The role of clay minerals on physical and chemical activities and soil structure and also on hydraulic and mechanical properties is important. Not only the quantity of mineral also kinds of clay affect physical and mechanical soil behavior. Among the 2:1 clays, minerals such as sepiolite and palygorskite are high prevalence in the arid region soils of Iran, but research about hydraulic and mechanical properties of these fibrous minerals is little. The goal of this research is to study the effect of adding sepiolite and palygorskite on soil water characteristic curve (SWCC) , indirect tensile strength (ITS) and penetration resistance (PR) of their sand mixtures. The samples of sand- clay mineral with 15% , 30% and 45% clay with 0.9 critical bulk density were tested. The SWCC was determined on samples with 5 cm diameter and 3 cm height, using sand box, sand kaolin box and pressure plate. ITS in cylindrical specimens with a diameter of 5 cm and height of 3 cm in different moisture levels were measured according to the Brezilian indirect test. Also the PR was measured using cone micro penetrometer with 30° angle and 4 mm diameter with 2 mm/min speed of penetration. Results showed that palygorskite in clay-sand mixture increased the ITS because this mineral bonded the sand particles and prevented disruption. Also with addition of clay and lower soil moisture the PR was rised. This effect was more pronounced for sepiolite mixtures which produced more frictions beetween cone of the penetrometer and the samples. By fitting van Genuchten-Mualem model to the SWCC data, plant available water (PAW) and least limit water range (LLWR) were calculated, for the 2 types of clay minerals. The results showed that, with increment in percent of clay, AW and LLWR was increased. It also showed that with increasing clay content and decreasing degree of saturation, the effective stress values were increased. The slope of the mechanical strength-effective stress curve was used as the hardsetting index (H index ) . With increasing in percent of clay the H index was increased, and the sepiolite treatment was more effective than palygorskite.
