Classification and identification of soils for general engineering purposes
1970 Edition

IS 1498 classifies soils based on particle size as follows: Coarse-grained soils contain over 50% by weight of particles larger than 75 microns (0.075 mm). Within this, gravels consist predominantly of particles greater than 4.75 mm, while sands have more than 50% of particles between 75 microns and 4.75 mm. Fine-grained soils are those with more than 50% of particles smaller than 75 microns. The fines fraction includes silts and clays smaller than this size. This classification aids in determining soil behavior and suitability for engineering applications.

IS 1498 differentiates silts, clays, and organic soils by integrating particle size, plasticity characteristics, and physical testing. Clay particles are smaller than 75 microns and exhibit plasticity and strength when air-dried. Silts fall between 75 and 425 microns with lower plasticity. Organic soils contain decomposed matter identifiable by reduced liquid limits after oven drying—if the liquid limit decreases to less than 75% of the original after drying, the soil is classified as organic. Additional manual tests such as dilatancy and dry strength assist in distinguishing these soil types based on their physical responses.

IS 1498 suggests several manual field tests for fine-grained soils (particles finer than 425 microns), including: the dilatancy test, which observes water appearance or disappearance upon shaking a nearly saturated soil pat; the dry strength test, assessing the soil's resistance to crumbling when dry; evaluation of organic content and color by smell and appearance; and the acid test, where dilute hydrochloric acid is applied to detect calcium carbonate cementation through effervescence. These tests provide immediate, qualitative insights into soil properties before laboratory analysis.

The degree of expansion in fine-grained soils is assessed based on Atterberg limits—namely, the liquid limit and plasticity index—along with shrinkage limit and free swell percentage. Table 8 in IS 1498 categorizes soils into expansion degrees (low, medium, high, very high) and severity levels (non-critical to severe) according to these parameters. Free swell measures the volume increase upon water immersion, with values above 50% indicating expansive soils critical for design considerations. This methodology helps engineers evaluate the swelling potential and plan appropriate foundation and pavement designs.

Boundary classifications in IS 1498 refer to soils exhibiting transitional characteristics between two recognized groups, leading to dual group symbol designations. For coarse-grained soils, boundaries exist between gravels and sands or between different gravel types; these are classified first by assuming the coarser soil and then the finer, assigning two group symbols accordingly (e.g., GW-GC for well-graded gravel with clay). Boundaries between coarse and fine soils also employ dual symbols such as SM-ML or SC-CL. Fine-grained soils with intermediate properties in plasticity or liquid limit are similarly denoted with paired symbols like ML-MI or CL-CI. This approach reflects the gradational nature of soils and assists in more precise classification.