Terms and Definitions in Soil Engineering

Various definitions of terms used in Geotechnical Soil Engineering are presented.

Volumetric Relationships of Soil

1. Void Ratio

Void ratio is the volume of voids to the volume of solids. It is denoted by ‘e’. 


It is expressed as a decimal.

2. Porosity

It is defined as the ratio of volume of voids to the total volume. It is denoted by ‘n’.


It is generally expressed as a percentage

1/n=V/Vv= (Vv+vs)/Vv

1/n= 1+ (1/e) = (1+e)/e

n=e/ (1+e) –> (a)

1/e= (1/n)-1= (1-n)/n

e=n/ (1-n) –> (b)

In equations (a) and (b), the porosity should be expressed as a ratio and not percentage.

3. Degree of saturation

The degree of saturation is the ratio of the volume of water to the volume of voids. It is denoted by ‘S’.


The degree of saturation generally expressed as a percentage. It is equal to zero when the soil is absolutely dry and 100% when the soil is fully saturated.

4. Percentage air voids

It is the ratio of volume of air to the total volume.

na= Va/V

It is also expressed as a percentage.

5. Air content

Air content is defined as the ratio of the volume of air to the volume of voids

ac= Va/Vv


na=n ac

6. Water content

The water content (w) is defined as the ratio of the mass of water to the mass of solids

w= Mw/Ms

It is also known as the moisture content (m). It is expressed as a percentage but used as a decimal in computation.

Volume Mass Relationship of Soils

7. Bulk Mass Density of Soil

The bulk mass density (?) is defined as the total mass (M) per unit volume (V)


8. Dry Mass Density

The dry mass density (?d) is defined as the mass of solids per unit total volume

?d= Ms/V

9. Saturated Mass Density

The saturated mass density (?sat) is the bulk density of the soil when it is fully saturated

?sat= Msat/V

10. Submerged Mass Density

When the soil exists below water, it is in a submerged condition. The submerged mass density (?’) of the soil is defined as the submerged mass per unit total volume.

?’= Msub/V

11. Mass Density of Solids

The mass density of solids (?s) is equal to the ratio of the mass of solids to the volume of solids


Volume-Weight Relationships of Soils

  2. DRY UNIT WEIGHT (?d) = Ws/V
  3. SATURATED UNIT WEIGHT (?sat) = Wsat/V
  4. SUBMERGED UNIT WEIGHT (?sub or ?’) = Wsub/V

12. Specific Gravity of Solids

The specific gravity of soil particles (G) is defined as the ratio of the mass of a given volume of solids to the mass of an equal volume of water at 4° C.

G = ?s/?w

The mass density of water ?w at 4°C is 1gm/ml, 1000 kg/m3 or 1 Mg/m3

Basic Soil Relationships

Sl NoRelationship in mass densityRelationship in unit weight
1n = e/(1+e)n = e/(1+e)
2e = n/(1-n)e = n/(1-n)
3na = n acna = n ac
4?= (G+Se)?w/(1+e)?= (G+Se) ? w/(1+e)
5?d = G?w/(1+e)?d = G ? w/(1+e)
6?sat = (G+e)?w/(1+e)?sat = (G+e) ? w/(1+e)
7?’ = (G-1)?w/(1+e)?’ = (G-1) ? w/(1+e)
8e = wG/se = wG/s
9?d = ?/(1+w)?d = ?/(1+w)
10?d = (1-na)G ?w/(1+wG)?d = (1-na)G ? w/(1+wG)