EFFECTS OF SOIL PROPERTIES ON THE STRENGTH OF WEAKLY STRUCTURED TROPICAL SOILS

This paper describes the relationship between strength and other prope rties of weakly structured tropical soils from Nigeria. Unconfined com pressive strength (T) and indirect tensile strength (Y) were measured at various matric potentials on cores of 40 mm in length and 20 mm in diameter. The relationship between strength and water content was desc ribed by a power function. The slope of this relationship varied with soil type and management. The values of slopes were positively related to clay content and water suspendable solids but not to bulk density or aggregate stability. At each matric potential, strength was positiv ely related to clay content, bulk density, and some fractions of water suspendable solids. The relationship with aggregate stability was als o significant but negative. No significant relationship was found betw een strength and organic carbon. From these relationships it was concl uded that the measured soil properties had a significant influence on the strength of these soils. Effective stress has been observed to inf luence markedly soil strength. The relationship between strength and e ffective stress was influenced by soil properties and management. Howe ver, the relationship between strength and effective stress was not li near for all but one soil as predicted by a model based on the Coulomb -Mohr theory and the concept of effective stress. The measured soil pr operties could not explain this discrepancy. For predictive purposes, improvements in the physical concepts on which the model is based need to be addressed.