The principal minerals in a deposit of clay tend to influence its engineering behaviour. For example, the plasticity of a clay soil is influenced by the amount of its clay fraction and the type of clay minerals present since clay minerals greatly influence the amount of attracted water held in a soil. The undrained shear strength is related to the amount and type of clay minerals present in a clay deposit together with the presence of cementing agents. In particular, strength is reduced with increasing content of mixed-layer clay and montmorillonite in the clay fraction. The increasing presence of cementing agents, especially calcite, enhances the strength of the clay. Geological age also has an influence on the engineering behaviour of a clay deposit. The porosity, water content and plasticity normally decrease in value with increasing depth, whereas the strength and elastic modulus increase. The engineering performance of clay deposits is also affected by the total moisture content and by the energy with which this moisture is held. For instance, the moisture content influences their consistency and strength, and the energy with which moisture is held influences their volume change characteristics. One of the most notable characteristics of clays from the engineering point of view is their susceptibility to slow volume changes that can occur independent of loading due to swelling or shrinkage. Differences in the period and magnitude of precipitation and evapotransportation are the major factors influencing the swell-shrink response of a clay beneath a structure. Generally kaolinite has the smallest swelling capacity of the clay minerals. Illite may swell by up to 15% but intermixed illite and montmorillonite may swell some 60–100%. Swelling in Ca montmorillonite is very much less than in the Na variety; it ranges from about 50 to 100%. Swelling in Na montmorillonite occasionally can amount to 2000% of the original volume. One of the most widely used soil properties to predict swell potential is the activity of a clay. Volume changes in clays also occur as a result of loading and unloading which bring about consolidation and heave, respectively. When a load is applied to a clay soil its volume is reduced, this being due principally to a reduction in the pressures are induced, the effective strength is increased, and the undrained strength is much higher than the drained strength—the exact opposite to a normally consolidated clay. When the negative pore-water pressure gradually dissipates the strength falls as much as 60 or 80% to the drained strength. Skempton (1964) observed that when clay is strained it develops an increasing resistance (strength), but that under a given effective pressure the resistance offered is limited, the maximum value corresponding to the peak strength. If testing is continued beyond the peak strength, then, as displacement increases, the resistance decreases, again to a limiting value which is termed the residual strength. In moving from peak to residual strength, cohesion falls to almost, or actually, zero and the angle of shearing resistance is reduced to a few degrees (it may be as much as 10° in some clays). Under a given effective pressure, the residual strength of a clay is virtually the same whether it is normally consolidated or overconsolidated (Figure 1.4). Furthermore, the value of residual shear strength (_’r) decreases as the amount of clay fraction increases in a deposit. Not only is the proportion of detrital minerals important but so is that of the diagenetic minerals. The latter influence the degree of induration of a deposit of clay and the value of _’r can fall significantly as the ratio of clay minerals to detrital and diagenetic minerals increases.