COMPRESSIBILITY AND STRENGTH CHARACTERISTICS OF A PARTIALLY SATURATED BLACK COTTON SOIL OF NORTH-EASTERN NIGERIA

Abstract

The consolidation and strength characteristics of a partially saturated clay soil are studied theoretically and experimentally. The Terzaghi's one-dimensional consolidation equation for saturated soils has been extended to embrace unsaturated soils using unsaturated soil principles and theories. The model equation obtained has been solved using the implicit method of numerical analysis for the assumed initial and boundary conditions. The proposed differential equation for onedimensional consolidation of a partially saturated soil was used to predict the rate of dissipation of excess pore pressure for partially saturated black cotton soil at various degrees of saturation. It was found that except at the initial stage the profiles of pore water pressure Uw, pore air pressure Ua and void ratio e follow a parabolic pattern during the process of consolidation. The rate of reaching complete saturation was found to be decreasing with height from the bottom of the clay layer as expected depending on the degree of saturation, also the lower the degree of saturation, the faster is the rate of change of the pore pressures. Oedometer, triaxial compression and suction tests were performed in the laboratory on samples of Black Cotton soil compacted by static compaction to various degrees of saturation and moisture content. The investigation indicated that significant changes in the volume and shear strength of a partially saturated soil would accompany slight variations in the moisture content of the soil when the degree of saturation is below 80%. This can also be inferred from the theoretical considerations which shows that the rate of change of the pore pressures is quite remarkable with degree of saturation, especially when this is below 80%. Equations 5.4 and 5.6 relate the pore pressures with compression index 'C;' and shear strength '''Cf'. The data xv obtained from the experimental programme also emphasize that, for design of pavements, especially in arid and semi-arid regions, one must consider not only material changes but also potential significant changes in soil suction for the same material. From the results, a more accurate prediction of the magnitude and rate of consolidation settlement, in advance of construction for Black Cotton soil at a specific state of soil moisture can now be determined. It is recommended that minimum variations in soil moisture and equilibrium moisture conditions should be maintained for highways with Black Cotton soil subgrade. It is also suggested that periodic measurement of soil suction for such highways can be used as a means of evaluating pavement performance.