Engineering Geology

Chapter 5
Table 5.6. Mixed coarse soil types
Description
Slightly sandy gravel
Sandy gravel
Very sandy gravel
Gravel/sand
Very gravelly sand
Gravelly sand
Slightly gravelly sand
Composition of the coarse fraction
Up to 5% sand
5–20% sand
Over 20% sand
About equal proportions of gravel and sand
Over 20% gravel
5–20% gravel
Up to 5% gravel
depending on its plastic properties (although it cautioned against the use of the
A-line on the plasticity chart as a reliable method of distinguishing between silts,
which are supposed to plot below, and clays that plot above). It also was recommended
that the terms should be mutually exclusive. In other words, terms such
as silty clay were regarded as redundant. Field identification of fine soils can be
made according to dilatancy, dry strength and toughness tests (see Table 5.2).
The terms outlined in Table 5.7 can be used to describe common soils that
include a mixture of soil types.
3. Geological formation, age and type of deposit.
4. Classification (optional).
When small amounts of organic matter occur throughout a soil, they can have a notable effect
on plasticity and therefore the engineering properties. Increase in the quantities of organic
matter can increase these effects. Nonetheless, soils in which the organic contents may be
up to 30%, by weight, behave primarily as mineral soils.
Table 5.7. Description common types of soils
Approximate proportion of
secondary constituent
Description Main soil type Coarse soil Coarse or fine soil
Slightly clayey or silty Sand — >5%
and/or sandy or gravelly
Clayey or silty and/or or — 5–20%
sandy or gravelly
Very clayey or silty Gravel — >20%
and/or sandy or gravelly
Very sandy or gravelly Silt >65%
Sandy and/or gravelly or 35– 65% —
Slightly sandy and/or gravelly Clay