A Strain-Based Percolation Model and Triaxial Tests to Investigate the Evolution of Permeability and Critical Dilatancy Behavior of Coal

Modeling the coupled evolution of strain and CH4 seepage under conventional triaxial compression is the key to understanding enhanced permeability in coal. An abrupt transition of gas-stress coupled behavior at the dilatancy boundary is studied by the strain-based percolation model. Based on orthogonal experiments of triaxial stress with CH4 seepage, a complete stress-strain relationship and the corresponding evolution of volumetric strain and permeability are obtained. At the dilatant boundary of volumetric strain, modeling of stress-dependent permeability is ineffective when considering the effective deviatoric stress influenced by confining pressure and pore pressure. The computed tomography (CT) analysis shows that coal can be a continuous medium of pore-based structure before the dilatant boundary, but a discontinuous medium of fracture-based structure. The multiscale pore structure geometry dominates the mechanical behavior transition and the sudden change in CH4 seepage. By the volume-covering method proposed, the linear relationship between the fractal dimension and porosity indicates that the multiscale network can be a fractal percolation structure. A percolation model of connectivity by the axial strain-permeability relationship is proposed to explain the transition behavior of volumetric strain and CH4 seepage. The volumetric strain on permeability is illustrated by axial strain controlling the trend of transition behavior and radical strain controlling the shift of behavior. A good correlation between the theoretical and experimental results shows that the strain-based percolation model is effective in describing the transition behavior of CH4 seepage in coal.

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Experimental Research into the Evolution of Permeability of Sandstone under Triaxial Compression

Energies ◽

10.3390/en13195065 ◽

2020 ◽

Vol 13 (19) ◽

pp. 5065

Author(s):

Liming Zhang ◽

Shengqun Jiang ◽

Jin Yu

Keyword(s):

Axial Strain ◽

Triaxial Compression ◽

Testing Machine ◽

Volumetric Strain ◽

Confining Pressure ◽

Rate Of Change ◽

Peak Strength ◽

Sudden Increase ◽

Seepage Pressure ◽

Confining Pressures

Failure tests on sandstone specimens were conducted under different confining pressures and seepage pressures by using an MTS triaxial rock testing machine to elucidate the corresponding correlations of permeability and characteristic stress with confining pressure and pore pressure during deformation. The results indicate that permeability first decreases and presents two trends, i.e., a V-shaped increase and an S-shaped trend during the non-linear deformation stage. The greater the seepage pressure, the greater the initial permeability and the more obvious the V-shaped trend in the permeability. As the confining pressure was increased, the trend in the permeability gradually changed from V- to S-shaped. Compared with the case at a high confining pressure, the decrease of permeability occurred more quickly, the rate of change becomes greater, and the sudden increase observed in the permeability happened earlier under lower confining pressures. Within the range tested, confining pressure exerted a greater effect on the permeability than the seepage pressure. In comparison with the axial strain, volumetric strain better reflected changes in permeability during compaction and dilation of sandstone. The ratio of crack initiation stress to peak strength ranged from 0.37 to 0.50, while the ratio of dilation stress to peak strength changed from 0.58 to 0.72. Permeabilities calculated based on Darcy and non-Darcy flow changed within the same interval, while the change in permeability was different.

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Mechanical Characteristics of Coal Samples under Triaxial Unloading Pressure with Different Test Paths

Shock and Vibration ◽

10.1155/2020/8870821 ◽

2020 ◽

Vol 2020 ◽

pp. 1-10

Author(s):

Yang Zhang ◽

Yongjie Yang ◽

Depeng Ma

Keyword(s):

Axial Strain ◽

Triaxial Compression ◽

Axial Loading ◽

Peak Stress ◽

Friction Angle ◽

Confining Pressure ◽

Triaxial Tests ◽

Compression Tests ◽

Leading Role ◽

Unloading Rate

In order to understand the influence of unloading path on the mechanical properties of coal, triaxial unloading confining pressure tests with different initial confining pressure and different unloading rate were carried out. The test results show that the triaxial unloading strength of coal samples under different test conditions is lower than conventional triaxial tests, but the brittleness characteristics are more obvious. This result indicates that the coal samples are easily damaged under unloading conditions. In the axial loading stage of the confinement unloading tests, the axial strain plays a leading role. However, during the confining pressure unloading stage, the circumferential deformation is large, which is the main deformation in this stage. Higher unloading rates of confining pressure are associated with shorter times between the peak stress position and sample complete failure. This shows that samples are more easily destroyed under higher unloading rates and the samples are more difficultly destroyed under lower unloading rates. In addition, with increasing unloading rate, the peak principal stress difference and confining pressure at failure decrease gradually, whereas the confining pressure difference at failure increases gradually. Compared with conventional triaxial compression tests, the cohesion of coal is reduced and the internal friction angle is increased under the condition of triaxial unloading test.

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Hydromechanical behaviour of overconsolidated unsaturated soil in undrained conditions

Canadian Geotechnical Journal ◽

10.1139/cgj-2018-0323 ◽

2019 ◽

Vol 56 (11) ◽

pp. 1609-1621 ◽

Cited By ~ 2

Author(s):

Shengshen Wu ◽

Annan Zhou ◽

Jie Li ◽

Jayantha Kodikara ◽

Wen-Chieh Cheng

Keyword(s):

Effective Stress ◽

Unsaturated Soils ◽

Axial Strain ◽

Triaxial Compression ◽

Volumetric Strain ◽

Degree Of Saturation ◽

Triaxial Tests ◽

Conventional Triaxial Compression ◽

Hydromechanical Behaviour ◽

Undrained Conditions

Hydromechanical behaviour of an unsaturated silt with various suctions and different overconsolidated ratios (OCRs) was investigated through a series of undrained triaxial tests (constant water contents, CW). All the samples were prepared from the slurry state. Different OCRs (= 1, 2, 4, and 8 in net stress) were achieved by unloading the samples to 400, 200, 100, and 50 kPa from an initial confining net pressure of 400 kPa. Then the samples were dried to various suctions (0, 100, 200, 300, and 400 kPa). Unsaturated samples with different OCRs were then sheared at CW conditions following the conventional triaxial compression (CTC) paths. Full hydromechanical responses including the changes in deviator stress, stress ratio, volumetric strain, suction, and degree of saturation with axial strain were monitored and are presented in this paper. Some key findings include (i) the critical state for unsaturated soils with different OCRs can be well defined by Bishop’s effective stress; (ii) the peak strength in Bishop’s effective stress increases with increase of OCR, but decreases with increase of suction in the undrained condition; and (iii) the volume change of unsaturated soils in undrained conditions is related to OCRs and the volume of pore air.

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Mechanical behaviour of powders using a medium pressure flexible boundary cubical triaxial tester

Proceedings of the Institution of Mechanical Engineers Part E Journal of Process Mechanical Engineering ◽

10.1243/095440803322328881 ◽

2003 ◽

Vol 217 (3) ◽

pp. 233-241 ◽

Cited By ~ 11

Author(s):

F Li ◽

V M Puri

Keyword(s):

Shear Modulus ◽

Mechanical Behaviour ◽

Triaxial Compression ◽

Three Dimensional ◽

Volumetric Strain ◽

Confining Pressure ◽

Alumina Powder ◽

Medium Pressure ◽

Mean Pressure ◽

Confining Pressures

A medium pressure (<21 MPa) flexible boundary cubical triaxial tester was designed to measure the true three-dimensional response of powders. In this study, compression behaviour and strength of a microcrystalline cellulose powder (Avicel® PH102), a spray-dried alumina powder (A16SG), and a fluid-bed-granulated silicon nitride based powder (KY3500) were measured. To characterize the mechanical behaviour, three types of triaxial stress paths, that is, the hydrostatic triaxial compression (HTC), the conventional triaxial compression (CTC), and the constant mean pressure triaxial compression (CMPTC) tests were performed. The HTC test measured the volumetric response of the test powders under isostatic pressure from 0 to 13.79MPa, during which the three powders underwent a maximum volumetric strain of 40.8 per cent for Avicel® PH102, 30.5 per cent for A16SG, and 33.0 per cent for KY3500. The bulk modulus values increased 6.4-fold from 57 to 367MPa for Avicel® PH102, 3.7-fold from 174 to 637 MPa for A16SG, and 8.1-fold from 74 to 597MPa for KY3500, when the isotropic stress increased from 0.69 to 13.79 MPa. The CTC and CMPTC tests measured the shear response of the three powders. From 0.035 to 3.45MPa confining pressure, the shear modulus increased 28.7-fold from 1.6 to 45.9MPa for Avicel® PH102, 35-fold from 1.7 to 60.5MPa for A16SG, and 28.5-fold from 1.5 to 42.8MPa for KY3500. In addition, the failure stresses of the three powders increased from 0.129 to 4.41 MPa for Avicel® PH102, 0.082 to 3.62 MPa for A16SG, and 0.090 to 4.66MPa for KY3500, respectively, when consolidation pressure increased from 0.035 to 3.45MPa. In addition, the shear modulus and failure stress values determined from the CTC test at 2.07, 2.76, and 3.45MPa confining pressures are consistently greater than those from the CMPTC test at the same constant mean pressures. This observation demonstrates the influence of stress paths on material properties. The CTT is a useful tool for characterizing the three-dimensional response of powders and powder mixtures.

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Experimental investigation of the effect of compliant pores on reservoir rocks under hydrostatic and triaxial compression stress states

Canadian Geotechnical Journal ◽

10.1139/cgj-2018-0133 ◽

2019 ◽

Vol 56 (7) ◽

pp. 983-991

Author(s):

Hua Yu ◽

Kam Ng ◽

Dario Grana ◽

John Kaszuba ◽

Vladimir Alvarado ◽

...

Keyword(s):

Pore Pressure ◽

Triaxial Compression ◽

Matrix Material ◽

Volumetric Strain ◽

Confining Pressure ◽

Differential Pressure ◽

Sandstone Sample ◽

Stress States ◽

Rock Springs ◽

Stage 1

The presence of compliant pores in rocks is important for understanding the stress–strain behaviors under different stress conditions. This paper describes findings on the effect of compliant pores on the mechanical behavior of a reservoir sandstone under hydrostatic and triaxial compression. Laboratory experiments were conducted at reservoir temperature on Weber Sandstone samples from the Rock Springs Uplift, Wyoming. Each experiment was conducted at three sequential stages: (stage 1) increase in the confining pressure while maintaining the pore pressure, (stage 2) increase in the pore pressure while maintaining the confining pressure, and (stage 3) application of the deviatoric load to failure. The nonlinear pore pressure – volumetric strain relationship governed by compliant pores under low confining pressure changes to a linear behavior governed by stiff pores under higher confining pressure. The estimated compressibilities of the matrix material in sandstone samples are close to the typical compressibility of quartz. Because of the change in pore structures during stage 1 and stage 2 loadings, the estimated bulk compressibilities of the sandstone sample under the lowest confining pressure decrease with increasing differential pressure. The increase in crack initiation stress is limited with increasing differential pressure because of similar total crack length governed by initial compliant porosity in sandstone samples.

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Triaxial Compressive Failure Characteristics and Constitutive Model Study of Jurassic-Cretaceous Weakly Cemented Sandstone

Advances in Civil Engineering ◽

10.1155/2020/8812575 ◽

2020 ◽

Vol 2020 ◽

pp. 1-12

Author(s):

Jinlong Cai ◽

Wei Zou

Keyword(s):

Constitutive Model ◽

Residual Strength ◽

Triaxial Compression ◽

Volumetric Strain ◽

Ductile Failure ◽

Mathematical Expression ◽

Confining Pressure ◽

Mining Area ◽

Peak Strength ◽

Correction Coefficient

A conventional triaxial compression test of Jurassic-Cretaceous typical weakly consolidated sandstone from a mining area in Ordos, China, was conducted using an MTS816 tester. Results showed that, before the peak, the rock had a distinct yield stage. When the specimen reached its peak strength, the strength decreased rapidly and showed an obvious brittle failure. When the confining pressure was increased to 15 MPa, the decrease of strength was slow and the rock tended toward ductile failure. With the increase of confining pressure, the cyclic strain initially increased slightly, whereas the volumetric strain increased greatly and the rock sample was in a compression state. When the load reached a critical value, the curve was reversely bent, resulting in volume expansion, whereas the peak strength, residual strength, and elastic modulus increased with confining pressure, and Poisson’s ratio decreased with the confining pressure. In the model based on macroscopic failure rock, the expression of the relationship between fracture angle and confining pressure provided a solid theoretical basis for the direction and failure mode of the macroscopic crack. Based on the rock strength theory and Weibull random distribution assumption of rock element strength, the damage variable correction coefficient was introduced when the residual strength was considered. Then, the mathematical expression of the 3D damage statistical constitutive model was established. Finally, the theoretical curve of the established constitutive model was compared with the triaxial test curve, which showed a high degree of coincidence.

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Microcracking and the failure of polycrystalline ice under triaxial compression

Journal of Glaciology ◽

10.3189/s0022143000009606 ◽

1992 ◽

Vol 38 (128) ◽

pp. 65-76 ◽

Cited By ~ 1

Author(s):

P. Kalifa ◽

G. Ouillon ◽

P. Duval

Keyword(s):

Triaxial Compression ◽

Peak Stress ◽

Confining Pressure ◽

Triaxial Tests ◽

Strain History ◽

Compression Tests ◽

Brittle Transition ◽

Polycrystalline Ice ◽

Strain Components

AbstractTriaxial and uniaxial compression tests have been carried out at –10°C on granular ice in order to study the role of microcracking on failure in the ductile-brittle transition zone. In the triaxial tests, the effect of confining pressure and strain rate on the crack population, as well as on strength and strain at the peak stress, was investigated. In the uniaxial tests, we measured the evolution of elastic and non-elastic components of deformation with the stress-strain history. The concept of effective stress, with a single scalar damage variable, was used to calculate the effect of microcracking on the strain components.

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Comparison of Mechanical Properties of Fiber-Reinforced Sand under Triaxial Compression and Direct Shear

Open Geosciences ◽

10.1515/geo-2015-0041 ◽

2015 ◽

Vol 7 (1) ◽

Cited By ~ 7

Author(s):

Reza Noorzad ◽

Seyed Taher Ghoreyshi Zarinkolaei

Keyword(s):

Shear Strength ◽

Axial Strain ◽

Triaxial Compression ◽

Polypropylene Fiber ◽

Triaxial Tests ◽

Direct Shear ◽

Test Results ◽

Peak Reduction ◽

Shear Tests ◽

Direct Shear Tests

AbstractThis research investigates the behavior of sand reinforced with polypropylene fiber. To do this, 40 direct shear tests and 40 triaxial tests were performed on the coastal beaches of Babolsar, a city in the North of Iran. The effect of parameters such as fiber content, length of fiber and normal or confining pressure on the behavior of Babolsar sand have been studied. In this study, four various fiber contents (0, 0.25, 0.5 and 1 percent), three different lengths of fiber (6, 12 and 18 mm) and four normal or confining pressures (50, 100, 200 and 400 kPa) have been employed. The test results show that fiber inclusion has a significant effect on the behavior of sand. In both direct shear and triaxial tests, the addition of fibers improved shear strength parameters (C, '), increased peak shear strength and axial strain at failure, and also limited the amount of post-peak reduction in shear resistance. The comparison of the test results revealed that due to better fiber orientation toward the direction of principal tensile strain in triaxial test as compared to direct shear tests, the fiber efficiency and its effect on soil behavior is much more significant in triaxial specimens.

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The Effect of Confining Pressure on the Mechanical Properties of Sand–Ice Materials

Journal of Glaciology ◽

10.1017/s0022143000031889 ◽

1973 ◽

Vol 12 (66) ◽

pp. 469-481 ◽

Cited By ~ 42

Author(s):

Bernard D. Alkire ◽

Orlando B. Andersland

Keyword(s):

Axial Strain ◽

Triaxial Compression ◽

Confining Pressure ◽

Silica Sand ◽

Creep Tests ◽

Ice Volume ◽

Coulomb Failure ◽

Confining Pressures ◽

Sand Material ◽

Cylindrical Samples

Cylindrical samples containing 0.59 mm to 0.84 mm diameter silica sand at about 97% and 55% ice saturation (the ratio of ice volume to sand pore volume) were tested at a temperature of −12° C in triaxial compression. Both constant axial strain-rate tests and step-stress creep tests provide information on the influence of confining pressure on the shear strength and creep behavior of the sand–ice material. Changes in the degree of ice saturation help show the influence of the ice matrix versus the sand material on the mechanical behavior. Data are discussed in terms of the Mohr–Coulomb failure law and creep theories. It is shown that the cohesive component of strength depends on response of the ice matrix, whereas the frictional component of strength responds in a manner very similar to unfrozen sand tested at high confining pressures. Experimental data show that creep rates decrease exponentially and creep strength increases with an increase in confining pressure.

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Small-strain behavior of frozen sand in triaxial compression

Canadian Geotechnical Journal ◽

10.1139/t95-047 ◽

1995 ◽

Vol 32 (3) ◽

pp. 428-451 ◽

Cited By ~ 29

Author(s):

Glen R. Andersen ◽

Christopher W. Swan ◽

Charles C. Ladd ◽

John T. Germaine

Keyword(s):

High Pressure ◽

Strain Rate ◽

Yield Stress ◽

Axial Strain ◽

Triaxial Compression ◽

Strain Curve ◽

Confining Pressure ◽

Stress Strain ◽

Strain Behavior ◽

Frozen Sand

The stress–strain behavior of frozen Manchester fine sand has been measured in a high-pressure low-temperature triaxial compression testing system developed for this purpose. This system incorporates DC servomotor technology, lubricated end platens, and on-specimen axial strain devices. A parametric study has investigated the effects of changes in strain rate, confining pressure, sand density, and temperature on behavior for very small strains (0.001%) to very large (> 20%) axial strains. This paper presents constitutive behavior for strain levels up to 1%. On-specimen axial strain measurements enabled the identification of a distinct upper yield stress (knee on the stress–strain curve) and a study of the behavior in this region with a degree of precision not previously reported in the literature. The Young's modulus is independent of strain rate and temperature, increases slightly with sand density in a manner consistent with Counto's model for composite materials, and decreases slightly with confining pressure. In contrast, the upper yield stress is independent of sand density, slightly dependent on confining pressure (considered a second order effect), but is strongly dependent on strain rate and temperature in a fashion similar to that for polycrystalline ice. Key words : frozen sand, high-pressure triaxial compression, strain rate, temperature, modulus, yield stress.

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