This research deals with the improvement of the mechanical properties of expansive clay soils by silicafume. The expansive soil was prepared in laboratory by mixing natural soil brought from (Nahrawan) citywith different percentages of Bentonite (30, 50 and 70%by weight). The test program included the effect of Bentonite on natural soil then study the effect of silicafume on prepared soil by adding different percentages of silicafume(3,5, and 7% by weight) to the prepared soils and the influence of these admixtures were observed by comparing their results with those of untreated soils (prepared soils). The properties chosen for this comparison were specific gravity, the consistency limits, swelling percent and swell pressure. The resultsshow that the plasticity index, the optimum moisture content, swelling percent and swell pressure increase with increasing the Bentonite percent, and the maximum dry density, specific gravity decrease with increasing the Bentonite percent. When the prepared soil treated with silicafume, the results show that plasticity index, specific gravity, maximum dry density swelling percent and swell pressure decrease with increasing the silicafume percent, while optimum moisture content and unconfined compressive strength increased with increasing silica fume percentages. And the perfect percentage of silica fume was obtained to be 7% which where decreased the swell and swell pressure in large percent.
The result showed that the addition of silicafume to expansive soils has in a positive effect to the geotechnical properties and these results will benefit the engineers or decision makers in using this additive.