DISPERSIVE SOILS - A REVIEW FROM A SOUTH-AFRICAN PERSPECTIVE

Dispersive soils deflocculate in the presence of relatively pure water 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 d ifferences in the clay contents of dispersive and non-dispersive soils . Several tests have been devised to recognize dispersive soils. Unfor tunately no one test is successful in identifying these soils in every instance. Because the principal chemical factors influencing dispersi ve behaviour in a soil are exchangeable sodium and cation exchange cap acity, these two parameters are frequently used in South Africa to hel p distinguish between dispersive and non-dispersive soils. Nonetheless it is recommended that more than one test is used to identify these s oils and perhaps a rating system could be adopted. Dispersive soils ar e widely distributed in South Africa and are derived from a variety of rock types. They are commonly found in regions where the annual rainf all is less than 850 mm. Suspicion of their presence is indicated by t he occurrence of erosion gullies and piping. In some parts of South Af rica 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 e arth dams. However, if an earth dam is properly constructed, incorpora ting filters, impermeable core and, in particular, correctly compacted , then it should perform satisfactorily. Dispersive soils sometimes ca n be stabilized by treatment with lime or gypsum.