Physical disintegration of aggregates as a response to externally imposed disruptive forces to soil or the physico-chemical dispersion and swelling of soil clays (soil intrinsic behavior) are often reported to cause deterioration in the hydraulic properties of soils. The joint effect of these two mechanisms on column hydraulic conductivity (HC) of different soils was studied. Three prewetting rates (PWR) of 1,6 and 30 mm/hr and a water quality characterized by sodium adsorption ratio (SAR)10 and total electrolyte concentration (TEC) of 0.5,0.05, 0.01 Molc L 1 and distilled water (DW) were used. The results showed that absolute values of HC and its relative changes over time depended on the type of soil, the PWR, the TEC, and the aggregate size. HC decreased with an increase in the silt and clay content, with an increase in the PWR, and with a decrease in the TEC of the percolating solution. Soils that slaked into microaggregates under the effect of fast PWR, showed a substantial decrease in HC with an increase in the PWR. In structurally unstable soils, fast PWR caused more slaking and physical disintegration of aggregates that restricted water flow leading to low HC. HC of the columns that sustained greater slaking also deteriorated more under the effect of dilute solutions. The larger the aggregate size fraction, the more pronounced was the PWR effect on HC apparently due to greater pore throttling by the slakes. development of cohesive forces between clay structural units with time (aging) was suggested to counteract HC deterioration as observed for smaller aggregate size fraction of this soil. HC of less aggregated or structurally stable soils were less affected by the PWR. The study also indicated that PWR effects on soil HC could be satisfactorily predicted from soil aggregate stability tests.
Gypsum is an useful soil amendment. However, application of gypsum sometimes enhances the dispersion of highly weathered acid soils. The mechanism of the effect of gypsum on the dispersion of a Japanese acid soil is discussed in terms of adsorbed ions and aggregate stability. Leaching of the soil column by NaCl,Na2 SO4, CaCh, and CaSC)4 electrolyte solutions showed that Ca2+ could displace Al3+ in the soil, and SO2- caused more cation retention of the soil. In the slaking experiment, the soil was most dispersive with the CaSO4, while it was least dispersive with the NaCl. The result goes against the effect that gypsum has on sodic soils. It was concluded that removal of Al3', as a binding agent of the acid soil, by Ca2+ and the change in charge characteristic due to SO42 adsorption were the mechanisms that enhanced dispersion of the Japanese acid soil following gypsum application.
For conservation of a valuable land with high-moor peat soil where the main vegetation is sphagnum, it is very important to make an index of the qualitative change of the soil due to the change in soil moisture. Therefore, in the present study, the moisture tension-moisture relation and the shrinkage-restoration characteristics due to the change in soil moisture as well as the fundamental physical-chemical properties of the Akaiyachi high-moor peat soil were attempted to be clarified. Especially, the shrinkage-restoration of soil is a conspicuous physical behavior due to the change in soil moisture. The principal results are as follows : The method followed in the study, namely, the pressure plate method is very useful to determine the moisture tension-moisture relation for low and medium moisture tension ranged from 0.981 kPa (pF 1.0) to 981 kPa (pF 4.0) in drying process. The changes of horizontal, vertical and volume shrinkage percentages of the high-moor peat soil consisting mainly of living sphagnum are small in the stage of low moisture tension and large in that of high one. Moreover, the change of its vertical shrinkage percentage is larger than that of its horizontal one in all stages of the moisture tension because of its subsidence by dead weight. The changes of the shrinkage percentages of the high-moor peat soil containing much inorganic dressing materials are large in the stage of the low moisture tension and small in that of the high one, while in the same soil under natural condition, the changes are large in both the stages of the low and the high moisture tension. Moreover, in these high-moor peat soils, the changes of their vertical shrinkage percentages are also larger than those of their horizontal ones in all stages of the moisture tension because of their subsidences by dead weight. To protect the land with high-moor peat soil from the qualitative change due to the shrinkage, it must be managed so that the moisture tension does not exceed about 41.8 kPa (pF 2.6) where the restoration after the shrinkage is perfect, namely, the moisture content is about more than the capillary one.
Maximum frost depth is an important index for design of engineering in cold region to prevent frost damage. When boundary conditions and soil type are given, the frost depth is affected by soil water content. But it is still not clear how the maximum frost depth is affected by the soil water content. The heat balance during freezing and thawing processes of soils are strongly affected by soil water content as well. The knowledge about the effect of soil water content on the maximum frost depth and heat balance is very important for control and prediction of freezing and thawing processes. However, the elucidation of these effects is difficult to deal with only by physical experimental method. Therefore in this paper, the numerical experiments for some different initial water contents were performed to analyze the effects on maximum frost depth and heat balance. As the results of these experiments the maximum frost depth takes the smallest value nearing 0.15〜0.20 (m3 m-3) of total water content (ice+water) in frozen layer, and increases both with the increasing and decreasing of the total water content under the boundary conditions and soil used in this paper. The ratio of accumulate latent heat to the accumulate heat outflow to the air increases with initial water content increases, the same ratio of accumulate sensible heat increases with initial water content decreases, and the same ratio of the accumulate heat inflow from the lower boundary keeps increasing with the freezing-thawing process advance.
The convection-dispersion equation (CDE) is widely used to predict solute transport in soils. The mechanical dispersion is described with the mathematically identical equation to the Fick’s law for molecular diffusion. Physically unrealistic backward solute mixing may occur for the CDE when a high concentration gradient exists in a soil. We propose a convective random-walk model (CRWM) which only allow solute particles to move in the convective direction. Solute particles move randomly according to an asymmetric probability density function (pdf) having identical mean and variance to the normal Gaussian pdf for the CDE. The CRWM is firstly applied to a solute leaching for a Dirac delta initial distribution. The CRWM can avoid backward mixing as was observed for the CDE during the early stages of leaching. The solute distribution for the CRWM converges to the distribution for the CDE regardless of the shape of the pdf for sufficiently large travel distances because of the central limit theorem. A stochastic input method for the CRWM assuming an additional hypothetical soil outside of the boundary is employed to describe solute input at the boundary in order not to overestimate the concentration near the soil surface. We then apply the CRWM to predict a salt accumulation process on the surface in a soil column having a shallow salty water table. All the flow and transport parameter values are independently determined. Molecular diffusion and salt crystallization at the surface are included for the model prediction. A good agreement between measured and predicted solute profiles is observed for the CRWM, whereas the CDE overestimates downward solute movement due to backward mixing.