Specific gravity is the ratio of the mass of a given volume of material at a temperature to the mass of an equal volume of distilled water at the same temperature. It is dimensionless and expresses ratio only. 

importance of specific gravity of soil

Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer. The test is performed to determine the specific gravity of soil passing 4.75 mm sieve. Soil passing 4.75 mm sieve can be determined by particle size distribution using sieve analysis of soil.

The importance of specific gravity is many. They are in the following:

  1. The phase relationships of soil are void ratio, degree of saturation, porosity, etc. Specific gravity is used to determine these relationships. 
  2. The specific gravity of soil is used to determine the density of the soil solids. This is done by multiplying its specific gravity by the density of water at the proper temperature.
  3. Specific gravity is used to understand the behavior of soil mineral composition and weathering.
Soil solids for these test methods do not include solids that can be easily altered by these test methods or highly organic soil solids such as fibrous matter that floats in water or contaminated with a substance that prohibits the use of these methods.

The balances, specimen masses, and pycnometer sizes are established to obtain test results with three significant digits.

Typical values of specific gravity help to primarily identify the soil specimen. The specific gravity of most inorganic soil solids falls in the range of 2.6 to 2.8. Sand particles are composed of quartz. They have specific gravity ranging from 2.65 to 2.67. Inorganic clays generally vary between 2.70 to 2.80. Some soils have large amounts of organic matter or porous particles (such as diatomaceous earth). They have low specific gravities. The value is below 2.60 and some fall below 2.00.