| Summary: | The present work determined the influence of superhydrophobic nano-SiO<sub>2</sub> on the hydraulic conductivity and pore size distribution of expansive soil, and analysed the mechanism of modification between superhydrophobic nano-SiO<sub>2</sub> and expansive soil from a microscopic view. Superhydrophobic nano-SiO<sub>2</sub> was added to expansive soil as a modifier. Our samples were of two types, i.e., unmodified (without nano-SiO<sub>2</sub>) and modified (with 0.2%, 0.4%, 0.6%, 0.8%, and 1.0% nano-SiO<sub>2</sub> by weight of the parent soil). The hydraulic conductivity decreased with increasing nano-SiO<sub>2</sub> content. Fourier transform mid-infrared test revealed that some silanols in soil and nano-SiO<sub>2</sub> were dehydrated and condensed to form siloxanes. We inferred that nano-SiO<sub>2</sub> can attach onto the surface of soil particles to form a hydrophobic membrane, which reduced the soil expansion and the change in pore size distribution. And microscopic tests showed that the pore volume and hydrophilicity of the soil samples decreased with increasing SiO<sub>2</sub> content. According to the Young–Laplace equation, the minimum permeable pore radius was calculated in the hydraulic-conductivity test. With increasing nano-SiO<sub>2</sub> content, the volume of permeable pore decreased. It had an excellent linear relationship with the hydraulic conductivity and permeable pore volume of samples containing different nano-SiO<sub>2</sub> contents. Therefore, superhydrophobic nano-SiO<sub>2</sub> could effectively reduce hydraulic conductivity by changing the pore size distribution of expansive soil.
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