Void Ratio, Porosity, Degree of Saturation

# Void ratio describes the relation between the free space between loose particles.

# Porosity describes the void directly at the surface of a particle.

Void ratio

Voids ratio (e) of a given soil sample is the ratio of the volume of voids to the volume of soil solids in the given soil mass.

e=Vv/Vs

Porosity

The porosity (n) of a given soil sample is the ratio of the volume of voids to the total volume of the given soil mass

n=Vv/V

The voids ratio e is generally expressed as a fraction, while the porosity n is expressed as a percentage and is, therefore, also referred to as percentage voids.

Figure (a) shows the soil element in terms of voids ratio e. If the volume of voids is taken equal to e, the volume of solids, by definition equation would be equal to 1, and the total volume equal to (1 + e).

Similarly, if the volume of the voids is taken equal to n in fig.(b), the total volume of the element will be 1(unity) and hence the volume of solids would be equal to (1 – n). From Fig. (a), we have by definition of porosity

n=Vv/V=e/1+e

Similarly, from Fig.(b), we get by definition of voids ratio,

e=Vv/Vs=n/1−n

Above two Equations  give two relations between n and e. After merging equations we get

n=e/1+e=e(1−n)

Degree of saturation

The degree of saturation S is defined as the ratio of the volume of water present in a given soil mass to the total volume of voids in it.Thus,

s=Vw/Vv

The degree of saturation is usually expressed as a percentage and is also known as percent saturation.

For a fully saturated sample Vw = Vv and hence S = 1.

For a perfectly dry sample, Vw = 0, and hence S = 0.

Percentage air voids

Percentage air voids (na) is defined as the ratio of the volume of air voids to the total volume of the soil mass and is expressed as percentage:

na=Va/V

Air content

The air content (ac) is defined as the ratio of volume of air voids to the volume of voids:

ac=Va/Vv

Since Va = Vv – Vw,

we getac=1−Vw/Vv=1−S

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