Three phase system of soil :-
A soil mass is a three phase system consisting of solid (soil grains
particles), water and air.
The void (i.e. "empty") space in the soil mass may be filled with air, with
water or partly with air and partly with water.
Three phase diagram is also known as Block diagram.
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| Figure 1: Constituents of Soil |
In general, a soil mass consists of solid particles, water and air.
The three constituents are blended together to form a complex soil structure
or material (Figure 1). 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 (relating to measurement by weight) 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 Figure 2.
In a 3-phase diagram, it is conventional to write volumes on the left side and the weight (or mass) on the right side (Figure 2).
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| Figure 2: Three Phase Diagram of Soil |
In Figure 2 :-
V = Total volume of soil = Va + Vw + Vs = Vv + Vs
Va = Volume of air
Vw = Volume of water
Vs = Volume of soil solid
W = Total weight of soil = Wa + Ww + Ws = Ww + Ws
Wa = Weight of air (weight of air is negligible = 0)
Ww = Weight of water
Ws = Weight of soil solid
Vv = Volume of voids = Va + Vw
Two phase system of soil :-
Although the soil is a three-phase system, it becomes a two-phase system in
the following two cases :-
(1) When the soil is absolutely dry, the water phase disappears (Figure 3).
(2) When the soil is fully saturated, there is no air phase (Figure 4).
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Figure 3: two phase diagram of soil |
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| Figure 4: two phase system of soil |
In dry condition of soil :-
Dry condition of soil generally occur when the soil mass gets completely dry
then the voids in between the soil are filled only with air(gaseous).
Va = Vv
Total Weight = Ws
Total Volume = Vs + Va
In fully saturated condition of soil:-
Vv = vw
Total Weight = Ws + Ww
Total Volume = Vs + Vw
FAQ :
1. What is phase diagram in soil?
Ans: The phase diagram is a simple, diagrammatic representation of real soil.
2. What is the use of phase diagram in soil?
Ans: It is extremely useful for studying the various terms used in soil engineering (soil mechanics or geotechnical engineering) and their inter-relationships.