A Study on the Permeability Characteristics of Modified Red-Bed Mudstone and a Prediction Model for Its Permeability Coefficient

When red-bed mudstone is directly used as fill material for building foundations, road subgrades, and similar applications, it is prone to seepage-induced deformation and instability. Amending it with montmorillonite bentonite can effectively regulate its permeability characteristics. Meanwhile, rapid and accurate prediction of the permeability coefficient is crucial for building foundations and road subgrade seepage analysis and stability assessment. This study investigated red-bed mudstone fill material modified with montmorillonite bentonite at different blending ratios. Soil–water characteristic curve tests, saturated/unsaturated permeability tests, and nuclear magnetic resonance (NMR) tests were conducted on the specimens to examine the pore evolution patterns and permeability characteristics of the modified red-bed mudstone, and a predictive model for coefficients was proposed. The results indicate that the incorporation of montmorillonite-based bentonite markedly affects the permeability properties of modified red-bed mudstone fillers. The NMR T2 spectrum exhibits a bimodal distribution; the incorporation of bentonite and the saturation process result in a marked reduction in large pores and an increase in microporosity. The predictive model achieves higher accuracy when the montmorillonite bentonite content is high. Sensitivity analysis revealed that the maximum pore radius has a far greater influence on permeability than the pore fractal dimension and tortuosity. The research findings provide experimental evidence and theoretical models for the rapid estimation of permeability and seepage stability analysis of modified red-bed mudstone fill materials.