Specific Gravity
Specific Gravity is the ratio of the weight of a given volume to the weight of standard fluid (water) of the same volume. It is represented as ‘G’.
There are two types of specific gravity.
Specific Gravity is the Unit less.
- True/Absolute Specific Gravity.
- Mass/Bulk/Apparent Specific Gravity.
True/Absolute Specific Gravity
It is the ratio of the weight of a solid of a given volume to the weight of a standard fluid (water) of the same volume. It is the Specific Gravity of a Solid. It is represented as ‘G’.
\(\mathrm{G} / \mathrm{GS}=\frac{\text { Weight of solid of given soil }}{\text { weight of standard fluid of same volume }}\)Standard Fluid=Water
\(\begin{array}{l}\mathrm{G} / \mathrm{Gs}=\frac{W s}{W w} \\
(\mathrm{~V}=\mathrm{Vw}) \\
\mathrm{G} / \mathrm{Gs}=\frac{W s}{W w}=\frac{W s \cdot V s}{V s \bullet W w} \\
\mathrm{G} / \mathrm{Gs}=\left(\frac{W s}{V s}\right)\left(\frac{V w}{W w}\right) \\
G=\frac{\gamma_{s}}{\gamma_{w}}=\frac{\rho_{s}}{\rho_{w}}
\end{array}\)
- It is the ratio of unit weight of solid (\(\gamma_{s}\)) to the unit weight of water(\(\gamma_{w}\)).
- It is the ratio of density of solid(\(\rho_{s}\)) to the density of water(\(\rho_{w}\)).
Mass/Bulk/Apparent Specific Gravity
It is the ratio of the weight of soil of a given volume to the weight of standard fluid (water) of the same volume. It is the Specific Gravity of Soil. It is represented as ‘Gm’.
\(\mathrm{Gm}=\frac{\text { Weight of soil of given soil }}{\text { weight of standard fluid of same volume }}\)Standard Fluid=Water
\(\begin{array}{l}\mathrm{G} \mathrm{m}=\frac{W}{W w} \\
(\mathrm{~V}=\mathrm{V} \mathrm{w}) \\
\mathrm{Gm}=\frac{W}{W w}=\frac{W \cdot V}{V \cdot W w}=\left(\frac{W}{V}\right)\left(\frac{V w}{W w}\right) \\
G m=\frac{\gamma}{\gamma_{w}}=\frac{\rho}{\rho_{w}}
\end{array}\)
- It is the ratio of the unit weight of soil(γ) to the unit weight of water(\(\gamma_{w}\)).
- It is the ratio of the density of soil (ρ)to the density of water(\(\rho_{w}\)).
Specific Gravity of Different Soil
- S- G of fine-grained ( Silt/ Clay) soil is generally more than coarse-grained (Sand/Gravel) soil.
- S- G of inorganic soil is generally more than organic soil.
- S- G increases with an increase in mineral content of the soil.
- As per Indian Standard “G” is reported at 27°C.
At any other temperature , \(\gamma_{s}=\text { constant. }\)
\(\begin{array}{l}\gamma_{s 27^{\cdot} C}=\gamma_{s T^{\bullet} C} \quad\left\{G=\frac{\gamma_{s}}{\gamma_{w}}\right\} \\
G_{27^{\circ} C} \gamma_{w 27^{*} C}=G_{T^{*} C} \gamma_{w T^{*} C}
\end{array}\)
If standard fluid is changed , \(\gamma_{s}=\text { constant. }\)
\(\begin{array}{l}\mathrm{G}^{\prime}=\frac{\gamma_{s}}{\gamma_{F}}, \mathrm{G}=\frac{\gamma_{s}}{\gamma_{w}} \\
\mathrm{G}^{\prime} \gamma_{F}=G \gamma_{w} \Rightarrow G^{\prime}=G\left(\frac{\gamma_{w}}{\gamma_{F}}\right)
\end{array}\) \(G^{\prime}=\frac{G}{G f}\)
For better understanding G’ is S-G of any materials (assume sand ) with respect to any fluid (assume kerosene). whereas G is S-G of the same materials (sand ) with respect to standard fluid(water). Gf is the S-G of fluid ( kerosene) with respect to standard fluid(water).
| Subject | Soil Mechanics |
| Unit | Soil Formation & Properties of Soil |
| Topic | Specific Gravity |
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