Pore Characterization of Sandy Silty Soil in Metro Surrounding Rock: A Synchrotron Small-Angle X-ray Scattering Experiment

To examine the pore structure characteristics of sandy silt in metro tunnel surrounding rock, the pore size distribution patterns and fractal dimension of sandy silt samples were determined through small-angle X-ray scattering (SAXS) experiments. This study also explored the relationship between the internal microscale pore structure of sandy silt and its macroscopic behavior. The results show that spherical pores within sandy silt predominantly exist in the forms of micropores and mesopores, with a concentrated distribution in the pore size range of 20–50 nm. Moreover, for scatterers with shapes like ellipsoids and disks, a smaller ratio of height to diameter or major axis to minor axis corresponds to a narrower range in observed pore diameter distribution, accompanied by a higher volume fraction. Meanwhile, the fractal dimensions for sandy silt samples B-1, B-2, and B-3 were 2.33, 2.33, and 2.38, respectively, aligning with the range of 2–3. The augmentation of fractal dimension corresponds to heightened surface irregularity or roughness, indicating a moderately smooth sandy silt surface. Additionally, the results show that the various deformation and strength characteristics possessed by the soil can be considered as a comprehensive reflection of the adjustment and evolution of its internal microscale pore structure elements. By utilizing the pore size distribution features and fractal dimension of the internal pore structure, a quantitative relationship between the internal pore structure of sandy silt and its macroscopic permeability or mechanical properties can be established.