The Influence of Varying Subgrade Soil Compaction on the Relationship between Soil Water Content and Resistivity

Quantifying the water content of subgrade soils is crucial for subgrade design and maintenance. As a noninvasive approach, the resistivity method is widely used for measuring the soil water content. However, the measuring accuracy strongly relies on the relationship between soil resistivity and water content. For compacted subgrade soils, the applicability of conventional relationship models remains unclear. To explore the influence of compactness on the relationship models, field subgrade soils (sandy loam and loamy sand) and improved soils (loam with lime and loam with sand) were prepared in this study. Considering the application scenarios of soil water content investigation for subgrade soils, compactness levels of 90%, 93%, and 96% were considered, which meets or is slightly below the required values of subgrade construction standards. The resistivities of compacted soil samples were measured and fitted using Archie’s, exponential, and logarithmic models. The result showed that Archie’s, exponential, and logarithmic models could all fit the measured soil resistivity and water content with high goodness of fit. Overall, Archie’s model had better prediction accuracy. A significant difference in the fitted parameters of the three relationship models was observed for soils with varying compactness levels, indicating that compactness played a notable role in the relationship models. The exponential and logarithmic models resulted in negative soil resistivity in the high soil water content range, and therefore, Archie’s model seemed more reasonable. However, attention should be paid to the parameters of Archie’s model for compacted soils. Compacted field soils have higher cementation but lower saturation exponents than conventional natural soils. The exponents of the compacted improved soils were basically consistent with those of the natural soils. These findings would be beneficial for model and parameter selection when measuring the soil water content of various compacted subgrade soils using the resistivity investigation method.