PARTICLE SIZE

Soils are assemblages of particles of various sizes and shapes with void spaces in between. They are formed primarily from decomposition of rocks. Based on the size of the particles present, soils can be described as gravel, sand, silt, or clay. Following are two grain size classification systems generally used by geo technical engineers:

• System of the American Association of State Highway and Transportation Officials (AASHTO)
  Gravel: 75 mm to 2 mm
  Sand: 2 mm to 0.075 mm
  Silt and clay: Less than 0.075 mm

• Unified System
  Gravel: 76.2 mm to 4.75 mm
  Sand: 4.75 mm to 0.075 mm
  Silt and clay: Less than 0.075 mm

The particle size distribution in a given soil is determined in the laboratory by sieve analysis and hydrometer analysis.

A typical particle size distribution curve is shown in Fig. 10.1. For classification purposes in coarse-grained soils, the following two parameters can be obtained from a particle size distribution curve:

• Uniformity coefficient:

  

• Coefficient of gradation:

  

  The coefficient of gradation is also sometimes referred to as the coefficient of curvature. The definitions of D ₁₀