In the present report, it is intended to clarify the physical behavior of various groups of paddy soil, as a problem of soil science connected with the readjustment of paddy field basis, and to obtain the technical guide for land consolidation, and soil improvement. In the following report, the physical properties of paddy soil are discussed item by item.
The soil hardness which is related to the bearing capacity of soil varies according to soil texture, soil structure, water regime, bulk density and the contents of humus and salt. In the organic paddy soil, Muck soil excels Peat soil in hardness as surface soil and plow sole. Such difference between the two is due to the height of underground water level and the accumulation mode of organic' matter.
In the inorganic paddy soil, it has been perceived that the soil hardness tends to increase in the order of Strong gley soil, Gley soil, Gray soil, Grayish brown soil, Yellowish brown soil and Ando low land soil. Namely, in organic paddy field, as ill-drained paddy field shifts to well-drained paddy field, the hardness of surface soil and plow sole clearly increases. Such difference in soil hardness is due to the change of physical
properties such as bulk density, water retainability and consistency owing to the height of underground water level and irrigation water.

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Paddy soil is under a condition that aggregation is continuously obstructed by the influence of irrigation water, and tends to be low in the degree of aggregation compared with upland field soil. The degree of aggregate of more than 0.25 mm is distributed in the range between 10-50% in the surface soil of various paddy field from which the feature of various groups of soil is hardly discernible.
However, a certain regularity of change seems to be noticeable between bulk density and the state of aggregate or aggregate index. Namely, with the bulk density of 0.9g/cc as the border line, in the region (A) of smaller than 0.9, the aggregate index decreases with the decrease of bulk density, while in the region (B) of greater than 0.9, the tendency of decreasing aggregate index is observed with the increase of bulk density. In region (A), the ill-drained paddy soil with high humus content and a portion of well drained paddy soil are included, and the region (B) is composed of well-drained paddy soil. Consequently, in order to accelerate the aggregation of the soil belonging to (A), it is necessary to carry out such soil improvement as to heighten bulk density, and on the other hand, for the soil belonging to region (B), such soil management as the tillage which lowers bulk density and dressing in of organic matter may be required.
The percentage of dispersion decreases from ill-drained paddy field towards well-drained paddy field. Negative correlation tends to be observed between the percentage of dispersion and aggregate index of the soil belonging to the group of same category, but the correlation coefficient between the two is small. Besides, although the aggregate index is same, the percentage of dispersion is higher in the case of more ill-drained paddy field, and the water stability of aggregate is greater in the case of more well-drained paddy field.

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As the specific feature of water retentivity of each soil group, the water retentivity of Strong gley soil in alluvial area and Gley soil in reclaimed land are remarkably high, and the tendency of lowering water retentivity in well-drained paddy field is seen. Especially in Yellowish brown soil in diluvial area, water retentivity is extremely low. In brief, in ill-drained paddy field, a great quantity of unfree water such as hygroscopic water and swelling water is retained on the interface of soil particles, and besides, more capillary water or film water equivalent to low matric suction is contained. In the ill-drained paddy field, the water film around primary particles being thick, the interposition of such water may lead to weaken the combination between soil particles.
As a feature of water retentivity of paddy field soil, the irreversible lowering of the water retentivity of soil by air drying is mentioned. The lowering of water retentivity by means of air drying tends to be generally greater in the case of more ill-drained paddy soil. The remarkable lowering of water retentivity in inorganic paddy field soil is odserved in the suction range below the matric suction of 10 bars, while in Ando low land soil, the decrease in the water retentivity of swelling water is conspicuous. Air drying, while changing the property of adsorbing force field at the interface of soil particles, strengthens the combination between soil particles, thereby reduces irreversibly micro pore space. For this reason, it is surmised that, in relation to rehydration, film water or capillary water comes to fail to be absorbed as before.
Ando low land soil, although being well-drained paddy field, has exceedingly high water retentivity, and also the decrease of water retentivity by air drying is great. Such a tendency is noteworthy especially in subsoil. This fact is closely related with the micro structure as well as water retentivity of allophene, composing the primary substance of the clay mineral of volcanic ash soil.

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In studying the relation of soil groups and their consistency, those points similar to the features stated in the item of water retentivity are observed. Namely, it is considered that Atterberg limit shows higher value especially in the case of more ill-drained paddy field, and that high plasticity and compression are shown.
Strong gley soil has high liquid limit W_L, and high plastic index I_p, and W_L, and I_p lower remarkably as the result of air drying. As for Gley soil, Gray soil, and Grayish brown soil, belonging to imperfectly drained paddy field, W_L and I_p are smaller than the former, and the decrease of Atterberg limit through air drying is comparatively less. In the Yellowish brown soil of diluvial area, the small values of W_L and I_p are observed, and both plasticity and compression are low.
In the organic paddy field soil, the compression due to external force changes characteristically, and plastic index is small in comparison with high liguid limit. Besides, liquid limit tends to decrease remarkably as the result of air drying.
The matric suction of soil water corresponding to Atterberg limit is contained in certain definite suction range, irrespective of the kind of soil. Liquid limit corresponds to the suction of about 30cm H₂O plastic limit to the suction of about 3 bars, and shrinkage limit to the suction of about 15 bars, respectively.
When the relation between the consistency index and matric suction of various soil is studied, although some difference exists owing to soil groups or air-drying, the linear correlation is deemed to exist on the whole. The matric suction when I_c =0 is 30cm H₂O, when I_c=0.5, it is 0.3 bars, and I_c=1, it corresponds to 3 bars, respectively.

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The viscosity (ηa) and volume concentration (φ) of the surface soil of paddy field undergo change to considerable extent in the course of years, and certain definite trend is not recognizable about them. However, it is observed that, during irrigation period, both ηa andφ show the trend of decrease in the course of years. As for the flowage of soil treatment plots, during non-irrigation perio, decrease is seen in the order of bare not-ploughed field, not-puddled field and puddled field, and in irrigation period, the difference in viscosity decreases.
The mechanical models of natural soil layer of paddy field are roughly divided into the Maxwell model in which Peat soil and Ando low-land soil are included, the three element model in which Gray soil and Yellowish brown soil are included, and the intermediate model in which Strong gley soil is included. The soil in ill-drained paddy field bears the property of viscous flow, while in well-drained paddy field, relatively elastic deformation takes place predominantly.
It is considered that in the stress relaxation due to the mold of surface soil, Peat soil and Ando low land soil, have smaller elastic constant and viscosity as compared with Strong gley soil and Gray soil, and possess the dynamic property which is comparatively apt to flow in viscous state.

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The change in vertical percolation during the period of rice-plant cultivation according to the existence or not of soil compaction was measured on natural soil layer, using an artificial soil layer lysimeter.
The solid volume rate of natural soil layer was largest is Strong gley soil and in Gley soil, and least in Ando low land soil and Peat soil. By the application of soil compaction by 2 kg/ cm², solid volume increased by 3-5%, while non-capillary pore decreased by 2-3%.
The variation of vertical percolation with the lapse of time showed V-form with august as the minlmum point, and especially striking V shape curve was indicated in poor-percolation soil. Vertical percolation showed the lowering of some 10^-1cm/sec order as the result of soil compaction.
Between solid volume or non-capillary pore space and percolation, a certain relation is observed in the case of the same soil, but when the soil is taken as a whole, no one-to-one correspondence is discernible between the two. In other words, it means that, in the evaluation of permeability, the form of water pathway affecting soil structure is significant.
The temporary lowering of permeability as the result of soil compaction gradually revives with the process of plant cultivation. The value of permeability K after one crop revives on the whole by the order of 10-1cm/ sec compared with K immediately after compaction.
When the relation between soil groups and percolation is studied within the soil column experiment, Grey soil, Greyish brown soil, Yellowish brown soil and Ando low land soil are included in optimum percolation class, and Strong gley soil and Gley soil belong to poor percolation class.

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The special features of soil groups have been made clear as mentioned above, and the summarized classification of soil has been attemped on the basis of these several physical and mechanical behavior. The system of classification is the presentation of the relative values of various physical and mechanical properties of soil groups included in the Casagrande's plasticity chart mentioned in the item of consitency.
A. Casagrande divided the plasticity chart into 6 ranges by means of the perpendiculars of liquid limit 30%, 50% and oblique “A” line, while in the soil classification of paddy field, the high plasticity range above 50% liquid limit is divided into two by means of liquid limit 90%, thus the division is made into 8 ranges in all. In paddy soil, because of the large Atterberg limits, the soil contained in the range of 50% or more liquid limit occupies the majority thereof. Consequently, in the normal plasticity chart, it becomes difficult to decide the physical and mechanical properties of soil groups. Accordingly, each soil group was made to belong to the 8 divisions by means of the perpendiculars of liquid limit 30%, 50%, 90%and “A” line.

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Profitable slopes of farm branch roads of an orange orchard at slope were calculated by OR. If the slope becomes steep, extension lines become short, and so the construction cost of roads, their mending expense and the orange decrease by collapese lands decrease.
When the slope becomes steep, the unit running cost increases, meanwhile the extension line becomes short. Therefore the whole running costs do not change too much. In view of the results so for achieved, the authers found that it is profitable to make slopes of farm roads steep to the limit of safety in descending slopes.

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In this paper the authers studied some mechanical problems of Uchinokura-hollow-gravity dam, which we have been interested in, and we have already reported on some of these previously. The Finite Element method was used here because it has been recognized that Finite Element method is a useful tool for analysing such complex problems as dealt with here; it can be used for expressing various boundary conditions easily; and accuracy of numerical solutions can be selected as desired. The proplems dealt with here are as follows.
1) The effect of elastic property of dam foundation on the stress distribution of dam.
2) The effect of construction joint of dam on the condition of the dam.
3) The effect of extent of improved ground (such as consolidation grouted zone) on the condition of the dam.
4) The effect of the excavation figure of ground in the vertical plane of the dam axis on the stability of dam.A summary of the results obtatined is as follows:
1) The effect of contact surface does not extend to over 1/5 of thedam height.
2) Extending the zone of improved ground vertically was better than extending it laterally.
3) There is possibility of arc shaped joint inducing tensile stress along the joint in case the ground condition is complex.
4) Terrace excavation of dam foundation on side slope improves stability of dam.

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Several studies have been made in the past on consolidation of soils due to vibrating load. The authors carried out more detailed investigation on this subject and clarified settlement theoretically by considering Burger's model as a mechanical model of soil. Also, the case of consolidation in whichinertia force is taken into consideration is described in the latter half of this paper.

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