Search Results (53)

Rock fracture mechanics and the associated energy-release behavior play a key role in ensuring safe extraction in underground coal mining. Hydraulic fracturing generates prefabricated fracture networks in competent rock strata, thereby modifying fracture propagation patterns and reducing the failure resistance of the strata. In this study, standardized three-point bending tests were conducted to investigate the fracture behavior of pre-cracked sandstone specimens with different crack morphologies, quantities, and spacings. New crack initiation occurred mainly at the midspan in specimens containing horizontal prefabricated cracks, whereas inclined prefabricated cracks promoted crack initiation from the crack tips. Although horizontal crack length did not exhibit a clear monotonic effect on load-bearing capacity, the overall capacity decreased with increasing crack density or decreasing crack spacing. Vertical cracks further reduced load-bearing performance, particularly at relatively small crack spacings. The strain response exhibited a non-monotonic relationship with horizontal crack parameters, increasing first and then decreasing with increasing crack length and spacing, while showing a positive correlation with vertical crack spacing. Dissipated energy was negatively correlated with prefabricated crack angle, accounting for 92.65%, 89.10%, and 94.03% of the total input energy. With increasing crack length, the proportion of dissipated energy first increased and then decreased, with values of 92.65%, 90.77%, 92.52%, and 96.13%. Energy dissipation decreased with increasing horizontal crack spacing but increased with vertical crack spacing. Numerical simulations further showed that both horizontal and vertical fractures generated by ground fracturing promoted timely strata failure, while vertical fractures were more effective in facilitating overburden fracture propagation and reducing the bearing capacity of the rock strata and advance coal body by more than 13%. These findings provide a mechanistic basis for the control of thick and competent hard-roof strata. Full article

Rock fracture behavior is strongly influenced by grain size and boundary effects, which complicate the determination of fracture parameters and the interpretation of size-dependent failure. This study investigates the fracture behavior of sandstone and diorite within the framework of the boundary effect model (BEM) using three-point bending tests, acoustic emission (AE), and digital image correlation (DIC). By varying the prefabricated crack length, different values of the structural geometric parameters a_e were obtained, and the fracture toughness K_IC and tensile strength f_t were identified by regression analysis. The results show that K_IC = 0.6841 MPa·m^0.5 and f_t = 4.5625 MPa for sandstone, whereas K_IC = 2.7233 MPa·m^0.5 and f_t = 21.8218 MPa for diorite. Increasing the prefabricated crack length reduces the peak load and prolongs the pre-peak damage evolution stage. Diorite, with a larger average grain size, exhibits higher AE energy release, a higher proportion of high-energy AE events, and a larger fracture process zone (FPZ) than sandstone. Moreover, the AE energy distribution along the crack propagation direction shows a distinct “three-stage” characteristic, consistent with the non-uniform distribution of local fracture energy g_f predicted by boundary effect theory. The results indicate that BEM can reasonably characterize the fracture behavior of rocks with different grain sizes, and the identified material parameters can be used to construct a BEM-based structural failure curve for estimating nominal failure stress over a wider range of structural geometric parameters. Full article

Addressing the engineering challenge of creep instability in weakly cemented fractured sandstones within extremely soft coal-bearing formations under long-term loading in western mining areas, using weakly cemented sandstone from a coal mine in Xinjiang as the study subject. This research employs uniaxial graded loading creep tests combined with full-information acoustic emission technology and DIC high-speed strain field observation to investigate the creep deformation patterns (The full name of “DIC” is the three-dimensional high-speed dynamic and static stress–strain analysis system of the DIC strain field measurement and analysis system. For the convenience of expression, this system will be uniformly referred to as DIC in the following text), damage evolution characteristics, and failure mechanisms of sandstone under intact, pre-fabricated 30° fractures, and pre-fabricated 60° fractures. Results indicate: Fractures significantly weaken rock strength and long-term stability. Unfractured specimens primarily exhibit columnar splitting tensile failure, while pre-fractured specimens show pronounced shear failure. Shear cracks accounted for 83.67% of failures in 30° pre-fractured specimens and decreased to 63.44% in 60° pre-fractured specimens. Intact specimens exhibited acoustic emission ringing responses during accelerated creep stages, whereas fractured specimens showed ringing responses as early as the first loading stage. During graded loading, ringing counts in pre-fractured specimens continuously accumulated, with cumulative counts significantly exceeding those of intact specimens. Pre-fabricated cracks induced significant stress concentration effects at the ends, causing failure cracks to propagate preferentially along the crack direction and forming a non-uniform deformation field bounded by the crack. The study revealed the micro-macro evolution patterns of progressive damage during creep in extremely weak fractured rock, providing theoretical support for early warning and control technologies against creep instability in tunnel rock masses of weakly cemented strata in western regions. Full article

To address the current research gap where studies on the failure mechanisms of fissured tunnels mainly focus on single tunnels with insufficient research on double tunnels, and to provide a scientific basis for disaster prevention and control of the Jinan Tunnel on Jinan Ring Expressway, this study investigates the mechanical behavior and failure characteristics of tunnel structures containing fissure–hole composite systems using experimental tests and numerical simulations. The crack initiation, propagation, and coalescence mechanisms are systematically analyzed to provide engineering references for tunnel design and stability assessment. Sand-based 3D printing technology was used to fabricate double-tunnel models with prefabricated fissures of different inclination angles α. Uniaxial compression tests were conducted, and crack evolution was monitored using DIC technology. Meanwhile, numerical simulation verification was performed based on the parallel bond (PB) model of the Discrete Element Method (PFC). The results show that the mechanical response of sand-based 3D-printed models conforms to the brittle characteristics of engineering rock masses. For models without fissures, cracks are preferentially initiated at the top and bottom of the tunnels. For models with fissures, the peak strength is the highest when α = 30° and 60°, and the lowest when α = 45° and 90°. As the fissure inclination angle increases, the tensile stress concentration shifts from the top and bottom of the tunnels and the middle of the fissure to the two ends of the fissure. The numerical simulation results are consistent with the experimental results and can accurately reproduce crack evolution. This study verifies the effectiveness of combining sand-based 3D printing with discrete element simulation, providing a reference for fissure prevention and control as well as operation and maintenance optimization of similar double-tunnel projects. Full article

The study of mechanical response and crack propagation behavior of layered composite rock mass is helpful for the efficient extraction of geological energy and the safety and stability of underground space structures. The shale is a heterogeneous rock, which is often mixed with mudstone and sandstone. Studying the propagation law of cracks in layered composite rock mass can better serve underground engineering. In this paper, three different strength rock materials (coarse sandstone, red sandstone, and gray sandstone) were spliced together to make three-point bending specimens with prefabricated cracks in the middle, and three-point bending experiments under different loading rates were carried out. The digital image correlation method was used to visualize the strain distribution in the three-point bending experiment, and the difference in crack propagation in different layered composite rock masses was studied. The numerical simulation is established by the cohesive element, and the correctness of the simulation is verified by the displacement-load data. Then the crack propagation speed under different conditions is studied. The results show that there are differences and similarities in the crack propagation process in different strength gradient composite rock masses. When the crack propagates from strong to weak, the crack tip receives more complex tensile shear force, which facilitates the crack crossing the interface. As the loading speed increases, the earlier the prefabricated crack initiates, the shorter the time it stays at the joint surface. When the crack propagates from strong to weak, the crack propagation is more penetrating. Full article

To investigate characteristic stress and energy evolution law of carbonaceous shale under dry–wet cycles and fissure angle, several samples with prefabricated fissure angles were prepared and subjected to the coupled influence of dry–wet cycles and loading. The results show that the closure stress, initiation stress, damage stress, and peak stress gradually increase with the increase in confining pressure, effectively suppressing the initiation and propagation of the crack. At the same time, the total energy, elastic energy, and dissipated energy at the crack characteristic stress are enhanced by a linear function relationship, significantly improving the bearing capacity and energy storage capacity of carbonaceous shale. The dry–wet cycle is regarded as the driving force of damage, reducing the crack characteristic stress and the total energy, elastic energy, and dissipated energy of crack characteristic stress. This results in a weakened capacity of the rock samples to store elastic strain energy, ultimately contributing to the damage degradation of carbonaceous shale. The anisotropic damage of rock is controlled by fissure angle. The crack characteristic stress and the total energy, elastic energy, and dissipated energy of crack characteristic stress with a 45° fissure angle is the smallest. Finally, the energy storage level at the damage stress (Kcd) can be used as an early warning indicator for rock failure. Full article

The long-term stability of grout-reinforced fractured rock masses in acidic groundwater environments after tunnel fires is critical for the safe operation of underground engineering. In this study, grouting reinforcement tests were performed on fractured diorite specimens using a high-strength fast-anchoring agent (HSFAA), and their mechanical degradation and microstructural evolution mechanisms were investigated under coupled high-temperature (25–1000 °C) and acidic corrosion (pH = 2) conditions. Multi-scale characterization techniques, including uniaxial compression strength (UCS) tests, X-ray computed tomography (CT), scanning electron microscopy (SEM), three-dimensional (3D) topographic scanning, and X-ray diffraction (XRD), were employed systematically. The results indicated that the synergistic thermo-acid interaction accelerated mineral dissolution and induced structural reorganization, resulting in surface whitening of specimens and decomposition of HSFAA hydration products. Increasing the prefabricated fracture angles (0–60°) amplified stress concentration at the grout–rock interface, resulting in a reduction of up to 69.46% in the peak strength of the specimens subjected to acid corrosion at 1000 °C. Acidic corrosion suppressed brittle disintegration observed in the uncorroded specimens at lower temperature (25–600 °C) by promoting energy dissipation through non-uniform notch formation, thereby shifting the failure modes from shear-dominated to tensile-shear hybrid modes. Quantitative CT analysis revealed a 34.64% reduction in crack volume (V_ca) for 1000 °C acid-corroded specimens compared to the control specimens at 25 °C. This reduction was attributed to high-temperature-induced ductility, which transformed macroscale crack propagation into microscale coalescence. These findings provide critical insights for assessing the durability of grouting reinforcement in post-fire tunnel rehabilitation and predicting the long-term stability of underground structures in chemically aggressive environments. Full article

With the advancement of deep engineering (e.g., deep resource development, tunnel excavation), the deep rock mass is in a high in situ stress environment, leading to a critical engineering challenge: traditional blasting often causes disordered blast-induced crack propagation (severe deviation from the target direction) and unstable expansion rates, which reduce the directional blasting efficiency, trigger over-excavation/under-excavation, and threaten construction safety. Water jet notching is a promising directional control technique, but its coupling effect with vertical stress (a dominant component of in situ stress) on blasting crack characteristics remains unclear—hindering its application in deep engineering. To address this problem, reveal the law of blasting crack expansion in deep rock, explore the mechanism of controlled blasting for deep rock fractures, and clarify the effect of deep environmental water jet notching on the blasting effect, this study carried out experimental research on the crack extension velocity of the directional blasting of prefabricated grooved concrete under vertical stress (based on the crack extension strain gauge test system and perimeter pressure loading system) and verified the results by numerical simulations. The main conclusions are as follows: (1) Within the experimental test range, with the increase in vertical stress, the deviation of cracks from the prefabricated groove center in the vertical direction gradually decreases, indicating that vertical stress can further guide the direction of the crack extension on the basis of prefabricated grooves. (2) The experimentally measured crack expansion velocity shows a decreasing trend with the increase in the crack expansion length; the average crack expansion velocity is enhanced with the increase in vertical stress, while the change in the crack tip velocity is suppressed as a whole and gradually tends to be flat at approximately 555.6 m/s. (3) Numerical simulation results (using a model replicating the experimental concrete specimens) further verify the accuracy of the experimental results: the increase in vertical stress further guides the vertical crack expansion, enhances the average crack expansion velocity, and slows down the decay of the crack extension velocity. The core value of this research lies in “converting theoretical experimental data into engineering control capabilities.” Its findings can be directly applied to key areas such as deep resource development, tunnel engineering, and water conservancy projects. While ensuring engineering safety, improving efficiency, and reducing costs, it also provides scientific support for engineering construction in complex geological conditions. Full article

Understanding the multi-crack coupling fracture behavior in brittle materials is particularly critical for aging dam infrastructure, where 78% of structural failures originate from crack network coalescence. In this study, we introduce the concepts of crack distance ratio (DR) and size ratio (SR) to describe the relationship between crack position and length and employ the discrete element method (DEM) for extensive numerical simulations. Specifically, a crack density function is introduced to assess microscale damage evolution, and the study systematically examines the macroscopic mechanical properties, failure modes, and microscale damage evolution of rock-like materials under varying DR and SR conditions. The results show that increasing the crack distance ratio and crack angle can inhibit the crack formation at the same tip of the prefabricated crack. The increase in the size ratio will promote the formation of prefabricated cracks on the same side. The increase in the distance ratio and size ratio significantly accelerate the rapid increase in crack density in the second stage. The crack angle provides the opposite effect. In the middle stage of loading, the growth rate of crack density decreases with the increase in crack angle. Overall, the size ratio has a greater influence on the evolution of microscopic damage. This research provides new insights into understanding and predicting the behavior of materials under complex stress conditions, thus contributing to the optimization of structural design and the improvement of engineering safety. Full article

With the accelerated development of urban underground spaces, prefabricated tunnel linings have become a research focus due to their advantages in construction efficiency and cost effectiveness. However, issues such as stress concentration at joints and insufficient overall stability hinder their broader application. This study investigates a cut-and-cover prefabricated tunnel project in the Chongqing High-Tech Zone through scale model tests and numerical simulations to systematically compare the mechanical behaviors of cast-in situ linings and three-segment prefabricated linings under surrounding rock loads. The experimental results show that the ultimate bearing capacity of the prefabricated lining is 15.3% lower than that of the cast-in situ lining, with asymmetric failure modes and cracks concentrated near joint regions. Numerical simulations further reveal the influence of joint stiffness on structural performance: when the joint stiffness is 30 MN·m/rad, the bending moment of the segmented lining decreases by 37.7% compared to the cast-in situ lining, while displacement increments remain controllable. By optimising joint pre-tightening forces and stiffness parameters, prefabricated linings can achieve stability comparable to cast-in situ structures while retaining construction efficiency. This research provides theoretical and technical references for the design and construction of open-cut prefabricated tunnel linings. Full article

To overcome the limitation of traditional elastic phase field models that neglect plastic deformation in rock compressive-shear failure, this study developed an elastoplastic phase field fracture model incorporating plastic strain energy and established a coupling framework for plastic deformation and crack evolution. By introducing the non-associated flow rule and plastic damage variable, an energy functional comprising elastic strain energy, plastic work, and crack surface energy was constructed. The phase field governing equation considering plastic-damage coupling was obtained, enabling the simulation of the energy evolution in rock from the elastic stage to plastic damage and unstable failure. Validation was carried out through single-edge notch tension tests and uniaxial compression tests with prefabricated cracks. Results demonstrate that the model accurately captures characteristics such as the linear propagation of tensile cracks, the initiation of wing-like cracks under compressive-shear conditions, and the evolution of mixed-mode failure modes, which are highly consistent with classical experimental observations. Specifically, the model provides a more detailed description of local damage evolution and residual strength caused by stress concentration in compressive-shear scenarios, thereby quantifying the influence of plastic deformation on crack driving force. These findings offer theoretical support for crack propagation analysis in rock engineering applications, including hydraulic fracturing and the construction of underground energy storage caverns. The proposed plastic phase field model can be effectively utilized to simulate rock failure processes under complex stress states. Full article

Fractures and voids are widely distributed in slope rock masses. These defects promote crack initiation and propagation, ultimately leading to rock mass failure. Investigating their damage evolution mechanisms and strength characteristics is of significant importance for slope hazard prevention. A numerical simulation study of Brazilian splitting tests on disk samples containing prefabricated holes and fractures was conducted using the Finite Element Method with Cohesive Zone Modeling (FEM-CZM) in ABAQUS by embedding zero-thickness cohesive elements within the finite element model. This 2021 study analyzed the effects of fracture angle and length on tensile strength and crack propagation characteristics. The results revealed that when the fracture angle is small, cracks initiate near the fracture and propagate and intersect radially as the load increases, ultimately leading to specimen failure, with the crack coalescence pattern exhibiting local closure. As the fracture angle increases, the initiation location of the crack shifts. With an increase in fracture length, the crack initiation position may transfer to other parts of the fracture or near the hole, and longer fractures may result in more complex coalescence patterns and local closure phenomena. During the tensile and stable failure stages, the load–displacement curves of samples with different fracture angles and lengths exhibit similar trends. However, the fracture angle has a notable impact on the curve during the shear failure stage, while the fracture length significantly affects the peak value of the curve. Furthermore, as displacement increases, the proportion of tensile failure undergoes a process of rapid decline, slow rise, and then rapid decline again before stabilizing, with the fracture angle having a significant influence on the proportion of tensile failure. Lastly, as the fracture angle and length increase, the number of damaged cohesive elements shows an upward trend. This study provides novel perspectives on the tensile behavior of fractured rock masses through the FEM-CZM approach, contributing to a fundamental understanding of the strength characteristics and crack initiation mechanism of rocks under tensile loading conditions. Full article

Efficiently extracting effective information from the massive experimental data from physical mechanics and accurately identifying the premonitory failure information from coal rock are key and difficult points of intelligent research on rock mechanics. In order to reveal the deterioration characteristics and the forewarning law of fractured coal rock, the digital image correlation method and the acoustic emission technology were adopted in this study to non-destructively detect the strain field, displacement field, and acoustic emission response in time and frequency domains. Additionally, by introducing the derivative functions of the multi-source information function for quantitative analysis, a comprehensive evaluation method was proposed based on the multi-source information fusion monitoring to forewarn red sandstone failure by levels during loading. The results show that obvious premonitory failure information, such as strain concentration areas, appears on red sandstone’s surface before macro-cracks can be observed. With an increase in the inclination angle of the prefabricated crack, the macroscopic failure mode gradually transforms from tensile splitting failure to tensile-shear mixed failure. Moreover, the dominant frequency signals of high frequency–low amplitude (HF–LA), intermediate frequency–low amplitude (IF–LA) and low frequency–low amplitude (LF–LA) are denser near the stress peak. The initial crack expansion time and failure limit time measured by multi-source information fusion are 20.72% and 26.71% earlier, respectively, than those measured by direct observation, suggesting that the forewarning of red sandstone failure by levels is realized with multi-source information fusion. Full article

In order to investigate the effect of fissure inclination on the mechanical properties, deformation, and crack evolution of the surrounding rock in the roadway, uniaxial compression experiments were conducted on sandstone-like materials with prefabricated fissures. The high-speed camera and DIC (digital image correlation) method were employed to analyze the strain distribution and the crack evolution of the specimen. The results demonstrated that the presence of fissures reduces the stress for crack initiation, with intact specimens producing new cracks from about 75% of peak strength and fissured specimens producing new cracks from 50% to 60% of peak strength. The fissure reduced the strength and elastic modulus of the specimen while increasing the strain. The fissure inclination of 45° exhibited the most significant changes compared to the intact specimen. The peak strength and elastic modulus decreased by 54.52% and 35.95%, respectively, and the strain increased by 151.42%. The intact specimen and specimen with 90° inclination are mainly distributed with the shear crack, tensile crack, and far-field crack, which are mainly tensile–tension damage; specimens with 0~75° inclination are mainly distributed with the wing crack, anti-wing crack, oblique secondary crack, and coplanar secondary crack, which are mainly shear slip damage. The direction of the extension of cracks is related to the fissure inclination. For specimens with 0° inclination, the new cracks mainly propagate in the direction perpendicular to the fissure; for specimens with 30° and 45° inclinations, the new cracks mainly propagate in the direction parallel and perpendicular to the fissure; for specimens with 60° and 75° inclinations, the new cracks propagate in the direction parallel to the fissure; and for specimens with 90° inclination, the new cracks propagate in the direction parallel to the fissure. Full article

To investigate the stratification effect on rock splitting and Mode I fracture characteristics, standard Brazilian splitting disk specimens and straight-crack disk specimens were subjected to splitting loading tests, and a high-speed camera system and acoustic emission (AE) system were used to study the rocks’ mechanical properties, fracture parameters, and AE characteristics. The results demonstrate the following: (1) The tensile strength and fracture toughness of the layered rock exhibit significant stratification effects, gradually decreasing with the increase in the number of layers and the layer angle. (2) The different angles of the stratification planes lead to the diversity of failure modes in the disk specimens. (3) The S-value and the cumulative AE count curve of specimens without prefabricated cracks show two types of pattern during loading: fluctuating increase mode, and “gentle–steep” increase mode. (4) Layered rock specimens exhibit a low ratio of rise time to voltage amplitude (RA) value and high average frequency (AF) characteristics during fracture, and the shear failure mainly occurs during the stable propagation phase after the initiation of macroscopic cracks. (5) The fracture process zone (FPZ)’s length at the peak point of the specimens decreases exponentially with the increase in the number of layers, but this reduction does not go on indefinitely, and there exists a minimum value. Within the range of 0° to 60°, the FPZ length decreases linearly with increasing stratification angle. Full article