Tropical alluvial soils have unusual engineering properties and their behavior is not typical of fine-grained soils. The literature regarding strength properties of alluvial soils is limited. This study presents an experimental evaluation of inherent anisotropy of undisturbed tropical alluvial soil samples by comparing the direct simple shear (DSS) test results of 52 specimens cut in horizontal and vertical orientations. The tests were carried out under consolidated undrained constant volume with excess pore water pressure measurements. The peak shear strength, effective stress paths, and secant shear modulus analyses showed that the difference in properties between horizontally and vertically cut specimens was negligible. Nonetheless, the results for vertically cut specimens presented a higher random error compared to the results for horizontal specimens in all aspects. A good relationship between the undrained shear strength and over-consolidation ratio was found but was not comparable to past DSS results for natural clays. Scanning electron microscopy images revealed flocculated edge-to-edge and edge-to-face particle associations. Directionality analysis of microfabric showed that the alluvial soils had low preferred orientation of particles. The general notion that all alluvial soils are highly anisotropic was not evident, based on the experiments reported in this study. The findings herein are useful for geotechnical engineers to better understand the strength behavior of tropical alluvial soils.

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