Experimental study on seepage properties, AE characteristics and energy dissipation of coal under tiered cyclic loading

Understanding the stress‐cracks‐permeability evolution law inside coal body under cyclic loading is significant to optimize the gas extraction technology and gas disasters prevention technology. In order to explore the influence of mining cyclic stress on the mechanical behavior and permeability variation of gas‐bearing seams, the experiments on permeability evolution, deformation law, and acoustic emission (AE) characteristics of coal samples under cyclic loading with different loading stress level and different loading frequency were carried out. These results indicate that the higher loading stress level, the shorter the fatigue life of coal sample. With the increase of the loading stress level, the change rate of permeability became higher and the relation curves between permeability and loading cycles developed from “U” to “V,” which can be defined and divided into three stages, for example, decrease stage, stable stage, and increase stage. And the permeability in the decrease stage can be modeled as a power function of the loading cycles, while the permeability in the increase stage can be modeled as an exponential function of the loading cycles. Besides, the loading stress level has a significant on the permeability, while the loading frequency has little impact on the permeability evolution. In the process of coal deformation under cyclic loading, the strain development of strain coal samples was in the shape of inverted “S,” while the variation of the peak values of corresponding AE ring counts presented “U” type. In addition, there was a good correlation between the permeability and the AE parameters of coal samples, which provide a new sight into the dynamic variation process of the stress‐cracks‐permeability evolution inside coal body under cyclic loading.

Section: Introductionmentioning

confidence: 99%

Mechanical response characteristics and permeability evolution of coal samples under cyclic loading

Yang

Cao

Cheng

et al. 2019

“…The experimental results showed that the dilatancy of sandstone under low confining pressure is more evident, and the failure mode of sandstone is shear failure. Jiang et al 5 performed the tiered cyclic loading experiments of axial stress on raw coal. It was found that the cumulative dissipated energy increased exponentially during cyclic loading-unloading.…”

Section: Introductionmentioning

confidence: 99%

Experimental study on strain behavior and permeability evolution of sandstone under constant amplitude cyclic loading‐unloading

Liu

Zhang

et al. 2019

Self Cite

The coupling of in situ stress, seepage, and fracture makes the strata exhibit complex strain behavior and permeability evolution under the stress path of cyclic loading-unloading. In this study, the cyclic loading-unloading experiments of constant amplitude axial stress of sandstone under the combination of different initial confining pressures and loading-unloading rates are conducted. The experimental resultsshow that the heterogeneity of sandstone, Poisson's ratio produced by adjacent loading-unloading, and confining pressure determine the deformation characteristics of sandstone in lateral and axial directions. Sandstone can produce significant shear dilatancy at a low rate; this is because stress perturbation can activate more particle slippage and fracture structure changes. However, the fracture preferentially propagates along the end or edge of the crack with strong stress sensitivity to form a shear failure plane at high rate. The normalized permeability of sandstone decreases with an increase in the loading-unloading rate before failure. The evolution of normalized permeability is closely related to the shear slip of fracture structure and sandstone particles. For the prevention of rock engineering disasters, the high loading-unloading rate results in lower normalized permeability, which favors the prevention of gastype disasters in rocks with higher gas potential energy; however, it is not conducive to the prevention of rockburst. K E Y W O R D Sconstant amplitude cyclic loading-unloading, loading-unloading rate, normalized permeability, shear effect, volumetric strain | 453 LIU et aL.

“…In load and unload axial stress process, the permeabilitydeviatoric stress curves show a variation of negative exponential function. During the unload confining pressure process, the permeability-deviatoric stress curves show a variation of exponential function, satisfying the equation (22). The permeability was taken at every 1 MPa stress in load and unload process, the fitting curves between the permeability and deviatoric stress were shown in Figure 8.…”

Section: Analysis Of the Permeability Evolutionmentioning

confidence: 79%

“…Assuming that the gas flow in the coal sample conforms to Darcy's law, the permeability of raw coal sample was continuously calculated by Jiang et al:

k = \frac{2 v μ l p_{2}}{A (p_{1}^{2} - p_{2}^{2})}

…”

Section: Test Results and Discussionmentioning

confidence: 99%

Test and study on transfusion law of gas in mining coal

Zhang

Yang

et al. 2019

In this paper, the seepage tests of mining coal were conducted by servo‐controlled seepage apparatus. These tests consist of conventional triaxial compression seepage tests, and load and unload seepage tests. It was observed that the peak strength and corresponding axial strain of raw coal samples gradually increase with the increase of confining pressure, which conforms to the Mogi‐Coulomb strength criterion, and the internal friction angle was calculated as φ = 42.65°, and the cohesion force was c = 3.56 MPa. The ultimate strength of coal samples after load and unload test was obviously lower than that of the triaxial compression test under the same confining stress conditions, and the deviatoric stress‐permeability curves consistent with the exponential function under two stress paths. In load and unload test, the damage degree of raw coal was characterized by the permeability damage rate and the maximum permeability damage rate. The permeability of coal seam was closely related to the mining stress, it presents a nonlinearly declining as the mining stress increases, and the permeability increases nonlinearly when the mining stress was released.