Abstract: Fractured rocks in underground engineering are generally in the state of triaxial stress, which are characterised by large porosity and high permeability. Under the action of the ground stress and high-water head, it is most likely to result in the failure of seepage instability and induce the disaster of water inrush. In this study, a self-developed triaxial permeability testing system was employed to study the permeability characteristics of the fractured sandstone under different triaxial stress conditions. The permeability tests were carried out on fractured sandstone with five different sizes of fragments by the steady-state permeability method. Finally, the permeability variations of fractured sandstone under triaxial stress states were obtained, and the relationship between effective stress and seepage velocity was deduced. The result showed that the relationship between the effective stress and seepage velocity of fractured sandstone under triaxial stress was approximately linear. When the axial displacement increased, the decreasing amplitude of seepage velocity became smaller with the increase of the effective stress. The pore pressure gradients and the seepage velocities of fractured sandstone with five different diameters of fragments under triaxial stress conditions were coincidenet with the Forchheimer equation. Moreover, the correlation coefficient between these two was above 0.95. When the axial displacement was constant, the permeability k of fractured sandstone decreased and non-Darcy flow β factor increased with increasing confining pressure. Under various axial displacements, the relationship between the permeability and confining pressure of fractured sandstone presented an exponential function. With the decrease of porosity, the permeability of fractured sandstone with five different diameters of fragments exhibited a downward trend, whereas non-Darcy flow β factor increased as a whole. In addition, the magnitude of the permeability was from 10-14 to10-11 m2 and the magnitude of non-Darcy flow β factor was from 106 to 1012 m-1.

Key words: three-dimensional stress, broken sandstones, permeability characteristics, effective stress, confining pressure, porosity