SMC Soil Classification (Soil Material Classification): A soil is a natural aggregate of mineral grains that can be separated by simple mechanical agitation or by agitation in water.
The most common terms used to describe a soil are: gravel, sand, silt, clay.
In their natural state, soils consist of a mixture of two or more of these constituents. Gravels and sands are known as coarse-grained soils, silts and clays as fine-grained soils.
Another distinction that can be made is between cohesive and cohesionless soils: the former exhibit non-negligible tensile strength when dry, while they lose all consistency after immersion in water; the latter always exhibit zero strength and tension.
Coarse-grained soils are recognized primarily based on grain size; gravel has grains with a diameter greater than 2 mm, while sand consists of grains with a diameter between 2 and 0.063 mm.
Among fine-grained soils, silts represent the coarser fraction and possess little or no plasticity and cohesion. From a particle-size perspective, silts are those between the lower limit of sands and 0.002 mm.
Clays, on the other hand, are an aggregate of microscopic and submicroscopic lamellar mineral particles, characterized by the typical colloidal properties of plasticity, cohesion, and ion absorption capacity. Distinguishing a silt from a clay based solely on particle size is not possible since the significant physical properties of the two materials are only indirectly linked to the particle sizes themselves, therefore other criteria are generally used in situ.
In order to provide an adequate description of soils, certain classification tests are used, leading to the definition of index properties.