The excavation-unloading damage effects of western high-geostress slopes on rock were explored by testing the pre-peak confining pressure unloading sandstone reloading mechanical properties. The ...deformation and failure mechanisms were studied from a mesoscopic perspective using the particle discrete-element method. (1) Approaching the unloading failure, confining pressure increased the specimen bearing capacity attenuation. (2) The confining pressure unloading promoted microdefect propagation and development; the specimens increased rapidly to the damage stress value after reaching the initiation stress value. The penetration fracture zone was more evident and expansive in the model, and the distribution of the dense crack areas was more concentrated in the fracture zone and area. (3) The average interval of the tangential contact force was the largest in the direction of crack expansion and propagation. The strong force chains were shown to primarily bear external loads, whereas the weak force chains played a key auxiliary role in maintaining stability. (4) The number of cracks developing in the confining pressure unloading damage process indicated that the loading process did not cause damage to the specimens. The fracture zones further propagated and formed on the dominant fractures based on the damage caused by the confining pressure unloading disturbance.
The objective of this paper is to study the effect of rock anisotropy on the initiation and propagation of fracture driven by fluid. For this purpose, an improved hydromechanical model considering ...rock structural anisotropy is established in the framework of the particle flow simulation by assuming that the anisotropic rocks are characterized by a matrix phase with non-persistent weak layers embedded. In this model, the mechanical behavior of rock matrix is described by bond contact while that of weak layers by smooth joint contact, and the fluid flow is reproduced through a new aperture evolution model of pipes redefined according to contact types and orientations. After the calibration of model’s parameters, the effectiveness of proposed model is assessed with the help of a typical case of fluid driven fracture around a borehole. The proposed model can successfully describe the local stress anisotropy and fracture reoriented propagation around borehole due to fluid injection. Some additional numerical simulations with different confining stress are also conducted for the typical case. Moreover, a series of sensitive analysis is further realized to investigate effects of inherent rock anisotropy including elastic, strength and permeability on the initiation and propagation of fractures.
It is difficult to accurately calculate the lump coal rate in a fully mechanized mining face. Therefore, a numerical simulation of the coal wall cutting process, which revealed the crack expansion, ...development, evolution in the coal body and the corresponding lump coal formation mechanism, was performed in PFC2D. Moreover, a correlation was established between the cutting force and lump coal formation, and a statistical analysis method was proposed to determine the lump coal rate. The following conclusions are drawn from the results: (1) Based on a soft ball model, a coal wall cutting model is established. By setting the roller parameters based on linear bonding and simulating the roller cutting process of the coal body, the coal wall cutting process is effectively simulated, and accurate lump coal rate statistics are provided. (2) Under the cutting stress, the coal body in the working face underwent three stages—microfracture generation, fracture expansion, and fracture penetration—to form lump coal, in which the fracture direction is orthogonal to the cutting pressure chain. Within a certain range from the roller, as the cutting depth of the roller increased, the number of new fractures in the coal body first increases and then stabilizes. (3) Under the cutting stress, the fractured coal body is locally compressed, thereby forming a compact core. The formation and destruction of the compact core causes fluctuations in the cutting force. The fluctuation amplitude is positively related to the coal mass. (4) Because the simulation does not consider secondary damage in the coal, the simulated lump coal rate is larger than the actual lump coal rate in the working face; this deviation is mainly concentrated in large lump coal with a diameter greater than 300 mm.
Flaws in the rock play an important role in the failure of rock engineering. In this article, some rock-like specimens prepared by transparent resin material were used to investigate the propagation ...and coalescence processes of pre-existing closed three-dimensional flaws, and a series of uniaxial compressive tests were carried out to study the effects of flaw dip, flaw area, flaw friction coefficient, and composite flaw spacing on mechanical properties and flaw propagation and coalescence processes. Based on the particle flow method, the experimental results were verified by numerical simulation. The results show that there is a good agreement on flaw propagation rule revealed by the experiments and numerical simulations. In the case of the specimens with single-flaw, with the increase of the inclination angle, the peak stress of the specimen first decreases and then increases, when the inclination angle is 45°, the peak stress is the smallest. Meanwhile, the peak stress decreases with the increase of flaw area but increases with the increase of flaw friction coefficient. The spacing of the composite flaw has a significant influence on the strength, and the impact on the elastic modulus is not obvious. The results can provide a reference for the mechanism of flaw propagation and coalescence.
The increase in mining depth causes the temperature of the coal seam to rise. Studying the effect of temperature on the mechanical property of coal is very necessary. In this paper, based on the ...results of conventional triaxial compression experiments of coal samples, the discrete element program PFC2D was used to conduct a conventional triaxial compression simulation of coal samples to obtain microscopic parameters. On this basis, the conventional triaxial simulation study of coal samples at different temperatures was carried out to explore the influence of temperature on the physical and mechanical properties of coal. In this process, acoustic emission and energy monitoring were carried out. The damage and failure process of coal is divided into three stages: the undamaged stage, the stable damage stage, and the rapid damage stage. The relationship between acoustic emission characteristic law, energy transformation law, and crack evolution in the damage and failure process of coal was analyzed in detail. The results indicate that the fracture evolution is affected by the increase in cross-sectional area and the decrease in distance between particles due to the expansion of particles, which corresponds to the phenomenon that the temperature increase produces new microcracks and the particles expand to fill the original cracks in the experiment. This indicates that PFC2D can be used as an effective numerical method for a coal mesoscopic study, and the results of these numerical experiments are also helpful to deepen the understanding of the damage mechanism of coal failure under thermo-mechanical coupling.
The paper represents a simulation investigation about the crack evolution and acoustic emission characteristics of coal specimen subjected to conventional compression loading with four confining ...pressures. From the simulation tests, the following conclusions can be drawn. (1) Under the confining pressure of 0.5, 1.0 and 1.5 MPa, at the stress peak point, the number of total cracks in coal specimen are 3.33, 4.28 and 4.50 times of that under uniaxial stress state, respectively. That is, with the increase of confining pressure, the number of total cracks at coal specimen failure gradually increases, but the increase rate gradually decreases. (2) Under the confining pressure of 0, 0.5, 1.0 and 1.5 MPa, at the stress peak point, the proportion of tensile cracks in the total cracks decreases from 100% to 95.95%, and the proportion of shear cracks increases from 0 to 4.05%. These show that with the increase of confining pressure, the coal specimen presents a transition trend from tensile failure to shear failure. (3) When the confining pressure is less than 1.0 MPa, the confining pressure has an obvious effect on the crack distribution of coal specimen subjected to conventional compression loading, but when the confining pressure exceeds 1.0 MPa, this effect gradually weakens. (4) The maximum value of acoustic emission impact counts slightly lags behind the time point corresponding to the peak strength, namely, the coal specimen is destroyed at some point in the post-peak phase.
Layered rock masses with holes are common in nature. Their mechanical behavior plays an important role in the safety and stability of engineering structures. However, previous studies have ...concentrated on a single lithological layer, and few studies have reported on the mechanical behavior of layered rock masses with holes. Based on the concept of symmetry, uniaxial compression tests and numerical simulations were performed on rock-like specimens with three layers and a hole in the interlayer. The hole was in the center of the sample and was symmetrical up and down. The influence of the thickness and strength of the interlayer on the mechanical behavior and failure processes of the layered rock masses with holes was investigated. The results show that the peak strength and elastic modulus were associated with the thickness and strength of the interlayer. Three failure modes were observed in the specimens, which were not only related to the thickness and strength of the interlayer, but also affected by the presence of the hole. When the thickness of the interlayer is small, mainly a single failure mode was observed (tensile failure or shear failure). However, when the interlayer was thick, the failure mode was tension-shear mixed failure. The failure mechanism of the specimens was primarily crack propagation at the edge of the hole. These research results can provide a basis for site selection, and the design of surrounding rock protection and support parameters, and thus have important practical significance for improving surrounding rock stability and ensuring construction safety.
A series of laboratory experiments and PFC numerical simulations for rock-like material specimens containing two unparallel fissures were carried out. On the basis of experimental and numerical ...results, the stress-strain curves, mechanical properties, AE events, cracking behavior and energy characteristics were analyzed to reveal the macro-mechanical behavior and meso-mechanism of pre-fissured specimens under different loading rates. Investigated results show that: 1) When the loading rate is relatively low, the stress-strain curves show a brittle response. When the loading rate is relatively high, the curve shows a more ductile response. Both of the peak strength and elastic mudulus increase with the increase of loading rate, which can be expressed as power functions. 2) Four crack types are identified, i.e., tensile crack, shear crack, far-field crack and surface spalling. Moreover, the tensile crack, far-field crack and surface spalling are under tensile mechanism, while the shear crack is under shear mechanism. 3) The drops of the stress-strain curves all correspond to the crack initiation or coalescence, which is also linked to a sudden increasing in the accumulated micro-crack curve. 4) Both of the maximum bond force and energy have the similar trend with the increase of loading rate to peak strength, which indicates that the trend of peak strength can be explained by the meso-mechanics and energy.
Geological dynamic hazards during coal mining can be induced by the structural instability of a composite structure of roof rock, coal and floor rock layers, whereas the joint in coal plays a vital ...role in the corresponding structural instability. In this paper, the effects of coal persistent joint on the uniaxial compression failure of a roof rock–coal–floor rock composite sample were analyzed using PFC
2D
software. The results show that with an increase of included angle
α
between the loading direction and coal persistent joint plane, the uniaxial compressive stress (UCS) and peak strain of the composite sample decreased firstly and then increased. The change of elastic modulus was not obvious with
α
. The UCS and peak strain at
α
of 45° were the smallest. Additionally, the coal bodies in composite samples were mainly destructed as a shear failure. The roof or floor rock presented a tensile failure or no damage. And four failure patterns were observed for composite samples after failure, i.e., inverted V-shaped shear failure in coal and tensile failure in roof or floor rock, M-shaped shear failure in coal and tensile failure in floor rock, shear failure along the joint plane in coal and tensile failure in roof and floor rocks, shear failure along partial joint plane without obvious damage for roof or floor rock.
In order to research the evolution of energy stored in the composite coal-rock structure and coal fragments’ burst characteristics, lateral pressure unloading numerical tests of composite coal–rock ...models with different Young’s modulus were carried by PFC2D software. The research showed that the accumulated strain energy and kinetic energy in the coal was greater than that in roof and floor, and particles in the coal had a longer burst distance. The variation of the kinetic energy in the model could be separated into initialized burst, rapid burst, stable burst and residual burst stage. With the increase of the coal’s Young’s modulus, the accumulated amount and releasing velocity of strain energy in the coal decreased in the form of a power function, while the strain energy in both roof and floor changed little. The maximum value of the kinetic energy in the coal was negative linearly related to the Young’s modulus of the coal, while that of the rock was positive linearly. When the Young’s modulus of the coal was increased, particles bursted with a shorter distance, and most particles accumulated near the coal wall.
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