To investigate crack propagation and the coalescence mechanism of a rock bridge under unloading condition induced by intensive excavation of rock mass, the direct shear test with unloading normal ...stress and corresponding particle flow code (PFC) simulation were conducted on the sandstone specimen containing a parallel fissure pair considering different fissure inclinations (varied from 0° to 90°) and initial shear stresses (varied from 4 to 7 MPa). Three failure patterns (i.e., shear failure, tensile failure, and tensile–shear mixed failure) are identified as experimental and numerical results. The failure pattern transforms in the order of a shear, tensile, and tensile–shear mixed failure pattern as the fissure inclination increases. Three displacement field types are summarized and correspond to different failure patterns. Comparing the shear strength, cracking process, and microscopic displacement field in the direct shear test with unloading normal stress and the conventional direct shear test, normal unloading weakens the shear strength of the specimen under the selected stress conditions (initial normal stress is 20 MPa, initial shear stress ranges from 4 to 7 MPa). Rebound deformation in the process of unloading promotes the high proportion of tensile cracks for the tested fissure inclinations.
Under the Castor earthquake, there is a risk of liquefaction instability of saturated tailings, and the evolution of dynamic pore pressure can indirectly reflect its instability process. Before ...applying dynamic loads, the static stress state of soil is one of the main factors affecting the development of soil dynamic strength and dynamic pore pressure, and there are significant differences in soil dynamic strength under different consolidation ratios. This paper conducted dynamic triaxial tests on saturated tailings silt with different consolidation ratios, and analyzed the dynamic strength variation and liquefaction mechanism of the samples using the discrete element method (PFC3D). The results showed that 1) as the Kc′ gradually increased, and there was a critical consolidation ratio Kc′ during the development of the dynamic strength of the sample. The specific value of Kc′ was related to the properties and stress state of saturated sand. The Kc′ in this research was about 1.9. When Kc < 1.9, dynamic strength was increased with the increase in Kc; when Kc > 1.9, dynamic strength was decreased with the Kc. 2) Under the impact of cyclic load, when samples were normally consolidated (Kc =1), the pore water pressure would tend to be equal to the confining pressure to cause soil liquefaction. In the case of eccentric consolidation (Kc > 1), the pore water pressure would be less than the confining pressure, thus, the soil liquefaction would not be induced, and the pore pressure value would decrease with the increase of consolidation ratio. This paper provides engineering guidance value for the study of dynamic strength and liquefaction mechanism of tailings sand and silt in Castor earthquake prone areas under different consolidation ratios.
To investigate the influence of geometric position distribution and interaction mechanisms of internal defects of the loaded coal body on its mechanical behavior, this study conducted a uniaxial ...loading test on double-fractured coal samples with different inclination angles of rock bridges. The compression failure process was scanned in stages using computed tomography equipment, and the dynamic evolution law and deformation damage characteristics of the internal fractures of the loaded coal samples were qualitatively analyzed by image processing technology, three-dimensional reconstruction technology, and digital volume correlation methods. Meanwhile, the PFC
3D
simulation program was used to realistically restore the stress field distribution characteristics of different stages of the loading process. Based on the experimental method and linear elastic fracture mechanics theory, the fracture characteristics of fractured coal in a three-dimensional state were discussed. The results show that: (1) With the increase of the inclination angle of the rock bridge, the macroscopic intensity of the coal sample gradually decreased, and the law was obvious. (2) Under uniaxial compression conditions, the internal microcrack development of coal samples with double fractures was mainly tensile cracks, supplemented by shear cracks. The number of tensile crack growth angles parallel to the loading direction is relatively large, and the development direction of shear microcracks is mainly concentrated in the direction parallel to the prefabricated fracture plane and the rock bridge dip angle. (3) In the early stage of loading coal samples containing fractures, the force form from the edge of the sample to the central area is first reduced and then increased. The force at the edge of the sample was the largest, and the stress distribution was more uneven, with a greater difference. When the sample reached the failure stage, the force form transformed into the middle area with the smallest force and the transition zone with the largest force. (4) The X, Z-direction average strain ratio of coal samples with different rock bridge inclination angles gradually increased with the increase of the square area, whereas the Y-direction strain ratio of coal samples with rock bridge inclination angles of 0° and 90° increased first and then stabilized, and those with rock bridge inclination angles of 30° and 60° showed a trend of first decreasing and then stabilizing. (5) The type I stress intensity factor of each crack tip was always greater than the type II and type III stress intensity factors, with a significant difference. With the increase of the inclination angle of the rock bridge, the type I stress intensity factor first decreased and then increased, while the type II stress intensity factor showed a trend of first increasing and then decreasing. Compared with the inner crack tip, the outer crack tip of coal samples with the same rock bridge inclination angle was more likely to crack.
Highlights
From a mesoscopic perspective, it further revealed the form and mechanism of mutual influence between fractures under different rock bridge inclinations.
The study investigated the internal strain evolution law and stress propagation path of a fracture-bearing coal body at a three-dimensional scale.
Make up for the current research shortfall in accurately solving the three-dimensional fracture tip stress intensity factor through experimental methods.
In this paper, we present a numerical study of hydraulic fracturing in brittle rock by using particle flow simulation. The emphasis is put on the influence of in situ stress, differential stress, ...fluid injection rate, fluid viscosity and borehole size on hydraulic fracturing behavior. To this end, an improved hydromechanical coupling model is first introduced to better describe fluid flow and local deformation of particle-based rocks. A series of parameter sensitivity studies are then conducted under the framework of particle flow simulation. Modelling results suggest that the breakdown pressure and time to fracture both linearly increase with confining stress, and hydraulic fracturing patterns present a distinct transition from brittle to ductile. Fluid injection rate and fluid viscosity have similar influences on hydraulic fracturing propagation, their value decrease leads to borehole pressure decrement and time to fracture prolongation. However, the former mainly controls the time to initial cracking, while the latter largely decides the duration of fracturing propagation. As for borehole radius, its increases can directly enhance the fluid diffusion zone, which further intensifies the nonlinear property of borehole pressure, leads to breakdown pressure decrease, prolongs time to fracture and forms more complex hydraulic fractures.
The random distribution of a complex joint network within a coal–rock mass has a significant weakening effect on its bearing capacity, making the surrounding rock of the roadway highly susceptible to ...instability and failure under the influence of in situ stress and mining-induced stress. This poses challenges in controlling the surrounding rock and seriously affects the normal production of mines. Consequently, it is imperative to conduct stability analysis on complex jointed roadway surrounding rock. Therefore, taking the transport roadway of Panel 11030 in the Zhaogu No. 2 Coal Mine as a case study, the microscopic contact parameters of particles and joint surfaces in each rock layer were calibrated through uniaxial compression and shear simulation tests using the particle flow simulation software PFC2D 5.0. Based on the calibrated microscopic contact parameters, a multilayered roadway surrounding rock model containing complex joints was established, and the joint density was quantified to analyze its effects on the displacement field, stress field, force chain field, and energy field of the roadway surrounding rock. The research findings indicate that as the distance to the sidewall decreases, the impact of joint density on the deformation of the surrounding rock of the roadway increases. The displacement of the roadway roof, floor, and sidewalls is affected differently by the joint density, predominantly contingent upon the properties of the rock mass. During the process of stress redistribution in the surrounding rock, the vertical stress of the roof and floor is released more intensively compared to the horizontal stress, while the horizontal stress of the sidewalls is released more intensively compared to the vertical stress. The increase in joint density leads to an increasing release rate of the surrounding rock stress, causing the load-bearing rock mass to transfer towards the deeper part. As the joint density increases, the force chain network gradually transitions from dense to sparse, resulting in a decrease in strong force chains and a decline in the bearing capacity of the surrounding rock, accompanied by an expansion in the range of force chain failure and deformation. With the continuous increase in joint density, the values of maximum released kinetic energy and residual released kinetic energy become larger. Once the joint density reaches a certain threshold, the kinetic energy stability zone consistently maintains a high energy level, indicating extreme instability in the roadway and sustained deformation. The results provide a valuable insight for analyzing the failure mechanism of complex jointed roadway surrounding rock and implementing corresponding support measures.
Backfill-strip mining is proposed as a sustainable mining method to address the shortage of backfill material and high filling costs at present. The overlying strata in backfill-strip mining are ...mainly supported by the combined support pillar (CSP) of the residual coal pillar and the filling body. The stability of the CSP in backfill-strip mining is important to control the surface subsidence and reduce surface environmental damage. The particle flow code (PFC) simulation method is used in this study to investigate the deformation characteristics, failure behaviours, and stress distribution of the CSP for assessing its stability. The different influencing factors of the stability of the CSP, including geological mining factors, backfilling mining techniques, and the sizes of the residual coal pillar and the working face, are discussed. The results show that the shape of the CSP looks similar to a saddle. The vertical stress of the coal pillars is larger than that of the filling body. The subsidence value of coal pillars is smaller than that of the filling body, but the horizontal movement value of the coal pillars is large. Among these influencing factors of the stability, the residual coal pillar width has the greatest influence on the CSP. The different widths of the residual coal pillar lead to changes of the support formation and the bearing stress weight of the CSP, which make a big difference in the stress distribution characteristics, the movement deformation characteristics, and the stability of the CSP. On this basis, the CSP is divided into four types according to the stability and the support characteristics of the CSP, and their deformation characteristics and stability are summarised. The research results are important for guiding the stability assessment of CSP in backfill-strip mining and preventing the subsidence disaster whilst promoting sustainable extraction of coal resources.
Stroke is the second leading cause of death worldwide. Nearly two-thirds of strokes are produced by cardioembolisms, and half of cardioembolic strokes are triggered by Atrial Fibrillation (AF), the ...most common type of arrhythmia. A more recent cause of cardioembolisms is Transcatheter Aortic Valve Replacements (TAVRs), which may onset post-procedural adverse events such as stroke and Silent Brain Infarcts (SBIs), for which no definitive treatment exists, and which will only get worse as TAVRs are implanted in younger and lower risk patients. It is well known that some specific characteristics of elderly patients may lower the safety and efficacy of anticoagulation therapy, making it a real urgency to find alternative therapies. We propose a device consisting of a strut structure placed at the base of the treated artery to model the potential risk of cerebral embolisms caused by dislodged debris of varying sizes. This work analyzes a design based on a patented medical device, intended to block cardioembolisms from entering the cerebrovascular system, with a particular focus on AF, and potentially TAVR patients. The study has been carried out in two stages. Both of them based on computational fluid dynamics (CFD) coupled with Lagrangian particle tracking method. The first stage of the work evaluates a variety of strut thicknesses and inter-strut spacings, contrasting with the device-free baseline geometry. The analysis is carried out by imposing flowrate waveforms characteristic of both healthy and AF patients. Boundary conditions are calibrated to reproduce physiological flowrates and pressures in a patient's aortic arch. In the second stage, the optimal geometric design from the first stage was employed, with the addition of lateral struts to prevent the filtration of particles and electronegatively charged strut surfaces, studying the effect of electrical forces on the clots if they are considered charged. Flowrate boundary conditions were used to emulate both healthy and AF conditions. Results from numerical simulations coming form the first stage indicate that the device blocks particles of sizes larger than the inter-strut spacing. It was found that lateral strut space had the highest impact on efficacy. Based on the results of the second stage, deploying the electronegatively charged device in all three aortic arch arteries, the number of particles entering these arteries was reduced on average by 62.6% and 51.2%, for the healthy and diseased models respectively, matching or surpassing current oral anticoagulant efficacy. In conclusion, the device demonstrated a two-fold mechanism for filtering emboli: while the smallest particles are deflected by electrostatic repulsion, avoiding microembolisms, which could lead to cognitive impairment, the largest ones are mechanically filtered since they cannot fit in between the struts, effectively blocking the full range of particle sizes analyzed in this study. The device presented in this manuscript offers an anticoagulant-free method to prevent stroke and SBIs, imperative given the growing population of AF and elderly patients.
This paper summarises a Doctoral Thesis which proposes a new approach for large-scale forest visualisation with geometrically diverse trees. The main contribution of the proposed method is an ...interactive visualisation of numerous trees without generating geometric data in advance, which is achieved by a new method for on-the-y tree skeleton synthesis with a specific level of detail, and by a new procedural volumetric tree crown visualisation which avoids geometry formation altogether. The proposed method enables visualisation of forests with millions of trees, thus allowing rendering more trees than geometry-based visualisation methods.
The symmetrical fissures located within the surrounding rock of the roadway (borehole) in tunnel engineering activities can easily induce damage and instability of the surrounding rock. Therefore, ...studying the impact of perforated symmetrical fissures on the mechanical properties of rock with a hole has significant practical significance. Based on indoor experimental results, conventional triaxial compression simulations were performed on symmetrical fissure-hole sandstone using PFC2D. The impact of the dip angle and length of symmetric fissures on the mechanical properties of the hole-containing sandstone was analyzed. Furthermore, the relationship between crack propagation and the macroscopic mechanical properties of the specimen was discussed. The results show that: (1) The deterioration effect of symmetric fissures on hole-containing sandstone can be controlled by increasing the fissure dip angle, suppressing the stress drop phenomenon. However, increasing the fissure length exacerbates the deterioration effect. (2) The effect of symmetrical fissure dip angle on the displacement field near the hole decreases with increasing dip angle while increasing fissure length exacerbates the effect of fissures on the displacement field. (3) As the angle between the fissure and the vertical principal stress increases, the degree of tensile failure weakens while the degree of shear failure increases. (4) During the crack development phase, the extension of the stress concentration zone drives rapid crack growth. It exhibits a stress drop in the macroscopic mechanical properties, followed by the evolution of the stress field with loading, allowing rapid expansion of the microcracks and eventually leading to rock destabilization damage.
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.
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