Plastic limit test of soil, procedure, and sample calculations
Plastic limit test of soil is a laboratory test internationally used to help identify or classify soils into a particular group. Starting the procedures simultaneously of plastic limit and liquid limit test are common. The tests are purposely executed at the same time in most cases. A. Atterberg, a Swedish Scientist, defines the “plastic limits” as a boundary between the plastic and solid state of soils.
These boundaries of soil means, the point at which soils begin to behave as a plastic. Using this method it becomes clear that the moisture of the soil (or water content) determines that boundary specifications. When the moisture content or water content of a soil increases above a certain limit then the soils will begin to behave as liquid. The same can be said when the moisture content or water content of soil drops below a certain level then the soil starts to act like a solid. As stated above the “limit” will refer to as the plastic limit of a soil. Meaning, the plastic limit of a soil is the point of moisture or water content as a percentage of its final weight when dried.
PLASTIC LIMIT TEST PROCEDURES
The apparatus required to perform the test are as follows:
laboratory dish (porcelain material), spatula, soil, balance (scale) that has an accuracy of 0.01g, wash/spray bottle, water that is distilled (tap water is not acceptable), oven for drying the sample, glass plate as well as moisture cans for the sample to be dried. You should follow the procedures below step by step.
- Before starting the test, measure the weights of the tares (moisture cans) also you should name the tares as A, B, C or 1, 2, 3 for easy identification, then record each of the weights with their respective names on a sheet of paper.
- Sieve the soil sample through a number 40 sieve.
- Water should be added to the soil sample that passed the no. 40 sieve using the wash/spray bottle. Add a little bit at a time while thoroughly mixing the soil sample with your spatula.
- Now, make a ball out of the moistened soil sample using the palms of your hand. (The ball should be non stick to your hands.)
- Then make a uniformed thread (like a long noodle) using the ball of soil by rolling it on the glass plate. Use your palms or fingers to make it even. When doing this you must provide enough force by moving your hand at the rate of 90 strokes per minutes. “Moving” means going in a forward and backward movement from the original starting point.
- Continue rolling until the diameter of the sample is 1/8 inches (3mm) in diameter. The thread should be broken into several pieces so you can repeat the same process using the steps above for the smaller “broken” pieces.
- This procedure should be repeated until the threads crumble.
- Measure the mass of the crumbled (broken) soil samples. Separating them into their own tares (moisture cans).
- Now, put the tares (moisture can) without their lids into the oven for a minimum of sixteen hours at a temperature of 240 degrees Fahrenheit (116 degrees Celsius).
- Finally, after the sixteen (16) hours in the oven, measuring each tare separately and record their individual weights on your sheet of paper.
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Use the following procedure to calculate the moisture content of each tare.
Atterbery calculation example
| Sample no | 1 | 2 |
| Mass of empty tare (grams) | 6.12 | 7.52 |
| Mass of tare, + wet soil (grams) | 14.22 | 15.54 |
| Mass of tare, + dry soil (grams) | 13.22 | 14.63 |
| Mass of soil solids (grams) | 7.10 | 7.11 |
| Mass of water/moisture (grams) | 1.00 | 0.91 |
| Water content, w% | 14.1 | 12.8 |
The Atterberg limit can be calculated as (14.1 + 12.8) / 2 = 13.45
See other soil testing methods here
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