Experimental study on seepage characteristics of coal under truly triaxial loading and unloading based on different minimum principal stress properties

Abstract

China’s coal-mining industry has developed from shallow mining to deep mining. Consequently, coal under unloading conditions is becoming increasingly complicated. In this study, seepage experiments are conducted for different minimum principal stress properties and levels of coal using a self-developed truly triaxial fluid–solid coupling seepage experimental system. In the experiment, the maximum and intermediate principal stresses correspond to the rigid stress and the minimum principal stress corresponds to the flexible stress. Experimental results show that the flexible minimum principal stress is conducive to coal deformation during axial compressive loading, whereas the rigid confining pressure inhibits coal deformation. For coal under stress unloading, the deformation and permeability of coal show a recovery change. In the initial stage of unloading, the permeability of coal shows a relatively large increase; this increase is evident in the unloading tests for rigid minimum principal stress. However, the level of the minimum principal stress is not related to the unloading effect. The findings of this study aid the prevention and control of dynamic disasters caused by coalbed gas in deep coal mining.