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Atterberg’s Limits - Significance and determination

What are Atterberg  limits?

Atterberg identified three limiting water contents, in between the soil states, that are commonly known as Atterberg’s limits. The upper and lower limit of water content within which a clay element exhibits a plastic behaviour are defined as liquid and plastic limits respectively.

Purpose: 

The liquid state of soil is the state at which soil will flow on its own weight.

The Plastic State of soil is the state at which soil can be remoulded in any shape without any cracks.

The Plasticity Index (PI) is the range of moisture contents where the silt or clay remains plastic (PI = LL – PL).

Procedure:

Casagrande’s Method to determine liquid limit

The device consists of a metal cup seated onto a hard rubber base and fixed to a crankshaft arrangement with a handle so that the cup can be alternately raised and dropped.

The soil is first dried sufficiently. It is then sieved using a 425-micron sieve and materials passing are taken for the limit tests. The soil sample is mixed thoroughly with water into a smooth thick paste. A portion of it is placed in the metal cup. The soil paste is leveled off in the cup to give a maximum depth of 8 mm.

A groove is formed in the soil along the midsection of the cup, using a standard grooving tool.

The handle is turned at two revolutions per seconds and the number of blows is counted until the two parts of the soil come in continuous contact at the bottom of the groove along about 12 mm.

A sample is taken from the closed portion of the groove for moisture content determination. The remaining soil is removed from the cup to mix with the original paste adding further water, and the test is repeated several times. The number of blows and moisture content in each trial are recorded. It is desirable to have the tests in the blow range of 15 to 35. The liquid limit according to this method, is defined as the moisture content at which 25 blows are required to close the groove. A semi-log plot of the points is obtained with a number of blows as abscissa in logarithmic scale as against moisture content in linear scale. A best-fitting straight line is drawn which is termed as flow curve. The moisture content corresponding to 25 blows is read from the flow curve as the liquid limit of soil.

Fig. Semi log curve for liquid limit

Conclusion: Soil can be of Low plasticity or Medium plasticity or High plasticity.

Plastic limit

The plastic limit is the minimum moisture content at which the soil can be deformed plastically. As standardised, it can be taken as the smallest water content at which the soil begins to crumble when rolled out into thin threads, approximately 3 mm in diameter. That is at the plastic limit the soil must gain some minimum stiffness or strength.