Flawed Rock Research Articles

Cracking evolution and failure mechanism of brittle rocks containing pre-existing flaws under compression-dominating stresses: Insight from numerical approach

Comprehending the cracking evolution and failure mechanism of flawed rocks under compression-dominating stresses is essential to evaluating the stability of rock engineering. In this work, a newly developed geometry-constraint-based non-ordinary state-based peridynamic (GC-NOSBPD) theory that can eliminate the numerical oscillation is applied to predict the development of new cracks of flawed rocks under compression-dominating stresses. The failure of bonds between particles is judged by the bond-associated stress-based fracture criteria. The cracking evolution trajectories of semi-disc and disc specimens containing flaws are traced. The effects of flaw inclination angle on the cracking trajectories are analyzed. The cracking evolution paths of rock-like specimens with two flaws under uniaxial and biaxial compression are molded. The effects of confining stress on the growth of new cracks are investigated. The confining stress restrains the propagation of tensile cracks, but it is easy to promote the growth of shear cracks. The concentration and transfer effects of stress can plainly revel the failure mechanism of flawed rocks. The present numerical method is reasonable to predict the tensile and shear failure modes of flawed specimens under compression-dominating stresses.

Experimental study on mechanical and acoustic emission characteristics of red sandstone containing combined flaws of hole-fissure under biaxial compression

To investigate the mechanical behaviour and failure mechanism of flawed rock masses under biaxial stress conditions, a series of biaxial compression experiments are conducted on red sandstone specimens containing combined flaws of the circular hole and perforated symmetric fissure combined with acoustic emission (AE) technology in this work. The results show a close association between the stress-strain curve morphology and confining stress, both the biaxial compression strength and elastic modulus exhibit an upward trend with increasing fissure angle and confining stress. The maximum AE energy and cumulative AE energy both increase with higher confining stress. The crack types in the specimens are identified by analyzing the AF/RA value distribution, revealing a gradual decrease in the percentage of tensile cracks with increasing fissure angle. An AE localization algorithm based on the least absolute value method is applied to pinpoint AE events during biaxial compression, and AE events distribution is analyzed through the Kernel density estimation (KDE). The crack extension behaviour is described based on the movement of the maximum kernel density points, which initially exhibits a progression from both ends of the specimen towards the central region and subsequently from the central fissure tip towards the two ends.

Compression-induced failure characteristics of brittle flawed rocks: Mechanical confinement-dependency

The flaw tips in brittle rocks are often the sources of crack initiation and growth due to the stress concentration, which commonly governs the rock strength. However, a unified framework identifying the compression-induced crack types, ultimate failure patterns and the cracking levels of brittle flawed rocks under different mechanical confinements is not yet available. This study conducts the laboratory compression experiments with the AE monitoring to explore the failure characteristics of flawed limestone and its confinement-dependency. Four new crack types including loop crack, secondary transverse crack, near-field transverse crack and far-field transverse crack are found experimentally, and then a modified crack type classification strategy is proposed. Four failure patterns including the σ1-axisymmetric flaw-disturbed spalling for uniaxial compression, the σ3-transverse-symmetric flaw-disturbed spalling for biaxial compression, the σ1 −axisymmetric flaw-disturbed shearing for conventional triaxial compression, and the mixed σ3-transverse-symmetric flaw-disturbed shearing and σ2-transverse-symmetric flaw-disturbed spalling for true triaxial compression, are documented for the first time. Moreover, an acousto-mechanics-based classification methodology of rock cracking levels is established, as well as an AF (average frequency)-RA (rising angle)-based Kernel density estimation method for interpreting the rock cracking nature and the strength mechanism. This paper gets insights into the mechanical confinement-dependency of the rock failure characteristics incorporating the pre-existing flaws and help interpret the field observations.

Crack Initiation and Propagation Mechanism of Flawed Rock Mass Based on Improved Maximum Distortion Energy Theory and PFC Numerical Simulation

Crack Initiation and Propagation Mechanism of Flawed Rock Mass Based on Improved Maximum Distortion Energy Theory and PFC Numerical Simulation

Effect of high-temperature and strain rate on the mechanical and cracking behaviors of flawed sandstone under dynamic impact loading

A deep understanding of how flawed rock cracks under high temperatures and dynamic impacts is crucial for safe deep rock engineering design and operation. Dynamic impact tests were conducted on sandstones with various flaw inclinations using a Split Hopkinson Press bar (SHPB) system at room temperature and after high-temperature treatment at 600 °C. The results show that the dynamic peak stress and energy consumption of rock were reduced by thermal treatment. The study identified the circular tensile- and compressive-tangential stress zones around the flaw boundary using elastic theory. The strain accumulation zones gradually shift from the tensile- to the compressive-tangential stress zones as the flaw inclination increases, resulting in a larger crack initiation angle and opening displacement, while also constraining the propagation path. Additionally, the high-temperature treatment strengthened the deformation in the circular tensile-tangential stress zone. In contrast to the severe failure of specimens at room temperature, the clear failure pattern of the specimens after high-temperature treatment merely consists of wing cracks. Finally, this study presents a detailed theoretical explanation of the cracking behavior under various conditions, which is consistent with the experimental results. This research enhances the understanding of dynamic crack propagation in deep high-temperature rocks and provides scientific references for engineering safety design and protection.

Intermediate Principal Stress Effects on the 3D Cracking Behavior of Flawed Rocks Under True Triaxial Compression

Crack initiation, growth, and coalescence in flawed rocks have been extensively studied for 2D (planar, penetrating) flaws under uniaxial/biaxial compression. However, little is known as to the mechanisms and processes of cracking from 3D flaws under true triaxial compression, where the intermediate principal stress (σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document}) is distinguished from the major and minor principal stresses. In this work, we systematically investigate the effects of σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document} on the 3D cracking behavior of rock specimens with preexisting flaws, through the use of mechanistic simulations of mixed-mode fracture in rocks. We explore how two characteristics of σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document}, namely, (i) its orientation with respect to the flaw and (ii) its magnitude, affect two aspects of the cracking behavior, namely, (i) the cracking pattern and (ii) the peak stress. Results show that the orientation of σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document} exerts more control over the cracking pattern than the flaw inclination angle. The peak stress becomes highest when σ1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _1$$\\end{document} is parallel to the flaw, whereas it becomes lowest when σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document} is parallel to the flaw. Also, the effects of σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document} magnitude are more significant when σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document} becomes more oblique to the flaw plane. On the basis of our observations, we propose mechanisms underlying the cracking behavior of 3D flawed rocks under true triaxial compression.HighlightsThe effects of the intermediate principal stress (σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document}) on the 3D cracking behavior of flawed rocks under true triaxial compression are systematically investigated.The orientation of σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document} exerts more control over the cracking pattern than the flaw inclination angle.The effects of σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document} magnitude become more significant when σ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _2$$\\end{document} are more oblique to the flaw plane.Mechanisms underlying the cracking behavior of 3D flawed rocks under true triaxial compression are proposed based on the observations made.

Testing method of rock structural plane using digital drilling

The rock mass consists of rock blocks and structural planes, which can reduce its integrity and strength. Therefore, accurately obtaining the characteristics of the rock mass structural plane is a prerequisite for evaluating stability and designing supports in underground engineering. Currently, there are no effective testing methods for the characteristic parameters of the rock mass structural plane in underground engineering. The paper presents the digital drilling technology as a new testing method of rock mass structural planes. Flawed rock specimens with cracks of varying widths and angles were used to simulate the rock mass structural planes, and the multifunctional rock mass digital drilling test system was employed to carry out the digital drilling tests. The analysis focuses on the variation laws of drilling parameters, such as drilling pressure and drilling torque, affected by the characteristics of prefabricated cracks, and clarifies the degradation mechanism of rock equivalent compressive strength. Additionally, an identification model for the characteristic parameters of rock mass structural planes during drilling is established. The test results indicate that the average difference of the characteristics of prefabricated cracks identified by the equivalent compressive strength is 2.45° and 0.82 mm, respectively. The identification model while drilling is verified to be correct due to the high identification accuracy. Based on this, a method for testing the characteristic parameters of the surrounding rock structural plane while drilling is proposed. The research offers a theoretical and methodological foundation for precise in situ identification of structural planes of the surrounding rock in underground engineering.

Deformation localization and crack propagation of sandstone containing different flaw inclination angles under different loading rates

Defects with varying geometric distribution in the rock mass play a crucial role in determining the stability of engineered rock masses. Previous studies have primarily investigated the initiation, propagation, and coalescence behaviors of flaws within rock masses. Nevertheless, there has been limited analysis of flawed rock masses under different loading rates. In this study, we present the deformation localization and cracking process of three types of inclined flawed sandstone specimens through conducting uniaxial compression and acoustic emission tests at four different levels of loading rate. The results supported the following findings: 1) Sandstone specimens with different flaw inclination angles exhibit the loading rate strengthening effect, and the strengthening effect gradually decreases with the increase of loading rate. 2) As the loading rate increases, the type of crack emergence changes from wing cracks to anti-tensile cracks, and the time of flaw initiation is shortened. 3) The cumulative acoustic emission counts were higher for the low-loading rate specimens than for the high-loading rate specimens. 4) Tensile cracks typically occur as the initial cracks. Anti-tensile cracks often coexist with wing cracks in rock specimens that have undergone tensile damage. Coplanar secondary cracks are the primary indication of shear damage formation in rock specimens. 5) The increase in loading rate promotes the transition of rock specimens from the mixed tensile-shear damage mode to the shear damage mode. These research results are of great theoretical significance and engineering value for understanding the failure mechanism of rock mass containing flaws and proposing effective measures to prevent cracking and ensure the safety of brittle solid structures.

Monitoring the Fracture Damage Evolution Process of Flawed Rocks and Its Temporal Fractal Characteristic

Monitoring the Fracture Damage Evolution Process of Flawed Rocks and Its Temporal Fractal Characteristic

Macro- and micromechanical properties of flawed sandstone and their degradation mechanisms under hydrodynamic scouring

To reveal the degradation mechanisms of the mechanical properties of a rock mass in a reservoir bank slope under hydrodynamic scouring, a hydrodynamic scour test device was developed in this study. Sandstone specimens containing prefabricated flaws were used as the research objects. Three conditions, namely, natural conditions, hydrostatic immersion, and hydrodynamic scouring, were considered. By considering the results of laboratory testing, digital image correlation (DIC), scanning electron microscopy (SEM), and thermodynamic analysis and fractal theory, the effects of different flow states on the mechanical properties of the flawed rock mass were systematically investigated in terms of the macromechanical parameters, strain field, strain energy, and microstructure. A crack initiation stress identification method based on the differentiation rate of the effective variance of strain fields was proposed. The energy conversion in the rock loading process was quantitatively characterized using three strain energy characteristic indicators. The internal microstructure of the rock was quantitatively characterized using a combination of SEM image characteristics and the fractal dimension. It was found that the uniaxial compressive strength, elastic modulus, crack initiation stress, energy storage limit, elastic strain energy conversion rate, and dissipated strain energy conversion rate of the hydrodynamically scoured specimen were the smallest, while the fractal dimension was the largest. There was a strong correlation among the mechanical parameters, energy characteristic indicators, and fractal dimensions of differently treated sandstone specimens because the macromechanical response and energy conversion characteristics of a rock mass are closely correlated with changes in its internal structure.

Influences of true 3D twin flaws on principal stress status and crack occurrence of brittle rock-like samples under uniaxial compression: Insights from experimental and DEM studies

Fracture structures commonly have negative impacts on the strength and safety of rock mass, and various combinations of 2D and 3D flaws make the destruction procedure becomes more complicated. Therefore, laboratory tests based on three types of flawed rocks (symmetric twin flaws with different 2D-3D occurrences) are finished to detect the mechanical, optical and acoustic characteristics of fractured rocks during the uniaxial compression test; PFC3D simulations are conducted for analyzing the deformation features, principal stress distributions, and detailed failure mechanism of the fractured rocks. The main results are as follows: 1) Different combinations of 2D & 3D preset flaws directly influence the results of deformation, stress distribution, and failure developments. 2) Rotation of the intermediate stress axes only appear near the flaw inclination boundary (when the monitoring position moves outwards), while the directions of intermediate principal stress axes remain stable near the flaw strike boundary. 3) Separating cracks by various occurrences helps to construct local failures along different directions, and make some local failures visible and clear; the traditional cracks (short line or thin oval shape) are induced by the uneven deformation in x-o-z planes, and the separated cracks (wide oval or standard disc shape) result from the uneven deformation in y-o-z planes. 4) Surrounding rocks at different positions suffer various modes of failures, the non-ignorable thickness of preset flaw leads to simultaneous Mode I and Mode II failure at the flaw tip, and the mixed Mode II&III failure only occurs in the rocks adjacent to the flaw strike boundary.

Mechanical behaviors and failure mechanism of flawed sandstone containing infillings under freeze-thaw cycles and uniaxial compression

The flawed rock masses with joints, fissures, and other defects in cold regions engineering are affected by freeze–thaw (F-T) cycles. Substantial efforts have been focused on the mechanical properties and failure characteristics of flawed rock masses under F-T cycles. However, only a few laboratory tests have considered the effect of fillers in the rock flaws under F-T cycles. In the present work, F-T cycling and uniaxial compression tests are conducted on intact sandstone, flawed sandstone containing infillings, and flawed sandstone without infillings to further investigate the effects of fillers and the number of F-T cycles on the mechanical behaviors and failure mechanism of flawed sandstone containing infillings. The experimental results indicate that filling the flaws can effectively inhibit mass loss and pore growth of flawed sandstone specimens in the early stages of F-T cycles. Fillers can effectively reduce the stress concentration at the flaws and improve the strength of flawed specimens. In the early stages of F-T cycles, crack propagation caused by uneven stress and deformation at the flaws leads to the failure of the specimen. As the number of F-T cycles increases, the peak stresses of the specimens are gradually decreased, and the reduction of the cementation strength between rock particles causes particle loss and cracking failure in the specimens. The failure characteristics of flawed specimens containing infillings are significantly different from those of other two types of specimens. Based on the test phenomena, a deterioration failure mode for F-T failure of flawed specimens containing infillings is proposed.

Cracking Characteristics and Damage Assessment of Filled Rocks Using Acoustic Emission Technology

Natural flaws containing fillings strongly affect the mechanical properties and failure process of rocks in practical engineering. In this work, uniaxial compression tests are performed on filled and unfilled red sandstone specimens containing two parallel dentate flaws. Two monitoring technologies––acoustic emission (AE) and high-speed camera––are employed to record the cracking process of the flawed rocks. The effects of filling and ligament angles on the mechanical properties of the flawed rocks are analyzed. The relationship between AE parameters and stress–strain curves is established in order to evaluate the cracking process. The fracture behaviors between the filled and unfilled specimens are qualitatively compared. An intensity analysis method, based on the historic index and severity, is employed to assess the damage level of the flawed rocks. Simultaneously, a judgmental design criterion, including safety, early warning, and instability, is developed, which can be employed to evaluate the stability of rock structures.

Numerical study on cracking behavior and fracture failure mechanism of flawed rock materials under uniaxial compression

AbstractThe numerical approach is a vital means for evaluating the stability of flawed rocks. In this paper, the extended non‐ordinary state‐based peridynamics (NOSB‐PD) theory is employed to simulate the fracture process of rocks containing two pre‐existing flaws. Two stress criteria, including the maximum tensile stress criterion and the Mohr–Coulomb criterion, are implemented into the NOSB‐PD numerical method to respectively judge the tensile and shear failure of rock materials. The effects of inclination angles and ligament angles on the crack initiation and coalescence modes for flawed rocks are investigated based on the numerical results. The numerical results are in good agreement with the previous experimental results. The fracture mechanism of flawed rocks is revealed based on the evolutionary distribution characteristics of the maximum principal stress field and shear stress field obtained by the extended NOSB‐PD theory.

Size effects on the strength and cracking behavior of flawed rocks under uniaxial compression: from laboratory scale to field scale

Size effects on the strength and cracking behavior of flawed rocks under uniaxial compression: from laboratory scale to field scale

Numerical simulation of size effect of defective rock under compression condition

The existence of various types of damage, small cracks, some large voids and the size of the sample in the rock will make the experimental results show great discreteness. In this paper, based on the results of laboratory experiments, a numerical model of large flawed rock samples is established by using particle flow software PFC2D, and the mechanical response of rocks with different length-diameter ratios and different flaw positions in uniaxial compression experiments is discussed. The results show that the specimen size has a significant effect on the crack characteristics, mechanical characteristics and energy characteristics of rock mass. From the perspective of energy and crack characteristics, the total number of cracks after the failure of the defective rock sample is slightly lower than that of the intact rock sample, resulting in a slightly lower peak strain energy during the rock failure process. From the mechanical properties of rock samples, the Poisson’s ratio of intact rock samples is slightly smaller than that of defective rock samples. The strength of the defective sample is weakened relative to the complete rock sample, and the relationship formula between the weakening range and the aspect ratio is obtained through analysis. Moreover, different defect locations lead to different crack processes and crack modes, resulting in different uniaxial compressive strength.

Mechanical Properties and Energy Characteristics of Flawed Samples with Two Non-Parallel Flaws under Uniaxial Compression

Unequal flaws are widespread in rock engineering, in which the extensions induced by excavation and unloading are the main factors leading to engineering instability. Rock deformation and failure are essentially the results of energy transmission. In order to study the influence of flaw type and angle on the mechanical properties and energy characteristics of the flawed rock mass, uniaxial compression tests were conducted using particle flow numerical analysis software. The results show that the crack initiation strength and peak strength of the sample increase with the increase in the flaw angle. For smaller flaw angles, the sample shows obvious plastic deformation during uniaxial compression. For larger flaw angles, the sample shows elastic–brittle properties, whose storage energy is larger. The peak strength of samples with flaws of unequal length presents an obvious decrease compared with that of flawed samples with flaws of equal length, and the extent of the reduction is 6.15% on average. Unequal flaws decrease the ability of the flawed sample to absorb strain energy. Compared with flawed samples with equal flaws, flawed samples with unequal flaws decrease the boundary energy and elastic strain energy by 9.29% and 9.95% on average, respectively. Flaws of unequal length in this sample can weaken the performance of the flawed sample.

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

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

Experimental investigation on mechanical, acoustic, and fracture behaviors and the energy evolution of sandstone containing non-penetrating horizontal fissures

Flaws are the weakest part of rock mass, which directly determine the stability of underground engineering. Existing investigations on the mechanical behaviors of flawed rock mainly emphasize on the geometric parameters of the fissure, such as inclination, width, and length, but few works have been conducted on the effect of fissure location on rock properties. This study explores the influences of two fissure layouts (distance between horizontal single fissure and specimen end, and horizontal double fissures spacing) on the sandstone properties under uniaxial compression. The properties studied include strength and deformation, failure mode, acoustic emission (AE) characteristics, and energy evolution. The findings indicated that pre-existing fissures significantly reduced the rock's strength, elastic modulus, and peak strain, and the rock's mechanical properties decreased as the distance between the horizontal single fissure and the specimen end decreased and as the double fissures spacing increased. Furthermore, it was observed that intact specimens mainly underwent shear failure and the flawed rocks under uniaxial compression experienced splitting failure or both. The flawed specimens presented multiple higher AE counts before the peak stress, corresponding to crack initiation, propagation and coalescence, which were distinctly distinct from the intact specimen. The energy evolution characteristics of the fissured sandstone reflected their damage process. The findings are promising to improve the understanding of mechanical mechanisms and energy evolution of flawed rock engineering.

Experimental investigation on crack coalescence and strength loss of rock-like materials containing two parallel water-filled flaws under freeze–thaw

The freezing of water-bearing flaws has a much more serious threat on the stability of fractured rock slopes, because saturated flaws provide more liquid water for frost expansion and thus induces frost cracking. The freeze–thaw collapse of alpine mountains is mainly caused by the frost cracking of rock bridges between flaws. In this research, the frost propagation and coalescence process of two parallel water-filled flaws was investigated by experiment, by considering the effects of flaw angle and bridge angle. It indicates that the maximum frost heaving pressure in these elliptical flaws is more than 4 MPa, which is large enough to drive flaws propagation and cause bridge cracking. A considerable uniaxial compressive strength (UCS) loss occurs after freeze–thaw cycles when the flaw or bridge angle is close to the inherent rupture angle of intact samples, because the loading rupture coincides with new frost heaving cracks. Therefore, more attentions should be paid on these flawed rocks with flaw or bridge angles equal to the rupture angle, which is easier to rupture and fall down from the mountains. In addition, the unidirectional freezing mode is more conducive to drive the flaw propagation than the uniform freezing mode, because the frost heaving pressure under unidirectional freezing is much larger due to the ice plug effect, which means that the solid ice is like a plug blocking the mouth of cracks and preventing the water flowing outside. This study can provide a better understanding of the frost propagation of water filled flaws in rock masses and the stability of rock engineering under freeze–thaw in cold regions.