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What is plastic limit of soil?

The plastic limit of a soil is the moisture content at which soil begins to behave as a plastic material. At this water content (plastic limit), the soil will crumble when rolled into threads of 3.2mm(1/8in) in diameter.

In this article test methods used for determination of soil plastic limit in accordance with ASTM D4148 will be presented.

Apparatus

  1. Ground Glass Plate: a ground glass plate at least 30 cm (12 in.) square by 1 cm (3?8 in.) thick for rolling plastic limit threads.
  2. Plastic Limit-Rolling Device (optional).
  3. Spatula or pill knife having a blade about 2 cm wide, and about 10 to 13 cm long.
  4. Drying Oven
  5. Metallic rod 3.2mm diameter and 100mm long
  6. Water Content Containers
  7. Balance, conforming to Specification D 4753, Class GP1 (readability of 0.01 g).
Set of apparatus used for plastic limit test

Fig.1: Set of apparatus used for plastic limit test

Soil sample preparation

  1. Select a 20-g or more portion of soil from the material prepared for the liquid limit test.

2. Reduce the water content of the soil to a consistency at which it can be rolled without sticking to the hands by spreading or mixing continuously on the glass plate or in the mixing/storage dish

  1. The drying process may be accelerated by exposing the soil to the air current from an electric fan.
Plastic limit sample

Fig.2: Plastic limit sample

Plastic limit test procedure

  1. select a 1.5 to 2.0 g from the plastic-limit specimen and form the selected portion into an ellipsoidal mass.

    Ellipsoidal plastic limit sample

    Fig. 3: Ellipsoidal plastic limit sample

  2. Roll the soil mass by one of the following methods (hand or rolling device):

Hand Method: roll the mass between the palm or fingers and the ground-glass plate with just sufficient pressure to roll the mass into a thread of uniform diameter throughout its length.

The thread shall be further deformed on each stroke so that its diameter reaches 3.2 mm, taking no more than 2 min.

Normally 80-90 stroke per minute is recommended. count a stroke as one complete motion of the hand forward and back to the starting position. The rate of rolling shall be declined for very fragile soils.

Rolling plastic limit sample

Fig. 4: Rolling plastic limit sample

performing plastic limit test on soil sample

Fig.5:performing plastic limit test on soil sample

Rolling Device Method: attach smooth unglazed paper to both the top and bottom plates of the plastic limit-rolling device. Place the soil mass on the bottom plate at the midpoint between the slide rails.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 that the top plate comes into contact with the side rails within 2 min. During this rolling process, the end the soil thread shall not contact the side rail.

  1. When the diameter of the thread becomes 3.2 mm, break the thread into several pieces. Squeeze the pieces together, knead between the thumb and first finger of each hand, reform into an ellipsoidal mass, and re-roll. Continue this alternate rolling to a thread 3.2 mm in diameter, gathering together, kneading and re-rolling, until the thread crumbles under the pressure required for rolling and the soil can no longer be rolled into a 3.2-mm diameter thread.

    Lean clay soil at plastic limit

    Fig.6: Lean clay soil at plastic limit

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

  1. Select another 1.5 to 2.0-g portion of soil from the plastic limit specimen and repeat step 1 and 2 until the container has at least 6 g of soil.
  2. Repeat step 1 to 5 to make another container that contains at least 6 g of soil.
  3. Determine the water content of the soil contained in the containers in accordance with Test Method D 2216.

Calculation

Calculate the average of the two water contents (trial plastic limits) and round to the nearest whole number; this value is the plastic limit, PL.

The test shall be repeated if unless the difference between the two trail plastic limit is smaller than Repeat the test if the difference between the two trial plastic limits is greater than the acceptable range for two results that is, 1.4 percentage points.

Table 1 Data Sheet for Plastic Limit Test

Sl. No. Observations and Calculations Determination No.
1 2 3
Observation
1Moisture content container number
2Mass of empty container (M1)
3Mass of container + wet soil (M2)
4Mass of container + dry soil (M3)
Calculations
5Mass of water = M2 – M3
6Mass of dry soil = M3 – M1
7Water content = w = [(5) / (6)] x 100

Result

Plastic limit of the soil = _____%

About Madeh Izat HamakareemVerified

Madeh is a Structural Engineer who works as Assistant Lecturer in Koya University. He is the author, editor and partner at theconstructor.org.