Thermal And Hydrological Processes In Clayey Soils As Related To Shrink-swell Behaviors During Wetting-drying Cycles

The phenomenon of moisture expansion and dry shrinkage occurs in clay soil during the alternate process of drying and wetting,resulting in vertical and horizontal shrinkage and expansion of the soil.The expansion and contraction of soil lead to soil deformation,which can cause groundwater pollution,exacerbate soil erosion,and even damage roads and buildings.Currently,research on soil swelling and shrinkage characteristics is mainly conducted indoors,with few studies on in-situ monitoring of soil swelling and shrinkage dynamics.The hydrothermal process and swelling and shrinkage changes of clayey soils interact with each other,with significant coupling.However,traditional hydrothermal prediction models do not consider the impact of soil swelling and shrinkage,resulting in errors between the prediction results and the actual situation.Therefore,exploring the changes in water and heat characteristics of clay soils,especially under natural conditions in the field,during the process of moisture expansion and drying shrinkage,and establishing a model of water and heat characteristics that comprehensively considers soil swelling and shrinkage characteristics,is conducive to in-depth understanding of the swelling and shrinkage characteristics of clay soils under natural conditions and their impact on water and heat characteristics,and has important theoretical significance and application value for studying the generation of soil fissures and preferential flows,and the coupled migration of water and pollutants.This study combined indoor and outdoor experiments to simulate and analyze the changes in water characteristic curves,hydraulic conductivity,and thermal properties of different types of clay soils under different initial bulk density,and explored the interaction and mechanism between soil expansion and contraction and water and heat transfer.Outdoor in-situ monitoring experiments were conducted on two different types of land use clayey soils（alluvial soil and paddy soil）,using thermal-TDR sensors and matrix potential sensors to monitor the changes in soil swelling and shrinkage and hydrothermal characteristics,in order to explore the swelling and shrinkage characteristics and hydrothermal spatiotemporal variation characteristics of clayey soils under natural conditions in the field.Finally,a three-dimensional prediction model for water and thermal properties of soil was established,which comprehensively considered soil expansion and contraction.The following main conclusions were obtained:（1）The thermal-TDR technology used in this paper can achieve in situ non-destructive monitoring of soil swelling and shrinkage dynamics and hydrothermal changes in the field.The dynamic soil bulk density measured by the thermal-TDR sensor is comparable to the results obtained by the ring knife method,with a root mean square error of 0.07 g·cm^-3compared to the volumetric method.The void ratio and moisture ratio of soil are calculated from unit weight and volumetric water content,so the thermal-TDR technology can obtain the soil shrinkage characteristic curve in situ.Field soil measurements showed that under natural conditions,paddy soil and alluvial soil mainly contracted in proportion,and the total volume of soil decreased rapidly in proportion to the decrease in water content.（2）Soil swelling and shrinkage and hydrothermal processes interact with each other.Under natural conditions,clayey soil exhibits a significant phenomenon of wet swelling and dry shrinkage,and paddy soil undergoes more intense swelling and shrinkage changes than alluvial soil.The surface soil has a higher water content in the early stage,but a more severe drought occurs in the later stage,which reduces the water content to a minimum,and undergoes more frequent dry and wet alternations,resulting in more significant shrinkage.The expansion and contraction changes of in-situ soil lead to changes in its hydrothermal characteristics that are different from those of rigid soil.Laboratory studies have also found that soil shrinkage results in a maximum deviation of 0.04 m³·m^-3 from the measured water characteristic curve,while the deviation of hydraulic conductivity can reach two orders of magnitude.Unlike rigid soils,during soil shrinkage,soil thermal capacity is negatively correlated with changes in bulk density,while soil thermal conductivity is negatively correlated with changes in water content,but positively correlated with changes in aerated porosity（except for alluvial soil）.（3）In this study,a three-dimensional water characteristic surface,hydraulic conductivity surface,and thermal conductivity surface model of clay soil with comprehensive expansion and contraction changes were established.The three-dimensional surface model of water and heat characteristics can more intuitively represent the synchronous changes in soil expansion and contraction and water and heat transfer,and the simulation results of the model indicate that the simulation effect of the model is better when considering soil expansion and contraction than when not considering soil expansion and contraction.