Geotechnical Engineering

2.1 Composition of Soil

Soil is a complex physical system. A mass of soil includes accumulated solid particles or soil
grains and the void spaces that exist between the particles. The void spaces may be partially or
completely filled with water or some other liquid. Void spaces not occupied by water or any
other liquid are filled with air or some other gas.
‘Phase’ means any homogeneous part of the system different from other parts of the
system and separated from them by abrupt transition. In other words, each physically or chemi-
cally different, homogeneous, and mechanically separable part of a system constitutes a dis-
tinct phase. Literally speaking, phase simply means appearance and is derived from Greek. A
system consisting of more than one phase is said to be heterogeneous.
Since the volume occupied by a soil mass may generally be expected to include material
in all the three states of matter—solid, liquid and gas, soil is, in general, referred to as a
“three-phase system”.
A soil mass as it exists in nature is a more or less random accumulation of soil particles,
water and air-filled spaces as shown in Fig. 2.1 (a). For purposes of analysis it is convenient to
represent this soil mass by a block diagram, called ‘Phase-diagram’, as shown in Fig. 2.1 (b). It
may be noted that the separation of solids from voids can only be imagined. The phase-dia-
gram provides a convenient means of developing the weight-volume relationship for a soil.

Water

Air

Solid

particles

(Soil)

Water around

the particles

and filling

up irregular

spaces between

the soil grains

Soil grains

Air in irregular

spaces between

soil grains

(a) (b)

Fig. 2.1 (a) Actual soil mass, (b) Representation of soil mass by phase diagram

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Chapter 2

COMPOSITION OF SOIL

TERMINOLOGY AND DEFINITIONS