To remedy the problems of civil engineering related to soils in recent years, various geo-synthetic materials have been used to improve the mechanical and physical soil characteristics. Besides these geosynthetic materials, different kinds of natural fibers have also been used to improve soil charac-teristics. On the other hand, while a basalt fiber, which is a natural material, has been used to reinforce the concrete; it is rarely available to reach a source in literature regarding the use for soil improvement. This study is about to investigate the behavior of a silty soil reinforced with basalt fibers. To conduct this study, a series of tests of unconsolidated undrained triaxial test (UU) were carried out on both unsaturated and saturated soils. Unsaturated soils were prepared in optimum water content and maximum dry density and saturated soils were prepared by consolidating under 200 kPa. Two parameters (the length and the content of the basalt fibers) were considered in this study. The fibers of 6, 12 and 24 mm length were dis-persed in silty soil at different contents (1%, 1.5% and 2%). The results of the unconsolidated undrained triaxial tests (UU) show that the addition of basalt fibers effectively improves the shear strength of silty soil without making any difference if the soil is saturated or unsaturated. For all of the lengths of the bas-alt fibers, the maximum improvement was obtained with a content of 1.5%. Moreover; the maximum im-provement was obtained for a content of 1.5% and 24 mm of basalt fiber.
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