Abstract
Dispersive soils deflocculate in the presence of relatively pure water to form colloidal suspensions and therefore are highly susceptible to erosion and piping. Such soils contain a higher content of sodium in their pore water than non-dispersive soils. However, there are no significant differences in the clay contents of dispersive and non-dispersive soils. The principal chemical factors influencing dispersive behaviour in a soil are exchangeable sodium and cation exchange capacity. These two parameters are frequently used in South Africa to help distinguish between dispersive and non-dispersive soils. It is recommended that more than one type of test is used to identify these soils. Dispersive soils are widely distributed in South Africa and are derived from a variety of rock types. They are commonly found in regions where the annual rainfall is less than 850 mm. In some parts of South Africa dispersive soils have to be used for the construction of earth dams since no other soil material is available. Several failures of earth dams due to piping have occurred in South Africa, most taking place on first filling of the reservoir. However, if an earth dam containing dispersive soils is properly constructed, with filters, an impervious core and correctly compacted, then it should perform satisfactorily. Dispersive soils sometimes can be stabilized by treatment with lime or gypsum. A case history of a failure of a dam is provided which illustrates some of the points mentioned.
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