Lesson 2/Learning Event 1
Learning Event 1
IDENTIFY SYMBOLS USED IN USCS
This learning event will introduce you to the Unified Soil Classification System (USCS) which allows
you to apply a short lettered symbol to a soil for the purposes of communication and identification of the
engineering characteristics.
CLASSIFICATION SYSTEM
In the Unified Soil Classification System, all soils are divided into three major categories:
Coarse-grained soils.
Fine-grained soils.
Organic soils.
Coarse-grained and fine-grained soils are differentiated by grain size. Organic soils are identified by the
presence of large amounts of organic material.
The Unified Soil Classification System further divides soils which have been classified into the major
soil categories by using arbitrary letter symbols consisting of two letters. For example, the letters for
sand, silt, and clay are S, M, and C, respectively, and the symbol for a soil which meets the criteria for a
clayey sand is designated by SC. In the case of borderline soils which possess characteristics of two
groups and cannot be classified by a single symbol, it may be necessary to use four letters such as SM-
SC which would describe a sand which contains appreciable amounts of fines on the borderline
classification for silt and clay. After the physical characteristics of a soil have been determined by use
of the appropriate tests and calculations described in Lesson 1, these characteristics are used to classify
the soil. The criteria for identification are presented in the form of a job aid in Table 2 later in this
lesson.
PRIMARY AND SECONDARY LETTERS
The Unified Soil Classification System uses a system of two letter abbreviations to describe the soil.
PRIMARY (lst letter) ...................................
identifies the predominant soil fraction
SECONDARY (2nd Letter)..........................
an adjective further describing the characteristics of
the predominant fraction
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