Size effect and anisotropy of rock fractures: insights from investigations on a large-scale fracture surface

The mechanical and hydraulic behaviors of rock fractures are strongly dependent on the properties of the surface morphology. In this study, a sandstone fracture surface is taken as the research object. The morphological properties of the fracture surface characterized by parameters Z2s, Z2, and SRv are analyzed under the different sampling windows and profile lines. The results show that the large sampling window is more conducive to obtaining stable values of the morphology parameter Z2s. The coefficient of variation (CV) and the percentage of variation (PV) are proposed to determine the stable values of the morphology parameter Z2s, which is defined as a representative elementary surface (RES). It is confirmed that there is a RES with a size of 220 mm in the study region. The large sampling window generates a long correlation length of the morphology parameter Z2s. The differential sensitivities of the morphology parameters Z2 and SRv are compared under the different profile lines. It is found that the parameter Z2 can better describe the random variation properties of the surface morphology, while the parameter SRv is more suitable for describing the spatial structural variation properties of the surface morphology. The findings of this work are of great significance for investigating the size effect and anisotropy of rock fracture permeability.