三轴应力下不同粒径破碎砂岩渗透特性试验

Permeability tests of fractured sandstone with different sizes of fragments under three-dimensional stress states

地下工程中破碎岩体往往处于三向应力状态,此类岩体具有孔隙率大、渗透性高等特点,在地应力与高水头作用下易发生渗流失稳破坏,诱发突水灾害。为研究三轴应力下破碎砂岩的渗透特性,运用自主研发的破碎岩石三轴渗流试验系统,采用稳态渗透法进行5种粒径破碎砂岩的渗流试验,得到了三轴应力下破碎砂岩渗透特性变化规律,推导了有效应力与渗流速度之间的关系。试验结果表明:三轴应力下破碎砂岩的有效应力与渗流速度呈线性关系,且轴向位移越大时,随有效应力的增加渗流速度减小的幅度越小;三轴应力下5种粒径破碎砂岩的孔压梯度与渗流速度服从Forchheimer关系,两者之间的相关系数达0.95以上;轴向位移恒定时,随着围压的增大,破碎砂岩渗透率k减小,非Darcy流β因子增大,各级轴向位移下,破碎砂岩的渗透率与围压之间呈指数函数关系;随着孔隙率的减小,5种粒径的破碎砂岩渗透率呈减小趋势,非Darcy流β因子整体增大,且渗透率量级为10^(-14)~10^(-11) m^2,非Darcy流β因子的量级为10~6~10^(12) m^(-1)。

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） to 10^（-11） m^2 and the magnitude of non-Darcy flow β factor was from 10~6 to 10^（12） m^（-1）.