Civil Engineering Reference
In-Depth Information
Classification of soils
5.1 Description and classification
Soils consist of grains, usually rock fragments or clay particles, with water and gas,
usually air or water vapour, in the void spaces between the grains. If there is no gas
present the soil is saturated and if there is no water it is dry, while if there is both
water and gas in the voids the soil is unsaturated. The mechanics of unsaturated soils
is very complicated and in this topic I will normally consider saturated or dry soils.
Fortunately, in civil engineering applications soils are mostly saturated, except in hot
dry environments or when compacted.
The mechanical properties of a soil (i.e. its strength and stiffness) depend principally
on the nature of the grains (i.e. what they are) and the state of the soil (i.e. how
the grains are packed together). You can dig up a sample of soil from your garden
or from the beach and describe what you see. You can describe its colour, the size
and shape of the grains (if you can see them) and some aspects of the behaviour,
such as its response to moulding in your fingers. To be useful, however, you will
need a scheme of classification that separates groups of soils with markedly different
behaviour. Any useful scheme of soil classification should be based on relatively simple
tests and observations.
It is important to distinguish between soil description and soil classification. Descrip-
tion is simply what you see and how the soil responds to simple tests; you may want
to describe only a single soil sample or a soil profile exposed in a cliff face, in an exca-
vation or from a number of samples from a borehole. A classification is a scheme for
separating soils into broad groups, each with broadly similar behaviour. There are var-
ious classification schemes for different purposes: there are agricultural classifications
based on how soils support crops and geological classifications based on the age of the
deposit or the nature of the grains. For civil engineering purposes soil classifications
should be based mainly on mechanical behaviour.
5.2 Description of soils
Soil description is essentially a catalogue of what the soil is and it is helpful to have
a simple scheme to describe the essential features. There are several such schemes
published in National Standards and to some extent these reflect the characteristics of
