Abstract: Coral sand is a crucial geotechnical material for the construction of islands and reefs in the South China Sea. In some reclamation areas, the particle size distribution of coral sand spans a wide range, encompassing sizes from sand particles to fine particles. 【Objective】During the reclamation of reef islands, coral sand and fine particles form a heterogeneous, layered structure. However, the influence of this stratified structure, resulting from the concentrated distribution of fine particles, on the strength and deformation of the coral sand layers remains unclear. 【Methods】This study conducted a series of undrained triaxial shear tests on coral sand-fine particle mixture samples to investigate the effects of effective confining pressure and the spatial distribution of fine particles on the mechanical properties of coral sand. 【Results】The experimental results indicate that, under the same fine particle content, the strength of samples with uniformly distributed coral sand and fine particles is significantly lower than that of samples with layered fine particle distribution. Both the apparent cohesion and internal friction angle of coral sand samples increase with the number of fine particle interlayers. Variations in the spatial distribution of fine particles lead to different undrained shear behaviors in the samples. On the e-p' plane, the critical state line of coral sand shifts upward as the number of fine particle layers increases. 【Conclusions】The stratified structure formed in coral sand islands and reefs under processes such as hydraulic sorting can enhance their overall strength. Conventional homogenization methods for fine particles may underestimate the actual bearing capacity of such stratified coral sand layers. This study provides theoretical insights for the treatment of foundations containing fine-grained coral sands in the South China Sea.