Seepage deformation characteristic of gap-graded coral sand in the South China Sea

Abstract

Foundation settlement and collapse disasters resulting from seepage deformation in hydraulic-filled islands and reefs have been observed in the South China Sea, but the underlying failure mechanism and characteristic remain unclear. This study aims to investigate the influence of compactness and fine particle content on the seepage deformation of gap-graded coral sand and revel the characteristics and mechanism of seepage deformation of gap-graded coral sand through laboratory seepage deformation tests. The results indicate that the seepage deformation failure mode of gap-graded coral sand is influenced by the content of fine particles which undergo an evolution process from continuous piping to discontinuous piping to boiling. Particle loss is affected by the constraints between coarse particles, and the ability of different particle contact forms to restrict the loss of fine particles is different. Moreover, irregular particle morphology increases intergranular constraints, enhancing the coral sand's resistance to seepage deformation compared to standard quartz sand. Based on these findings, the instability coefficient was used to consider the influence of particle morphology and inter-particle contact on the seepage deformation. A hydraulic criterion for the internal stability of coral sand was established, demonstrating its versatility. Furthermore, the applicability of existing geometric criteria in evaluating coral sand was analyzed. The existing methods were found to be inaccurate in evaluating the internal stability of coral sand specimens with a fine particle content below 20 %.