Rock Failure Process Research Articles

DDA Simulation Study on Mechanical Failure of Heterogenous Rock

Heterogeneity is an important characteristic that affects the mechanical behavior of rock. In the present work, a statistical rock mesoheterogeneity model based on the Weibull distribution function is introduced into the discontinuous deformation analysis (DDA) method to simulate the mechanical failure of heterogeneous rock, in which the general heterogeneity degree is controlled by a heterogeneity index and the mechanical property of each subblock element is randomly assigned. Brazilian disc and uniaxial compressive rectangular specimens are simulated as examples. Results show that it is more reasonable to consider the heterogeneity of elasticity properties (the elastic modulus and Poisson’s ratio) and strength properties (the tensile strength, cohesion, and friction angle) simultaneously in the heterogeneity model. It is also shown that with a larger heterogeneity index, which means a lower degree of heterogeneity, the reproducibility of the macroscopic response curves of a specimen gets better, while the exact cracking always differs but with less scattered cracks, and the global fracturing failure pattern and mode are weakly influenced by the heterogeneity. Moreover, with the increase in the heterogeneity index, the macroscopic equivalent modulus and strength get larger and approach those of a homogeneous specimen. This work indicates the importance of heterogeneity for rock mechanical behaviors including the macroscopic equivalent response and the fracturing failure. By the subblock DDA method to simulate fracturing realistically, the fracturing failure process of heterogeneous rock can be successfully reproduced, which builds good foundation for the simulation study of heterogeneous rock fracturing in practical problems, e.g., coal and rock fracturing in fluidization mining in the future.

Research on the progressive failure process and fracture mechanism of rocks with the structural evolution perspective

The progressive failure process and fracture mechanism of rocks has been a key scientific issue which needs to be solved. Therefore, the progressive failure process and fracture mechanism of rocks under compression are comparatively analyzed based on the perspectives of energy evolution, rock burst starting and mesoscopic characterization. Then, the structural evolution theory to reveal the progressive failure process and fracture mechanism of rocks is proposed. Additionally, the evolution characteristics of four stages and progressive failure mechanism of rocks under compression are better verified according to the results captured by multiple methods. The main conclusions are as follows: (1) The fundamental reason caused the stress changed and transferred is the change of system structure, and the stress is only the external manifestation of system structure changed. (2) The macroscopic instability of rocks occurs in the post-peak sub-instability stage. (3) Critical slowing down theory can provide sufficient evidence for the structural evolution perspective. (4) The driving force of the structure self-adjustment in rocks comes from the activation of its self-organizing and the energy consumption. The conclusions above can provide new ideas and concepts for the fundamental theoretical research of deep rock mechanics, thus provide a certain of theoretical reference for the practice of field engineering.

Particle Flow Analysis of Macroscopic and Mesoscopic Failure Process of Salt Rock under High Temperature and Triaxial Stress

In order to reveal the mechanism of thermal-induced deformation and fracture development of salt rock under high temperature, the particle flow program PFC2D was used to study the triaxial compression failure process of salt rocks under different temperatures; at the same time, a combination model of Burge and Linearpbond was proposed to simulate plastic deformation and heat conduction of salt rock. Finally, the simulation results were compared with the experimental results to verify the validity of the conclusion. The simulation results show that the elastic limit points of rock gradually descend, the dilatancy points rise gradually, and the plastic deformation characteristics of salt rock become more obvious with the increase of temperature. Due to the damage of the sample, the strong chains break and disappear, increasing the proportion of the weak chains, and the high temperature intensifies the rupture of the contact between the particles in the salt rock. As the temperature increases from 50°C to 120°C, the strong chains in the rock sample decrease significantly, and the damage gradually increases; when the temperature is 150°C, the contact force decreases sharply, and the damage of salt rock is significant. According to the particle displacement cloud diagrams, it is found that the expansion direction from the middle part of the rock sample to the left and right ends is 12.08°, 9.55°, 8.2°, 6.33°, and 0°, respectively. The displacement directions of the rock sample show obvious radial expansion tendency, and the higher the temperature, the more obvious the “drum-shaped” failure phenomenon in the middle of the rock sample. During the heating process, the thermal cracks are mainly tensile cracks, and transverse cracks are gradually formed in the middle of the model. The cementation failure points at the top and bottom of the model expand in an oblique direction and form oblique cracks of about 45°. From the three different mathematical models of macroscopic and mesoscopic views, it is concluded that the effect of temperatures on salt rock is more significant after 90°C. This research is important for exploring the macroscopic and microscopic mechanics evolution of salt rock and provides a reference for determining the long-term mechanical strength of salt rock.

Experiment on Strength and Failure Behavior of Sandstone Containing Pre-Existing Cracks Under Brazilian Compression

Determination of the mechanical properties of rock containing pre-existing cracks under tension condition is of great significance to understand the failure process of rock in engineering. This paper presents the experimental results of sandstone containing pre-existing cracks under Brazilian compression. The characteristics of the microcracks were analyzed by a scanning electron microscope. The results show that the rock containing pre-existing cracks has an obvious anisotropic characteristic. When the crack inclination is 45°, the rock has the minimum tensile strength and the weakest axial deformation resistance.

Study on the Variation Rule and Characteristics of Pore Water Pressure in the Failure Process of Saturated Rock

With the development of tunnels and other engineering constructions into the deep strata, rock masses are more prone to dynamic damage such as rock bursts under in situ conditions and excavation disturbances. The pore water in the rock mass will produce pressure changes during this process. According to the relationship between the change of pore water pressure and the development of rock mass damage, the variation rule and precursor characteristics of pore water pressure in the process of rock mass failure can be found. In this paper, through mechanical analysis, the evolution law of pore water pressure during the failure process of saturated rock is obtained. The study found that, in the process of rock failure, the pore water pressure presents three stages of linear growth, transition, and decrease. The rise and fall of pore water pressure are closely related to rock damage and influence each other. Through the observation of pore water pressure during coal mining, it is found that the coseismic effect of pore water pressure is significant. It is proved that there is a close correlation between the evolution of the stress field in the surrounding area of the stope and the change of pore water pressure in the surrounding area under the effect of mining disturbance. During the engineering practice, dynamic monitoring can be carried out on the change of pore water pressure inside the rock mass according to the law, and the precursor information of rock mass instability and failure can be explored.

Global Wave Velocity Change Measurement of Rock Material by Full-Waveform Correlation.

Measuring accurate wave velocity change is a crucial step in damage assessment of building materials such as rock and concrete. The anisotropy caused by the generation of cracks in the damage process and the uncertainty of the damage level of these building materials make it difficult to obtain accurate wave velocity change. We propose a new method to measure the wave velocity change of anisotropic media at any damage level by full-waveform correlation. In this method, the anisotropy caused by the generation of cracks in the damage process is considered. The accuracy of the improved method is verified by numerical simulation and compared with the existing methods. Finally, the proposed method is applied to measure the wave velocity change in the damage process of rock under uniaxial compression. We monitor the failure process of rock by acoustic emission (AE) monitoring system. Compared with the AE ringing count, the result of damage evaluation obtained by the proposed method is more accurate than the other two methods in the stage of increasing rock heterogeneity. These results show that the proposed method is feasible in damage assessment of building materials such as rock and concrete.

Characterization of damage processes in Montney siltstone under triaxial compression using acoustic emission and diagnostic imaging

SUMMARY Crack nucleation and rock failure processes in a fine-grained siltstone (Montney Formation) under triaxial compression are investigated using combined diagnostic techniques, including ultrasonic-wave measurement, acoustic-emission (AE) monitoring, computed tomography (CT) scanning, and thin-section imaging. The sample displays a weak-to-moderate inherent seismic anisotropy and noticeable stress-induced anisotropy prior to failure. No AE event was detected until the applied axial stress reached 95 per cent of the peak value. The signal-to-noise ratio is relatively low, however, and detectable AE events are more diffuse than those observed in highly brittle rocks. The AE locations correlate with a shear fracture zone imaged by CT scanning. AE moment-tensor analysis reveals that events with larger relative magnitudes are characterized by high volumetric (tensile or compressive) components, and the initiation of the failure zone is dominated by combined shear-tensile (ST) failure. Stress inversion of the AE events with high tensile components is in good agreement with the known applied stress. Microscopic imaging of thin sections from the failed sample shows that the failure zone is an en echelon structure consisting of a major fracture with branching micro and minor cracks. This failure mechanism is consistent with a ST source mechanism and is interpreted to be associated with the fine granular structure and mineral composition of Montney siltstone.

Effects of shear rates on the damaging behaviors of layered rocks subjected to direct shear: Insights from acoustic emission characteristics

It is believed that the loading rate significantly affects the damaging behaviors of layered rocks. However, previous studies have mainly focused on the inherent bedding structure rather than the loading rate, particularly in the case of shearing. In this paper, the discrete element method (DEM) was used to simulate the rate-dependent shear behaviors of layered rocks under direct shear tests. Then, the rate-dependent acoustic emission (AE) characteristics were systematically investigated based on the moment tensor theory. The results showed that the shear rate significantly affects the failure process and AE characteristics of layered rocks. With the increase in the shear rate, the average released AE energy increased, while the total AE number and b-values decreased, which indicates that the crack propagation rate and the proportion of high-energy AE events increased. As a result, a macroscopic failure occurred for the rocks at the peak strength point under low-rate shearing and at the postpeak stage under high-rate shearing. In addition, a high-energy AE event corresponded to the crack damage point defining the boundary between the stable and unstable failure stages, thus providing an efficient method for capturing the damage precursors of rock failures. Under this premise, the characteristic stress and strain thresholds were investigated to discuss the feasibility of predicting the failure of layered rocks while considering the shear rate. Overall, this study provides new insights with regard to the effects of the shear rate on the damaging behaviors of layered rocks from the viewpoint of AE characteristics.

Dynamic properties and fracture damage characteristics of granite under impact load

The goal of this research is to study the dynamic properties and fracture damage laws of granite at the meso-scale under impact load. The quasi-static and dynamic mechanical properties of granite were studied through laboratory tests (Uniaxial compression, split Hopkinson pressure bar). The establishment of three-dimensional coupled numerical model of rock specimen and split Hopkinson pressure bar was realized by new coupling technology of PFC-FLAC. Three wave superposition and end stress distribution were discussed to determine the stress balance of the specimen during the loading process. The dynamic compression mechanics response characteristics and crack growth laws of granite under different strain rates were studied. The results indicated that PFC-FLAC coupling technology can solve the problems of tedious modeling process and long calculation time in the past three-dimensional discrete element model, and can better describe the dynamic mechanical behaviour of rock. There is obvious rate dependency in the dynamic strength of rock material, and the dynamic compressive strength of rock increases with the increase of strain rate. The macro fracture of rock mass is the result of the continuous development, expansion, aggregation and connection of the internal microcracks. With the increase of strain rate, the increase and cross propagation of cracks lead transformation of rock from intact to crushing destruction. Under dynamic loading, the failure process of rock is crack initiation, propagation, crossing, transfixion and rupture.

Failure of Rock Slope with Heterogeneous Locked Patches: Insights from Numerical Modelling

Rock slope stability is commonly dominated by locked patches along a potential slip surface. How naturally heterogeneous locked patches of different properties affect the rock slope stability remains enigmatic. Here, we simulate a rock slope with two locked patches subjected to shear loading through a self-developed software, rock failure process analysis (RFPA). In the finite element method (FEM)-based code, the inherent heterogeneity of rock is quantified by the classic Weibull distribution, and the constitutive relationship of the meso-scale element is formulated by the statistical damage theory. The effects of mechanical and geometrical properties of the locked patches on the stability of the simulated rock slope are systematically studied. We find that the rock homogeneity modulates the failure mode of the rock slope. As the homogeneity degree is elevated, the failure of the locked patch transits from the locked patch itself to both the interfaces between the locked patched and the slide body and the bedrock, and then to the bedrock. The analysis of variance shows that length and strength of locked patch affect most shear strength and the peak shear displacement of the rock slope. Most of the rock slopes exhibit similar failure modes where the macroscopic cracks mainly concentrate on the interfaces between the locked patch and the bedrock and the slide body, respectively, and the acoustic events become intensive after one of the locked patches is damaged. The locked patches are failed sequentially, and the sequence is apparently affected by their relative positions. The numerically reproduced failure mode of the rock slope with locked patches of different geometrical and mechanical properties are consistent with the laboratory observations. We also propose a simple spring-slider model to elucidate the failure process of the rock slope with locked patches.

Progressive Failure Process of Anisotropic Rock: Insight from Full-Field Strain Evolution

Rocks with layered structure (bedding or foliation) usually exhibit different levels of anisotropy in terms of mechanical properties. The structural anisotropy has pronounced influence on the failure process of anisotropic rock. However, relatively few studies have been carried out on the subject. In this paper, the failure processes of a foliated gneiss with different schistosity orientations under uniaxial compression were studied based on digital speckle correlation method. The results show that the evolution process of full-field strain of the gneiss is closely related to the schistosity orientation. More specifically, when β = 0°, the strain concentration zone mainly originates from the microstructure, and the potential failure plane cannot be observed before total failure. When β = 30° or 60°, there are several strain concentration strips before failure. The ultimate failure is due to the interaction among these strips under the action of local stress field. When β = 90°, the initiation and evolution of strain concentration strip is relatively stable. When tensile failure occurs along the schistosity, the crack opens abruptly during loading. In contrast, the crack presents a gently stable growth trend, when the shear failure occurs along the schistosity.

Experimental and numerical investigation of the failure mechanism and permeability evolution of sandstone based on hydro-mechanical coupling

The exploitation of natural gas and oil in the low-permeability reservoir is a severe challenge in the field of deep underground energy extraction. Therefore, a thorough understanding of the failure mechanism of reservoir sandstone under hydro-mechanical coupling is of great significance. The purpose of this study is to carry out triaxial compression tests of reservoir sandstone under different confining pressures and seepage pressures to investigate the failure mechanism and permeability evolution of reservoir sandstone. Laboratory experimental results show that the permeability of sandstone first decreases to a minimum and then increases with deviatoric stress, and finally reaches the maximum when a macroscopic fracture is formed. In addition, the permeability of sandstone gradually decreases with the increase of effective confining pressure, showing a good exponential function relationship. Axial strain stiffness is proposed to characterize the ability of the rock to resist deformation and failure, and an empirical equation of maximum axial strain stiffness, confining pressure and seepage pressure is established. Subsequently, the failure process of rock is reproduced in COMSOL based on elastic damage mechanics, porous elastic medium theory and the effective stress principle. The failure mechanism and the distribution of seepage pressure of rock feature points under different hydro-mechanical coupling are discussed. As a perfect tool, the distribution of streamline can intuitively reflect the impact of damage on porosity and permeability.

Development of excavation damaged zones around a rectangular roadway under mining-induced pressure

This paper presents an investigation of the development of excavation damaged zones (EDZs) around a rectangular coal roadway under mining-induced pressure in a longwall mining process. Field stress monitoring at a coal mine in Shaanxi Province, China was carried out to obtain stress distribution induced by mining in the surrounding rock mass. During longwall mining, a high-stress zone with a limited scope is distributed in front of longwall face, and a relatively low-stress increment occurs near the roadway wall due to the failure of rock mass. A borehole camera was employed to clearly observe the development state of cracks in the damaged zones around the roadway to capture the characteristics of the EDZ. To investigate the rock failure process and development patterns of the EDZ under mining-induced loading conditions, three numerical simulation cases considering the heterogeneity of rock were conducted with a proposed loading mode similar to the mining-induced pressure. The results of the simulations show that the complexity of EDZ development under the proposed loading conditions involves tensile and shear failures, the development of remote cracks, and the effect of roadway geometry. The development patterns of the EDZ around the rectangular roadway show agreement with field observations.

Effects of confining pressure on acoustic emission and failure characteristics of sandstone

In this study, uniaxial and triaxial compression acoustic emission (AE) tests were implemented to investigate the AE effect and failure characteristics of sandstone under different confining pressures (σ3). The evolution of AE parameters in the rock failure process and fracture fractal dimension characteristics after failure were analyzed. The results revealed that the activity of the AE signal is strongly related to σ3. The evolution of the Ib value can be divided into the I-fluctuation, II-stability, and III-decrease stages. In the first stage, the Ib value of the AE was relatively high, and the AE energy was low. Then, the Ib value tended to be stable; however, the fluctuation amplitude decreased, and the AE energy rapidly increased. In the stage of decrease, the AE energy sharply increased before the load approached the peak value, and the Ib value significantly decreased and dropped to the lowest point before the peak value. As σ3 increased, the rock’s failure mode changed from tensile failure to shear failure and became more coordinated. As the confining pressure increased, the shape dimension decreased, and the order degree of rock failure increased. The confining pressure exerted a certain control effect on the rock failure.

Discontinuous deformation analysis with distributed bond for the modelling of rock deformation and failure

Fracture and fragmentation of rock under various loadings or in multi-field conditions are important topics in many rock engineering scenarios. Numerical tools are commonly applied to evaluate the rock deformation and progressive failure process, while the accuracy and efficiency of the analysis are closely related to the applied fracture models. In this paper, an improved discontinuous deformation analysis (DDA) program with the distributed bond (DB) is proposed for rock deformation and rock fragmentation analysis. The jointed rock model is regarded as an assembly of triangular elements connected with DBs and contacts. The DB uses distributed constraints to bond the co-edge elements for continuous deformation modeling, and the fracturing and fragmentation of rock material is fulfilled by progressive DB failure with linear and nonlinear bond constitutive models. The theory, formulation, and implementation of DB in DDA framework are given in detail. The feasibility and accuracy of the proposed DB in simulating rock deformation and failure process are verified by several examples.

Effects of thermally-induced cracks on acoustic emission characteristics of granite under tensile conditions

Temperature can affect the physical and mechanical properties of rocks. With the aim of better understanding the effect of thermally-induced cracks on rock failure process, we heated granite disks (up to 800 °C) to induce different degrees of thermal damage, which were then used for Brazilian tests . The acoustic emission (AE) technique was applied to collect AE signals throughout each test. Specifically, the effect of thermally-induced cracks on AE characteristics (AE spatio-temporal characteristics, damage parameters D AE , amplitude-frequency characteristics, and b -value) was studied. Our results show that the thermally-induced cracks increase with the treated temperature, leading to more AE damage occurring at the early stage of the loading process. The existence of a large amount of thermally-induced cracks causes the scattered AE distribution on the disks treated at 600 °C and 800 °C. The average dominant frequency of AE signals decreases with the treated temperature. The decreasing of high-frequency signals and the increasing of low-frequency signals can be explained by the different mechanisms of thermally-induced cracks and stress-induced cracks. The b -value increases with treated temperature and decreases with the increasing applied stress.

The material-structure duality of rock mass: Insight from numerical modeling

The roots of rock engineering disaster lie not only in cross-scale deterioration and failure of rock material, but also in large deformation and instability of rock discontinuities and structural bodies. The material-structure duality of rock mass influences the mechanical behavior of rock mass, resulting in continuous-discontinuous characteristics and singularity phenomena. Nevertheless, the cognition of the dominant transformation and cooperative effect between these complex mechanisms is still vague, and there is a lack of a numerical scheme considering both material and structral effects of rock. In this study, we constructed a unified numerical scheme (Rock failure and instability analysis, RFIA) that considers both material failure and structural instability of rock mass. By establishing the statistical meso-damage constitutive model, the damage and failure of rock materials were reflected, and by developing the finite deformation numerical formation considering current state response, the large deformation and instability of rock structural body were described. The proposed numerical scheme realized the synchronous simulation of both material failure induced instability, and structural instability induced failure of rock mass. The size effect on rock under uniaxial compression and the local collapse mechanism of the surrounding rock in deep slabbing tunnel were investigated by RFIA. Meanwhile, Rock failure analysis (RFA) that only considers rock material failure and Rock instability analysis (RIA) that only considers structural instability were performed in the simulations. The results show that there are dominant and competitive mechanisms between material failure and structural instability during failure and instability process of rock. For the rock structral body with dominant geometric features, its instability plays a decisive role in inducing catastrophe. The cooperative effect of material-structure is an important mechanism, which may result in large displacement and rotation of rock mass and the abrupt release of massive energy once instability induces the failure (e.g., in situation of rockburst).

Comparison of strength-based rock brittleness indices with the brittleness index measured via Yagiz’s approaches

Rock brittleness is one of the most significant properties of rock having a major impact not only on the failure process of intact rock but also on the response of rock mass to rock excavation. In fact, the brittleness is a combination of rock properties including not only the uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) but also density and porosity of rocks. Due to that, the brittleness should be examined very carefully for any excavation projects, i.e., mechanized excavation, drilling and blasting. The aim of this paper is to compare the strength-based brittleness indices with both the rock brittleness index (BIo), directly obtained via punch penetration test (PPT) and also estimated via Yagiz’s approach (BI1) as a function of strengths and density of rocks. For the aim, database including more than 45 tunnel cases are used to compute common rock brittleness indices (BI2, BI3, BI4), different combination of UCS and BTS. Further, these indices are compared with both BIo and BI1 as well as each other. It is found that the BIo and BI1 have a significant relations (ranging of determination coefficients (r2) from 0.69 to 0.88 with strength-based brittleness indices commonly used in practice. Also, based on findings, several rock brittleness classifications are also revised herein.

Investigation into the gas adsorption-loading coupling damage constitutive model of coal rock

Coal and gas outburst is the result of comprehensive action of in situ stress, gas and mechanical properties of coal rock. The coupling effect of loading and gas adsorption eventually leads to the coal rock failure. Based on the principle of strain equivalence and considering the coupling effect of gas adsorption and stress loading, an adsorption-loading coupling damage model is established which breaks through the bottleneck of only considering a single influencing factor. Taking briquette samples with controllable properties as the research object, uniaxial compression tests of coal rock at different gas adsorption pressures are carried out and the model is verified based on the test results. The results of model calculation and tests show that the meso-damage stage of the coal body can well correspond to the macroscopic deterioration phenomenon and it is in good agreement with the stress–strain curve. It is proved that the model has good applicability and can accurately describe the damage and failure process of coal rock.

The tensile strength of volcanic rocks: Experiments and models

The tensile strength of volcanic rock exerts control over several key volcanic processes, including fragmentation and magma chamber rupture. Despite its importance, there is a paucity of laboratory data for the tensile strength of volcanic rocks, leading to an incomplete understanding of the influence of microstructural parameters, such as pore size and shape (factors that vary widely for volcanic rocks), on their tensile strength. To circumvent problems associated with the variability of natural samples, we provide here a systematic study in which we use elastic damage mechanics code “Rock Failure Process Analysis” to perform numerical experiments to better understand the influence of porosity, pore diameter, pore aspect ratio, and pore orientation on the tensile strength of volcanic rocks. We find that porosity and pore diameter exert a first-order control on the tensile strength of volcanic rocks, and that pore aspect ratio and orientation also influence tensile strength. Tensile strength is reduced by up to a factor of two as porosity is increased from 0.05 to 0.35 or as pore diameter is increased from 1 to 2 mm. Small, but systematic, reductions in tensile strength are observed as the angle between the loading direction and the major axis of an elliptical pore is increased from 0 to 90°. The influence of pore aspect ratio (the ratio of the minor to major axis of an ellipse) depends on the pore angle: when the pore angle is 0°, a decrease in pore aspect ratio, from 1 (a circle) to 0.2, increases the tensile strength, whereas the same decrease in pore aspect ratio does not substantially change the tensile strength when the pore angle is 90°. These latter numerical experiments show that the tensile strength of volcanic rocks can be anisotropic. Our numerical data are in broad agreement with new and compiled experimental data for the tensile strength of volcanic rocks. One of the goals of this contribution is to provide better constrained constitutive models for the tensile strength of volcanic rocks for use in volcano modelling. To this end, we present a series of theoretical and semi-empirical constitutive models that can be used to determine the tensile strength of volcanic rocks, and highlight how tensile strength estimations can influence predictions of magma overpressures and assessments of the volume and radius of a magma chamber.