Shrinkage limit The shrinkage limit (SL) is the water content where further loss of moisture will not result in a reduction in volume. The test to determine the shrinkage limit is
ASTM International D4943. The shrinkage limit is much less commonly used than the liquid and plastic limits.
Plastic limit The plastic limit (PL) is determined by rolling out a thread of the fine portion of a soil on a flat, non-porous surface. The procedure is defined in
ASTM Standard D 4318. If the soil is at a moisture content where its behavior is plastic, this thread will retain its shape down to a very narrow diameter. The sample can then be remolded and the test repeated. 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). A soil is considered non-plastic if a thread cannot be rolled out down to 3.2 mm at any moisture possible.
Liquid limit The liquid limit (LL) is conceptually defined as the water content at which the behavior of a clayey soil changes from the
plastic state to the
liquid state. However, the transition from plastic to liquid behavior is gradual over a range of water contents, and the
shear strength of the soil is not actually zero at the liquid limit. The precise definition of the liquid limit is based on standard test procedures described below.
Casagrande's method 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. Casagrande subsequently standardized the apparatus (by incorporating a crank-rotated cam mechanism to standardize the dropping action) and the procedures to make the measurement more repeatable. Soil is placed into the metal cup (Casagrande cup) portion of the device and a groove is made down at its center with a standardized tool of width. 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 number of blows for the groove to close is recorded. The moisture content at which it takes 25 drops of the cup to cause the groove to close over a distance of is defined as the liquid limit. The test is normally run at several moisture contents, and the moisture content which requires 25 blows to close the groove is interpolated from the test results. The liquid limit test is defined by ASTM standard test method D 4318. The test method also allows running the test at one moisture content where 20 to 30 blows are required to close the groove; then a correction factor is applied to obtain the liquid limit from the moisture content.
Fall cone test Another method for measuring the liquid limit is the
fall cone test, also called the cone penetrometer test. It is based on the measurement of penetration into the soil of a standardized stainless steel cone of specific apex angle, length and mass. Although the Casagrande test is widely used across North America, the
fall cone test is much more prevalent in Europe and elsewhere due to being less dependent on the operator in determining the liquid limit. Advantages over Casagrande Method • It is easier to perform in laboratory. • The results from the cone penetrometer do not depend on the skills or the judgement of the operator. So, the results obtained are more reliable. • The results can be used to estimate the undrained shear strength of soils. ==Derived limits==