# Void Ratio and Porosity of Soil | Properties of Soil | Soil Mechanics

Contents

## Void Ratio

It is defined as the ratio of the volume of voids (Vv) to the volume of solids(Vs) present in a given soil mass. It is represented as ‘e’.

Voids Ratio (e)  = Volume of voids (Vv) / Volume of solids (Vs).

Voids Ratio is the Unit less.

Voids Ratio (e) >0.

$$e_{\min }$$=0.35,

$$e_{\min }$$=0.91.

⇒Volume of the void cannot be zero for soil as it is at least a two-phase system, however it can be zero for rock.

e = Vv / Vs .                    Add 1 on both sides.

e+1 = Vv/Vs +1.

e+1 = (Vv+Vs)/Vs.          We know that Vv+Vs= V (total volume of soil).

e+1 =V/Vs

Vs= V / (e+1)

⇒Void ratio can also be used to represent the degree of denseness of soil. (Denseness: Degree of packing.)

V1 = V2    V= Total volume of soil

e2 > e1.

Vv2 > Vv1.

Vs2 < Vs1.

(Denseness)2 < (Denseness)1.

Denseness ∝ 1/e.

⇒ Though the volume of one void is more for a coarse-grained but total volume of voids is more for fine-grained soil, hence void ratio & water content of fine-grained soil is more than coarse-grained soil. ( For comparison make sure the volume of fine-grained soil & volume of coarse-grained soil  are the same.)

⇒ Whenever two or more soil samples are mixed, the quantity ( volume& weight ) of solids will not change.

If , Soil (A) +Soil (B) = Soil (C).

1. Vs(A) + Vs (B) = Vs (C).
2. Ws(A) +Ws(B) = Ws(C).

Quantity of solid remains the same If soil is displaced from one to another location.

## Porosity (η)

It is defined as the ratio of the volume of voids(Vv) to the volume of soil (V)  present in the given soil mass. It is represented as ‘η’.

η= Vv / V *100

Porosity is the Unit less.

0% < η < 100%.

η = Vv / V

η = Vv/ (Vv+Vs) =  (Vv/Vv)/{(Vv/Vv)+(Vs/Vv)}.

η = 1/(1+1/e).

η = e / (1+e).

or e = η / (1-η).

⇒ Both e & Significant the water storage capacity of soil as they represent volume of voids in it.

⇒Volume of solid is comparatively stable in comparison to volume of soil as it does not change with change in volume of voids , hence engineering significance of  e>η .

NOTE:

• With increasing Void ratio & Porosity, water storing capacity is increased
• Voids Ratio of Coarse-grained soil is less and fine-grained soil is more of the same volume of soil.
• Porosity of Coarse-grained soil is less and fine-grained soil is more of the same volume of soil.
• The denseness of Coarse-grained soil is more and fine-grained soil is less of the same volume of soil.
• The water storing capacity of Coarse-grained soil is more and fine-grained soil is less of the same volume of soil.

 Subject Soil Mechanics Unit Soil Formation & Properties of Soil Topic Void Ratio and Porosity of Soil Next Topic Degree of Saturation, Air Content, Percentage Air Voids Previous Topic Soil Water & Types of Soil Water

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