The mechanism of drainage of the clayey paddy field soil at Gyôda City, Saitama Pref., Japan was clarified as follows :
1) The investigated paddy fields were severely waterlogged and it was dificult to plough there. The tile drains were settled and physical and chemical properties of soil were improved for plant growth and tillage.
2) Columnar structure is developed in the subsoil and cracks which join each other are observed in the upper 50 cm. The feature of cracks is varied in different stages of drying process.
3) In non-waterlogging season the permeability of subsoil including cracks is vary large. Excessive rain water flows into tile drains through cracks in mold.
4) In irrigation period, the mold becomes impervious, thus cracks remain dry and permeable.
By drying up paddy field in early August, cracks are created in mold, so it becomes very permeable. Then irrigation water flows rapidly through mold into cracks, but subsoil still remains permeable.
5) Therefore, consideration must be given to cracks in subsoil on estabilishing the design of drainage in such a clayey soil. Tile drains should be settled when cracks are developed in subsoil by the action of open ditches. In design, the optimum tile depth for laterals is limited by the depth of cracks. It is of no use to settle tile drains in the layer below the cracks.
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