Procedure of Test for Determining the Plastic Limit and Plasticity Index of Soils

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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