This part of IS 2720 covers the method for determining the quantitative particle size distribution in soils. It specifies mechanical sieve analysis for coarse-grained fractions and sedimentation analysis (using either the hydrometer or pipette method based on Stokes' law) for fine-grained fractions passing the 75-micron sieve.
Prescribes methods for determining the grain size distribution of soil samples, crucial for track bed design and foundations.
Quick Reference Values
boundary coarse and fine soil75 microns
boundary gravel and sand4.75 mm
sodium hexametaphosphate dose33 g/litre
sodium carbonate dose7 g/litre
standard test temperature27°C
Key Formulas
D = sqrt([30 * η * He] / [(G - 1) * γw * t]) — Stokes' Law for equivalent particle diameter
N = (W_D / W) * 100 — Percentage finer than a given diameter
Practical Notes
Inadequate chemical dispersion or omitting the dispersing agent causes flocculation of clay particles, heavily underestimating the actual clay content.
Always apply temperature, meniscus, and dispersing agent corrections to hydrometer readings for accurate grain-size calculations.
Ensure the soil is washed gently over the 75-micron IS sieve during wet sieving to prevent the breakdown of individual fragile particles.