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There are various methods for improving the engineering properties of weak soils among which the usage of chemical additives is one of the most important ones. Considering sepiolite as an environmental friendly material and its unique physiochemical characteristics, this substance has been used as an additive for soil improvement in this study. For this purpose, in addition to soil identification tests, in order to examine the effect of changes in the percentage of sepiolite additive, a set of tests has been done including Atterberg limits, modified compaction, uniaxial compressive strength, pH, direct shear and permeability test on sandy clay and clayey sand soils. Furthermore, scanning electron microscopy (SEM) and x-ray diffraction (XRD) tests have been performed to investigate the microstructure of non-stabilized and stabilized soil. According to the results, with increasing the additive percentage and the curing time, the uniaxial compressive strength of the soil increased. The uniaxial strength in the clayey sand soil rose up to six times larger than that of the initial strength of the soil. However, the maximum strength enhancement in the sandy clay soil was up to three times the initial value. Sepiolite (above 5%) has improved soil shear strength parameters including cohesion and internal friction angle. The increase in the additive content led to diminished permeability in soil. Further, the elevation of the sepiolite content in the soil resulted in enhanced liquid and plastic limits of soil. Microstructural investigations indicated that the reason of improvement in strength parameters of soil was the incidence of chemical reactions including ion exchange, flocculation and production of calcium-silicate-hydrate (CSH) and calcium-aluminate-hydrate (CAH) gels. Further, sepiolite fiber resulted in enhanced strength by filling the pores and void spaces of soil. • Sepiolite is an environmentally friendly material to stabilize the soils. • Increasing the sepiolite content during the curing time, increases UCS of the soil. • Sepiolite fibers act like bridges and connect different parts of soil. • CSH and CAH are products of chemical reactions between sepiolite and soil. [ABSTRACT FROM AUTHOR]