Abstract: The physical and engineering mechanical behaviors between coral sand and terrigenous sandy soils are considerably different. To study these behaviours, a series of undrained multistage strain-controlled cyclic triaxial tests was conducted on saturated coral sand from Nansha Islands, South China Sea. The influence of effective confining pressure and relative density Dr on the dynamic shear modulus and damping ratio of coral sand was studied. Compared with the test results of coral sand and terrigenous sandy soil and gravel, significant differences were found in the maximum shear modulus Gmax, the shapes and the upper and lower boundaries of shear modulus ratio G/Gmax curves, reference shear strains , the shapes and the upper and lower boundaries of damping ratio ? curves. The maximum shear modulus Gmax of coral sand is higher than that of terrigenous sandy soil and gravel, and the Gmax of coral sand predicted by empirical equations of terrigenous sandy soil is underestimated by 30%. The nonlinearity of coral sand is slightly weaker than that of terrigenous sandy soil and gravel. The empirical formulas for predicting G/Gmax and ? of terrigenous sandy soil and gravel are not applicable for Nansha coral sand. The empirical formulas for predicting G/Gmax and ? of coral sand are proposed.

Key words: Nansha coral sand, the maximum shear modulus, shear modulus ratio, damping ratio, cyclic triaxial test