Determine the liquid limit and plasticity index of soil by casagrande apparatus | LL PI of soil

Determine the liquid limit and plasticity index of soil by casagrande apparatus | LL PI of soil

Determine the liquid limit and plasticity index of soil by casagrande apparatus | LL PI of soil
casagrande apparatus for LL PI test of soil
casagrande apparatus
Test Standard : IS 2720-Part 5:1985, Reaffirmed 2010
 Apparatus :
  1. Mechanical Liquid limit device 
  2. Grooving tool
  3. Casagrande’s apparatus
  4. Evaporation dishes
  5. Air tight containers
  6. Spatula
  7. 425 micron IS sieve
  8. 45cm square
  9. 10mm thick glass plate or equivalent
  10. Pallet knives
  11. Balance of 0.1 gm accuracy
  12. Beaker
  13. Hot air oven.

Liquid Limit of Soil

First Clean the liquid limit device , dry and check good working condition, the cup falls freely and it does not have too much play at its hinges. The height through which the cup is lifted and dropped shall be adjusted with the adjustment plate so that the point of cup that contacts the base falls exactly 1cm per one revolution of handle.

Procedure :

  1. Take 120gm of soil from the material sieved from 425 micron IS sieve in evaporation dish and mix with water thoroughly to form paste, which attains a consistency of 30-35 strokes to cause the required closer of the standard groove.
  2. Place a portion of paste in the liquid limit device cup, squeeze down and spread the soil with few strokes of soil and trim it to a depth of 1 cm at the point of maximum thickness of cup, returning the excess soil to the dish. 
  3. The soil in the cup than be divided by firm strokes of grooving tool, at the centre line of cup, so that a clean sharp groove of proper dimensioned is formed. 
  4. The cup shall be lifted and dropped by rotating the crank at the rate of 2 revolutions/sec, until the two halves are come into contact by 12 mm
  5. The number of drops required to cause the groove close for length of 12 mm shall be recorded.
  6. Take 25-30gm of the above soil and keep it in oven for moisture determination. Now add a little more soil to the left over wet soil and repeat the above steps to determine different moisture contents between different numbers of drops 25-35, 15-20, 10-15.
Draw the Graph for Liquid limit
Draw a semi-logarithmic ‘flow curve’ graph with water content on arithmetical scale and number of drops on logarithmic scale. Liquid limit ids the water content from the graph corresponding to 25 drops.

Plastic Limit of soil
  1. Take 30gm of soil from the above representative soil, mix with water on the glass plate till the soil mass becomes plastic enough to be easily moulded between fingers. 
  2. A ball weighing approximately 8 gm shall be formed, rolled between fingers and the glass plate with just sufficient pressure to roll the mass of soil to form a uniform diameter of 3 mm throughout its length, at a rate of 80-90 strokes/minute
  3. The soil is kneaded together to a uniform mass and rolled again, this process of kneading and rolling shall be continued until the thread of 3 mm diameter just crumbles with no pressure and the soil no longer can be rolled. The pieces of crumbled threads are to be collected in an airtight container and moisture content is to be determined. 

Note : The two tests are carried out on each soil sample. 

Calculation :Plastic limit is the average of two such determinations

Plasticity Index (%) = Liquid Limit (%) – Plastic Limit (%)

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