560 ENGINEERING GEOLOGY/Problematic Soils Figure Failed earth dam constructed of dispersive soil, showing piping outlets on the downstream side, near Ramsgate, South Africa that if an earth dam is built with careful construction control and incorporates filters, then it should be safe enough, even if it is constructed with dispersive soil Alternatively, hydrated lime, pulverised fly ash, gypsum, or aluminium sulphate have been used to treat dispersive soils used in earth dams Humid Tropical Zone Soils In humid tropical regions, weathering of rock is more intense and extends to greater depth than in other parts of the world Residual soils develop in place as a consequence of weathering, primarily chemical weathering The mineralogy of residual soils is partly inherited from the parent rock from which they were derived and partly produced by the processes of weathering Hence, the mineralogy varies widely, as does grain size and unit weight The particles and their arrangement evolve gradually as weathering proceeds In addition, weathering of parent rock in situ may leave behind relict structures that may offer weak bonding even in extremely weathered material Low strength along relict discontinuities may be attributable to particles being coated with low-friction iron/manganese organic compounds Reproducible results from some standard tests may be difficult to obtain from residual tropical soils Different results can be obtained depending upon whether the soil is pre-dried prior to testing or kept close to its natural moisture content Also, disaggregation of the soil structure, especially in relation to particle size analysis has proved problematic Consequently, conventional index tests frequently have been modified in an attempt to make them more applicable for use with tropical residual soils Of course, it would be wrong to assume that all tropical soils behave differently from those found in other climatic regions For instance, alluvial clays and sands behave in the same manner and have similar geotechnical properties, regardless of the climatic conditions of the region of deposition Drying brings about changes in the properties of residual clay soils in that it initiates two important effects, namely, cementation by the sesquioxides and aggregate formation on the one hand, and loss of water from hydrated clay minerals on the other In the case of halloysite, the latter causes an irreversible transformation to metahalloysite Drying can cause almost total aggregation of clay size particles into silt and sand size ranges, and a reduction or loss of plasticity Cycles of wetting and drying may increase the stiffness of the soil fabric, which increases its shear strength and decreases its compressibility Laterite can be regarded as a highly weathered material that forms as a result of the concentration of hydrated oxides of iron and aluminium in such a way that the character of the deposit in which they occur is affected These oxides may be present in an unhardened soil, as a hardened layer, as concretionary nodules in a soil matrix or in a cemented matrix enclosing