Experimental study on the anisotropy of the effective stress coefficient of sandstone under true triaxial stress

The effective stress principle plays an important role in the study of the permeability evolution and mechanical behavior of coal and rocks. A key to evaluating the effective stress is determining the evolution of the effective stress coefficient α. However, conventional triaxial stress-path tests are not suitable for reproducing the actual stress state of coal and rocks and thus determining the effective stress coefficient. Therefore, this study was conducted to investigate the anisotropic characteristics of the effective stress coefficient for sandstone under different true triaxial stress and pore pressure conditions. The results show that the effective stress coefficient exhibits anisotropy due to the different principal strains in three directions and the anisotropy of pore structure, and it is closely linked with the pore pressure and principal stress. An increase in pore pressure causes sandstone particles that block the seepage channel to migrate, which reduces the sensitivity of permeability to principal stress, increasing the effective stress coefficient. When principal stress increases, pores and fractures are compressed, which reduces the sensitivity of permeability to pore pressure, resulting in a decrease in α. However, as principal stress continues to increase, effective stress coefficients can increase in the directions of major and intermediate principal stresses, while an α3 beyond unity appears in the direction of minor principal stress. This is related to the pore fluid flowing through highly compressible clay aggregates, in which the pore area is large, and the sensitivity of permeability to pore pressure is enhanced. The effective stress coefficient increases with an increase in permeability due to the effect of porosity. A new formula for calculating the volumetric strain of a linear elastic isotropic porous medium is established based on the anisotropy of the effective stress coefficient, which can be applied to experimental results.