PHYSICAL STATES AND INDEX PROPERTIES OF FINE-GRAINED SOILS
The physical and mechanical behavior of fi ne-grained soils is linked to four distinct states: solid, semisolid, plastic, and liquid, in order of increasing water content. Let us consider a soil initially in a liquid state that is allowed to dry uniformly. If we plot a diagram of volume versus water content as shown in , we can locate the original liquid state as point A. As the soil dries, its water content reduces and, consequently, so does its volume (see Figure ). At point B, the soil becomes so stiff that it can no longer fl ow as a liquid. The boundary water content at point B is called the liquid limit;
it is denoted by LL. As the soil continues to dry, there is a range of water content at which the soil can be molded into any desired shape without rupture. The soil at this state is said to exhibit plastic behavior—the ability to deform continuously without rupture. But if drying is continued beyond the range of water content for plastic behavior, the soil becomes a semisolid. The soil cannot be molded now without visible cracks appearing. The water content at which the soil changes from a plastic to a semisolid is known as the plastic limit, denoted by PL. The range of water contents over which the soil deforms plastically is known as the plasticity index, PI:
As the soil continues to dry, it comes to a fi nal state called the solid state. At this state, no further volume change occurs since nearly all the water in the soil has been removed. The water content at which the soil changes from a semisolid to a solid is called the shrinkage limit, denoted by SL. The shrinkage limit is useful for the determination of the swelling and shrinking capacity of soils. The liquid and plastic limits are called the Atterberg limits after their originator, Swedish soil scientist A. Atterberg (1911).
We have changed the state of fi ne-grained soils by changing the water content. Since engineers are interested in the strength and deformation of materials, we can associate specifi c strength characteristics with each of the soil states. At one extreme, the liquid state, the soil has the lowest strength and the largest
deformation. At the other extreme, the solid state, the soil has the largest strength and the lowest deformation. A measure of soil strength using the Atterberg limits is known as the liquidity index (LI) and is expressed as
The liquidity index is the ratio of the difference in water content between the natural or in situ water content of a soil and its plastic limit to its plasticity index. Table shows a description of soil strength based on values of LI. Typical values for the Atterberg limits for soils are shown in Table 4.5. The Atterberg limits depend on the type of predominant mineral in the soil. If montmorillonite is the predominant mineral, the liquid limit can exceed 100%. Why? Recall that the bond between the layers in montmorillonite is weak and large amounts of water can easily infi ltrate the spaces between the layers.
In the case of kaolinite, the layers are held relatively tightly and water cannot easily infi ltrate between the layers in comparison with montmorillonite. Therefore, you can expect the Atterberg limits for kaolinite to be, in general, much lower than those for either montmorillonite or illite. Skempton (1953) showed that for soils with a particular mineralogy, the plasticity index is linearly related to the amount of the clay fraction. He coined a term called activity (A) to describe the importance of the clay fractions on the plasticity index. The equation for A is