The liquefaction of coral sands caused by the accumulation of excess pore-water pressure is a major factor contributing to catastrophic events on coral reefs, and accurately estimating this excess pore-water pressure accumulation holds significant importance. High-quality laboratory test results are essential for analytical or numerical calculations. In this study, a new test method is employed to conduct a series of undrained, multistaged, stress-controlled multidirectional hollow cylinder tests on saturated coral sand under complex loading conditions. The concept of threshold strain (gamma t) and the method for determining gamma t of saturated coral sand specimens under complex loading conditions are proposed. The test results demonstrate that gamma t of saturated coral sand remains insensitive to cyclic loading conditions (including frequency, stress path, and mode) but increases with increasing relative density. The range of volumetric threshold strain, degradation threshold strain, and flow threshold for saturated coral sand under different initial states and cyclic loading conditions are 0.0183%- 0.0341%, 0.0242%-0.0454%, and 1.006%-1.614%, respectively. This research provides a novel approach for accurately determining input parameters required for resolving and implementing coupled models in numerical modeling.

In this study, a series of cyclic triaxial tests were conducted on two poorly graded sands with different particle shapes. The experimental results were integrated with test results reported in the literature to construct a comprehensive database for investigating the effects of particle shape and relative density on excess pore water pressure (EPWP) generation. The sand types in the database were divided into three groups based on the particle shape: subrounded, subangular, and angular. The test results showed that the particle shape and relative density influenced the axial strain development and EPWP generation during undrained cyclic loading. As the relative density increased, the effect of the particle angularity on the shape of the EPWP generation curves decreased. The normalized EPWP generation curves of the subangular particles (C306 sand) exhibited a strong dependence on the relative density. However, the angular and subrounded particles exhibited a weaker influence than the subangular particles on EPWP change for different relative densities. A model was established to predict the EPWP generation with variations in the particle shape and relative density.