In order to address the issues of settlement and collapse of coral sand foundations on island reefs caused by rainfall infiltration

this study conducted top-down unidirectional penetration tests on gap-graded coral sand with varying D/d ratios (D represents the particle size corresponding to a cumulative percentage of 15% for soil particles smaller than a certain size in the coarse-grained group

while d represents the particle size corresponding to a cumulative percentage of 85% for soil particles smaller than a certain size in the fine-grained group.) and fine particle contents using a self-designed permeameter. It identified the migration characteristics and fundamental conditions for fine particle loss

revealed the patterns of permeability change in coral sand

and analyzed the underlying mechanisms through microscopic methods. The results show that: (1) For coral sand with a soil skeleton particle size ranging from 2 mm to 10 mm

particles measuring 0.25 mm to 0.50 mm are critical for ensuring stable seepage erosion. (2) Coral sand foundations with a D/d ratio below 10 and a fine particle content between 20% and 30% exhibit better stability against infiltration erosion. (3) Compared to quartz sand

the unique mineral composition and particle morphology of coral sand contribute to its increased resistance to particle migration and stronger resilience against seepage erosion. (4) In engineering applications

measures such as improving relative density

optimizing particle gradation

or implementing grouting consolidation can enhance the stability of foundations against seepage erosion. The research findings provide a scientific basis for the design of coral reef foundations with resistance to seepage erosion.