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Fine-grained sand deposits in subtropical regions are valued for their low settlement percentages, providing high load-bearing capacity for foundations. Despite existing literature, certain behaviors of these deposits, particularly under cyclic loading in unsaturated conditions with stiffness control, still need to be better understood. Given the potential for shaft friction degradation in cyclically loaded axial piles, this gap is critical. This paper comprehensively investigates the mechanical properties of PE-Sand from a Southern Brazil deposit under cyclic loads, integrating laboratory and field analyses. Characterization involved granulometric, specific gravity, consolidation, matric suction, microscopic, mineralogical, and chemical tests. Uncommon unsaturated direct shear tests were performed, evaluating shear interface degradation under one-way and two-way cyclic loads. Standard penetration and seismic piezocone tests at 25m depth contrasted field properties. Findings identified uniform micaceous quartz sand. A novel conceptual model for PE-sand cohesion emerged from stiff-direct shear tests. Cyclic tests revealed heightened interface degradation with fewer cycles at increased load amplitudes. Fatigue results converged to a cumulative damage value, signifying material criticality limiting sand-sand interface resistance. Field exploration confirmed a heterogeneous sand deposit with geotechnical properties akin to laboratory findings. This study contributes nuanced insights into fine sand behavior under cyclic loading in unsaturated conditions, bridging gaps in existing knowledge. [ABSTRACT FROM AUTHOR]