Lesson 2/Learning Event 5
field identification methods generally requires extensive field experience. If the dust, wash, and smear
tests are first practiced on soils of known silt contents, they can be used to produce a fairly accurate
result. In the dust test, when a completely dry sample of soil (with the gravel portion removed) is
dripped from a height of one or two feet onto a solid surface, a silt content higher than 10 percent will
generally cause a fairly large amount of dust to be produced. In the wash test, an identical soil sample,
as above, is placed in the palm of the hand, and covered with 1/8 inch of water. If the water becomes
completely discolored and hides the sand grains, this indicates that the soil sample contains a silt content
higher than 5 percent. In the smear test, a sample of soil, again with the gravel portion removed, is
moistened to just below the sticky limit, and then smeared between the thumb and forefinger. When it
produces a gritty, harsh feel, this indicates that it contains a silt content of less than 10 percent. A rough,
less harsh feel, however, indicates that the sample contains more than 10 percent silt. All of the above
tests can be considered as reliable indicators of the silt contents only if the engineers performing the
tests are experienced.
Now look at Figure 11 on the next page. It illustrates the suggested procedure for using the tests
described in the first three parts of this lesson. You will see that all the tests in the diagram can be
performed in the field with no laboratory equipment.
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