Chloe Zhang
The plastic limit of soil is the moisture content at which it begins to behave like a plastic material. This is the point at which the soil will crumble when rolled into a thread. The plastic limit is used, along with the liquid limit, to calculate the flow index, toughness index and plasticity index of the soil. The plasticity index can be calculated as PI = LL ‑ PL, where LL is the liquid limit and PL is the plastic limit. To determine the plastic limit of a soil sample, a series of steps must be followed, including rolling the soil into a ball and then into a thread of uniform diameter. This procedure is repeated three times to calculate an average plastic limit for the sample.
| Characteristics | Values |
|---|---|
| Definition | The plastic limit of soil is the moisture content at which soil begins to behave as a plastic material. |
| Plastic Limit Test | One of the laboratory tests used internationally to differentiate or classify soils into groups. |
| Plastic Limit Test Apparatus | Laboratory porcelain dish, spatula, soil sample, balance to an accuracy of 0.01g, wash bottle with distilled water, drying oven, glass plate and similar weight moisture cans. |
| Plastic Limit Test Procedure | Mix soil sample with water, form a ball, roll into a thread of 3mm or 1/8-inch diameter, break and repeat procedure until thread crumbles, dry soil in an oven, obtain dry weight of soil samples. |
| Plasticity Index | Calculated as PI = LL ‑ PL, where LL = liquid limit and PL = plastic limit. |
| Plastic Limit and Engineering Behavior | The liquid limit, plastic limit, and plasticity index of soils are used to correlate with engineering behavior such as compressibility, hydraulic conductivity, shrink-swell, and shear strength. |
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What You'll Learn
Plastic limit test procedures
The plastic limit of soil is the moisture content at which it starts to behave like a plastic material. At this water content, the soil will crumble when rolled into threads. The plastic limit is an important measure to ascertain the plasticity index of the soil.
The following apparatus is required to carry out the test: a laboratory porcelain dish, spatula, soil sample, balance to an accuracy of 0.01g, wash bottle with distilled water, drying oven, glass plate, and similar-weight moisture cans.
Firstly, measure the weights of the moisture cans and record their weights on a sheet. Take a soil sample of around 20g that has passed through a sieve and add distilled water gradually while mixing with a spatula. Form a ball from the watered soil sample using your palms—the ball should not be sticky.
Next, form a uniform thread from the soil ball by rolling it on a glass plate using your palms or fingers. Apply enough pressure by exerting 80-90 strokes per minute, with a stroke being one complete forward and backward movement of your hand. The thread should be rolled until it reaches a diameter of 3mm or 1/8 inches. Then, break the thread into pieces and repeat the procedure for those broken pieces. The procedure can be repeated until the thread crumbles.
Finally, measure the weight of the crumbled soil collected in the moisture cans. Keep the cans in an oven for at least 16 hours to dry them. Obtain the dry weights of all soil samples separately by measuring them at a temperature of 115 degrees Celsius.
Repeat the test at least three times and take the average of the results calculated to the nearest whole number. This average is the plastic limit of the soil.
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How to prepare a soil sample
To prepare a soil sample for a plastic limit test, the following steps can be taken. It is important to note that the plastic limit of a soil is the moisture content at which it begins to behave like a plastic material.
Firstly, obtain the dry weight of the soil sample by heating it to a temperature of 115 degrees Celsius. This step should be repeated for all soil samples. The moisture content of the soil sample can then be calculated. The plastic limit is the average moisture content of the samples.
Next, sieve the soil sample. Place around 250g of air-dried soil through a #40 sieve into an evaporating dish. Add water to the sieved soil sample. This should be done gradually, mixing the sample with a spatula until a uniform paste is formed. The paste should be smooth and not sticky.
Then, place the soil in Casagrande's cup and use a spatula to smooth the surface so that the maximum depth is around 8mm. Use a grooving tool to cut a groove in the centre line of the soil cup.
Finally, crank the device at a rate of 2 revolutions per second until there is a clear visible closure of 12.7mm in the soil. Count the number of blows (N) that caused the closure. The paste should be made so that N begins with a value higher than 35.
The plastic limit test is used to differentiate and classify soils and is used internationally. The test is important to ascertain the plasticity index of the soil. The plastic limit is the boundary between the plastic and solid state of the soil.
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How to roll a soil sample
The plastic limit of a soil sample is the moisture content at which the soil begins to behave as a plastic material. This is when the soil will crumble when rolled into threads of 3.2 mm (1/8 in) in diameter.
To calculate the average plastic limit, you must first prepare your soil sample:
Preparing the soil sample
- Place about 250 g of air-dried soil through a # 40 sieve into an evaporating dish.
- Add water to form a uniform paste.
- Place the soil in a Casagrande’s cup and use a spatula to smooth the surface so that the maximum depth is about 8 mm.
- Using the grooving tool, cut a groove at the centre line of the soil cup.
- Crank the device at a rate of 2 revolutions per second until there is a clear visible closure of 1/2” or 12.7 mm in the soil pat placed in the cup.
- Count the number of blows that caused the closure.
Rolling the soil sample
- Take a small portion of the soil sample and form it into a ball. The ball should be non-sticky.
- Roll the ball into a uniform thread on a glass plate using your palms or fingers.
- Apply enough pressure by exerting 80-90 strokes per minute.
- Roll the thread until it reaches a diameter of 3 mm or 1/8 inches.
- Break the thread into pieces and repeat the procedure for the broken pieces.
- Continue until the thread crumbles.
- Measure the weight of the crumbled soil.
- Place the soil in an oven for at least 16 hours to dry it.
- Obtain the dry weight of the soil sample by measuring it after heating to a temperature of 115-degree celsius.
The plastic limit can be determined by the moisture content at which the thread breaks apart at a diameter of 3.2 mm.
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How to calculate the plasticity index
The plasticity index (PI) is a measure of the plasticity of a soil sample. It is calculated using the liquid limit (LL) and plastic limit (PL) of the soil. The plastic limit of a soil sample is the moisture content at which the soil begins to behave like a plastic material. In other words, it is the moisture content at which the soil can no longer be remoulded without cracking.
To calculate the plasticity index, follow these steps:
Step 1: Gather the necessary equipment
You will need a laboratory porcelain dish, a spatula, a soil sample, a balance with an accuracy of 0.01 g, a wash bottle with distilled water, a drying oven, a glass plate, and moisture cans.
Step 2: Prepare the soil sample
Take a portion of the soil sample that passed through the No. 40 sieve pan. Add water from the wash bottle little by little while mixing the sample with the spatula. Continue mixing until the soil is moist enough to form a non-sticky ball when rolled between your palms.
Step 3: Form a thread
Roll the soil ball on the glass plate using your palms or fingers, applying enough pressure to form a uniform thread. Continue rolling until the thread reaches a diameter of approximately 3 mm or 1/8 inches. If the thread becomes too dry and brittle, you may need to add a small amount of water to the soil ball and repeat this step.
Step 4: Dry the soil
Break the thread into smaller pieces and place them into the moisture cans. Remove the lids from the cans and place them in the oven to dry for at least 16 hours at a temperature of 115 degrees Celsius. This step ensures that any remaining moisture in the soil evaporates.
Step 5: Weigh the dried soil
After drying, weigh each soil sample separately and record their weights. These weights represent the dry weights of the soil after the water has evaporated.
Step 6: Calculate the plastic limit
The plastic limit can be calculated by subtracting the percentage of water lost during drying from 100. This value represents the moisture content at which the soil transitions from a plastic to a solid state.
Step 7: Determine the liquid limit
The liquid limit is the moisture content at which the soil transitions from a plastic to a liquid state. This can be determined by gradually adding water to the soil sample and measuring the moisture content at which the soil flows like a liquid.
Step 8: Calculate the plasticity index
Finally, the plasticity index can be calculated by subtracting the plastic limit from the liquid limit. This value provides a measure of the plasticity or workability of the soil. A higher plasticity index indicates a higher plasticity, while a lower index suggests a lower plasticity.
It is important to note that the plasticity index is just one aspect of soil behaviour, and other factors such as grain size distribution, mineralogy, and organic matter content also play a significant role in determining the overall behaviour and characteristics of soil.
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The Atterberg limit
The plasticity index (PI) is a measure of the plasticity of soil and is calculated as the difference between the liquid and plastic limits (PI = LL-PL). Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 (non-plastic) tend to have little or no silt or clay. The liquidity index (LI) is used to scale the natural water content of a soil sample to the limit and can be calculated as a ratio of the difference between natural water content, plastic limit, and liquid limit: LI = (W-PL)/(LL-PL), where W is the natural water content.
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Frequently asked questions
The plastic limit of a soil is the moisture content at which soil begins to behave as a plastic material.
The plastic limit is used to ascertain the Plasticity Index (PI) of the soil. The liquid limit, plastic limit, and plasticity index of soils are used extensively to correlate with engineering behavior such as compressibility, hydraulic conductivity, and shear strength.
The procedure involves rolling a clay sample to a 3mm diameter cylindrical shape. The clay sample loses moisture and cracks start to appear. The water content at which cracks appear is defined as the plastic limit.
The plastic limit is the average of the three water contents obtained from three plastic limit tests.
