Liquid Limit of soil

Liquid limit is defined as that minimum water content at which it has a tendency to flow. All soil at liquid limit possesses similar shear strength which is negligible.


Determination of Liquid Limit

  1. Casagrande's Tool

    Casagrande tool to be used in the determination shall conform to IS 9529 and test procedure is as per IS 2720 Part 5.
    Source: MATHalino

    Procedure:


    1. Take soil sample of 120 gram from the thoroughly mixed portion of material passing from 425-micron sieve.
    2. Mixed it thoroughly with distilled water in an evaporating dish or on a flat glass plate to form a uniform paste.
      In case of clayey soil, left it for 24 hours so as to ensure uniform distribution of moisture throughout the soil mass.
    3. Remix the soil mass just before the test and place the paste in the cup and squeezed it down as shown in the figure.
    4. Turn the crank at a rate of 2 revolutions per minute until the two halves of the soil cake come in contact with the bottom groove along a distance of about 12 mm.
    5. Record the number of drops at which this 12 mm distance is covered.
    6. Take a representative slice of soil for determination of water content.
      At least two consistent consecutive closures shall be observed before taking the sample for the determination of water content.


    Water content at which 25 blows closes the groove is called liquid limit. It is obtained from the semi-log curve between water content and number of blows and this curve is known as flow curve

             Flow Index, ${{I}_{f}}=\frac{{{W}_{1}}-{{W}_{2}}}{{{\log }_{10}}{{N}_{2}}-{{\log }_{10}}{{N}_{1}}}$



  2. Cone Penetration Method


    This method is also based on IS 2720 Part 5 and its apparatus shall conform to IS 11196.

    Procedure:


    1. Take soil sample of 150 gram from the thoroughly mixed portion of material passing from 425-micron sieve and mixed it with distilled water to make a uniform paste.
    2. Put the wet paste in the cylindrical cup of penetration apparatus ensuring no air is trapped in the process.
    3. Adjust the penetration cone such that it just touches the top of the soil paste.
    4. Release the vertical clamp and let it penetrate under its own weight for 5 seconds and reading shall be noted to the nearest millimetre.
    5. If the difference in penetration is between 14 and 28 mm, the test shall be repeated with suitable adjustments to moisture content either by adding some water or exposure of the spreading paste on a glass plate for a reduction in moisture content. And the test shall be repeated to have at least 4 sets of values of penetration in the range of 14 to 28 mm.


      6. A graph representing water content on the y-axis and cone penetration on the x-axis is prepared and the best fitting straight line is drawn. The water content corresponding to cone penetration of 20 mm shall be taken as the liquid limit.





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