Abstract:
This thesis investigates the effectiveness of using cow dung ash and lime as additives for expansive subgrade soil stabilization. Expansive soils exhibit a large strength and volume changes (swelling and shrinkage) due to moisture content fluctuation. This leads to the premature failure of the structures founded on them, followed by repeated maintenance schedule, and budget. Methods adopted in Rwanda to deal with expansive subgrade soils are the replacement of such soils with selected stable soils and lime stabilization. The feasibility of removal and replacement of poor subgrade soils depends on the availability of suitable fill materials within economic distances and the activation depth. Lime is used worldwide for expansive subgrade soil treatment. In this research work, an attempt has been made to assess the effectiveness of utilizing cow dung ash (CDA) and lime in expansive soil stabilization. The main focus of this experimental study is to evaluate the application of cow dung ash in such poor subgrade soil and to use lime as optional additive depending on the target, therefore minimizing the consumption of lime and emission of CO2 resulting from in lime production. The subgrade investigated in this work is a clay soil with high plasticity and high expansion potential that has been treated with cow dung ash at varying content (0%, 6%, 10%, 14%, 20%, and 25% by dry weight of the soil), lime (0%, 5%, 7%, and 9%) and a mixture of 5% lime with varying ash content. Effectiveness of stabilizer has been appraised by measuring the change between untreated and treated subgrade soils through atterberg limits tests, linear shrinkage tests, compaction tests, free swell index, CBR tests, and UCS tests. Also, by using X-Ray Fluorescence, the chemical compositions of lime, CDA, and natural soil have been identified. In general, the results obtained show that the sample soils treated with CDA had relatively higher strength, lower PI, and swelling potential than those of the untreated soils. The blend of lime and CDA significantly improve the bearing strength and shearing strength, lower PI, and the swelling potential became negligible. Finally, based on the results obtained, the use of cow dung ash alone did not show the desired result. However, the combinations of cow dung ash and lime showed an effective improvement of the different properties of expansive clay soil together with the reduction of the consumption of lime in soil stabilization.
