Page 252 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 247

Figure 12.1
Schematic
representation of
transitions between
solid, plastic, and
viscous liquid
behaviors defined
by liquid and
plastic limits. These
tests are basic to
engineering
classifications and
emphasize
influences of clay
mineralogy and
capillarity.

The numerical difference between the two limits therefore represents a range in
moisture contents over which the soil is plastic, and is referred to as the plasticity
index or PI. By definition,
PI ¼ LL PL ð12:1Þ
where PI is the plasticity index and LL and PL the liquid limit and plastic limit,
respectively. This relationship is shown in Fig. 12.1. Because the plasticity index
is a difference in percentages and not in itself a percentage, it is expressed as a
number and not a percent. Also shown in the figure is the shrinkage limit, which is
discussed later in the chapter.

12.1.3 Preparation of Soil for Testing
As discussed in relation to clay mineralogy, drying a soil can change its
adsorptive capacity for water and therefore can change the liquid and plastic
limits. If the soil contains the clay mineral halloysite, dehydration from
air-drying is permanent, so to obtain realistic data the soil must not be dried
prior to testing. A similar change can occur in soils that have a high content of
organic matter.

Air-drying nevertheless is still an approved method because it is more convenient
for storing soil samples and for dry sieving, because only the portion of a soil
passing the No. 40 (425 mm) sieve is tested. Also, many existing correlations were
made on the basis of tests of air-dried samples. If a soil has been air-dried it should
be mixed with water for 15 to 30 minutes, sealed and stored overnight, and
re-mixed prior to testing. Details are in ASTM D-4318.

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