Study on anisotropic characteristics of rock joints considering fractal features of actual contact surface morphology

The anisotropy parameter of joint surfaces (JS) reflects the mechanical and physical properties of joint surfaces in different directions, which significantly affects their stability, permeability, and the design reliability of rock engineering applications. To quantitatively characterize the anisotropic properties of JS, a novel method is proposed. This method integrates the morphological distribution characteristics of the actual contact JS with fractal theory. Firstly, the three-dimensional morphological features of the JS at various sampling intervals were acquired using 3D laser scanning. The results reveal that the surface morphology exhibits significant anisotropic characteristics and a pronounced sampling interval effect (SIE). Subsequently, the anisotropy of the 3D roughness parameter proposed by Ban et al. is investigated. To investigate the SIE on the accuracy of roughness characterization, a new 3D fractal roughness parameter is developed by incorporating fractal theory. Finally, a method is proposed to quantify the anisotropy based on the fractal features of the ACJS according to the variation of the fractal parameter in different directions. Furthermore, the size effect (SE) on fractal parameters in different shear directions is further investigated. The results demonstrate that the impact of the SE on fractal parameters depends on shear direction, and the parameters tend to stabilize with increasing observation scale. The anisotropy parameter proposed in this study is influenced by both normal stress and joint material properties, and it reflects the anisotropy of the actual contact 3D roughness under specific loading and material conditions.