Lesson 2/Learning Event 3
This means that all of the particles in the gravel and sand ranges are visible to the naked eye. All of the
silt particles and all of the clay particles are smaller than this size and are therefore invisible to the naked
eye. Material smaller than 0.07 millimeters will pass the #200 sieve.
GRAIN SIZE DISTRIBUTION
The laboratory mechanical analysis must be performed whenever the grain size distribution must be
determined accurately. However, an approximation of the grain size distribution can be made by visual
inspection. Spread a portion of the dry sample on the flat surface; then, using your hands or a piece of
paper, attempt to separate the material into its various grain size components. By this method, the gravel
particles and some of the sand particles can be separated from the remainder. This will at least give the
observer an opportunity to estimate whether the total sample is to be considered coarse-grained or fine-
grained, depending on whether or not more than 50 percent of the material would pass the #200 sieve.
Coarse-grained Soils
If the material is believed to be coarse-grained, then there is less than 5 percent passing the #200 sieve;
and, the fines are nonplastic. If both these criteria can be satisfied and there appears to be a good
representation of all grain sizes from largest to smallest, the material may be said to be well-graded (GW
or SW). If any intermediate sizes appear to be missing, or if there is too much of any one size, then the
material is poorly graded (GP or SP).
Fine-grained Soils
Estimating the grain size distribution of a sample using no equipment at all is probably the most difficult
part of field identification. It depends to a great extent on the experience of the individual making the
estimate. A better approximation of the relative proportions of the components of the finer soil fraction
may sometimes be obtained by shaking a portion of this sample into a jar of water and then allowing the
material to settle to the bottom. The material will settle in layers; the gravel and coarse sand particles
settling out almost immediately, the fine sand particles within a minute, the silt particles requiring as
much as an hour, and the clay particles remaining in suspension indefinitely, or until the water is clear.
In using this method, keep in mind that the gravel and sand will settle into a much more dense formation
than will either the silt or clay.
GRAIN SHAPE
The grain shape of the sand and gravel particles can be determined by close examination of the
individual grains. The grain shape affects the stability of the soil. Remember that increased resistance
to displacement is found in the more irregular particles. A material whose grains are rounded has only
34
Previous Page
