ABSTRACT
This project involved investigating the possibility of stabilizing poor lateritic soil with varied percentage of rubber wood ash (RWA) and lime for use as Civil Engineering construction material especially to improve subgrade soils. This will considerably reduce the cost of construction as well as reducing environmental pollution in terms of disposal of Rubber Wood Ash (RWA).
The work was carried out in three stages, by testing the natural soil material; mixing with rubber wood ash and thereafter mixing with combinations of rubber wood ash and lime in varied proportions. Stabilization of the soil was carried out by mixing each of the additives with the soil in varied proportion of 0%, 2%, 4% and 6% by weight of the soil after which each corresponding RWA percentage was mixed with 0.5%, 1%, 1.5% and 2% lime content. The laboratory tests carried out includes natural moisture content, specific gravity, particle size distribution, compaction, Atterberg’s limits and California bearing ratio. The reliability of the laboratory test results was assessed using the Cronbach Alpha Coefficient verified by means of Analysis of Variance (ANOVA). To determine the optimum value of rubber wood ash and lime required to stabilize a given weight of soil, statistical design of experiment using central composite design (CCD) and optimize by way of numerical optimization method was done.
The soil was classified as A-6 soil group according to AASHTO soil classification system possessing the following characteristics in its natural state; average moisture content of 18.7%, average liquid limit of 34.72%, average plastic limit of 22.02%, average plasticity index of 12.70% and average specific gravity of 2.83. The soil sample has a California Bearing Ratio of 2.47% for the natural soil sample. Test results also show that increase in RWA content increased the optimum moisture content but decreased the maximum dry density. It was also observed that increase in RWA content, reduced plasticity and increased strength of the soil. Every decrease in MDD indicates that the compaction energy is less than its natural state. The addition of 2% constant RWA with varying lime from 0.5%-2% lime shows that the MDD decreases consistently from 1.89g/cm3 to 1.63g/cm3 with an increase in OMC from 14.00% to 21.50%. The addition of 4% constant RWA with varying lime from 0.5%-1.5% lime reduce the MDD from the initial value of 1.63g/cm3at 2% RWA to a constant value of 1.62g/cm3 and then increases to 1.68g/cm3 at 2% lime variation when the additives became too much and vice versa for the OMC. The CBR value of the virgin soil is (2.47%) shows that the sample is very poor as subgrade material. The addition of RWA only shows a little improvement but the addition of RWA and lime gave a better result. For this research, the optimum mix ratio for economy and effectiveness was 4% RWA mixed with 1.5% lime which gave a result of 11.34% for soaked sample and 14.30% for the unsoaked sample respectively. The percentage improvement observed is 359%. From the result of numerical optimization, it was revealed that; RWA value of 3.05%, lime concentration of 1.67% will be required to stabilize 96.76% of natural soil resulting to a soaked CBR of 10.0423%.
