Classification of soils - The Mechanics of Soils and Foundations - page 59

Figure 5.4 Limits of consistency of soils.

An important parameter for clay soils is the plasticity index ( I P ), defined as

I P = w L − w P (5.8)

This defines the range of water content for a soil and is related to the maximum volume
change (or compressibility) of the soil. Similarly, the difference between the maximum
and minimum densities is related to the relative compressibility. These limits depend
on the grading and on the mineralogy, shape and surface texture of the grains, so they
describe the nature of the soil. The Atterberg limits are measured on the fraction of a
soil sample smaller than 425

µ m so they describe only part of a well-graded soil. The

activity A is defined as

I P
%by weight of clay

A = (5.9)

This is closely related to the specific surface and to the mineralogy of the clay. Typical
values for common clay minerals are given in Table 5.1.