Plastic and Liquid Limits Test
(Atterberg Limit)
Index
1- Scope.
2- Definition of plastic and liquid limit.
3- Apparatuses.
4- Test Preparation.
5- Plastic Limit Test Procedure.
6- Liquid Limit Test Procedure.
7- Calculations.
1- Scope.
This test is used to determine the plasticity index (PI) of clay and silt or fine part passing through a sieve No - 40 of coarse soil.
2- Definition of plastic and liquid limit.
2.1- Definition of plastic limits.
Plastic limit is the water content at which soil will just begin to crumble when rolled into a thread approximately 3 mm in diameter.
Also, can definition as the water content corresponding to an arbitrary limit between the plastic and semi sold consistency states of a soil.
2.2- Definition of liquid limit.
Liquid limit is the water content at which a pat of soil, cut by a groove of standard dimension, will flow together for a distance of 13 mm under the impact of 25 drops by a standard liquid limit device.
Also, can definition as the water content corresponding to any arbitrary limit between the liquid and plastic consistency states of a soil.
3- Apparatuses.
For a plastic and liquid limits test, you will need the following.
3.1- Cazagrande Device.
This device consists of a metal plate with a hard rubber base that can to rise in a specific way by rotating the hand for a distance of 1 cm that can be adjusted before starting the experiment and then the plate falls free to the base.
3.2- Grooving tool.
A combination grooving tool and height calibration gauge.
3.3- Drying Containers.
Small containers about 1 1/2 inches in diameter tared and numbered with indelible ink.
3.4- Scale.
Accurate to 0.01 gram.
3.5- Ground glass plate.
At least 12 inches square by 3/8 inch thick.
3.6- Rubber-Tipped pestle and mortar.
3.7- Mixing Bowl.
Porcelain bowl 4 to 5 inches in diameter.
3.8- Spatula.
Having a flexible blade approximately 3 inches long by 1/2 inch to 3/4 inch wide.
4- Test Preparation.
A natural sample of the soil is used and allow it to air dry or oven dry at 60 C.
Provided that it doesn't contain large grains in general, if the soil has large grains then the sample must be sifted sieve No- 40 and the passing from sieve is taken to be considered the sample that will be tested.
Mixing the sample with water in the mixing bowl.
Use the spatula during mixing water with the sample. Continue to mix, knead, and chop the sample while adding water as needed to bring the soil consistency to somewhere between the liquid and plastic limit.
Break off a portion of the sample for the plastic limit test approx 20 gram and place it in an airtight bag or plastic container to cure.
Seal the remaining portion of the sample in another container. This portion will be used for the liquid limit test.
Waiting for 16 hours to allow both samples to cure. This will ensure that the water and soil practices are thoroughly blended together.
5- Plastic Limit Test Procedure.
Remove half of the soil to be used for the plastic limit test from the container.
Twisted the sample on a glass plate using the fingers. It is noted that the rope start to crack if its diameter is smaller than a certain size according to the strength of the sample.
The twisting process of the rope is repeated until the rope begins to crack or cut at diameter 3 mm. Immediately place the sample into a container and record the weight.
Place the sample in the oven to dry at 110 C. Repeat this procedure for the remaining half of the sample.
The point at which the soil cracks apart and can no longer be twisted into 3 mm thread is the plastic limit.
Any sample that cannot be twisted into a 3 mm should be considered Non-Plastic.
6- Liquid Limit Test Procedure.
6.1- Step 1.
Remove from the container the soil to be used for the liquid limit test. Mixing sample with around 3 to 5 ml and thoroughly mix by chopping, mixing, and kneading.
6.2- Step 2.
Part of the sample is placed in about one-third of the front of a plate using a spatula, level the soil pat to a thickness of 1 cm.
A sample in the plate is divided into two parts using the grooving tool (there are two models for these tools, each arming for certain types of soil according to its viscosity). This tool makes the distance between the two sections about 3 mm and at the same times confirms that the thickness of the sample in the plate is exactly 1 cm.
6.3- Step 3.
The hand of the device is managed at a rate of two turns per second, which is the rate of knocking the plate on the base. Which makes the two divided soil parts flow to each other. So, the distance between them with the succession of beats is narrowed until they are applied to each other with a distance of 13 mm.
If a closure of 13 mm is achieved at between 30 and 35 drops, proceed with step 4. If more than 35 drops are necessary to obtain the 13 mm closure, add more moisture and remix as described in step 1.
If sample requires fewer than 30 blows for proper closure, then the sample is too wet. In that case continue to mix the sample until it has sufficiently dried back. Don't add more soil to help dry the sample back, adding dry soil will create an inconsistently mixed sample (from lack of curing), thus putting the accuracy of the test in question.
Repeat step 1 through step 3 until the proper closure is achieved at between 30 and 35 blows.
6.4- Step 4.
Remove a portion of the soil pat by using the spatula (approximately 10 to 15 gram) from the 13 mm closure. Place the sampling in a numbered container.
Weight the sample and record it with the number of drops on the test sheet as per below figure.
Place the sample in the oven to dry.
Repeat step 1 through step 4 as needed to obtain two more samples. One sample closing between 20 and 30 drops and one sample obtained from between 15 and 20 drops. This will allow three liquid limit samples to be plotted as points on a graph.
6.5- Step 5.
Weight all of the plastic and liquid limit samples that have been dried in the oven.
Then draw an average straight line through the three points. The graph location at which the sloped line intersects the 25 drop line will correspond with a moisture at the left side of the graph.
This corresponding moisture percentage is the liquid limit.
Where LL is the liquid limit and PL is the plastic limit.
6.2- Step 2.
Part of the sample is placed in about one-third of the front of a plate using a spatula, level the soil pat to a thickness of 1 cm.
A sample in the plate is divided into two parts using the grooving tool (there are two models for these tools, each arming for certain types of soil according to its viscosity). This tool makes the distance between the two sections about 3 mm and at the same times confirms that the thickness of the sample in the plate is exactly 1 cm.
6.3- Step 3.
The hand of the device is managed at a rate of two turns per second, which is the rate of knocking the plate on the base. Which makes the two divided soil parts flow to each other. So, the distance between them with the succession of beats is narrowed until they are applied to each other with a distance of 13 mm.
If a closure of 13 mm is achieved at between 30 and 35 drops, proceed with step 4. If more than 35 drops are necessary to obtain the 13 mm closure, add more moisture and remix as described in step 1.
If sample requires fewer than 30 blows for proper closure, then the sample is too wet. In that case continue to mix the sample until it has sufficiently dried back. Don't add more soil to help dry the sample back, adding dry soil will create an inconsistently mixed sample (from lack of curing), thus putting the accuracy of the test in question.
Repeat step 1 through step 3 until the proper closure is achieved at between 30 and 35 blows.
6.4- Step 4.
Remove a portion of the soil pat by using the spatula (approximately 10 to 15 gram) from the 13 mm closure. Place the sampling in a numbered container.
Weight the sample and record it with the number of drops on the test sheet as per below figure.
Place the sample in the oven to dry.
Repeat step 1 through step 4 as needed to obtain two more samples. One sample closing between 20 and 30 drops and one sample obtained from between 15 and 20 drops. This will allow three liquid limit samples to be plotted as points on a graph.
6.5- Step 5.
Weight all of the plastic and liquid limit samples that have been dried in the oven.
7- Calculations.
7.1- Calculate the percent moisture.
percent moisture = (W/D) * 100
where,
W, weight of water = (wet weight of soil and container) - (dry weight of soil and container).
and,
D, weight of dry soil = (dry weight of soil and container) - (weight of container).
7.2- Determine the plastic limit.
Plastic limit (PL) = (percent moisture of plastic limit sample # 1) + (percent moisture of plastic limit sample # 2) / 2.
7.3- Determine the liquid limit.
Proceed as follows:
Plot the percent moisture versus the number of drops for each of the three samples on the liquid limit and plasticity graph as per below figure.
Then draw an average straight line through the three points. The graph location at which the sloped line intersects the 25 drop line will correspond with a moisture at the left side of the graph.
This corresponding moisture percentage is the liquid limit.
7.4- Calculate the plasticity index.
PI = LL - PLWhere LL is the liquid limit and PL is the plastic limit.