Measurement of Specific Gravity of Soils

Introduction

Soil is a three-phase material that consists of solid particles and voids which are filled with water and air. The specific gravity (G_S) of a soil refers to the ratio of the solid particles’ unit weight to the unit weight of water. G_S should not be confused with the soil density since it is a dimensionless unit and expresses the ratio of two particular densities.

G_S is a significant parameter of soil mechanics since it can be associated with the soil’s mineral composition and weathering. It is also used to derive several important soil parameters such as the porosity, the dry and saturated density and the degree of saturation. 

Testing Equipment

To derive the Specific Gravity of a soil, the following equipment is needed:

1. Volumetric flask marked with a thin ring at a specific point of its neck (graduation mark)
2. Balance sensitive to 0.01g
3. Vacuum pump
4. Distilled water
5. Funnel
6. Spatula
7. Drying oven

Testing procedure

The Specific Gravity is computed as the ratio of the weight in air of a given volume of soil particles at a stated temperature to the weight in air of an equal volume of distilled water at the same temperature. The procedure that is followed towards that goal is the following:

1. Weigh the empty and clean volumetric flask (W₁).
2. Fill the flask with distilled water up to the graduation mark.
3. Clean and dry the inside (above the water level) and the outer part of the flask and weigh it (W₂).
4. Empty and dry the Flask
5. Weigh around 50 grams of soil material.
6. Use the funnel to carefully place the soil into the flask and weigh it (W₃).
7. Fill around 2/3 of the flask with distilled water.
8. Use a vacuum pump to gradually apply vacuum and remove the entrapped air while spinning the flask to remove the air bubbles. The procedure should last for about 2-3 minutes for sands and 10-15 minutes for clays.
9. Remove the vacuum, clean and dry the flask and add distilled water up to the mark.
10. Weigh the flask (W₄).
11. Use the thermometer to derive the temperature of the water.

Calculations

To better understand how the Specific Gravity is calculated, the aforementioned measured quantities are presented in Figure 1.

Based on the temperature of the water that was recorded in the last step, the density of the distilled water ρ_w is derived through specific tables (i.e., ρ_w=998.23 kg/m³ at T=20°C)

Therefore, the Specific Gravity G_S is calculated as:

A correction is utilized to adjust the results at a reference temperature T=20°C:

where K is the temperature correction factor.

The Specific Gravity of soils is usually between 2.65-2.80 with finer soils having higher values than coarser ones.