Pre-existing Fissures Research Articles

Modelling the Blast Cracking Processes of Rock Masses Using a Total Lagrange Meshless Method

The dynamic damage of surrounding rock masses has a great impact on the tunnel stability during the drilling and blasting construction processes. However, traditional FEM has some difficulties in simulating the blast damage processes of rock masses during mesh re-divisions. In our work, the smoothing kernel function in traditional SPH method has been improved, and the momentum equation considering the blasting load is derived, which realizes the modeling of the dynamic damage evolution processes under the SPH framework. Firstly, two typical examples are numerically simulated: 1) Upper slow and lower steep “Y” shaped fissure and 2) Upper short and lower long “Y” shaped fissure. The simulation results are compared with previous experimental results to verify the correctness of the SPH method. Then, the progressive blast damage processes are simulated, and the effects of different stress wave loading modes, different pre-existing fissure distances, different fissure dip angles as well as different ground stress levels on the blast cracking morphologies are investigated. Results show that: The blast cracks first appear around the blast hole, then the radial cracks propagate to the model boundary; The radial crack length increases with the increase of peak blast stress wave; The length of the horizontal radial crack decreases and the length of the vertical radial crack increases with the increase of the horizontal ground stress; The existence of pre-existing fissure prevents the further extension of the blasting cracks, and the increase of the fissure dip angle leads to the crack propagation at the fissure tips; The increase of fissure distance leads to the larger crack propagation in the around the fissure; The damage count increases with the increase of peak blast stress wave; The increase of the ratio of horizontal stress to vertical stress decreases the damage count; The damage counts increase with the increase of fissure dip angles; The damage degree of the model increases with the increase of the fissure distance.

Influences of Pre-Existing Fissure Angles and Bridge Angles on Concrete Tensile Failure Characteristics: Insights from Meshless Numerical Simulations.

The existence of cracks is a key factor affecting the strength of concrete. However, traditional numerical methods still have some limitations in the simulation of crack growth in fissured concrete structures. Based on this background, the numerical treatment method of particle failure in smoothed particle hydrodynamics (SPH) is proposed, and the generation method for concrete meso-structures under the smoothed particle hydrodynamics (SPH) framework is developed. The concrete meso-models under different pre-existing micro-fissure inclinations and bridge angles (the inner tip line of the double pre-existing micro-fissure is defined as a bridge, and the angle between the bridge and the horizontal direction is defined as the bridge angle) were established, and numerical simulations of the crack propagation processes of concrete structures under tensile stress were carried out. The main findings were as follows: The concrete meso-structures and the pre-existing micro-fissures all have great impacts on the final failure modes of concrete. The stress-strain curve of the concrete model presents four typical stages. Finally, the crack initiation and propagation mechanisms of fissured concrete are discussed, and the application of smoothed particle hydrodynamics (SPH) in crack simulations of fissured concrete is prospected.

Modeling fracture in multilayered teeth using the finite volume-based phase field method

The present work, utilizing the finite volume-based phase field method (FV-based PFM), aims to investigate the initiation and propagation of cracks in the second molar of the left mandible under occlusal loading. By reconstructing cone beam computed tomography scans of the patient, the true morphology and internal mesostructure of the entire tooth are implemented into numerical simulations, including both 2D slice models and a realistic 3D model. Weibull functions are introduced to represent the tooth's heterogeneity, enabling the stochastic distribution characteristics of mechanical parameters. The results indicate that stronger heterogeneity leads to greater crack tortuosity, uneven damage distribution, and lower fracture stress. Additionally, different cusp angles (50° and 70°) and pre-existing fissure morphologies (i.e., U-shape, V-shape, IK-shape, I-shape, and IY-shape) also significantly affect the mechanical performance of the tooth. The study reveals that different cusp angles affect the location of crack initiation. Overall, this work demonstrates the utility of the FV-based PFM framework in capturing the complex fracture behavior of teeth, which can contribute to improved clinical treatment and prevention of tooth fractures. The insights gained from this study can inform the design of dental crown restorations and the optimization of cusp inclination and contact during clinical occlusal adjustments.

Effects of fissure length and angle on the fracture modes of 3D printed teeth model: Insights from DIC-based fracture tests and meshless numerical simulations

To investigate the influences of teeth fissure properties on their failure modes, 3D Printing technology is used to prepare the teeth models. The strain distributions of the teeth model surfaces at each moment of the loading processes are obtained by the DIC technique. And the progressive failure processes as well as the stress distributions of the teeth models are simulated by the improved Smoothed Particle Hydrodynamics (SPH) Method. Experimental results show that under the action of the steel ball, the teeth models mainly produce two types of cracks: The tensile cracks along the pre-existing fissures and the shear cracks along both sides of the teeth model. The existence of prefabricated fissures greatly reduces the peak strength of the teeth models. Compared with the circumstances containing no pre-existing fissures, the peak strength of d = 1 cm, d = 2 cm and d = 3 cm decreases by 22.33%, 31.79% and 18.94%, respectively, and the peak strength of θ = 30°, θ = 45°, θ = 60° decreases by 10.78%, 44.01% and 34.3%, respectively. Numerical results show that the initiations of tensile cracks are induced by the high tensile stress concentrations at the pre-existing fissure tips, while the shear cracks are caused by the high tensile stress concentrations in the low tensile stress concentration areas after the initiation of tensile cracks. The research results can provide some references for the understandings of teeth failure mechanisms as well as the applications of SPH method into teeth crack propagation simulations.

Triple-phase-field modeling and simulation for mixed-mode fracture of bedded shale

Comprehending the intricate mechanics and fracture characteristics of shale holds significant importance in the realm of controlling the reconstruction of shale gas reservoirs. It is noteworthy that shale typically exhibits a distinctive bedding structure which sets it apart from other geological formations. Previously, single-mode cracks in shale have been widely studied by former researchers. However, its matrix and bedding may undergo various fracture modes under loading due to the bedded structure. To effectively capture the mixed-mode fracture behavior observed, a triple-phase-field model is proposed in this paper. Three phase-field variables are introduced herein to track pure-tension, tensile-shear, and compressive-shear cracks in the matrix and bedding, respectively. The energetic driving forces governing the propagation of pure-tension and tensile-shear cracks are formulated through the spectrum decomposition of positive strain tensor with its volumetric and deviatoric components; while the energetic driving force that propels the progression of compressive-shear cracks is derived by employing the Mohr-Coulomb criterion within the context of the negative strain tensor. For the bedding plane, lower-dimensional interfacial elements are adopted to capture the mixed-mode fracture. Based on the above triple-phase-field model, both 2D and 3D problems are modeled with good results. The heterogeneity of material is also considered. The simulations reveal that the compression strength and fracture mode of bedded shale are highly related to the orientations of the bedding plane and pre-existing fissure.

An numerical investigation of the three dimensional multi-level force chain network of the sample with a single fissure under uniaxial compression

A novel three-dimensional grain-based model based on Particle Flow Code is proposed. Within this model, both quantitative calculations and multi-level classifications of force chain networks are achieved. A batch of numerical samples with a single pre-existing fissure with different inclination angle αs is constructed and then applied to the uniaxial compression. The result shows that the general force chains (GF) are approximately uniformly distributed in each orientation. The number of high-strength force chains (HF) reaches its maximum at the peak load moment, and the main orientation is consistent with the loading direction. The average value of the force chain in the tip regions of the fissure at any characteristic moment is the highest. In the whole region and the upper and lower regions of the fissure, the level of the force chain network rises as αs increases. Conversely, within the tip regions of the fissure, both the average value and sum value of GF decrease. The fracture resistance of intragranular structures is the highest, surpassing that of both all structures and intergranular structures. The fracture resistance within the tip regions of the fissure is the lowest compared to the entire region and the upper and lower regions of the fissure.

Mechanical properties and failure behaviour of anchored granite containing two filled edge-notched fissures in triaxial compression

High in-situ stress commonly exists in deep rock engineering and causes great difficulties in underground mining support systems. To elucidate the relationship between the anchoring effect of bolts and confining pressures, triaxial compression tests were performed to study the deformation characteristics, mechanical properties, and failure behaviour of anchored granite with two pre-existing fissures. The results demonstrated that the deformation characteristics or mechanical properties of anchored granite, including triaxial compressive strength, elastic modulus, and stored or dissipated energy, increased with increasing confining pressure under low confining pressures. Rockbolt boreholes significantly weaken the mechanical properties and failure behaviour under high confining pressures and have a negative impact on the peak axial strength and stored or dissipated energy of the anchored granite. The fissure angle and confining pressure have a direct impact on the failure mode of anchored granite. With increasing confining pressures or fissure angles, shear cracks and anti-wing cracks instead of tensile wing cracks are more likely to cause ultimate failure. In addition, four modes of crack coalescence were identified: no coalescence, two types of direct crack coalescence, and indirect crack coalescence.

Analysis of Mechanical Properties and Crack Failure Characteristics of Rock with Pre-Existing Fissures Using Particle Flow Simulation

Analysis of Mechanical Properties and Crack Failure Characteristics of Rock with Pre-Existing Fissures Using Particle Flow Simulation

Quantitative investigation of crack propagation and fracture mechanism of fissured granite from the mesoscopic perspective

Natural rock masses comprise numerous fissures, which affect the stability of rock mass engineering. It is crucial for rock engineering to comprehend the crack propagation and fracture mechanisms of fissured rock. In this study, uniaxial compression tests were conducted on fissured granite specimens with various flaw inclinations (0°, 30°, 45°, 60°, and 90°). After specimen failure, the cracks originating from pre-existing fissures were classified (tensile, shear, and tensile-shear cracks). To link the macroscopic failure behavior and mesoscopic fracture mechanism in fissured granite specimens, a quantitative method combining deep learning and scanning electron microscope (QMDL-SEM) was employed for the identification of the mesoscopic fracture mechanism of macroscopic cracks. Identified results suggested that the failure of fissured specimens was attributed to secondary cracks characterized by tensile stress (failure area caused by tensile stress accounting for more than 87%) and far-field cracks dominated by shear stress (failure area caused by shear stress accounting for more than 60%). Additionally, for secondary cracks, as the transition went from tensile crack to tensile-shear crack and then to shear crack in a macroscopic scale, the proportion of failure area caused by shear stress on the failure surface gradually increased. However, tensile stress still dominated the failure area.

Mechanical response of sandstone exposed to monotonic and multilevel fatigue loading: Insights from deformation, energy and acoustic emission characteristics

Rock masses containing intermittent discontinues under dynamic loading have rarely been investigated, although step-path failure frequently occurs in field engineering. This work aims to explore the action mechanism of dynamic cyclic loading and discontinuities on step-path failure. Four treatments including the control (Intact sandstone), pre-flawed samples with assemblage of two 30° dip fissures (type-I), assemblage of 10° and 65° dip fissures (type-II) and assemblage of two 65° dip fissures (type-III) were replicated. Laboratory experiments consisted of conventional uniaxial compression tests (UCT) and multilevel constant-amplitude fatigue tests (MLFT). Real-time acoustic emission (AE) monitoring was conducted along with scanning electron microscopy (SEM) observation on the fractured surfaces. Results indicated that the mechanical properties, energy evolution, AE characteristics and microscopic features of sandstones were significantly influenced by pre-existing fissures. The weakened rates of peak strength ranged from 2.80% to 85.63%, and regardless of loading methods, always followed the sequence of type-I > type-II > type-III. We contribute the weakening effect to the reduction in the effective bearing area and the increase in the dip angle of the rock bridge. With the same pre-fissure morphology, the sandstone under MLFT has enhanced its deformation resistance compared to under UCT. In addition, the maximum AE count/energy during the whole cyclic stage were always significantly lower than those during the monotonic rising process. SEM images revealed that long-term fatigue loading lead to numerous micro pores and transgranular cracks. Ultimately, a simple constitutive model was proposed based on the statistical damage mechanics and geometric damage theory.

Experimental and numerical simulation study on the relationship between cutting depth of high-pressure water jet with high traverse speed and disc cutter penetration of TBM in hard rock tunnel

The disc cutting load of a TBM is insufficient to induce crack propagation in hard rock tunnels due to the high strength of the rock. High-pressure water jets, considered an auxiliary rock-breaking method, enhance the disc cutter's capacity to penetrate cracks by creating pre-existing fissures. In order to examine the correlation between cutting depth achieved by a high-pressure water jet operating at high traverse speeds and the penetration capability of TBM disc cutters, we conducted rock cutting experiments employing varying traverse velocities for the water jet. Additionally, we performed linear disc cutting experiments using full-size disc cutters equipped with grooves. These experiments were conducted using high-strength granite from China's Bei Shan region. We utilized the Discrete Element Software PFC2D to construct a rock breaking model based on Tyson polygons, enabling us to investigate the microfine influence mechanism of grooves with symmetry. The experiments involving high-pressure water jet grooving at high traverse velocities revealed a logarithmic decrease in groove depth as the nozzle's traverse velocity increased. These findings suggest that when high-pressure water jets are employed as an auxiliary method in TBM operations, increased distance from the center of the disc hob leads to higher traverse velocities, resulting in less efficient rock breaking. Nonetheless, increasing the nozzle diameter can enhance the rock-breaking effect. Linear disc cutting experiments indicate that, at a penetration depth of 4 mm, grooves with depths less than 4 mm have minimal impact on assisting in disc cutter rock breaking. However, when the groove depth exceeds 4 mm, there is a noticeable improvement in the disc cutter's breaking performance. The numerical simulation results provide initial validation for the linear disc cutting experiment. Lastly, considering the specific conditions of the Beishan tunnel, we propose a design scheme that involves the zoned arrangement of water jets. Additionally, we recommend appropriately reducing the rotational speed when employing high-pressure water jets as an auxiliary method for rock breaking. The research findings presented in this paper can serve as valuable references for the design and implementation of high-pressure water jet-assisted TBM cutter rock breaking.

The influence of environmental humidity change on the stability of rock caves in Longyou Grottoes

Silty sandstone grottoes are suffered climate changes. The Longyou Grottoes are typical argillaceous siltstone. This paper utilized the coupled humid-mechanical-damage model to investigate the influence of environmental humidity change on the deformation and stability of Cave 4 and Cave 5 in Longyou Grottoes. It is found that the change in environmental humidity significantly influences the stability of caves in Longyou Grottoes. The water saturation of surrounding rock changes with the increase or decrease of ambient humidity. The water swelling and dry shrinkage processes greatly affect the stress state and mechanical properties of the surrounding rock, resulting in micro-cracks which may form fractures that disturb the structural integrity and aggravate the geological hazard of weathering. The cave roof is the most dangerous position during the humidity variation. Pre-existing fissures act as shortcuts for humidity diffusion and stress generation. In turn, the fast diffusion rate and high stress cause further propagation of the fissures. Repairing pre-existing fissures and applying waterproof measurements on the inner wall of caves contributes to the stability and durability of the grottoes. This study improves the understanding of the relationship between the changes in environmental humidity and the weathering of grottoes and provides theoretical support for the subsequent disaster risk management and preservation of Longyou Grottoes.

Water Erosion and Extension of Ground Fissures in Weihe Basin Based on DEM-CFD Coupled Modeling

The Weihe Basin is one of the regions with the highest concentrations of ground fissure development and the most serious disasters in China. Hidden ground fissures are formed in the shallow soil layer due to preexisting fissures generated by tectonism, hidden ground fissures suddenly exposed to the surface after heavy rainfall. Because the details of the water erosion process cannot be replicated by geological survey methods, the erosion and extension mechanism has not fully developed and the discontinuous medium numerical simulation method is well suited for simulating large deformations and facilitates a microscopic perspective in elucidating the underlying causal mechanisms. This paper deploys the discrete element method (DEM)–computational fluid dynamics (CFD) fluid–solid coupled method modeling the growth process of hidden fissures containing different soil types (sand and clay) under heavy rainfall, revealing a mechanism for the development of hidden fissures into surface fissures. The findings include: (1) the emergence process of hidden fissures into surface fissures under heavy rainfall can be summarized into four stages: subsurface erosion into hidden holes, traction into arches, collapse into sinkholes, and horizontal extension; (2) the emergence process of clay is slower than that of sand due to the constraint of cohesion; (3) the shape of the bottom seepage point affects the fissure emergence process, which is an important factor in the macroscopic performance of the exposed surface fissures. The intuitive and reproducible DEM-CFD coupled modeling used in this paper possesses important reference value for the study and prevention of water erosion ground fissures.

Study on the creep properties and fracture behavior of three different rock types with a single preexisting fissure

Study on the creep properties and fracture behavior of three different rock types with a single preexisting fissure

Macro and meso crack evolution of granite specimens with non-straight fissures: A comparison between two bond models

Various flaws in rock mass can weaken mechanical properties of the rock and pose a risk to the safety of underground engineering, with significant implications for design, construction and maintenance. A study of the strength and failure modes of rocks with non-straight fissures can provide a better understanding of fracture mechanisms of rocks. In this study, uniaxial compression tests were carried out on granite specimens with three types of fissures, including straight and arc fissures. The experiments showed that the stress–strain curves and failure modes of the granite specimens were influenced by pre-existing fissures. Compared to straight fissures, arc fissures had less influence on mechanical parameters of the specimens. In order to further investigate the effects of arc fissures, PFC models were constructed, based on calibrated PBM and FJM parameters. Stress–strain curves, mechanical parameters and failure modes of specimens with pre-existing fissures were simulated. The initial cracks around the arc fissure were shifted from the tip of the fissure to the highest point of the arc fissure. Finally, the dip angle of cracks, the stress distribution in the specimens and the local stress evolution around the fissures were further discussed, and the advantages and disadvantages of the two contact models were summarized.

Mechanical and fractal characteristics of sandstone with Pre-existing fissures of different lengths under varying impact loads in SHPB tests

The Split Hopkinson Pressure Bar (SHPB) with a 100 mm diameter was used to investigate the impact of pre-existing fissure length on the mechanical properties of sandstone under different launch pressure (P = 0.3 MPa, P = 0.45 MPa, and P = 0.6 MPa). Subsequently, CT scanning was conducted on sandstone samples with pre-existing fissure after failure to examine the relationship between pre-existing fissure length (Lc) and fractal characteristics under different launch pressures. Our findings indicate that, compared to intact sandstone, the proportion of plastic deformation in the stress–strain curve of sandstone with pre-existing fissure increases. As Lc increases, the initial plastic deformation stress and secant modulus decrease. After CT scanning, sandstone failure was classified into three conditions for different Lc: (1) Axial-splitting-tensile failure is caused by the development of natural defects, and in this case, the degree of development of pre-existing fissure is low, (2) Radial-splitting -tensile-failure is caused by the development of pre-existing fissure, in which the degree of development of pre-existing fissure is high and dominates the macroscopic failure of sandstone, while the degree of development of natural defects is low, and; (3) Combined-splitting-tensile failure: The failure of sandstone includes both axial-splitting-tensile failure and radial –splitting-tensile failure. At this time, both the degree of development of natural defects and pre-existing fissure are high, which jointly dominate the macroscopic failure of sandstone. With increasing launch pressure (P) and Lc, the failure mode of sandstone transitions from axial splitting-tensile failure to radial splitting-tensile failure, and eventually to combined splitting-tensile failure. However, local compression-shear failure occurs with increased launch pressure. Moreover, Lc significantly affects the fractal characteristics of sandstone. Under 0.3 MPa launch pressure, the fractal dimension (DH) on different height (H) slices initially increases and then decreases with increased Lc. Under 0.45 MPa launch pressure, the DH on different H slices exhibits two changes with increasing Lc. For H = 70 mm and H = 100 mm, the variation of DH is decreasing-increasing–decreasing-increasing, while for H = 0 mm and H = 30 mm, the variation of DH is increasing–decreasing-increasing. Under 0.6 MPa launch pressure, the DH on different H slices increases first and decreases secondly with the increase of Lc, but Lc is different when DH reaches its maximum.

Experimental and numerical study on peak strength, coalescence and failure of rock-like materials with two folded preexisting fissures

Natural preexisting fissures are often overlapped, and these preexisting fissures greatly influence the stability of rock mass engineering; hence, it is necessary to investigate the strength and mechanical behaviour of rock materials with folded preexisting fissures. In this paper, for rock-like materials containing two folded preexisting fissures at different inclination angles and undulation angles, the folded preexisting fissures were overlapped by several preexisting fissures at the ends. The peak strength, coalescence patterns and failure modes were comprehensively investigated. It can be clearly found that the peak strength monotonically increased with increasing of the inclination angle; however, the influence of the undulation angle on the peak strength is not obvious through comparison with the inclination. Four coalescence modes can be summarized, and the failure mode can be classified into two types. By combining the crack number-strain curve and stress–strain curve, the failure process of the specimen with two folded preexisting fissures can be divided into four stages. In stage I, no cracks were generated; subsequently, some cracks were generated in stage II; however, the increment of the crack number was small. Many microcracks appear mainly in stage III and stage IV.

Study on the creep properties and crack propagation behavior of single-fissure sandstone based on the damage bond model

As geotechnical engineering expands to deeper formations, the creep behavior of surrounding rock becomes more significant. To better understand the creep deformation and creep propagation of fissured rocks, this study conducts triaxial compression tests and triaxial creep tests of sandstone samples containing a single fissure. The experimental results show that fissured sandstone has a time-dependent failure mode, and creep loading not only causes the crack that dominates the failure of the fissured sandstone to change from a wing crack to an anti-wing crack but also produces more secondary cracks, which are obvious in samples with a low-angle fissure. The steady-creep rate of fissured samples is higher than that of intact samples and first increases and then decreases with increasing fissure angle. The sample with a 45° fissure produces the maximum axial deformation before entering the accelerated creep stage. The theory of the damage bond model (DBM) applied in Particle Flow Code software is proposed, combining the Kachanov damage theory and linear parallel bond model. The DBM effectively simulates the creep deformation and crack propagation process of sandstone in all creep stages, and the failure mode is consistent with the test results. During the initial and steady-state creep stages, the number of microcracks slowly increases, generating a dispersed distribution that rarely exhibits coalescence. Then, the microcracks propagate unstably and coalesce macroscopic cracks, and the accelerated creep stage starts, which is both stress- and time-dependent. Under the same deviatoric stress, the creep simulation samples generate more microcracks than the triaxial compression samples and exhibit a greater reduction in the width of the preexisting fissure after failure. Creep damage accumulation with time significantly affects the evolution of contact forces and displacement vectors. A stress concentration is more likely to occur along the propagation path of an anti-wing crack in creep samples, and the anti-wing cracks present a trend of shear failure. This study plays an important role in predicting the deformation of fissured surrounding rock in underground engineering and explaining the mechanisms of catastrophic events due to the evolution of damage.

Characteristics of stress, crack evolution, and energy conversion of anchored granite containing two preexisting fissures under uniaxial compression

Characteristics of stress, crack evolution, and energy conversion of anchored granite containing two preexisting fissures under uniaxial compression

Experimental investigation on mechanical, acoustic, and fracture behaviors and the energy evolution of sandstone containing non-penetrating horizontal fissures

Flaws are the weakest part of rock mass, which directly determine the stability of underground engineering. Existing investigations on the mechanical behaviors of flawed rock mainly emphasize on the geometric parameters of the fissure, such as inclination, width, and length, but few works have been conducted on the effect of fissure location on rock properties. This study explores the influences of two fissure layouts (distance between horizontal single fissure and specimen end, and horizontal double fissures spacing) on the sandstone properties under uniaxial compression. The properties studied include strength and deformation, failure mode, acoustic emission (AE) characteristics, and energy evolution. The findings indicated that pre-existing fissures significantly reduced the rock's strength, elastic modulus, and peak strain, and the rock's mechanical properties decreased as the distance between the horizontal single fissure and the specimen end decreased and as the double fissures spacing increased. Furthermore, it was observed that intact specimens mainly underwent shear failure and the flawed rocks under uniaxial compression experienced splitting failure or both. The flawed specimens presented multiple higher AE counts before the peak stress, corresponding to crack initiation, propagation and coalescence, which were distinctly distinct from the intact specimen. The energy evolution characteristics of the fissured sandstone reflected their damage process. The findings are promising to improve the understanding of mechanical mechanisms and energy evolution of flawed rock engineering.