Introduction
1.1 Preamble
Geotechnical engineers are often faced with the problem of constructing road beds on soil which do not have adequate strength to support the wheel loads imposed upon them either in construction or during the service life of the pavement. It is sometimes necessary to treat these soils, termed expansive soils to give a stable sub-grade or working platforms for the construction of the pavement. Treatment of soil are generally classified into two processes: soil modification or soil stabilization. The purpose of sub-grade modification is to provide a working platform for construction equipment, while sub-grade stabilization is meant to increase the strength and durability of the soil.
1.1.1 Black cotton soil
Expansive soil can be found almost any where in the world but they are basically confined to semi arid regions of the tropical /temperate zones. These areas are usually characterized by marked dry and wet seasons with low rainfall, poor drainage and exceedingly great heat. The precipitation is such that the annual evaporation exceeds the precipitation (Chen, 1988).
Two groups of parent rock materials have been associated with the formation of expansive soils. The first group comprises sedimentary rock of volcanic origin, which can be found in North America, South Africa and Israel (Ola, 1978). The second group of parent materials are basic igneous rocks found in India, Nigeria and South Western U.S.A (Plait, 1953). Tropical black clays are formed by weathering (Morin, 1971).
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