It is an important problem in soils physics to develop a technique making hard pans which have suitable permeability and sufficient bearing capacity for working machine in paddy fields. The fundamental properties ( deformation, strengeth and hardness) of soils concerning this technique are reviewed, partially including the author’s speculation.
1)On the basis of the considerations on the relation between incompressibility and poisson’s ratio, logarithmic strain is reasonable to express deformation of soils.
2) It is preferable to adopt deviatoric stress tensor rather than principal stress ratio or principal stress differences in order to express shear stress for soils.
3) A process of deformation to failure of soils can be expressed by the simplified model: generation of uniform strain, deformation of shear band and formation of a continuous shear failure band.
4) It is very difficult to physically define hardness of soils, so that it is practical to define it by the methods of measurement which are widely applied.
5) To develop the technique making hard pans so that their properties can be controlled, such investigations will be required on the relation between hardness and not only density of soils but also qualitative differences of bond force among soil particles, and on the physical relation between soil moisture and bond force among soil particles.
Increasing use of heavy machinery in intensive arable production has increased the probability of compaction and structural degradation of agricultural soils. Examples are given in this paper to show that relatively small increase in soil bulk density and small loss of air - filled porosity upon compaction could result in appreciable increase in mechanical resistance and serious deteri-oration of gas diffusion in the soils. Volume change characterictics of soils under compressive stress are affected by soil water con-tent as well as structural stability of the soils. It is stressed that the magnitude of the change in total porosity (or bulk density) of soils depends upon the previous history of compression and is not always a useful index of the soil physical state after the compression. Determination ofs oil water characteristic curves for compressed and uncompressed soils has revealed that even when soils with the same initial suction are compressed with the same magnitude of compressive stress, the size of the pores lost or collapsed differs significantly among the soils. Compression susceptibility x of soil is introduced to explain the different response of the soils in the alteration of pore size characteristics and defined as
where upf and σt are the overburden water pressure at the end of compression, and the applieds tress, respectively. Soils which exhibit normal shrinkage over a wide range of water content havei n general greater values of x, and are expected to be liable to structural damage by compression. The need for research on the destruction of soil structure by shear forces as well as on the envi-ronmental consequense of soil compaction is briefly discussed.
The factors which change the permeabiliyty (k) of soils was examined on three compacted ino-rganic cohesive soils.
During prolonged percolation the degree of saturation (Sr ) was measured by a back pressure method developed by the auther. On an initial stage, the water percolation induced a rapid increase in Sr and k, but when Sr reached 100% k began to decrease immediately. This phenomenon suggested that there was a factor which decreased k at the beginning of percolation. An intermittent percolation method was adopted to examine the contribution of dispersion and swelling to the reduction of k. k of untreated soil specimen decreased during the period of perc-olation, but it recoverd with the pause of percolation. The reason of this reversibility of k might be attributed to the flocculation of soil particles which had been dispersed by water flow during the percolation period. Whereas the reversibility of k of 0. 5N NaCl-treated specimen was not signifi-cant. The reduction of k of NaCl-treated specimen might be mainly caused by swelling and/or destruction of aggregates. The reduction of k by microorganisms was significant when nutrient matter was dissolved in influent water and the temperature was 21°C, but it was not so in the case of no nutient and/or 1.3°C. The reduction of k which followed consolidation by self weight and/or seepage force was calc-ulated using a new equation of one- dimensional consolidation. The effect of seepage force was superior to that of self weight in the case that simulated the experimental results of puddled soilperformed by Adachi.
A near-infrared reflectance spectrophotometer with optical fiber was developed to determine soil moisture content in soil layer. The major components of the measurement system were a halogen lamp, a feed and return fiber optic cable, a spectrophtometer, and a personal computer as a system controller. Light was induced in a feed optical fiber to soil surface and reflect ray was also induced in a return optical fiber to spectro-photometer. The percent refrectances of soil surface at 1.94 /zm wavelength were correlated with the soil moisture contents. However, soil particle size deviation affects the percent reflectance. To remove this influence, a reflectance ratio (reflectance at 1.7zim/l.9/zm) was adopted as dependent varia-ble of moisture content instead of percent reflectance. The reflectance ratio was not affected by soil temperature range 10 to 50 °C and salt concentration of soil solution. Reflectance ratio vs. soilmoisture content curves were best fit for exponential curve. Furthermore, each soil appears to have a unique moisture-reflectance curve. It was concluded that this technique of soil moisture measurement is feasible and deserved further research.
I n this paper, following aspects are discussed so as to describe quantitatively the role of cracks on water movement in a clayey agricultural field with shrinkage cracks.
(1) Hydrological properties of drainage according with rainfall events.
(2) Quantitative estimation of the cracks which developed in the field.
(3) P hysical properties and routes of water flow through an undisturbed soil block (1.5m X5.0m).
(4) P hysical properties of water flow in the entire field estimated by recession curve of drain flow.
A test field was 30mX70m in area and has a impervious plastic sheet at 0.7m deep. A drain pipe, 60mm in diameter and 70m long was installed on the sheet and mole drains, 100mm in dia-meter, were constructed at about 0.35m deep and at 1.2m spacing. Soil texture was heavy clay (48%). Soil profile was able to be divided into two layers, plowed layer (about 0.1m thick) and subsoil. The subsoil had large prismatic structure due to shrinkage cracks penetrating to the bottom of the field. Matrix of the subsoil showed a massive structure and saturated hydraulic conductivity was no more than the order of 10-6 to 10-7 cm/s. Results of the experiments are as follows.
(1) Most of rain water was drained from the field by pipe drainage. The pipe drainage started soon after beginning of rainfall, and the drain flow responded quickly on the rainfall patterns.
⑵ In the subsoil of the field, cracks 1—2mm wide developed and the length of the cracks on horizontal sections was 20m/m2 at the depth shallower than the mole drains and 10m/m2 at the depth deeper than the mole drains. From these value, volume of the cracks could be estimated 1.5—3 % of the volume of entire field, and 1—2 % of the volume of the field deeper than 35cm.
⑶ Experiment by using the undisturbed soil block showed that water flowed only along the mole drains and cracks. When the water table was high and water flowed mainly along mole drains, rapid and non- Darcian flow occurred. On the other hand, when the water table was low and water flowed along only cracks, the flow showed D arcian property and the hydraulic conductivity was in the order of 10-2cm/s.
(4) Recession curve of the flow of the pipe drainage could be divided in to 2 parts. When water table was high, flow rate of the pipe drainage decreased linnearly with time. This was similar to flow property from an orifice of a water tank, and was interpreted by rapid flow along the mole drains. On the other hand, when wather table was low and water flows along the cracks, the flow rate decreased exponentially with time. This could be described very well by the equation of unsteady Darcy flow, and volume of the cracks and hydraulic conductivity was calculated 2% and in the order of 10—2cm/s respectively. From these results, it can be said that most of water flowed only along the mole drains and the crack in this clayey field, and that when the water flows along the cracks, the flow showed D ar ci an property and the hydraulic conductivity was in the order of 10-2cm/s.