Soil properties are mainly divided into properties and engineering properties. The Index properties are mainly used in the classification and identification of soils. For coarse grained soil, the main indexed properties are particle size distribution and relative density. For fine-grained soil, the main indexed properties are atterberg limits. These index properties also give the rough assessment of engineering properties. In the present chapter, the various indexed properties are discussed. The classification of soil based on indexed properties is also discussed here.
Index Properties Of Soil :
Particle Size Analysis it also known as mechanical analysis, it is a method of separation of soils into different fractions based on particle size. Mechanical analysis is done in two stages like Sieve & Sedimentation analysis.
Sieve analysis is used for coarse-grained soils (size >75µ) and Sedimentation analysis or wet analysis is used for fine grained soils (size < 75µ). Sedimentation analysis is based on Stokes’ law, which gives the terminal velocity of a sphere settling in a fluid of infinite extent. A dispersion solution is added to have proper dispersion of soil in preparation of suspension for sedimentation analysis. Dispersion solution is obtained after adding 33 g of sodium hexa meta phosphate and 75 g of sodium carbonate in distilled water to make one liter of solution.
If the soil contains organic matter and calcium compounds, it should be pre treated before adding the dispersing agent.
Pipette method and hydrometer method are used for the particle size analysis of fine grained soil.
Pipette method is a standard laboratory method. It is a highly accurate method for quick particle size analysis, hydrometer method is used.
Hydrometer reading is taken corresponding to the upper level of meniscus. It increased in downward direction towards the center of the bulb the hydrometer readings are corrected as follows:
1.Meniscus correction: As the marking on stem increases downward, meniscus correction is positive, constant for a hydrometer.
2. Temperature correction: The hydrometer is generally calibrated at 27°C. If temperature is more than 27°C, the suspension is lighter and actual reading will be less than the corrected reading. The temperature correction is positive. If temperature is less than 27°C, the temperature correction is negative.
3. Dispersion agent correction: It is always negative.