1. objective
From liquid limit test, the compression index may be estimated, which is used in settlement analysis.
If the natural moisture content of soil is higher than liquid limit, the soil can be considered
as soft and if the moisture content is lesser than liquid limit, the soil is brittle and stiffer.
The value of liquid limit is used in classification of the soil and it gives an idea about
plasticity of the soil.
Theory: The liquid limit is the moisture content at which the groove formed by a standard tool
into the sample of soil taken in the standard cup, closes for 12 mm on being given 25 blows in
a standard manner. At this limit, the soil possess low shear strength.
2. apparatus required
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conforming to 1S : 9259-1979.
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conforming to IS : 9259-1979.
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The balance to be used must be sensitive to the extent of 0.01 g
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Thermostatically controlled with interior of non-corroding material to maintain the temperature around 1050C and 1100C
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12 to 15 cm in diameter
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3. reference
IS 2720(Part 5):1985 Methods of test for soils: Determination of Liquid
and Plastic limit (second revision). Reaffirmed- May 2020.
4. procedure
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About 120 gm of air-dried soil from thoroughly mixed portion
of material passing 425 micron I.S sieve is to be obtained.
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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 12 mm length.
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For clayey soil leave it for 24 hours prior to test to ensure distribution of Moisture
throughout the soil mass.
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The soil should then be remixed thoroughly before the test. A portion of the paste is placed in the cup of Mechanical Liquid Limit
device and spread into portion with few strokes of spatula as possible.
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At the same time, trim it to a depth of 1 cm at the point of maximum thickness
and return excess of soil to the dish.
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The soil in the cup shall be decided by the firm strokes of the
grooving tool (Indian Standard/ Casagrande's Grooving Tool for Clayey soils
and ASTM Grooving Tool for sandy soils) along the diameter through the centre line of the
follower so that clean sharp groove of proper dimension is formed.
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Lift and drop the cup by turning crank at the rate of two revolutions
per second until the two halves of soil cake come in contact with each
other for a length of about 12 mm by flow only.
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The number of blows required to cause the groove close for about 12 mm
shall be recorded.
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A representative portion of soil is taken from the cup for water content
determination.
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Repeat the test with different moisture contents at least three more
times for blows between 15 and 35.
5. observation and recording
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Details of the sample: .............
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Natural moisture content: ........
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Room temperature: .................
Parameters |
Observation No. 1 |
Observation No. 2 |
Observation No. 3 |
Observation No. 4 |
Container number |
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Weight of container |
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Weight of container + wet soil |
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Weight of container + dry soil |
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Weight of water |
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Weight of dry soil |
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Moisture content (%) |
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No. of blows |
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Table 1 : Determination of Liquid Limit of Soil
6. calculation
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‘A flow curve’ shall be plotted on a semilogarithmic graph representing
water content on the arithmetical scale and the number of drops on the
logarithmic scale. The flow curve is a straight line drawn as nearly
as possible through the four or more plotted points. The moisture
content corresponding to 25 drops as read from the curve shall be rounded
off to the nearest whole number and reported as the liquid limit of the soil.
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Flow index, If = (W1-W2)/ log10(N2/N1)
where,
If = Flow Index
W1 = Moisture content in percentage corresponding to N1 drops and
W2 = Moisture content in percentage corresponding to N2 drops
7. graph
Fig. 7 : Plot of Moisture Content v/s No. of Blows
8. video