From an engineering perspective, soil is any un-cemented or weakly cemented accumulation of mineral particles formed by the weathering of rocks and contains void spaces between particles, which are filled by water, and air (Craig, 1998). Also, Bell (1993) defined soil as a material having three components, which includes: solid particles, air and water.
The geological formation is based on rock weathering which can occur either chemically when the minerals of a rock are altered through a chemical reaction with rain water, or mechanically through climate effects such as freeze – thaw and erosion. Laterite is a soil group, which are formed under weathering systems productive of the process of laterization (decomposition of ferro alumino – silicate minerals, leaching of the combined silica and base; and the permanent deposition of sesquioxide within the profiles (Wooltorton, 1975). The silica that is left unleached after laterization will form secondary clay silicate minerals. Laterites usually form a poor soil full of concretionary lumps and very unfertile because the potash and phosphate has been removed in solution, while only iron and silica are left behind (Gidigasu, 1976). 2.1.1 Definition and Classification Typical for the genesis of lateritic soils are the hot and wet climatic conditions of the tropical and subtropical regions.
Lateritic soils are products of a three-stage in-situ weathering and decomposition process (Ganssen, 1965 and Nwaiwu; osinibu, 2005). Their color is generally reddish to yellowish, dependant on the water regime during genesis, and on the mineralogical composition of the parent rocks (Glaser, 2005). Their name is derived from “Ferrum” and “Alumen” that are Latin for iron and aluminum.
Usually the in-situ weathered lateritic soil does not show any distinctive stratification (ICOLD, 2008). ICOLD (2008) proposes a classification of residual soils based on a consideration of their mineralogical influence. The problems of using classical, existing classification systems are complex and may be summarized as follows: 1. Laboratory tests may be misleading due to the particular importance of the in-situ structure of lateritic soils. 2.
Existing classification systems were not established for tropical in-situ formed residual soils but mainly for transported and remolded soils. 3. Existing correlations and empirical relationships describing engineering parameters may not be valid for residual soils. Wesley L.D.
and Irfan T.Y. (Blight, 1997) proposed a practical system for classifying all residual soils, based on the mineralogical composition and soil micro- and macrostructure. Wesley?s classification system is intended to provide an orderly division of residual soils in to groups which belong together because of common factors in their formation and/or composition which can be expected to give them similar engineering properties.