Atterberg limits

[1] They were later refined by Arthur Casagrande, an Austrian geotechnical engineer and a close collaborator of Karl Terzaghi (both pioneers of soil mechanics).

The amount of expansion is related to the ability of the soil to take in water and its structural make-up (the type of minerals present: clay, silt, or sand).

Thus these tests are used widely in the preliminary stages of designing any structure to ensure that the soil will have the correct amount of shear strength and not too much change in volume as it expands and shrinks with different moisture contents.

The shrinkage limit (SL) is the water content where further loss of moisture will not result in more volume reduction.

The plastic limit (PL) is determined by rolling out a thread of the fine portion of a soil on a flat, non-porous surface.

As the moisture content falls due to evaporation, the thread will begin to break apart at larger diameters.

The plastic limit is defined as the gravimetric moisture content where the thread breaks apart at a diameter of 3.2 mm (about 1/8 inch).

Atterberg's original liquid limit test involved mixing a pat of clay in a round-bottomed porcelain bowl of 10–12 cm diameter.

A groove was cut through the pat of clay with a spatula, and the bowl was then struck many times against the palm of one hand.

The cup is repeatedly dropped 10 mm onto a hard rubber base at a rate of 120 blows per minute, during which the groove closes up gradually as a result of the impact.

The moisture content at which it takes 25 drops of the cup to cause the groove to close over a distance of 12.7 millimetres (0.50 in) is defined as the liquid limit.

It is based on the measurement of penetration into the soil of a standardized stainless steel cone of specific apex angle, length and mass.