Investigation on the Anisotropy of Mechanical Properties and Brittleness Characteristics of Deep Laminated Sandstones

Abstract Rock brittleness is a crucial mechanical property and essential for fracability evaluation and fracturing scheme design in unconventional reservoirs. However, the influence of inherent anisotropy on deep laminated sandstone’s mechanical properties and brittleness characteristics is rarely investigated. The energy transformation and damage evolution reflected by complete stress-strain curves are analyzed during the entire process of rock rupture under compressions. A new brittleness index is established based on energy evolution during sandstone failure. Its advantages involve comprehensively considering the energy transformation characteristics at both pre-peak and post-peak stages and the capability to characterize the effect of confining pressure and bedding plane (BP) geometry on sandstone brittleness. The triaxial compression tests on sandstones are conducted to validate the reliability and accuracy of the new brittleness index. Numerical simulations are then performed to further investigate the manner in which BP angle, BP density, and confining pressure control the brittleness anisotropy of deep laminated sandstones based on the finite element method. Then the acoustic emission (AE) characteristics of anisotropic sandstone and correlations between AE mode and brittleness index are discussed. The results indicated that the anisotropy of mechanical properties and brittleness of deep laminated sandstones were significantly affected by BP angle, BP density, and confining pressure. With the increase of BP angle, the brittleness index of deep laminated sandstone decreases first and then increases, showing a U-shape variation law, whose maximum and minimum values are obtained at φ =0° and φ =45°, respectively. The AE characteristics were closely related to rock brittleness, which was jointly controlled by BP geometry and confining pressure. The results provide a basis for the brittleness and fracability evaluation and optimum hydraulic fracturing design in deep laminated sandstones.

Download Full-text

Effect of Bedding Angle and Confining Pressure on the Brittleness of Geomaterials: A Case Study on Slate

Advances in Materials Science and Engineering ◽

10.1155/2019/1650170 ◽

2019 ◽

Vol 2019 ◽

pp. 1-17

Author(s):

Xianjie Hao ◽

Quansheng Xu ◽

Dequan Yang ◽

Shaohua Wang ◽

Yingnan Wei

Keyword(s):

Energy Release ◽

Triaxial Compression ◽

Great Influence ◽

Confining Pressure ◽

Brittleness Index ◽

Compression Tests ◽

External Energy ◽

Whole Process ◽

Bedding Angle ◽

Rock Brittleness

Brittleness is one of the most significant properties of geomaterials. However, very few studies have been conducted on factors influencing the rock brittleness indices. In this paper, conventional triaxial compression tests were carried out to investigate the effects of confining pressure and bedding angle on the brittleness of slate. From the perspective of energy, brittleness is an index that could reflect the release rate of energy that accumulated in the slate under the effect of external energy after reaching peak strength. Therefore, a new brittleness index of slate based on postpeak energy release is proposed herein. The applicability of this index is illustrated by comparing with other five existing brittleness indices. The following results can be obtained. (1) The confining pressure exerts a great influence on the brittleness of slate. With the increase of confining pressure, the brittleness of slate decreases significantly. The dispersion of brittleness values of slate declines with increasing confining pressure. (2) There is a parabolic relationship between slate brittleness and bedding angle. As bedding angle increases, the brittleness is intensified and reaches its maximum at a bedding angle of about 45° and then decreases gradually. (3) In contrast to the previous indices, the brittleness index proposed in this paper can describe the whole process of the postpeak stage through an index of the energy release, which makes this measure more suitable for rock that has the characteristics of step-drop or bench-drop at the postpeak stage.

Download Full-text

Mechanical Properties of Bentonite-Sand Mixtures Under Triaxial Stress Condition

MRS Proceedings ◽

10.1557/proc-353-307 ◽

1994 ◽

Vol 353 ◽

Cited By ~ 1

Author(s):

M. Umedera ◽

A. Fujiwara ◽

N. Yasufuku ◽

M. Hyodo ◽

H. Murata

Keyword(s):

Mechanical Properties ◽

Shear Strength ◽

Water Content ◽

Triaxial Compression ◽

Confining Pressure ◽

Strength Properties ◽

Compression Tests ◽

Optimum Water Content ◽

Triaxial Compression Tests ◽

Optimum Water

AbstractA series of triaxial compression tests is being conducted under the drained condition on bentonite and sand mixtures, known as buffer, in saturated and optimum water content states to clarify the mechanical properties of the buffer.It was found that the mechanical properties of bentonite and sand mixtures are strongly influenced by water and bentonite contents: shear strength in a saturated state is less than that in an optimum water content state; shear strength decreases rapidly with increasing bentonite content. Strength properties are much dependent on confining pressure.

Download Full-text

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.

Download Full-text

An Evaluation Method of Rock Brittleness Based on the Prepeak Crack Initiation and Postpeak Stress Drop Characteristics

Mathematical Problems in Engineering ◽

10.1155/2021/5639649 ◽

2021 ◽

Vol 2021 ◽

pp. 1-9

Author(s):

Meiben Gao ◽

Tianbin Li ◽

Lubo Meng

Keyword(s):

Crack Initiation ◽

Stress Drop ◽

Evaluation Method ◽

Triaxial Compression ◽

Experimental Tests ◽

Confining Pressure ◽

Brittleness Index ◽

Comparison Results ◽

Confining Pressures ◽

Rock Brittleness

Recent research shows that the brittleness of rock is closely related to the initiation and propagation of internal microcracks, but there are few brittleness evaluation indices considering the characteristics of rock initiation. Based on the theoretical analysis of brittleness and the characteristics of rock initiation, this study proposes an evaluation method of rock brittleness based on the prepeak crack initiation and postpeak stress drop characteristics. First, based on the description and definition of brittleness by George Tarasov and Potvin et al., the feasibility of an evaluation method based on the prepeak crack initiation and postpeak stress drop is theoretically analyzed. Second, the component Bi representing the prepeak brittleness of rock and component Bii representing the prepeak brittleness of rock are constructed, and the product of the two is the brittleness index BI, representing the prepeak crack initiation and postpeak stress drop. Finally, experimental tests of granite and marble were conducted to evaluate the new index, and the brittleness indices of different methods are calculated and compared. The results show that, like other brittleness indices (B1∼B5), the brittleness index BI can effectively reflect the effects of different confining pressures and loading modes on rock brittleness. The brittleness of marble decreases with increasing confining pressure from 5 MPa to 35 MPa. At a confining pressure of 5 MPa, the brittleness of granite during a triaxial unloading test is greater than that during a triaxial compression test. The calculated results are consistent with the experimental results. By tests and comparison results, the reliability of this evaluation method was verified, which provides a way to evaluate rock brittleness from the perspective of crack initiation and is helpful to enrich the analysis and evaluation of rock brittleness in the laboratory.

Download Full-text

Experimental Study on Rock-Like Specimens with Single Flaw under Hydro-Mechanical Coupling

Applied Sciences ◽

10.3390/app9163234 ◽

2019 ◽

Vol 9 (16) ◽

pp. 3234 ◽

Cited By ~ 4

Author(s):

Jinquan Xing ◽

Cheng Zhao ◽

Songbo Yu ◽

Hiroshi Matsuda ◽

Chuangchuang Ma

Keyword(s):

Mechanical Properties ◽

Shear Failure ◽

Water Pressure ◽

Triaxial Compression ◽

Confining Pressure ◽

Tensile Failure ◽

Compression Tests ◽

Cracking Behavior ◽

Mechanical Coupling ◽

High Confining Pressure

In order to study the mechanical characteristics and cracking behavior of jointed rock mass under hydro-mechanical coupling, a series of uniaxial compression tests and triaxial compression tests were carried out on cylinder gypsum specimens with a single pre-existing flaw. Under different confining pressures, water pressure was injected on the pre-existing flaw surface through a water injection channel. The geometrical morphology and tensile or shear properties of the cracks were determined by X-ray computed tomography (CT) and scanning electron microscope (SEM). Based on the macro and micro observation, nine types of cracks that caused the specimen failure are summarized. The results of mechanical properties and crack behavior showed that the confining pressure inhibited the tensile cracks, and shear failure occurred under high confining pressure. The water pressure facilitated the initiation and extension of tensile crack, which made the specimens prone to tensile failure. However, under the condition of high confining pressure and low water pressure, the lubrication effect had a significant effect on the failure pattern, under which the specimens were prone to shear failure. This experimental research on mechanical properties and cracking behavior under hydro-mechanical coupling is expected to increase its fundamental understanding.

Download Full-text

Study on Mechanical Behavior of Jurassic Frozen Sandstone in Western China Based on NMR Porosity

Journal of Chemistry ◽

10.1155/2020/2936932 ◽

2020 ◽

Vol 2020 ◽

pp. 1-11

Author(s):

Maoyan Ma ◽

Yishun Huang ◽

Guangyong Cao ◽

Jian Lin ◽

Shiliang Xu

Keyword(s):

Mechanical Properties ◽

Growth Rate ◽

Low Temperature ◽

Rock Strength ◽

Tracking System ◽

Triaxial Compression ◽

Confining Pressure ◽

Western China ◽

Compression Tests ◽

Rock Mechanical Properties

Study of frozen rock mechanical properties is necessary for safe application of the artificial ground freezing method in excavation of Chinese western water-rich soft rock layers. Triaxial compression tests and NMR test for samples from the western Jurassic sandstone were performed to investigate rock mechanical properties affected by low temperature and confining pressure. The results show mechanical parameters such as peak strength, cohesion, internal friction angle, residual strength, and elasticity modulus increased with the decreasing temperature under stable pressure, and the above parameters increased with the increasing confining pressure at a certain temperature. In particular, the growth rate of the rock strength would decline when the temperature was below −10°C in this study. Strength attenuation coefficients increased with the decreasing temperature, which indicated higher brittleness, whereas plastic characters got more obvious with the increasing confining pressure at a stable temperature. Furthermore, during the first two freezing stages, porosity decreased sharply with obvious increase of pore (crack) ice content, while porosity varied little at the third stage, which was the reason for the growth rate of rock strength declining with continuous low temperature from microcosmic point of view.

Download Full-text

A New Rock Brittleness Evaluation Method Based on the Complete Stress-Strain Curve

Lithosphere ◽

10.2113/2021/4029886 ◽

2021 ◽

Vol 2021 (Special 4) ◽

Author(s):

Chenyang Liu ◽

Lizhi Du ◽

Xiaopei Zhang ◽

Yong Wang ◽

Xinmin Hu ◽

...

Keyword(s):

Stress Drop ◽

Evaluation Method ◽

Triaxial Compression ◽

Strain Curve ◽

Confining Pressure ◽

Brittleness Index ◽

Peak Strain ◽

Stress Strain ◽

Rock Brittleness ◽

Drop Rate

Abstract Brittleness is a crucial parameter of rock mass and the key indicator in rock engineerings, such as rockburst prediction, tunnelling machine borehole drilling, and hydraulic fracturing. To solve the problem of using present brittleness indexes, the existing rock brittleness indexes were firstly summarised in this paper. Then, a brittleness index (BL), which considers the ratio of stress drop rate and stress increase rate and the peak stress, was proposed. This new index has the advantage of simplifying the acquisition of key parameters and avoiding dimensional problems, as well as taking the complete stress-strain curves into account. While applying the BL, the peak strain is used to describe the difficulty of brittle failure before the peak point, and the ratio of stress drop to strain increase can reflect the stress drop rate without dimension problem. In order to verify the applicability of BL, through the PFC2D, the microparameters and confining pressure were changed to model different types of rock numerical specimens and different stress condition. The results show that the BL can well reflect and classify the brittleness characteristics of different rock types and characterise the constraint of confining pressure on rock brittleness. Moreover, the influence of microparameter on macroparameter was studied. In order to further verify the reliability of the brittleness index (BL), this study conducted uniaxial and triaxial compression tests (30 MPa) on marble, sandstone, limestone, and granite under different confining pressure.

Download Full-text

Mechanical Properties of Sandstone under Loading and Unloading Conditions

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.852.441 ◽

2014 ◽

Vol 852 ◽

pp. 441-446 ◽

Cited By ~ 3

Author(s):

Xing Xia Wang ◽

Wen Juan Ma ◽

Jian Wen Huang ◽

Zai Yi Liao

Keyword(s):

Mechanical Properties ◽

Triaxial Compression ◽

Deformation Modulus ◽

Confining Pressure ◽

Rock Deformation ◽

Test Results ◽

Compression Tests ◽

Triaxial Compressive Strength ◽

Unloading Confining Pressure ◽

Triaxial Compression Tests

The mechanical properties of rock mass under unloading conditions are essentially different from that under loading conditions. Triaxial compression tests and unloading confining pressure tests are conducted, and test results show that unloading failure is more brittle, and rock samples suffer more damage under unloading failure. The larger the initial confining pressure is, the easier of unloading failure is occurred. The increasing or decreasing values of rock deformation modulus under unloading conditions are within 10% of rock triaxial compressive strength. Unloading failure leads to deterioration of rock deformation modulus, which decreases gradually with confining pressure decreasing, and the decrease rates get bigger and bigger with unloading ratio of confining pressure increasing. Deformation modulus is only 24-34% of that under loading condition when rock strength goes down to residual strength.

Download Full-text

Influence of Clay Mineral Content on the Mechanical Properties and Microfabrics of Tailings

10.21203/rs.3.rs-1066120/v1 ◽

2021 ◽

Author(s):

Chao Zhang ◽

Zhenkai Pan ◽

Changkun Ma ◽

Lei Ma ◽

Xueting Li

Keyword(s):

Mechanical Properties ◽

Mechanical Behavior ◽

Clay Minerals ◽

Clay Mineral ◽

Mineral Content ◽

Triaxial Compression ◽

Clay Content ◽

Nitrogen Adsorption ◽

Confining Pressure ◽

Compression Tests

Abstract Clay mineral content has an important influence on the mechanical behavior of tailings, and the mechanical behavior of tailings directly affects the stability of tailings dams. XRF and XRD tests were carried out on tailings from three different regions. The chemical and mineral compositions of the tailings are analyzed. The strength and failure deformation of tailings were studied by carrying out laboratory triaxial compression tests. The effect of clay content on the stress path of tailings was investigated. The microfabric of tailings samples was examined with scanning electron microscopy (SEM) and nitrogen adsorption tests. The results show that the confining pressure corresponding to the samples exhibiting strain hardening increases with increasing clay mineral content in the tailings. The cohesion of tailings increases linearly, and the specific surface area decreases as the content of clay minerals increases. Nitrogen adsorption test results reveal from a microscopic point of view that changes in pore structure are associated with the content of clay minerals. The higher the content of clay minerals is, the higher the proportion of micropores (aggregated interior). Macroscopically, the overall porosity decreases and the fineness of the pores increases with clay content, which will directly affect the mechanical properties of tailings.

Download Full-text

Study on Mechanical Properties and Damage Characteristics of Red Sandstone under Freeze-thaw and Load

Advances in Civil Engineering ◽

10.1155/2021/8867489 ◽

2021 ◽

Vol 2021 ◽

pp. 1-13

Author(s):

Lei Shi ◽

Yang Liu ◽

Xiangzhen Meng ◽

Huimei Zhang

Keyword(s):

Mechanical Properties ◽

Triaxial Compression ◽

Confining Pressure ◽

Damage Variable ◽

Mechanical Parameters ◽

Compression Tests ◽

Internal Damage ◽

Red Sandstone ◽

Freeze Thaw ◽

Total Damage

To analyze the effects of freeze-thaw cycles and confining pressure on the mechanical properties of red sandstone, through freeze-thaw cycles and triaxial compression tests, full stress-strain curves of different freeze-thaw cycles and different confining pressures were obtained. The degradation degree of red sandstone was quantitatively considered from different mechanical parameters of ultimate stress, elastic modulus, and Poisson’s ratio. Based on summarizing the characteristics of rock under freeze-thaw and load, the total damage variable of rock was determined by the reasonable measurement of freeze-thaw damage variable and load damage variable, and a damage constitutive model under freeze-thaw and load was established. The research showed that the freeze-thaw cycles aggravate the degree of rock damage deterioration, the rock stiffness and strength were reduced, and the characteristics of plastic deformation and ductile failure were more obvious. The confining pressure inhibited red sandstone internal damage, and with the increase of confining pressure, the stiffness and strength and the plastic characteristics were increased. In the overall trend, the mechanical parameters had different sensitivity to the degradation effect of freeze-thaw cycles and confining pressure. Regardless of the increase in the number of freeze-thaw cycles or confining pressure, the strain softening modulus tended to decrease gradually, and red sandstone plastic damage became more obvious after the stress peak. The total damage evolution path of red sandstone reflected the nonlinear influence of freeze-thaw and load on the total damage propagation. The research results provide theoretical support for the improvement of the technology of the effluent coal rock in Balasu Coal Mine.

Download Full-text