Page 255 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
250 Geotechnical Engineering

as the liquid limit. Tests usually are performed in duplicate and average results
reported.

A ‘‘one-point’’ test may be used for routine analyses, in which the number of
blows is between 20 and 30 and a correction that depends on the departure
from 25 is applied to the moisture content. See AASHTO Specification T-89 or
ASTM Specification D-423 for details of the liquid limit test.

12.3 MEASURING THE PLASTIC LIMIT

12.3.1 Concept

Soil with a moisture content lower than the liquid limit is plastic, meaning that
it can be remolded in the hand. An exception is clean sand, which falls apart on
remolding and is referred to as ‘‘nonplastic.’’ It is the plasticity of clays that allows
molding of ceramics into statues or dishes. At a certain point during drying,
the clay can no longer be remolded, and if manipulated, it breaks or crumbles;
it is a solid. The moisture content at which a soil no longer can be remolded
is the plastic limit,or PL.

The standard procedure used to determine the plastic limit of a soil is deceptively
simple. The soil is rolled out into a thread, and if it does not crumble it is then
balled up and rolled out again, and again, and again ... until the thread falls apart
during remolding. It would appear that a machine might be devised to perform
this chore, but several factors make the results difficult to duplicate. First, the soil
is continuously being remolded, and second, it gradually is being dried while being
remolded. A third factor is even more difficult—the effort required to remold the
soil varies greatly depending on the clay content and clay mineralogy. Despite
these difficulties and the lack of sophistication, the precision is comparable to or
better than that of the liquid limit test.

12.3.2 Details of the Plastic Limit Test

The plastic limit of a soil is determined in the laboratory by a standardized
procedure, as follows. A small quantity of the soil-water mixture is rolled out with
the palm of the hand on a frosted glass plate or on a mildly absorbent surface such
as paper until a thread or worm of soil is formed. When the thread is rolled to a
1
diameter of 3 mm ( in.), it is balled up and rolled out again, the mixture gradually
8
losing moisture in the process. Finally the sample dries out to the extent that it
becomes brittle and will no longer hold together in a continuous thread. The
moisture content at which the thread breaks up into short pieces in this rolling
process is considered to be the plastic limit (Fig. 12.4). The pieces or crumbs
therefore are placed in a small container for weighing, oven-drying, and
re-weighing. Generally at least two determinations are made and the results

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