This book presents a new theory of effective stresses in soils, which takes into account the internal stresses caused by the molecular, electrostatic, and structural mechanical forces. These forces exist in thin hydrate films of adsorbed water molecules at the contacts of structural elements, producing the so-called disjoining effect. They can be evaluated by incorporating theoretical achievements of molecular physics and colloidal chemistry. The novel theory described in this book considers not only external but also internal stresses and gives different results for effective forces compared with the Terzaghi theory widely applied in soil mechanics. The book provides equations for the actual effective stresses at the contacts of particles, thus improving the Terzaghi theory in physicochemical context.
The concept of effective stress and the effective stress equation is fundamental for establishing the theory of strength and the relationship of stress and strain in soil mechanics and poromechanics. However, up till now, the physical meaning of effective stress has not been explained clearly, and the theoretical basis of the effective stress equation has not been proposed. Researchers have not yet reached a common understanding of the feasibility of the concept of effective stress and effective stress equation for unsaturated soils. Effective Stress and Equilibrium Equation for Soil Mechanics discusses the definition of the soil skeleton at first and clarifies that the soil skeleton should include a fraction of pore water. When a free body of soil skeleton is taken to conduct internal force analysis, the stress on the surface of the free body has two parts: one is induced by pore fluid pressure that only includes normal stress; the other is produced by all the other external forces excluding pore fluid pressure. If the effective stress is defined as the soil skeleton stress due to all the external forces excluding pore fluid pressure, the effective stress equation can be easily obtained by the internal force equilibrium analysis. This equation reflects the relationship between the effective stress, total stress and pore fluid pressure, which does not change with the soil property. The effective stress equation of saturated soils and unsaturated soils is unified, i.e., o ̃=o ̃t –Seuw–(1–Se)ua. For multiphase porous medium, o ̃=o ̃t –u*,u*=Seiui(i=1,2,...,M). In this book, a theoretical formula of the coefficient of permeability for unsaturated soils is derived. The formula of the seepage force is modified based on the equilibrium differential equation of the pore water. The relationship between the effective stress and the shear strength and deformation of unsaturated soils is preliminarily verified. Finally, some possibly controversial problems are discussed to provide a better understanding of the role of the equilibrium equation and the concept of effective stress.
With the application of the effective stress concept, the strength and volumetric behavior of saturated materials was clearly understood. For the case of unsaturated materials, a universally accepted effective stresses equation is still under debate. However, the use of the effective stress concept to develop constitutive models for unsaturated soils is becoming increasingly popular not only because the hydro-mechanical coupling observed in these materials is implicit in the formulation but also because simpler and more precise models can be established. Towards A Unified Soil Mechanics Theory demonstrates that the same strength and volume change equations used in saturated soil mechanics can be used for the case of unsaturated soils. In other words, the constitutive models developed for saturated soils can be used for unsaturated materials once the effective stress equation has been defined. In this book, an analytical equation for the effective stress for unsaturated soils is established. This equation requires the knowledge of the superficial area of solids affected by the capillary phenomenon. In other words, we need to know how water distributes in the pores of the soil. This distribution can be modeled using a solid-porous model built on a regular network. However, the size of the network required to correctly simulate the structure of a small soil sample cannot be managed with a common PC. For that reason, a probabilistic porous-solid model is developed. This model uses the pore size distribution in the form of a probabilistic function which in addition to the Laplace equation and the principle of continuity can be expressed as the probability of a pore of certain size to be filled or dry at suction during a wetting or drying path, respectively. In this way, the soil-water retention curves can be simulated and the effective stress at any suction during wetting or drying processes can be determined. Based on this approach, it is shown that unsaturated soils behave under the same principles for strength and volumetric behavior as saturated soils. This revised edition brings additional information about an elastoplastic framework for expansive soils, hydro-mechanical coupling and a fully coupled model. The additional chapters also cover the experimental parameters used to derive the models. An updated set of references and a list of abbreviations is also included in this edition. Towards A Unified Soil Mechanics Theory paves the way for a universal theory of soil mechanics. The volume will be a valuable reference to civil engineers, earth scientists and hydrologists interested in soil mechanics at both academic and professional levels.
Soil is fundamentally a multi-phase material – consisting of solid particles, water and air. In soil mechanics and geotechnical engineering it is widely treated as an elastic, elastoplastic or visco-elastoplastic material, and consequently regarded as a continuum body. However, this book explores an alternative approach, considering soil as a multi-phase and discrete material and applying basic Newtonian mechanics rather than analytical mechanics. It applies microscopic models to the solid phase and fluid phases, and then introduces probability theory and statistics to derive average physical quantities which correspond to the soil‘s macroscopic physical properties such as void ratio and water content. This book is particularly focused on the mechanical behaviour of dry, partially saturated and full saturated sandy soil, as much of the physicochemical microscopic characteristic of clayey soil is still not clear. It explores the inter-particle forces at the point of contact of soil particles and the resultant inter-particle stresses, instead of the total stress and effective stress which are studied in mainstream soil mechanics. Deformation and strength behaviour, soil-water characteristic curves, and permeability coefficients of water and air are then derived simply from grain size distribution, soil particle density, void ratio and water content. A useful reference for consultants, professional engineers, researchers and public sector organisations involved in unsaturated soil tests. Advanced undergraduate and postgraduate students on Unsaturated Soil Mechanics courses will also find it a valuable text to study.
"With the application of the effective stress concept, the strength and volumetric behavior of saturated materials was clearly understood. For the case of unsaturated materials, a universally accepted effective stresses equation is still under debate. Howe"
Fundamentals of Continuum Mechanics of Soils provides a long-needed general scheme for the study of the important yet problematic material of soil. It closes the gap between two disciplines, soil mechanics and con- tinuum mechanics, showing that the familiar concepts of soil mechanics evolve directly from continuum mechanics. It confirms concepts such as pore pressures, cohesion and dependence of the shear stress on consolidation, and rejects the view that continuum mechanics cannot be applied to a material such as soil. The general concepts of continuum mechanics, field equations and constitutive equations are discussed. It is shown how the theory of mixtures evolves from these equations and how, along with energetics and irrevers- ible thermodynamics, it can be applied to soils. The discussion also sheds light on some aspects of mechanics of materials, especially compressible materials. Examples are the introduction of the Hencky measure of strain, the requirement of dual constitutive equations, and the dependence of the spent internal energy on the stored internal energy. Researchers in engineering mechanics and material sciences may find that the results of experiments on soils can be generalized and extended to other materials. The book is a reference text for students familiar with the fundamentals of mechanics, for scholars of soil engineering, and for soil scientists. It is also suitable as an advanced undergraduate course in soil mechanics.
This publication is an assemblage of selected papers that have been authored or co-authored by D.G. Fredlund. The substance of these papers documents the milestones of both the science of unsaturated soil mechanincs and the career of the author during his tenure as a faculty member in the Department of Civil Engineering at the University of Saskatchewan, Saskatoon, Canada.
Unsaturated Soils: Research and Applications contains 247 papers presented at 6th International Conference on Unsaturated Soils (UNSAT2014, Sydney, Australia, 2-4 July 2014). The two volumes provide an overview of recent experimental and theoretical advances in a wide variety of topics related to unsaturated soil mechanics:- Unsaturated Soil Behavi
