ABSTRACT

Red tropical soils, though readily available in the tropics and relatively cheap, are often substandard due to poor engineering qualities. They are therefore, problematic and tend to deviate in terms of engineering behavior from the expectations of conventional soil mechanics principles for temperate zone soils. Consequently, soil mechanics test results on intrinsic properties of red tropical soils are generally not reproducible when compared with temperate zone soils. This could be attributed to either mechanical instability or thermal instability on account of their morphology and mineralogy which are linked to the peculiar pedogenetic processes of tropical weathering from which the soils resulted. However, research has shown that the soils can be stabilized with hydrated calcitic lime resulting in improved index and engineering characteristics.

Following field studies, 16 samples of the red tropical soils were collected from designated locations from parts of Central and Southern Nigeria and subjected to various tests including the basic engineering classification tests, physico-chemical, chemical and mineralogical identification tests in accordance with procedures stipulated in BS 1377 (1990) as well as chemical and mineralogical analyses. The soils were subsequently treated with varying percentages (4.5 – 6.0%) of hydrated calcitic lime and cured for periods of up to 28 days in order to determine their pozzolanic activity. They reacted, showing a wide variation of maximum unconfined compressive strength ranging from 460 – 1980 KN/m².

Based on two theoretical models available in the literature (dissolution of soil minerals and reactions through diffusion controlled process) it is believed that reaction between the soil and lime is, essentially, a diffusion controlled process. Results show that most of the reactions occur within the first five to seven days during which period the reactions obey Jander’s diffusion equation for solid state reactions. On the basis of constant rate of reaction obtained from this equation, an activity index, Ai is presented and its validity tested against the unconfined compressive strength of the soil-lime system. The pozzolanic activity of the soils implies the formation of cementitious reaction products upon which the increase in strength of the soil-lime system is dependent. A valid statistical correlation exists linking the proposed activity index, Ai with the empirical reactivity parameter of Thompson. In spite of the increase in strength recorded for each soil upon stabilization, there is a resultant increase in the porosity of the soil-lime system. Classification of the red tropical soils from North Central and Southern Nigeria based on lime stabilization and activity index is a major contribution this research has achieved.