In order to classify a soil for engineering purposes, one needs to know the distribution of the size of grains in a given soil mass. Sieve analysis is a method used to determine the grain-size distribution of soils. Sieves are made of woven wires with square openings. Note that as the sieve number increases, the size of the openings decreases.

Soils having particles larger than 0.075mm size are termed as coarse grained soils. In these soils more than 50% of the total material by mass is larger than 75 micron. Coarse grained soils may consist of boulder, cobble, gravel and sand.

The following particle classification names are given depending on the size of the particle:

1. BOULDER: Particle size is more than 300mm.
2. COBBLE: Particle size in range of 80mm to 300mm.
3. GRAVEL (G): Particle size in range of 4.75mm to 80mm.
   1. Coarse Gravel: 20 to 80mm.
   2. Fine Gravel: 4.75mm to 20mm.
4. SAND (S): Particle size in range of 0.075mm to 4.75mm.
   1. Coarse sand: 2.0mm to 4.75mm
   2. Medium Sand: 0.425mm to 2mm
   3. Fine Sand: 0.075mm to o.425mm.

Name of the soil is given depending on the maximum percentage of the above components.

Broadly, sands can also be classified as:

1. Uniformly graded - Majority percentage of the particles fall within the same range of size.
2. Well graded - The soil contains almost equally distributed percentage of each size of particles.

We can plot the distribution of soil particle sizes as percentage of soil finer vs particle size and obtain the following quantities to help us classify the soil:

1. Coefficient of curvature: C_c = D₃₀² / (D₆₀ * D₁₀)
   

   where, D₃₀ is particle size for which 30% of the soil is finer, D₆₀ is particle size for which 60% of the soil is finer and D₁₀ is particle size for which 10% of the soil is finer. All these values are obtained from the plot.

2. Uniformity coefficient: C_u = D₆₀ / D₁₀
   

   where, D₆₀ and D₁₀ have the same definitions as mentioned above for the coefficient of curvature formula.

Coefficient of curvature (C_c) should lie between 1 and 3 for well graded gravel and sand.

Similarly, the uniformity coefficient (C_u) value should be less than 4 for poorly/uniformly graded soil, more than 4 for well graded gravel and more than 6 for well graded sand. Higher value of C_u indicates that the soil mass consists of soil particles with different size ranges.