Shear Failure Zone Research Articles

Design of 3D printing osteotomy block for foot based on triply periodic minimal surface

The ankle joint, which connects the lower limbs and the sole of the foot, is prone to sprain during walking and sports, which leads to ankle arthritis. Supratroleolar osteotomy is an ankle preserving operation for the treatment of ankle arthritis, in which the osteotomy is an important fixing and supporting part. In order to avoid stress shielding effect as much as possible, the osteotomy block is designed as a porous structure. In this study, the osteotomy block was designed based on three-period minimal surface, and the designed structure was manufactured by 3D printing. The mechanical properties of different structures were studied by mechanical test and finite element simulation. In mechanical tests, the Gyroid structure showed a progressive failure mechanism from bottom to bottom, while the Diamond structure showed a shear failure zone at 45° Angle, which was not conducive to energy absorption and was more prone to brittle fracture than the Gyroid structure. Therefore, the Gyroid structure is valuable for further research in the development of porous osteotomy.

Plowing mechanism of rapid flow-like loess landslides: Insights from MPM modeling

Plowing refers to the phenomenon by which a landslide displaces and pushes the path material forward. Despite its importance in shaping the dynamics of many rapid flow-like loess landslides, the mechanism of plowing is not fully understood. Therefore, this paper presents an improved numerical model based on the material point method (MPM). This model takes into account the interactions between the landslide mass and the path material. The reliability of this model was validated by simulating two granular column collapse experiments involving interaction with an erodible mass. The model was then applied to analyze the plowing dynamics of the Ximiaodian loess landslide in Jingyang County, Shaanxi Province, China. A series of well-designed simulations were conducted to investigate this specific landslide event, contributing to a deeper understanding of the plowing phenomenon in loess landslides. The simulation results show that our model can satisfactorily simulate the plowing process of the Ximiaodian landslide. The plowing process of a rapid-flow loess landslide is proposed to consist of four distinct stages: 1) the collision stage, when the landslide strikes the terrace, resulting in the bending of its strata; 2) the shear failure stage, characterized by significant plastic shear deformation within the bent strata, resulting in the formation of multiple shear failure planes; 3) the steady plowing stage, during which the deformation structure in the affected terrace stabilizes and plowing continues in a steady pattern; and 4) the stopping stage, when the propagation of the disturbed terrace gradually stops. Furthermore, the disturbed terrace zone exhibits three distinct deformation structures. In the rear part, a stable zone develops where the terrace layers are compressed. However, due to the compressive forces exerted by the overlying loess, no significant shear deformation is observed. The middle part is the shear failure zone, characterized by the formation of multiple shear failure planes. The front part is the upheaval zone, where the terrace layers are slightly bent and tilted.

Failure Behavior and Fracture Evolution Mechanism of Coal with Fissures around Holes.

The fracture of coal is the main channel of gas flow and an important factor affecting the stability and efficiency of gas drainage boreholes. The coal structure, the development of hole cracks, and the degree of deformation are different. It affects the strength and mechanical deformation characteristics of coal to a great extent. In order to investigate the law of fracture evolution around the borehole of fractured coal, uniaxial and triaxial compression tests of raw coal samples have been carried out. The stress field evolution characteristics of fractured coal under compression were analyzed by Particle Flow Code (PFC2D). The strength, deformation, and fracture evolution behavior of fractured coal around boreholes under different confining pressures were studied. The results show that the compressive strength and fracture morphology evolution characteristics of coal around the hole are obviously related to the confining pressure and fracture occurrence of raw coal. The borehole structure itself has an important influence on the distribution location of the shear failure zone of the fracture around the hole, and its influence degree increases with the decrease of borehole confining pressure. During the deformation of coal with cracks around the hole, the initiation, propagation, and union behavior of cracks are related to the crack angle β. The cracks with β 0 and 180° are most easily closed during compression and the cracks with β 90° have little effect on the crack propagation zone. When the crack angle β is 45°, it is most easy to sprout and expand at the end; when the coal is compressed to the ultimate strength, the increase rate of the tensile crack increases, and the polymerization and combination behavior of the crack is more obvious. The evolution cloud map of the stress field can better reflect the evolution characteristics of fracture development, expansion, and fracture in the process of coal loading. Studying the failure behavior and fracture evolution mechanism of the coal around the hole can better predict and control the gas migration and extraction effect, which is of great significance to prevent the occurrence of gas accidents.

An Experimental Investigation of the Effects of Block Proportion on Bimrocks, Considering Different Block-to-Matrix Strength Ratios.

The rock block proportion is one of the most important factors affecting the mechanical properties of bimrocks. Under different block-to-matrix strength ratios, the influence of rock block proportion is different. To explore the influence of rock block proportion on the mechanical properties of specimens under different block-to-matrix strength ratios, a new indoor test method for making bimrocks was proposed. A uniaxial compression test and a direct shear test were carried out on specimens with different rock block proportions. The results show that this method can control the block-to-matrix strength ratio well, and the influence of rock block proportion is obviously different under different block-to-matrix strength ratios. The strong matrix sample will decrease significantly after reaching the peak compressive strength, while the weak matrix will decrease slowly after reaching the peak strength. The rock block proportion is negatively correlated with the uniaxial compressive strength of strong matrix samples (the reduction was 12.53%) and is positively correlated with the uniaxial compressive strength of weak matrix samples as a whole, but it changes when block proportion is more than 50%. With the increase in normal stress and rock block proportion increases from 30% to 60%, the shear failure zone of the weak matrix sample increases, and the cracks are inclined, while the strong matrix sample has more secondary cracks. The results of this study also show that the effect of volumetric block proportion (VBP) on the internal friction angle and cohesion of the sample is less related to the block-to-matrix strength ratio.

Ultimate Load-Bearing Capacity and Sustainable Performance of Pile Foundations of Yanji Suspension Bridge in Fault Zone Based on Refined Geological Model

Pile foundation is the most important foundation type of long-span bridges, of which the ultimate load-bearing capacity affects the safety and sustainable performance of bridges. When constructing large-span bridges, the bridge site may be close to the adjacent fault zones, which seriously affects the safety and long-term performance of pile foundations, causing the failure and unsustainability of long-span bridges in their life-cycle service life. At present, there are no engineering design rules or methods for assessing the load-bearing capacity of the pile foundation near the fault zones. To study the influence of the fault zone on the loading-bearing capacity and sustainable performance of pile foundations, triaxial compression tests were carried out on the mylonite at the Yanji suspension bridge site near the Xiangfan–Guangji fault zone in Hubei Province. The mechanical properties of mylonite were reflected by the Mohr–Coulomb yield criterion, and a topographic and geological modeling method based on the multi-platform was established. Then, the ABAQUS finite element software was used to study the deformation, stress, failure modes, and sustainable performance of the pile foundation under different bridge load levels, analyze the safety of the pile foundation in the fracture zone, and summarize the ultimate bearing characteristics of the pile foundation. The results show that the whole pile and surrounding rock are basically elastic under the pressure of the designed load, the plastic zone of the pile foundation is mainly concentrated at the pile bottom, and the shear stress concentration zone of the pile is mainly manifested in the joint of the cap and pile and the interface between soft and hard rock. When the load is increased to 4 times the designed load, the stress concentration area of the pile body gradually shifts upward from the pile bottom, and the surrounding rock at the bottom forms an “X-shaped” shear failure zone. After 100 years of operation, the maximum compressive stress of piles reaches 28.6 MPa, which is 120% higher than that at the beginning of the bridge construction, indicating that the sustainable performance of the piles can withstand the effect of the fault zone over the designed service years.

Experimental Study on Sliding at the Soil-Structure Interface of a Shallow Foundation

Abstract The interaction between a foundation and its supporting soil involves several different energy dissipation mechanisms. For example, sliding of a foundation element is a soil-structure interaction component that can alter the seismic demand on a foundation as well as its superstructure. Experimental data from tests with realistic boundary and stress conditions is required to characterize the frictional properties at the soil-foundation interfaces in order to model sliding of shallow foundations. To this end, a series of large-scale monotonic laboratory shear tests were performed to study the interface between a shallow concrete foundation element and Ottawa F-55 sand under varying normal pressures, shearing rates, and foundation surface roughnesses and textures. The soil-foundation interface behavior was evaluated through a combination of global measurements of force and displacement, as well as by capturing the localized effects around the interface. This article describes the dependency of the interface behavior on each of the aforementioned variables. The results indicate that when subrounded fine sand comes into contact with surfaces of varying roughness, it exhibits different shear resistance behaviors. The findings further reveal insights into the dependency of the interface frictional characteristics and shear failure zone on the real area of contact, which includes irregularities on the surface. The data further utilized to study the impact of adopting different interface friction assumptions on the finite element model’s interface shear response being subjected to monotonic loading. Using the internal friction angle, or a factored down value, as an estimation for interface friction may lead to an erroneous or unconservative prediction of the superstructure response.

Non-contact model test and numerical simulation of plastic zone in sandy soil foundation

Reasonable determination of the ultimate bearing capacity of the foundation is of great significance in engineering applications. Based on the self-developed non-contact testing device for the plastic zone of shallow foundation, this paper carries out model test research on the plastic zone of sandy soil shallow foundation and analyzes the development law of plastic zone under the action of foundation load. The main advantages and functional characteristics of the foundation plastic zone test system are that it can realize the non-contact test of displacement deformation of soil, the dynamic development process of shear deformation, and the plastic zone of model foundation can be obtained intuitively. The development process, morphological boundary characteristics, and foundation failure mode of the plastic zone of the sand foundation are obtained. The plastic zone of the foundation starts from the edges of both sides of the bearing plate; with the increase of the load, the plastic zone gradually develops downward and approaches to the center line, and finally crosses through at the bottom until the local shear failure of the foundation occurs. The measured plastic zone is a symmetrical spindle shape with thin ends and a bulge in the middle; the sand on both sides of the bearing plate partially bulges, forming a “V” shaped shear failure zone, and the bottom of bearing plate forms a triangular compression zone, the foundation failure mode of sandy soil represents a typical punching shear failure mode. In order to study the distribution characteristics of the plastic zone range of foundation under different foundation loads and foundation widths, the Universal Distinct Element Code (UDEC) based on the discrete element method is used for numerical simulation research. By conducting parameterized numerical simulation test for the plastic zone of foundation and analyzing the results, with the changes in the foundation load and the foundation width, the variation law of (1) the depth of the plastic zone on the foundation bottom, (2) the width of the plastic zone on both sides of the foundation, (3) the ratio for the depth of plastic zone to the width of foundation, (4) the ratio for the width of plastic zone to the width of foundation and (5) the ratio for the width of plastic zone to the depth of plastic zone is obtained. The research has significant guiding significance for the study of the development law of plastic zone and foundation design.

Crack instability in deep coal seam induced by the coupling of mining unloading and gas driving and transformation of failure mode

Coal and gas outburst is a violent mechanical instability of gas-containing coal, and its mechanism is crucial for prevention of gas dynamic disasters. The macroscopic instability of gas-containing coal is the result of the expansion and connection of micro cracks driven by unloading induced tensile stress and gas pressure. This study is aimed at solving the problem of crack instability of gas-containing coal during mining unloading. To this end, first, the governing equations of unloading-induced tensile stress and gas pressure evolutions in cracks were built, and the dynamic evolutions and main controlling factors of tensile stress and gas pressure were revealed. Furthermore, the evaluation indexes of crack stability under tensile stress and shear stress were established, and the variation of crack stability during unloading was investigated. Finally, the phase diagram of crack failure mode was drawn, and the transformation of crack failure mode was analyzed. The results show that unloading-induced tensile stress is generated when θ > 54.73°, and it rises gradually with the increase of original stress, unloading rate, unloading time and crack size and dip angle. Overall, gas pressure in cracks increases first and then decreases during unloading. It increases gradually with the increase of initial gas pressure, gas desorption rate and original stress, while it decreases as the unloading rate gets higher (k from 10−4 to 10−2 s−1) and the dip angle gets larger (θ from 50 to 90°). With the increase of unloading ratio, the crack stability declines slowly first and then plunges rapidly until the crack finally becomes unstable. In addition, burial depth of coal seam, unloading rate, gas desorption rate and mechanical strength of coal have significant effects on this process. Different factors have important influences on the crack failure mode. With the increase of burial depth from 100 to 2000 m, the crack failure mode gradually transforms from tensile failure to shear failure, and the higher the unloading rate is, the more easily tensile failure occurs. With the increase of cohesion (0.1–2 MPa), the scope of the shear failure zone shrinks gradually, and the crack is less prone to shear failure. The desorption rate has a notable effect on the scope of the non-failure zone. Under a higher gas desorption rate, the scope of the non-failure zone becomes smaller, which is due to the greater gas pressure in crack for a given time. The research in this paper has certain reference value for further grasping of the instability failure of gas-containing coal and the mechanism of coal and gas outburst.

Impacts of different cooling methods on the dynamic tensile properties of thermal-treated granite

Hot dry rock (HDR) is a kind of clean, renewable energy and has received widespread attention worldwide in recent years. It is significant to explore the dynamic tensile properties of thermal-treated granite using different cooling methods for mining HDR. In the current work, a series of tests were performed containing ultrasonic testing and Split Hopkinson Pressure Bar (SHPB) tests on granite at different temperatures (25, 200, 400, 500, 600, and 800 °C) after different cooling methods (including natural, water, and liquid nitrogen cooling). Then the dynamic tensile properties, thermal shock damage, and energy evolution process of granite using different cooling methods were compared and analyzed. The high-speed camera and Digital Image Correlation (DIC) technology were used to analyze the tensile failure mode and the failure mechanism under different thermal treatment and cooling conditions. According to the experimental results, with the increasing thermal treatment temperature by adopting the three cooling methods, the P-wave velocity, dynamic tensile strength, and final absorbed energy of granite decreased, and thermal shock damage increased. In addition, the effect of liquid nitrogen cooling on thermal-treated granite was the most obvious among the three methods, followed by water cooling and natural cooling. Moreover, with the increasing treatment temperature, the time for the occurrence of central cracks in the specimen after impact was prolonged, and the areas of the shear failure zone and tensile failure zone increased; the influence of different cooling methods was also evident (natural cooling < water cooling < liquid nitrogen cooling). The experimental findings were expected to offer a reference for understanding the dynamic tensile properties and failure mechanism of thermal-treated granite after rapid cooling. By studying the degree of deterioration of rock dynamic properties by different cooling methods, the exploitation efficiency of geothermal wells can be improved.

Dynamic failure characteristics of surrounding rocks under different lateral pressure coefficients in deep tunnel transient excavation

A novel transient unloading testing system was adopted to simulate the transient excavation of tunnels under different lateral pressure coefficients (k0). The results show that the transient excavation of a tunnel induces significant stress redistributions and concentrations, particle displacements and vibrations to the surrounding rocks. The decrease of k0 enhances the dynamic disturbance of transient tunnel excavation, and especially when k0 = 0.4 and 0.2, the tensile stress can be observed on the top of the tunnel. The peak particle velocity (PPV) of the measuring points on the top of the tunnel decreases with the increasing distance between the tunnel boundary and measuring point. The transient unloading wave is generally concentrated on lower frequencies in the amplitude-frequency spectrum under the same unloading conditions, especially for lower k0 values. In addition, the dynamic Mohr–Coulomb criterion was used to reveal the failure mechanism of a transient excavated tunnel by involving the loading rate effect. It is found that the excavation damaged zone (EDZ) of the tunnel is dominated by the shear failure, and the number of the shear failure zones increases with the decrease of k0. The EDZ of tunnels after transient excavations varies from ring-shape to egg-shape and X-type shear with the decrease of k0. The evolution of the EDZ induced by the transient unloading is associated with k0, i.e., the shear failure of surrounding rocks mainly occurs in the stress redistribution stage under high k0 (1.0–0.7), while the dramatic destruction of surrounding rocks is more prone to occur after the transient unloading process when k0 ≤ 0.6.

Numerical simulation on the excavation damage of Jinping deep tunnels based on the SPH method

In order to systematically evaluate the damage degree in Jinping deep tunnels during excavation, an improved SPH algorithm with good Lagrangian characteristics is introduced. By modifying the derivative of the traditional SPH kernel functions, the brittle failure processes of particles are realized. The ‘Fissure Searching Generating Method’ (FSGM) has also been used to determine the establishment of joints and the tunnel excavation parts. Based on the Jinping tunnel engineering practices, numerical models are set up, and the damage degrees of the tunnel excavation under different buried depths, joint distances, positions and inclination angles are numerically simulated, and compared with the engineering site pictures to verify the rationality. (1) Typical “V” shaped shear failure zones are observed when no joints exist in the model, and the damage degree increases with the increasing tunnel buried depth. (2) The increase of joint-tunnel distance causes the damage degree to decrease, and the failure modes between the tunnel and the joint transform from tensile to shear failure. (3) The stability of the tunnel is less affected when the joint appears in the vault, north and south arch shoulder, but the tunnel is prone to becoming unstable when the joint appears in the north and south walls. (4) Steep and gentle dips of the joints greatly influence the excavation stability of the tunnel, while the excavation damage range becomes the smallest when the joint inclination angle is 45°. (5) The improved method can be well applied to predict and evaluate surrounding rock damage in Jinping tunnels.

Numerical investigation on the debonding of cement anchorage in clay rocks based on a hybrid FEM/DEM model

In this study, the failure process of the cement mortar-clay rock (C–C) binary was analyzed using a composite hybrid FEM/DEM model. First, an improved pressure-related cohesive element was used to simulate the mechanical properties of the binary interface. Then, a numerical model of the cement anchorage body in clay rocks was constructed based on the new interfacial cohesive element and FDEM model of clay rock. Finally, the debonding and slip failure process of the anchorage was analyzed by the numerical model and a corresponding physical model test. The results show that: except for the debonding of the interface, the fracture of weaker side material (clay rock) is also a potential failure mode of the C–C binary. The clay rocks have a larger internal friction angle but smaller cohesion, so the failure mode changes from the shear failure of clay rock to the debonding of the interface when the normal pressure is up to 200 kPa. The failure of the anchor may also cause by the generation of the shear failure zone in soft rock instead of the interface debonding. Therefore, the shear strength of the rock should also be considered in the calculation of anchorage resistance.

Dynamic Monitoring of the Standard Penetration of PHC Tubular Piles and Analysis of the Construction Effect Based on Monocular Visual Digital Photography

The penetration of large-diameter tubular piles and their relevant construction effect have significant influences on the bearing capacity of piles and nearby buildings (structures). This study focuses on developing an indoor test model device for the dynamic field monitoring of the construction effect of penetrated large-diameter tubular piles based on monocular visual digital photography. The results show that the relationship between the penetration of the tubular piles and time function (blow counts) changes from a logarithmic function to a linear function when piles penetrate from the loose layer to the sandy layer, and then to the silty soil layer. The penetration rates differ significantly under different formation conditions. There are obvious plugging and squeezing effects as tubular piles penetrate different strata. The plugging effect radiates outward in a rectangular shape. The influence sphere of the squeezing effect is divided into the shear failure zone, radial squeezing zone, and hemispherical expansion zone. According to the measurement data, the squeezing effect increases first and then weakens during the construction of tubular piles. This makes the adjacent pile deviate from the initial position by 17.4 mm, making the next pile deviate from the initial position by 6.4 mm, to the maximum extent. This further verifies the superiority of pile-jumping construction. The research conclusions can provide reasonable suggestions and a reference basis to improve the penetration parameters of tubular piles and optimise their construction techniques.

Characterization of arching effect and failure mode of longwall panel floor

The stress arching mechanism of longwall panel floor were studied by theoretical analysis, numerical simulation and field observation. The study results show that: the direction of the principal stress on the floor is deflected in the process of mining, so the concentrated stress of floor transmits along an arching line. The goaf floor went through a process of high unloading, which intensifies the degree of crack propagation and fracture of the floor. Microcracks in floor are likely to connect with each other to form a macroscopic fracture surface, which leads to a significant volume expansion of goaf floor. There is a terrace-like tensile plastic zone in the goaf floor and an arched shear failure zone exists outside the tensile plastic zone. Field monitoring results reveal that the goaf floor is dominated by unloading tensile failure, and the floor in front of the mining face is dominated by shear failure.

Research on energy dissipation and damage evolution of dynamic splitting failure of basalt fiber reinforced concrete

Based on split Hopkinson bar (SHPB) loading system and digital image correlation (DIC) technology, a series of dynamic splitting experiments of basalt fiber reinforced concrete (BFRC) were carried out to reveal the effects of basalt fiber content and loading level on the dynamic tensile properties, energy dissipation and damage evolution. The results show that the dynamic tensile strength and dynamic increase factor (DIF) have obvious strain rate effect, and the appropriate basalt fiber content (0.39%) can remarkably improve the dynamic tensile strength of concrete. According to the changing trend of dissipated energy curve, the dynamic splitting process can be divided into compaction stage, elastic stage, plastic yield stage and softening stage. Appropriate basalt fiber content (0.39%) can effectively improve the dissipated energy and dissipated energy rate of BFRC. The loading level only promotes the dissipated energy of BFRC, but has no obvious effect on the dissipated energy rate. Adding an appropriate basalt fiber content (0.39%) can reduce the shear strain, thereby reducing the damage degree of the shear failure zone, and delaying the cracking time in the center of the specimen to a certain extent.

CMT welding-brazing of al/steel dissimilar materials using cycle-step mode

Cycle step (CS) process is an advanced mode in cold metal transfer (CMT) welding technology, in which allows to set both the numbers of individual droplets and the pause time between cycles. This novel mode enables a reduced weld heat input and exhibits clear-rippled beads with high degree of reproducibility and high tolerance for workpiece gaps. In the present study, the CMT-CS process was used for welding-brazing of AA5052 aluminum alloy to galvanized low carbon steel with Al-5Si filler wire. A visually acceptable weld with clear fish-scale ripples was successfully obtained in a lap joint configuration. The influence of CMT cycle and interval time on the weld heat input, weld morphology, microstructure and mechanical properties of aluminum/steel dissimilar joints was explored. The results showed that the increase of CMT cycles led to the increase of weld heat input and it in turn increased the braze width and deceased the contact angle; while, the increase of interval time resulted in the decrease of weld heat input and it increased the weld spot spacing and decreased the density of the fish-scale ripples. The change of CMT cycle and interval time had very limited impact on the phase composition of the interfacial reaction layer. The joint strength basically increased with the increased CMT cycle; while, it increased first then deceased with the increased interval time. Two kinds of fracture modes were observed, namely interfacial shear failure and fusion zone failure. The bending angle decreased with the increase of weld heat input.

A three-dimensional DEM modelling of triaxial test on gas hydrate-bearing sediments considering flexible boundary condition

The discrete element method (DEM) has been widely used to investigate the mechanical properties of gas hydrate-bearing sediments (GHBS). However, a flexible membrane boundary has not been incorporated into three-dimensional DEM modelling. Moreover, the roles of hydrates in stress transmission have not been comprehensively interpreted from a particulate scale point of view. This study adopted the finite difference method (FDM)-discrete element method (DEM) coupling method to establish the flexible boundary condition to capture the failure modes of GHBS and categorized the overall contact set into three contact subsets (i.e., soil-soil, soil-hydrate and hydrate-hydrate contact subsets) in a way that the stress transmission characteristics of hydrates can be identified. The simulated macroscale mechanical responses demonstrate a favourable agreement against the experimental results. The results show that the sample with a low hydrate saturation shows a single slant shear failure zone, while the sample with a high hydrate saturation shows an X-shape shear failure zone. The particulate scale analysis shows that the soil-hydrate contact subset not only plays a critical role in stabilizing the strong force chains but also has a significant contribution to the deviatoric stress. By contrast, the hydrate-hydrate contact subset accounts for a small contribution to the deviatoric stress.

Numerical Study of Fracture Characteristics of Deep Granite Induced by Blast Stress Wave

To study the characteristics of rock fracture in deep underground under blast loads, some numerical models were established in AUTODYN code. Weibull distribution was used to characterize the inhomogeneity of rock, and a linear equation of state was applied to describe the relation of pressure and volume of granite elements. A new stress initialization method based on explicit dynamic calculation was developed to get an accurate stress distribution near the borehole. Two types of in situ stress conditions were considered. The effect of heterogeneous characteristics of material on blast-induced granite fracture was investigated. The difference between 2D models and 3D models was discussed. Based on the numerical results, it can be concluded that the increase of the magnitude of initial pressure can change the mechanism of shear failure near the borehole and suppress radial cracks propagation. When initial lateral pressure is invariable, with initial vertical pressure rising, radial cracks along the acting direction of vertical pressure will be promoted, and radial cracks in other directions will be prevented. Heterogeneous characteristics of material have an obvious influence on the shear failure zones around the borehole.

Investigations of Coal-Rock Parting-Coal Structure (CRCS) Slip and Instability by Excavation

Slip and instability of coal-rock parting-coal structure (CRCS) subjected to excavation disturbance can easily induce coal-rock dynamic phenomena in deep coal mines. In this paper, the failure characteristics and influencing factors of CRCS slip and instability were investigated by theoretical analysis, numerical simulations, and field observations. The following main results are addressed: (1) the slip and instability of CRCS induced by excavation are due to stress release, and the damage of the rock parting is partitioned into three parts: shear failure zone, slipping zone, and splitting failure zone from inside to outside with slip; (2) the slip and instability process of CRCS is accompanied by initiation, expansion, and intersection of shear and tensile cracks. The development of the cracks is dominated by shear behaviour, while the tensile crack is the main factor affecting fracture and instability of CRCS; and (3) slip and instability of CRCS are characterized by stick-slip first and then stable slip, accompanied with high P-wave velocity and rockburst danger coefficient based on microseismic tomography.

A three-dimensional discrete element model of triaxial tests based on a new flexible membrane boundary

Based on a new elastic clump model, a flexible membrane is proposed for the discrete element numerical simulations of triaxial tests. Conversional triaxial tests of sandstone under the confining pressures of 2 MPa and 8 MPa were carried out, in order to validate the effectiveness of the proposed numerical simulation method. The numerical model is validated by comparing the numerical results with the test results. The deformation and failure process of numerical model is analyzed by stress–strain curves, micro fractures, displacement fields, stress fields and energy fields. The model shows an X-shape shear failure zone, of which the angle is very close to that of the test; the dip angle of most shear fractures is close to the angle of the internal friction; and there is a large amount of slipping frictional heat generated on the failure surface. During the loading process, the stress chain and stress concentration appear in the middle of the model, which lead to displacement zoning in the model. The failure of the model is associated with the growth of the micro tensile- and shear fractures. This study provides an effective tool for the macro–micro investigation of rock failure processes.