Experimental study on vertical seepage of coarse-grained calcareous sand

Calcareous sand, a special geotechnical material employed as foundation fill in numerous reef constructions, exhibits susceptibility to seepage deformation in complex marine dynamic environments. Its permeability characteristics are notably distinct from conventional terrestrial soil, making the understanding of these properties critical for further applications. This study conducted vertical seepage tests on coarse-grained calcareous sand samples with varying coefficients of uniformity (Cu), coefficients of curvature (Cc), relative compactions (Dr), and soil skeletal structures. These tests utilized a custom-designed apparatus, supplemented with image binarization and particle image velocimetry techniques. Based on the results, the findings revealed a three-stage vertical seepage process: steady seepage, hydraulic adjustment, and seepage failure. The permeability coefficient k20 of the calcareous sand samples was observed to transition from a decreasing to an increasing trend with rising Cu and Cc values. Moreover, the k20 of tightly compacted and medium-tightly compacted samples exceeded that during the steady seepage stage upon entering the second stage, while the opposite holds true for relatively loose samples. The spatial organization of the sample skeleton had a considerable impact on its permeability, with coarse-grained calcareous sand imposing a greater constraint on fine particle migration compared to river sand. •Multi-factor consideration of seepage deformation characteristics of coarse calcareous sand.•Self-designed laboratory calcareous sand seepage model box with sufficient size.•Unique seepage characteristics observed in calcareous sand with different gradations.•Inconsistent permeable deformation characteristics observed in different soil layers of seepage path.•Permeability characteristics of calcareous sand crucial for offshore construction design.