Rock Failure Behavior Research Articles

Mechanical properties and failure behavior of rock with different flaw inclinations under coupled static and dynamic loads

The deep fissured rock mass is affected by coupled effects of initial ground stress and external dynamic disturbance. In order to study the effect of internal flaw on pre-stressed rock mechanical responses and failure behavior under impact loading, intact granite specimens and specimens with different flaw inclinations are tested by a modified split Hopkinson pressure bar (SHPB) and digital image correlation (DIC) method. The results show that peak strain and dynamic strength of intact specimens and specimens with different flaw angles (α) decrease with the increase of axial static pressure. The 90° flaw has weak reduction effect on peak strain, dynamic strength and combined strength, while 45° and 0° flaws have remarkable reduction effect. Specimens with 90° flaw are suffered combined shear and tensile failure under middle and low axial static pre-stresses, and suffered shear failure under high axial static pre-stresses. Specimens with 45° and 0° flaws are suffered oblique shear failure caused by pre-existing flaw under different axial static pre-stresses. Besides, based on digital image correlation method, it is found that micro-cracks before formation of macro fractures (include shear and tensile fractures) belong to tensile cracks. Tensile and shear strain localizations at pre-existing flaw tip for specimen with 45° and 0° flaws are produced much earlier than that at other positions.

Simulation of rock fracture process based on GPU-accelerated discrete element method

The Discrete Element Method (DEM) is increasingly used to study the failure behavior of rock. Despite DEM's intrinsic capability to capture the mechanical behavior of discontinua, there remains several open questions that include the numerical modelling of the meso-fracture evolution and behavior of brittle rock bodies. A Cohesive Fracture Model (CFM) designed explicitly for polyhedral shaped DEM particles is proposed for the simulation of the fracture and behavior of brittle rock bodies. A rock body is discretized into a series of rigid polyhedral blocks which are bonded along the boundary faces along the normal and shear directions. A cohesive criterion dictates the normal and shear break strengths of the bonds for the CFM. The computational efficiency of the CFM for polyhedral shaped DEM simulations is enhanced by parallelization over multiple graphical processing units (GPUs) in the Blaze-DEM framework. A series of numerical and laboratory tests are conducted. For marble, these include three-point bending and uniaxial tests to investigate the relationship between the meso-mechanical parameters and macro-mechanical behavior. Following a sensitivity analysis of the meso-mechanical parameters, an inversion procedure is established to estimate the numerical meso-mechanical parameters of the CFM model. Two different failure modes for different rocks are proposed.

The failure, slaking, and self-sealing behavior of clay-bearing rocks

Clay-bearing rocks with different thickness are commonly encountered in sedimentary rocks during many important engineering applications such as tunneling, underground and open-pit mining, and road cut or dam site constructions. Clay minerals are the main constituent of the mineralogical composition of such sedimentary rocks, and are the most important minerals that affect the strength and deformation behavior of these rocks. Due to the presence of clay minerals, the physico-mechanical features of these sensitive rocks are not constant and eventually change with variations in moisture content when subjected to natural environmental conditions. Many natural disasters and engineering projects–based problems such as landslide, slope instability, embankment failures, open-pit and underground excavation problems, and undercutting-related rockfalls generally have direct or indirect relationships with clay-bearing rocks due to their low durability and strength characteristics as well as high swelling tendency as a result of contact with water. Despite these natural disasters and engineering problems, due to their low characteristics of hydraulic conductivity and self-sealing behavior, the thermo-hydro-mechanical properties of clay-bearing rocks (e.g., Opalinus Clay and Boom Clay) have also been investigated up to now for the purpose of being used as host rocks for high-level radioactive waste. Particularly in the last two decades, the source of energy found trapped within shale formations has begun to be important in the popularity of these rocks. The reductions of conventional fossil fuels and worldwide high energy demands as well as occasional disruptions in energy supply have led scientists to do more investigation to achieve alternative energy sources from clay-bearing rocks. Considering low durability and strength dependent clay-bearing rocks–based engineering problems and the importance of these rocks for extraction shale gas and storage nuclear waste, the physico-mechanical and self-sealing characteristics of these rocks are comprehensively discussed and presented in this study.

An Experimental and Numerical Study on the Influence of Filling Materials on Double-Crack Propagation

Filling materials such as clay or sand widely exist in natural rock joints and work as weak bonds between the joint surfaces. The fillings affect rock deformation and failure behavior, and show different influences in terms of single crack or multiple cracks. While most of the literature have focused on unfilled cracks in brittle materials, this study aims to investigate various filling materials on the crack behavior, e.g., initiation, secondary cracks and peak strength. In this paper, the crack propagation in rock-like specimens with double-filled and unfilled cracks are investigated experimentally and numerically. Uniaxial compression tests were conducted and the experimental observations indicate that the peak stress and first crack initiation stress of the specimens vary with different geometries and different filling materials, while the crack initiation location and the pattern of crack coalescence show similar behavior between filled and unfilled cracks. In parallel to the experimental tests, numerical simulations were carried out using a modified phase field model (PFM) to complement the experiments and provide a new perspective. The PFM is found to produce consistent stress–strain curve, strength, and crack patterns with those observed in the experimental tests for both unfilled and filled cracks.

Effects of cyclic freeze-thaw treatments on the fracture characteristics of sandstone under different fracture modes: Laboratory testing

For engineering projects in cold regions, the failure behaviour of jointed rocks is affected by freeze–thaw (F-T) cycle action. To reveal the fracture characteristics of rock under the influence of cyclic F-T treatments, fracture tests on semicircular specimens with different F-T cycle numbers were conducted. The fracture toughness values in different modes were investigated by SCB specimens with different crack inclinations (0°, 5°, 15°, 30°, 45° and 54°). The experimental results show that cyclic F-T treatments can lead to degradation in three kinds of fracture toughness values. Specifically, the value of KIC (fracture toughness in mode I) decreases by 3.1, 7.2, 15.0 and 24.3% after 20, 40, 60, and 80 F-T cycles, respectively. When the number of cycles increases from 20 to 80, the value of KIIC (fracture toughness in mode II) of the sandstone decreases by 8.43, 12.0, 18.8 and 27.9%. For the specimen with crack inclinations of β = 5°, 15°, 30° and 45°, after 80 F-T cycles, the Keff (effective fracture toughness) value decreases to approximately 83.7, 78.3, 79.1 and 76.7% of the initial value, respectively. The mixed-mode fracture toughness ratios Keff/KIC were calculated and compared with the theoretical solution. The experimental results of Keff/KIC agree well with the theoretical results calculated based on the MMTS criterion. Moreover, after F-T treatments, the fracture paths of the SCB specimens are more tortuous than those in specimens without F-T treatment. In addition, the fracture surface morphological characteristics of the SCB specimen were obtained by a 3D laser scanner. Overall, the fractal dimension value increases gradually with additional F-T cycles. This indicates that the F-T treatments lead to rough fracture paths and fracture surfaces.

Full- and Local-Field Strain Evolution and Fracture Behavior of Precracked Granite under Coupled Static and Dynamic Loads

Flaws and discontinuities play a crucial role in the failure process of rocks. To investigate the fracturing mechanism of rock with combined flaws composed of crack and hole, the digital image correlation (DIC) method is used to record and analyze the rock failure behavior. Coupled static and dynamic loads are applied on granite specimens with prefabricated flaws by a modified split Hopkinson pressure bar (SHPB) device. The dynamic mechanical properties of the granite specimens are affected by the flaw inclinations with the loading directions. With the inclination angle increasing, the combined strength and peak strain both decrease first and then increase. Full- and local-field strain evolution of the granite specimens is analyzed in a quantitative way by using DIC technique. The specimens with a flaw angle of 45° are broken relatively evenly with homogenous small particle sizes. The variation trend of fragment sizes is consistent with that of combined strength and absorption energy of the specimens.

Strength and failure characteristics of jointed rock mass with double circular holes under uniaxial compression: Insights from discrete element method modelling

Underground projects are typically performed in nonpersistent jointed rock masses. Excavation of tunnels or chambers causes crack initiation, propagation and coalescence, resulting in the instability and destruction of surrounding rocks. The failure behaviour of rocks is complicated owing to the interaction between holes and joints. Therefore, understanding the failure characteristics of holes in jointed rock masses is essential. In this paper, the mechanical characteristics of a jointed rock mass with double circular holes under uniaxial loading was investigated using the discrete element method. The effects of different joint parameters on the strength and failure behaviour of the jointed rock mass with double circular holes were studied. The results show that the existence of joints degrades the mechanical behaviour of the rock mass. Specifically, the peak strength and elastic modulus of the specimens show a “U” shape change with the joint dip angle and reach the minimum value when the angle is 30°. With an increase in joint spacing, the peak strength and elastic modulus increase gradually. An investigation of the crack coalescence process and displacement field of the specimens reveal the crack propagation mechanism based on the interaction between holes and joints under uniaxial loading. Five types of crack coalescence among the holes and adjacent joints were summarized. Four typical failure modes were observed: tensile failure across joints, block rotation failure around holes, shear failure through joint planes and tensile–shear mixed failure through holes.

MULTIFRACTAL OF ELECTROMAGNETIC WAVEFORM AND SPECTRUM ABOUT COAL ROCK SAMPLES SUBJECTED TO UNIAXIAL COMPRESSION

During coal and rock loading, a significantly large number of electromagnetic signals are generated as a result of fracture appearance and crack expansion. The generation of electromagnetic signal is the comprehensive embodiment of the coal rock failure behavior. Therefore, the generated signals contain complex and rich messages that can reflect the damage process and degree of coal and rock. In this work, the multifractal theory is applied to analyze the nonlinear characteristics of the electromagnetic wave and its spectrum induced during coal rock, which present good correlation with failure process. The failure process of coal rock is non-uniform, non-continuous and nonlinear, during which, there is a good synchronization and correlation between the electromagnetic pulses and the stress drop, rather than the stress. Both waveform and its spectrum of electromagnetic signal have multifractal characteristics, the larger the fracture scale is, the more significant the multifractal characteristic of electromagnetic signal is, and the multifractal characteristic of electromagnetic signal from coal is higher than that from sandstone. The difference of fracture energy and size can be represented by the maximum of the multifractal dimension [Formula: see text] of the electromagnetic wave and its spectrum during coal rock failure. In the electromagnetic spectrum, small signals are always dominant, and the dominant frequency is only a few isolated points. What is more, with the increase of fracture size, the difference between the dominant frequency and the non-dominant frequency is gradually enhanced.

Performance of rock crack stress thresholds determination criteria and investigating strength and confining pressure effects

Identifying crack stress thresholds of rocks under different loading conditions, e.g. long-term monotonic or cyclic loads, are crucial in the design of rock excavations and underground openings. The evaluation of the performance of damage thresholds, i.e. crack closure, crack initiation and crack damage stresses, for a variety of rocks, is crucial to assess the most suitable method to capture the stages of rock strength degradation for rock having different strengths subjected to different confining pressures expected to in the field. In this study, several uniaxial and triaxial tests on different rock specimens (Uniaxial compressive strengths from 5 to 278 MPa) were performed to investigate the application and performance of different criteria to determine damage threshold values, using their stress–strain response. It was found that the volumetric strain response of the rock is the most promising parameter to identify crack stress thresholds. For the rocks having different strength values and subjected to different confining pressures, on average, ratios of crack closure, crack initiation and crack damage to the peak strength are found to be 0.21, 0.33 and 0.80 respectively. Effect of rock strength on crack closure and crack initiation stress thresholds was found to be negligible. Crack damage threshold, however, increases with a rock strength increase. This was mainly attributed to the brittle failure behaviour of stronger rocks when compared to weaker rocks. In triaxial compressive testing, crack closure partially occurs during rock consolidation and therefore, crack closure stress decreases with confining pressure increase. Furthermore, a limiting effect on the generation and further growing of lateral cracks in the rocks did lead to an increase in the stress to induced crack damage with an increase in the confining pressure. On the other hand, the effect of confining pressure on the crack initiation stress was observed to be different in different rocks.

Experimental Study on Physical-mechanical Properties and Fracture Behaviors of Saturated Yellow Sandstone Considering Coupling Effect of Freeze-Thaw and Specimen Inclination

The effect of freeze-thaw on the physical-mechanical properties and fracture behavior of rock under combined compression and shear loading was crucial for revealing the instability mechanism and optimizing the structure design of rock engineering in cold regions. However, there were few reports on the failure behavior of rock treated by freeze-thaw under combined compression and shear loading due to the lack of test equipment. In this work, a novel combined compression and shear test (C-CAST) system was introduced to carry out a series of uniaxial compression tests on saturated yellow sandstone under various inclination angles (θ = 0°, 5°, 10°, and 15°) and the number of freeze-thaw cycles (N = 0, 20, 40, and 60). The test results showed that the P-wave velocity dramatically decreased, while the rock quality and porosity increased gradually as N increased; the peak compression strength and elastic modulus obviously decreased with the increasing θ and N, while the peak shear stress increased gradually with the increasing θ and decreased with the increase of N, indicating that the shear stress component can accelerate the crack propagation and reduce its resistance to deformation. The acoustic emission (AE) results revealed that the change of crack initiation (CI) stress and crack damage (CD) stress with the θ and N had a similar trend as that of the peak compression strength and elastic modulus. Particularly, the CI and CD thresholds at 60 cycles were only 81.31% and 84.47% of that at 0° cycle and indicated a serious freeze-thaw damage phenomenon, which was consistent with the results of scanning electron microscopy (SEM) with the appearance of some large-size damage cracks. The fracture mode of sandstone was dependent on the inclination angle. The failure mode developed from both the tensile mode (0°) and combined tensile-shear mode (5°) to a pure shear failure (10°–15°) with the increasing inclination angle. Meanwhile, the freeze-thaw cycle only had an obvious effect on the failure mode of the specimen at a 5° inclination. Finally, a novel multivariate regression analysis method was used to predict the peak compression strength and elastic modulus based on the initial strength parameters (θ = 0°, N = 0). The study results can provide an important reference for the engineering design of rock subjected to a complex stress environment in cold regions.

Damage Evolution and Failure Behavior of Post-Mainshock Damaged Rocks under Aftershock Effects

Rock stability has long been a hot topic during underground energy exploitation, but the failure process of rock materials under earthquake effects is extremely complicated, and the failure mechanism still remains unclear. In order to investigate the fatigue damage and failure behavior of rocks under aftershock effects considering the post-mainshock damage states, a series of laboratory tests were conducted on marble specimens subjected to stepwise cyclic loading. Four levels of peak stress (i.e., 10, 30, 50, and 70 MPa) were applied in the first cycle, to simulate mainshock damage. The results indicate that, with the increase of initial cycle amplitude, mainshock damage has a significant effect on deformation behavior, dissipated energy, P-wave velocity, and AE characteristics of tested specimens during aftershock process. The increasing amplitude of initial cycle enhances irreversible deformation and weakens the resistance to deformation, which accelerates the expansion of specimen volume and results in the reduction of bearing capacity. Furthermore, the increasing amplitude of initial cycle obviously changes the failure morphologies and intensifies the final macro-fracture scale of tested specimens, which are verified by acoustic emission AF-RA value and b-value, respectively.

Failure behaviour and acoustic emission characteristics of different rocks under uniaxial compression

Abstract In the present study, mechanical behaviour of three types of rocks has been investigated under uniaxial compression. The stress–strain responses of the tested rocks are composed of four regions: the compaction stage; elastic stage; strain hardening stage and strain softening stage. The elastic modulus and uniaxial compressive strength increase in order from the result of the coal to the mudstone and then to the sandstone, while the Poisson ratio shows a reverse order. In coal and mudstone, volumetric dilation behaviour vanishes gradually with an increase in damage degree, while it remains stable in the sandstone where the strain hardening behaviour is less obvious. Regarding acoustic emission (AE) characteristics, AE hit and AE energy show a similar evolutionary trend in the loading process. A sudden increase in AE energy is accompanied with drastic drop or local fluctuation in the load-bearing capacity of the rock while AE hit fails in predicting such variation in the load-bearing capacity. A quiet stage of AE signal prior to the peak stress is captured, which can be taken as a precursor for rock failure. Failure pattern of the coal, mudstone and sandstone varies from shear faulting to tension-shear mixed fracturing and then to axial splitting, which is in good accordance with that deduced from AE location analysis.

Experimental investigation on strength and failure behavior of carbonate rocks under multistage triaxial compression

Conventional or single stage triaxial compression experiments for obtaining failure envelope of rocks require several specimens to be tested under different confining pressures. As a replacement, multistage triaxial tests using one specimen have been employed in experimental rock mechanics for situations where preparing several specimens is difficult or expensive. The results obtained by the multistage triaxial test may deviate from those obtained by the single stage. However it is believed that using multistage triaxial test can decrease the cost and time and the obtained results are still reliable due to less variability in the rock properties. Several methods have been proposed for conducting multistage triaxial test including ISRM suggested method and a modified version of the ISRM method proposed by Crawford and Wylie. The results of this study demonstrate that both multistage methods give reasonable results. However, the modified method provides closer results to the single stage, because the specimen is subjected to less damage. As the number of the loading stages rises, both ISRM and the modified version provide lower strength parameters comparing to the single stage.

A cohesive grain based model to simulate shear behaviour of rock joints with asperity damage in polycrystalline rock

A cohesive grain based model is proposed to simulate the fracture behaviour of polycrystalline rocks. The model was employed to characterize the cracking response of both inter- and intra-grain contacts in the grain based model (GBM). The model was implemented in distinct element codes and its ability to mimic the mechanical and failure behaviour of polycrystalline rocks was demonstrated through calibration of several granitic rocks including Adelaide black granite, Eibenstock II granite, and Aue granite. The calibrated model of Aue granite was used to assess the effect of joint roughness coefficient (JRC) on asperity damage and shear mechanism of rock joints (i.e. grain crushing) under constant normal load (CNL) and constant normal stiffness (CNS) conditions. The bond-break in intra-grain contacts contributed to asperity damage in form of grain crushing. The numerical observations showed that under CNS condition asperity damage increased with increasing initial normal stress and JRC, and rough rock joints exhibited more dilative response.

Complex deformation of naturally fractured rocks

Strain is a key parameter in the evaluation of rock mechanical properties and failure behavior. Strain is usually measured using strain gauges, however, this laboratory method has inherent uncertainty in the attachment of sensitive gauges to the rock surface and it is limited to one-point strain measurement. In complex geometries such as rock samples with natural fractures or lamination, that classical approach cannot provide the whole picture of strain development in such complex geometries. To overcome these challenges, the application of optical technique with image processing is proposed. The optical technique has the advantage of being a non-contact method and it provides quantitative visualization of full-field strain measurement.To examine quantitative visualization of strain development, DIC technique was first conducted on a homogeneous sandstone sample as a benchmark. The results verify that the maximum tensile strain is developed at the central vertical line of the sample as it was expected from theoretical solutions. This DIC measurement matches the post-failure fracture pattern of the specimen. Then the technique was applied on the Buda Limestone samples with horizontal, sub-horizontal or multiple complex natural fractures (sealed or partially open) and inclusions while the samples were under diametrical compression experiment. The results of this study provide further insight into the role of natural fractures on induced fracture initiation and propagation under this specific testing condition. Horizontal natural fractures are planes of maximum compressive strain which can delay the propagation of vertical tensile fractures. The maximum amount of tensile strain usually initiates at the boundary of these natural fractures and vertical tensile fractures usually deviate toward horizontal fracture for a short distance before turning back to propagate vertically following the maximum stress path. Full-field surface deformations are mapped optically in the crack–inclusion vicinity where DIC results showed that crack front is arrested by the inclusion for the complete fracture event. In contrast to the failure pattern in homogenous sandstone specimen, complex fracture pattern was observed on several specimens with multiple fracture network. Fracture slippages along the natural fracture, fracture deviation was observed in samples with steep inclined natural fractures.The results of this study can help us to better understand the complex failure mechanism in naturally fractured media and elaborate on various mechanisms of failure under diametrical compression tests. DIC is a powerful technique in providing the dynamic strain development over the surface of the sample in real-time, thus it can provide the fracture initiation and propagation in such heterogeneous rock media.

Triaxial Loading and Unloading Tests on Dry and Saturated Sandstone Specimens

The brittle failure of hard rock due to the excavation unloading in deep rock engineering often causes serious problems in mining and tunneling engineering, and the failure process is always affected by groundwater. In order to investigate the effects of stress paths and water conditions on the mechanical properties and failure behavior of rocks, a series of triaxial compression tests were conducted on dry and saturated sandstones under various loading and unloading paths. It was found that when the sandstone rock samples are saturated by water, the cohesion, the internal friction angle and the Young’s modulus will decrease but the Poisson′s ratio will increase. The fracturing characteristics of the sandstone specimens are related to the initial confining pressure, the stress paths and the water conditions from both macroscopic and microscopic viewpoints. The failure of sandstone in unloading test is more severe than that under loading test, particularly for dry sandstone samples. In unloading test, the energy is mainly consumed for the circumferential deformation and converted into kinetic energy for the rock bursts. The sandstone is more prone to produce internal cracks under the effect of water, and the absorbed energy mainly contributes to the damage of rock. It indicates that the possibility of rockburst in saturated rock is lower than the samples in dry condition. It is important to mention that water injection in rock is an effective way to prevent rockburst in deep rock engineering.

Novel 3D Failure Criterion for Rock Materials

AbstractThe failure criterion of rock material has been used in geotechnical engineering to assess the failure behavior of rocks under complicated stress conditions. In the present study, a new fai...

A phenomenological modelling of rocks based on the influence of damage initiation

It is recognized that the stress-induced damage impacts the progressive failure behavior of rocks. A phenomenological model for the compressive failure of rocks is thus presented in this study. The model addresses the progressive growth of damage that leads to the strength weakening on a macroscopic scale. Considering dramatic difference between uniaxial compression and tension strengths for rocks, the admitted Mises–Schleiche Drucker–Prager strength criterion is adopted to characterize the damage initiation. On this basis, a two-parameter Weibull-type probability function is used to define the strength distribution of representative volume elements, followed by the use of damage variable for addressing the accumulated probability of failure. The proposed damage variable essentially characterizes both the critical stress level of damage initiation and progressive damage evolution law. Detailed comparisons have been carried out between the predictions and experimental observations, and issues related to the damage evolution are particularly addressed. In addition, the results further validate the proposed model considering damage initiation.

Crack Initiation, Propagation, and Failure Characteristics of Jointed Rock or Rock‐Like Specimens: A Review

Rock masses are heterogeneous materials containing a large number of discontinuities, and the failure of the natural rock mass is induced by the crack propagation and coalescence of discontinuities, especially for the rock mass around tunnel or underground space. Because the deformation or failure process of jointed rock mass exhibits strongly nonlinear characteristics, it is also very difficult to predict the strength and failure modes of the rock mass. Therefore, it is very necessary to study the failure mechanisms of jointed rock mass under different stress conditions. Apart from the stress condition, the discontinuities geometry also has a significant influence on the mechanical behavior of jointed rock mass. Then, substantial, experimental, and numerical efforts have been devoted to the study of crack initiation, propagation, and coalescence of rock or rock‐like specimens containing different kinds of joints or fissures. The purpose of this review is to discuss the development and the contribution of the experiment test and numerical simulation in failure behavior of jointed rock or rock‐like specimens. Overall, this review can be classified into three parts. It begins by briefly explaining the significance of studying these topics. Afterwards, the experimental and numerical studies on the strength, deformation, and failure characteristics of jointed rock or rock‐like materials are carried out and discussed.

Numerical analysis of the compressive and shear failure behavior of rock containing multi-intermittent joints

The failure behavior of intermittent jointed rocks is dependent on joint configurations. Joint inclination angle and continuity factor determined the joint arrangement in a rectangular numerical sample that was established by using the particle flow code approach. To identify the differences in the failure processes of identical intermittent jointed samples, uniaxial compressive and shear loads were applied on each sample. The crack growth path presented the four typical crack coalescence patterns identified via compressive and shear numerical tests. The crack coalescence pattern was associated with joint slant angle and continuity factor. The observed crack coalescence patterns of every sample with the same inclination angle and continuity factor were partially identical under compressive and shear loading. The differences in the crack patterns of the compressive and shear failure processes were described and compared. Typical compressive and shear failure processes were illustrated. Four compressive and three shear failure modes were identified. The cracking location and number of cracks in each failure mode were different. Additionally, the contact force evolution among particles during shear and compressive loading was different and likely accounted for the differences in cracking patterns. Under compressive or shear loading, the contact force concentration in each sample underwent the following stages: uniform distribution before loading, concentrated distribution, and scattered distribution after failure.