Soil classification systems (USCS)
The terms and letters used in the USCS system are detailed in Table 1.4 (alternative). The boundary
between coarse and fine soils is taken to be 50% fines (i.e. particles smaller than 0.075 mm, No. 200
sieve). The liquid and plastic limits are used to classify fine-grained soils, employing the plasticity chart
shown in Figure 1.12 (alternative). The axes of the plasticity chart are plasticity index and liquid limit;
therefore, the plasticity characteristics of a particular soil can be represented by a point on the chart.
Classification letters are allotted to the soil according to the zone within which the point lies. The chart is
divided into two ranges of liquid limit, low (L) and high (H). The A-line may be mathematically represented by Equation 1.5:
Ip = 0.73(wL – 20)
SILT (M) plots below the A-line and CLAY (C) above the A-line on the plasticity chart, i.e. silts
exhibit plastic properties over a lower range of water content than clays having the same liquid limit.
The letter denoting the dominant size fraction is placed first in the group symbol. Organic silts and
clays (with a low to moderate organic content) have their own group symbol (O), as shown in Table 1.4
(alternative). Highly organic soils (e.g. peat) are defined by PT. A group symbol may consist of two or
more letters, for example:
SW – well-graded SAND
CL – Inorganic CLAY of low plasticity
Coarse-grained soils with fines between 5% and 12% must be classified using dual symbols (i.e.
describing both the grading of the coarse fraction (W or P) and the type of fines (M or C). Similarly,
fine-grained soils which plot in the shaded zone in Figure 1.12 (alternative) are described using dual
symbols (CL + ML).
Table 1.4 Descriptive terms for soil classification (BS 5930)
Main terms
Qualifying terms
GRAVEL
SAND
G
S
FINE SOIL, FINES
SILT (M-SOIL)
CLAY
F
M
C
PEAT
Pt
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Well graded
Poorly graded
Uniform
Gap graded
Of low plasticity (wL < 35)
Of intermediate plasticity (wL 35–50)
Of high plasticity (wL 50–70)
Of very high plasticity (wL 70–90)
Of extremely high plasticity (wL > 90)
Of upper plasticity range (wL > 35)
Organic (may be a suffix to any group)
W
P
Pu
Pg
L
I
H
V
E
U
O
Soil classification
Plasticity increasing
70
e
lin
60
A-
Clays
Plasticity index
50
40
CH
30
CL
20
MH & OH
10
7
4
6
0
0
CL + ML
10
20
Silts & organic soils
ML & OL
30
40
50
60
70
Liquid limit
80
90
100
110
120
Figure 1.12 Plasticity chart: British system (BS 1377–2: 1990).
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