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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 ne soils is taken to be 50% nes (i.e. particles smaller than 0.075 mm, No. 200

sieve). The liquid and plastic limits are used to classify ne-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.

Classication 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 repre-

sented 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 rst 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 dened 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 nes between 5% and 12% must be classied using dual symbols (i.e.

describing both the grading of the coarse fraction (W or P) and the type of nes (M or C). Similarly,

ne-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 G Well graded W

SAND S Poorly graded P

Uniform PuGap graded Pg

FINE SOIL, FINES F Of low plasticity (wL< 35) L

SILT (M-SOIL) M Of intermediate plasticity (wL3550) I

CLAY C Of high plasticity (wL5070) H

Of very high plasticity (wL7090) V

Of extremely high plasticity (wL> 90) E

Of upper plasticity range (wL> 35) U

PEAT Pt Organic (may be a suffix to any group) O

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Figure 1.12 Plasticity chart: British system (BS 13772: 1990).

Liquid limit

00

6

10

20

30

40

50

60

70

10 20

CL + ML ML & OL

MH & OH

CH

Clays

CLPlasticity

index

Silts & organic soils

Plasticity increasing

A-li

ne

30 40 50 60 70 80 90 100 110 120

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