Analysis of seasonal change in paddy soil structure based on the elasto-plastic deformation model

Abstract

Paddy soil, puddled and submerged for a few months every year, shows remarkable shrinkage when it dries, but cannot swell up to its original volume when it is submerged again. Such volumetric change induces desiccation cracks, which significantly affect the hydrological behavior of paddy fields. The present study aimed to reveal the basic behavior of cyclic shrinkage and swelling of clayey paddy soil with regard to water management during cultivation and precipitation in the fallow season. The study also aimed to examine the applicability of the elasto-plastic deformation model to the temporal changes in water retention and volume change of paddy soil. Undisturbed soil samples from a clayey experimental paddy were taken repeatedly from the top soil and hardpan layers in a rice monoculture paddy field and a rotationally cropped paddy field, both having clayey soil. Sampling was performed three times a year from July 2005 to April 2007: 1) at the end of two months of continuous flooding after puddling, 2) after drying by the release of poding water, and 3) after the wet fallow period in winter. The samples were subject to measurement of Water Retention Characteristics (WRCs) by multiple saturation and dehydration treatment in the laboratory. The samples were saturated first and dehydrated at 3.1, 10, 20, 40 and 80 kPa in a pressure chamber. They were then saturated again and dehydrated at 3.1 and 80 kPa. From the difference in the retained water at 3.1 kPa of suction between the first and second dehydration, the amount of pore volume subject to irreversible volumetric shrinkage was evaluated. The amount of water retained by puddled soil decreased irreversibly due to desiccation, which can be attributed to the plastic deformation of the pores. The water reduction due to drying was higher in the rice monoculture paddy field than in the rotationally cultivated paddy field. However, the continuous wet conditions after harvest induced restoration of 23% to 78% of the pores that evaluated as having been plastically compressed. These observations revealed the limitation of applying the elasto-plastic model to temporal changes in paddy soil structure. The mechanisms which cause time-dependency in shrinkage and swelling should be studied to obtain effective control of the mechanical and hydrological behavior of paddy soil. Application of alternative mechanical models that take into account the time-dependent behavior should also be explored.