This is a summary of collaboration conducted by 10 coworkers on engineering behaviors of clayey soils under external forces, such as deformation due to compression, softening and hardening due to remolding, contraction due to drying, changes of consistency due to airdrying, compaction, water permeability and so on.
Materials mainly used were the Kanto Loam, a kind of allophane volcanic-ash soils, as well as various kinds of non-volcanic ash soils as controls.
Previous soil mechanics were lacking in researching engineering behaviours of soils based on physical properties of soils, which seems to have led recent soil mechanics to a dead lock. With such view in mind, the authors intended to clarify engineering behaviors of soils, in terms of their physical properties, especially of pF and rheology. They wish this paper serves to make a mood of constructing a new aspect in soil engineering, combining soil mechanics with soil physics.

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The energy index pF of the soil water, which has been established by Schofield and the followers, plays an important role in interpreting the structure and the behaviour of the soil. In this paper, it is noted that there are four important factors determining the energy states of the soil water, the capillary action, the osmotic pressure, the interaction between the soil particles and the water, and the intersticial pressure. The pF-water content relation was calculated under the simplified assumption of the dispersed uniform particles. Thus it was revealed that the pF-water content relation is greatly affected by the degree of dispersion of the soil particles or the structure of the soil. The Kanto Loam has a well dispersed and close-packed structure, because they mainly consist of fine particles and have highly active surfaces. The water confined in the well-dispersed soil aggregates is made free by the disturbance. The existence of such confined water in a large amount is one of the characteristics of the Kanto Loam.

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The soil is considered as a colloidal clay system of visco-elastic body. Force applied to the system causes the change of chemical potential of water in soil, and when the free energy overcomes the barrier, the visco-elastic body becomes a Newtonian liquid. pF of the system is equal to the magnitude of barrier (the yield value) required to break the particle links.
The hydrophilic and the hydrophobic colloid correspond to the visco-elastic gel and the viscoelastic sol respectively. This concept can be refered to as the flow condition of a colloidal system. The results are summarized as follows.
1) The yield value corresponds to the transition point from solid to liquid. Under the stress smaller than the yield value, the soil behaves as a Voigt body, and under the larger stress, as a Maxwell liquid or a Newtonian liquid.
2) The behaviour of hydrophoilic olloid mainly depends on the entropy elasticity, and that of hydrophobic colloid on the energy elasticity.
3) The negative shear rate thixotropy flow is a phenomenon of rate process. When the rate of hydration is greater than that of dehydration in course of the decrease in stress. The hydrophobic property changes to the hydrophilic one, and the system behaves as a visco elastic body. The entropy production accelerates the rate of structure-building in the colloidal clay system.

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Terzaghi (1925) treated the elasticity of soil as pseudo high elasticity that was owed to the “honey-comb” structure. We, now, can put that Terzaghi's model is the first approximation which corresponds to the Hooke-body. From the rheological point of view the behaviour of soil is undoubtedly visco-elastic.
Soil can be treated as Burgers body in mechanical behaviourand “honey-comb structure made of gel”, which is composed of the Maxwell model and the Voigt model linked in series. In this treatment both initial set and strain hardening are however omitted from the behaviour of soil.
The unit system in soil is gel which is regarded as hydrophilic colloid. Gel behaves as the Voigt model and is entropy-elastic from the view point of thermodynamics. On the other hand, the Maxwell model corresponds to the porous structure of soil. The dashpot (η_M) of the Maxwell model linked in series to the Voigt model may be caused by pores in soil in the manner of the Hole Theory of liquid. And the spring (G_M) of the Maxwell model that represents instantantaneous elasticity depends on the honey-comb structure consisted of gel.
Initial set (ε_I) results from the compression by diagonal sum of stress tensor and the amount of initial set (ε_I) is one third of volume strain εV. Strain hardening occurs on all the model constants. Particularly it has much effect on η_M, because of the break-down of the pore structure,
The yiela value τ_f of the Burgers body can be determined by the pF value, and log (τ_f) is nearly equal to the pF value.

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The soil-water systems were classified on the basis of the pF value by SUDO et al (1, 2) and a few transition points for mechanical properties were determined.
In this paper we studied on these transition points from the viewpoint of the behavior of soil as the Burgers model (13, 6) and considered on the significance of liquid limit and on the mechanism of softening or hardening of soil.
On the basis of the pF scale, transition points are determined as follows:
(1) At pF=-1.5 (State of sediment in water) stands second-order transition point in relation to yield value θ. viscosity η and elasticity G.
(2) At pF=1.5 (state of liquid limit) stands the first-order transition point in relation to θ, η, G.
On the L. L. test in case of ω>ωL, Bingham flow occurs. In case of ω<ωL, flow according to ηM-element on the Burgers model occurs, then hardening (Fig. 5) is observed.
Softening is caused by the mobilization of pore water due to the breakdown of the structure of kinetic units in case of ω>ωL, and the plastic viscosity η_ρl is lowered. In case of ω<ωL, softening occurs in the only process in which the stress progressively increased.
The process of hardening is concerned with non-equilibrium i. e. rate process in the soil-water system. If pF_s of water inside kinetic units is higher than pF_ω of pore water or bulk water, pore (or bulk) water will be immobilized with the increase of hydration, and hardening occurs.
The thixotropy (softening) and hardening of soils were, for the first time, explained theoretically in this paper, according to both the classification of the states of the soil-water system and the introduction of the concept of transition points.

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Previously, the investigation of shrinkage of soil were focussed only on the measurement of the amount of shrinkage.
The author introduced the pF concept to the investigation of shrinkage and clarified the importance of the rate process.
Tests were conducted with the Kanto Loam valcanic ash soil and the paddy field soil. Results are as follows:-
1. The power that causes shrinkage is indicated by the pF value. The yield value of soil is closely related to the critical pF value when shrinkage is observed for the first time.
2. The amount of shrinkage is affected by the rate of drying.
3. The shrinkage of Tachikawa Loam (the Kanto Loam volcanic ash soil) continues up to the high pF value. This fact is due to the soil structure and its affinity to water.
4. The shrinkage of soil is closely related to the one axial strengh, the viscosity and the rigidity. The investigation of shrinkage of soil should be developed further on the basis of the relaxation process.

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The Kanto Loam (volcanic ash soil) has many physical characteristics, from which result particular engineering properties of the soil.
(1) The compaction curve of the Kantii Loam is also unusual, but its types can be expounded by several factors (shrinkage, expansion, lubricating action of water, rupture of soil particles etc.).
(2) The permeability decreases with decreasing of dry bulk density if the Kanto Loam is compacted in drying process. Therefore the sample compacted nearly at the natural moisture content has the lowest permeability notwithstanding small bulk density. This phenomenon can be explained, from the structual properties, that is the fact that there are no large pores and much non-free water.

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(1) The Kanto Loam has the minimum permeability if it is compacted at natural moisture content (cf. the report I). One of these causes is the fact that the Kanto Loam fresh soil has much nonfree water.
(2) Because of this reason, the saturated permeability (k_sat) of compacted fresh soil changes with hydraulic gradient. The value of k_sat is constant in the range where hydraulic gradint is small, but in the range of great gradient, the value of k_sat increases proportionaly with a hydraulic gradient, and beyond the latter range k_sat decreases.
But k_sat is constant in the case where oven dry soil is compacted. This phenomenon can be explained by the hypothesis that the volume of free water (being able to flow) and non free water (being unable to flow) change with hydraulic gradient.
(3) This phenomenon can be modelled with the combination of two formulae by the Hagen-Poiseuille and the Buckigham.

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Atterberg's limit with the fresh soil is differentfrom that with the air-dried soil.
The authors investigated the influence of air-drying uponAtterberg's limit with several kinds of soil.
Results are as follows:-
1. The Kanto loam volcanic ash soil is located separately from the non-volcanic ash soil in the plasticity map.
2. Compared with the surface soil, the effect of air-dryingupon consitency is severer with the deep soil.
3. The younger is the age of soil formation, the severer isthe influence of air-drying upon consistency of the Kanto Loam volcanic ash soil.
Generally speaking, the liquid limit and the plastic limitare decreased by air-drying. These facts result from the decrease of affinity to water. Accordingly the preparation and testing of the hydrophilic soil in our country should be reconsidered.

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1) 土壌の示す剛性率Gは測定降伏値θ よりも大きい。水分含量が増せばG=θになる。ダイラタンシを示す土壌ではGがθよりも小さい場合がみられる。
2) ビンガム領域で剛性率Gと容積分率濃度Φ とはG=G_ste^{A₂ (φ-φ₀)} で示される。
3) 降伏値の対数logθ と遠心pF値とはチクソトロピ効果を補正すればほぼ等しい。すなわち土壌強度がpFとその土壌のチクソトロピ係数kできまる。
α・logθ+logk=pF (α=1)
4) LLにおけるlogθ', pFは約1.5である。PLではlogθ'は3.0 (前後) である場合が多いからpF'はほぼ3.0であると考えられる。θ'は補正降伏値
ここでは粘性についてはふれなかった。またチクソトロピの測定についても検討すべきものが多い。これらについては別報6) にゆずる。なお本研究は東大山崎教授を中心として行なわれ, 主として文部省科学研究費によった。

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The hardening and softening of the soils by kneading is a basic study of soil engineering, concerning with earth works. The soils are not always softened by kneading, but some soils, one of alluvial clay, and the Kanto Loam are hardened. These phenomena can be investigated with the change of water suction of soil (pF). This suction relates to the rigidity which determines a factor of yield value discussed on the previous paper. The mechanisms of hardening and softening of the soils are grouped with 5 factors, that is:
1 The capillary tension between kinetic units is decreased, as the units are aproached by the force applied. In this case, pF is dropped irreversibly (softening irreversible).
2. The kinetic units are aparted, and pF is increased (dilatancy).
3. The water in the structure joined with pore water by breakdown of soil structure by kneading, and pF is dropped (softening irr.)
4. The kinetic unit is broken down to increase the specific surface absorbing water (hardeningirr.)
5. Immobilized water absorbed by the soil particles is freed with the increment of free energy in the soil water system, in result of external force (thixotropy).
The Kanto Loam shows the softening, when the kneading is not so great as to break down the soil structure (factor 1, 5), but otherwise hardening is observed (factor, 2, 4). The factors are not independent each ether.
The Kanto Loam is generally (believed to be) devided into three layers, which are the Tama, the Musashino, and the Tachikawa in order of age.
In earth work by bulldozer, the pF of the Kanto Loam is decreased in the undisturbed zone, but contrarily increased in the banking. The increment of pF is notable, when the Tachikawa Loam, which containes allophane in the clay fraction is set after kneading. The Tama Loam, which containes halloisite, with a large amount of immobilized water, is softened irreversibly. The Musashino is similar to the Tama in consistency.
These properties, that. the kneading give rise to change the suction in soil water system, must be considered in the pedological engineering.

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Coulomb-Mohr's yield condition (IV-theory) had been abandoned in former days, but in the field of soil mechanics it has been considered to be valied up to the present. To-day's yield condition is based on Mises-Hencky's theory (V-theory). This theory was generalized by Schleicher-Mises et al, as the equation σ_oct= f (σ_oct) which can be applied to the plain twodimensional case, where σ2= σ8, to get the same result as shown by Rendulic (1937). From these points the authors obtained the yield condition poet (σ1, σ2, σ3) = f (σ_oct).
On the basis of IV-theory the yield condition is determine only by the principal shear stress σf. However, from the view point of V-theory, it is to be put that σf is proportional to τ_oct, and τ(= (τ1+ τ3)/ 2) is nearly equal to the diagonal sum of the stress tensor (=τ1+τ2+τ3=τ_oct). In this way coulomb-Mohr's yield condition can be approximately reduced to Mises-Hencky's condition.
In the state of the stress tensor, r_oct is lowered according to the decrease of the chemical potential μ of the soil water that is due to σ_oct. On the other hand, σ_oct is made higher by the compacting action of τ_oct. If the soil structure is massive and saturated with water, the yield value τf can be determined only the pF value of the soil water. Here, log τf is nearly equal to pF. Dilatancy in the case of over-consolidated soil is due to the closest parking of the soil colloid in the gel-state, where micelle water is thinner than that of normal consolidated soil-gel and comes to have the higher pF value.

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The Kanto Loam is the volcanic ash soil distributed over Kanto District, Japan, and it is classified into the next four formations: Tachikawa Loam, Musashino Loam, Shimosueyoshi Loam and Tama Loam in the order from the upper to the lower.
(1) The physical properties of the Kanto. Loam are characteristic. The remarkable characteristics are as follows; for Tachikawa loam porosity is about 80%, natural moisture content is 150%, the moisture content at pF 4.2 by centrifugal method and Atterberg limit are higher than other soils. The upper formation of the Kanto Loam has the distingished values except for surface layer and the physical properties are classified into Tachikawa Loam, Musashino Loam and Tama loam (Shimosueyoshi Loam was not examined)
(2) The distingished physical properties among the formations of the Kanto Loam are the differences of Atterberg limit and centrifugal pF 4.2 water content between fresh soil and air dried soil. By comparing the difference, we can know which formation the sample belongs to, and how the sample has been dried.
(3) On the other hand, by knowing the formation of soil its physical and engineering properties can be estimated. There so-called Pedological Engineering comes out as a research method.

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The Kanto Loam volcanic ash soil has abnormal engineering properties. Compared with non-volcanic ash soil, earth moving and compaction of the KantO Loam is more difficult. The authors investigated the relationships between the soil condition and the traction performance of tractors in the field and in the laboratory.
Results are as follows:-
1. The strength of the KantO Loam is decreased extremely by disturbance, which inevitably results in the decrease of the traction performance of tractors.
2. The mechanism of generating the power of traction is clarified.
3. The compaction of the fill up ground of the Kanto Loam is very difficult. This fact should be considered in planning the earth moving.
4. The engineering properties of soil are closely related to the history of the soil formation. We must create the field of the engineering pedology for the improvement of the earth work.

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Agricultural roads of the Kanto district are mainly composed of the Kanto Loam which is widely distributed on the surface layer. The chief ingredient of the Kanto Loam is Allophane from the clay mineralogical viewpoint. Allophane can easily hold a large quantity of water, because it has no crystalline structure and is electrically unsaturated. Once the Kanto Loam with higher water content is remoulded. by wheels, its engineering properties become weak and then it's trafficability decreases. In this paper, at first the author studied the kneading number and reduction of strength that have most important relations to its trafficability, and further studied the methods of soil stabilization treatment as a countermeasure to improve its. trafficability.
(1) As the kneading number increases, the elastic properties of the Kanto Loam extremely decreases, so Sr, qu, C, E reach the inflection point in about 170-200 numbers, and then it enters the plastic region after that.
(2) The values of E and qu decrease about 1/5-1/6 and 1/10 times respectively, after theundisturbed samples are perfectly remoulded.
(3) The recovery of strength due to thixotropic phenomenon is nearly proportional to the logarithm of curing days.
(4) Ca (OH) ₂ 10%+CaSO₄·2H₂O 5-10% is more effective than Cement 10%+CaSO₄·2H₂O 1.5% +CaCl₂ 1.5%, because Ca (OH) ₂ does no injury to the organophilic effect, and it can be expected to show the cementation during a long period due to a pozzolanic action.

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