UNIT: 2
PHYSICAL PROPERTIES OF SOIL
The relative proportion of Solid, Liquid/Water and Air/void remaining in the soil sample represents the combination of 3 materials:- Solid, Liquid, and Gas. This type of combination is called Three Phase system of soil. The relative proportion may vary in any soil deposit. A schematic diagram of the three-phase system is given below:-
In the above figure, we can see that soil samples have solid particles of a certain quantity, water has a certain amount and Gas is in a certain amount by volume.
On the Right side, V = Vs + Vw + Va
Where V = volume of soil sample
Vs = Volume of solid in the soil sample
Vw = Volume of water in the soil sample
Va = Volume of air in the soil sample
On the left side, W = Ws + Ww + Wa
Where, W = Weight of sample
Ws = Weight of solid particle in the soil sample
Ww = Weight of water in the soil sample
Wa = Weight of air in soil sample = 0 (negligible)
Water Content
Water content is denoted by ‘w’. It is defined as the ratio of the weight of water to the weight of dry soil,
i.e., W = Ww / Wd
The dry weight used in the above expression is the weight of the soil when water contained in it is lost. The water is lost when the soil is heated to a temperature of 105° to 110° for 24 hours. This is a test for water content in soil done in the laboratory. It is called Oven Dry test.
Water content and determination of water content by oven dry method.
1. A sample of wet soil is placed into a container and weighed.
2. Now, the weight of the empty container is taken.
3. After that, the wet soil with the container is kept in the oven and the temperature is set to 105°C to 110°C.
4. The soil is left there for 24 hours.
5. The dry soil with the container is taken out and weighed.
The following calculations are made to determine water content.
1. Weight of empty container= W1
2. Weight of container with wet soil = W2
3. Weight of container with dry soil = W3
4. Weight of wet soil= W2 – W3
5. Weight of dry soil = W3 – W1
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Now, Water content of soil (W) = [(W2 – W3)/(W3 – W1)] X 100%
Physical proportions of soil
1. Void ratio:-
The void ratio is denoted by the symbol 'e'. It is defined as the ratio of volume of void to the volume of solid i.e. e = Vv / Vs = (Va+Vw) / Vs
2. Porosity:-
Porosity is denoted by a symbol η. It is denoted in percentage. it is defined as the ratio of the volume of the void to the total volume i.e. η = (Vv/ V) X 100%.
3. Degree of saturation:-
It is denoted by the symbol 'Sr'. It is also expressed in percentages. It is defined as the ratio of the volume of water to the volume of void i.e. Sr = (Vw/Vv) X 100%.
4. Density Index:-
It is denoted by 'I'. It is also called relative density or degree of density. It is defined as the ratio of the
difference between the void ratio of the soil in its loosest state and its natural void to the difference
between the void ratios in the loosest and densest states.
I = (Emax - e) / (Emax- Emin)
Where Emax = void ratio in the loosest state
Emin = void ratio in the densest state
e = natural void ratio of deposit