ENGINEERING CHARACTERISTICS OF SOME NIGERIAN RESIDUAL SOILS FOR HIGHWAY CONSTRUCTION

The engineering characteristics of residual soils from the three major geological zones of Nigeria are analyzed and discussed. The characteristics of the parent rocks seemed to have some influence on the observed textural differences among the three groups of soils. There was also a fair degree of correlation between weathering, as indicated by grain size, and their consistency and swelling characteristics. The group A soils, which are nonplastic and granitic in origin, compact well at relatively low optimum moisture contents. Their CBR values at natural moisture contents, which are lower than optimum, are much higher than those at their optimum moisture contents. The soils are considered suitable as base and subbase materials because they compact well and have high CBR values. The group B soils are sedimentary in origin and have a high plasticity and relatively low CBR values. Unlike the group A soils, their natural moisture contents are higher than their optimums. There appears to be some correlation between the natural moisture content and the maximum dry density. Their high plasticity may make it uneconomical to stabilize them with chemical additives. The group C soils are lacustrine in origin, have very low CBR values, and exhibit a high degree of swelling. Therefore they are not considered suitable as base and subgrade materials. However, appreciable improvements in the compaction characterstics and increases in CBR values were recorded when the soils in this group were blended with local sand.