Geotechnical Engineering

DHARM

14 GEOTECHNICAL ENGINEERING

The general three-phase diagram for soil will help in understanding the terminology

and also in the development of more useful relationships between the various quantities. Con-

ventionally, the volumes of the phases are represented on the left-side of the phase-diagram,

while weights are represented on the right-side as shown in Fig. 2.3.

Porosity

‘Porosity’ of a soil mass is the ratio of the volume of voids to the total volume of the soil mass.

It is denoted by the letter symbol n and is commonly expressed as a percentage:

n =

V

V

v× 100 ...(Eq. 2.1)

Here Vv = Va + Vw ; V = Va + Vw + Vs

Void Ratio

‘Void ratio’ of a soil mass is defined as the ratio of the volume of voids to the volume of solids in

the soil mass. It is denoted by the letter symbol e and is generally expressed as a decimal

fraction :

e =

V

V

v

s

...(Eq. 2.2)

Here Vv = Va + Vw

‘Void ratio’ is used more than ‘Porosity’ in soil mechanics to characterise the natural

state of soil. This is for the reason that, in void ratio, the denominator, Vs, or volume of solids,

is supposed to be relatively constant under the application of pressure, while the numerator,

Vv, the volume of voids alone changes ; however, in the case of porosity, both the numerator Vv

and the denominator V change upon application of pressure.

Degree of Saturation

‘Degree of saturation’ of a soil mass is defined as the ratio of the volume of water in the voids
to the volume of voids. It is designated by the letter symbol S and is commonly expressed as a
percentage :

S =

V

V

w

v

× 100 ...(Eq. 2.3)

Here Vv = Va + Vw

For a fully saturated soil mass, Vw = Vv.

Therefore, for a saturated soil mass S = 100%.

For a dry soil mass, Vw is zero.

Therefore, for a perfectly dry soil sample S is zero.

In both these conditions, the soil is considered to be a two-phase system.

The degree of saturation is between zero and 100%, the soil mass being said to be ‘par-

tially’ saturated—the most common condition in nature.

Percent Air Voids

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

volume of the soil mass. It is denoted by the letter symbol na and is commonly expressed as a

percentage :