Revisiting scale effect on joint roughness coefficient and shear strength considering sampling methods and geometric characteristics

The scale effect on shear strength of rock joints is well-documented. However, whether scale effects are negative, positive, or even exist or not is still controversial. Joint roughness significantly influences the shear strength of rock joints. Compared to the shear tests, using the joint roughness coefficient (JRC) and its roughness parameters offers a more convenient method for describing the scale effect on shear strength. However, it is crucial to understand that the scale effect mechanisms of JRC are distinct from those of shear strength. Therefore, this paper aims to clarify these distinct mechanisms. By digitally extracting roughness parameters from granite samples, it is found that the scale effect of roughness parameters mainly comes from the sampling methods and the geometric characteristics of parameters. Furthermore, a full data sampling method considering heterogeneity is proposed to obtain more representative roughness parameters. To reveal the scale effect mechanisms of shear strength, Gaussian filtering is firstly used to separate the waviness and unevenness components of roughness, facilitating a deeper understanding of the geometric characteristics of roughness. It is suggested that the wavelength of the waviness component can reflect the scale effect on shear strength. Secondly, numerical simulations of ideal artificial joint models are conducted to validate that the wavelength of the waviness component serves as the dividing point between positive and negative scale effects. The mechanical mechanisms of positive and negative scale effects are also interpreted. Finally, these mechanisms successfully elucidate the occurrence patterns of the scale effect on natural joint profiles. • Different scale effects of joint roughness were investigated and interpreted. • A full data sampling method was proposed to obtain accurate JRC characteristic parameters. • Scale effects mechanisms of roughness and shear strength were studied. • Contributions of waviness on rock joint shear behaive were evaluated.