Geotechnical Engineering

DHARM

COMPOSITION OF SOIL TERMINOLOGY AND DEFINITIONS 13

When the soil voids are completely filled with water, the gaseous phase being absent, it
is said to be ‘fully saturated’ or merely ‘saturated’. When there is no water at all in the voids,
the voids will be full of air, the liquid phase being absent ; the soil is said to be dry. (It may be
noted that the dry condition is rare in nature and may be achieved in the laboratory through
oven-drying). In both these cases, the soil system reduces to a ‘two-phase’ one as shown in
Fig. 2.2 (a) and (b). These are merely special cases of the three-phase system.

Water

Solid

particles

(Soil)

Solid

particles

(Soil)

Air

(a) (b)

Fig. 2.2 (a) Saturated soil, (b) Dry soil represented as two-phase systems

2.2 Basic Terminology

A number of quantities or ratios are defined below, which constitute the basic terminology in
soil mechanics. The use of these quantities in predicting the engineering behaviour of soil will
be demonstrated in later chapters.

Water

Air

Solids

(Soil)

V

Vv

Va

Vw

Vs

Wa

Ww

Ws

Wv

W

Volume Weight

Va = Volume of air Wa = Weight of air (negligible or zero)

Vw = Volume of water Ww = Weight of water

Vv = Volume of voids Wv = Weight of material occupying void space

Vs = Volume of solids Ws = Weight of solids

V = Total volume of soil mass W = Total weight of solid mass

Wv = Ww

Fig. 2.3. Soil-phase diagram (volumes and weights of phases)