Experimental study on liquefaction flow characteristics of saturated coral sand considering the influence of particle gradation

Under seismic loading, saturated coral sand is prone to liquefaction, leading to damage of structures on coral reefs. Due to hydraulic sorting, coral sand in different sites often has varying particle gradations. This study reinterprets liquefiable coral sand as a non-Newtonian fluid and systematically investigates the rheological characteristics controlled by mean particle size ( d ₅₀ ), coefficient of uniformity ( C _u ), and fines content ( F _c ) through undrained cyclic triaxial tests. Experimental results show that apparent viscosity ( η ) correlated with the number of cyclic loading cycles ( N ) are significantly influenced by particle gradation. The decay rate of the η - N curve decelerate with increasing d ₅₀ , accelerate with increasing C _u , and initially accelerate then decelerate with increasing F _c . When F _c = 30%, the rates are highest. The apparent viscosity gradient (∆ η / η ) initially remain steady, then rapidly increase, and sharply decrease with increasing excess pore water pressure ratio ( r _u ). When r _u reaches about 0.9, the saturated coral sand changes from solid-liquid phase transition state to fully liquid state. This transition process is not influenced by particle gradation. It is suggested that a value of r _u reaching 0.9 be adopted as the liquefaction criterion of coral sand sites.