Large part of Free State in South Africa is prone to subgrade problems resulting from expansive activities. These soils are known for their shrinkage and swelling potential that causes damage to infrastructure; such as pavement, rail track and airport runways. This study presented systematical laboratory method, adopted for expansive soils treatment using activated fly ash (AFA). Base on the laboratory findings, the AFA significantly improved the shear strength and suction characteristics of the reactive subgrade, thus, quantified these expansive soils to be suitable as backfill for highway subgrades.
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