Extension of unsaturated soil mechanics and its applications

Atsushi Iizuka,Shun Nomura,Tomohide Takeyama,Hideki Ohta,Shinya Tachibana,Yuri Sugiyama

doi:10.1680/jgere.18.00004

Abstract

Although the soil, water and air coupling theory has been deductively derived from the three-phase mixture theory, assumptions and interpretations inherent in soil mechanics are also conveniently introduced in this theory. Mechanical properties of unsaturated soils are phenomenologically integrated into the constitutive equation for a deformation problem, with effective stress expressed as a combination of partial stresses acting on each phase of the mixture. The governing equations of two seepage problems describing the movements of pore water and pore air are formulated by a priori introduction of Darcy’s law. Consequently, the coupling theory consists of a deformation problem and two seepage problems, in which the soil-water retention characteristics play the role of a mediator connecting these problems. If the theoretical structure is clarified in this way, an extension of this theory is easily made. In this paper, the extension of the theory to each of four examples and their performance is demonstrated. The features of settlement caused by absorption of soil water due to vegetation; dissolution, advection and diffusion of substances in groundwater; desaturation and state change due to vaporisation of dissolved gas from pore water; and the liquefaction potential of unsaturated soil are presented.

Highlights

If the construction of earth structures is regarded as an artificial mechanical action using soil materials, unsaturated soil mechanics is the basic logical tool guiding the thinking process
According to Sheng (2011), the following three points should be considered in the theoretical framework for unsaturated soil mechanics: (a) volume change behaviour associated with suction or saturation change, (b) shear strength behaviour associated with suction or saturation change and (c) hydraulic behaviour associated with suction or saturation change
This paper presents some trials to extend the theory of unsaturated soil mechanics to applications to boundary value problems

Summary

Introduction

If the construction of earth structures is regarded as an artificial mechanical action using soil materials, unsaturated soil mechanics is the basic logical tool guiding the thinking process. After summarising the theoretical framework of unsaturated soil mechanics based on the three-phase mixture theory, unsaturated soil mechanics is understood as a coupled formulation of a deformation problem and two seepage problems This articulation highlights the constitutive equation in the deformation problem with the use of Darcy’s law in the dynamics of pore water and pore air flow. L− 1þ k e0 ln p0 xp0sat þ l−k ð1 þ e0ÞnE h*nE M evp in which l is the compression index; k is the swelling index; M is the critical state parameter introduced in the Cam-Clay model; p0 is the effective mean stress; p0sat is the effective mean stress at the pre-consolidation state under the fully saturated condition; e is the void ratio; e0 is the void ratio at the reference; evp is the plastic component of volumetric strain; h* is the generalised deviator stress ratio originally introduced by Sekiguchi and Ohta (1977) to describe the dilatancy behaviour due to deviator stresses; and nE is a parameter to adjust the rate of dilatancy development, as described by Ohno et al (2007, 2013) In this model, the yielding effective mean stress p0c at the unsaturated state is assumed as.

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Concluding remarks