Soil and rock terminology for description and classification for engineering purposes - Practical Engineering Geology - page 362

Table C1 Basic soil types - based on GCO (1988) but generally compatible with much international

practice.

SOIL TYPE PARTICLE SIZE (mm) IDENTIFICATION

BOULDERS - >200 Only seen complete in pits or exposures.

COBBLES - 60 - 200 Often dif cult to recover by drilling.

Coarse 20 - 60

Visible to naked eye; little or no cohesion but where
cemented should state so; particle shape and grading
can be described.
Well-graded means wide range of grain sizes.
Poorly graded is the opposite.
Can use the terms uniform or gap-graded as
appropriate.

Medium 6 - 20

GRAVEL

Fine 2 - 6

Coarse 0.6 - 2

Medium 0.2 - 0.6

SAND

Fine 0.06 - 0.2

Coarse 0.02 - 0.06 Coarse silt is barely visible to naked eye easily

seen with hand lens; exhibits little plasticity (can

-

t
roll into a cohesive sausage) and marked dilatancy
(when wet and squeezed, it increases in volume so
that water will disappear); slightly granular or silky
to the touch. Disintegrates in water; lumps dry
quickly; may possess cohesion but can be powdered
between

'

-

Medium 0.006 0.2

-

Fine 0.002 0.006

SILT

fingers. Silt is often detrital quartz (not clay

minerals).

- <0.002 Dry lumps can be broken by hand but not

powdered between the

fingers. Disintegrates in
water more slowly than silt; smooth to the touch;
exhibits plasticity but no dilatancy; sticks to the

fingers and dries slowly; shrinks appreciably on
drying, usually showing cracks. These properties
more noticeable with increasing plasticity. Quartz of
clay size (often glacial rock

CLAY

flour) has very different
properties than true clay minerals.

ORGANIC
CLAY, SILT
OR SAND

- varies Contains much organic vegetable matter; often has

a noticeable smell and changes to different colours
because of oxidation (red/yellow) or reducing
environment (green/blue/black).

PEAT - varies Predominantly plant remains; usually dark brown or

black in colour; often with distinctive smell; low bulk
density.

are presented in Tables C4 and C5 and the potential for confusion is
obvious. The engineering geologist must be cautious, especially where
using an empirical guideline, say for allowable bearing pressure or
rippability of rock based on intact rock strength (e.g. Table 6.1). Refer
to the actual UCS in MPa and not the descriptive term that could
mean different things, depending on who has logged the sample and
in which country.