3. Classification of Soils Based on Consistency. Atterberg limit and a plasticity chart may be used to classify
relatively fine-grained soils (those that pass a Number 40 sieve) on the basis of their consistencies.
a. Atterberg Limits. Atterberg limits describe the moisture content of a soil exhibiting certain physical
properties. Three parameters define the Atterberg limits of a soil--the plastic limit (PL), the liquid limit (LL), and
the plasticity index (PI).
(1) Plastic Limit. If water is slowly added to a dry, fine-grained soil, the soil will eventually be
transformed from a semisolid state to a plastic state, as evidenced by its ability to be rolled into thin, 3-millimeter-
diameter threads. The water content, expressed as a percentage by weight, of the soil at that point is called the
(2) Liquid Limit. If additional water is added to a soil already in its plastic state, the soil will eventually
behave as a liquid. The water content, expressed as a percentage by weight, of the soil at that point is called the
(3) Plasticity Index. The difference in moisture content between the LL and the PL of a soil is known as
the PI. That is:
PI = LL -PL
The PI represents the range of moisture content over which the soil will exhibit plastic behavior. For example, a
soil with a high PI is one that will behave plastically over a wide range of moisture contents.
Complete procedures and details for conducting tests to determine the PL and LL of a soil are outlined in TM 5-
b. Plasticity Chart. The Casagrande Plasticity Chart (Engineer (ENG) Form 4334)(see figure 3-4) may be
used to plot the LL (horizontal axis) and the PI (vertical axis) of a specific soil sample. Depending on the location
of the plot, a fine-grained soil may be considered to be either a clay or a silt or an organic-type material.
4. Unified Soil Classification System. As mentioned earlier, soils commonly consist of particles of various
sizes mixed in varying proportions. Each individual component, therefore, contributes its own unique
characteristics to the mixture. In order to describe the engineering classifications of such heterogeneous mixtures,
the US Army uses what is known as the USCS (see table 3-1, page 3-11). Under this system, soils are divided
into two categories-coarse-grained and fine-grained.
a. Coarse-Grained Soils. If at least half of the material, by weight, of a soil is made up of particles that are
larger than the openings in a Number 200 sieve, the soil is considered to be coarse-grained. (A particle the size of
an opening in a Number 200 sieve is about the smallest particle size distinguishable to the unaided eye). Coarse-
grained soils are further classified as gravels or sands.
(1) Gravels. A coarse-grained soil is classified as a gravel if at least half of the coarse fraction, by
weight, consists of particles larger than the opening. In a Number 4 sieve (that is,