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To accurately capture crack trajectory and failure morphology with continuum mesoscopic model-based methods in three-dimensional space, conventional global uniform refinement schemes may incur significant computational cost. This paper first presents a three-dimensional structured mesh refinement strategy to achieve the hierarchical discretization for the focused region. To deal with the mismatching problem caused by different-sized meshes, a coupling interface technique based on the discontinuous Galerkin framework is proposed. The validity and advantages of the present strategy are first verified by a benchmark example for elastic analysis. By strongly collaborating with the statistical meso-damage smear method, we further simulate the spatial crack growth (including a through-thickness crack, an embedded crack, and a surface crack) and the local failure process of a D-shaped tunnel. Great agreements between the simulation results and the available experiment or numerical reference results demonstrate that the present method can effectively simulate the rock fracturing process as well as save considerable computational time and memory storage. Highlights: A three-dimensional structured mesh refinement strategy is presented for the geometric and scale restrictions of rock meso-fracturing simulations. An effective coupling interface technique is introduced to accurately capture the interface effects and couple different-sized meshes. Different crack configurations, including a through-thickness crack, an embedded crack, and a surface crack are simulated. The present method can achieve an ideal trade-off between accuracy and computing effort. [ABSTRACT FROM AUTHOR]

The stability of the roof in coal mining plays a crucial role in ensuring safe excavation. After coal extraction, the roof experiences complex and unbalanced stress conditions, leading to diverse roof failure behaviors, particularly in the case of thick roofs. This study examines the failure behaviors of roofs with different thicknesses through a combination of on-site measurements, physical modeling, and numerical modeling. The on-site results indicate that the failure of thick roof leads to larger magnitudes, increased magnitude variance, and energy compared to thin roof. A self-developed three-dimensional roof fracture experimental platform was utilized to conduct loading experiments on roofs with different thicknesses. The experimental results reveal that roof thickness significantly impacts peak load, deformation characteristics, crack patterns, and acoustic emission (AE) signals. Thin roof failure only generates tensile cracks, while thick roof failure produces both tensile and shear cracks. This suggests that an increase in roof thickness leads to a transition in the roof's failure mode from pure tensile failure to tensile-shear mixed failure or shear failure. The energy of AE events has a threshold. When below this threshold, more AE events occur for thin roofs, and when above it, thick roofs exhibit more AE events. The digital image correlation (DIC) results and AE location indicate roof failure initiates from the upper surface of the roof, where "O" shaped cracks form around the periphery, followed by gradual development of cracks on the lower surface. Thin roofs exhibit regular "X" shaped crack on lower surface, while thick roofs display additional branching and inflection points based on "X" shaped crack on lower surface. Numerical modeling using four polygonal particle roof models confirms the influence of roof thicknesses on peak load, deformation, and crack distribution, aligning with experimental and on-site results. These findings offer valuable theoretical support for controlling the stability of surrounding rock conditions in the presence of different roof thicknesses. Highlights: Roof thickness significantly affects coal mine safety, leading to diverse roof failure behaviors. On-site measurements, physical experiments, and numerical simulations reveal insights into roof behaviors. An experimental platform was established to explore the correlation between roof thickness and failure behavior. Thicker roofs exhibit higher peak loads, reduced deflection, complex crack propagation, and a tensile-shear mixed failure or shear failure. Numerical models of polygonal particle roofs further corroborate the impact of roof thickness on failure behavior, offering practical support for coal mine stability. [ABSTRACT FROM AUTHOR]

In cold regions, rock fractures can expand under the force of frost heave pressure, potentially triggering severe geological catastrophes such as collapses and landslides on rock slopes. Nevertheless, a comprehensive study of the evolution patterns and causes of frost heave pressure within rock fractures is currently lacking. To address this gap, this paper focuses on single-cracked sandstone and identifies four distinct types of frost heave pressure curves by conducting monitoring tests under various conditions. Each type of curve was thoroughly analyzed to understand its evolution and the underlying causes. It was found that the rupture and drop type curve can be segmented into five stages: incubation, eruption, drop, equilibrium, and dissipation. The drop stage is predominantly attributed to the fracturing of the crack influenced by frost heave pressure. Compared with the rupture and drop type curve, the stable type curve, where the crack does not expand during the freeze–thaw cycle, lacks a drop stage. Furthermore, samples displaying a stable descent type curve possess a higher freezing temperature, causing a noticeable delay in the emergence of their frost heave pressure. This curve, when compared with the stable type, shows a faster reduction in frost heave pressure during the equilibrium stage. For the secondary frost heave type curve, secondary frost heave emerges within the rock fracture at the onset of thawing, with the rupture of ice proving critical to this phenomenon. The frost heave pressure distributed along the crack surfaces of the four types of curves displays a lack of uniformity. This unevenness is observed to correlate with the inconsistent growth of ice according to its self-purification principle. Finally, based on the experimental results, a novel criterion for frost heave pressure and rock fracture cracking is proposed. Highlights: Four distinct frost heave pressure curves are obtained, and the underlying causes of each are examined. The uneven growth of ice causes non-uniform frost heave pressure distribution within the fracture. Ice rupture is pivotal to the formation of secondary frost heave. A novel criterion for frost heave pressure and rock fracture cracking is proposed. [ABSTRACT FROM AUTHOR]

Cracks are commonplace on slope crests. It has been found that cracks have a significant influence on the stability of slopes in soil with strength governed by a linear criterion; rock slopes with cracks are analyzed in this study using a nonlinear rock model. The kinematic approach of limit analysis is utilized because it straightforwardly considers open cracks and provides rigorous bounds to limit loads. The depth range of a vertical crack in the Hoek–Brown rock mass is derived for dry and wet crack boundary conditions. The stability number and factor of safety are provided by assuming the most adverse crack location, and the presence of water is considered as pore-water pressure acting on the boundaries of the crack and failure surface. When the inclination angle of the rock slope is less than 60°, the influence of cracks on the slope stability is negligible (< 5%). However, it increases to 33.9% when the slope angle is increased to 85°. Based on the critical collapse mechanism, the most adverse vertical cracks appear to be deeper in steeper slopes. Examinations of stress vectors on rupture surfaces and principal stresses indicate that the tensile stress components are eliminated by the presence of cracks, whereas the compressive stresses are maintained as they are. Cracks introduced in rock slopes cause the removal of tension in slopes without adjusting the strength envelope to eliminate the tensile strength of rock. Highlights: Stability analysis of rock slopes with cracks are performed by means of parametric form of Hoek–Brown failure criterion. The range of vertical crack depth in rock mass is investigated based on dry and wet crack boundary conditions. Most adverse crack depths and locations are identified for various slope geometry and rock properties. Introduced cracks in rock slopes eliminate tensile stress on failure surfaces. [ABSTRACT FROM AUTHOR]

Electrohydraulic shockwave drilling (EHSD) technology can effectively break rocks in the laboratory. However, there are few studies on the effect of this technology on bottom-hole rock, which limits the practical application. It is of great significance to study the bottom-hole rock-breaking law by establishing a model. In this context, a model for calculating the stress wave in the bottom-hole rock under electrohydraulic shockwave was established based on the stress wave theory, and coaxial electrode, bottom-hole stress field, and rock surface crack were considered. Then, the distribution law of stress wave peak value was studied. Based on the key parameters of rocks obtained by experiments, the influence laws of electrical parameters, location parameters, and well depth on the stress wave peak value were further studied. The results showed that EHSD breaks the bottom-hole rock mainly through compression and shear stress. When there are internal boundaries such as surface cracks or bedding planes, tensile stress failure also occurs near these boundaries. With the increase of capacitance and voltage, the damage effect of EHSD increases. With the decrease of the distance between the electrode, rock, and crack, the maximum stress peak value increases. With the increase of well depth, the shear and compressive stress increases, but the effective tensile stress decreases as a whole. Finally, a confirmatory experiment was carried out on granite and limestone samples with coaxial electrodes. The results showed that there are special orthogonal tensile cracks and "bell mouth" shear cracks, which are consistent with the model calculation. The research can improve our understanding of how EHSD breaks bottom-hole rock during drilling, which can promote the application of this technology in auxiliary drilling. Highlights: A calculation model for calculating the stress wave peak value in the bottom-hole rock is established. Coaxial electrode, bottom-hole stress field, and rock surface crack were considered in the model. The effects of electrical parameters, location parameters, and well depth on the stress waves were studied. The rationality of the model was verified by a confirmatory experiment of broken granite rock sample. [ABSTRACT FROM AUTHOR]

Traditional grouting technology only provides a bonding effect to surrounding rock fissures. Therefore, an expansion agent was added to the grouting material to provide compressive stress to the rock mass to make it more stable. Based on Mohr–Coulomb theory, the expression of the stress state of rock crack surface under different reinforcement methods and specimen strength under biaxial compression were derived. The action mechanism of grouting reinforcement on improving the strength of rock mass and the failure mode of rock mass after grouting are revealed. With the aid of the Digital Image Correlation (DIC) technique and ABAQUS software, the biaxial compression test and numerical simulation of expansive slurry-inclined layered rock mass composite (ESRC) with different contents of expansive agent were carried out, respectively. The results show that the strength of slurry-rock mass composite is higher with the expansive slurry grouting as compared with the regular slurry grouting. When the expansion agent contents are from 0 to 6%, the strength of the ESRC increased with an increase in the expansion agent content, the failure of ESRC occurred in the component with the lowest strength first. When there is little difference in strength between the weakest member and the sub-weak member, the failure occurred in these two components, while there is large difference in strength, the failure only occurred in the weakest component. Highlights: Through theoretical derivation, the strength expression of composite rock mass under different grouting conditions is obtained. The mechanism of grouting to improve the mechanical strength of rock mass is analyzed. And its failure mode is analyzed, which is of great significance to judge the strength and failure mode of composite rock mass. The theoretical analysis and numerical simulation results are consistent with the lab test results, and the lab test results show that expansive slurry can improve the strength of fractured rock mass better than regular slurry. [ABSTRACT FROM AUTHOR]

Every macroscopic crack that leads to rock failure is a product of microcracks. Similar to fault events that occur at different locations during earthquakes, microfractures show chaotic and concentrated distributions within rocks and in different macroscopic fracture planes, respectively. Spatial distribution is used to describe the crack propagation process. In this study, we conducted true triaxial unloading and compression tests on cuboid granite samples to simulate real stress conditions, and the spatial coordinates of the microfractures were acquired using acoustic emission (AE) monitoring. Based on graph theory, we constructed the minimum spanning tree structure of AE events, analyzed the crack propagation according to the spatial correlation, and introduced two indexes of spatial correlation length and spatial correlation degree. Strongly correlated AE events exhibited a stronger ability to expand, and this clustering of AE events is called the fracture source. Furthermore, the local lowest point of the spatial correlation length corresponded to the formation time window of the fracture source on the macroscopic crack fracture surface. Results indicated that these fracture sources were distributed in the fracture surface space of each macroscopic crack and corresponded with the final crack morphology. Additionally, the spatial distribution of the fracture sources under different time windows demonstrated the evolution of the crack propagation path with time. Furthermore, we discussed the variation in the AE-dominant frequency in this critical time window, and the final result strongly verified the above statement. Highlights: The minimum spanning tree structure of acoustic emission events was constructed based on graph theory. The temporal variation of spatial correlation length was analyzed and its value described the degree of microfracture aggregation. The spatial correlation degree was proposed and strong correlation microcracks characterize the spatial evolution of macroscopic crack. [ABSTRACT FROM AUTHOR]

Water is a crucial factor that influences the mechanical behavior of rock and can induce failure. The aim of this study was to investigate the effects of water content and water distribution on the mechanical behavior of sandstone subjected to triaxial compression. Sandstone samples under different water states, including dry, unsaturated, saturated, and long-term saturated, were prepared. Some saturated samples were dried in air for different periods to prepare samples that were dry on the outside but remained wet on the inside. The triaxial compression tests were performed on the samples under four confining pressures: 5, 10, 15 and 25 MPa. The results indicated that the water state of a sandstone sample can be characterized by its water content, soaking time, water distribution, and long-term saturation. The different water states can affect remarkably the strength, deformation, and failure pattern of the sandstone samples subjected to triaxial compression. Several empirical equations were established to describe the evolution of the mechanical behavior of sandstone with increasing water content and soaking duration. The nonuniform distribution of water promotes the generation of tensile cracks, while increasing confining pressure promotes the development of shear cracks. The strength of sandstone samples with dry outside and wet inside are lower than that of samples with wet outside and dry inside. What is more, a long-term saturated state increases the strain-softening of the sandstone samples, thereby increasing their deformation and lowering their strength. Highlights: Influence of water state including water content and water distribution on sandstone is comprehensively investigated by triaxial experiments. A series of predictive empirical equations are established to describe the evolutions of the mechanical properties of sandstone with water content/soaking duration. The strengths of the sandstone samples in the dry outside but wet inside state are smaller than those of samples in the wet outside but dry inside state. A long-term saturated state increases the softening of the rock, thereby increasing the deformation and lowering the strength. Water especially nonuniformly distributed water promotes the propagation of tensile cracks near the surface of the rock. [ABSTRACT FROM AUTHOR]

The dynamic fracture toughness of rock materials under blast load is the basis for studying the stability of rock blasting engineering. To study the dynamic fracture characteristics of sandstone with an explosive load, the initiation and propagation times of pre-crack samples with different lengths were obtained through physical experiments. The experimental–numerical method was used to obtain the stress intensity factor (SIF) curve and the crack initiation toughness of the specimens with different pre-crack lengths. By comparing and analyzing the dynamic SIF and explosion load curves at different pre-crack lengths, the following results and conclusions were obtained: (1) a polyvinylidene fluoride pressure gauge effectively measured the blast pressure curves at the blast hole wall and the blast hole with a diameter of 40 mm in the presence of a coupling water medium that could effectively avoid the formation of crushed zones and reduce the attenuation of the blast energy. (2) The crack-tip SIF curves were well correlated to the blast pressure curves with a minimal SIF lagging of 0.3–2.9 μs after the peak of the blast pressure curve and a maximum SIF lagging of 0.1–22.9 μs after the time when the peak blast pressure decayed to zero, and the lagging time increased with the pre-crack length. (3) The crack was affected by the reflected wave during the propagation process, and the reflected wave did not affect the crack propagation within 0–109.8 μs. The longer the pre-crack length was, the longer the reflected wave influence time was. (4) The SIF at the pre-crack tip was controlled to a degree such that the first wave was superposed on the surface wave of the crack surface. Both the peak value of the stress intensity factor curve and the crack initiation toughness value decreased with the increase of the pre-crack length, and an obvious separation inflection point appeared on the stress intensity factor curve with the increase of the pre-crack length. (5) The implications of these findings particularly regarding the test method for dynamic fracture of materials tested by explosion method are discussed. Highlights: The water coupling technology for loading hole effectively avoids the formation of crushing circle in loading hole. The pressure time range of the loading hole is accurately measured by a PVDF pressure sensor. The stress intensity factor curves of internal single-cracked square plate specimens under blast loading are significantly influenced by the superposition of the first wave and the surface wave on the crack surface. The degree of superposition between the first wave and the surface wave on the crack surface is significantly affected by the length of the prefabricated crack. The average loading rate of the stress intensity factor curve varies from 1036 GPa·m1/2·s−1 to 2198GPa·m1/2·s−1 with the change of pre-crack length. [ABSTRACT FROM AUTHOR]

Directional fracturing technology for coal and rock is crucial for preventing accidents resulting from the rupture of the hard roof in coal mines. Supercritical carbon dioxide (SC-CO2) fracturing, a green and waterless technology, has garnered significant research interest due to its ability to form complex crack networks. This technology holds promise for mitigating the risk of hard roof collapse in coal mines. Understanding the mechanisms to induce optimal crack propagation through SC-CO2 fracturing is highly valuable. This study employs the true triaxial test platform for SC-CO2 fracturing to examine the propagation law of cracks induced by directional grooving. A series of SC-CO2 fracturing tests were performed under varying in-situ stress differentials, groove numbers, and spacing between them, and a method for characterizing the directional fracturing effects in terms of inducing cracks was proposed. The morphology of crack propagation in fractured samples was photographed, and the types, lengths, and directions of surface cracks were statistically analyzed. A three-dimensional model of cracks in the fractured samples was reconstructed, revealing the evolution characteristics of hole-line spacing and the main crack-induced angle of deflection. The influence of grooves on crack propagation during SC-CO2 fracturing under various conditions was determined, and the interaction mechanism of the stress field at the crack tip within the grooved area was analyzed. These results provide theoretical guidance for applying SC-CO2 fracturing technology in coal mines. [ABSTRACT FROM AUTHOR]

Microwave-induced hard rock fracturing technology has potential application prospects in tunnel boring machine (TBM) hard tunnel excavation. It is of great significance to research the hard rock cracking induced by rotary microwave applicator irradiation to promote the application of microwave–TBM coupling. Based on a rotary open microwave irradiation hard rock fracturing true triaxial test system independently developed by Northeastern University, Chifeng basalt is taken as the research object. The crack propagation characteristics of hard rock irradiated by a microwave applicator at different rotation speeds under the conditions of no stress and true triaxial stress are studied, and the fracturing mechanism of hard rock irradiated by rotary microwave applicator is revealed. The results show that the temperature in the same area of hard rock rises in a "step shape" under rotary microwave applicator irradiation. The faster the microwave applicator rotation speed is, the shorter the intermittent irradiation time of the same area of hard rock, the quicker the temperature rise is, and the earlier the hard rock cracking time is first detected. The degree of crack propagation and the decrease in compressional wave velocity are positively correlated with the microwave applicator rotation speed. In the process of rotary microwave applicator irradiation, the cracks in the irradiated hard rock surface develop and expand from the high principal strain region at the boundary to the central region of the irradiated surface. Under the condition of no stress, tensile failure in hard rock is induced by rotary microwave applicator irradiation. Under the condition of true triaxial stress, tensile failure induced by rotary microwave applicator irradiation mainly occurs in hard rock, although shear failure occurs at the same time. This makes the crack development in the irradiated hard rock surface denser and the crack network more complex. This research provides a new perspective for microwave–TBM coupled tunneling in hard rock tunnel engineering. Highlights: A rotary open microwave irradiation hard rock fracturing system is developed independently. The influence of microwave applicator rotation speed on cracking effect of hard rock under a true triaxial stress condition is studied. The fracturing mechanism of hard rock irradiated by rotary microwave applicator is revealed. This study provides a new perspective for microwave–TBM coupled rock breaking in hard rock engineering. [ABSTRACT FROM AUTHOR]

The rock fracture behavior has an important impact on the breakage performance of Tunnel Boring Machines (TBMs). To model the rock fracture process, a novel constitutive law is proposed for the cohesive zone model. In this constitutive law, the Mohr–Coulomb criterion is adopted as the crack initiation criterion, and followed by the development of a traction–separation model under both the compression and tension states to describe the strongly pressure-dependent mechanics of rocks. After the complete failure of a cohesive zone, an updated friction model based on the Coulomb formulation is used to determine the contact interaction between the free surfaces of cracks. Three rock fracture examples, Brazilian, uniaxial compression, and triaxial compression tests, are simulated using the proposed constitutive law with zero-thickness cohesive elements. The reliability and validity of the proposed model are demonstrated by comparing the simulation results with the experiments. In addition, parametric studies on the indentation cutting of two identical TBM cutters are numerically conducted to investigate the effects of confining stress, cutter spacing, and cutter velocity on the rock fracture behavior and the contact interaction between the cutter and the rock. [ABSTRACT FROM AUTHOR]

It is of great significance to reveal the penetration damage behavior of hard rock under confining and hydrostatic column pressure for improving the rate of penetration of percussion drilling. In this paper, experiments of single spherical button penetration into granite rock under dynamic impact loading were conducted. The cavity expansion model (CEM) was utilized to predict the radius of the elastic–plastic boundary and compared with the experimental results. The essential parameters of the crater were tested and analyzed, and the damage of the sample was evaluated. The quantitative study of crack and damage zone revealed the failure mechanism. The results indicate that both confining pressure and hydrostatic column pressure have adverse effects on the damage of hard rock, but confining pressure is the dominant factor. The change of fractal dimension is mainly caused by the transformation of crack type. The radius of the elastic–plastic boundary reveals that confining pressure can enhance the strength of the rock, shorten the length of the radial crack, and increase the difficulty of lateral crack propagation to the free surface. Although hydrostatic column pressure can aggravate the internal damage of the rock, the lateral cracks cannot extend to the free surface to form radial cracks due to the distant location of the lateral crack initiation, which makes it impossible to form a large crater. [ABSTRACT FROM AUTHOR]

Extension fractures are typical for the deformation under low or no confining pressure. They can be explained by a phenomenological extension strain failure criterion. In the past, a simple empirical criterion for fracture initiation in brittle rock has been developed. In this article, it is shown that the simple extension strain criterion makes unrealistic strength predictions in biaxial compression and tension. To overcome this major limitation, a new extension strain criterion is proposed by adding a weighted principal shear component to the simple criterion. The shear weight is chosen, such that the enriched extension strain criterion represents the same failure surface as the Mohr–Coulomb (MC) criterion. Thus, the MC criterion has been derived as an extension strain criterion predicting extension failure modes, which are unexpected in the classical understanding of the failure of cohesive-frictional materials. In progressive damage of rock, the most likely fracture direction is orthogonal to the maximum extension strain leading to dilatancy. The enriched extension strain criterion is proposed as a threshold surface for crack initiation CI and crack damage CD and as a failure surface at peak stress CP. Different from compressive loading, tensile loading requires only a limited number of critical cracks to cause failure. Therefore, for tensile stresses, the failure criteria must be modified somehow, possibly by a cut-off corresponding to the CI stress. Examples show that the enriched extension strain criterion predicts much lower volumes of damaged rock mass compared to the simple extension strain criterion. [ABSTRACT FROM AUTHOR]

In this study, numerical simulations of the behaviour of large-size sandstone specimens exposed to wood crib fires were conducted. The developed numerical model, which innovatively considers the reversible quartz expansion behaviour during heating–cooling cycles, can replicate the temperature distributions, cracking behaviours, and strength variations of sandstone samples under various thermal loads (e.g., fires). Results show that the cracking mechanisms during heating and cooling are different, and the spalling is due to their combining effects. During the heating process, tensile stresses are induced mainly inside the sample due to significant expansions of the outer layer. The induced tensile microcracks beneath the surface will be furtherly extended and widened due to the re-distributed local stresses caused by local failures. After the fire, significant tensile stresses appear in the outer part of the sample due to the increased thermal gradients (i.e., volumetric expansions) of the inner part, leading to crack widening on the surface. These widened cracks will coalesce with the heating-induced cracks in the earlier time and boost the fracturing and spalling behaviour until the temperature across the sample reaches an equilibrium state. The findings are essential for understanding the failure mechanism of sandstone during real fire development. The good agreements between lab tests and numerical simulation also reveal the feasibility of using numerical models to predict the fire-induced damage in sandstone elements in architecture. [ABSTRACT FROM AUTHOR]

The rapid and accurate measurement of in situ stress fields is one of the essential steps in geological engineering. Indoor acoustic emission tests and the Kaiser effect provide a promising prior peak stress measurement method. This paper aims to qualitatively explore the time delay mechanism of the Kaiser effect under uniaxial compressive stress. Acoustic emission tests were conducted with natural and water-soaked sandstone specimens to examine the time delay phenomenon of the Kaiser effect, and the typical Griffith fracture mechanics theory was adopted to explain the mechanism. The time delay of the Kaiser effect under uniaxial compressive stress was affected by the friction coefficient on the crack surfaces and the initial crack angle β . The time effect was indirectly induced by the changes in the friction coefficient on the crack surfaces, and the friction coefficient was controlled by the water content in the rock specimen. A higher initial friction coefficient led to an earlier and easier time delay of the Kaiser effect. Consequently, from the aspect of the Kaiser effect, it was suggested that when testing in situ stress, measures should be taken to maintain the original moisture contents in the rock cores after they are drilled underground. Otherwise, the in situ stress should be tested within 7 days. [ABSTRACT FROM AUTHOR]

It is significant to conduct anchorage on the non-persistent jointed rock mass by bolts to inhabit the unstable fracture evolution of rock engineering. In this research, a series of experiments are done on 70 specimens to investigate the effect of anchorage method on strength and deformation behavior of a non-persistent jointed rock mass. First, based on the stress–strain curves of anchorage jointed specimens, the effect of anchorage method on the peak strength and elastic modulus of non-persistent jointed rock mass is investigated. The experimental results show that the peak strength and elastic modulus of jointed specimens changes with anchorage method for the same joint angle, while first decreases and then increases from 0° to 90° for the same anchorage method. Second, the effect of pretightening force on the strength and deformation behavior of anchorage jointed specimens is analyzed. The axial stress-axial strain curves of anchorage jointed specimens with various pretightening forces can be characterized into five types: (I) strain softening after the specimen drops to yield platform from peak strength; (II) strain hardening after the specimen drops to yield platform from peak strength; (III) strain hardening after the specimen yields; (IV) stepwise strain softening after the peak strength; (V) single stress drop after the peak strength. The peak strength and elastic modulus of jointed specimens for the same dip angle all increases nonlinearly with the pretightening force. And then, based on a series of surface failure mode of non-persistent jointed rock specimen, it can be seen that the initiation, propagation and coalescence of surface cracks depend on not only the joint angle, but also the magnitude of pretightening force. Twelve crack coalescence types are identified to analyze the surface crack mode of anchorage non-persistent jointed rock specimens. Finally, the effect of pretightening force on brittleness index of non-persistent jointed rock mass is made a discussion. [ABSTRACT FROM AUTHOR]

Crack velocity, gas ejection, and stress waves play an important role in determining delay time, designing a blast and understanding the mechanism of rock fragmentation by blasting. In this paper, the emerging times of the earliest cracks and gas ejection on the lateral surfaces of cylindrical granite specimens with a diameter of 240 mm and a length of 300 mm were determined by high-speed photography, and the strain waves measured by an instrument of dynamic strain measurement during model blasting. The results showed that: (1) the measured velocity of gas penetration into the radial cracks was in a range of 196–279 m/s; (2) the measured velocity of a radial crack extending from the blasthole to the specimen surface varied from 489 to 652 m/s; (3) the length of strain waves measured was about 2800 µs, which is approximately 1000 times greater than the detonation time. At about 2850 µs after detonation was initiated, gases were still ejected from the surface cracks, and the specimens still stood at their initial places, although surface cracks had opened widely. [ABSTRACT FROM AUTHOR]

Rock masses contain pre-existing cracks. These cracks are not usually considered when predicting the maximum injection pressure, i.e. the breakdown pressure, in hydraulic fracture stimulations. The conventional approach to predict breakdown pressure is to use the maximum tensile stress failure criterion to calculate the pressure when a point on the borehole wall reaches the tensile strength of the rock. In addition, a pre-existing crack intersecting a hydraulically pressurized section of a borehole may produce a non-planar crack propagation surface. It is important to predict these non-planar crack propagation surfaces to design productive hydraulic fracturing stimulations and to mitigate risks associated with uncertainties of the resultant crack propagation. To gain a better understanding of this problem, a series of hydraulic fracturing experiments were conducted to investigate the breakdown pressures and crack propagation surfaces of a pressurized circular crack represented by a thin notch, subjected to different external triaxial stresses. The results show that the breakdown pressures under the shear stress conditions studied can be estimated using only the resultant normal stress on the plane of the crack. As the material properties of the experimental specimens are well defined and the crack propagation surfaces were mapped, the experimental results presented in this study provide a very useful measured dataset for the validation of various modelling approaches. The propagation surfaces from experiments were found to align closely to the computational predictions based on the maximum tangential stress criterion. Finally, this study gives evidence in three-dimensions that via the hydraulic fracturing process, the propagation of an initially arbitrarily oriented crack will eventually realign to be perpendicular to the minor principal stress direction. [ABSTRACT FROM AUTHOR]

The relative tangential displacement (or slip) and relative normal displacement of flaw surfaces are two of the main factors driving crack initiation and propagation of rocks. In this study, uniaxial compression experiments are performed in flawed marble under different cases, i.e., open flaw and flaw filled by gypsum, cement and resin, and different flaw inclinations, to study the influence of infilling conditions on the flaw surface relative displacement (i.e., relative tangential and normal displacement) and the induced cracking behaviors, such as crack initiation, propagation and crack patterns. The digital image correlation (DIC) technique is used to trace the evolution of relative deformation across the flaw surface and cracking process of flawed specimens. The results reveal that the difference in flaw surface relative displacement, at a given load, is introduced by different infilling materials. Among the four types of flaws, the filled flaw with smaller relative displacement is less likely to initiate cracks than the unfilled flaw with larger relative displacement. Moreover, the crack initiation position, angle, and types are also dependent on the relative deformation between the flaw surfaces. With the increase of the stiffness of the filler or flaw inclination angle, the effect of relative normal displacement on the cracking behavior is weakened, while the effect of relative tangential displacement increasingly dominates. As a result, the filled flaw with smaller relative normal displacement makes wing crack initiation points closer to flaw tips. Moreover, the filler with higher stiffness makes the wing crack initiation angle smaller, improves the possibility of the quasi-coplanar crack initiation, and suppresses the initiation of anti-wing cracks from the flaw tips in hard rock. [ABSTRACT FROM AUTHOR]

Based on the frequent occurrence of stress-induced hazards in deep rock tunnels, a physical model experimental study of the development process, failure mechanism, and onset conditions of deep hard rock spalling in horseshoe-shaped tunnels is carried out. The test process mainly includes model preparation, loading, excavation, and monitoring. A high-speed camera, distributed fiber optic sensing, acoustic emission (AE) system, and visual imaging correlation-2D (VIC-2D) techniques are integrated to monitor the mechanical response of the surrounding rocks. The test results indicate that the physical model test successfully simulates the spalling phenomena of the deep hard rock. The integrated monitoring system successfully captures the information on deformation, AE signals, and crack propagation in the instantaneous process of the surrounding rock spalling failure, which provides a sound basis for a more comprehensive understanding of the mechanism. Microcracks and dilation appear first on the sidewall surface; the cracks then continue to propagate and coalesce, forming thin rock flakes and spalling in a lamellar manner from shallow to deep. The quantitative relationships between the onset stresses, the critical strains, and the initial boundary stresses of the spalling failure are established. At the same depth from the excavation boundary, the radial strain of the sidewall is larger than that of the crown, and it decreases away from the excavation boundary to the interior. The increasing loading in vertical direction leads to the stress concentration in the shallow part at the sidewall and a sharp increase of deformation. There is a positive correlation between the spalling failure degree and the initial boundary stresses. [ABSTRACT FROM AUTHOR]

The failure mechanisms of rocks as a result of hydro-mechanical coupling effects have not been fully understood due to the limited abilities of conventional triaxial test equipment in measuring both the internal and external damage of rocks simultaneously in real time. This study presents an innovative triaxial testing system for detecting the internal and external damage of rocks. The system consists of an innovatively designed built-in acoustic emission sensor, an optical measurement system and a fully transparent pressure chamber. This work shows that the developed system can capture the internal and external damage behaviour of rocks using triaxial tests. The results demonstrate that the developed built-in acoustic emission sensors can measure the internal damage of rock specimens in an aqueous environment during a load test, while the proposed configuration of the optical measurement system together with the developed imaging construction technique can capture the surface crack development of samples. In addition, the acoustic ring-down counts and event counts can be used to detect the internal damage of the rock specimens, and the ring-down counts reach a significantly high level when the maximum axial force is reached. Furthermore, damage to rock specimens due to triaxial loading first occurs internally, and damage then extends externally. The critical failure point of a specimen can be determined when large fluctuations in the crack fractal dimension and ring-down counts occur simultaneously. [ABSTRACT FROM AUTHOR]

To explore the law of ground deformation from shallow-buried close-distance multi-seam mining, an observation station was built in the Bulianta Coal Mine to measure and record the periodic variation of related parameters about ground subsidence and surface cracks with the advancement of working face. From the data observed from the field, it can be found that, when lower seam mining, the ground subsidence above the previously mined area was deeper and steeper than that above the left pillar; besides, the influence scope of the former was larger than that of the latter. In terms of ground cracks, the ground cracks were formed ahead of the working face and developed rapidly during the period of the breakage of the immediate roof. Besides, the average interval of the ground cracks above the previous gob was 14.75 m, and still existed and hardly changed after the advancement of the working face; while that above the left pillar was 27.8 m and most of them were closed. In addition, when the advance rate of the working face was 12.8 m/day, the advance influence distance of the mining surface crack reached the minimum of 13.6 m. This finding is helpful for protecting the surficial environment in mining area during and after mining operations and is also of significance to conduct green mining in other mining areas. [ABSTRACT FROM AUTHOR]

The structural plane of a rock mass is the indispensable premise for studying and understanding cataclastic texture rock masses, and it is the main reason for the intense heterogeneity of cataclastic structural rock masses. The plane is also the key to classifying the characteristics of cataclastic texture rock masses and quantifying the degree of fragmentation. In this paper, through a thorough investigation of structural plane characteristics, a statistical analysis of the geometric characteristics of structural planes, a network simulation and an analysis of structural planes, a quantitative description index and a quantitative evaluation index are proposed and analyzed from the "point-pine-plane" perspective. A quantitative description index for trace line node density, trace line segment length and crack surface polygon area are proposed based on the structural characteristics of cataclastic texture rock masses. To evaluate fragmentation degree, a trace line node index, a trace line segment length index and a crack surface polygon index are proposed as quantitative evaluation indices. Results show that the relevant indices can effectively identify the structural characteristics and fragmentation degree of cataclastic rock masses. According to the "classification of rock mass from the structural characteristics and fragment degree" approach, a cataclastic texture rock mass classification method is proposed. Taking the example of the cataclastic texture rock mass in the Daguangbao landslide, the quantitative evaluation index and the cataclastic texture rock mass classification method are applied to effectively solve the problem of quantitative evaluation and classify the cataclastic texture rock. [ABSTRACT FROM AUTHOR]

To provide insight into the understanding of step-path failure in rocks, gypsum specimens containing various configurations of preexisting flaws are used to investigate the crack behaviors of step-path failure in rock slopes under compression in terms of the inclination angle and number of flaws. The results show that step-path failure is usually caused by wing cracks (or tensile cracks) when the preexisting flaws are isolated or when two preexisting flaws are adjacent. Additionally, with an increase in the number of flaws, step-path failure is more likely to be caused by mixed wing-secondary crack formation (or tensile-shear crack formation), indicating that the mechanism of step-path failure is associated with the characteristics of the flaw (including the number of flaws, flaw inclination angle, flaw spacing, etc.). In the process of step-path cracking and failure, wing cracks can initiate at or near the middle of a preexisting flaw when the preexisting flaw has a small inclination angle (e.g., 0° or 15°), and secondary cracks usually propagate in three directions: (1) in the horizontal direction of the specimen; (2) coplanar or quasi-coplanar to the preexisting flaw; (3) in the direction perpendicular to that of wing crack propagation. Additionally, the closer the inclination angle is to 90°, the harder the secondary crack initiates. In addition, four types of coalescence cracks are identified from the specimens containing two preexisting flaws. The peak load of the specimen is influenced by the inclination angle; the closer the inclination angle is to 90°, the greater the peak load of the specimen. Furthermore, the equation describing the wing crack path of an isolated flaw, two types of spalling formations and the characteristics of the wing and secondary crack surfaces are determined and discussed. [ABSTRACT FROM AUTHOR]

With the purpose to investigate the influence of joint dip angle and spacing on the TBM rock-breaking efficacy and cracking behaviour, experiments that include miniature cutter head tests are carried out on sandstone rock material. In the experiment, prefabricated joints of different forms are made in rock samples. Then theoretical analysis is conducted to improve the calculating models of the fractured work and crack length of rock in the TBM process. The experimental results indicate that lower rupture angles appear for specimens with joint dip angles between 45° and 60°. Meanwhile, rock-breaking efficacy for rock mass with joint dip angles in this interval is also higher. Besides, the fracture patterns are transformed from compressive shear mode to tensile shear mode as the joint spacing decreases. As a result, failure in a greater extent is resulted for specimens with smaller joint spacings. The results above suggest that joint dip angle between 45° and 60° and joint spacing of 1 cm are the optimal rock-breaking conditions for the tested specimens. Combining the present experimental data and taking the joint dip angle and spacing into consideration, the calculating model for rock fractured work that proposed by previous scholars is improved. Finally, theoretical solution of rock median and side crack length is also derived based on the analytical method of elastoplastic invasion fracture for indenter. The result of the analytical solution is also in good agreement with the actual measured experimental result. The present study may provide some primary knowledge about the rock cracking character and breaking efficacy under different engineering conditions. [ABSTRACT FROM AUTHOR]

This paper develops a numerical approach to determine the mechanical behavior of discrete fractures network (DFN) models based on digital image processing technique and particle flow code (PFC). A series of direct shear tests of jointed rocks were numerically performed to study the effect of normal stress, friction coefficient and joint bond strength on the mechanical behavior of joint rock and evaluate the influence of micro-parameters on the shear properties of jointed rocks using the proposed approach. The complete shear stress-displacement curve of the DFN model under direct shear tests was presented to evaluate the failure processes of jointed rock. The results show that the peak and residual strength are sensitive to normal stress. A higher normal stress has a greater effect on the initiation and propagation of cracks. Additionally, an increase in the bond strength ratio results in an increase in the number of both shear and normal cracks. The friction coefficient was also found to have a significant influence on the shear strength and shear cracks. Increasing in the friction coefficient resulted in the decreasing in the initiation of normal cracks. The unique contribution of this paper is the proposed modeling technique to simulate the mechanical behavior of jointed rock mass based on particle mechanics approaches. [ABSTRACT FROM AUTHOR]

Researchers have recently realized that the natural fractures in shale reservoirs are often cemented or sealed with various minerals. However, the influence of cement characteristics of natural fracture on fracturing network propagation is still not well understood. In this work, laboratory-scaled experiments are proposed to prepare model blocks with discrete orthogonal fractures network with different strength of natural fracture, in order to reveal the influence of cemented natural fractures on the interactions between hydraulic fractures and natural fractures. A series of true triaxial hydraulic fracturing experiments were conducted to investigate the mechanism of hydraulic fracture initiation and propagation in model blocks with natural fractures of different cement strength. The results present different responses of interactions between hydraulic and natural fractures, which can be reflected on the pump pressure profiles and block failure morphology. For model blocks with fluctuated pump pressure curves, the communication degree of hydraulic and natural fractures is good, which is confirmed by a proposed new index of 'P-SRV.' The most significant finding is that too high and too low strength properties of cemented natural fracture are adverse to generate complex fracturing network. This work can help us better understand how cemented natural fractures affect the fracturing network propagation subsurface and give us reference to develop more accurate hydraulic fracturing models. [ABSTRACT FROM AUTHOR]

Fracture growth in underground rock structures occurs under complex stress states, which typically include the in- and out-of-plane sliding deformation of jointed rock masses before catastrophic failure. However, the lack of a comprehensive theoretical and experimental fracture toughness study for rocks under contributions of out-of plane deformations (i.e. mode III) is one of the shortcomings of this field. Therefore, in this research the mixed mode I/III fracture toughness of a typical rock material is investigated experimentally by means of a novel cracked disc specimen subjected to bend loading. It was shown that the specimen can provide full combinations of modes I and III and consequently a complete set of mixed mode I/III fracture toughness data were determined for the tested marble rock. By moving from pure mode I towards pure mode III, fracture load was increased; however, the corresponding fracture toughness value became smaller. The obtained experimental fracture toughness results were finally predicted using theoretical and empirical fracture models. [ABSTRACT FROM AUTHOR]

The convective heat transfer coefficient (HTC) is a useful indicator that characterizes the convective heat transfer properties between flowing fluid and hot dry rock. An analytical method is developed to explore a more realistic formula for the HTC. First, a heat transfer model is described that can be used to determine the general expression of the HTC. As one of the novel elements, the new model can consider an arbitrary function of temperature distribution on the fracture wall along the direction of the rock radius. The resulting Dirichlet problem of the Laplace equation on a semi-disk is successfully solved with the Green's function method. Four specific formulas for the HTC are derived and compared by assuming the temperature distributions along the radius of the fracture wall to be zeroth-, first-, second-, and third-order polynomials. Comparative verification of the four specific formulas based on the test data shows that the formula A corresponding to the zeroth-order polynomial always predicts stable HTC values. At low flow rates, the four formulas predict similar values of HTC, but at higher flow rates, formulas B and D, respectively, corresponding to the first- and third-order polynomials, predict either too large or too small values of the HTC, while formula C, corresponding to the second-order polynomial, predicts relatively acceptable HTC values. However, we cannot tell which one is the more rational formula between formulas A and C due to the limited information measured. One of the clear advantages of formula C is that it can avoid the drawbacks of the discontinuity of temperature and the singular integral of HTC at the points (± R, 0). Further experimental work to measure the actual temperature distribution of water in the fracture will be of great value. It is also found that the absorbed heat of the fluid, Q, has a significant impact on the prediction results of the HTC. The temperatures at the inlet and the outlet used for Q should be consistent with the assumptions adopted in the derivation of its corresponding HTC formula. A mismatched value of Q might be the reason that some existing HTC formulas predict negative or extremely large HTCs at high flow rates. [ABSTRACT FROM AUTHOR]

The article presents information on a device developed for testing the direct shear strength of rock materials. Topics including the use of shear tests in geotechnical engineering to detect rock failures, using hydraulic jack as an apparatus to construct the test device, and assessing the crack behavior using this device, are also analyzed.

The article presents a study of micro-cracks in shale gas plays from stress sensitivity of ultrasonic wave velocities. Key highlights of the study include the physico-mechanical parameters of shale samples, comparison of stress sensitivity coefficient and crack density of shale samples, and elastic anisotropy and mineralogy of the samples.

The article discusses the acoustic emission (AE) characteristics and fracture mechanical behavior of the sandstone under the uniaxial compression conditions. Topics discussed include the use of sandstone in geotechnical engineering applications, the increase of the AE signals with the increase of load, and the identification of the location of the crack with the use of AE three-dimensional positioning technology.

The article focuses on the study conducted by the authors on effects of smaller or larger cracks on strength and stability of rock specimens. Topics discussed include unconfined compression tests which demonstrate stress–stain behavior, effect of the bridge length on the strength of specimens and dependence of stress on width of cracks, regardless of type of specimen.

The article focuses on a study regarding the use of Flattened Brazilian disk test for measuring tensile strength of rocks and rock-like materials. It mentions that the use of stress distribution for tensile strength, and also states the reliability of modified tensile strength formula. It highlights that geometrical factors and material factors of central crack.

Generation, propagation, and coalescence of the shear and tensile cracks in the gob-side filling wall are significantly affected by the location of the fracture of the critical rock block. The Universal Discrete Element Code software was used to investigate crack evolution characteristics in a gob-side filling wall and the parameter calibration process for various strata and the filling wall was clearly illustrated. The cracks in both the filling wall and the coal wall propagate inward in a V-shape pattern with dominant shear cracks generated initially. As the distance between the fracture and the filling wall decreases, the number of cracks in the filling wall decreases, and the stability of the filling wall gradually improves; thus, by splitting the roof rock at the optimal location, the filling wall can be maintained in a stable state. Additionally, we conducted a sensitivity analysis that demonstrated that the higher the coal seam strength, the fewer cracks occur in both the filling wall and the coal wall, and the less failure they experience. With the main roof fracturing into a cantilever structure, the higher the immediate roof strength, the fewer cracks are in the filling wall. With the critical rock block fracturing above the roadway, an optimal strength of the immediate roof can be found that will stabilize the filling wall. This study presents a theoretical investigation into stabilization of the filling wall, demonstrating the significance of pre-splitting the roof rock at a desirable location. [ABSTRACT FROM AUTHOR]

An analytic solution is introduced for the stress field developed in a circular finite disc weakened by a central slit of arbitrary ratio of its edges and slightly rounded corners. The disc is loaded by radial pressure applied along two finite arcs of its periphery, anti-symmetric with respect to the disc's center. The motive of the study is to consider the stress field in a disc with a mechanically machined slit (finite distance between the two lips) in juxtaposition to the respective field in the same disc with a 'mathematical' crack (zero distance between lips), which is the configuration adopted in case the fracture toughness of brittle materials is determined according to the standardized cracked Brazilian-disc test. The solution is obtained using Muskhelishvili's complex potentials' technique adopting a suitable conformal mapping function found, also, in Savin's milestone book. For the task to be accomplished, an auxiliary problem is first solved, namely, the infinite plate with a rectangular slit (in case the resultant force on the slit is zero and also the stresses and rotations at infinity are zero), by mapping conformally the area outside the slit onto the mathematical plane with a unit hole. The formulae obtained for the complex potentials permit the analytic exploration of the stress field along some loci of crucial practical importance. The influence of the slit's width on the local stress amplification and also on the stress concentration around the crown of the slit is quantitatively described. In addition, the role of the load-application mode (compression along the slit's longitudinal symmetry axis and tension normal to it) is explored. Results indicate that the two configurations are not equivalent in terms of the stress concentration factor. In addition, depending on the combination of the slit's width and the load-application mode, the point where the normal stress along the slit's boundary is maximized 'oscillates' between the central point of the slit's short edge (intersection of the slit's longitudinal axis with its perimeter) and the slit's corners. [ABSTRACT FROM AUTHOR]

Modal analysis theory of rock is proposed and the modeling of vibration characteristics of fractured rock is undertaken in this study. The modeling includes two aspects, namely, the natural frequency of rock with a single fracture and the crack expansion energy of rock with multiple fractures. Also, the results of numerical analysis are presented. Four main control parameters are considered, including the material properties, crack size, crack trend and the number of cracks. It is confirmed that the natural frequency of rock will be reduced by the cracks in it. The expansion energy of a single crack is inversely proportional to the number of such cracks in the rock. Namely, the more the similar cracks are, the less the energy required for a single crack expansion is and the smaller the excitation frequency needed for rock resonance is. The vibration characteristics of fractured rock are validated by numerical analysis. The natural frequency of rock increases with the increase of elastic modulus, and decreases with the increase of the angle, the length, the width and the number of cracks. [ABSTRACT FROM AUTHOR]

This paper presents a new analytical criterion for brittle failure of rocks and heavily over-consolidated soils. Griffith's model of a randomly oriented defect under a biaxial stress state is used to keep the criterion simple. The Griffith's criterion is improved because the maximum tensile strength is not evaluated at the boundary of the defect but at a certain distance from the boundary, known as half of the critical distance. This fracture criterion is known as the point method, and is part of the theory of critical distances, which is utilised in fracture mechanics. The proposed failure criterion has two parameters: the inherent tensile strength, σ, and the ratio of the half-length of the initial crack/flaw to the critical distance, a/ L. These parameters are difficult to measure but they may be correlated with the uniaxial compressive and tensile strengths, σ and σ. The proposed criterion is able to reproduce the common range of strength ratios for rocks and heavily overconsolidated soils ( σ/ σ = 3-50) and the influence of several microstructural rock properties, such as texture and porosity. Good agreement with laboratory tests reported in the literature is found for tensile and low-confining stresses. [ABSTRACT FROM AUTHOR]

The article offers a discussion on the evaluation of rocks's indirect tensile strength using different types of jaws coupled with compressive test equipment. It offers information on the standard Brazilian jaw, variation in the contact area of standard jaws and effect of friction between discs and plates. It concludes that the first crack should appear at the centre of the discs while the cracks should appear under the induced tensile stresses.

A novel meshless numerical method, called general particle dynamics (GPD), is proposed to simulate samples of rock-like brittle heterogeneous material containing four preexisting flaws under uniaxial compressive loads. Numerical simulations are conducted to investigate the initiation, growth, and coalescence of cracks using a GPD code. An elasto-brittle damage model based on an extension of the Hoek-Brown strength criterion is applied to reflect crack initiation, growth, and coalescence and the macrofailure of the rock-like material. The preexisting flaws are simulated by empty particles. The particle is killed when its stresses satisfy the Hoek-Brown strength criterion, and the growth path of cracks is captured through the sequence of such damaged particles. A statistical approach is applied to model the heterogeneity of the rock-like material. It is found from the numerical results that samples containing four preexisting flaws may produce five types of cracks at or near the tips of preexisting flaws including wing, coplanar or quasi-coplanar secondary, oblique secondary, out-of-plane tensile, and out-of-plane shear cracks. Four coalescence modes are observed from the numerical results: tensile (T), compression (C), shear (S), and mixed tension/shear (TS). A higher load is required to induce crack coalescence in the shear mode (S) than the tensile (T) or mixed (TS) mode. It is concluded from the numerical results that crack coalescence occurs following the weakest coalescence path among all possible paths between any two flaws. The numerical results are in good agreement with reported experimental observations. [ABSTRACT FROM AUTHOR]

Granite rock mass is used for various rock engineering purposes. To ensure long-term stability, information about the subcritical crack growth (SCG) and an estimate of the long-term strength (LTS) of the rock are necessary. The influence of the anisotropy of granite on its LTS has not yet been clarified. In this study, the anisotropy of the long-term rock strength was investigated for two types of granite rocks, Oshima granite and Inada granite. Specifically, the effect of the anisotropy in crack propagation on the LTS was examined. The results showed that the LTS of granite is anisotropic, as are the fracture toughness and Brazilian tensile strength measured in this study. The LTS was lowest when crack propagation occurred parallel to the rift plane, where most of the microcracks occur. For Inada granite, which has an anisotropic SCG index, the degree of anisotropy of the LTS increased as the time-to-failure increased. This suggests that the LTS of granite is anisotropic. [ABSTRACT FROM AUTHOR]

The article discusses the proposed identification model for the outburst of coal and gas. Topics include the experiments carried to obtain the threshold of the initial expansion energy of released gas (IEERG) indicator, the mechanical conditions which must be met by occurrence of coal and gas outburst, and the potential for the gas in soft coal to transport through the cracks of the barrier layer.

We present a formulation of a discontinuous cellular automaton method for modeling of rock fluid pressure induced fracture propagation and coalescence without the need for remeshing. Using this method, modelers discretize a discontinuous rock-mass domain into a system composed of cell elements in which the numerical grid and crack geometry are independent of each other. The level set method, which defines the relationship between cracks and the numerical grid, is used for tracking the crack location and its propagation path. As a result, no explicit meshing for crack surfaces and no remeshing for crack growth are needed. Discontinuous displacement functions, i.e., the Heaviside functions for crack surfaces and asymptotic crack-tip displacement fields, are introduced to represent complex discontinuities. When two cracks intersect, the tip enrichment of the approaching crack is annihilated and is replaced by a Heaviside enrichment. We use the 'partition of unity' concept to improve the integral precision for elements, including crack surfaces and crack tips. From this, we develop a cellular automaton updating rule to calculate the stress field induced by fluid pressure. Then, the stress is substituted into a mixed-mode fracture criterion. The cracking direction is determined from the stress analysis around the crack tips, where fracture fluid is assumed to penetrate into the newly developed crack, leading to a continuous crack propagation. Finally, we performed verification against independent numerical models and analytic solutions and conducted a number of simulations with different crack geometries and crack arrangements to show the robustness and applicability of this method. [ABSTRACT FROM AUTHOR]

Experiments on man-made flawed rock-like materials are applied extensively to study the mechanical behaviour of rock masses as well as crack initiation modes and crack coalescence types. A large number of experiments on specimens containing two or three pre-existing flaws were previously conducted. In the present work, experiments on rock-like materials (formed from a mixture of sand, plaster, limestone and water at mass ratio of 126:9:9:16) containing multiple flaws subjected to uniaxial compression were conducted to further research the effects of the layout of pre-existing flaws on mechanical properties, crack initiation modes and crack coalescence types. Compared with previous experiments in which only three types of cracks were found, the present experiments on specimens containing multiple flaws under uniaxial compression revealed five types of cracks, including wing cracks, quasi-coplanar secondary cracks, oblique secondary cracks, out-of-plane tensile cracks and out-of-plane shear cracks. Ten types of crack coalescence occurred through linkage among wing cracks, quasi-coplanar secondary cracks, oblique secondary cracks, out-of-plane shear cracks and out-of-plane tensile cracks. Moreover, the effects of the non-overlapping length and flaw angle on the complete stress-strain curves, the stress of crack initiation, the peak strength, the peak strain and the elastic modulus were also investigated in detail. [ABSTRACT FROM AUTHOR]

The diametrical compression of a circular disc (Brazilian test) or cylinder with a small eccentric hole is a simple but important test to determine the tensile strength of rocks. This paper studies the failure mechanism of circular disc with an eccentric hole by a 3D numerical model (RFPA3D). A feature of the code RFPA3D is that it can numerically simulate the evolution of cracks in three-dimensional space, as well as the heterogeneity of the rock mass. First, numerically simulated Brazilian tests are compared with experimental results. Special attention is given to the effect of the thickness to radius ratio on the failure modes and the peak stress of specimens. The effects of the compressive strength to tensile strength ratio ( C/T), the loading arc angle (2 α), and the homogeneity index ( m) are also studied in the numerical simulations. Secondly, the failure process of a rock disc with a central hole is studied. The effects of the ratio of the internal hole radius ( r) to the radius of the rock disc ( R) on the failure mode and the peak stress are investigated. Thirdly, the influence of the vertical and horizontal eccentricity of an internal hole on the initiation and propagation of cracks inside a specimen are simulated. The effect of the radius of the eccentric hole and the homogeneity index ( m) are also investigated. [ABSTRACT FROM AUTHOR]

The axial, crack and total volumetric strains, porosity, elastic constants, crack damage stresses, uniaxial compressive strengths, as well as fracture apertures and number of fracture traces in rock samples surface after compression were defined for different chalk, basalt, dolomite, granite, limestone and sandstone samples exhibiting negative total volumetric strain at failure. It is established that the total (summed) aperture of vertical fractures obtained on the lateral surface of rock sample is related to three characteristic strain parameters: axial strain at the onset of negative total volumetric strain, axial failure strain and negative total volumetric strain at failure. The relation is based on Descartes folium equation, where the length of the loop of folium is equal to axial strain coordinate at the onset of negative total volumetric strain. This relation shows that the total aperture increases according to power law with increasing difference between axial failure strain and axial strain at the onset of negative total volumetric strain. Simultaneously, an increase in this difference leads to an increase in the value of negative total volumetric strain at failure. It is found that a direct correlation between total aperture of fractures and negative total volumetric strain at failure is relatively weak. Nevertheless, total aperture of fractures tends to increase with increasing absolute value of negative total volumetric strain at failure. It is revealed that there is no connection between the number of fracture traces and negative total volumetric strain at failure. [ABSTRACT FROM AUTHOR]

Splitting failure, which is recognized as a special engineering geology phenomenon, occurs continually in the brittle rock mass of caverns during underground excavation. In this paper, a splitting model of linear slippage crack groups is built with fracture mechanics, energy analysis, and crack extension theories. Considering intrinsic cracks in rock mass and change of outer stress, intrinsic cracks propagate into macroscopical splitting cracks that are approximately parallel to the side wall of caverns. The splitting criterion of cavern rock mass and the method for predicting displacement in view of splitting opening displacement are proposed. In the end, the forecasting method is applied to the Jinping-I Hydropower Station, underground caverns engineering in China, the splitting failure zone and forecasting displacement are accordant with the monitoring data. The new forecasting displacement method is proven to contribute to the construction of similar underground caverns. [ABSTRACT FROM AUTHOR]