Plasticity is a property of the fine-grained portion of a soil that allows it to be deformed beyond the point of recovery without cracking or changing volume appreciably. Some minerals, such as quartz powder, cannot be made plastic no mat-ter how fine the particles or how much water is added. All clay minerals, on the other hand, are plastic and can be rolled into thin threads at a certain moisture content without crumbling. Since practically all fine-grained soils contain some clay, most of them are plastic. The degree of plasticity is a general index to the clay content of a soil.

The term fat and lean are sometimes used to distinguish between highly plastic and slightly plastic soils. For example, lean clay is only slightly plastic, whereas fat clay is highly plastic. In engineering practice, soil plasticity is determined by observing the different physical states that a plastic soil passes through as the moisture conditions change. The boundaries between the different states, as described by the moisture content at the time of change, are called consistency limits or Atterberg limits.

The liquid limit (LL) is the moisture content corresponding to the arbitrary limit between the liquid and plastic states of consistency of a soil. Above this value, the soil is presumed to be a liquid and behaves as such by flowing freely under its own weight. Below this value, it deforms under pressure without crumbling, provided the soil exhibits a plastic state.

The plastic limit (PL) is the moisture content at an arbitrary limit between the plastic and semisolid state. It is reached when the soil is no longer pliable and crumbles under pressure. Bet-ween the liquid and plastic limits is the plastic range. The numerical difference in moisture con-tent between the two limits is called the plasticity index (PI). The equation is PI = LL PL. It defines the range of moisture content within which the soil is in a plastic state.

The shrinkage limit is the boundary in moisture content between the solid and the semisolid states. The solid state is reached when the soil sample, upon being dried, finally reaches a limiting or minimum volume. Beyond this point, further drying does not reduce the volume but may cause cracking. The limit tests are described later in this chapter.

Moisture affects coarse-grained soils much less than fine-grained soils. One reason for this is that coarser soils have larger void openings, and, as a rule, drain more rapidly. Capillarity is practically nonexistent in gravels and in sands containing little fines. These soils, if they are above the groundwater table, will not usually retain large amounts of water. A second reason is that since the particles in gravelly and sandy soils are relatively large (in comparison to clay and silt particles), they are, by weight, heavy in comparison to the films of moisture that might surround them.  On the other hand, the small (sometimes microscopic) particles of fine-grained soil weigh so little that water in the voids has considerable effect. It is not unusual, for example, for clays to undergo large volume changes with variations in moisture content, as witness the shrinkage cracks in a dry lake bed. Consequently, unpaved clay roads, though hard enough when sun-baked, often lose stability and turn into mud in rainy weather.

Not only do clays swell and lose stability when they become wet, but they also, because of their flat, platelike grain shapes and small size, retard the drainage of water. Since drainage is of the greatest importance in (for example) the construction of airfield pavement, design engineers must know whether or not subsurface clay exists. Plasticity is, as you know, the characteristic by which clay is primarily identified.

Soils of organic origin are formed either by the growth and subsequent decay of plant life or by the accumulation of inorganic particles of skeletons or shells of organisms. The term of vegetable matter. An organic soil may be an organic silt or clay, or it may be a HIGHLY ORGANIC soil, such as peat or meadow mat. Organic soils are most often black in color, and usually have a characteristic musty odor. These soils are usually compressible and have poor load-maintaining properties.

In summary, soil characteristics area measure of the suitability of the soil to serve some intended purpose. Generally, a dense, solid soil withstands greater applied loads (has greater bearing capacity) than a loose soil. Particle size has a definite relation to this capacity. From empirical tests, it has been found that well-graded, coarse-grained soils generally can be compacted to a greater density than fine-grained soils. This is because the smaller particles tend to fill the spaces between the larger ones. The shape of the grains also affects the bearing capacity. Angular particles tend to interlock, forma denser mass, and become more stable than the rounded particles, which can roll or slide past one another. Poorly graded soils, with their lack of one or more sizes, leave more or greater voids and comprise a less dense mass. Moisture content and the consistency limits aid in describing the suitability of the soil. A coarse-grained sandy or gravelly soil generally has good drainage characteristics and may be used in its natural state. A fine-grained clayey soil with a high plasticity index may require considerable treatment, especially if used in a moist location.

As can be inferred from the previous discussions in this chapter, soil types are important factors to consider when selecting the proper location on which to construct any structure or facility. With the soil accurately identified and described, its suitability for supporting traffic as a subgrade, base, or foundation material or as an aggregate, a filler, or a binder for a mixture can be evaluated.