The size of the particles in a certain soil can be represented graphically in a grain size diagram, see Figure below.
Such a diagram indicates the percentage of the particles smaller than a certain diameter, measured as a percentage of the weight. A teep slope of the curve in the diagram indicates a uniform soil, a shallow slope of the diagram indicates that the soil contains particles of strongly different grain sizes. For rather coarse particles, say larger than 0.05 mm, the grain size distribution can be determined by ieving. The usual procedure is to use a system of sieves having different mesh sizes, stacked on top of each other, with the coarsest mesh on top and the finest mesh at the bottom. After shaking the assembly of sieves, by hand or by a shaking machine, each sieve will contain the particles larger than its mesh size, and smaller than the mesh size of all the sieves above it. In this way the grain size diagram can be determined. Special standardized sets of sieves are available, as well as convenient shaking machines. The example shown in Figure 2.1 illustrates normal sand. In this case there appear to be no grains larger than 5 mm. The grain size distribution can be characterized by the quantities D60 and D10. These indicate that 60 %, respectively 10 % of the particles (expressed as weights) is smaller than that diameter. In the case illustrated in Figure above it appears that D60 ~ 0.6 mm, and D10 ~ 0.07 mm. The ratio of these two numbers is denoted as the uniformity coefficient Cu,
In the case of Figure this is about 8.5. This indicates that the soil is not uniform. This is sometimes denoted as a well graded soil . In a
poorly graded soil the particles all have about the same size. The uniformity coefficient is than only slightly larger than 1, say Cu = 2.
For particles smaller than about 0.05 mm the grain size can not be determined by sieving, because the size of the holes in the mesh would become unrealistically small, and also because during shaking the small particles might fly up in the air, as dust. The amount of particles of a particular size can then be determined much better by measuring the velocity of deposition in a glass of water. This method is based upon a formula derived by Stokes. This formula expresses that the force on a small sphere, sinking in a viscous fluid, depends upon the viscosity of the fluid, the size of the sphere and the velocity. Because the force acting upon the particle is determined by the weight of the particle under water, the velocity of sinking of a particle in a fluid can be derived. The formula is
Because for very small particles the velocity may be very small, the test may take rather long.
(SOIL MECHANICS By Arnold Verruijt : Delft University of Technology, 2001)
