Hydraulic Conductivity of Partially Saturated Semi-Arid Tropical Black Clay from Consolidation Tests

Oedometer and suction tests were performed in the laboratory on samples of black cotton soil of North-Eastern Nigeria compacted by static compaction to various degrees of saturation and moisture content to determine the compressibility and hydraulic conductivity characteristics at different states of stress in the pore-water. The goal is the evaluation of pavement performance and hydraulic barrier behaviour of semi-arid/arid soils under varied soil moisture and applied stress conditions. From the results, the basic index properties of the black cotton soil such as fines content, plasticity index, clay activity and predominant clay mineral were discussed against the backdrop of the pollution barrier potential of this tropical black clay. Soil suction decreased from 5.2 pF at 20% saturation to 1.8 pF at 100% saturation. Soil suction has been identified as a major factor controlling the expansive nature of black cotton soil i.e swelling- shrinkage properties. The coefficient of consolidation, which is an indication of the rate of consolidation, increased from 0.088 at 20% saturation to 0.324 at 100% saturation. It was observed that the hydraulic conductivity increased only by an order of magnitude from 2.83 x 10^-8 m/s to 2.15 x 10^-7 m/s as the degree of saturation increased from 20% to 100% when voids ratio varied between 0.305 and 0.338. This reveals that change in the hydraulic conductivity of the black cotton soil with increase in degree of saturation was not as remarkable as the change in compressibility. However the highest hydraulic conductivity value at 100% saturation (2.15 x 10^-7 cm/s) is within the upper acceptable limit requirement for leak-proof waste containment liners (1 x 10^-7 cm/s).