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The Caspian Sea is regularly visited by many tourists and the surrounding lands are densely populated. Accordingly, many tall buildings and heavy structures are constructing over this coastal area. The region is overlaid by poorly graded clean sand which seems to be susceptible to liquefaction occurrence. However, there is no documentation and field observation to ensure liquefaction triggering in the uniform rounded- shape deposits of the region during an actual earthquake. Hence, lots of research works are expected to be carried out to recognize cyclic behavior of these coastal deposits before the probable scenario earthquake and the consequent disaster happen. The undrained shear strength of granular soils is one of the most important parameters to evaluate flow liquefaction, which is prone to produce beneath the shallow foundations bearing considerable levels of static shear stress. Mechanism of flow liquefaction is commonly studied using triaxial apparatus to obtain a better understanding of the parameters controlling the phenomenon. Soils experiencing the flow type of liquefaction commonly undergo large deformations because the driving stresses tolerated by the soil exceed the shear strength reduced by generation of excess pore pressure in earthquake condition. There exist numerous studies for evaluation of undrained behavior of mixed soils comprising of gravels, sands, silts, and clays. Majority of the previous studies have focused on the homogeneous mixtures of soils while natural soils are generally found in the nature with levels of non-homogeneity. In this study, mechanical behaviors of homogeneous and heterogeneous mixtures of the Babolsar sand and gravels are compared under different conditions such as initial effective stress, relative density, and heterogeneity. Several triaxial experiments were conducted to evaluate effects of stratified sedimentation and increase of heterogeneity on undrained shear strength and pore pressure development of sand-gravel mixtures in various relative densities and confining pressure. A simple formula has been defined to specify level of heterogeneity of the sample, which varies from zero to one percent. The results demonstrate that shear strength, the potential of pore water pressure, and internal friction angle of homogeneous samples are quite different from heterogeneous samples. The undrained shear strength of sand-gravel mixtures increases with increasing degree of heterogeneity even in identical gravels content. Excess pore water pressure in sand - gravel mixed samples with higher degrees of heterogeneity reduces and tends towards dilative behavior, with respect to the more homogeneous samples. The undrained shear strength of the samples is proportional to initial relative density and effective confining pressure, as expected and observed in other granular materials. It is found from the results of the tests that the type of mixed sedimentation has significant impact on the soil's friction angle. In fact, increase of heterogeneity level increases the internal friction angle of the mixed soils while the gravels content is kept identical.

Keywords: Undrained shear strength, Sand – gravel mixture, Static triaxial tests, Flow liquefaction

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