This standard covers the classification and identification of soils for general engineering purposes based on their index properties. It establishes a unified soil classification system categorizing soils into coarse-grained, fine-grained, and highly organic soils using both laboratory tests and field identification methods.
Provides a system for classification and identification of soils based on their engineering properties and behaviour for general engineering purposes.
Quick Reference Values
coarse grained soil boundary> 50% retained on 75-micron IS Sieve
fine grained soil boundary> 50% passing 75-micron IS Sieve
gravel sand boundary4.75 mm IS Sieve
low plasticity liquid limit< 35%
medium plasticity liquid limit35% to 50%
high plasticity liquid limit> 50%
dual symbol fines percentage5% to 12% fines
Key Formulas
Ip = 0.73(wL - 20) — Equation of the A-line for plasticity chart
Cu = D60 / D10 — Coefficient of uniformity
Cc = (D30)^2 / (D10 * D60) — Coefficient of curvature
Practical Notes
Always use the A-line chart in conjunction with liquid limit and plasticity index to accurately classify fine-grained soils into clays or silts.
Dual symbols (e.g., SW-SM) must be used when a coarse-grained soil contains between 5% and 12% fines.
Field identification methods like dilatancy (shaking test), toughness (threading test), and dry strength are critical for quick on-site preliminary classification.