Specific Gravity of Soil

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.

                                   $G=\frac{{{\gamma }_{s}}}{{{\gamma }_{w}}}$

Determination of Specific Gravity of Soil Solids by Pycnometer Method

A pycnometer is a glass jar of about 1-litre capacity and fitted with a brass conical cap. This cover has a small hole of 6 mm diameter at its apex.

1. Weigh the empty pycnometer, say ${W}_{1}$.
2. Fill the pycnometer with dry soil and weigh it, say ${W}_{2}$.
3. The remaining volume of pycnometer is gradually filled with distilled water and weighed, say ${W}_{3}$. The entrapped air should be removed either by slight heating or vigorous shaking or by applying vacuum.
4. Empty the bottle and clean it thoroughly and filled with distilled water and weighed, say ${W}_{4}$.

                              $G=\frac{\text{Weight of solids}}{\text{Weight of equivalent volume of water}}$

                              $G=\frac{{{W}_{2}}-{{W}_{1}}}{({{W}_{4}}-{{W}_{1}})-({{W}_{3}}-{{W}_{2}})}$

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