Methods of test for soils, Part 4: Grain size analysis
1985 Edition

The sieve analysis under IS 2720 Part 4 involves preparing the soil sample retained on the 4.75 mm sieve, weighing it, and then sieving through a series of IS standard sieves. The sieves should be agitated so that particles roll irregularly without forcing them through. Retained material is gently rubbed using a rubber pestle to separate aggregates before weighing. This process is repeated through successive sieves. Mechanical sieve shakers may be used for at least 10 minutes to enhance efficiency. Wet sieving can also be performed by soaking the soil and washing it over a 75-micron sieve until the wash water clears. Maximum allowable sample masses per sieve size must be adhered to for accuracy.

Prior to grain size analysis, soil samples must be prepared by separating fractions larger and smaller than 4.75 mm. Material retained on the 75-micron sieve should be oven-dried at 105–110°C. Chemical dispersants such as sodium hexametaphosphate or a combination of sodium hydroxide and sodium carbonate are added to the soil suspension and thoroughly mixed. The soil is then soaked and washed over nested sieves until the water runs clear, followed by oven drying each fraction. For soils containing insoluble calcium salts, acid treatment followed by peroxide oxidation may be necessary. These steps ensure accurate dispersion and prevent particle aggregation during testing.

Hydrometer testing requires a calibrated hydrometer conforming to IS specifications, with smooth graduations and minimal scale error. A 1000 ml measuring cylinder is used for calibration and testing. Calibration involves determining the sectional area of the cylinder by measuring the volume between graduations divided by the distance between them. The hydrometer bulb volume is established either by water displacement or by weighing. Key distances on the hydrometer, such as from the neck to calibration marks and bulb height, are measured to calculate the effective depth for readings. A calibration curve plotting effective depth against hydrometer readings is prepared to ensure precise particle size determination.

For soils exhibiting soluble salt contents exceeding 1% (as identified per IS 2720 Part 21), the standard mandates washing the soil with water containing dispersing agents like sodium hexametaphosphate or a combination of sodium hydroxide and sodium carbonate. The soil is washed over nested sieves until the effluent is clear, ensuring minimal loss of particles. Organic soils require hydrogen peroxide treatment prior to washing. In calcium-rich soils, hydrochloric acid treatment follows peroxide application to remove calcium compounds. This pre-treatment removes soluble salts that could otherwise interfere with accurate grain size analysis.

Sedimentation tests must be conducted in a constant temperature bath maintained at 27 ± 0.1°C, with the sedimentation tube immersed to the 500 ml mark. The bath should be free from vibrations to avoid disturbing the soil suspension. Suspension temperature must be recorded initially within the first 15 minutes and subsequently at each hydrometer reading with an accuracy of ±0.5°C. Hydrometer readings in distilled water at the test temperature are also taken to calculate temperature corrections. Tubes must equilibrate to bath temperature before shaking, and care should be taken to avoid agitation during sampling and readings to maintain test integrity.