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There is a very few experimental data on the mechanical behavior of unsaturated sand-fine mixture, particularly in constant water content conditions, because of the technical difficulties and time-consuming nature of measuring suction and deformation. This paper presents the results of a series of constant water constant triaxial tests on the specimens of an unsaturatedclayey silty sand in low confining pressure.The effective confining pressures selected in this study are 25, 50 and 100kPa. In comparison with the majority of experimental works, existing in the literature, these levelsof confining pressures are relatively low and this aspect is fewer focused.The effect of matric suction on stress-strain behavior of selected soil in different density and confining pressure is the main aim of the present study. The tests were carried out as Constant water content (CW test). This type of test represents field un-drained condition, where, the rate of loading is much quicker than the rate at which the pore water is able to drain out of the soil. Axis translation technique and double-walled triaxial cell have been used to measure the soil matric suction and variation of pore air volume respectively. Test specimens were prepared at two different compaction conditions prior to testing to achieve different initial density. According to the obtained results, it is found that the mechanical behavior of soil mainly depends on the initial density, the mean net stress and the initial matric suction. Also the volume and pore water pressure changes are totally different in specimens with different initial condition. However, the results of tests indicated that the shearing strength of sand-fine mixture increase non-linearly with matric suction.The results of tests show that the soil shear strength is increased with increasing in both effective confining and matric suction. However the effect of confining pressure is more significant than matric suction. The matric suction and confining pressure play an important role on volume changes of unsaturated soil, however the effect of confining pressure is more considerable. For all of the tests, the specimens were subjected to large axial strain. This allowed us to calculate the deviatoric and mean net stresses at critical state. The comparison of results in q: p-uacoordinate, indicate that, the position of critical state lines for unsaturated soil changes as a function of matric suction. However, these lines can be considered as parallel to each other.Deviatoric stress (q) intercept of critical state line increases with increasing matric suction. Besides, initial density for unsaturated specimens influences the shear resistance corresponding to large strains. However the tests results on saturated specimens, according to the classical critical state soil mechanics, indicated that, soil resistance at critical state is not a function of soil density.

Keywords: Shear Strength, Matric suction, Unsaturatedsoil, Density, Confining stress

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