33 research outputs found

    Analysis on deformation characteristics and energy dissipation of marble under different unloading rates

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    Ispitivanja oÅ”tećenja provodila su se na mramoru kod različitih brzina ograničenja tlaka pri rasterećenju da bi se dobila krivulja promjene energije cijeloga procesa deformacije i oÅ”tećenja. S povećanjem brzine rasterećenja, razlike vrÅ”nog naprezanja kod oÅ”tećenja mramora su se smanjile, razlike ograničenja tlaka su se povećale, omjeri prirasta ograničenja tlaka kod svakog naprezanja su bili manji, a deformacija volumena je bila osjetljivija na promjene ograničenja tlaka kod rasterećenja. OÅ”tećenje kod rasterećenja nastalo je Å”irenjem volumena, i Å”to je veća bila brzina rasterećenja, lakÅ”e je dolazilo do oÅ”tećenja stijene. S porastom brzine rasterećenja, prirast ukupne apsorbirane energije, prirast elastične energije i prirast disipacije energije smanjili su se u stadiju rasterećenja mramora. Prirast rasipanja energije u postupku rasterećenja bio je pet puta veći od onoga u postupku opterećenja dok je prirast elastične energije pokrivao svega 10 % ukupne akumulirane energije. Postupak rasterećenja pokazao je porast rasipanja energije, a stanje geo-naprezanja stijenske mase odredilo je nivo energije otpuÅ”tene u oÅ”tećenju.Failure tests were conducted on marble under different unloading confining pressure rates to obtain the energy change curve of whole-process deformation and failure. With increasing unloading rate, the peak stress differences in marble failure were reduced, confining pressure differences in failure increased, the increment ratios of each stress confining pressure were smaller, and the volume deformation was more sensitive to changes in unloading confining pressure. Unloading failure was caused by volume expansion, and the greater the unloading rate was, the easier the rock failure was. With the increasing unloading rate, total absorbed energy increment, elastic energy increment and dissipated energy increment were reduced in the unloading stage of marble. The dissipated energy increment in the unloading process was more than five times that in the loading process, while the elastic energy increment only accounted for 10 % of total stored energy. The unloading process showed increasing dissipated energy, and the geo-stress state of the engineering rock mass determined the level of energy released in failure

    Energy dissipation mechanism and damage model of marble failure under two stress paths

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    Marble conventional triaxial loading and unloading failure testing research is carried out to analyze the elastic strain energy and dissipated strain energy evolutionary characteristics of the marble deformation process. The study results show that the change rates of dissipated strain energy are essentially the same in compaction and elastic stages, while the change rate of dissipated strain energy in the plastic segment shows a linear increase, so that the maximum sharp point of the change rate of dissipated strain energy is the failure point. The change rate of dissipated strain energy will increase during unloading confining pressure, and a small sharp point of change rate of dissipated strain energy also appears at the unloading point. The damage variable is defined to analyze the change law of failure variable over strain. In the loading test, the damage variable growth rate is first rapid then slow as a gradual process, while in the unloading test, a sudden increase appears in the damage variable before reaching the rock peak strength. According to the deterioration law of damage and the impact of confining pressure on the elastic modulus, a rock damage constitutive model is established, which has a better fitting effect on the data in the loading and unloading failure processes

    3-D Numerical Study on Progressive Failure Characteristics of Marbles under Unloading Conditions

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    3-D particle-based discrete element method (PB-DEM) was employed to numerically study the mechanical and progressive failure characteristics of pre-fissured marble specimens under conventional triaxial unloading conditions. The microscopic parameters of PB-DEM for marble materials were calibrated using comparison with the previous experimental data. To systematically investigate the mechanical properties and the progressive failure characteristics of pre-fissured marble specimens under the unloading conditions, a series of numerical specimens were simulated. The effects of fissure geometric conditions, initial confining pressures, and unloading rates on the mechanical and failure behaviors were explored via simulations. The present numerical results indicate that peak strength increased as the initial confining pressures increased or the unloading rate decreased. Crack coalescence types and the ultimate failure modes in the pre-fissured marble specimens were significantly affected by the unloading stress paths. The present numerical results provide a better understanding of unloading mechanical and failure characteristics to scientists and engineers in rock mechanics and rock engineering

    Instability Risk Assessment for Deep Excavation of Soilā€“Rock Combinations Containing Groundwater

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    Dynamic risk assessment is a pivotal tool for enhancing construction safety and minimizing the potential for partial failure during deep and extensive excavation projects. To enhance the efficacy of dynamic risk assessment in deep excavation, this study introduces a novel risk assessment model designed to evaluate instability risk in extensive excavations. It comprises a risk factor selection model for identifying the most pertinent factors and an instability risk assessment model for gauging the extent of instability risk throughout the construction process. Then, the model was deployed in the construction of Anshan Road Station of the Qingdao Metro. To pinpoint the factors with the most pronounced impact on excavation instability, a risk factor selection model was employed, yielding a comprehensive risk evaluation index system. For real-time assessment of risk, the monitoring data were used as the primary source of evidence. A comprehensive comparative analysis involving actual data and predictions from conventional RBF and back propagation neural networks was performed. The outcome of this analysis underscored the superior accuracy and predictive capabilities of the assessment model. The instability risk assessment model offers the ability to dynamically evaluate the instability risk associated with extensive excavations featuring a combination of soil and rock. It can serve as a valuable methodological tool, furnishing essential support for the systematic prevention and mitigation of excavation instability disasters

    Study on the Reinforcing Effects of the FRP-PCM Method on Tunnel Linings for Dynamic Strengthening

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    In recent years, fiber-reinforced plastic (FRP) has been widely used in the reinforcement of concrete structure fields due to its favorable properties such as high strength, low weight, easy handling and application, and immunity to corrosion, and the reinforcing effects with FRP grids on tunnel linings should be quantitatively evaluated when the tunnels encounter an earthquake. The aim of the present study is to estimate the reinforcing effects of fiber-reinforced plastic (FRP) grids embedded in Polymer Cement Mortar (PCM) shotcrete (FRP-PCM method) on tunnel linings under the dynamic load. A series of numerical simulations were performed to analyze the reinforcing effects of FRP-PCM method quantitatively, taking into account the impacts of tunnel construction method and cavity location. The results showed that the failure region on lining concrete is improved obviously when the type CII ground is encountered, regardless the influences of construction method and cavity location. With the increment of ground class from CII to DII, the axial stress reduction rate RĻƒ increases from 13.18% to 48.60% for tunnels constructed by the NATM, while for those tunnels constructed by the NATM, RĻƒ merely varies from 0.72% to 2.11%. RĻƒ decreases from 43.35% to 34.80% when a cavity exists on the shoulder of lining, while decreasing from 14.7% to 0.12% when a cavity exists on the crown of lining concrete. All those conclusions could provide valuable guidance for the reinforcing of underground structures

    Fractal characteristics of acoustic emission of gas-bearing coal subjected to true triaxial loading

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    As coal mining proceeds deeper, coal and rock are subjected to growing stresses and gas pressures. Consequently, coal and rock gas dynamic disasters pose more and more considerable threats and hazards which should be warned in advance. Coal and rock in the field is actually in the true triaxial stress state. Study on deformation characteristics and precursory information of coal and rock under this state is meaningful for disaster warning. In this paper, experiments on the deformation, failure and acoustic emission (AE) characteristics of gas-bearing coal under true triaxial loading conditions were carried out. Besides, the variation law and fractal characteristics of AE under different gas pressures and confining stresses were analyzed. Furthermore, the effects of gas pressure and confining stress on the deformation, failure and fractal characteristics of coal were discussed. The results show that the process of AE variation under true triaxial loading conditions can be divided into two stages, namely the slow growth stage and the accelerated growth stage. A higher gas pressure corresponds to a shorter duration of slow growth stage, while a higher confining stress corresponds to a longer duration of slow growth stage. AE time series has fractal characteristics, and the correlation dimension can characterize the damage degree of a loaded coal sample. The dynamic changes, i.e., fluctuation-increase-decrease, in correlation dimension can accurately reflect the damage evolution process of a coal sample. In addition, the gradual reduction of correlation dimension can be used as the precursor information of coal sample instability and damage. The research results boast instructive significance for preventing the occurrence of coal and rock gas dynamic disasters and for reducing casualties and property loss in coal mines

    Influence of Confining Pressure on Nonlinear Failure Characteristics of Coal Subjected to Triaxial Compression

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    The stress of a coal seam increases with an increase in the mining depth, which makes the failure mechanism of a coal mass more complex. To reveal the deformation and failure law of deep coal, a series of triaxial experiments was carried out via laboratory experiments and numerical simulation experiments to analyze the influence of the confining stress on the nonlinear failure characteristics of coal. Based on the crack-propagation model, the values for the inelastic flexibility S1 and the damage variable D were calculated. The results showed that the value of S1 decreased with an increase in the confining stress, which indicated that the increase in the confining pressure could inhibit the crack propagation and that the inhibitory effect was more obvious when the confining pressure increased in a small range of 4 to 12 MPa. The damage variable decreased with an increase in the confining pressure at the yield point; moreover, with an increase in the initial confining pressure, the damage rate gradually decreased. The coal body changed from the compression state to the expansion state when moving from the yield point to the peak point, and the compression value of the yield point and the dilation value of the peak point increased with the increase in the confining pressure. After the coal body entered the yield stage, the change in the confining pressure had a more significant effect on the damage to the coal body
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