Classification and Physical Properties of Soils - Elements of Soil Mechanics - page 14

Fig. 1.7 Relationship between I p and clay percentage (after Skempton, 1953) .

1.6 Activity of a clay

In addition to their use in soil classification, the w L and w P values of a plastic soil also give an indication
of the types and amount of the clay minerals present in the soil.
It has been found that, for a given soil, the plasticity index increases in proportion to the percentage
of clay particles in the soil. Indeed, if a group of soils is examined and their I P values are plotted against
their clay percentages, a straight line, passing through the origin, is obtained.
If a soil sample is taken and its clay percentage artificially varied, a relationship between If P and clay
percentage can be obtained. Each soil will have its own straight line because, although in two differing
soils the percentages of clay may be the same, they will contain different minerals.
The relationship between montmorillonite, illite, kaolinite and the plasticity index is shown in Fig. 1.7.
The plot of London clay is also shown on the figure and, from its position, it is seen that the mineral
content of this soil is predominantly illite. London Clay has a clay fraction of about 46 per cent and consists
of illite (70%), kaolinite (20%) and montmorillonite (10%). The remaining fraction of 54 per cent consists
of silt (quartz, feldspar and mica: 44%) and sand (quartz and feldspar: 10%).
In Fig. 1.7 the slope of the line is the ratio

I
clay

P

%

Skempton ( 1953) defined this ratio as the activity of the clay. Clays with large activities are called active
clays and exhibit plastic properties over a wide range of water content values.

1.7 Soil classification and description

1.7.1 Soil classification systems

Soil classification systems have been in use for a very long time with the first recorded use being in China
over 4000 years ago. In 1896 a soil classification system was proposed by the Bureau of Soils, United
States Department of Agriculture in which the various soil types were classified purely on particle size and
it is interesting to note that the limiting sizes used are more or less the same as those in use today. Further