Evaluation of the heat pulse technique for monitoring the shrinkage process of clayey soils

Shrinkage is one of the most critical behaviors in clayey soils. It is often characterized by volume reduction, as the water contents decrease during drying. Accurately measuring soil shrinkage is essential to optimizing land practices. In this study, the heat pulse technique was utilized to monitor the shrinkage process in clayey soils. Four representative soil types were also selected from the widely distributed areas in China: Shajiang black soil, paddy soil, red soil, and alluvial soil. The shrinkage characteristic curves were determined for each soil type under two initial bulk densities (1.0 and 1.2 g/cm³). A comparison was then made of the measurements from the conventional volume method in order to validate the efficacy and accuracy of the heat pulse technique. An enhanced Hyprop device was equipped with a dual-needle heat pulse sensor. A series of experiments was carried out to continuously and simultaneously monitor the soil thermal properties alongside the gravimetric water content during drying. Soil bulk density (ρ_b) was derived from the soil thermal conductivity (λ) data. The void ratio and water ratio were calculated to characterize the shrinkage dynamics. The results show that there were distinct patterns in the thermal properties during drying. Soil volumetric heat capacity (C) decreased generally as the moisture content declined, indicating its strong dependence on water content. There were some variations among the soil types. The paddy and alluvial soil also exhibited a slight initial increase in C followed by a gradual decrease, whereas Shajiang black soil and red soil shared a consistent reduction during drying. In contrast, the soil λ increased with drying, driven by a higher proportion of the solid phase, as the water was lost. Soil shrinkage was dominated by multiple factors, including the clay content, organic matter, and initial bulk density. Shajiang black soil, paddy soil, and alluvial soil exhibited parallel and comparable shrinkage curves under the two initial bulk densities, indicating relatively consistent behavior. Red soil demonstrated the more pronounced linear shrinkage at the lower initial bulk densities, with the highest shrinkage capacity ranging from 19.5% to 28.8%. The elevated shrinkage in red soil was attributed to its high clay content (42.0%) and significant organic matter levels. Conversely, the paddy soil exhibited the lowest shrinkage capacity (17.1%-17.8%), likely due to the differences in its composition and structure. The heat pulse technique has been proven highly effective in monitoring the shrinkage process of clayey soils. Dynamic changes were accurately captured in the proportions of solid, liquid, and gas phases during drying under different initial bulk densities. There was strong agreement with the derived ρb values from the heat pulse technique and the conventional volume method. There was a coefficient of determination (R²) exceeding 0.90 and a root mean square error (RMSE) ranging from 0.038 to 0.185 g/cm³, indicating the high precision and reliability. The heat pulse technique can offer continuous, non-destructive monitoring and high-resolution data collection. A valuable tool can provide for the soil water-heat transfer and practical applications, such as irrigation optimization, as well as soil and water conservation.