Atterberg Limits – Lab

Objective

For fine-grained soils the expected behavior is not strictly dependent particle size.  Rather, fine-grained soil is mostly affected by the water content of the soil. To assess the way the fine- grained soil will behave, it is necessary to determine the liquid and plastic limit of the soil. The liquid limit describes the point at which the water content alters the consistency of the soil from plastic to liquid.  The plastic limit defines the point at which the soil transitions from brittle to plastic.  Knowing these figures provides an index of plasticity that allows a soil to be easily classified.

Procedure

A fine-grained soil, or clay is considered to be dry soil that passes through sieve #40. To Approximately 200 grams of this soil will be needed to complete this experiment.  Moisture cans will be needed to weigh and dry each sample. Record the weights of about ten of these cans.

To determine the liquid limit of the soil a device called a Casagrande will be needed.  Make sure the device is completely cleaned of all residues.  Once cleaned, the Casagrande should be calibrated so that just before the bowl is about to fall there is a 1 cm gap (the width of the grooving tool) between the base and the bowl. Take roughly two thirds of the weigh soil sample and place it in a mixing dish.  Gradually add water to the soil, till it has a paste-like consistency. Use the spatula to transfer the sample to the bowl of the Casagrande.  Apply the sample so that it covers the lower two thirds of the bowl.  Make sure the sample is flat and smooth in the bowl.  With the grooving tool wedge, slowly groove the sample. Remove the sample until the wedge can pass through without pushing excess sample of the edge. Once a good groove has been cut, crank the Casagrande at 2 blows per second. Count the number of blows it takes to reduce the gap of the groove to half an inch. Once closed take a small portion of the sample, place it in one of the weighed moisture can, and measure how much it weighs.  Clean the Casagrande. Adjust the water content of the soil by adding soil or water.  Repeat the explained process for samples consisting of a range of blows e.g. 10, 20, 30, and 40 blows.

The soil and water mixture that requires approximately 40 blows should be used for the plastic limit test.  Using this sample, roll threads of clay to about 3mm in thickness.  Each thread should be rolled until they begin cracking.  Take three of these threads and place them in a moisture can.  Record how much the can and sample weighs.  Repeat this so that you have three moisture cans of soil threads.

Place all of the weighed samples, for both the liquid and plastic test, in the oven.  Let the samples dry for 24 hours.  Once dried, weigh and record the masses of the samples.

Calculations

To obtain the plastic index of the soil, the liquid and plastic limits need to be determined. After obtaining the masses of the wet soil, the containers, and the dry mass.   The mass of the dry soil can be obtain by subtracting the mass of the dry soil and can by the mass of the can (appendix 1).  The mass of the water can be determined by subtracting the mass of the   wet soil and can by the mass of the dry soil and can (2). To determine the limits of the soil sample the water content of the soil is needed.  The water content is the mass of the water divided by the mass of the dry soil, this number multiplied by 100 the percentage of the soil that is water. The liquid limit of a soil is the water content for a soil sample that takes 25 blows in the Casagrande test.  A plot of the blows and respective water contents (fig. 1) provides the ability determine the water content of at 25 blows.  Excel can be used to make a linear approximation of the data set.  Using the equation for the linear approximation, the exact water content at 25 blows can be determined (3).  The plastic limit of the soil is the average water content of the threads (4).  The plastic index is simply the difference between the liquid limit by the plastic limit (5).

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