Pore water pressure characteristics of saturated intact loess under constant stress ratio stress paths
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The undrained triaxial shear tests for K_0 consolidated saturated intact loess were carried out to study the pore water pressure characteristics under two types of constant stress ratio stress paths including axial loading-radial unloading and axial unloading-radial loading.It is found that: 1.the pore water pressure is positive and increases quickly at first and tends to slowly later under both types of stress paths;2.the stress path may significantly affect the pore water pressure characteristics,it increases with the increase of stress ratio under axial unloading-radial loading stress path but decreased with the increase of stress ratio under axial loading-radial unloading stress path;3.the pore water pressure of saturated intact loess is produced by both the spherical stress increment and deviator stress increment.The pore water pressure produced by the spherical stress increment is independent of the non-linear property of soil,but the pressure due to shear stress is linearly related to shear stress.Keywords:
Stress path
Cylinder stress
Effective stress
Radial stress
Overburden pressure
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Using the self-developed three-axis servo fluid-solid coupling system with gas-solid coupling of gas-bearing coal, the variation law of the permeability of gas coal under the stress cycle loading and unloading path was studied. The qualitative and quantitative relationships between permeability, axial force, and radial stress of gas-bearing coals were established, and the variation law of permeability of gas-bearing coals was discussed. The results show that (1) different cyclic loading and unloading stress paths correspond to the permeability characteristics of different gas-bearing coals. (2) Permeability of gas-bearing coal decreases with the increase of axial stress and radial stress, and it has a logarithmic function with axial stress and radial stress. This shows that axial stress and radial stress are important factors affecting the permeability characteristics of gas-bearing coal. (3) Under the same stress loading and unloading conditions, the axial stress is less than radial stress on the permeability of gas-bearing coal. In the cyclic loading and unloading axial stress process, the permeability of the gas-bearing coal varies by a smaller extent than the cyclically unloaded confining force. (4) The cumulative damage rate of gas-bearing coal under axial stress gradually increases with the increase of the number of cycles of loading and unloading, and the rate of the cumulative damage rate of permeability is less than the corresponding rate of radial stress.
Cylinder stress
Overburden pressure
Effective stress
Radial stress
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Excess pore pressure generation potential within shear zone by employment of the ring shear tests have been investigated on the saturated samples which have quite different grain crushing susceptibility. In naturally drained tests, the sample the grains of which are crushable showed the great reduction of apparent internal friction during shearing to generate rapid flow phenomena. Based on the results of permeability and grain size analyses, it is estimated that the crushed particles formed the less-permeable shear zone and excess pore pressure generation rate exceeded the dissipation rate to yield flow behaviour, and it occurred heavily in high shear speed or normal stress tests. And to express the susceptibility of the samples against flows, the new parameter "potential for rapid flow phenomena" was proposed as the ratio of internal friction angle (φm) to apparent friction angle (φa).In order to estimate the excess pore pressure in undrained condition from the drained tests, the concept of "equivalent normal stress" was introduced as the ratio of volumetric strain to the compressibility. Equivalent normal stress using compressibility which was obtained by changing normal stress during shearing was almost equal to excess pore pressure at the same shear displacement up to 1 m at which steady state was reached. And Excess pore pressure ratio in undrained tests and the logarithm of equivalent normal stress divided by normal stress with compressibility obtained during consolidation process showed the linear relationship.
Shearing (physics)
Consolidation
Effective stress
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Overburden pressure
Effective stress
Stress path
Radial stress
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Effective stress
Constant (computer programming)
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