Abstract

Expansive clay soils swell when wetted and shrink when they dry out, henee giving rise to ground movements which, depending on their severity, can cause structural damage to buildings, especially low-rise buildings. The occurrence of potentially expansive soils in Natal has long been recognized and over the past two years, because of the drought conditions, low-rise buildings have suffered structural damage because of soil shrinkage. In the case of shallow founded buildings, soil shrinkage gives rise to an “edges down” effect and the pattern of cracks which develop in the buildings is similar to that which develops when soil beneath them swells. Shrinkage settlement of embankments has also occurred to the detriment of roads. In addition to evaporation, loss of moisture from soil is very much influenced by transpiration from plants, both of which exert control on the suction pressure developed in soil. The development of soil suction leads to shrinkage of the soil. When vegetation is cleared from a site prior to it being developed, its desiccating influence is removed and so the soil may swell. Empirical methods, soil suction methods and oedometer methods have been used to predict the degree to which soils swell. However, the results obtained by different methods can differ significantly. Nonetheless such results and the evidence obtained from site investigations are used to help design foundations for buildings. These vary from reinforced strip footings to rafts and bored piles, depending on the likely severity of the resultant ground movements. The soil can also be stabilized, the most common agent used being lime. A number of case histories are provided to illustrate the problems involved.

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