Geotechnical Engineering

DHARM

COMPOSITION OF SOIL TERMINOLOGY AND DEFINITIONS 15

na =

v

V

a× 100 ...(Eq. 2.4)

Air Content

‘Air content’ of a soil mass is defined as the ratio of the volume of air voids to the total volume

of voids. It is designated by the letter symbol ac and is commonly expressed as a percentage :

ac =

V

V

a

v

× 100 ...(Eq. 2.5)

Water (Moisture) Content

‘Water content’ or ‘Moisture content’ of a soil mass is defined as the ratio of the weight of water

to the weight of solids (dry weight) of the soil mass. It is denoted by the letter symbol w and is

commonly expressed as a percentage :

w =

W

WW

w

sd()or

× 100 ...(Eq. 2.6)

=

()WW

W

d

d

− × 100

...[Eq. 2.6(a)]

In the field of Geology, water content is defined as the ratio of weight of water to the

total weight of soil mass ; this difference has to be borne in mind.

For the purpose of the above definitions, only the free water in the pore spaces or voids

is considered. The significance of this statement will be understood as the reader goes through

the later chapters.

Bulk (Mass) Unit Weight

‘Bulk unit weight’ or ‘Mass unit weight’ of a soil mass is defined as the weight per unit volume
of the soil mass. It is denoted by the letter symbol γ.

Hence, γ = W/V ...(Eq. 2.7)

Here W = Ww + Ws

and V = Va + Vw + Vs

The term ‘density’ is loosely used for ‘unit weight’ in soil mechanics, although, strictly

speaking, density means the mass per unit volume and not weight.

Unit Weight of Solids

‘Unit weight of solids’ is the weight of soil solids per unit volume of solids alone. It is also

sometimes called the ‘absolute unit weight’ of a soil. It is denoted by the letter symbol γs:

γs =

W

V

s

s

...(Eq. 2.8)

Unit Weight of Water

‘Unit weight of water’ is the weight per unit volume of water. It is denoted by the letter symbol

γw :

γw =

W

V

w

w

...(Eq. 2.9)

It should be noted that the unit weight of water varies in a small range with tempera-

ture. It has a convenient value at 4°C, which is the standard temperature for this purpose. γo is

the symbol used to denote the unit weight of water at 4°C.