Geotechnical Engineering

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COMPOSITION OF SOIL TERMINOLOGY AND DEFINITIONS 17

Mass Specific Gravity
The ‘Mass specific gravity’ of a soil may be defined as the ratio of mass or bulk unit weight of
soil to the unit weight of water at the standard temperature (4°C). This is denoted by the letter
symbol Gm and is given by :
Gm =

γ
γo

...(Eq. 2.14)

This is also referred to as ‘bulk specific gravity’ or ‘apparent specific gravity’.
Specific Gravity of Solids
The ‘specific gravity of soil solids’ is defined as the ratio of the unit weight of solids (absolute
unit weight of soil) to the unit weight of water at the standard temperature (4°C). This is
denoted by the letter symbol G and is given by :


G =

γ
γ

s
o

...(Eq. 2.15)

This is also known as ‘Absolute specific gravity’ and, in fact, more popularly as ‘Grain
Specific Gravity’. Since this is relatively constant value for a given soil, it enters into many
computations in the field of soil mechanics.


Specific Gravity of Water
‘Specific gravity of water’ is defined as the ratio of the unit weight of water to the unit weight
of water at the standard temperature (4°C). It is denoted by the letter symbol, Gw and is given
by :


Gw =

γ
γ

w
o

...(Eq. 2.16)

Since the variation of the unit weight of water with temperature is small, this value is
very nearly unity, and in practice is taken as such.
In view of this observation, γo in Eqs. 2.14 and 2.15 is generally substituted by γw, with-
out affecting the results in any significant manner.


Water

Solids

V

Vv

Va

Vw

Vs

W0a»

W=V.wwwg

W = V. = V .G.sssswgg

Air

W = V. = V.G .ggmw

Volume Weight

Fig. 2.4. Soil phase diagram showing additional equivalents on the weight side

2.3 Certain Important Relationships


In view of foregoing definitions, the soil phase diagram may be shown as in Fig. 2.4, with
additional equivalents on the weight side.
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