Lesson 2/Learning Event 5
You can perform the same test described above after separating out the #40 sieve soil fraction. Use a
procedure similar to that outlined above except pour off the water within one or two seconds after
completion of agitation. The suspended portion will then include the particles of the fine sand range.
A difficulty that you will encounter with many clay soils stems from the fact that the clay particles often
form small lumps (flocculate) that will not break up in water. Usually this condition can be detected by
examining the coarse fraction of the soil after several repetitions of the test. If substantial amounts of
clay are still present, the sand will have a somewhat slippery feel and further mixing and grinding with a
wood stick will be necessary to break up these lumps.
The cast test refers to the strength of a moist soil sample when squeezed in the hand. It is used to
indicate the approximate type and quantity of fines present in the sample. The correct amount of water
to add to the soil must be estimated by trial and error. Generally stated, the maximum cohesion or
attraction between the individual soil particles normally will occur when the soil is damp but not sticky.
The test consists of compressing a handful of the moist soil into a ball or cigar-shaped cast and
observing its ability to withstand handling without crumbling. While experience is desirable in making
predictions based upon this test, Table 4 serves as a general guide of the behavior of different soil types
when formed into a cast and tested.
TABLE 4. CAST TEST REACTIONS
WASH, DUST, AND SMEAR TESTS
A small amount of silt (less than 5 percent), when intermixed with a coarse-grained soil, normally will
not lessen the value of the soil as a construction material. However, increasing the quantity of silt will
sharply reduce the strength and interfere with the free drainage characteristics of the coarse-grained soil.
This makes it less desirable as a road or airfield construction material. To decide what constitutes a
harmful concentration of silt by the