Last 40 years, most of soil mechanics researches and practices based on saturated and dry soils mechanics. Unsaturated soil is barely been discussed in details, but last 20 years the development and understanding been gradually increase as there are growing concerns on subject.

An unsaturated soil has commonly been refereed to as a three-phase system containing solid, water and air phases. However, Fredlund & Rahardjo (1993) have postulated that unsaturated soil is a four phase material containing three phases as mentioned above along with the addition of a “contractile skin” which represents the interfacial surface tension between water and air.

The natural process of de-saturation or practices adopted in geotechnical engineering such as placement and compaction of fills are where the possibility of unsaturated soils to occur. Basically, soils become unsaturated in three different types which are evapo-transpiration from the ground surface, compacted soils and gassy soils.

To be continued…

Fig1: Evapo-transpiration from the ground surface