Determining the Plastic Limit and Plasticity Index of Soil - Soil Properties: Testing, Measurement, and Evaluation - page 94

diameter of the ellipsoidal mass nearer to the required 3.2-mm final
diameter. If crumbling occurs when the thread has a diameter greater
than 3.2 mm, this shall be considered a satisfactory end point, pro-
vided the soil has been previously rolled into a thread 3.2 mm in di-
ameter. Crumbling of the thread will manifest itself differently with
the various types of soil. Some soils fall apart in numerous small ag-
gregations of particles, others may form an outside tubular layer that
starts splitting at both ends. The splitting progresses toward the mid-
dle, and finally, the thread falls apart in many small platy particles.
Fat clay soils require much pressure to deform the thread, particu-
larly as they approach the plastic limit. With these soils, the thread
breaks into a series of barrel-shaped segments about 3.2 to 9.5 mm
( 1 ⁄ 8 to 3 ⁄ 8 in.) in length.

(4) Gather the portions of the crumbled thread together and place
in a container of known mass. Immediately cover the container.

(5) Select another 1.5- to 2.0-g portion of soil from the plastic limit
specimen and repeat the operations described in (1) and (2) until
the container has at least 6 g of soil.

(6) Repeat (1) through (5) to make another container holding at
least 6 g of soil. Determine the water content of the soil contained in
the containers in accordance with ASTM Method D 2216 (see
Chapter 4).

As indicated in the preceding section, the plastic limit is the water
content that exists when the thread crumbles and the soil no longer can
be rolled into another thread. Hence, the only data collected and recorded
are those required to determine the two water contents (see Chapter 3),
namely:

DATA

Mass of container, M c
Mass of container plus moist soil, M cws
Mass of container plus oven-dried soil, M cs

After computing the water content of the soil contained in each con-
tainer [Eq. (4-2)], the average of the two water contents is found. If the
difference between the two water contents is greater than the accept-
able range for two results listed in Table 8-1 for single-operator preci-
sion, the test must be repeated. The plastic limit is the average of the
two water contents.
The plasticity index ( PI ) is determined simply by subtracting the
plastic limit ( PL ) from the liquid limit ( LL ) (the latter to have been
determined previously as explained in Chapter 7). In equation form,

CALCULATIONS

PI LL PL (8-1)