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.
- 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.
- Plastic Limit-Rolling Device (optional).
- Spatula or pill knife having a blade about 2 cm wide, and about 10 to 13 cm long.
- Drying Oven
- Metallic rod 3.2mm diameter and 100mm long
- Water Content Containers
- Balance, conforming to Specification D 4753, Class GP1 (readability of 0.01 g).
Soil sample preparation
- Select a 20-g or more portion of soil from the material prepared for the liquid limit test.
- The drying process may be accelerated by exposing the soil to the air current from an electric fan.
Plastic limit test procedure
- select a 1.5 to 2.0 g from the plastic-limit specimen and form the selected portion into an ellipsoidal mass.
- Roll the soil mass by one of the following methods (hand or rolling device):
- 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.
- 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.
- Repeat step 1 to 5 to make another container that contains at least 6 g of soil.
- Determine the water content of the soil contained in the containers in accordance with Test Method D 2216.
CalculationCalculate 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||Moisture content container number|
|2||Mass of empty container (M1)|
|3||Mass of container + wet soil (M2)|
|4||Mass of container + dry soil (M3)|
|5||Mass of water = M2 – M3|
|6||Mass of dry soil = M3 – M1|
|7||Water content = w = [(5) / (6)] x 100|