stress, seismic-derived static stress, seismic-derived dynamic stress, and impact-derived dynamic stress. These monitoring strategies have been implemented in a Chinese coal mine to understand the mechanism of a rock burst incident.
The effect of thermal treatment on several physical properties and the tensile strength of Laurentian granite (LG) are measured in this study. Brazilian disc LG specimens are treated at temperatures ...of up to 850 °C. The physical properties such as grain density, relative volume change per degree, and P-wave velocity are investigated under the effect of heat treatment. The results indicate that both the density and the P-wave velocity decrease with the increase in heating temperature. However, the relative volume change per degree is not sensitive below 450 °C, while a remarkable increase appears from 450 to 850 °C. All cases are explained by the increase in both number and width of the thermally induced microcracks with the heating temperature. Brazilian tests are carried out statically with an MTS hydraulic servo-control testing system and dynamically with a modified split Hopkinson pressure bar (SHPB) system to measure both static and dynamic tensile strength of LG. The relationship between the tensile strength and treatment temperatures shows that static tensile strength decreases with temperature while the dynamic tensile strength first increases and then decreases with a linear increase in the loading rate. However, the increase in dynamic tensile strength with treatment temperatures from 25 to 100 °C is due to slight dilation of the grain boundaries as the initial thermal action, which leads to compaction of rock. When the treatment temperature rises above 450 °C, the quartz phase transition results in increased size of microcracks due to the differential expansion between the quartz grains and other minerals, which is the main cause of the sharp reduction in tensile strength.
The deformation and permeability of coal are largely affected by the presence and distribution of natural fractures such as cleats and bedding planes with orthogonal and abutting characteristics, ...resulting in distinct hydromechanical responses to stress loading during coal mining processes. In this research, a two-dimensional (2D) fracture network is constructed based on a real coal cleat trace data collected from the Fukang mine area, China. Realistic multi-stage stress loading is designed to sequentially mimic an initial equilibrium phase and a mining-induced perturbation phase involving an increase of axial stress and a decrease of confining stress. The geomechanical and hydrological behaviour of the fractured coal under various stress loading conditions is modelled using a finite element model, which can simulate the deformation of coal matrix, the shearing and dilatancy of coal cleats, the variation of cleat aperture induced by combined effects of closure/opening, and shear and tensile-induced damage. The influence of different excavation stress paths and directions of mining is further investigated. The simulation results illustrate correlated variations among the shear-induced cleat dilation, damage in coal matrix, and equivalent permeability of the fractured coal. Model results are compared with results of previous work based on conventional approaches in which natural fracture networks are not explicitly represented. In particular, the numerical model reproduces the evolution of equivalent permeability under the competing influence of the effective stress perpendicular to cleats and shear-induced cleat dilation and associated damage. Model results also indicate that coal mining at low stress rates is conducive to the stability of surrounding coal seams, and that coal mining in parallel to cleat directions is desirable. The research findings of this paper have important implications for efficient and safe exploitation of coal and coalbed methane resources.
Influence of unloading disturbance on adjacent tunnels Li, Xibing; Cao, Wenzhuo; Tao, Ming ...
International journal of rock mechanics and mining sciences (Oxford, England : 1997),
April 2016, 2016-04-00, 20160401, Letnik:
84
Journal Article
Recenzirano
A theoretical model was first established in order to evaluate the physical possibility of a dynamic response occurring around an existing tunnel under conditions of unloading wave incidence. Based ...on the steady state solution of the wave expansion approach, transient solutions subjected to unloading waves with different unloading times were obtained. A three-dimensional numerical model was then constructed in order to simulate dynamic responses around an existing tunnel under unloading disturbance forces. In the simulation, dynamic unloading was carried out through a new excavation after stress redistribution had been finalized in the examined tunnel, and parametric studies were then conducted for various unloading times, stress levels and tunnel spacing configurations. The results show that dynamic effects are induced around an existing tunnel under high levels of initial stress and at high unloading rates. The results of the dynamic analysis show that the PPV of unloading-induced microseism can be as high as that of an explosion-induced wave, and damage can be inflicted on existing tunnel walls (especially on the incident side of a tunnel). This dynamic effect diminishes dramatically with an increase in tunnel spacing and unloading times.
•Unloading-induced stress concentration around adjacent tunnels was examined.•Dynamic responses of tunnels subjected to unloading disturbance were modelled.•The influencing factors of unloading disturbance were investigated.•Unloading excavation under high stress can serve as a part of vibration excitation.
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