Procedure of Test for Determining the Plastic Limit and
Plasticity Index of Soils
Standard Method of Test for Determining the Plastic Limit and Plasticity
Index of Soils
AASHTO
AASHTO
Designation: T 90-16
Technical
Section: 1a, Soil and Unbound Recycled
Material
Release:
Group 3 (OCTOBER)
6. PROCEDURE
6.1 Determine and record the mass of
moisture content container.
6.2 Select a 1.5 to 2.0 g portion from
10g mass of soil prepared in accordance with Section
5. Form the selected portion into an ellipsoidal mass.
6.3 Use one of the following methods to
roll the soil mass into a 3-mm thread at a rate of 80 to 90 strokes per minute,
counting a stroke as one complete motion of the hand forward and back to the
starting position again.
6.3.1. Hand Rolling Method- Roll the mass
between the palm or fingers and the ground – glass plate or unglazed paper with
just sufficient pressure to roll the mass into a thread of uniform diameter
throughout its length. Deform the thread further on each stroke until its
diameter reaches 3mm. Take no more than 2 min to roll the soil mass to the 3-mm
diameter.
Note 4-- The amount of hand or finger pressure
required will vary greatly, according to the soil. Fragile soils of low
plasticity are best rolled under the outer edge of the palm or at the base of
the thumb.
6.3.2. Alternate Procedure, Plastic Limit
Device Method--- Place the soil mass on the bottom plate. Place the
top plate in contact with the soil mass. Simultaneously apply a slight downward
force and back and forth motion to the top plate so the plate comes in contact
with the side rails within 2 min. During this rolling process, do not allow the
soil thread to contact the side rails.
Note 5—in most case, more than one soil mass
(thread) can be rolled simultaneously in the plastic limit rolling device.
6.4 When the diameter of the thread
reaches 3mm, squeeze the thread between the thumbs and fingers and form the
mass back into a roughly ellipsoidal shape. Repeat the rolling process
described in Section 6.3.1 or 6.3.2 until the soil can no longer be
rolled into a thread and begins to crumble. Reform the soil into an ellipsoidal
mass after each rolling of the thread to a 3-mm diameter. The crumbling may
occur when the thread has a diameter greater than 3mm.This is considered a
satisfactory end point, provided the solid has been previously rolled into a
thread 3mm diameter (Note 6). Do not attempt to produce failure
at an exact 3-mm diameter by allowing the thread to reach 3 mm, then reducing
the rate of rolling or the hand pressure, or both, and continuing the rolling
without further deformation until the thread falls apart (Note 7)
Note 6— the crumbling will manifest itself
differently with various types of soil. Some soils fall apart in numerous small
aggregations of particles; others may form an outside tubular layer that start
splitting at both ends. The splitting may progress toward the middle, and the
thread may finally fall apart in many small platy particles. Heavy clay soils
require much pressure to deform the thread, particularly as they approach the
plastic limit. These types of soil may break into a series of barrel-shaped
segments each about 6 to 9mm in length
Note 7— for feebly plastic soils, it may be
necessary to reduce the total amount of deformation by making the initial
diameter of the ellipsoidal-shaped mass nearer to the required 3-mm final
diameter.
6.5 Gather the portions of the crumbled
soil together and place in the moisture content container. Immediately cover the container with a
close-fitting lid to prevent additional loss of moisture.
6.6 Repeat the operations described in Section 6.3 through 6.5 until
the entire 10-g specimen is tested. Place all of the crumbled portions into the
same moisture content container.
6.7 Determine the
moisture content of the soil in the container in accordance with T 265, and
record the result.
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