Page 265 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
260 Geotechnical Engineering

compacted clay is less effective, perhaps because it interrupts the continuum (Katti
and Katti, 2005).

12.9 PLASTICITY INDEX VS. LIQUID LIMIT

12.9.1 Concept

The relationship between PI and LL reflects clay mineralogy and has an advan-
tage over the activity index because a particle size analysis is not required. Because
the liquid limit appears on both sides of the relationship, data can plot only within
a triangular area defined by the PL ¼ 0 line shown in Fig. 12.11.

12.9.2 The A-Line
A line that approximately parallels the PI versus LL plot for particular soil groups
is called the A-line, which was proposed by A. Casagrande and for the most part
separates soils with and without smectite clay minerals. However, the separation is
not always consistent, as can be seen in Fig. 12.11 where loess crosses the line.
At low clay contents loessial soils also are more likely to show collapse behavior
instead of expanding.

12.10 A SOIL CLASSIFICATION BASED ON THE A-LINE

12.10.1 Background
During World War II, Arthur Casagrande devised a simplified soil classification
system for use by the armed forces. The objective was a system that could be used
to classify soils from visual examination and liquid/plastic behavior. In 1952

Figure 12.11
Representative
relationships
between PI and
LL. (Adapted from
U.S. Dept. of
Interior Bureau of
Reclamation,
1974.)

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