Abstract
Rice husks are frequently used as envelope and filler material for subsurface drainage in Japan. Recently, occurrence of deep slots along the subsurface drainage caused by decomposition of rice husk filler are getting apparent, requiring coping with the problem. The objectives of this study are to clarify the state of decomposition of rice husk filler in clayey paddy fields and to estimate the influence of them on the formation of gapping voids and efficiency of drainage. The rice husk fillers were sampled at 38 plots of clayey paddy fields which were selected considering the year and the ward of subsurface drainage installation and history of cultivation. Decomposition Index (DI), Filtered Soil Content (FSC), and in-situ dry bulk density were measured for the samples. The degree of decomposition depends on the ward as well as the year after the installation, but does not on the history of cultivation. Although FSC increases with the year after the installation of subsurface drainage, it is not observed that the filtered soil seriously affects a decrease in water permeability. The gapping voids can be formed from 8 years and very probably made in 12 years after installation. Compression of decomposed husk causes the formation of gapping voids. Reduction of trench widths, which is more probable in wet clayey soil, lightens the risk of gapping void formation.
