Abstract:

This study systematically investigates the swelling behavior of red-bed mudstone from the Jurassic in Central Sichuan, under overlying loads using macroscopic water-absorption expansion tests, microstructural observations, and molecular dynamics simulations. The results demonstrate that the expansion process of red-bed mudstone can be divided into three distinct stages: rapid, decelerated, and stable. With increasing overlying load, both the macroscopic expansion force and water absorption rate generally exhibit a linearly decreasing trend; however, an anomalous peak appears at 100 kPa. This anomaly is attributed to critical loading-induced microcracking, which facilitates water penetration and enhances mineral hydration. Microstructural analysis indicates that within the micropore range, the fractal dimension of the pores reaches a maximum value at 1.88 under 100 kPa. This finding reflects a positive correlation between microstructural complexity and macroscopic expansion. Molecular dynamics simulations further show that, at the nanoscale, applied loading inhibits the diffusion of interlayer water molecules and ions in clay minerals. The diffusion coefficients of water molecules and Na⁺ decreased to 3.3×10⁻⁷ cm²/s and 2.0×10⁻⁷ cm²/s, respectively. This inhibition enhances the stability of the hydrated structure. This study elucidates the internal mechanism and evolutionary patterns of water-absorption expansion in red-bed mudstone across macro–micro–nano scales. It provides a theoretical basis for geological hazard prevention and engineering stability assessment in red-bed strata.

Key words: red-bed mudstone, overlying load, expansion characteristics, multi-scale evolution, molecular dynamics