Although we sometimes encounter soil strata that consist of nearly pure clay, most clays are mixed with silts and/or sands. Nevertheless, even a small percentage of clay significantly impacts the behaviour of a soil. When the clay content exceeds about 50 percent, the sand and silt particles are essentially floating in the clay, and have very little effect on the engineering properties of the soil [10]
Plasticity of Clay
Because of the controlling importance of the effect of surface activity on the behaviour of fine grained soils, description of these materials by reference to their particle sizes is practically meaningless. The practical distinction between silt and clay is made, not on the basis of an arbitrary size distinction, but on the basis of material behaviour in the presence of water. The consistency of fine soil varies according to the amount of water present. Completely dry, the soil may be hard (solid), while at high water contents it may be almost a slurry (liquid). Intermediate states of consistency are semisolid and plastic states. A plastic material is one that deforms readily without cracking or rupture. The boundaries of these states of consistency are defined in terms of soil water content [9].
Different soils may be distinguished by their plasticity characteristics because these characteristics vary with surface activity of the constituent particles. The more active soils (claylike) are more plastic than the inactive soils (silts). This phenomenon may be explained by examining the nature of the water near the surface of a clay particle. Since water's molecular structure is dipolar, the water near the clay particle is effectively immobilized by the surface charge. It is adsorbed and may be considered essentially solid. As distance from the particle surface increases, the orientation of water is reduced in degree until, at the boundary of the particle's influence (limit of diffuse double layer), the viscosity is that of free water. With an abundance of water, soil particles would be separated by free water and the mixture would be fluid. As the amount of water decreases, the particles are separated by increasingly stiffer water. The mixture becomes like a solid. The soil water system, therefore, has a range of water contents over which it may be plastic. Changing the chemical composition of the constituent phases (soil type or fluid) causes a change in the plasticity range or the plasticity index. This permits to distinguish among soils on the basis of their plasticity [9].
Expansion of Clays
The generic name for clay minerals that swell (expend) as change in soil water occur is smectite. Clays expand in volume if the soil water content is below a stability value when becomes available. The volume change is related to the thickness and mobility of the water film adsorbed onto or surrounding the clays particle, being increased relatively easily during natural wetting conditions. Probably most of swelling that occurs is due to water moving in. A requirement for significant swelling is that the soil have many fine pores in the 0.001 mm to 0.002mm range. Pores of this size permit rapid acceptance.
The expensive force created by a clay undergoing an increase in water content and volume can be considerable, being capable of lifting heavy structures and imposing lateral pressure that can move retaining walls and basement walls. Swelling pressures in excess of 500 kPa have been measured. High swelling pressures can occur even in already high-moisture clays if additional water is adsorbed. Damage to structure can result when a clay swells.
Shear Strength Theory
The strength of soil is a variable and elusive property. Strength characteristics of some materials are represented by concepts such as yield point or tensile strength. For compacted fills, engineers deal with soils as they are in nature. Because they are not manufactured products, variations in properties are the rule, not the exception. They are natural materials, and in most cases engineers use what is available, adjusting designs and working methods to accommodate conditions [9].
Strength is the measure of the maximum stress state that can be induced in a material without it failing. Although strength can be stated in terms of compressive stress or tensile stress, fundamentally it's the ability to sustain shear strength that provides strength. The shear strength of a soil, τ is the internal resistance per unit area that the soil mass can offer to resist failure and sliding along any plane inside it. The shear strength of a soil is indicative of the stability and strength of the soil under various conditions of loading, compaction, and moisture content [2].
The shear strength of a soil is the maximum resistance which it can offer to shear stress. When the maximum has been research the soil is regarded as having failed, its strength having been fully mobilized. However, the shear strength value determined experimentally is not a unique constant which is characteristic of the material but varies with the method of testing [13].
