Characteristics of Geologic Materials and Formations - Geotechnical Engineering Investigation - page 387

FIGURE 5.11
Shrinkage pattern development in
clayey alluvium with silt lenses.
Shrinkage is caused by capillary tension
developing in porewater during drying
(Fronteira, Piaui, Brazil).

purity. Clay soils are commonly identified by the relationship between the plasticity index
and the liquid limit, as given in Figure 3.12.
Activity refers to an affinity for moisture, resulting in large volume changes with an
increase in moisture content (swelling) or decrease in moisture content (shrinking) (Figure
5.11), which is due to the crystal structure and chemistry. The degree of activity is related
to the percent of the clay fraction in the specimen and the type of clay mineral, and has
been defined as the ratio of the plasticity index to the percent by weight finer than 2

m.
A clay classification based on activity is given in Table 5.29. Expansive materials and their
characteristics are described in Section 10.6.2.

µ

Clay Mineralogy and Chemistry
Clay Minerals
Clays are hydrous aluminum silicates that are classified into a number of groups based on
their crystal structure and chemistry. Common groups include kaolinite, halloysite, illite,
and montmorillonite. Less common groups include vermiculite and chlorite, which
although common in decomposing rock masses, transform readily to the other types.
Characteristics of the common clay minerals are summarized in Table 5.30. Classification
of the clay minerals based on chemistry and crystal structure is given in Table 5.28.

Clay Chemistry
Classes : Clays are also classified on the basis of the cations adsorbed on the particle sur-
faces of the mineral (H, Ca, K, Mg, or Na). Sodium clays may be the product of the depo-
sition of clay in seawater, or of their saturation by saltwater flooding or capillary action.

TABLE 5.29
Clay Activity a

Activity b Classification

0.75 Inactive clays (kaolinite)

0.75-1.25 Normal clays (illite)

1.25 Active clays (montmorillonite)

a

After Skempton, A.W., Proceedings of the 3rd International Conference on Soil Mechanics and Foundation
Engineering , Switzerland, Vol. I, 1953, pp. 57-61.

b

Ratio of PI to percent by weight finer than 2 m µ.