Three Phase System of Soil is nothing but the soil mass made up of solid, liquid and gaseous matter. The solid particles present are called as soil grains.
The free space or void present between the solid particles is generally filled partially with water and air.
Here liquid phase is generally referred as to water and gaseous phase as to air. Liquid phase fills the void partly or wholly and gaseous phase fill the voids which are not filled by water.
In general, these three phases are randomly mixed forming a complex soil structure or material. They do not occupy separate spaces for each phase.
Relative percentage and the arrangements of these materials are responsible for the soil mass properties. Hence it is very much important to study the volumetric and gravimetric proportions of solids, water and air in solid mass.
Generally we cannot separate the various phases in soil mass, but for better understanding the behaviour of soil mass it is necessary to separate them in three phase diagram as shown in below figure.
It is always conventional to write volume on the right side and mass (weight) on the left side of the three phase diagram.
Three Phase Diagram of Soil
Here,
W = total weight of soil mass
Wa = weight of air
Ww = weight of water
Ws = weight of soil
V = total soil mass volume
Va = volume of air
Vw = volume of water
Vv = volume of voids
Vs = volume of solids
Also Read : Soil Stabilization Methods & Necessity
Also Read : Bearing Capacity of Soil and It’s Importance
Types of Conditions in Three Phase System of Soil
Dry Condition :
This type of condition is also called as 2 phase system of dry soil. In this condition the combination of air and solid particles takes place.
Generally when the soil mass gets completely dry then the free spaces or voids in between are filled only with air (gaseous). In this case liquid phase remains absent.
Here,
Saturated Condition :
Now in this 2 phase system of saturated soil the voids are only filled with water.
Here,