Daizo Karube

The role of pore water in the mechanical behavior of unsaturated soils

Deformation and failure of soils are governed by the stresses acting on the soil skeleton. The isotropic stress acting on the soil skeleton can be divided into two components. One is the stress component which is transmitted through the soil skeleton. This skeleton stress is influenced by the pore water (“bulk water”) in the soil. The other is the internal stress component which does not contribute to equilibrium with a given external force. The internal stress is induced by the capillary tension of meniscus water clinging to the contact point of soil particles and acts so as to connect the soil particles tightly. Therefore, in modeling the stress and strain relations for unsaturated soils, it is of much importance to quantitatively evaluate how the pore water exists in the soil. This paper discusses the role of pore water on the mechanical behaviour of the soil. In particular, the significance of the water retention curve is emphasized from a mechanical viewpoint. Essential features required in modeling of the constitutive relations for unsaturated soils are discussed and presented.

New Concept of Effective Stress in Unsaturated Soil and Its Proving Test

Triaxial compression tests, including anisotropic consolidation tests, were performed on compacted kaolin clay by an automatically controlled apparatus. Test results were analyzed in terms of suction and (σ - u a ), where σ is total normal stress and u a is pore air pressure, recognizing that when soil is saturated, u a becomes equal to u w , pore water pressure. When suction is kept constant, the relationships among the observed shear strength, stress-strain, and stress-water content change can be formulated in equations similar to those of saturated soil, in which suction can be regarded as a factor contributing to soil constants. When both suction and all-around pressure are varied, the plastic volumetric strain can be expressed by an equation in which (σ - u a ) is multiplied by a function of suction. Therefore suction can be regarded as a factor contributing to soil constants in this case too. These facts mean that the suction need not be regarded as one of the principal stress components such as (σ - u a ) in the stress-strain equations of unsaturated soil.