Atterberg Limits
We know that the particles of the clay fraction may consist of finely ground rock flour or clay minerals. Because of their crystal structure and shape, clay minerals have strong surface forces that are predominant over the gravity forces. The surface forces attract water molecules to the clay particles. Those closest to the clay particles are tightly held to the clay and their properties are quite different from those of ordinary water. This is called adsorbed water and is considered to give clays their cohesive and plastic properties. Very little water is adsorbed by rock flour, which is nonplastic. Thus we see that the amount of fines present in a soil is a poor indicator of what the soil behavior may be like. Two clay soils have very different properties if one is composed primarily of quartz and the other of clay minerals such as illite. Furthermore, there exist several kinds of clay minerals whose properties and surface forces differ over a wide range. The Atterberg limits are designed to serve as an index of the property of the clay fraction.
Beginning at a very low water content, a clay soil is first a solid and then becomes plastic as the water content increases. The word plastic refers to the ability of the soil to be molded into various shapes without breaking up. The various states of solid, plastic, and liquid reflect the stiffness, or consistency, of the soil. The Atterberg limits are the water contents at which soil consistency changes from one state to another. They are called the shrinkage, plastic, and liquid limits, respectively (Fig.2.2).
The liquid limit (LL) is the water content at which the soil on two sides of a groove flows together after the dish that contains the soil has been dropped through a distance of 1 cm 25 times. The dimensions of the soil pat and groove and the dish are shown in Fig.2.3. According to Casagrande (1932) this test is analogous to a strength test, and at the liquid limit the soil has strength of approximately 1 g/cm2. The plastic limit (PL) is defined as the water content at which the soil crumbles when it is rolled down to a thread 1/8 in. in diameter. The different between the liquid and plastic limits is called the plasticity index (Ip).