Page 258 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 253

the range of moisture content within which it exhibits the properties of a plastic
solid is not measured by the standard laboratory tests.

12.5.3 Nonplastic Soils
Drying and manipulating a truly nonplastic soil such as a clean sand will cause it
to abruptly change from a liquid state to an incoherent granular material that
cannot be molded. If it is not possible to roll soil into a thread as small as 3 mm in
diameter, a plastic limit cannot be determined and the soil is said to be nonplastic,
designated as NP in test reports.

12.6 ACTIVITY INDEX AND CLAY MINERALOGY

12.6.1 Definition of Activity Index
The activity index was defined by Skempton to relate the PI to the amount of clay
in a soil, as an indication of the activity of the clay and therefore the clay mineral-
ogy: the higher the activity index, or AI, the more active the clay. The activity index
was defined as the PI divided by the percent 0.002 mm (or 2 mm) clay:
PI
AI ¼ ð12:3Þ
C 002
where AI is the activity index, PI the plasticity index, and C 002 the percent 2 mm
clay determined from a particle size analsysis. The basis for this relationship
is shown in Fig. 12.5.

12.6.2 Relation to Clay Mineralogy
The relationship between activity index and clay mineralogy is shown in
Table 12.1. Clay mineral mixtures and interlayers have intermediate activities.
Data in the table also show how the activity of smectite is strongly influenced by
the adsorbed cation on the plasticity index.

12.6.3 Modified Activity Index

The linear relationships in Fig. 12.5 do not necessarily pass through the origin.
This is shown in Fig. 12.6, where about 10 percent clay is required to generate
plastic behavior. This also has been found in other investigations (Chen, 1988).

It therefore is recommended that for silty soils eq. (12.3) be modified as follows:
PI
A ¼ ð12:4Þ
C 002 k
where A is the activity, C is the percent of the soil finer than 0.002 mm, and
k is a constant that depends on the soil type. For silty soils k ¼ 10. If this equation

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.