The paper deals with some engineering aspects of residual soils in the tropics including experience of halloysitic soils during the construction of the Ruiru Dam in Kenya and the more detailed studies of residual soils carried out during the construction of the Sasumua Dam. Such soils, as compacted, have an unusually low dry density and high optimum moisture content but high shear strength. Engineering properties are apparently inconsistent with the index properties; moreover, standard control tests show marked variations.

Mineralogical studies can explain some of these apparent irregularities. Water is bound to the halloysite crystals and the measured proportion of bound water varies according to the treatment in the test method. Further, the grading as determined by hydrometer tests varies markedly according to the dispersing agent used. The halloysite particles are collected in robust silt-sized clusters imparting to the soil a high shear strength and many of the properties of a granular material.

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