scholarly journals Permeability evolution of unloaded coal samples at different loading rates

2014 ◽  
Vol 18 (5) ◽  
pp. 1497-1504 ◽  
Author(s):  
Ze-Tian Zhang ◽  
Zhang Ru ◽  
Jian-Feng Liu ◽  
Xiao-Hui Liu ◽  
Jia-Wei Li
Keyword(s):  
Triaxial Compression ◽  
Axial Loading ◽  
Peak Stress ◽  
Initial Permeability ◽  
Differential Method ◽  
Permeability Evolution ◽  
Compression Tests ◽  
Evolution Law ◽  
Loading Rates ◽  
Conventional Triaxial Compression

As coal mass is often at unloading status during mining process, it is of great significance to push on the research on permeability evolution of unloaded coal samples at different loading rates. A series of triaxial unloading experiments were conducted for initially intact coal samples using an improved rock mechanics testing system, and the permeability was continuously measured by the constant pressure differential method for methane. Permeability evolution law of unloaded coal samples and the influence mechanism of loading rates on that were studied. The results of triaxial unloading experiments indicate that the permeability of coal samples increases throughout the whole testing process without a descent stage, which is different from the permeability evolution law in conventional triaxial compression tests. The maximum permeability of unloaded coal sample, which is 4 to 18 times to its initial permeability, often appears before reaching the peak stress and increases with the decrease of axial loading rate. Stress state corresponding to the surge point of permeability of the unloaded coal samples is also discussed.

scholarly journals Mechanical Behavior and Permeability Evolution of Coal under Different Mining-Induced Stress Conditions and Gas Pressures

Energies ◽  
2020 ◽  
Vol 13 (11) ◽  
pp. 2677
Author(s):  
Zetian Zhang ◽  
Ru Zhang ◽  
Zhiguo Cao ◽  
Mingzhong Gao ◽  
Yong Zhang ◽  
...  
Keyword(s):  
Triaxial Compression ◽  
Volumetric Strain ◽  
Coal Mass ◽  
Gas Pressure ◽  
Stress Conditions ◽  
Permeability Evolution ◽  
Compression Tests ◽  
Induced Stress ◽  
Conventional Triaxial Compression ◽  
Gas Pressures

The gas permeability and mechanical properties of coal, which are seriously influenced by mining-induced stress evolution and gas pressure conditions, are key issues in coal mining and enhanced coalbed methane recovery. To obtain a comprehensive understanding of the effects of mining-induced stress conditions and gas pressures on the mechanical behavior and permeability evolution of coal, a series of mining-induced stress unloading experiments at different gas pressures were conducted. The test results are compared with the results of conventional triaxial compression tests also conducted at different gas pressures, and the different mechanisms between these two methods were theoretically analyzed. The test results show that under the same mining-induced stress conditions, the strength of the coal mass decreases with increasing gas pressure, while the absolute deformation of the coal mass increases. Under real mining-induced stress conditions, the volumetric strain of the coal mass remains negative, which means that the volume of the coal mass continues to increase. The volumetric strain corresponding to the peak stress of the coal mass increases with gas pressure in the same mining layout simulation. However, in conventional triaxial compression tests, the coal mass volume continues to decrease and in a compressional state, and there is no obvious deformation stage that occurs during the mining-induced stress unloading tests. The theoretical and experimental analyses show that mining-induced stress unloading and gas pressure changes greatly impact the deformation, failure mechanism and permeability enhancement of coal.

Plasticity of Ice and Sand-Ice Systems

1975 ◽  
Vol 97 (2) ◽  
pp. 479-484 ◽  
Author(s):  
Leonard L. Smith ◽  
J. B. Cheatham
Keyword(s):  
Triaxial Compression ◽  
Plasticity Theory ◽  
Compression Tests ◽  
Loading Rates ◽  
Conventional Triaxial Compression ◽  
Confining Pressures ◽  
Von Mises ◽  
Comparison Of The Results ◽  
Force Displacement ◽  
Triaxial Compression Tests

The experimental deformation of ice and sand-ice systems is compared with predictions based upon plasticity theory. Properties of the materials were determined under various temperatures, confining pressures, and loading rates using conventional triaxial compression tests. Samples were indented at atmospheric pressure using flat punches and sharp wedges at two loading rates and calculated force-displacement relationships were determined for the von Mises, coulomb, and parabolic yield conditions. Comparison of the results of the experiments with the computations indicates that the force-displacement relationships for ice and sand-ice samples can be approximated using plasticity theory.

scholarly journals Undrained monotonic triaxial loading behaviors of a type of iron ore fines

2018 ◽  
Vol 55 (9) ◽  
pp. 1349-1357 ◽  
Author(s):  
Hailong Wang ◽  
Junichi Koseki ◽  
Fei Cai ◽  
Tomoyoshi Nishimura
Keyword(s):  
Iron Ore ◽  
Initial Conditions ◽  
Triaxial Compression ◽  
Axial Loading ◽  
Sandy Soils ◽  
Peak Stress ◽  
Confining Pressure ◽  
Compression Tests ◽  
Static Liquefaction ◽  
Iron Ore Fines

Concerning the static liquefaction properties of an industrial cargo, i.e., iron ore fines (IOF), undrained monotonic behaviors of a type of IOF are revealed through conducting triaxial compression tests. It is found that IOF exhibit some similar behaviors as those of common sandy soils, while some very unusual behaviors are also observed. All IOF specimens with compaction degree of 84%–95% and confining pressure of 50–200 kPa exhibit dilative behavior from the beginning of axial loading until the deviator stresses reach their peaks (qpk). Then the dilative behavior transforms to a contractive behavior, and the contractive behavior continues until reaching the residual stress without observation of phase transformation and quasi steady state. These behaviors are not usually observed for common sandy soils based on extensive previous works. More studies may be necessary as these unusual behaviors imply that flow failure, similar to the undrained monotonic behavior of very loose sand, may be triggered regardless of the density of IOF. In addition, this study also establishes the relationships of IOF between its initial conditions, peak stress conditions, and residual conditions by employing classical knowledges developed for sandy soils.

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

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.

scholarly journals Microcracking and the failure of polycrystalline ice under triaxial compression

1992 ◽  
Vol 38 (128) ◽  
pp. 65-76 ◽  
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.

scholarly journals Study of Acoustic Emission and Mechanical Characteristics of Coal Samples under Different Loading Rates

2015 ◽  
Vol 2015 ◽  
pp. 1-11 ◽  
Author(s):  
Huamin Li ◽  
Huigui Li ◽  
Baobin Gao ◽  
Dongjie Jiang ◽  
Junfa Feng
Keyword(s):  
Acoustic Emission ◽  
Elasticity Modulus ◽  
Loading Rate ◽  
Peak Stress ◽  
Test System ◽  
Strong Impact ◽  
Peak Strain ◽  
Compression Tests ◽  
Loading Rates ◽  
The Relationship

To study the effect of loading rate on mechanical properties and acoustic emission characteristics of coal samples, collected from Sanjiaohe Colliery, the uniaxial compression tests are carried out under various levels of loading rates, including 0.001 mm/s, 0.002 mm/s, and 0.005 mm/s, respectively, using AE-win E1.86 acoustic emission instrument and RMT-150C rock mechanics test system. The results indicate that the loading rate has a strong impact on peak stress and peak strain of coal samples, but the effect of loading rate on elasticity modulus of coal samples is relatively small. When the loading rate increases from 0.001 mm/s to 0.002 mm/s, the peak stress increases from 22.67 MPa to 24.99 MPa, the incremental percentage is 10.23%, and under the same condition the peak strain increases from 0.006191 to 0.007411 and the incremental percentage is 19.71%. Similarly, when the loading rate increases from 0.002 mm/s to 0.005 mm/s, the peak stress increases from 24.99 MPa to 28.01 MPa, the incremental percentage is 12.08%, the peak strain increases from 0.007411 to 0.008203, and the incremental percentage is 10.69%. The relationship between acoustic emission and loading rate presents a positive correlation, and the negative correlation relation has been determined between acoustic emission cumulative counts and loading rate during the rupture process of coal samples.

scholarly journals A Nonlinear Strength Criterion for Frozen Sulfate Saline Silty Clay with Different Salt Contents

2018 ◽  
Vol 2018 ◽  
pp. 1-8 ◽  
Author(s):  
Yanhu Zhao ◽  
Yuanming Lai ◽  
Jing Zhang ◽  
Chong Wang
Keyword(s):  
Triaxial Compression ◽  
Strength Criterion ◽  
Experimental Results ◽  
Saline Soils ◽  
Silty Clay ◽  
Compression Tests ◽  
Frozen Soils ◽  
Conventional Triaxial Compression ◽  
Confining Pressures ◽  
Pressure Melting

It has been proven that the mechanical properties of frozen saline soils are different from frozen soils and unfrozen saline soils. In this paper, in order to study the effects of the salt contents on the strength characteristics of frozen soils, a series of conventional triaxial compression tests are carried out for frozen saline silty clay with Na2SO4 contents 0.0, 0.5, 1.5, and 2.5% under confining pressures from 0 MPa to 18 MPa at −6°C, respectively. The experimental results show that the strength of frozen saline silty clay presents obvious nonlinearity, the strength of frozen saline silty clay increases with increasing confining pressures at first, but with a further increase in confining pressures, the strength decreases because of pressure melting and crushing phenomena under high confining pressures, and salt contents have an important influence on strength of frozen saline silty clay. A strength criterion is proposed on the basis of the experimental results. The strength criterion could well reflect the nonlinear strength characteristic of frozen saline silty clay and the influence of salt contents on frozen saline silty clay.

scholarly journals Analysis of Failure Characteristics and Strength Criterion of Coal-Rock Combined Body with Different Height Ratios

2020 ◽  
Vol 2020 ◽  
pp. 1-14
Author(s):  
Tuo Wang ◽  
Zhanguo Ma ◽  
Peng Gong ◽  
Ning Li ◽  
Shixing Cheng
Keyword(s):  
Shear Failure ◽  
Underground Mining ◽  
Triaxial Compression ◽  
Peak Stress ◽  
Strength Criterion ◽  
Confining Pressure ◽  
Experimental Results ◽  
Compression Tests ◽  
Rock Body ◽  
Coal Rock

In underground mining and roadway support engineering of coal mine, the coal and rock layers bear loads together; therefore, the deformation and mechanical characteristics of the coal-rock combined bodies are not the same as those of the pure coal or rock bodies. In this paper, conventional triaxial compression tests of coal-rock combined bodies with different height ratios were conducted. And the stress and deformation characteristics of coal-rock combined body were studied and the experimental results were analyzed with different strength criteria. The results show that the peak stress, elastic modulus, and strength reduction coefficient of coal-rock combined body are negatively correlated with the ratio of coal to coal-rock combination height and positively correlated with the confining pressure; the coal-rock combination shows obvious ductility under 10 MPa confining pressure. Under the conventional triaxial condition, the shear failure was the main cause of the lateral deformation of the coal body in the coal-rock combination, which was much larger than that of the rock body. The circle deformation value, volume strain value, and the deformation rate in the postpeak stage of coal-rock combination are much higher than those in the prepeak stage. Mohr–Coulomb and general Hoek–Brown strength criterion fit the experimental results well.

scholarly journals Experimental Study on the Permeability of Weakly Cemented Rock under Different Stress States in Triaxial Compression Tests

Geofluids ◽  
2018 ◽  
Vol 2018 ◽  
pp. 1-9 ◽  
Author(s):  
Gangwei Fan ◽  
Mingwei Chen ◽  
Dongsheng Zhang ◽  
Zhen Wang ◽  
Shizhong Zhang ◽  
...  
Keyword(s):  
Underground Mining ◽  
Axial Strain ◽  
Triaxial Compression ◽  
Confining Pressure ◽  
Initial Permeability ◽  
Compression Tests ◽  
Mining Operations ◽  
Stress Strain ◽  
Loading And Unloading ◽  
Stress States

Mudstone and shaly coarse sandstone samples of Jurassic units in northwestern China were collected to study the seepage mechanism of weakly cemented rock affected by underground mining operations. Samples were studied using seepage experiments under triaxial compression considering two processes: complete stress-strain and postpeak loading and unloading. The results show that permeability variations closely correspond to deviatoric stress-axial strain during the process of complete stress-strain. The initial permeability is 7 times its minimum, contrasting with lesser differentials of initial, peak, and residual permeability. The magnitude of permeability ranges from 10−17 to 10−19 m2, representing a stable water-resisting property, and is 1 to 2 orders lower in mudstone than that in shaly coarse sandstone, indicating that the water-resisting property of the mudstone is much better than that of the shaly coarse sandstone. Permeability is negatively correlated with the confining pressure. In response to this pressure, the permeability change in mudstone is faster than that in shaly coarse sandstone during the process of postpeak loading and unloading. Weakly cemented rock has lower permeability according to the comparison with congeneric ordinary rocks. This distinction is more remarkable in terms of the initial permeability. Analyses based on scanning electron microscope (SEM) observations and mineral composition indicate that the samples are rich in clay minerals such as montmorillonite and kaolin, whose inherent properties of hydroexpansiveness and hydrosliming can be considered the dominant factors contributing to the seepage properties of weakly cemented rock with low permeability.

Thermodynamic Properties of Soft Sedimentary Rock in Geotechnical Engineering

2012 ◽  
Vol 170-173 ◽  
pp. 687-691 ◽  
Author(s):  
Jing Yin Wang ◽  
Fang Liu
Keyword(s):  
Thermodynamic Properties ◽  
Sedimentary Rock ◽  
Geotechnical Engineering ◽  
Triaxial Compression ◽  
Soft Rock ◽  
Compression Tests ◽  
Creep Tests ◽  
Oil Drilling ◽  
Viscoplastic Model ◽  
Conventional Triaxial Compression

In geotechnical engineering such as supporting of deep and soft rock roadway ,oil drilling , and construction of military underground defensive facility, thermodynamic properties of soft sedimentary rock has guiding significances .The study of the thermal constitutive behavior of rock under temperature has been done for many years, and many achievement have been got on this issue. In this paper, some experimental researches on the thermo-mechanical characteristics of soft sedimentary rock have been presented. Some test results have been simulated with a thermo-elasto-viscoplastic model. The results show that the proposed thermo-elasto-viscoplastic model reflects the visco-elastoplastic properties of rock，and can describe the thermo-mechanical behaviors of soft shale rocks in not only drained conventional triaxial compression tests but also drained triaxial creep tests. So it can be used for analysis of theology and stability of rock engineering.

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