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The mechanics behaviors, failure modes and acoustic emission characteristics of rectangular green sandstone specimens containing a single joint with various inclination degree under different confining pressure have been determined in this article. The samples were tested over a confining pressure range of 0 to 40 MPa, and the inclination of the joint was varied between 0° and 50°. The test results indicated that: Mohr–Coulomb criterion is fit to peak strength of intact samples and residual strength of jointed samples with 40°. But the peak strength of jointed sample with 40° increases nonlinearly with confining pressure, which can be fitted by Hoek–Brown criterion. And the confining pressure has a reduction effect on anisotropy for jointed rock. When the inclination angle was less than 40°, the joint has a small negative influence on the strength and deformation of the jointed sample, as the inclination angle increased from 40° to 50°, a drop decrease in peak strength and Young's modulus can be observed. The failure modes can be classified into T-mode, S-mode and SD-mode. Finally, the evolution laws of AE count for green sandstone specimens has been discussed. [ABSTRACT FROM AUTHOR]

Abstract The geometry and orientation of cavities in a rock mass has a significant influence on the failure mechanism and fracture propagation. The theoretical study of these behaviors is of great importance to enable fundamental understanding of unstable failure in rock mechanics. A very limited number of experimental studies have been conducted for sandstone specimens containing oval cavities. Sensitive parameters include the short axis (b) to long axis (a) ratio. Here, we enhance the present understanding of crack coalescence behavior around an oval cavity by applying uniaxial compression tests to sandstone cores containing manually inserted single oval cavity, combined with photographic and acoustic emission monitoring. The b : a ratio was varied from 1/4 to 1/1, thus the cross-sectional cavity shape evolved from oval to a fully circular opening. The experimental results showed that the stress–strain curves of specimens containing cavities exhibited multiple stress reductions prior to peak strength, which led to a sudden increase in the accumulated AE count-rate. As the b : a ratio was increased, the uniaxial compressive strength, crack initiation stress and peak strain of cavity specimens all decreased linearly. Tensile fractures, shear fractures, and compound shear-tensile fractures were all observed to emanate from the cavities. Compared to the intact specimen, the failure mode of cavity specimens comprised a mixture of tensile and shear cracks, emanating from the tips (or surfaces) of the oval cavity. For specimen with a cavity angle of 45°, when the b : a ratio was below 1/2, no tensile crack from the surface of short axis was initiated, whereas tensile crack was observed clearly after b : a = 1/2. This study not only increases our understanding of mechanical failure behavior in fractured rock, but also enhances our knowledge of underground tunneling or roadway stability. [ABSTRACT FROM AUTHOR]

In the rock engineering, it is very significant to focus on the failure mechanical behavior of rocks containing all kinds of flaws. In this research, the strength, deformation and crack evolution behavior of sandstone containing a single oval flaw under uniaxial compression were evaluated by experiment and numerical simulation using a two-dimension particle flow code (PFC 2D ). The experimental results show that the peak strength and elastic modulus first decrease and subsequently increase with increasing oval flaw angle ( α ); the lowest value appears when α  = 15°, but the oval flaw angle has little effect on peak strain. The oval flaw angle has a great effect on crack initiation and propagation, as observed from photographic monitoring and the AE technique. The AE counts of flawed specimens show several larger peak values prior to peak strength, which correspond to crack initiation and propagation in the specimen. And then, PFC 2D was used to simulate sandstone containing a single oval flaw under uniaxial compression, and the simulated results are in good agreement with the experimental results. By analyzing the stress surrounding the oval flaw periphery, we explain that the wing tensile crack initiation area transfers from the middle of the oval flaw periphery to the tip of the oval with increasing oval flaw angle, shear crack is easily initiated at the tip of the oval flaw, and the stress of the first wing crack increases with increasing oval flaw angle. Finally, the variance of the stress of the measurement circle along the crack propagation path with axial strain was analyzed and combined with the crack evolution process and stress–strain curve. As a result, the crack evolution mechanism and cracking type in sandstone containing a single oval flaw under uniaxial compression are revealed. [ABSTRACT FROM AUTHOR]

To understand the fracture mechanism in all kinds of rock engineering, it is important to investigate the fracture evolution behavior of pre-fissured rock. In this research, we conducted uniaxial compression experiments to evaluate the influence of ligament angle on the strength, deformability, and fracture coalescence behavior of rectangular prismatic specimens (80 × 160 × 30 mm) of brittle sandstone containing two non-coplanar fissures. The experimental results show that the peak strength of sandstone containing two non-coplanar fissures depends on the ligament angle, but the elastic modulus is not closely related to the ligament angle. With the increase of ligament angle, the peak strength decreased at a ligament angle of 60°, before increasing up to our maximum ligament angle of 120°. Crack initiation, propagation, and coalescence were all observed and characterized from the inner and outer tips of pre-existing non-coplanar fissures using photographic monitoring. Based on the results, the sequence of crack evolution in sandstone containing two non-coplanar fissures was analyzed in detail. In order to fully understand the crack evolution mechanism of brittle sandstone, numerical simulations using PFC were performed for specimens containing two non-coplanar fissures under uniaxial compression. The results are in good agreement with the experimental results. By analyzing the stress field, the crack evolution mechanism in brittle sandstone containing two non-coplanar fissures under uniaxial compression is revealed. These experimental and numerical results are expected to improve the understanding of the unstable fracture mechanism of fissured rock engineering structures. [ABSTRACT FROM AUTHOR]

• Conduct a series of triaxial tests on rectangular solid sandstone containing two non-coplanar fissures. • Analyze the effect of ligament angle on triaxial deformation parameters of fissured rectangular solid sandstone. • Evaluate triaxial damage threshold and peak strength behavior of rectangular solid sandstone containing two non-coplanar fissures. • Investigate the crack type and internal damage behavior of rectangular solid sandstone containing two non-coplanar fissures. Fissures are generally existing in engineering rock mass, and cracking pattern observed in specimens with multiple fissures is analogous to the pattern obtained in specimens with two fissures, whereas the cracking coalescence of real rock specimen under triaxial compression have rarely been investigated. Therefore, thirty fissured rectangular specimens containing two non-coplanar fissures with different ligament angles were prepared to perform triaxial compression in this work, and the stress-strain curves, strength and deformation parameters, ultimate failure modes and Acoustic Emission (AE) characteristic were investigated. The experimental results indicate that pre-existing fissures have a significant effect on the stress-strain curves, more stress drops were observed before and after the peak strength, whereas confining pressure cannot reduce stress-drop. Cohesion (C) of peak strength and damage threshold has different trend with ligament angle (β), and the C of the peak strength is larger than that of the damage threshold. Pre-existing fissures have more effect on damage threshold than peak strength; cracks are easier to initiate in fissured specimen. It is easier to coalesce when ligament angle is parallel to the maximum principal stress, and cracks are easier to expand in width direction than that in thickness direction. The coalescence of the two non-coplanar fissures is classified as five typical modes, and the CT result shows that crack distribution in depth direction is similar. Crack classification by AE parameter and k-mean method can reflect the cracking process in a certain extent. It is more drastic when shear crack initiates, and tensile crack propagates in a stable way. [ABSTRACT FROM AUTHOR]

To reproduce the nonlinear damage of sandstone under cyclic loading, a nonlinear parallel-bonded stress corrosion (N-PSC) model is developed based on the linear parallel bond (LPB) model in particle flow code (PFC). The damage caused by cyclic loading is simulated by nonlinearly changing the bond diameter according to the nonlinear changes in the axial plastic strain of sandstone. The results show that the N-PSC model can effectively reproduce the main mechanical characteristics observed in laboratory tests, including the stress-strain curve, peak strength, axial plastic deformation, elastic modulus, and energy evolution characteristics. The sandstone specimens are damaged slowly during the elastic cyclic loading stage and rapidly after the plastic stage. In the first stage of post-peak cycling, the number of cracks inside the specimen increases dramatically as microcracks expand, coalesce, and form macroscopic fracture zones, resulting in a sharp release of energy. The specimens exhibit shear fracture under triaxial cyclic loading. However, in addition to the main shear fracture, tensile cracks are visible at the top and bottom of the specimen, which are caused by stress concentration extrusion. The N-PSC model is a powerful tool for investigating the evolution of rock damage under cyclic loading. [ABSTRACT FROM AUTHOR]

Rock masses containing intermittent discontinues under dynamic loading have rarely been investigated, although step-path failure frequently occurs in field engineering. This work aims to explore the action mechanism of dynamic cyclic loading and discontinuities on step-path failure. Four treatments including the control (Intact sandstone), pre-flawed samples with assemblage of two 30° dip fissures (type-I), assemblage of 10° and 65° dip fissures (type-II) and assemblage of two 65° dip fissures (type-III) were replicated. Laboratory experiments consisted of conventional uniaxial compression tests (UCT) and multilevel constant-amplitude fatigue tests (MLFT). Real-time acoustic emission (AE) monitoring was conducted along with scanning electron microscopy (SEM) observation on the fractured surfaces. Results indicated that the mechanical properties, energy evolution, AE characteristics and microscopic features of sandstones were significantly influenced by pre-existing fissures. The weakened rates of peak strength ranged from 2.80% to 85.63%, and regardless of loading methods, always followed the sequence of type-I > type-II > type-III. We contribute the weakening effect to the reduction in the effective bearing area and the increase in the dip angle of the rock bridge. With the same pre-fissure morphology, the sandstone under MLFT has enhanced its deformation resistance compared to under UCT. In addition, the maximum AE count/energy during the whole cyclic stage were always significantly lower than those during the monotonic rising process. SEM images revealed that long-term fatigue loading lead to numerous micro pores and transgranular cracks. Ultimately, a simple constitutive model was proposed based on the statistical damage mechanics and geometric damage theory. [ABSTRACT FROM AUTHOR]

The mechanical behavior of reservoir rock saturated in brine is of great interest to scientists and engineers for geological CO 2 sequestration in deep saline aquifers. However, the mechanical behavior of brine-saturated sandstone under triaxial compression has not been fully investigated, particularly with numerical simulations. In this study, an experimental study and numerical simulation were carried out on brine-saturated sandstone specimens to explore the mechanical properties in sandstones subjected to various brine salinities. Laboratory uniaxial compression tests were conducted on the brine-saturated sandstone specimens, and enhancements in strength and elastic modulus were observed with increasing brine salinity. Scanning electron microscopy results showed NaCl crystals deposited in the pore spaces, which provides evidence of strength and modulus variation in sandstone after brine saturation. Microparameters in a PFC3D numerical model were calibrated for each brine salinity condition, and the numerically simulated results were compared with the experimental results. The numerical results agreed well with the triaxial compression test results, indicating the appropriateness and reasonability of the calibrated models. [ABSTRACT FROM AUTHOR]

Highlights • The numerically simulated results of sandstone specimens containing two flaws were in agreement with the experiment. • The crack evolution process for pre-flawed specimen under triaxial compression was numerically investigated. • The internal crack patterns in pre-flawed specimens were revealed by X-ray CT scanning and PFC3D simulation. Abstract Flaws widely exist in engineering rock masses, which significantly influence the overall mechanical behaviors (strength, elastic modulus and failure, etc.) of rock masses. The study of crack coalescence behavior of rock specimen containing pre-existing flaws enhances the understanding of failure mechanism of rock. In this study, a series of PFC3D simulations on numerical specimens were carried out to investigate the crack initiation, propagation and coalescence process under conventional triaxial compression. First, numerical model for cylindrical sandstone specimen was constructed and a set of micro-parameters were calibrated by comparing with the experimental results of intact sandstone with different confining pressures. Then, two pre-existing open flaws which were the same as the tested sandstone were created by deleting balls. Triaxial compression simulations were conducted on pre-flawed specimens for a range of confining pressures (10–40 MPa). The numerically simulated results were compared with the experimental results, which showed that the simulations were in agreement with the laboratory tests. The processes of crack initiation, propagation and coalescence for pre-flawed specimens were investigated by program in PFC3D. The relationships among the cracking process, stress-strain curve and number of micro-crack were analyzed in detail. Finally, the internal crack patterns in pre-flawed specimens were revealed by analyzing the cross-sections from X-ray CT scanning and PFC3D. [ABSTRACT FROM AUTHOR]

• Conduct a series of experiments and simulations on cracking process of the brittle sandstone specimens with different shapes of holes. • Analyze the effect of the shape of hole on the uniaxial compression strength and failure behavior of brittle sandstone by experiment. • Reveal the crack evolution mechanism of brittle sandstone containing different shapes of holes by force distribution. Defects, such as joints, fissures and holes, are common in natural rocks. These pre-existing defects have significant effects on the mechanical properties of rock masses. Researchers have performed several studies on jointed or fissured rocks; however, rock specimens with one hole of different shapes have not been studied systematically. In this research, the mechanical properties and cracking behaviors of specimens with a hole under uniaxial compressive loading are explored through laboratory experiments. The corresponding numerical simulations using two-dimensional particle flow code (PFC2D) are conducted and the force field distribution before and during cracking, which is significant for exposing the cracking mechanism but hard to be measured in laboratory tests, were captured. The results demonstrate that the shape of hole has a considerable impact on the uniaxial compression strength and failure mode but minimal effects on the elastic modulus. The force field distribution before crack initiation varies with the shape of the pre-existing hole, and differences primarily exist in the configuration of the pure tension triangle and the low force region. During compressive loading, the crack initiation and propagation are influenced by the force field distribution, which will be affected by the newly formed cracks in return. The macro-cracks, generated in the specimens with a hole under compression, can be assigned to two categories: tension-induced cracks and compression-induced cracks. The former type of crack is typically thin and comparatively orderly, and it generally propagates along the maximum loading direction. The latter type of crack is relatively disorderly and thick, and it typically forms from the coalescence of several short but thick cracks. The compression-induced crack is a mixture of tension and shear cracks whereas the tension-induced crack is a pure tension crack. [ABSTRACT FROM AUTHOR]

To increase the understanding of strength and failure mechanism of fractured rock, a series of uniaxial compression tests on red sandstone specimens containing two pre-existing oval flaws was carried out using a rock mechanics servo-controlled testing system. On the basis of the experimental results, the influences of the coplanar flaw angle and ligament angle on the mechanical parameters and fracture process of sandstone specimens containing two oval flaws were analyzed in detail. The mechanical parameters including peak strength, peak strain and elastic modulus of pre-flawed specimens are all much lower than those of the intact specimens. For the specimens containing two coplanar oval flaws, the peak strength is distinctly related to the coplanar flaw angle, while the peak strain and elastic modulus are not obviously dependent on the coplanar flaw angle. For the specimens containing two non-coplanar oval flaws, the peak strength, peak strain and elastic modulus all have a nonlinear relationship with the ligament angle. By adopting photographic and AE monitoring techniques, the crack initiation, propagation and coalescence process and the AE evolution behavior of sandstone specimens were all observed and characterized. The stress drops in the stress-time curve correspond to a sudden increase in accumulated AE counts in the accumulated AE-time curve, resulting from the initiation and coalescence of cracks. The ultimate failure modes of sandstone specimens containing two pre-existing oval flaws are a mixture of several cracks (wing crack, anti-wing crack, secondary crack, shear crack and far-field crack and surface spalling), which depend on the flaw geometry. The failure patterns of pre-flawed specimens can be identified as the following four distinct modes: no crack coalescence failure, indirect crack coalescence outside the bridge area failure, single crack coalescence inside the bridge area failure and tensile crack coalescence outside the bridge area failure. The last failure mode was only observed in the specimens containing two non-coplanar oval flaws for β = 120°and 150° due to the two pre-existing overlapping flaws. [ABSTRACT FROM AUTHOR]

• The mechanical parameters of sandstone specimens containing three pre-existing flaws at various inclination angles were investigated. • The evolution processes of three-dimensional crack of sandstone specimens containing three pre-existing flaws were explored. • The evolution of internal stress surrounding the pre-existing flaw during fracture process was discussed. Rock materials contain various scales of pre-existing flaws that greatly influence the stability of rock engineering. The study of strength and failure behavior of pre-flawed rock enhances the understanding the fracture mechanism of rock. In the present study, uniaxial compression tests were carried out on sandstone specimens containing three pre-existing flaws at various inclination angles. The experiment results indicated that the stress–strain curves, mechanical paprameters, acoustic emission and failure pattern of pre-flawed specimen were affected by the pre-existing flaws. To verify the test results, a PFC modelling was conducted on pre-flawed specimen using a set of calibrated parameters. The stress–strain curve, mechanical paprameters and failure mode of specimen containing three flaws were reproduced in the numerical simulation. Three types of crack, i.e., tensile crack, shear crack and mixed tensile-shear crack, were identified based on the laboratory micro observations and numerical displacement field. As the flaw angle increased from 0° to 90°, the tensile stress concentration gradually deteriorated and transferred from the middle to the tip of pre-existing flaw. The evolution of internal stress surrounding the pre-existing flaw during the crack initiation, propagation and coalescence process was further discussed. This experimental and numerical study aims to investigate the crack characteristic and internal stress evolution in fractured rock. [ABSTRACT FROM AUTHOR]

• Investigate the effect of high-temperature damage on tensile behavior of intact and fissured sandstone. • Analyze triaxial deformation behavior of fissured sandstone subjected to high-temperature damage. • Explore the influence of fissure angle on triaxial strength and deformation parameters of sandstone. • Discuss the crack and permeability behavior of fissured sandstone subjected to high-temperature damage. The quantitative temperature (T)-dependent relationships between the strength properties and elastic modulus of both intact and fissured sandstone are presented. Based on the experimental results, it can be concluded that tensile strength (TS) and mode I fracture toughness (K IC) decrease nonlinearly with increasing T. The elastic modulus (E) values of the intact and fissured sandstone specimens have different correlations with T and confining pressure (σ 3), whereas the Poisson's ratio does not depend on the temperature. As T increases, crack closure stress, crack initiation stress and crack damage threshold first increase and then decrease, however, these properties have linear correlations with σ 3. The cohesion (C) of the intact specimen decreases nonlinearly with increasing T , whereas that of the fissured specimen first increases and then decreases. However, the internal friction angle (φ) of the intact and fissured specimens has nonlinear correlations with T. The permeability (k) of the intact sandstone decreases during the compaction-dominated stage and then increases at dilation-dominated stage under constant strain compression at σ 3 = 30 MPa. Moreover, k decreases slightly as T increases to 250 °C but then increases with further increasing T. The crack types of intact and fissured specimens under different σ 3 and temperature conditions are also discussed. [ABSTRACT FROM AUTHOR]

• Analyze the effect of fissure inclinations on the macro-and meso-mechanical properties of sandstone under uniaxial compression. • Comparing the energy evolution characteristics of sandstone containing different fissure inclinations under uniaxial compression. • Investigate the failure process of fractured sandstones under uniaxial compression by the evolution of microcracks. For the rocky slope with potential sliding hazard, there are a larger number of flaws such as fractures and joints, which greatly affect the stability and safety of the slope. Thus, it is important and relevant to the behavior of fractured rock from a macroscopic and mesoscopic perspective by experiments and numerical simulation. In this paper, the mechanical properties and crack evolution behavior of grey sandstone containing two parallel fissures under the uniaxial compression tests were evaluated by experimental and bond-based Peridynamics (BB-PD) simulation. The results show that with the increase of fissure dip angle, the peak strength, elastic modulus and Poisson's ratio of pre-cracked sandstone samples under uniaxial compression condition gradually increase. At the same time, the total energy, dissipated energy and elastic peak energy of fractured sandstone also increase with the increase of fissure dip angle. In order to validate the experimental findings, a set of calibration parameters were used to model the intact samples using BB-PD. The numerical simulation reproduced the stress–strain curves, mechanical parameters and failure modes of double-fractured sandstone. Based on the results of indoor experiments and numerical simulations, it is concluded that the failure mode of double-fractured sandstone has three types under uniaxial compression: tensile failure, shear failure and conjugate shear failure. As the fissure dip angle increases, the main control factor for failure of sample gradually changes from rock bridge failure to secondary crack coalescence, and the stress distribution of fractured sandstone gradually changes from high stress concentration in the rock bridge zone to uniform stress distribution as a whole. [ABSTRACT FROM AUTHOR]

• Conduct the experiment and simulation on red sandstone containing a non-straight fissure. • Compare numerical and experimental results of red sandstone containing a non-straight fissure. • Analyze the mechanical parameters of red sandstone containing a non-straight fissure. • Explore the crack evolution behavior of red sandstone containing a single non-straight fissure. It is widely accepted that the strength, deformability, and failure behaviors of rock material are strongly influenced by widespread natural fissures. Research focusing on the crack initiation, propagation, and coalescence behavior in fissured rock material is important to ensure the reliability and predictability of rock engineering. However, the types and forms of pre-existing fissures are complex and variable; thus, mechanical experiments have rarely been conducted on rock samples containing non-straight prefabricated open fissures. In this research, the strength, deformability, and failure behaviors of sandstone samples containing a single non-straight fissure were systematically investigated by a series of uniaxial compression experiments and numerical simulations. First, the influence of the fissure angle (α) on the strength and deformation characteristics of red sandstone containing a single non-straight fissure was evaluated. Then, the ordinary state-based peridynamics model was used to investigate the fracture behavior of red sandstone specimens with respect to various α values. The results indicate that for α = 0°–15° and 75°–90°, tensile cracks first initiated from the convex points of the prefabricated fissure, and the crack initiation position transferred from the convex point to the tip of the prefabricated fissure for α = 30°–60°. In addition, for the α = 0°–90°, the cracks initiated in the form of tensile cracks, and the ultimate failure modes were mixed tensile and shear failure. Secondary cracks always appeared in the form of shear crack. Moreover, for α ≥ 75°, fewer cracks were initiated before the peak strength was reached; afterward, the crack initiations were more prolific, and more acoustic emission counts were observed. [ABSTRACT FROM AUTHOR]