The conventional triaxial loading in gas seepage studies does not reflect the actual stressing state in the coal seam. Thus, a true triaxial seepage experimental apparatus is developed before seepage tests are conducted with it. The first group of tests is carried out with continuous axial loading under uniform lateral constrain on moulded coal specimen: one test under constant-velocity axial loading, two tests under monotonic variable-velocity axial loading, and two tests under single-cycle variable-velocity axial loading. The tests reflect the multi-step, discontinuous, stressing condition with variable-loading rate and strength in a coal seam. The law of coal deformation and gas permeability variation under constant-velocity and variable-velocity axial loading is obtained. Then, the multi-step seepage tests under different non-uniform lateral pressures are conducted, focusing on permeability variation under different triaxial stresses versus gas pressure and intermediate principal stress. The permeability increase with gas pressure decrease is confirmed and interpreted. Moreover, the turning point between Klinkenberg effect and gas wedging effect in the coal seam is determined. A simplified concept of average effective stress is put forward, and the dominant influence of this average effective stress on coal seam permeability is identified based on the concept.

中文翻译：

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基于真三轴渗流试验装置的连续阶梯加载气体渗流

瓦斯渗流研究中的常规三轴加载并不能反映煤层中的实际应力状态。因此，在进行渗流测试之前，开发了一个真正的三轴渗流实验装置。第一组试验对型煤试样进行均匀横向约束下的连续轴向载荷试验：1次等速轴向载荷试验，2次单调变速轴向载荷试验，2次单循环变速试验轴向载荷。试验反映了煤层中具有可变加载速率和强度的多步、不连续、应力条件。得到了等速和变速轴向载荷作用下煤体变形和瓦斯渗透率变化规律。然后，进行了不同非均匀侧压下的多步渗流试验，重点研究了不同三轴应力下的渗透率变化与气压和中间主应力的关系。渗透率随着气压的降低而增加被证实和解释。此外，确定了煤层中克林伯格效应和瓦斯楔入效应之间的转折点。提出了平均有效应力的简化概念，并在此基础上确定了平均有效应力对煤层渗透率的主要影响。确定了煤层中克林伯格效应和瓦斯楔入效应之间的转折点。提出了平均有效应力的简化概念，并在此基础上确定了平均有效应力对煤层渗透率的主要影响。确定了煤层中克林伯格效应和瓦斯楔入效应之间的转折点。提出了平均有效应力的简化概念，并在此基础上确定了平均有效应力对煤层渗透率的主要影响。