Lesson 2/Learning Event 4
described by such terms as hard, stiff, brittle, friable, sticky, plastic, or soft. The soil is then remolded
by working it in the hands, and changes, if any, are observed. By this test, the natural water content is
estimated relative to the liquid or plastic limit of the soil. Clays which turn almost liquid on remolding
are probably near or above the liquid limit. If the clay remains stiff, and crumbles upon being remolded,
the natural water content is below the plastic limit.
The term texture refers to the degree of fineness and uniformity. It is described by such expressions as
floury, smooth, gritty, or sharp, depending on the sensation produced by rubbing the soil between the
fingers. Sensitivity may be increased by rubbing some of the material on a more tender skin area such
as the wrist. Fine sand will feel gritty. Typical dry silts will dust readily, and feel relatively soft and
silky to the touch. Clay soils are powdered only with difficulty but become smooth and gritless like
The shine test is another means of measuring the plasticity characteristics of clays. A slightly moist or
dry piece of highly plastic clay will give a definite shine when rubbed with a fingernail, a pocket knife
blade, or any smooth metal surface. On the other hand, lean clay will not display any shine, but will
The roll or thread test uses a soil sample prepared by adding water until the moisture content is such that
the sample may be easily remolded without sticking to the fingers. This is sometimes referred to as
being just below the "sticky limit." Using a nonabsorbent surface, such as glass, this sample is rolled
rather rapidly into a thread approximately 1/8 inch in diameter.
If a moist soil can be rolled into such a thread at some moisture content, it is said to have some
plasticity. Materials which cannot be rolled in this manner are nonplastic, or have very low plasticity.
Figure 19 shows the roll test being performed.
After reaching the plastic limit, the degree of plasticity may be one of the choices below:
High plasticity (CH). The soil may be remolded into a ball and the ball deformed under extreme
pressure by the fingers without cracking or crumbling.
Medium Plasticity (CL). The soil may be remolded into a ball, but the ball will crack and easily
crumble under pressure of the fingers.