Objective

For determination of the liquid limit of soil using casagrande apparatus.

Reference Standard

IS : 2720(Part 5)-1985- Methods of test for soils : Determination of liquid and plastic limit.

Equipment & Apparatus

• Oven
• Balance (0.01g accuracy)
• Sieve [425 micron]
• Casagrande apparatus

Preparation sample 

After receiving the soil sample it is dried in air or in oven (maintained at a temperature of 60⁰C). If clods are there in soil sample then it is broken with the help of wooden mallet. The soil passing 425 micron sieve is used in this test.

Procedure

1. About 120 gm. of air dried soil from thoroughly mixed portion of material passing 425 micron IS sieve is obtained.
2. Distilled water is mixed to the soil thus obtained in a mixing disc to form uniform paste. The paste shall have a consistency that would require 30 to 35 drops of cup to cause closer of standard groove for sufficient length.
3. A portion of the paste is placed in the cup of casagrande device and spread into portion with few strokes of spatula.
4. It is trimmed to a depth of 1 cm. at the point of maximum thickness and excess of soil is returned to the dish.
5. The soil in the cup is divided by the firm strokes of the grooving tool along the diameter through the centre line of the follower so that clean sharp groove of proper dimension is formed.
6. Then the cup is dropped by turning crank at the rate of two revolutions per second until two halves of the soil cake come in contact with each other for a length of about 12 mm. by flow only.
7. The number of blows required to cause the groove close for about 12 mm. is recorded.
8. A representative portion of soil is taken from the cup for water content determination.
9. The test is repeated with different moisture contents at least 3 times for blows between 10 and 40.

Calculation

• A ‘flow curve’ is to be plotted on a semi-logarithmic graph representing water content in arithmetic scale and the number of drops on logarithmic scale.
• The flow curve is a straight line drawn as nearly as possible through four points
• The moisture content corresponding to 25 blows as read from curve is the liquid limit of that soil

Report

The liquid limit is to be reported to the nearest whole number.

Safety & Precautions

• Soil used for liquid limit determination should not be oven dried prior to testing.
• In LL test the groove should be closed by the flow of soil and not by slippage between the soil and the cup
• After mixing the water to the soil sample , sufficient time should be given to permeate the water throughout out the soil mass
• Wet soil taken in the container for moisture content determination should not be left open in the air, the container with soil sample should either be placed in desiccators or immediately be weighed.