pF Concept Upon Softening and Hardening of Soil

Relation Between Engineering Properties of Soil and pF (II)

Abstract

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.