This study estimated water content of saturated clay using soft X-ray digital radiography (SXDR). The study focused on the relationship between water content and clay density. The water content of saturated clay can be calculated from its wet bulk density. The dry bulk den-sity of dried clay can be calculated by examining the aver-age gray levels of pixels in its soft X-ray image. The water content of saturated clay can also be estimated from its soft X-ray image if the wet bulk density of saturated clay can be estimated using the same technique. Experiments were conducted with the soft X-ray images of saturated kaolin and bentonite samples. The gravimetric and volumetric water contents estimated from these images were com-pared with observed values. The estimated values were in agreement with the observed values. This fact proved that the water content of saturated clay can be estimated by SXDR.
The effects of channel formation on settling ve-locity have not yet been clarified for high-density clay sus-pensions. In this study, we examined the effects of ver-tical channel formation conditions on channel shape and channel flow rate by a method where artificial bubbles were passed upward through a column containing a sus-pension of kaolinite. Specifically, settling velocity, chan-nel diameter, and channel number density were measured by photographic methods. These data were used to cal-culate channel flow rates, which were then compared with the values expected for Hagen-Poiseuille flow. At a high solid concentration (1.1 % 〜4.0 % by volume), vertical channels did not form unless artificial bubbles were ap-plied, because the suspension became highly viscous. The number of channels formed by applying bubbles decreased sharply with increasing solid concentration, whereas verti-cal channel diameter increased with increasing solid con-centration. As a result, there was a peak settling velocity at a solid concentration of 2.2 % by volume. Under con-ditions without structural force, the relation was examined between measured diameter and flow rate diameter (i.e., the mean of diameter to the 4th power) calculated from set-tling velocity using Darcy’s law and the Hagen-Poiseuille equation. The calculated diameter of channels formed by artificial bubbles agreed well with the measured diameter at low solid concentration (< 2.2 %), but was lower than the measured diameter at high solid concentration (> 2.2 %). These results show the importance of structural force in high-density clay suspensions.
To improve the grain self-sufficiency ratio in Japan, application of the rice - winter barley - soybean triple-cropping two-year system to clayey paddy fields is required in a Hokuriku region with heavy snowfall in win-ter. The clayey soils are hardly drained because of the low hydraulic conductivity; besides, the soils have limited amount of available water. The FOEAS (Farm-oriented en-hancing aquatic system) water table management system was developed to overcome these problems. We inves-tigated the discharge of water, nitrogen, hosphorus and suspended matter from FOEAS fields with the crop rota-tion system. We also evaluated effect of the groundwater level control on the discharge of water, nutrients and sus-pended matter. Rice was cultivated by the V-furrow no-till direct seeding method. The amount of nitrogen, phospho-rus, and suspended matter ischarge in the V-furrow no-till direct seeding rice period were same or smaller than in fields planted with the conventional methods includ-ing puddling and transplantation. During the crop rotation period, 94 % of water was discharged to the subsurface drains in the free drainage plot (FD). In the groundwater level control plot (GC), 32 % of water was discharged via surface runoff, demonstrating insufficient development of shrinkage cracks in the subsoil, which act as a pathway for the water. During the winter barley period, the concentra-tions of total nitrogen and nitrate in the discharge tended to be high, particularly in the pre-snow period, the concen-trations and the amount of discharge of nitrogen and sus-pended matter were the lower in GC than in FD, suggest-ing that denitrification was encouraged, and nitrification and shrinkage crack formation were suppressed in GC. In contrast, in the case of phosphorus, the concentration and the amount of discharge from GC was higher than FD, sug-gesting reduction condition in the subsurface. During the winter barley period, 70 % - 94 % of total water, nitrogen, phosphorus, and suspended matter discharge occurred in the crop rotation system.