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© 2012 Taylor & Francis 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 repre- sented by Equation 1.5: Ip = 0.73(w L – 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 G Well graded W SAND S Poorly graded P Uniform Pu Gap graded Pg FINE SOIL, FINES F Of low plasticity (w L < 35) L SILT (M-SOIL) M Of intermediate plasticity (w L 35–50) I CLAY C Of high plasticity (w L 50–70) H Of very high plasticity (w L 70–90) V Of extremely high plasticity (w L > 90) E Of upper plasticity range (w L > 35) U PEAT Pt Organic (may be a suffix to any group) O

Soil classification systems (USCS)

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Page 1: Soil classification systems (USCS)

© 2012 Taylor & Francis

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 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 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 SANDCL – 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 G Well graded WSAND S Poorly graded P

Uniform PuGap graded Pg

FINE SOIL, FINES F Of low plasticity (wL < 35) LSILT (M-SOIL) M Of intermediate plasticity (wL 35–50) ICLAY C Of high plasticity (wL 50–70) H

Of very high plasticity (wL 70–90) VOf extremely high plasticity (wL > 90) EOf upper plasticity range (wL > 35) U

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

Page 2: Soil classification systems (USCS)

Soil classification

2 © 2012 Taylor & Francis

Figure 1.12 Plasticity chart: British system (BS 1377–2: 1990).

Liquid limit0

0

6

10

20

30

40

50

60

70

10 20

CL + ML ML & OL

MH & OH

CH

Clays

CLPlas

tici

ty in

dex

Silts & organic soils

Plasticity increasing

A-line

30 40 50 60 70 80 90 100 110 120

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