Soil mechanics is an important subject in the field of civil engineering. This subject is not only important in the design and analysis of natural and man-made geotechnical structures, but also in the development of infrastructure such as earthen dams, reservoirs etc. Most of the available text books on soil mechanics deal with the " mechanics of saturated soils " which is useful for understanding the engineering behavior of soils under the idealized condition by assuming a two-phase system (soil solids and water) . The soils that are present in the nature and in the man-made geotechnical- structures do experience changes in the pore constituents to a varying degree due to environmental factors . Changes in the moisture content due to environmental or climatic conditions and changes in the pore-fluid characteristics during the exposure to the environment are significant. The recent research developments reveal that the amount of water and pore-fluid characteristics in the pore matrix of the soils significantly control the behavior of soils and important for geotechnical engineering applications. Therefore, several aspects of classical soil mechanics are re-addressed and extended to deal with the partly saturated soils with the available knowledge on the physico-chemical behavior of unsaturated soils. This web-based course is dedicated to deal with the fundaments aspects of unsaturated soils and their engineering significance.

Unsaturated soil mechanics

A few decades ago, the mechanics of soils was understood to be the application of laws of mechanics and hydraulics to predict the engineering behavior of soils. The prominence of unsaturated soil mechanics in various geotechnical engineering applications refines this classical definition by introducing the physic-chemical mechanism. Unsaturated soils contain water-contents below their full-saturation levels. Therefore, in the unsaturated soils the pore spaces of the soil matrix are occupied by both water and the air as shown in the phase diagram, Fig. 1.1. The quantitative illustration of mass and volume of air, water, and solids with subscripts a, w, and s, respectively, are given in the same figure.

Fig. 1.1 Soil phase diagram