Abstract

Dispersive soils deflocculate in the presence of relatively pure to form colloidal suspensions and are therefore highly susceptible to erosion and piping. Such soils contain a higher content of sodium in their pore water than other soils. However, there are no significant differences in the clay contents of dispersive and non-dispersive soils. Unfortunately no one test is successful in identifying these soils in every instance. Because 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. Nonetheless it is recommended that more than one test is used to identify these soils and pehaps a rating system could be adopted. 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. Suspicion of their presence is indicated by the occurrence of erosion gullies and piping. In some parts of South Africa dispersive soils have to be used for construction purposes since no other soil material is available. Hence dispersive soils have been used in the construction of earth dams and embankments. Serious piping and failures have occurred, especially when they have been used for earth dams. However, if an earth dam is properly constructed, incorporating filters, impermeable core and, in particular, correctly compacted, then it should perform satisfactorily. Dispersive soils sometimes can be stabilized by treatment with lime or gypsum.

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