Influence of gradation on shear behavior and particle breakage of coral sand under undrained conditions

This study explores the combined influence of particle size distribution (PSD) and relative density on the shear behavior and particle breakage characteristics of coral sand. Consolidated undrained (CU) triaxial shear tests were performed to examine the interactive effects of gradation and relative density on the shear response and particle fragmentation of coral sands collected from the South China Sea. The shear characteristics and particle breakage behavior of different gradations under varying relative densities and confining pressures were explored. The results revealed a complex interplay between gradation and relative density, significantly affecting the shear strength, modulus, and particle breakage behavior of the coral sand. Wide-graded materials exhibited enhanced shear strength due to improved particle interlocking, whereas narrow-graded materials showed increased particle breakage owing to higher localized stress concentrations. Notably, the 1–2 mm particle size fraction was identified as critical within the gradation. The relative breakage mass (Δ M ) was employed to quantify particle breakage, revealing a strong correlation between the gradation coefficient ( D 50 ) and the particle breakage. Furthermore, critical particle size ( d c ) was introduced, representing the point at which a dynamic equilibrium between particle breakage and the generation of fines is achieved. Based on these findings, a formula combining the gradation parameter D 50 , relative density, shear strength, and particle breakage characteristics was developed. These insights enhance the understanding of the mechanical behavior of coral sand under undrained conditions, offering valuable implications for the design of marine structures founded on coral sand. • CU triaxial tests explored gradation-density coupling effects on coral sand shear and breakage. • Gradation-density interaction governed strength, modulus, and breakage of coral sand. • 1–2 mm particle fraction is the critical particle size and D 50 strongly correlated with quantified breakage. • Model linking D 50 , density, strength, breakage aided coral sand structure design.