Rock Fracture Evolution Research Articles

Thermal-damage effects on fracturing evolution of granite under compression-shear loading

Shear-dominant instability disasters in high-temperature conditions pose a significant challenge for deep underground rock projects, necessitating a thorough understanding of rock fracture evolution and the impacts of thermal damage. To investigate how thermal damage influences shear fracturing behavior and geophysical responses, experiments were conducted on thermally-treated granite under compression-shear loading conditions. The detailed failure process was meticulously monitored using integrated acoustic emission (AE), electric potential (EP), and digital image correlation (DIC) techniques. The effects of thermal treatment on the patterns of AEs, EPs, and DIC strain fields during the loading process were analyzed. Micromechanics related to the influence of thermal treatment on mechanical properties and geophysical responses were explored using X-ray diffraction (XRD) and scanning electron microscopy (SEM). By defining damage variables through multiparameter analysis using AE, EP, and DIC, the study delved into predictive insights on the failure process of thermal-damage granite. The results indicate that the physical properties of granite undergo significant changes as the treatment temperature increases, with the peak stress gradually decreasing and the plastic characteristics increasing. These changes can be attributed to thermal damage, which causes internal water escape, expansion of mineral particles, and crack formation. Additionally, increasing the thermal treatment temperature leads to a decrease in both AE and EP intensity, as well as a reduction in the occurrence of high-magnitude AE events, while the cumulative AE count rapidly increases during the plastic stage. Notably, thermal treatment induces significant alterations in mineral composition, intergranular fractures, and the formation of a gradual zigzag crack path, resulting in the intensified development of shear microcracks. This observation is supported by the varying of AE characteristic parameter AF/RA and the analysis of DIC displacement decomposition. Moreover, the three defined damage variables exhibit accelerated growth prior to catastrophic failure, with the order of precursor occurrence observed as AE, DIC, and EP. However, the occurrence of the precursor time for these three geophysical signals becomes earlier with increasing treatment temperature. These research findings shed light on the mechanism of compression-shear failure of thermal-damage rock and provide valuable references for predicting precursors of the shear-dominant instability disasters such as induced earthquakes.

Damage mechanism and fracture evolution of rock containing defects with LCEM-GFEM method under explosive load

Damage mechanism and fracture evolution of rock containing defects with LCEM-GFEM method under explosive load

Stability Analysis of Support Anchor Bolt in the Process of Surrounding Rock Fracture Evolution

Stability Analysis of Support Anchor Bolt in the Process of Surrounding Rock Fracture Evolution

Identifying rock fracture precursor by multivariate analysis based on the digital image correlation technique

Estimating the initiation and propagation of rock fractures and identifying fracture precursors are significant for rock failure analysis. This study performed numerical uniaxial compression tests of rock samples with an excavated hole and laboratory tests of rock-like samples. Statistical methods were used to analyze the precursory indices of full-field strain information based on digital image correlation (DIC) technology. An identification method for rock fracture initiation based on the absolute standard deviation (ASD) and absolute variation coefficient (AVC) of the strain field was proposed. The results indicated that a sudden change in the shear strain field during the loading process was closely related to the occurrence of internal cracks in the rocks. The variation laws of the ASD and AVC of the strain field were highly correlated with the rock fracture evolution. The progress in the variation rate of the strain field corresponded to the rock fracture evolution, and the change points can be used as precursors of the rock fracture. The precursory points of the excavated and rock-like samples in each crack propagation stage were at least 0.84% and 1.06% earlier than those of the current stage, respectively. The precursory points of each stage were 2.48% and 2.11% earlier than those of the final failure stage. The first precursory points were 45.45% and 42.11% earlier than the final failure time.

Fracture evolution and fracture mechanism of tunnel surrounding rock: A case study based on laboratory tests and theoretical analysis

AbstractTo explore the fracture characteristics and fracture mechanism of the surrounding rock of straight‐walled arched tunnels, this study first carried out biaxial compression tests on rock samples and analyzed the disaster evolution characteristics under loading. Then, a stress distribution function was established using a complex function, and the distribution characteristics of the surrounding rock stress field were determined. Finally, combining the stress field with results obtained by computational tomography and acoustic emission measurements, the dynamic evolution of the main fracture mechanism of the tunnel surrounding rock was comprehensively analyzed, and the fracture mechanism of the surrounding rock of the straight‐walled arched tunnel was elucidated. The research results strengthen the understanding of rock fracture evolution during tunnel construction and operation and provide a basis for tunnel construction and surrounding rock stability analysis.

Numerical study of the effect of infilling material in a hole-like flaw on acoustic emission characteristics and damage evolution in sandstone

Per-existing flaws are wide existence in rock mass, which have a significant effect on mechanical properties and fracturing behaviors. To investigate the effects of flaw type, infilling material, and flaw shape on the mechanical properties and fracture evolution of rock, a series of uniaxial compression tests were conducted on sandstone specimens. The bonded-particle model (BPM) and acoustic emission (AE) techniques were adopted to study the cracking process and AE characteristics of sandstone. The results indicate that relatively rigid infilling can significantly improve the mechanical properties of the rock. The AE simulation in the BPM revealed its fracture evolution, and there is a good consistency between fracture evolution and AE events. The cracking process and crack type around the hole-like flaw are influenced by the type of infilling material and its shape due to changes in the stress state around the hole-like flaw. Tensile cracks in nature usually initiate from the top or the bottom of a hole-like flaw. The shear and compaction cracks in nature prefer to initiate within the infilling material or from the lateral sides of the hole-like flaw. The cracking process of rocks can be characterized by the b-value, the variation of which is comparable with fracture behaviors and energy dissipation.

Structural control within flawed rock specimens under external loading as visualized through repeating nucleation on multiple sites by acoustic emission (AE)

SUMMARYThe behaviour of discontinuities in rock mass have been experimentally investigated extensively for decades by a type of laboratory analogue of flawed rock specimens, taking the artificially prepared flaw(s) as the analogue of rock mass discontinuity(ies). The role of macroscale flaw(s) as controlling structure has been generally neglected. Here, we conduct detailed characterizations of the acoustic emission (AE) from the rock fracture process on the pre-flawed specimens under loading. Though the same phenomena from the literature can be reproduced within our tests, detailed spatial-temporal investigation on the AE events at the nucleation point suggests a different physical interpretation. Artificial flaw(s) play a non-negligible role as a controlling structure for the following rock fracture process including a series of nucleation points alteration via stress transfer. The interlocking parallelogram (IP) control and single-large-structural (SLS) control are two fundamental mechanisms initiated by the artificial flaws. Each of these two mechanisms are observed under a wide range of flaw settings and can potentially coexist within the across length scales. The scale-invariance feature is observed to be fundamentally different within the scale of specimens under SLS and/or IP control, suggesting the incompleteness at the specimen-scale for IP control. Therefore, no ‘fracture coalescence’ zone can be separately investigated without considering its external rock fracture evolution and confirming scale-invariance. Therefore, structural controlling mechanisms must be considered due to the pattern and AE characterization results.

Research on Mechanical Properties and Macroscopic Fracture Evolution of Rock with Different Joint Plane Features

To analyze the safety of geological engineering, experimental research on the mechanical properties of the rock joint plane is critical. In this paper, triaxial compression tests are carried out on rock specimens with the different prefabricated joint planes, including different dip angles, roughness, confining pressure and surrounding rock strength, and the strength characteristics, failure modes, stress evolution laws, and crack propagation laws are revealed. The test results show that when the specimen is damaged, the strain, peak stress, and residual strength all increase with the increase of roughness, confining pressure, and strength of surrounding rock but decrease significantly with the increase of joint plane angle. With the increase of stress, the stress-strain curve shows multiple fluctuations, indicating that the specimen occurs multiple slip-stable phenomena during the loading process. With the increase of the angle, the multiple cracks of the specimen gradually merge, and the cracks are always vertical to the joint plane, showing splitting failure. As the roughness of the joint plane increases, the possibility of the joint plane sliding becomes smaller, and the splitting mode of the specimen develops from multiple fractures to a single fracture. As the strength of the surrounding rock increases, the slope of the stress-strain curve gradually increases, and the splitting mode develops from a small number of single splits to multiple splitting penetration failures. The increase of the confining pressure significantly improves the failure strength of the specimen, and the crack number of the damaged specimen also increases significantly. The research results provide theoretical support for scientific analysis of the stability of underground engineering under the disturbance of fissures or faults.

Numerical Study of Rock Damage Mechanism Induced by Blasting Excavation Using Finite Discrete Element Method

In this paper, the mechanism of rock damage induced by blasting excavation is numerically studied by using an FDEM-based multiphysics fracture analysis software, MultiFracS. Based on the drainage channel project of Guanggu 1st Road to Gaoxin 4th Road, a numerical model considering the near-field fracture process is established to study the influence of a millisecond delay and construction technology on the blasting excavation. Firstly, the double side drift method model is established to analyze the influence of different millisecond delays on the peak blasting vibration velocity. Then, the rock fracture process of the surrounding rock around the blast holes under the blasting excavation construction technology of the double side drift method, the reserved core soil method, and the CRD method is studied, respectively. The numerical simulation results show that the mainshock phases of the blasting vibration velocity waveform generated by different bores overlap when the millisecond delay is small. With the increase in the millisecond delay, the mainshock phase is gradually separated, and the superposition effect of the blasting vibration is weakened. When the millisecond delay is greater than 40 ms, the peak blasting vibration velocity is not affected by the millisecond delay. In the three kinds of blasting excavation construction technologies, the double side drift method has a better effect on the deformation and the fracture control of the surrounding rock. The optimal millisecond delay and the rock fracture evolution process of the surrounding rock around blast holes with different blasting excavation construction technologies are obtained.

Study on the “Pressure Relief and Yielding Support” Joint Prevention and Control Technology for the Weak Impact Roadway in the Extremely Thick Coal Seam

Weak impact occurs during roadway excavation in some extremely thick coal seams in China. Although this hazard is not enough to destroy the roadway, it will cause fracturing and large deformation of the roadway surrounding rock, resulting in the fracturing of bolts and anchor cables and bringing great difficulties to roadway support. In the hope of solving this problem, firstly, the reason for impact occurrence in the roadway of the extremely thick coal seam is analyzed from the perspective of energy. Then, the surrounding rock fracture evolution in such a roadway is explored by means of numerical simulation, microseism, and borehole observation. Furthermore, the “pressure relief and yielding support” joint prevention and control technology is proposed and applied to Yili No. 1 Coal Mine. The field engineering application results show that the joint prevention and control technology can effectively reduce the impact energy and ensure the stability of the roadway surrounding rock in the extremely thick coal seam. The research findings can provide a theoretical foundation for the roadway support of the same type.

Laboratory Investigation on the Effects of Natural Fracture on Fracture Evolution of Granite Exposed to Freeze-Thaw-Cyclic (FTC) Loads

The natural fractures in rock mass are susceptible to damage evolution when subjecting to repeated freeze-thaw (F-T) weathering in cold regions, which can lead to the instability of rock engineering and even occurrence of geological hazards. Knowledge of how natural fracture impacts the overall fracture evolution of freeze-thawed rock is important to predict the stability of rock structure. In this work, we reported uniaxial experimental measurements of the changes in strength, deformation, acoustic emission (AE) pattern, and Felicity effect during increasing amplitude stress-cycling conditions on granite. The results show that the change of fracture aperture is related to the fracture openness and filling characteristics, open-type fracture is sensitive to F-T treatment, and its aperture increases faster than the close-type and fill-type fracture. In addition, strength decreases, and the damping characteristics first decrease and then increase with increasing natural fracture volume. AE activities also present different responses during sample deformation. The proportion of AE signals having low-frequency characteristics increases with increasing natural fracture volume, and the shear sliding along natural fracture results in the surge of AE activities. Moreover, the Felicity effect indicates that the Felicity ratio presents a fluctuation decreasing trend, and the preexisting fractures alter the stress memory characteristics of rock. It is suggested that the changes of the geomechanical and AE pattern are the interactions between the natural fracture and the newly stimulated fracture. The testing results are expected to improve the understanding of the influence of natural fractures on rock fracture evolution and can be helpful to predict the stability of rock structures and rock mass in cold regions.

Dynamic mechanical behaviors of interbedded marble subjected to multi-level uniaxial compressive cyclic loading conditions: An insight into fracture evolution analysis

Rocks in civil and mining engineering usually experience rather complex stress disturbance. Rock dynamic mechanical behaviors under constant stress amplitude condition have been widely studied. However, the rock fracture evolution characteristics subjected to multi-level cyclic loading conditions are not well understood. In this work, multi-level cyclic compressive loading experiments were performed on marble with interbed orientation of 0°, 30°, 60° and 90°. Anisotropic fracture evolution characteristics were revealed using dynamic stress strain descriptions and post-test CT scanning technique. Results show that rock fatigue deformation, strength, lifetime, dynamic elastic modulus and damping ratio are all impacted by rock structure. The interbed structure plays a dominant role in fracture evolution compared with natural fracture and pyrite band. Rock stiffness degrades and damping ratio increases with the increase of interbed orientation. In addition, a two-phase damage accumulation pattern was found for marble under multi-level cyclic loading condition. A damage evolution model was first established to model damage accumulation, the proposed model fits well to the experimental data. Moreover, post-test CT scanning reveals the internal crack coalescence pattern and failure mode is impacted by the interactions of interbeds, natural fracture and pyrite bands. The rock bridge structure in marble with 60° and 90° interbed orientation alters the crack propagation path. Although the crack pattern is relatively simple at rock bridge segment, much more energy is needed to drive crack propagation. The testing results are expected to improve the understanding of the influence of rock structure on fracture evolution when subjected to variable stress amplitude loading conditions.

Non‐Local Triggering in Rock Fracture

AbstractCombining multiphysics observations in the laboratory, we present novel phenomena for the analogue between rock fracture and seismicity. We show, for the first time in a laboratory setting, how a large‐scale flaw in rock can facilitate “non‐local triggering”—remotely triggered damage in rock mass. Results prove analogues of rock fracture evolution to natural seismicity including several lab‐analogues on seismicity model predictions beyond power law. We observe under specimen‐scale criticality a relatively small AE perturbation occurring at a seemingly random location can trigger cascading events whose spatial span can traverse the entire rock fracture system. The inclination angle of the non‐local triggering pairs shows dependence on the large‐scale flaw, and the subsequent generations of triggered AEs by these non‐local triggers decline significantly slower than that of the general triggers. The rock fracture system evolves to a state where the resistance to non‐local triggering continuously reduces, bridging the gap between an idealized lab setting and a natural fault. The non‐locally triggered events can also be reactivated as the rock fracture system evolves towards increased complexity. These lab‐observations as well as the understanding on the analogue relationship between rock fracture and seismicity may shed new light on the insight of the periodical, dynamic, and unpredictable nature of the multiscale rock fracture process.

On the effect of stress amplitude on fracture and energy evolution of pre-flawed granite under uniaxial increasing-amplitude fatigue loads

Rock mass containing discontinues is often subjected to fatigue loads in many mining and civil engineering. The stress amplitude (SA) as an important factor impacts rock fracture evolution during fatigue loading condition. Existing effects to investigate the impacts of SA on rock fatigue behaviors were mainly on constant SA loading condition, the influences of incremental stress amplitude (ISA) for rock subjected to cyclic uniaxial increasing-amplitude conditions is not well understood. In this work, uniaxial increasing-amplitude fatigue loads and post-test 3D CT scanning experiments were conducted on granite containing two pre-existing flaws. Our experimental results reveal the impact of four applied ISA (i.e., 5, 10, 20 and 30 MPa) on rock fracture evolution, energy conversion and crack network pattern at rock bridge segment. The damage accumulation and volumetric deformation increase with the increase of applied ISA. The accelerated damage evolution stage is different for sample with different ISA, and it starts to appear the earliest for a sample subjected to ISA of 30 MPa. Energy conversion analysis reveals that the dissipated energy used to drive the crack propagation is relatively large for a sample with higher ISA. Post-test 3D CT visualization reveals a most striking finding that the crack network pattern becomes complicated as the applied ISA increases. It is suggested that crack coalescence between the flaws is easy to be formed for rock subjected to low ISA, rock bridge segment fracturing is stress amplitude dependent. The testing results are expected to improve the understanding of the influence of stress amplitude on the fatigue behaviors of pre-flawed rock when rock mass is subjected to increasing-amplitude stress disturbance.

Characterization of Dynamic Mechanical Parameters of Rock Fracture Evolution Based on Dry Coupling Point Contact Ultrasonic Technology

The nonlinear characteristics of deep rock masses are very prominent. Accurately obtaining the dynamic mechanical parameters of rock masses and crack damage evolution can provide the necessary data basis for studying the nonlinear behavioral changes of the stress release core during the pressure relief of rock masses. An ultrasonic acquisition system based on a compact dry-coupled acoustic wave probe can adapt to the surface of the rock being monitored with various curvatures. It can monitor the ultrasonic wave at multiple locations during the entire process of uniaxial failure from the field sandstone standard sample. It can also comprehensively collect the ultrasonic wave changes in different areas of the rock during the entire process of failure. Furthermore, the evolution of cracks characterized by dynamic mechanical parameters of sandstone samples is analyzed. The results show that the dynamic Poisson ratio is relatively stable at the beginning and then gradually increases during crack development. While the dynamic elastic wave begins to accelerate down during crack develop steady gradually accelerates to decrease, and falls rapidly when the rock damage. This can serve as a precursor to damage, providing certain feasibility for the characterization of onsite rock damage.

Energy dissipation and damage evolution for dynamic fracture of marble subjected to freeze-thaw and multiple level compressive fatigue loading

For rock engineering in cold regions, rock is often subjected to coupled fatigue conditions of freeze-thaw (F-T) and stress disturbance. Rock fracture evolution and energy mechanism under room temperature and constant stress amplitude loading condition have been widely investigated. Yet the rock energy dissipation and damage evolution characteristics subjected to multiple level cyclic loading conditions are not well understood. In this work, multiple level cyclic compressive loading experiments were conducted using GCTS RTR 2000 rock mechanics system on marble with F-T treatment of 0, 20, 40 and 60 cycles. The fracture evolution and energy dissipation mechanism were analyzed as well as the damage evolution characteristics. The results indicate that F-T treatment strongly influences the fatigue mechanical behaviors of marble, with both fatigue strength and strain energy decreases and irreversible volumetric deformation increases with increasing F-T cycles. The incremental rate of dissipated energy becomes faster as cyclic loading level grows. In addition, a F-T-fatigue loads coupling damage variable was proposed by using the input total strain energy and the dissipated strain energy to describe the rock damage evolution after F-T treatment and experiencing fatigue loading. Moreover, a damage evolution model was first established based on the obtained coupling damage variable to describe the two-phase damage accumulation characteristics. Damage accumulation curve presents a first steady increase and then faster increase trend, the damage evolution model can good fit the experimental data.

Stress distribution and fracture evolution around a trapezoidal cavity in sandstone loaded in compression

To understand the failure mechanism of trapezoidal opening in rock engineering, the fracture evolution of rock containing a cavity under compression was investigated analytically and experimentally in this study. Firstly, the complex variable theory was employed to find the general solution for rock stress around a trapezoidal cavity under biaxial stresses. Secondly, a series of uniaxial compression tests were carried out on prismatic sandstone specimens with a circular or a trapezoidal cavity. During the tests, the acoustic emission (AE) and digital image correlation (DIC) techniques were jointly applied to record and analyze the damage and fracturing behavior. The results showed that both the cavity shape and lateral stress ratio have a significant effect on the stress distribution around the cavity. The fracture evolution starts from primary tensile cracks through compressive fractures and secondary tensile cracks to shear cracks, which can be well explained by the stress distribution. The primary and secondary tensile cracks are not responsible for the ultimate macro failure, which is dominated by the shear cracks. Compared to the specimens with a circular cavity, the crack initiation stress of the specimens containing a trapezoidal cavity is large, but the crack closure and the crack damage stresses are small, indicating relatively poor mechanical properties.

Mechanical properties and fracture evolution of sandstone specimens containing different inclusions under uniaxial compression

To investigate the effects of inclusions filled in holes, including inclusion strength, stiffness and shape, on the mechanical properties and fracture evolution of rock, a series of uniaxial compression tests were conducted on prismatic sandstone containing a prefabricated hole filled with different types of inclusions using a servo-hydraulic machine. Digital image correlation (DIC) and acoustic emission (AE) techniques were applied jointly to record and analyze the deformation and crack process of sandstone. The results indicate that both the inclusion type and shape are important factors affecting the strength and deformation properties of sandstone. The relatively rigid inclusion not only significantly improves the mechanical properties of pre-holed specimens, but also can provide a better support for structural integrity of rock after failure. Observations show that both DIC and AE techniques are capable of revealing the fracture evolution of specimens, and there is a good consistency between the mechanical behavior and surface strain fields and internal AE signals. Based on the strain localization characteristics, the crack sequences and relative stress levels for each group of specimens are summarized, especially the crack development inside the inclusion is first observed. The crack behavior and crack type around inclusions are greatly influenced by the inclusion type and shape due to dramatic changes in stress distribution around inclusions. After rock failure, the crack extent around inclusions can be characterized by the cumulative AE counts, and the final failure modes of specimens under different filling conditions can be categorized as tensile failure, shear failure and mixed tensile/shear failure.

Hydraulic properties and energy dissipation of deep hard rock under H-M coupling and cycling loads

The permeability of deep rock is closely related to the stability and safety of underground engineering. The rocks in deep stratum are mostly with high stress and high osmotic pressure. Therefore, it is necessary to consider the coupling effect between porewater pressure and in situ stress on rock mass. A series of triaxial cyclic loading and unloading experiments under hydraulic-mechanics coupling conditions are carried out to studied the mechanical and hydraulic properties of granite in the depth of 1300 m to 1500 m. Especially, the effect of the disturbance on the permeability of fractured rocks are investigated by unloaded the confining pressure. Tests results presented that the stress-strain curves of deep granite showed typical brittle characteristics. The principal stress of granite exhibited a linear relationship under the high confining pressure of 34-40 MPa and high osmotic pressure of 13-15 MPa. Dissipated energy of the rock decreased to a relatively low level after 2-3 loading cycles and then slowly increased. Permeability showed a decreasing trend as the loading and unloading cycles increase. Finally, acoustic emission technology was used to monitor the fracture evolution in rocks, the acoustic emission signal released as the fractures develop and energy dissipated. The results would provide basic data for the exploitation and excavation in the deep galleries.

Shear fracture evolution in rocks examined using a novel shear device associated with acoustic emissions

This study constructs a new shear test method composed of an open-box-style shear device with a crack-opening-displacement (COD) extensometer associated with a non-destructive acoustic emission (AE) detection system for examining the complete load behavior and the shear fracture triggering position and subsequent fracture propagation of two rock types (Charcoal granite and Berea sandstone). The complete macroscopic view loading curve presents that the pre-peak shear stiffness and shear strength of granite are higher than those for sandstone. The post-peak load behavior of both rocks trend towards the unstable fracture type (Class II), whereas in the microscopic view, the localization of AE events occurred before the peak and is located near the middle of the specimen. The AE localization spatial distribution is in good agreement with the macro-crack position. This suggests that the AE localization distribution is related to the macro-crack initiation position. After localization, the AE events then propagate following the shear direction. The results obtained in this study provide a better understanding of the shear fracture mechanism of brittle rock material.