Freezing point depression of soil water depending on its non-uniform nature in pore water pressure

•Freezing point is affected by water or solute contents, external loading and others.•Pore water pressure increases with decreasing distance to the soil particle surface.•The freezing point depression depends on increased pore water pressure.•Clausius-Clapeyron equation for bulk water is still applicable to soil water.•An inversion of pore water pressure and its components was investigated. The freezing point (FP) of soil water is a vital parameter for hydrothermal coupling simulations in cold regions, and is frequently depressed during soil freezing. Many studies have widely investigated the rules of FP depression affected by the soil type, water content, solute concentration, and external loading; however, few studies have clarified the underlying mechanisms. Water in soils is spatially non-uniform in nature, and the pore water state (PWS) (e.g., pressure) is essential to the FP depression. However, less attention has been paid to the FP depression considering spatially non-uniform nature of soil water. In this study, the FP depression was investigated considering the spatially varied pore water pressure (PWP) and non-uniform nature of soil water. The results showed that the FP depression depends on spatially increased PWP, and the Clausius-Clapeyron equation (CCE) for the phase transition of bulk water can be applied to soil water. These results were verified by two groups of typical experiments of FP with clay soil (external loading Pe = 0, 2, 4, 6, 8, and 10 MPa; gravimetric water content θ = 25, 30, 34, and 38%) and silty clay soil (salt concentration C = 0, 0.5, 1; gravimetric water content θ = 15, 20, 30, 40, and 50%). In addition, the PWP and its components were investigated using the proposed empirical formula.