There is an extensive distribution of organo-volcanic ash soils (Kuroboku soils) and a partial distribution of organic clayey soils (non-volcanic ash soils) in Japan. These soils have particular characteristics of physical and engineering properties.
In this paper, the authors have described the physical characteristics and problems regarding the measurements of these soils, and proposed several new measuring methods. However, we have described mainly the particular characteristics of the physical properties for organo-volcanic ash soils from the viewpoint of their relationship with organic content and changes due to the drying procedure in this area.
(1) Dry bulk density in the field is very small and it decreases with an increase in the organic content, but increases with air-drying.
(2) Natural water content is very high and it increases with an increase in the organic content.
(3) Specific gravity is small and it decreases with an increase in the organic content.
(4) Water retention is very large and it increases with organic content. But there is a large decrease in water retention after air-drying.
(5) Liquid and plastic limits have very large water contents, and these increase with organic content.
(6) Liquid and plastic limits vary with initial water content. In the first step, the values of the liquid and plastic limits are constant in spite of the initial water content of a soil sample, then those values decrease with a decrease in the initial water content. We defined the decreasing points of liquid and plastic limits as the first critical point and the second critical point of the initial water contents, respectively. The first critical point of the initial water content is always higher than the second critical point of the initial water content. Also, liquid limit decreases more sharply by air drying than in the case of the plastic limit.
(7) The first critical point of the initial water content is equivalent to about pF 4. 2, and it corresponds to the plastic limit and the turning point of the shrinkage curve for a fresh soil sample. Moreover, the plastic limit corresponds to the turning point of the shrinkage curve at any initial water content of a soil sample.

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The authors studied the thermal properties of Kuroboku soils (Organo-volcanic ash soils) greatly related to the water retention of soils. In this paper, the authors investigated thermal conductivity, volumetric heat capacity and thermal diffusivity in the thermal properties. Soil samples, three Kuroboku soils, one volcanic ash soil, one alluvial soil and one heavy clayey soil were used (Table 1).
The results obtained are summarized as follows:
(1) The thermal conductivity of Kuroboku soil is much smaller than that of volcanic ash soil, alluvial soil and heavy clayey soil (Fig. 3). This can be explained by the very large water retention of Kuroboku soil and the small thermal conductivity of organic matter which Kuroboku soils contain in large quantities (Table 1 and Fig. 1).
(2) Though the thermal conductivity incr eases with an increase in the solid ratio and liquid ratio of the soil sample, in the case of Kuroboku soil, the increase in thermal conductivity with liquid ratio is smaller compared with the other soils (Figs. 4 and 5).
(3) The thermal conductivity decreases with an increase in the air ratio of the soil sample, and the decrease in thermal conductivity is small for Kuroboku soil (Fig. 6), When the air ratio is zero and the soil sample is fully saturated with water, the thermal conductivity of Kuroboku soil is nearly equal to that of water (1. 43×10^-3 cal/cm·s·°C, 20°C) (Fig. 6). This is due to the characteristics of the heat conduction route of Kuroboku soil.
(4) The authors considered the heat conduction route as follows (Fig. 2). That is the heat conduction route of soil is composed of (Aggregate)-(Aggregate) route (direct heat conduction route) and (Aggregate)-(Water)-(Aggregate) route (indirect heat conduction route), but in the case of Kuroboku soil the heat conduction route is mainly composed of theY latter because of the very large amount of water retention.
(5) The thermal conductivity of soil decreases due to air-drying, and the decrease in thermal conductivity becomes much larger beyond pF 4. 2 (Figs. 7 and 8). This point corresponds to the critical point of the intial water content that is the decreasing point of the liquid limit for the relationship between intial water content and liquid limit. The decrease in thermal conductivity due to airdrying for Kuroboku soil is smaller in comparison with the other soil (Figs. 7 and 8).
(6) The volumetric heat capacity increases linearly with the dry density of the soil sample. The increase in volumetric heat capacity is very large for Kuroboku soil because of the large water retention (Fig. 9).
(7) The thermal diffusivity increases with water content, but that is constant beyond 30% in water content by volume (Fig. 11). The thermal diffusivity of Kuroboku soil is much smaller than that ofthe other soils (Fig. 11).

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In this paper the instantaneous insertion method for simple and easy determination of soil heat conductivity is proposed, and its usefulness is confirmed from the application of this method for some soil samples.
This method has other advantages that effects of the contact resistance and the destroy of soil structure are removed, in comparison with the conventional probe method, in addition to simple instrument and easy operation. Then, this method is concluded to be very useful in studies of heat conduction for the organic soil and the natural soil in which the soil structure is well developed.

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The heat conductivity is a physical characteristic of soil which varies remarkably due to the following factors; concentration of soil minerals and organic matters, soil structure which represents distribution pattern of these elements, and soil moisture state. Therefore, the value of heat conductivity becomes an useful index to explain the condition of soil. Based on this understanding, in order to determine the internal structure of soil, especially, soil moisture distribution in the sufficient dried regime, authors will try to analyze the observed data of the soil heat conductivity and its change depending on the soil water content.
The conclusion is summarized as follows.
(1) Under the condition of lower soil water content and relatively higher pF value such as air dried soil, the apparent soil heat conductivity can be prescribed only by the thickness of air surrounding soil particles and the air heat conductivity, while the existence of soil water contributes only to control the thickness of air film (Fig. 4).
(2) The soil water regime in which the soil heat conductivity mainly varies according to the above mentioned mechanism, is confineded at the water content for pF 4. 2. It corresponds to the well known soil water constant, at which various soil physical properties remarkably change (Fig. 6-8).
(3) In case of organic soil, the soil has much amount of adsorption water due to high activity of organic matter to water. Accordingly, the above mentioned dried regime with regard to the heat conductivity enlarges remarkably for the organic soil. Farthermore, this characteristic is strongly related to soil organic matter content.

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Soil is a non-uniform mixture consisting of gas, liquid and solid, which have quite different heat conductivites. The apparent heat conductivity of soil varies in a complicated manner with a wide range in accordance with the concentration and distribution of each phase.
In this paper, first, the effects of concentration and distribution on apparent heat conductivity are investigated using the Kunii & Smith model and the De Vries model, which are based on the packed or the dispersed system in regard to the uniform diameter particles. Though these two models are based on the quite different assumptions, they correspond well in two-phase system. It has been confi rmed that the heat conductivities calculated from these two models correspond to those measured under the air dry condition.
Next, in the three-phase system, it is indicated that there are three water content ranges in which the soil's water behaves in a different manner. Those ranges are distinguished by the two transient points; the water contents correspond to pF 4. 0 and pF 1. 8. These points have already been confirmed to be very important soil moisture constants in other physical phenomena.
If we take the different soil water contribution in each range into consideration, we can get a good estimation of soil heat conductivity in the three-phase system (unsaturated condition) by the combination of De Vries equation for the two phase system.

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The authors studied the stability of aggregates due to soil humus of volcanic and nonvolcanic ash organic soils, so-called Kuroboku soil, through analysis of water-stable aggregates, water holding capacity and penetrated depth by kneading.
The results obtained are summarized as follows:
1) The aggregates of each soil are reduced in size by kneading. However, coarse aggregates of nonvolcanic ash organic soil become smaller than those of volcanic ash organic soil.
2) The difference between volcanic and nonvolcanic ash organic soil are observed in the low region of soil moisture suction. In case of volcanic ash organic soil, pF increases due to kneading. On the other hand, in case of nonvolcanic ash organic soil, pF decreases as a result of kneading.
3) The penetrated depth by kneading of volcanic ash organic soil is much the same as that for nonkneading. But the penetrated depth due to kneading of nonvolcanic ash organic soil remarkably increases.
From the above results, we considered that coarse aggregates of volcanic ash organic soil are more stable than those of nonvolcanic ash organic soil.

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In this paper, the stability of aggregates was examined by the remolding of the Kuroboku soil, which can be found throughout the Kanto region and regarded as volcanic ash soil. The main results are as follows:
1) The model of the aggregate structure can be seen in Fig. 2. The unit of the aggregate structure is composed of a humus-clay composition.
2) The coarse aggregates sampled from fresh soil was easily changed to fine-aggregates by remolding (Fig. 3, 7), however, the fine-aggregates held their stability.
3) The relationship between water content and pF of the aggregates has been changed to a large extent. The aggregates (behavioral unit) are broken down to increase the specific surface absorbing water.

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The authors studied the characteristics of soil compaction for organo-volcanic ash soils (Kuroboku soils) from the side of the “Compaction Effect”. In this paper, the authors defined the “Compaction Effect” as the difference between the dry density by a standard compaction test and that by the vibration method.
The soil samples, four Kuroboku soils, two volcanic ash soils and one alluvial soil (non-volcanic ash soil) were used (Table 1).
The results obtained are summarized as follows:
(1) The compaction curves of the Kuroboku soils and volcanic ash soils do not have maximum dry density and optimum water content in the drying process, but in the wetting process the compaction curves do as well as non-volcanic ash soil (Figs. 2 and 3). Therefore, the compaction curvesof Kuroboku soils indicate hysteresis in the drying and wetting processes.
(2) The dry density of the compacted Kuroboku soils is smaller than that of the other soils (Fig. 4). This is due to the stability of the aggregated structure formed in Kuroboku soils against the compactive effort.
(3) Kuroboku soils and volcanic ash soils have their maximum “Compaction Effect” value in the drying process as well as the non-volcanic ash soil (Fig. 6). The maximum “Compaction Effect” appears at about pF 3 for Kuroboku soils and volcanic ash soils, while it appears at about pF 2 for non-volcanic ash soil (Fig. 10).
(4) The “Compaction Effect” of Kuroboku soils is smaller than that of the other soils (Fig. 6). And it decreases rapidily during the air-drying process in the range of pF 4-5, and finally approaches zero beyond pF 5 (Figs. 10 and 11). This can be explained by the remarkable aggregation of the soil particles (Fig. 11).

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The authors studied the hydraulic conductivity of compacted organo-volcanic ash soils (Kuroboku soil) in relation to the “Compaction Effect” in comparison with other soil samples.
Soil samples, three Kuroboku soils, two volcanic ash soil and one alluvial soil were used (Table 1).
The results obtained are summarized as follows:
(1) The undisturbed Kuroboku soils have large saturated hydraulic conductivity compared with the other soils (Table 1), but it decreases remarkably with soil compaction (Fig. 3).
(2) The hydraulic conductivity of compacted Kuroboku soils variy with the water content, and when the “Compaction Effect” is maximum, the compacted Kuroboku soils have its minimum saturated hydraulic conductivity in the drying and wetting processes (Fig. 4).
(3) The minimum saturated hydraulic conductivity of Kuroboku soils is larger in the wetting process than in the drying process (Fig. 4). This is explained by the formation of large pores that is due to the aggregation of the soil particles as a result of air-drying (Fig. 5).

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The authors studied the behavior of aggregates of organo-volcanic ash soil, so-called Kuroboku soil, through the analysis of water-stable aggregates, permeability and strength characteristics with compaction, as compared with non-organic soil.
The compaction curve of volcanic ash soil is unusual and factors determining it are deformation, shrinkage with drying and rupture of aggregates due to the compaction force. And these factors are defined by soil moisture at compaction. Difference between organo-volcanic ash soil and non-organic soil is also observed in the rupture of aggregates at high region of soil moisture suction, but especially in the deformation of aggregates at low region of soil moisture suction.
This was explained by the stability of the aggregates in Kuroboku soil.

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In this paper, the deformation behavior of the organic soil named Kuroboku soil under laterally constrained compression was discussed in relation to soil structure. The main results obtained are as follows:
(1) The difference in soil structure between the soils sampled was found. Kuroboku soil consisted o soft and porous aggregated particles, whereas sandy soil consisted mainly of hard and nonporous particles. Under maximum stress at σ1=4.9×105Pa (5 kgf/cm²), a 30% and 20% increase in Pd was observed for Kuroboku soil and sandy soil, respectively.
(2) The relationship between σ and ε in the process of σ: 0→σ_f at N=1 was varied due to the soil's structural characteristics. By modelling the deformation and arrangement of aggregated particles, it was estimated that the partly singurality in the σ-ε curve came from the failure of the aggregates.
(3) In the case of repeated compression at σ_f=4.9×10⁵ Pa (5 kgf/cm²) which is larger than the pc of aggregates, the σ-ε curves demonstrated an orderly pattern. Then, the loops of the curves progressed with an increase in ε. In case of a σ_f<<pc, the loops, however, showed a restriction tendency within the narrow limits of ε.
(4) Dissipated energy ω_d caused more of an influence on the deformation behavior under compression than conserved energy ω_e did. When ω_d≤ω_e, as mentioned above, the loops congressed and when ω_d approached zero, the loops restricted.

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