Strength and Failure Characteristics of Saturated Sandstone Under Loading and Unloading

Abstract Water has effects on the strength and failure characteristics of the sandstone in natural environment. Conventional triaxial compressive or unloading confining pressure experiments were conducted on sandstone specimens. Experimental results indicate that the compressive strength of sandstone decreases significantly under saturated conditions in comparison with dry conditions, the strength parameters of saturated specimens under unloading confining pressure are also lower than those of dry rock samples; for the sandstone with the same water content, the strengths under triaxial unloading confining pressure is slightly higher than those under triaxial compressive condition; compared with the stress path of triaxial compression, the stress path of unloading confining pressure makes cracks propagate more easily along the axial direction, and the angle between fracture surface and axial direction is smaller. Under triaxial unloading confining pressure, there failure modes of dry sandstone are tension failure and shear failure, while that of saturated sandstone is mainly shear failure. In the process of water saturation, the bond and friction characteristics between grain particles are degraded due to water weakening the cementation between the grain particles and softening grains boundary, and the expansion of clay minerals in the sandstone, which leads to the decrease of macroscopic mechanical strengths.

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Cracking Behaviors and Mechanical Properties of Rock-Like Specimens with Two Unparallel Flaws under Conventional Triaxial Compression

Advances in Civil Engineering ◽

10.1155/2019/5849703 ◽

2019 ◽

Vol 2019 ◽

pp. 1-15 ◽

Cited By ~ 1

Author(s):

Huilin Le ◽

Shaorui Sun ◽

Chenghua Xu ◽

Liuyang Li ◽

Yong Liu

Keyword(s):

Mechanical Properties ◽

Inclination Angle ◽

Failure Modes ◽

Shear Failure ◽

Triaxial Compression ◽

Confining Pressure ◽

Critical Value ◽

Tensile Failure ◽

Rock Bridge ◽

Bridge Length

Flaws existing in rock masses are generally unparallel and under three-dimensional stress; however, the mechanical and cracking behaviors of the specimens with two unparallel flaws under triaxial compression have been rarely studied. Therefore, this study conducted comprehensive research on the cracking and coalescence behavior and mechanical properties of specimens with two unparallel flaws under triaxial compression. Triaxial compressive tests were conducted under different confining pressures on rock-like specimens with two preexisting flaws but varying flaw geometries (with respect to the inclination angle of the two unparallel flaws, rock bridge length, and rock bridge inclination angle). Six crack types and eleven coalescence types in the bridge region were observed, and three types of failure modes (tensile failure, shear failure, and tensile-shear failure) were observed in experiments. Test results show that bridge length and bridge inclination angle have an effect on the coalescence pattern, but the influence of bridge inclination angle is larger than that of the bridge length. When the confining pressure is low, coalescence patterns and failure modes of the specimens are greatly affected by flaw geometry, but when confining pressure rose to a certain level, the influence of confining pressure is larger than the effect of flaw geometry. The peak strength of the specimens is affected by flaw geometry and confining pressure. There is a critical value for the bridge length. If the bridge length is larger than the critical value, peak strengths of the samples almost keep constant as the bridge length increases. In addition, as the bridge inclination angle increases, there is an increase in the probability of tensile cracks occurring, and with an increase in the confining pressure, the probability of the occurrence of shear cracks increases.

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Experimental Investigation on Failure Modes and Mechanical Properties of Rock-Like Specimens with a Grout-Infilled Flaw under Triaxial Compression

Shock and Vibration ◽

10.1155/2019/4909534 ◽

2019 ◽

Vol 2019 ◽

pp. 1-14

Author(s):

Huilin Le ◽

Shaorui Sun ◽

Feng Zhu ◽

Haotian Fan

Keyword(s):

Mechanical Properties ◽

Rock Mass ◽

Epoxy Resin ◽

Failure Modes ◽

Shear Failure ◽

Fractured Rock ◽

Triaxial Compression ◽

Friction Angle ◽

Confining Pressure ◽

Compression Tests

Flaws existing in rock mass are one of the main factors resulting in the instability of rock mass. Epoxy resin is often used to reinforce fractured rock mass. However, few researches focused on mechanical properties of the specimens with a resin-infilled flaw under triaxial compression. Therefore, in this research, epoxy resin was selected as the grouting material, and triaxial compression tests were conducted on the rock-like specimens with a grout-infilled flaw having different geometries. This study draws some new conclusions. The high confining pressure suppresses the generation of tensile cracks, and the failure mode changes from tensile-shear failure to shear failure as the confining pressure increases. Grouting with epoxy resin leads to the improvement of peak strengths of the specimens under triaxial compression. The reinforcement effect of epoxy resin is better for the specimens having a large flaw length and those under a relatively low confining pressure. Grouting with epoxy resin reduces the internal friction angle of the samples but improves their cohesion. This research may provide some useful insights for understanding the mechanical behaviors of grouted rock masses.

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Experimental Study on the Mechanical Properties and Seepage Characteristics of Red Sandstone with a Single Persistent Joint under Triaxial Compression

Geofluids ◽

10.1155/2021/3884605 ◽

2021 ◽

Vol 2021 ◽

pp. 1-13

Author(s):

Wei Wang ◽

Shifan Liu ◽

Chong Shi ◽

Shanxi Zheng ◽

Qizhi Zhu

Keyword(s):

Mechanical Properties ◽

Pore Pressure ◽

Failure Modes ◽

Shear Failure ◽

Triaxial Compression ◽

Confining Pressure ◽

Strengthening Effect ◽

Red Sandstone ◽

Inclination Angles ◽

Seepage Characteristics

In this research, the conventional triaxial compression experiments for intact red sandstone specimens and the specimens with a single persistent joint at different inclination angles, i.e., 0°, 30°, 45°, and 90°, were conducted at first. Based on the results of the conventional tests, the effects of the confining pressure and the joint inclination angle on the mechanical properties including deformation behavior and strength parameters were summarized and analyzed, respectively. We find that the strength and deformation of jointed red sandstone are enlarged due to the increment of confining pressure, and the mechanical parameters of specimens show a U-shaped development with the rise of the joint angle. Besides, to investigate the effects of the pore pressure on seepage characteristics of rocks with joint angles at 0°, 45°, and 90°, a series of triaxial compression drainage tests on the jointed red sandstone were performed. The results show that the pore pressure has a weakening effect on the strength of jointed specimens, which can reduce the strengthening effect induced by confining pressure. Meanwhile, the tested specimens mostly present shear failure modes. As a result, the mechanical responses, seepage characteristics, and cracking modes in red sandstone containing a single persistent joint under triaxial compression are revealed.

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Experimental Investigation on the Mechanical Characteristics and Deformation Behaviour of Fractured Rock-Like Material with One Single Fissure under the Conventional Triaxial Compression

Shock and Vibration ◽

10.1155/2018/2608639 ◽

2018 ◽

Vol 2018 ◽

pp. 1-11 ◽

Cited By ~ 3

Author(s):

Taoli Xiao ◽

Mei Huang ◽

Cheng Cheng ◽

Yunlong He

Keyword(s):

Mechanical Characteristics ◽

Failure Modes ◽

Shear Failure ◽

Early Stage ◽

Fractured Rock ◽

Deformation Behaviour ◽

Triaxial Compression ◽

Strain Curve ◽

Confining Pressure ◽

Material Specimen

An experimental study was carried out on a rock-like material specimen containing a single fissure to investigate its mechanical characteristics and deformation behaviour under triaxial compression. The mechanical characteristics, such as peak strength and residual strength, are discussed. The confining pressure had a distinct effect on the ductility characteristics of the specimen. “A distinct stress drop” occurred in the early stage of the stress-strain curve when the length fissure was relatively long. The I-crack, II-crack, and III-crack are all observed under triaxial compression, and the III-crack is commonly observed under triaxial compression. Confining pressure plays an essential role in affecting the failure mode of the specimen. There are three kinds of failure modes in the triaxial compression experiment on a rock-like material specimen with one single fissure: tensile-shear comprehensive failure, “X”-shaped shear failure, and shear failure along the fissure plane. These results are important and fundamental to understand the fracture mechanism of rock engineering.

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Mechanical Properties and Failure Modes of Thick-Walled Cylinder Granites with Different Apertures under Triaxial Compression

Advances in Civil Engineering ◽

10.1155/2020/8897086 ◽

2020 ◽

Vol 2020 ◽

pp. 1-18

Author(s):

Wei Chen ◽

Wen Wan ◽

Shuailong Lian ◽

Senlin Xie ◽

Yu Zhou ◽

...

Keyword(s):

Failure Modes ◽

Shear Failure ◽

Triaxial Compression ◽

Confining Pressure ◽

Outer Wall ◽

Surrounding Rock ◽

Compression Tests ◽

Deformation And Failure ◽

Triaxial Compressive Strength ◽

Walled Cylinder

Roadway excavation changes the original equilibrium stress state of the rock mass, resulting in the loading and unloading of the surrounding rock near the free surface. After the excavation, the tangential stress increases and the radial stress decreases, which mainly cause deformation and collapse of the roadway. In order to study the strength characteristics of the surrounding rock after the excavation, one effective way is to carry out triaxial compression tests on small surrounding rock samples. Therefore, this paper focuses on the triaxial compression mechanical propertiesof thick-walled cylinder granites with an electrohydraulic, servo-controlled rock mechanics testing system (MTS-815). It studies how different pore sizes and confining pressures affect the triaxial compressive strength (TCS), deformation, and failure modes of granite samples. The results are as follows: (1) Under triaxial compression, the stress-strain curves have no obvious yield stage, and the peak TCS increases with the confining pressure (σ3). When σ3 is low, there is little difference in the TCS between the complete specimen and the thick-walled cylinders. When σ3 reaches 30 MPa∼40 MPa, the TCS of samples with apertures of 15 mm and 20 mm are obviously lower. The σ3 has an obvious influence on the elastic modulus of thick-walled cylinder granites. (2) Shearing and splitting are the main failure modes under triaxial compression. When σ3 is low, shear failure appears. As σ3 reaches 30 MPa∼40 MPa, split failure occurs. The area of the fracture surface increases with σ3. (3) As σ3 grows, the influence parameter (m) of the three-dimensional Hoek-Brown criterion increases. Under the same σ3, the value of m presents a decreasing trend from the outer wall to the inner wall, which means the bearing capacity keeps getting lower and lower. As a result, the inner wall is most likely to be damaged. The theoretical analysis results agree well with the tests.

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Experimental Study on Mechanical and Acoustic Emission Characteristics of Rock Samples under Different Stress Paths

Shock and Vibration ◽

10.1155/2018/4813724 ◽

2018 ◽

Vol 2018 ◽

pp. 1-9 ◽

Cited By ~ 4

Author(s):

Tao Qin ◽

Hongru Sun ◽

Heng Liu ◽

Junwen Zhang ◽

Tao Li ◽

...

Keyword(s):

Acoustic Emission ◽

Failure Mode ◽

Shear Failure ◽

Rock Specimen ◽

Axial Strain ◽

Failure Process ◽

Confining Pressure ◽

Stress Path ◽

Initial Value ◽

Unloading Confining Pressure

A series of tests on characteristics of acoustic emission have been performed on sandstone under uniaxial, conventional, and triaxial conditions and the unloading confining pressure path. The failure mode of rock specimen has been scanned by CT and a three-dimensional reconstruction was made. The differences on characteristics of AE, mechanics, and the failure mode of sandstone during the failure process under three paths are studied. The results show that the deformation of rock specimen is bigger, and axial strain and circumferential strain have a deformation platform at peak point of stress under the unloading confining pressure path. Characteristics of AE ringing are significantly affected by the confining pressure and stress path. AE ringing counts peak value, and accumulative ringing on the breaking moment as well as cumulative release energy is higher, which indicates that the rock failure is more violent under the unloading confining pressure path. The failure mode of rock specimen was dominated by shear failure under the conventional triaxial stress path. The tension failure is the main form at a lower initial value of unloading confining pressure, and the shear failure is more prominent at a higher initial value of unloading confining pressure.

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The meso-fracturing mechanism of marble under unloading confining pressure paths and constant axial stress

Scientific Reports ◽

10.1038/s41598-021-97359-4 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Linna Sun ◽

Liming Zhang ◽

Yu Cong ◽

Yaduo Song ◽

Keqiang He

Keyword(s):

Shear Failure ◽

Axial Stress ◽

Confining Pressure ◽

Particle Flow Code ◽

Shear Cracks ◽

Confining Pressures ◽

Tension Cracks ◽

Trial And Error Method ◽

Unloading Confining Pressure ◽

Fracturing Mechanism

AbstractFailure tests on marble during unloading confining-pressure under constant axial stress and simulations with the particle flow code were performed. The influence mechanism of the unloading rate of the confining pressure, initial unloading stress, and confining pressure on the failure characteristics of, and crack propagation in, marble was studied. By using the trial-and-error method, the conversion relationship between the unloading rates of confining pressures in laboratory tests and numerical simulations was ascertained. Micro-cracks formed in the unloading process of confining pressure are dominated by tension cracks, accompanied by shear cracks. The propagation of shear cracks lags that of tension cracks. As the confining pressure is increased, more cracks occur upon failure of the samples. The proportion of shear cracks increases while that of tension cracks decreases. The failure mode of samples undergoes a transition from shear-dominated failure to conjugated shear failure.

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An Evaluation Method of Brittleness Characteristics of Shale Based on the Unloading Experiment

Energies ◽

10.3390/en12091779 ◽

2019 ◽

Vol 12 (9) ◽

pp. 1779 ◽

Cited By ~ 2

Author(s):

Xiaogui Zhou ◽

Haiming Liu ◽

Yintong Guo ◽

Lei Wang ◽

Zhenkun Hou ◽

...

Keyword(s):

Evaluation Method ◽

Shear Failure ◽

Confining Pressure ◽

Shear Zones ◽

Loading Path ◽

Mechanical Parameters ◽

Failure Characteristics ◽

Unloading Rate ◽

Uplift And Erosion ◽

Unloading Effect

Shale reservoir has an initial unloading effect during the natural uplift and erosion process, which causes the shale brittleness to change, affecting the design of the fracturing scheme. To consider this, the axial compression loading and confining pressure unloading experiment of shale is carried out, and then the influence of unloading rate on the mechanical parameters, failure characteristics, and the brittleness of rock are analyzed. What is more, a new evaluation method of brittleness characteristics that take the unloading effect into consideration is proposed. The conclusions are as follows: (1) The unloading rate has a weakening effect on the mechanical parameters, such as the destructive confining pressure and the residual strength of the samples. (2) The failure characteristics of shale specimens are a single shear failure in an oblique section under low unloading rate, and multiple shear zones accompanied with bedding fracture under high unloading rate. (3) The brittleness of shale samples is well verified by the brittleness index B d 1 and B d 2 during the loading path; nevertheless, it has shortage at the unloading path. This paper proposes a new brittleness evaluation method which can consider the influence of the different unloading rates and unloading points. Furthermore, there is a nice characterization between the brittleness damage and this method.

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Experimental Study on Strength and Failure Characteristics of Reinforced Concrete Plate under Complex Stresses Conditions

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.357-360.699 ◽

2013 ◽

Vol 357-360 ◽

pp. 699-704 ◽

Cited By ~ 1

Author(s):

Guang Yang ◽

Zuo Zhou Zhao ◽

Xiao Gang He

Keyword(s):

Reinforced Concrete ◽

Failure Modes ◽

Shear Failure ◽

Biaxial Tension ◽

Reduction Factor ◽

Failure Characteristics ◽

Concrete Plate ◽

Principal Tensile Stress ◽

Force Ratio ◽

Practical Engineering

To study the biaxial strength and failure characteristics of reinforced concrete in the state of biaxial tension-compression stresses and provide some suggestions for the practical engineering design, 3 one-third scale reinforced concrete (RC) plate specimens are tested. The results indicate that, in biaxial tension-compression stresses, reinforced concrete cracks in the direction normal to the principal tensile stress direction and presents the characteristics of shear failure modes. The test device could simulate the required stress condition. The compression strength of reinforced concrete is obviously lower than the uniaxial strength fc. The reduction factor k is about 0.55~0.75 which varies with different tension-compression force ratio. Concrete material takes part in the tension process of reinforced concrete and shares part of the tension, while the share ratio decreases as concrete cracks gradually.

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Experimental Study on the Mechanical Behavior of Yunnan Limestone in Natural and Saturated States

Advances in Civil Engineering ◽

10.1155/2021/6614412 ◽

2021 ◽

Vol 2021 ◽

pp. 1-16

Author(s):

Xutao Zhang ◽

Mingyang Ren ◽

Zhaobo Meng ◽

Baoliang Zhang ◽

Jinglong Li

Keyword(s):

Elastic Modulus ◽

Mechanical Behavior ◽

Water Content ◽

Failure Modes ◽

Shear Failure ◽

Mechanical Response ◽

Deformation Modulus ◽

Confining Pressure ◽

Hyperbolic Function ◽

Test Results

Rock material is a kind of mineral assemblage with complex structural heterogeneity, whose mechanical behavior is strongly affected by water or moisture content. In this work, we carried out a series of laboratory tests to investigate the mechanical response (e.g., deformation, strength, and failure characteristics) of Yunnan limestone in natural and saturated states. Our test results show that (1) after saturation, the stiffness and strength of Yunnan limestone degenerate considerably. Compared with the natural condition, the elastic modulus, deformation modulus, and tensile modulus decrease by about 30% on average, and uniaxial compressive strength and tensile strength also decrease by about 15% and 20%, respectively. While Poisson’s ratio is less affected by water content, it can be regarded as a constant; (2) the elastic modulus and deformation modulus of Yunnan limestone are significantly affected by confining pressure, and the relationship between them and confining pressure satisfies the law of hyperbolic function; (3) the peak strength envelope of Yunnan limestone has significant nonlinear characteristics, which can be well described by generalized Hoek-Brown strength criterion. However, the generalized Hoek-Brown criterion does not apply to the residual strength, which shows a linearly increasing trend with the increasing confining pressure; (4) the failure modes of Yunnan limestone are significantly dependent on confining pressure but insensitive to water content. With the increasing confining pressure, the failure modes of Yunnan limestone transform from splitting failure, tension-shear mixed failure, single inclined plane shear failure to Y-shaped or X-shaped conjugated shear failure. The test results can provide important experimental data for the establishment of the constitutive model of Yunnan limestone, which will contribute to obtain more reliable results for stability assessment of Xianglu Mountain Tunnel.

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