Soil classification - Geotechnical Investigation and Design Tables - page 22

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Laboratory testing is essential in borderline cases, eg silty sand vs sandy silt.
- Once classified many inferences on the behaviour and use of the soil is made.
- Medium Plasticity uses symbols mixed or intermediate symbols eg CL/CH or
CI (Intermediate).

2.8 Particle description
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The particle description is usually carried out in the field.

Table 2.8 Particle distribution.

Particle description Subdivision

Large size (Boulders, cobbles, gravels, sands) Coarse/medium/fine

Fine size (Silts, clays) Plasticity

Spread (gradation) Well/poorly/gap/uniform

Shape Rounded/sub-rounded/sub-angular/angular

- These simple descriptions can influence the design considerably. For example

an angular grain has a larger frictional value than a rounded grain.

2.9 Gradings
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While some field descriptions can be made on the spread of the particle
distribution, the laboratory testing provides a quantitative assessment for design.

Table 2.9 Gradings.

Symbol Description Comments

D 10 (mm) Effective size - 10% passing sieve

D 60 (mm) Median size - 60% passing sieve

U Uniformity coefficient D 60 /D 10 Uniformly graded U 5

= <

C Coefficient of curvature D 30 /(D 60 D 10 ) Well graded U 5 and C 1to3

= > =

2.10 Colour

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Colour Charts may be useful to standardise descriptions and adjacent to core
photos.

Table 2.10 Colour description.

Parameter Description

Tone Light/dark/mottled

Shade Pinkish/reddish/yellowish/brownish/greenish/bluish/greyish

Hue Pink/red/yellow/orange/brown/green/blue/purple/white/grey/black

Distribution Uniform/non - uniform (spotted/mottled/streaked/striped)