Specific gravity of soil solids, commonly denoted by $G_s$, is one of the fundamental index properties of soil. It is defined as the ratio of the density of soil solids to the density of distilled water at a specified temperature. Since it is a ratio of two densities, specific gravity is a dimensionless quantity.

$$G_s=\frac{\rho_s}{\rho_w}$$

where,

• $\rho_s$ = density of soil solids,
• $\rho_w$ = density of distilled water.

Since density is the ratio of mass to volume, the above expression may also be written as:

$$G_s=\frac{\text{Mass of soil solids}}{\text{Mass of an equal volume of water}}$$

The specific gravity of most inorganic soils generally lies between 2.60 and 2.75, although it may vary depending on the mineral composition and the presence of organic matter or heavy minerals.

Importance of Specific Gravity

The specific gravity of soil solids is an important engineering property because it is used in determining the weight-volume relationships of soils.

Knowledge of the specific gravity helps in the calculation of:

• Void ratio.
• Porosity.
• Degree of saturation.
• Unit weight of soil.
• Dry density of soil.

The density of individual soil particles depends on their mineral composition. Most mineral particles have densities ranging from 2.60 to 2.75 g/cm³. Dense minerals may have higher values, whereas organic soils may exhibit considerably lower values.

If the specific gravity and bulk density of a soil are known, important soil characteristics such as porosity and the amount of air and water occupying the void spaces can be estimated. These properties influence the storage and movement of water, heat transfer, root penetration, and the engineering behaviour of soils.

Principle of the Pycnometer Method

The pycnometer method is a standard laboratory method for determining the specific gravity of soil solids.

The test is based on comparing the mass of a known volume of soil solids with the mass of an equal volume of water. A pycnometer is filled with soil and water, and a series of mass measurements are taken to determine the specific gravity of the soil particles.

The specific gravity is calculated using:

$$G_s=\frac{W_2-W_1}{(W_2-W_1)-(W_3-W_4)}$$

where,

• $W_1$ = mass of the empty pycnometer,
• $W_2$ = mass of the pycnometer with dry soil,
• $W_3$ = mass of the pycnometer containing soil and water,
• $W_4$ = mass of the pycnometer filled with water only.

The pycnometer method is suitable for coarse-grained soils and is widely used because of its simplicity and accuracy.

Other Methods of Determining Specific Gravity

Specific gravity of soil solids can also be determined using:

1. Density bottle method.
2. Specific gravity flask method.
3. Pycnometer method.

The density bottle method is suitable for a wide range of soil types, while the specific gravity flask and pycnometer methods are commonly used for coarse-grained soils.

Engineering Applications

Determination of the specific gravity of soil solids is an important part of geotechnical investigations and laboratory testing. It is commonly used in:

• Phase relationship calculations.
• Soil classification.
• Determination of void ratio and porosity.
• Compaction studies.
• Estimation of degree of saturation.
• Foundation and pavement engineering.
• Earth dam and embankment construction.

Since specific gravity is a basic index property used in many soil mechanics calculations, its accurate determination is essential for the analysis and design of geotechnical engineering projects.