This part of IS 2720 prescribes methods for the quantitative determination of the particle size distribution in soils. It details the mechanical sieving method for coarse-grained soils (larger than 75 microns) and sedimentation techniques (pipette and hydrometer methods) based on Stokes' Law for fine-grained soils.
Describes methods for determining the quantitative distribution of grain sizes in a soil sample by sieve analysis and hydrometer analysis.
Quick Reference Values
boundary for sedimentation test75 microns
boundary between sand and gravel4.75 mm
standard test temperature27 °C
sodium hexametaphosphate concentration33 g per litre of water
sodium carbonate concentration7 g per litre of water
max sieve size typically used100 mm
Key Formulas
Percentage finer = (Mass of soil passing the sieve / Total mass of sample) x 100
V = (Gs - Gv) * D^2 * g / (18 * viscosity) — Stokes' Law for sedimentation velocity
D = sqrt((30 * viscosity * H) / (980 * (Gs - 1) * t)) — Particle diameter in hydrometer test
Practical Notes
Proper washing over the 75-micron sieve is critical; otherwise, fine clays adhering to coarse sand/gravel will distort the analysis.
Using the correct dispersing agent (sodium hexametaphosphate) is essential in sedimentation to prevent clay particles from flocculating and settling prematurely.
Hydrometer readings must be corrected for meniscus, temperature, and the dispersing agent.
Ensure the specific gravity of the soil (determined via IS 2720 Part 3) is known beforehand to calculate particle size accurately in sedimentation.