Page 253 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
248 Geotechnical Engineering

12.1.4 Liquidity Index
The liquidity index indicates how far the natural soil moisture content has
progressed between the plastic and liquid limits. If the soil moisture content is at
the plastic limit, the liquidity index is 0; if it is at the liquid limit, it is 1.0.

The formula for the liquidity index is
w PL
LI ¼ ð12:2Þ
LL PL
where LI is the liquidity index, w is the soil moisture content, PL is the plastic limit,
and LL is the liquid limit. The liquidity index also is called the relative consistency.

12.2 MEASURING THE LIQUID LIMIT

12.2.1 Concept
The concept of the liquid limit is simple: keep adding water to a soil until it flows,
and measure the moisture content at that point by oven-drying a representative
sample. Two difficulties in application of this concept are (1) the change from
plastic to liquid behavior is transitional, and (2) flow can be prevented by
thixotropic setting.

In order to overcome these limitations, Atterberg suggested that wet soil be placed
in a shallow dish, a groove cut through the soil with a finger, and the dish
jarred 10 times to determine if the groove closes. While this met the challenge
of thixotropy, it also introduced a personal factor. Professor A. Casagrande
of Harvard University therefore adapted a cog arrangement invented by
Leonardo da Vinci, such that turning a crank drops a shallow brass cup
containing wet soil 10 mm onto a hard rubber block, shown in Fig. 12.2.
The crank is turned at 2 revolutions per second, and the groove is standardized.

Casagrande defined the liquid limit as the moisture content at which the groove
would close after 25 blows, which increased the precision of the blow count
determination. Different amounts of water are added to a soil sample and stirred
in, and the test repeated so that the blow counts bracket the required 25. As it is
unlikely that the exact number will be achieved at any particular moisture content,
a graph is made of the logarithm of the number of blows versus the moisture
content, a straight line is drawn, and the liquid limit read from the graph where
the line intersects 25 blows (Fig. 12.3).

12.2.2 Procedure for the Liquid Limit Test
A quantity of soil passing the No. 40 sieve is mixed with water to a paste
consistency and stored overnight. It is then re-mixed and placed in a standardized

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