Definition of Liquid limit:
It is the minimum water content at which the soil is still in the liquid state, but has a small shearing strength against flow.
Liquid limit is defined as the minimum water content at which a part of soil cut by a groove of standard dimension will flow together for a distance of 12 mm (1/2 inch) under an impact of 25 blows in the device
Liquid limit: The boundary between the liquid and plastic states;
Definition of Plastic limit:
The water content corresponding to an arbitrary limit between the plastic and the semisolid state of consistency of a soil; water content at which a soil will just begin to crumble when rolled into a thread approximately 1/8 inches (3mm) in diameter. The minimum amount of water in terms of percent of oven-dry weight of soil that will make the soil plastic.
Plastic limit: The boundary between the plastic and semi-solid states;
Definition of Shrinkage limit:
The maximum water content at which the reduction in water content will not cause decrease in total volume of soil but the increase in moisture content will cause an increase in moisture content
Definition of Plasticity Index (PI):
The range of water content over which a soil behaves plastically. It is defined as “the range of consistency with in which the soil exhibit plastic properties”
It is also defined as “the numerical difference between the liquid limit and plastic limit”.
Mathematically, Plasticity index = Liquid Limit – Plastic Limit It is denoted by Ip and Ip = LL – PL
- Ip value (0-3)- Non-plastic
- Ip value (3-15) – Slightly plastic
- Ip value (15-30) – Medium plastic
- Ip value >30 – Highly plastic
The Liquidity index (LI):
The liquidity index (LI) is used for scaling the natural water content of a soil sample to the limits. It can be calculated as a ratio of difference between natural water content, plastic limit, and liquid limit:
LI=(w-PL)/(LL-PL) where ‘w’ is the natural water content
The consistency index (CI):
It indicates the consistency (firmness) of a soil. It is calculated as CI = (LL-w)/(LL-PL), where ‘w’ is the existing water content. Soil at the liquid limit will have a consistency index of 0, while soil at the plastic limit will have a consistency index of 1.
Acceptable Limit as per MoRT&H, GoI’s Specifications (5th Revision):
- Plasticity Index (PI) is 25 & Liquid Limit (LL) is 50 are the limits for clay in case of embankment materials as per MoRT&H, Govt of India specification, 5th revision (ref. Clause No. 305.2.1.1, page: 63)
- Plasticity Index (PI) is 20 & Liquid Limit (LL) is 40 are the limits for clay in case of soil back fill materials as per MoRT&H, Govt of India specification 5th revision (ref. Clause No. 305.4.1, page: 73)
- Physical requirement for materials for Granular sub-base (GSB) vide table 400-2, page 110
Liquid Limit test of soil using Casagrande apparatus (As per IS-2720-Part-5-1985)
For determination of the liquid limit of soil using Casagrande apparatus.
- Balance (0.01g accuracy)
- Sieve [425 micron]
- Casagrande apparatus etc.
Preparation test specimen
After receiving the soil sample it is dried in air or in oven (maintained at a temperature of
60°C). 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.
- About 120 gm. of air dried soil from thoroughly mixed portion of material passing 425 micron IS sieve is obtained.
- 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.
- A portion of the paste is placed in the cup of Casagrande device and spread into portion with few strokes of spatula.
- It is trimmed to a depth of 10 mm. at the point of maximum thickness and excess of soil is returned to the dish.
- 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.
- 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.
- The number of blows required to cause the groove close for about 12 mm. is recorded.
- A representative portion of soil is taken from the cup for water content determination.
- The test is repeated with different moisture contents at least 3 times for blows between 10 and 40.
• 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
The liquid limit is to be reported to the nearest whole number.
• 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.
• In case where grooving tool, Type “A” does not give a clear groove as in sandy soils, grooving tool type “B” or type “C” may be used.
Determination of Liquid Limit by one point method
The water content wN of the soil of the accepted trial shall be calculated. For the range of blows between 15 and 35 the Liquid Limit water content shall be calculated employing the
wL = Liquid Limit = wN /(1.321 5- 0.23 log N)
where, wN= Water content for trial & N=No. of blow
About 50 gm of soil sample to be taken for test.
Plastic Limit test of soil (As per IS-2720-PART-5-1985)
For determination of the plastic limit of soil.
• Balance (0.01 g accuracy)
• Sieve [425 micron]
• 10 mm thick & 45 cm square or larger sized flat glass surface for rolling etc.
Preparation of test specimen
After receiving the soil sample of 20 gram it is dried in air or in oven (maintained at a temperature of 60°C). 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.
- A soil sample of 20 gm. passing 425 micron IS sieve is to be taken.
- It is to be mixed with distilled water thoroughly in the evaporating dish till the soil mass becomes plastic enough to be easily moulded with fingers.
- It is to be allowed to season for sufficient time, to allow water to permeate throughout the soil mass.
- 10 gm of the above plastic mass is to be taken and is to be rolled between fingers and glass plate with just sufficient pressure to roll the mass into a thread of uniform diameter throughout its length. The rate of rolling shall be between 60 and 90 stokes per minute.
- The rolling is to be continued till the thread becomes 3 mm. in diameter.
- The soil is then kneaded together to a uniform mass and rolled again.
- The process is to be continued until the thread crumbled with the diameter of 3 mm.
- The pieces of the crumbled thread are to be collected in an air tight container for moisture content determination.
The Plastic limit is to be determined for at least three portions of soil passing 425 micron IS sieve. The average of the results calculated to the nearest whole number is to be reported as the plastic limit of the soil.
Determination of Plasticity Index
From the value of Liquid limit (wL), Plastic limit (wL) & Plasticity Index (IP), we may be determined of the soil grain category by using the Plasticity chart given below.
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