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Figure 4.7. Particle-size fractions

The principal fraction of a composite soil should be indicated by a capital letter (e.g. Sa for sand or SAND) and the secondary fraction by lower-case letters (e.g. gr for gravelly). Some examples of these abbreviations are:

saGr = sandy gravel or (in the UK) sandy GRAVEL

msaCl = medium sandy clay or (in the UK) medium sandy CLAY

The shape of a soil's particle size distribution (or grading curve) is described by the terms multi-graded, medium-graded, even-graded, and gap-graded and quantified by the uniformity coefficient CU and coefficient of curvature CC, defined as:

d10 d10 X d60 where dn is the particle size where n% of the soil is smaller than this size.

Shape of grading curve

Multi- Medium- Even- Gap-graded graded graded graded

4.3.2 Density of coarse soils

The density of coarse soils is classified in EN ISO 14688-2 by the terms very loose, loose, medium dense, dense and very dense, according to the value of their density index ID, defined as:

e — e max min where e is the soil's void ratio, emax its maximum voids ratio, and emin its minimum. See Figure 4.8.

Density index Id

Very loose |
Loose |
Medium dense |
Dense |
Very dense |

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