How to Determine the Atterberg Limits of Clay Soil
The Atterberg Limits measure the graininess and strength of silt and clay soil. They define the behavior of soil by measuring the shrinkage limit, liquid limit and plastic limit. Usually only the liquid (LL) and plastic limits (PL) are determined. The liquid limit is the amount of water that is needed to liquify the soil. The plastic limit is the amount of water that makes the soil become malleable. The plastic index (PI) is very useful, it characterizes the soil and it is the difference between the liquid and the plastic limit (PI=LL-PL). A low plastic index (0 to 10) indicates silt, while a high PI (more than 35) indicates clay. The liquid and plastic limit devices, needed to do the determinations, can be purchased from companies in the U.S. such as Geotest Instrument Corporation (http://www.geotestusa.com) or from global companies such as Associated Scientific and Engineering Works (http://testinginstrumentsindia.com). Does this Spark an idea?
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Things You'll Need
- Weighing balance
- Casagrande's liquid limit device
- Grooving tool
- Oven
- Wash bottle
- TxDOT plastic limit device
- Weighing containers
- Paper
- Porcelain dish
Instructions
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Determining the Liquid Limit
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1
Place a cylindrical clay sample (about 9 ounces) into a porcelain dish and add water if necessary to produce a pasty soil consistency. Detailed methods to determine the Atterberg Limits can be found at Geotech: http://geotech.
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2
Place the clay sample into the Casagrande cup; the soil sample should be about five-sixteenths of an inch deep.
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3
Use the grooving tool to cut a groove through the center of the soil sample.
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4
After cutting the sample, rotate the handle on the Casagrande device to drop the cup with the soil sample onto a rubber base; a drop is called a "blow". Count the number of blows it takes for the cut parts of the soil sample to come together.
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5
Take a small sample from the closed groove and determine its water content by weighing the sample when it is wet. Use an electric oven or other device to dry the same sample. Subtract the dry weight from the wet weight of the soil sample and calculate the percent of water weight of the sample.
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6
Repeat steps one through five about five times and plot the percent of water content versus the number of blows to generate a "flow" curve. Read the liquid limit (percent water content) from the curve at the point of 25 blows.
Determining the Plastic Limit
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7
Take about one ounce of air-dried clay and add some water to moisten it.
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8
Prepare several small thread-like rolls of soil (about one-third of an ounce each) and place them in the plastic limit device.
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9
Roll the upper part of the plastic limit device to generate threads of soil. Let the threads fall onto clean paper.
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10
Keep kneading soil samples to express water until the emerging thread becomes crumbly.
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11
Weigh the crumbled soil sample and again after the sample has been dried. The water weight content of the difference between the crumbly soil sample and the dry sample is the plastic limit, when expressed as percent of water.
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1
Tips & Warnings
Repeat all determinations three times to get more accurate values.
When you determine the plastic limit and the soil threads can be reduced to less than one-sixteenth of an inch without cracking, it means the water content is higher than the plastic limit.
Devices other than the Casagrande's device and the TxTOT device can also be used to determine Atterberg Limits.
It may take considerable pressure with some clay samples to deform the thread in the plastic limit test.
References
- Photo Credit clay image by Pavel Korsun from Fotolia.com
