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Basic Principles of Unsaturated Soil Mechanics In nature, unsaturated soils exist and are essentially a three-phase system. In the void space of the three-phase system, there are two phases. In the air-water interface of the soil, surface tension may develop to provide the soil with additional strength. The additional strength protects the soil from failing. Unsaturated soil mechanics can be defined as the application of the laws of mechanics, hydraulics, interfacial physics, and physio-chemical mechanisms to engineering problems dealing with partially saturated soils. Phenomena prediction requires a mathematical representation of the given problem. The Basic Steps in Predicting Phenomena are:

Observe the problem physically
Invoke governing equations and then Identify state variables
Construct constitutive relationships and then Identify material constants
Predict the phenomena
Compare the predicted phenomenon to the physical observation in Step 1

Phases of Unsaturated Soils – Concept The unsaturated soil is a multi-phase system with pore-air, pore water, and solids as its three phases. The density of air acts as a driving force for the pore-air floor, and vapour phase transport in the soils. Water density can directly influence the physical and mechanical behaviour of unsaturated soils because many unsaturated soil parameters depend on the water density. The flow characteristics of water through soils can be improved with the low viscosity as the conductivity of the water increases. Phases of Unsaturated Soils Thermal equilibrium occurs between the different phases of unsaturated soils. None of the phases is pure. Some components of one phase can be found in the other phase. Humidity represents the state of thermodynamic equilibrium between air and water.  Vapour pressure depends on many factors such as temperature, purity of water, the interaction between solid and water, etc. Absolute humidity expresses the amount of water vapour present in the air. It is the ratio of the mass of water vapour to the unit volume of total air. It doesn’t provide much information about the outside ambient conditions. Relative humidity defines how close a vapour pressure is to its saturated vapour pressure under a given temperature.  Equilibrium between Air and Water Phases The experimental facilities that determine the state variables in soil mechanics are directly or indirectly dependent on the principles of phase change phenomenon. Cavitation is the formation of vapour bubbles within liquid at low-pressure regions. To overcome cavitation in soils, a back-pressure must be maintained.
The density of moist air decreases with an increase in relative humidity when temperature and atmospheric pressure are constant.
The density of moist air decreases with the increase in temperature when relative humidity and atmospheric pressure are constant.
The density of moist air increases with an increase in atmospheric pressure, and when relative humidity and temperature are constant.