Cracks In Rock Mass Research Articles

Study of Impact Load Directions on Tunnel Stability in a Cracked Rock Mass

Study of Impact Load Directions on Tunnel Stability in a Cracked Rock Mass

Numerical simulation of blasting behavior of rock mass with cavity under high in-situ stress

With the shift of coal seam mining to the deep, the in-situ stress of coal and rock mass increases gradually. High ground stress can limit the generation of rock cracks caused by blasting, and blasting usually shows different crushing states than low stress conditions. In order to study the blasting expansion rule of rock mass with cavity under high ground stress and the rock mass fracture state under different side stress coefficients. In this paper, the effective range of blasting and the stress distribution under blasting load are analyzed theoretically. The RHT (Riedel-Hiermaier-Thoma) model is used to numerically simulate the blasting process of rock mass with cavity under different ground stress, and the influence of ground stress and lateral pressure coefficient on the crack growth of rock mass is studied. The results show that when there is no ground stress, the damage cracks in rock mass are more concentrated in the horizontal direction and the fracture development tends to the direction where the holes are located, which confirms the guiding effect and stress concentration effect of the holes in rock mass, which helps to promote the crack penetration between the hole and the hole. The length difference of horizontal and vertical damage cracks in rock mass increases with the increase of horizontal and vertical stress difference. Under the same lateral stress coefficient, the larger the horizontal and vertical stress difference is, the stronger the inhibition effect on crack formation is. For blasting of rock mass with high ground stress, the crack formation length between gun holes decreases with the increase of stress level, and the crack extends preferentially in the direction of higher stress. Therefore, the placement of gun holes along the direction of greater stress and the shortening of hole spacing are conducive to the penetration of cracks between gun holes and empty holes. The research can provide reference for rock breaking behavior of deep rock mass blasting.

Numerical study on fractal characteristic of blast-induced cracks in deep rock mass

The quantitative representation on the characteristics of blasting-induced cracks in rock mass is of great significance to guide the fine blasting design in practice. However, the quantitative relation between blasting-induced cracks of rock mass and in-situ stresses is currently lacking. To provide the reference for the fine blasting design to excavate rock mass with different in-situ stresses, the present study adopts the fractal theory to quantitatively represent the characteristics of blasting-induced cracks in rock mass. The effects of in-situ stress levels and lateral coefficients on the modes and fractal dimensions of blasting-induced cracks are investigated by numerical simulation combined with image processing. The results show that with the increase of in-situ stresses and lateral coefficients, the fractal dimensions of blast-induced cracks gradually decrease. In addition, quantitative relations between the fractal dimensions of blasting-induced cracks and in-situ stresses are established to provide the reference for the design of rock blasting.

Pressure-Shear Crack Initiation and Expansion Mechanism of Complex Cracked Rock Mass under the Seepage Stress

This work was to analyze and discuss the propagation mechanism of compressive shear initiation of complex fractured rock mass with seepage stress. The dense marble of Daye Iron mine with bulk density of 26.6 kN/m3 and uniaxial compressive strength of 52.4 MPa was selected as the material, and the upper and lower fracture surfaces were polished smoothly. The crack initiation criterion under compressive shear stress state is analyzed by taking the theory of fracture mechanics and classical mechanics. The coupling equation in the extended finite element simulation is established. The influence of lateral pressure on the crack propagation law, the relationship between lateral pressure and fracture, the initial expansion angle and pressure change law, and the effect of working face length on the crack expansion are analyzed. Results. The initial expansion angle of cracks increases with the increase of lateral pressure, and that of a single crack decreases with the increase of pressure. When other conditions are constant, the crack angle of the crevice also shows a trend of increasing with the increase of lateral pressure. When the lateral pressure becomes smaller, the initial expansion angle is relatively small. With the progress of the step size, the expansion angle shows a gradually decreasing trend, that is, the initial expansion angle gradually decreases with the increase of water pressure. The smaller the working face length, the smaller the expansion length of the floor crack. Conclusion. The expansion of the floor cracks is mainly formed by the tensile shear failure, and the fracture water pressure will reduce the initiation stress, which makes the rock mass more prone to the fracture failure.

Cracking behaviors and mechanism of pre-cracked rock specimens under coupled THM fields with chemical processes

In deep rock engineering, natural cracks in rock mass subjected to thermo-hydro-mechanical-chemical (THMC) loads have tendency to initiate and propagate, and result in potential safety hazards. Current literature on cracking behaviors and mechanism of pre-cracked rock under multi-field loads is focused on single (M), two (TM, HM) or three fields (THM, HMC), less on THMC fields. In this study, a series of self-designed triaxial compressive tests were performed to study different THMC fields influencing on mechanical behaviors and failure patterns of pre-cracked red sandstone specimens. A new THMC fracture criterion, recently proposed by our group team, is used to further analyze the crack initiation mechanism of red sandstone under THMC loads. Research results suggest that the stress-strain curve is greatly influenced by the temperature, confining pressure and acidity and alkalinity of chemical solution, but relatively less by the hydraulic pressure. As the temperature and hydraulic pressure increase, the confining pressure decreases, and the acidity or the alkalinity of chemical solutions becomes strong, both the elastic modulus, crack initiation stress and peak strength of specimen are decreased, and the failure pattern is changed from shear failure to tensile failure. The crack initiation stress and peak strength usually have the largest, middle, smallest values in the shear, tension-shear and tensile failures, respectively, while the peak strain has largest value in the mixed tensile-shear failure. The acid solution has greater weakening effect than the alkaline solution with the same concentration. Theoretical results indicate that the temperature and chemical fields influencing on crack initiation is mainly attributable to its weakening of rock fracture toughness, while seepage and stress fields on crack initiation to its change of the stress state at crack-tips. The THMC mechanism of crack initiation can be well revealed by the new THMC fracture criterion.

Experimental study on strength and failure characteristics of mortar specimens with prefabricated cracks under uniaxial and triaxial stress

Cracks in rock masses have adverse effects on their mechanical characteristics and the stability of rock mass engineering. For this experiment, uniaxial compression and true triaxial compression unloading tests were conducted on mortar specimens. The strength of the specimens was the lowest when the dip angle of the crack was 45°. Under uniaxial compression, the failure mode was mainly tensile failure. With an increase in the crack dip angle of the specimens, the types of cracks changed from shear to tensile cracks. Under true triaxial compression, the confining pressure considerably increased the strength and deformation characteristics of the specimens and weakened the influence of the crack dip angle on the strength. The failure mode when the crack dip angle was 15° was mainly tensile. The failure mode when the crack dip angles were 45° and 75° was mainly tensile–shear composite. With the increase in the crack dip angle, the failure degree of the specimens was more severe. Under the triaxial unloading confining, the failure mode was mainly shear. Compared with uniaxial compression, the failure degree of the specimens was more severe under true triaxial compression and true triaxial unloading. The innovation of this experiment is the use of a special mold to prepare specimens. We recommend that scholars conduct experimental and numerical simulation studies on different crack geometries.

Review on stress evolution and crack propagation mechanism of double-crack specimens

The distribution pattern of cracks in rock masses, known as the geometric distribution, holds significant significance in understanding the progressive failure mechanism and stress concentration can lead to instability and even failure of the specimens. However, the macroscopic and microscopic mechanisms of crack coalescence and the failure of the sample are not yet clear. Thus, the discrete element method was used to investigate the influence of stress distribution on the fracture coalescence and failure of dual-crack specimens with different arrangements under uni-axial compression in this study. The obtained results were compared and analyzed with laboratory test results. The results show: When the double-crack specimens are under overlapping arrangement, the stress concentration area only appears at the end of the crack, which is conducive to crack propagation, resulting in no coalescence. However, if the two preexisting cracks were arranged in a co-planar or non-overlapping manner, the stress concentration of the sample would not only occur at the end of the cracks, but also between the cracks. As the stress increases, the cracks will gradually expand and connect, forming shear coalescence and mixed mode coalescence (shear and tensile) respectively. In addition, the arrangement of cracks greatly changes the influence of stress concentration on the failure mode of the specimen, leading to the possibility that the coalescence of the specimen may occur in three periods: before, during, and after the peak stress. The findings of this research hold immense importance in comprehending the mechanisms behind the coalescence and failure of rock fractures. Moreover, they provide valuable insights for the analysis and design of rock engineering projects, offering practical guidance for ensuring stability.

Characteristics, numerical analysis and countermeasures of mud inrush geohazards of Mountain tunnel in karst region

The excavation of tunnels in karstic rocks inevitably intersects with karst caves even with reasonable geological survey and alignment design. The filler in karst caves directly affect the harmfulness of mud inrush geohazards to tunnels. Therefore, it is essential to understand the properties and failure characteristics of the filler in karst caves before treating karst caves. Based on a tunnel mud inrush case in Guangxi Province, China, failure characteristics and mechanism of clay filler in karst cave were analyzed. Besides, geological survey and geological prospecting indicate that the clay filler was formed by the deposition of surface clay entering the karst cave from the cracks of rock mass. The failure evolution process of clay filler was demonstrated based on discrete element method. The numerical simulation results show that excavation induced sliding force and insufficient shearing strength of clay were the mainly triggering of instability of clay filler in the karst cave. Advanced grouting, construction method improving were used to prevent mud inrush from bursting again and achieved satisfactory reinforcement effect. The lessons and analysis results in this case has reference value for the prevention of tunnel mud inrush in karst area.

Analysis of P-wave propagation in filled jointed rock mass with viscoelastic properties

Stress wave propagation in filled jointed rock mass is an important basis for the analysis of rock dynamic stability. The presence of small cracks in rock mass often causes the rock to exhibit viscoelastic behavior, resulting in the amplitude attenuation and time delay of the stress wave. In view of the incomplete understanding of stress wave propagation in viscoelastic media, this study presents an analysis on stress wave propagation in filled jointed rock mass with viscoelastic properties based on the time-domain recursive analysis method. By introducing the quality factors of stress wave in viscoelastic medium, the propagation equation of plane P-wave in filled jointed rock mass with viscoelasticity is established. Subsequently, the transmission and reflection characteristics of P-wave are accordingly analyzed. The results indicate that the transmission and reflection coefficients of P-wave through the filled joint decrease obviously due to the attenuation effect of rock viscoelasticity on wave amplitude, but the coefficients gradually stabilize as the quality factor increases. As the propagation distance of the P-wave increases in viscoelastic rock masses, the transmission and reflection coefficients decrease gradually. The normal stiffness of joint contact interface has a decisive influence on the wave propagation. Specifically, a higher normal stiffness leads to stronger transmission and poorer reflection. The larger the thickness of filled joint, the more obvious the attenuation of P-wave becomes. Additionally, the viscoelasticity of filling layer further aggravates the influence of joint thickness on the attenuation of P-wave. The study comprehensively takes into account the effect of viscoelasticity of both the rock mass and the filling layer on transmission and reflection of waves. The research results provide certain theoretical support for the dynamic response and stability analysis of rock mass containing structural planes.

Theoretical analysis of two cracks in sandstone under the simultaneous action of pressure and shear stress

AbstractThe cracked rock mass could slide along the weak side under the action of compressive loads and shear loads in landslides; thus, the stability evaluation of the crack in rock mass media is of particular importance. The formulas of stress intensity factors (SIFs) for each crack tip are theoretically derived by implementing complex function theory. The results of the analytical solutions are then compared with the predicted results of the finite element analysis via ABAQUS software, revealing a slight relative error. Photo elastic experiment and static compression tests are conducted to verify the correctness of theoretical results. The obtained results reveal that the internal pressure leads to the increasing SIFs of mode II at crack tips. Further, the larger the difference between the horizontal and vertical compressive stresses, the more likely the occurrence of shear failure in the cracked rock mass. For the two cracks, the most dangerous point is the inner crack tip of the longer crack.

Study on the global stress field of the rock mass under compression and shear considering three kinds of crack parameters

AbstractThe mechanical behavior of the cracked rock mass is controlled by the crack initiation and propagation under compression and shear. The key to predicting the wing crack initiation angle is to correctly describe the distribution of the stress field at the crack tip. First, the previous studies are reviewed, and their inadequacies are discussed in detail. Next, according to the experimental results, three kinds of crack parameters (i.e., geometry, strength, and deformation) are introduced, and a stress transfer model on the crack face under compression and shear considering crack parameters is proposed. Then, the relevant formulae of stress intensity factors (KI and KII) and non‐singular term (T‐stress) at the crack tip are obtained. Finally, the proposed model is verified by other researchers' test results, and the effects of three kinds of crack parameters and the rock property on the distribution of tangential stress at the crack tip are discussed.

Numerical Study of Slip Distribution at Pre-existing Crack in Rock Mass using Extended Finite Element Method (XFEM)

Numerical Study of Slip Distribution at Pre-existing Crack in Rock Mass using Extended Finite Element Method (XFEM)

Effect of reflected stress wave on dynamic crack propagation and arrest behavior of sandstone specimens under impact loading

The interaction of stress waves generated by blasting or excavation disturbance with cracks or joints significantly affects the mechanical behavior of rock masses with some pre-existing cracks. To better understand the dynamic damage evolution mechanism of fractured rock mass, the influence of stress waves on cracks in rock mass must be investigated. Thus, to study the effect of reflected stress wave on dynamic fracture behavior of fissured rock mass, a large-size arc-shape boundary single cleavage triangle (ABSCT) configuration sandstone specimen was proposed in the present study. A series of impact tests were performed on the specimen with 0°, 65°, 95°, and 125° ABSCT using the drop plate impact testing system to determine dynamic fracture properties. The test was numerically simulated using the modified finite difference software and the traditional finite element software. The effect of the reflected stress wave caused by the arc-shape boundary of the ABSCT specimen on the crack propagating speed, crack fracture toughness, and compressive stress along the crack trajectory under dynamic loads was analyzed and discussed. The results show that the ABSCT specimen has a crack arresting effect on a propagating crack. The average crack propagation speed of three specimens with arc-shape boundaries is almost 0.63 times that of the flat-bottom specimen, indicating that the arc boundary has an obvious effect on the crack propagating velocity. The average crack propagating speed decreases with the increase in arc boundary radian. The compressive stress on the crack path of three arc boundary specimens is found to be greater than that of the flat-bottom specimen. Furthermore, the crack arrest toughness is greater than the average crack propagating toughness at the arc-shape boundary specimen.

Experimental Study on the Evolution Law of Coal Mine Underground Reservoir Water Storage Space under the Disturbance and Water—Rock Interaction Effect

The void of the cracked rock mass of the goaf is the main water storage space of underground reservoirs, which is in a time-space dynamic evolution process. Before the formation of the underground reservoir, the water storage space was primarily affected by disturbances. After the safe operation of the coal mine underground reservoir, the water level of the mine rises and falls repeatedly and the water storage space is affected by the water-rock interaction. To study the void evolution law of a cracked rock mass under mining disturbance and the compaction and void deformation characteristics of caving gangue under the effect of the water-rock interaction, a simulation test of a coal mine underground reservoir is conducted. Furthermore, the rupture motion law and movement deformation characteristics of the overburden during coal mining are analyzed. The digital image method and fractal theory are introduced to describe the fractal characteristics of the rock mass void of the caving zone, fracture zone, and entire goaf during the mining process. Five prototype gangue samples with different immersion times are prepared with the same grain size grading as the similar model caving gangue. The influence of the immersion times on the compaction characteristics and evolution law of the void rate of the gangue are also studied. Based on the parameter fitting method, the stress–strain relationship equation of the gangue sample and void rate-stress relationship equation of the cylindrical gangue sample, considering the influence of the immersion times, are constructed. The results show that the overburden of the model is of a “two zone” structure and the entire structure moves and sinks asymmetrically in a “∩” shape. As the advancing distance of the working face increased, the fractal dimensions of the rock mass void of the caving zone and entire goaf increased logarithmically, and the fractal dimension of the rock mass void of the fracture zone first increased rapidly (60–80 cm) and then decreased linearly (80–200 cm). As the immersion time increased, the saturated moisture content and density of the gangue samples increased logarithmically and exponentially, respectively. Under the same stress, the strain of the gangue sample increased gradually, and the void rate decreased gradually (except for the initial loading).

A Novel Continuous-Discontinuous Multi-Field Numerical Model for Rock Blasting

During blasting, rock failure is caused by blasting wave and explosive gas pressure, as a multi-field coupled process. Most numerical models focus on the effect of blasting wave where the gas pressure is commonly accounted for by empirical relations, ignoring the penetration and permeation of gas flow in cracks. This can underestimate the failure region. In this work, a novel multi-field model is developed in the framework of a continuous-discontinuous element method (CDEM), which is a coupled finite-discrete method with explicit integration strategy. The deformation and cracking of rock mass and the distribution of gas pressure are captured. The proposed method is verified by comparing the results to other results provided in published literature. Especially, by simulating the cases with blocked and unblocked blasting hole, we found that: (i) The fracture degree of the case with blocked blasting hole was 30% higher than that of the unblocked blasting hole. (ii) The radial main cracks in the fracture area are mainly caused by the explosive gas, and the tiny and dense cracks near the hole are induced by the explosion stress wave. (iii) The explosion crushing zone is mainly formed by the action of explosion stress wave, while the crack zone is formed by the combined action of the explosion stress wave and explosive gas. The proposed method provides a useful tool to properly simulate a rock blasting process.

Time-dependent propagation and interaction behavior of adjacent cracks in rock-like material under hydro-mechanical coupling

Under the long-term coupling effects of seepage pressure and geo-stress, the interaction and propagation of cracks in rock mass show typical time-dependent characteristics, and directly control the failure mode of rock mass. The laboratory tests under long-term hydro-mechanical coupling were carried out based on the cement mortar specimen containing adjacent pre-existing cracks, and the micro crack evolution in the specimen was analyzed by using low-frequency nuclear magnetic resonance technology, the corresponding numerical simulation was carried out to study the propagation and interaction behavior of adjacent cracks and the time-dependent mechanism of creep failure of rocks. The results show that: The creep failure of rock mass with adjacent cracks presents a tension-shear composite mode; The macro fracture is formed by the propagation of one crack and does not connect with the adjacent crack; Due to the interaction effect between cracks, a large number of micro cracks are formed in the specimen, and the propagation of adjacent pre-existing cracks is non-uniform, and the propagation of upper and lower wing cracks of the dominant crack is asymmetric; In the crack initiation stage, the crack propagation is controlled by the shear fracture mode, and the interaction shows the inhibition of adjacent cracks; After propagating to a certain extent, the dominant crack propagation is controlled by the tensile fracture mode, and its propagation rate increases, which accelerates the failure of rock mass.

Experimental study on the energy evolution law during crack propagation of cracked rock mass under impact loads

Rock dynamic fracture is usually accompanied by numerous energy loss, to reveal energy evolution laws during rock dynamic fracture, the single cleavage triangle (SCT) specimens made by two types of sandstone are implemented to conduct dynamic fracture tests by using the split Hopkinson pressure bar (SHPB) device. The dynamic stress distribution law and the energy evolution law can be obtained according to the strain history of cracked rock mass subjected to impact loads. Crack propagation gauges (CPG) were applied to measure crack propagation speed, dynamic stress intensity factor (DSIF) and dynamic energy release rate can be determined by using an experimental–numerical method with the help of the software COSMOL. The experimental results show a physical pseudo-healing of the crack due to the short closure of the crack after its arrest under impact loads. The crack propagation speed and the DSIF show a certain oscillatory with the increase of crack length, the DSIFs at crack initiation are 0.2 ∼ 0.8 of the maximum value. The history of dissipated energy has a slow decrease after they reach a peak during 300 μs ∼ 350 μs, whose curves after the peak contain two stages: stage I is that the absorbed energy of the SCT specimen is mainly transformed into the transmitted energy; stage II is that it will be reflected energy. The input energy overtakes the consumed energy by new crack formation to drive the crack propagation and the test results of sandstone specimens show that 2.11 ‰ to 4.44 ‰ dissipated energy is implemented to drive crack propagation. The energy and stress are released more significantly around the crack tip before the crack arrest.

Research on dynamic cracking properties of cracked rock mass under the effect of thermal treatment

Deep fractured rock mass is more prone to failure with the coupling effect of suddenly dynamic disturbances loads (explosions, seismic waves, etc.) and high temperature environment conditions, these factors have a great effect on the dynamic stability of fractured rock mass. In the present study, some traditional cracked straight through Brazilian disc (CSTBD) specimens made of granite and sandstone were heated from 25 °C to 700 °C to obtain dynamic fracture properties of high temperature soft and hard rocks. A modified traditional split Hopkinson pressure bar (SHPB) was applied for dynamic fracture experiments under impact loads, high speed camera, digital image correlation (DIC) and fracture surface scanning technology were used to study the fracture morphology of the specimens, the dynamic damage mechanism of high temperature soft and hard rocks under impact loads was obtained. The test results indicate that high temperature can greatly affect the roughness of the fracture surfaces under impact loads during crack formation and propagation, and the fractal dimension increases with the increase of temperature. Observing the dynamic fracture strength and fracture toughness of the CSTBD specimens, it can be found that the strength of the granite decreases gradually with high temperature treatment, and the dynamic fracture strength of the green sandstone increases firstly and then gradually decreases. At 500 °C–600 °C, the dynamic fracture strength of the two types of rocks decreases rapidly. Dynamic critical crack tip opening displacement of the specimens with different thermal temperatures increases with temperature, but the fracture process was similar and the cracks would follow the loading direction.

The strain characteristics of cracked anchored soft rock under short-term creep

Abstract The short-term creep properties of cracked soft rock are the basis for analyzing and predicting the long-term deformation characteristics of engineering rock mass. Based on orthogonal test design and by means of physical testing and theoretical analysis, this paper systematically studies the short-term creep strain properties of soft rock mass with a vertical anchor form under different combinations of crack number, crack discontinuity degree and crack inclination. The results show the following: (i) as the stress increases, the growth rate of the instantaneous strain of the anchored soft rock slows down, the growth rate of the creep strain increases and the growth rate of the total strain decreases at first and before it then increases. Based on this, the idea is proposed that the stress corresponding to the turning stress of the total strain change rate, from small to large, is the turning stress of the strain rate. This stress value is approximately 50% of its uniaxial compressive strength (UCS). (ii) Statistical analysis shows that for the same cracked rock mass, the three characteristics of cracks have different effects on the instantaneous strain, creep strain and total strain of anchored soft rock, and a certain number of cracks under specific geometric conditions are beneficial to the long-term deformation control of anchored soft rock. (iii) A relative creep model based on mutation is proposed. Considering the effects of the test instrument, the model can be divided into the power function model, the logarithmic function model and the exponential function model, according to goodness of fit. All three models can describe the initial and stable creep stage of anchored soft rock with cracks.

Mechanical behavior of non-persistent joints with different geometric configurations and roughness in solid rock and concrete material

The shear behavior and failure mechanism of non-persistent joints or cracks in rock mass and concrete material are determined by both the rock bridges and the joint geometric configurations. In the present study, the synthetic rock mass (SRM) numerical models are established based on the particle flow code 2D (PFC2D), and numerical direct shear tests are then conducted to investigate the mechanical behavior of non-persistent rock joints. Effects of joint geometric configuration, normal stress, joint roughness, and joint aperture on the shear behavior of non-persistent joints are discussed. Results show that shape of the shear stress curves is greatly influenced by the joint geometric configuration. Besides, specimens with a non-persistent joint in the central have the largest shear modulus, the lowest peak shear strength and the smallest number of microcracks as compared with the other two joint types. The average residual shear stress is not significantly influenced by the joint geometric configuration, but fluctuates more dramatically with the increase in JRC value. For the same joint configuration, the microcrack propagation is restrained under higher normal stress, leading to the decrease of the maximum length of cracks with increasing normal stress. The peak shear strength decreases linearly with the increment of joint aperture under a low normal stress, while it tends to be stable when the joint aperture is larger than 2.0 mm under a high normal stress. The shear modulus of joint specimens is rarely influenced by the JRC values and the joint apertures. The obtained conclusions can provide valuable guidance for the stability analysis and micro-seismic (acoustic emission) monitoring of rock slopes, concrete dams and foundations with embedded non-persistent joints or cracks.