Abstract: Loading pattern and stress path have great influences on liquefaction resistance of saturated sand. Using GDS hollow cylinder torsion shear apparatus, a series of undrained cyclic loading tests was performed under complex loading conditions on isotropic consolidated saturated coral sand in a Nansha Island, South China Sea. The influence of the inclination angle of principal stress ( ) on liquefaction resistance of coral sand was discussed under 90° jump rotation of principal stress path. It is found that: when the cyclic stress ratio (CSR) is used as the stress level index, has no significant effect on the liquefaction resistance of coral sand as the change in intermediate principal stress coefficient (b) is not controlled; when b always maintains at 0.5, the liquefaction resistance of coral sand shows a tendency to decrease first and then increase with the increase of , and the liquefaction resistance is the lowest when 45°. Based on analyzing the change of the major and minor cyclic principal stresses and caused by complex cyclic loading, a unit cyclic stress ratio (USR) as a new index is defined for cyclic resistance of coral sand. A virtually unique correlation between USR and Nf can be established under different cyclic loading patterns and stress paths considered. The recompiled original experimental data of four types of cohesionless soils in the published literature independently verified the general applicability of USR to characterize liquefaction resistance.

Key words: saturated coral sand, complex cyclic loading conditions, liquefaction resistance, unit cyclic stress ratio