The present work determined the influence of superhydrophobic nano-SiO₂ on the hydraulic conductivity and pore size distribution of expansive soil, and analysed the mechanism of modification between superhydrophobic nano-SiO₂ and expansive soil from a microscopic view. Superhydrophobic nano-SiO₂ was added to expansive soil as a modifier. Our samples were of two types, i.e., unmodified (without nano-SiO₂) and modified (with 0.2%, 0.4%, 0.6%, 0.8%, and 1.0% nano-SiO₂ by weight of the parent soil). The hydraulic conductivity decreased with increasing nano-SiO₂ content. Fourier transform mid-infrared test revealed that some silanols in soil and nano-SiO₂ were dehydrated and condensed to form siloxanes. We inferred that nano-SiO₂ 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 SiO₂ content. According to the Young–Laplace equation, the minimum permeable pore radius was calculated in the hydraulic-conductivity test. With increasing nano-SiO₂ 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-SiO₂ contents. Therefore, superhydrophobic nano-SiO₂ could effectively reduce hydraulic conductivity by changing the pore size distribution of expansive soil.

Understanding the failure mechanism of soft rock roadways in high-humidity environments, as well as designing appropriate control measures, are critical for ensuring their stability. In this study, swelling and triaxial compression tests on argillaceous sandstone were first performed. Results show that the triaxial compressive strength (TCS), elastic modulus, cohesion, and internal friction angle of argillaceous sandstone are all decreased due to the water weakening effect. Then, a self-developed finite-element-based numerical code was employed to elucidate the role of humidity diffusion in the deformation and failure of soft rock roadways. Simulation results indicate that under the influence of humidity, high stress concentration zones develop, initiating microcracks within these regions. As humidity continues to diffuse, the high stress concentration zones expand and migrate deeper into the surrounding rock, causing microcracks to propagate and accelerating humidity diffusion. This cyclical process repeats, ultimately resulting in macroscopic fracturing. The failure of roadway exhibits a tensile–shear mixed mode during humidity diffusion. A comparative analysis of four control measures reveals that conventional non-waterproof shotcrete primary support is of limited effectiveness in ensuring the stability of high-humidity soft rock roadways. It is essential to promptly establish a closed waterproof support structure. Furthermore, localized support defects significantly impact control effectiveness.