The engineering behaviour of soil is greatly influenced by the size and distribution of its particles. Grain size distribution analysis is used to determine the proportion of different particle sizes present in a soil sample and forms the basis for soil classification.

Sieve analysis is the standard laboratory method used for determining the particle size distribution of coarse-grained soils. A set of standard sieves with progressively smaller openings is arranged in descending order, and the soil sample is mechanically shaken through the sieves.

According to the particle size, coarse-grained soils are generally classified as:

1. Boulder: Particle size greater than 300 mm.
2. Cobble: Particle size between 80 mm and 300 mm.
3. Gravel: Particle size between 4.75 mm and 80 mm. 3.1 Coarse gravel: 20 mm to 80 mm. 3.2 Fine gravel: 4.75 mm to 20 mm.
4. Sand: Particle size between 0.075 mm and 4.75 mm. 4.1 Coarse sand: 2.0 mm to 4.75 mm. 4.2 Medium sand: 0.425 mm to 2.0 mm. 4.3 Fine sand: 0.075 mm to 0.425 mm.

A soil is termed coarse-grained if more than 50% of its particles are larger than 0.075 mm.

Importance of Grain Size Distribution

The distribution of particle sizes influences:

• Soil permeability.
• Shear strength.
• Compressibility.
• Compaction characteristics.
• Drainage properties.

Grain size analysis helps engineers classify soils and predict their engineering behaviour.

Principle of Sieve Analysis

The sieve analysis test separates soil particles according to their size by passing the sample through a stack of standard sieves.

The percentage finer for each particle size is determined and used to construct the grain size distribution curve.

From the curve, important parameters are obtained:

$$C_u=\frac{D_{60}}{D_{10}}$$

where,

• $D_{60}$ = particle size corresponding to 60% finer,
• $D_{10}$ = particle size corresponding to 10% finer.

The coefficient of curvature is given by:

$$C_c=\frac{D_{30}^2}{D_{60}D_{10}}$$

where,

• $D_{30}$ = particle size corresponding to 30% finer.

Generally,

• Well-graded gravel: $C_u>4$ and $1<C_c<3$.
• Well-graded sand: $C_u>6$ and $1<C_c<3$.

Engineering Applications

Grain size analysis is widely used in:

• Soil classification.
• Filter design.
• Pavement engineering.
• Foundation design.
• Earth dam construction.
• Drainage studies.

Particle size distribution is one of the basic index properties used in geotechnical engineering.