Abstract: The accumulation of excess pore water pressure (EPWP) under cyclic loading may induce partial or complete liquefaction of saturated coral sands, posing significant threats to the safety of structures and foundations. In numerical simulations and analyses, accurate prediction of EPWP development is essential, with the determination of threshold strain serving as a critical step. A novel method has been developed to determine the threshold strains (pore pressure threshold strain γ_tp, stiffness degradation threshold strain γ_td, and flow threshold strain γ_tf) for the EPWP generation and stiffness degradation in saturated coral sands under complex stress paths. This was achieved isotopically consolidated, undrained single-stage and multistage cyclic shear tests, including 90° jumps and continuous rotations of principal stress. The findings indicate that while γ_tp, γ_td, and γ_tf are relatively insensitive to the cyclic stress, they are significantly influenced by the initial relative density (Dr). Additionally, the gap between γ_tp and γ_td widens as D_r increases. Under varying cyclic loading conditions and initial physical states, γ_tf corresponds to the EPWP ratio of approximately 0.9, with a corresponding stiffness index of around 0.10. The proposed method for determining γ_tp, γ_td, and γ_tf can effectively reduce the number of required cyclic tests, making it suitable for use as input values in numerical calculations or analytical methods, and for characterizing soil behavior under stress and strain conditions.

Key words: saturated coral sand, complex stress path, threshold strain, excess pore water pressure generation, stiffness degradation