The object of this experiment is to determine the Liquid and Plastic Limits of a given sample of cohesive soil and hence classify the soil according to Casagrandes Classification Chart.

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By Tejinder Sohel

HNC Civil Engineering

Soil Mechanics

Atterberg Limits

Object

The object of this experiment is to determine the Liquid and Plastic Limits of a given sample of cohesive soil and hence classify the soil according to Casagrandes Classification Chart.

These Limits are defined as: -

 

Liquid Limit (LL): - The minimum moisture content at which the soil will flow under its own weight.

The liquid limit is the empirically established moisture content at which a soil passes from the liquid state to the plastic state. It provides a means of classifying a soil, especially when the plastic limit is also known.

Plastic Limit (P.L): - The minimum moisture content at which the soil can be rolled into a tread 3mm diameter without breaking up.

The plastic limit is the empirically established moisture content at which a soil becomes too dry to be plastic. It is used together with the liquid limit to determine the plasticity index which when plotted against the liquid limit on the plasticity chart provides a means of classifying cohesive soils.

Apparatus

Procedure

The soil sample has been air dried and mixed. Then was sieved through a 425μm BS sieve. It was then kept in an airtight container until the test was to be executed.

Liquid Limit Procedure

  1. Take a sample of about 300g of soil. Transfer the soil to a flat glass plate. Add distilled water and mix thoroughly with two palette knives until the mass becomes a thick homogeneous paste.
  2. Mix the paste for at least 10 minutes using the two palette knives. If necessary add more distilled water so that the first cone penetration reading is about 15mm.
  3. Push a portion of the mixed soil into the cup with a palette knife taking care not to trap air. Strike off excess soil with the straightedge to give a smooth level surface.
  4. With the penetration cone locked in the raised position lower the supporting assembly so that the tip of the cone just touches the surface of the soil. When the cone is in the correct position a slight movement of the cup will just mark the soil surface. Lower the stem of the dial gauge to contact the cone shaft and record the reading of the dial gauge to the nearest 0.1mm.
  5. Release the cone for a period of 5 seconds. After locking the cone in position lower the stem of the dial gauge to contact the cone shaft and record the reading of the dial gauge to the nearest 0.1 mm. Record the difference between the beginning and end of the drop as the cone penetration.
  6. Lift out the cone and clean it carefully to avoid scratching.
  7. Add a little more wet soil to the cup, taking care not to trap air, strike off excess soil with the straightedge to give a smooth level surface. Repeat the cone penetration test (procedure 4 as stated above).
  8. If the difference between the first and second penetration reading is not more than 0.5 mm record the average of the two penetration. If the second penetration is more than 0.5 mm and less than 1 mm difference from the first, carry out a third test. If the overall range is then not more than 1mm record the average of the three penetrations. If the overall range is more than 1mm remove the soil from the cup, remix and repeat procedure 3 to 8 until consistent results are obtained.
  9. Take a moisture content sample from the area penetrated by the cone and place in a clean and dry sample bowl (weigh the sample bowl before on a balance).
  10. Weigh sample bowl on balance with the sample. Place the sample bowl in the microwave for approximately 3 minutes.
  11. After drying remove sample bowl and contents from microwave and leave to cool. Weigh sample bowl with contents on a balance to the nearest 0.01 g. Once these weights are obtained the moisture content of the sample can be calculated (see calculations).    
  12. Repeat procedure 3 to 11 at least three more times using the same sample of soil to which further increments of distilled water have been added. Proceed from the drier to the wetter condition of the soil. The amount of water added shall be such that a range of penetration value of approximately 15mm to 25mm is covered by the four or more test runs and is evenly distributed. Each time soil is removed from the cup for the addition of water, wash and dry the cup.
  13. If at any time during the procedure the soil has to be left for a while on the glass plate cover the soil with the damp cloth to prevent the soil drying out.

 

Plastic Limit Procedure

  1. Take a sample of about 20g of soil. Transfer the soil to a flat glass plate. Add distilled water and mix thoroughly with two palette knives until the mass becomes a thick homogeneous paste.
  2. Allow the soil to dry partially on the plate until it becomes plastic enough to be shaped into a ball
  3. Mould the ball of soil between the fingers and roll it between the palms of the hands until the heat of the hands has dried the soil sufficiently for slight cracks to appear on its surface. Divide this sample into two subsamples of about 10g each and carry out a separate determination on each portion. Divide each subsample into four more or less equal parts and treat each part as stated in procedure 4.
  4. Mould the soil in the fingers to equalize the distribution of moisture, then form the soil into a thread about 6mm diameter between the first finger and tumb of each hand.
  5. Roll the thread between the fingers, form fingertip to the second joint, of one hand and the surface of the glass rolling plate. Use enough pressure to reduce the diameter of the thread to about 3mm in five to 10 complete, forwards and backwards, movements of the hand. Some heavy clays will require 10 to 15 movements when the soil is near the plastic limit because the soil hardens at this stage. It is important to maintain a uniform rolling pressure; do not reduce the pressure as the thread diameter approaches 3mm.
  6. Pick up the soil, mould it between the fingers to dry it further, form it into a thread and roll it out again as specified in procedure 5.
  7. Repeat procedure 6 until the thread shears both longitudinally and transversely when it has been rolled to about 3 mm diameter. Do not gather the pieces of soil together after they have crumbled, in order to reform a thread and to continue rolling; the first crumbling point is the plastic limit.
  8. Gather together the portions of the crumbled soil thread, transfer them to a suitable container and replace the lid immediately (weigh container before sample is put in container).
  9. Repeat procedure 4 to 8 on the other three portions of soil, placing them all in the same container.
  10. Weigh sample bowl on balance with the samples. Place the sample bowl in the microwave for approximately 3 minutes.
  11. After drying remove sample bowl and contents from microwave and leave to cool. Weigh sample bowl with contents on a balance to the nearest 0.01 g. Once these weights are obtained the moisture content of the sample can be calculated (see calculations).
  12. Repeat procedure 3 to 11 to gain second set of results to reaffirm first set of calculations.
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Using the calculations for the plastic limit and liquid limit; the Plasticity Index value can be calculated and therefore the soil can be catalogued.

Discussion

During the experiment several errors could occur. One of the basic errors is misreading the dial gauge scale for the Penetrometer test, fortunately we where briefed thoroughly about the scale from our tutor so that was avoided.

The Penetrometer cone is a crucial part of the apparatus when undertaking the penetration test to work out the liquid limit. These cones have to bee sharp so that they can adequately penetrate the soil ...

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