Abstract:  Under low hydraulic gradients, low-permeability media often exhibit low-velocity flow. Investigating the mechanisms of low-velocity seepage is crucial for groundwater environmental protection research. Fluid flow in low-permeability porous media is influenced by a combination of multiple factors. This study systematically investigates the effects and mechanisms of low-velocity seepage in different types of cohesive soils at varying temperatures through laboratory constant-head permeability tests and nuclear magnetic resonance (NMR) experiments. The results indicate: 1) Temperature elevation significantly enhances the hydraulic conductivity of clayey soils. Within the 5-30 ℃ range, increasing temperature raised the hydraulic conductivity of white clay, loess, and silty clay by 289.9%, 210.4%, and 156.2%, respectively, while simultaneously decreasing the critical hydraulic gradient from approximately 10 to around 4. 2) Temperature elevation promotes the conversion of weakly bound water to free water. When temperature increased from 5 ℃ to 30 ℃, the bound water mass fraction of white clay, loess, and silty clay decreased by 23.1%, 22.8%, and 25.6%, respectively. Reduced bound water mass fraction increases effective pore space and lowers flow resistance, thereby enhancing hydraulic conductivity and decreasing the critical hydraulic gradient. 3) Temperature regulation of permeability coefficients exhibits a viscosity-bound water coupling effect, but bound water dominates permeability influence. Within the 5-30 ℃range, the contribution of bound water to hydraulic conductivity coefficients in cohesive soils increases with rising temperature. At constant temperatures, white clay soil exhibits the strongest influence of bound water on hydraulic conductivity, followed by loess, with silty clay showing the weakest effect.

Key words: clay soil, low-velocity non-darcy flow, temperature, nuclear magnetic resonance, bound water