558 ENGINEERING GEOLOGY/Problematic Soils transpiration from large trees, and significant damage to property on potentially expansive clay can be caused during notably dry summers Generally, the clay fraction of expansive clay exceeds 50%, silty material varying between 20 and 40%, and sand forming the remainder Montmorillonite normally is present in the clay fraction and is the principal factor determining the appreciable volume changes that take place in these soils on wetting and drying The depth of the active zone in expansive clays (i.e., the zone in which swelling and shrinkage occurs in wet and dry seasons, respectively) can be large For instance, the maximum seasonal changes in the moisture content of expansive clays in Romania are around 20% at 0.4 m depth, 10% at 1.2 m depth, and less than 5% at 1.8 m depth The corresponding cyclic movements of the ground surface are between 100 and 200 mm Surface heaves approaching 500 mm have been recorded in some expansive clays in South Africa During the dry season, profiles in some regions can dry out to depths of 15 to 20 m The potential for volume change in expansive clay soils depends on the initial moisture content, initial voids ratio, the microstructure, and the vertical stress, as well as the type and amount of clay minerals present The type of clay minerals are responsible primarily for the intrinsic expansiveness, whilst the change in moisture content or suction controls the actual amount of volume change that a soil undergoes at a given applied pressure Changes in soil suction are brought about by moisture movement through the soil, due to evaporation from its surface in dry weather, by transpiration from plants, or alternatively by recharge consequent upon precipitation Alternating wet and dry seasons may produce significant vertical movements as soil suction changes The rate of expansion depends upon the rate of accumulation of moisture in the soil In semi-arid regions, it is limited by the availability of water and this, together with the available void volume, governs the rate of penetration of the heave front in the soil When these expansive clays occur above the water table, they can undergo a high degree of shrinkage on drying Seasonal changes in volume also produce shrinkage cracks so that expansive clays are often heavily fissured Sometimes the soil is so desiccated that the fissures are wide open and the soil is shattered or micro-shattered Transpiration from vegetative cover is a major cause of water loss from soils in subtropical semiarid regions Indeed, the distribution of soil suction in soil is controlled primarily by transpiration from vegetation and this represents one of the most significant changes made in loading (i.e., to the state of stress in a soil) The suction induced by the withdrawal of moisture fluctuates with the seasons, reflecting the growth of vegetation The maximum soil suction that can be developed is governed by the ability of vegetation to extract moisture from the soil The level at which moisture is no longer available to plants is termed the permanent wilting point In fact, the moisture content at the wilting point exceeds that of the shrinkage limit in soils with high clay contents and is less in those possessing low clay contents This explains why settlement resulting from the desiccating effects of trees is more notable in low to moderately expansive soils than in highly expansive ones These volume changes can give rise to ground movements that may result in damage to buildings Low-rise buildings are particularly vulnerable to such ground movements since they generally not have sufficient weight or strength to resist Be that as it may, three methods can be adopted when choosing a design solution for building on expansive soils Firstly, a foundation and structure can be provided that can tolerate movements without unacceptable damage; secondly, the foundation and structure can be isolated from the effects of the soil; and thirdly, the ground conditions can be altered or controlled These soils also represent a problem when they are encountered in road construction, and shrinkage settlement of embankments composed of such clay soils can lead to cracking and breakup of the roads they support (Figure 4) Dispersive Soils Dispersive soils occur in subtropical semi-arid regions, normally where the rainfall is less than 850 mm annually Dispersion occurs in such soils when the repulsive forces between clay particles exceed the attractive forces, thus bringing about deflocculation so that in the presence of relatively pure water the particles repel each other to form colloidal suspensions In nondispersive soil there is a definite threshold velocity below which flowing water causes no erosion By contrast, there is no threshold velocity for dispersive soil, the colloidal clay particles going into suspension even in quiet water Therefore, these soils are highly susceptible to erosion and piping Dispersive soils contain a moderate to high content of clay material but there are no significant differences in the clay fractions of dispersive and non-dispersive soils, except that soils with less than 10% clay particles may not have enough colloids to support dispersive piping Dispersive soils contain a higher content of dissolved sodium (up to 12%) in their pore water than ordinary soils The clay particles in soils with high salt contents exist as aggregates and coatings around silt and sand particles (Figure 5) For a given eroding fluid the boundary between the flocculated and deflocculated states depends on the