General

While it is possible to have a range of values from 2.2 to 3.5, most soils have specific gravity from 2.60 to 2.80. Any values outside of this latter range should be viewed skeptically and a re-test should be made to verify the value. Where specific values are not available, the following can be assumed for local soils:

Sand and gravels: 2.65
Silts and clay: 2.78

By definition the equation for specific gravity is:

Equation Set 1.1

The specific gravity test uses a flask of known volume (500 ml typically) called a pycnometer. Based on the measured masses Mpw, Mpws and Ms, specific gravity can be calculated as below. Remember to be consistent with units (typically grams and milliliters). Pycnometer masses listed are without the mass of the flask itself.

Equation Set 1.2

However, the variation of the density of water with temperature must be accounted for between measuring pycnometer mass full of water and full of water and soil. In addition, the volume of the pycnometer itself changes with temperature. Two methods are commonly used to compensate:

1. The easiest way is to determine both masses at the same temperature - we'll do it this way!
2. In a production environment, the mass of water alone in the pycnometer can be calibrated experimentally with water over a range of temperature and plotted. Once this plot is made only the mass with soil and water and the temperature is necessary for each test.

Apparatus

500 ml pycnometer flask

Thermometer readable to 0.1 C

Balance sensitive to 0.01 gm

Spoon and Scoop

Drying pan larger than 500 ml

#4 sieve

Vacuum pump and hoses

Procedure

Procedure and data sheets [right-click | "save target as" Excel Office 2000

References

Al-Khafaji & Andersland (1992), pages 493-497.

Lambe (1951), 15-21.

List of Minerals Specific Gravities