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.

Marine clay may experience stiffness degradation and catastrophic failure when subjected to complex ocean dynamic loadings. This can result in instability, destruction, or capsizing of offshore structures. In this study, marine clay was regarded as a non-Newtonian fluid with shear-thinning behaviour, and the mechanism of progressive stiffness degradation during cyclic loading was discussed from the perspective of fluid dynamics. A series of cyclic direct simple shear tests were conducted on undisturbed marine clay obtained from three offshore sites. Further, the stiffness degradation and flow characteristics under different plasticity index (I-P) and cyclic stress ratio (CSR) conditions were investigated and quantified using the stiffness degradation index (delta) and average flow coefficient (kappa), respectively. The results revealed that the decrease in delta with the increasing number of cycles (N) in a semi-log scale can be categorised into three modes: (1) linear (nonfailure), (2) fast-linear-fast (failure), and (3) linear-stable (failure). Consequently, a two-parameter model was proposed to predict the delta of failure marine clay from different sea areas with varying I-P and CSR values. Moreover, with the increase in N, kappa of the nonfailure marine clay increased gradually in a very limited range, thus exhibiting illiquidity characteristics; by contrast, kappa of the failure marine clay exhibited a slow linear-exponential-rapid linear growth pattern, thus indicating a change in liquidity from weak to strong. Finally, a unified model linking the stiffness degradation and flow characteristics of marine clay under different types and conditions was proposed, where kappa at the cyclic failure state (the failure criterion is a double-amplitude shear strain of 15%) was denoted as kappa(f). Evidently, all data points of kappa/kappa(f) similar to delta were distributed in a narrow range, and a virtually negative exponential relationship was observed between kappa/kappa(f) and delta.

Dynamically loaded soils can exhibit large-deformation flow liquefaction or limited-deformation cyclic mobility mechanisms, depending on the initial state of the soil. Undrained cyclic triaxial tests were performed on saturated calcareous and silica sand specimens prepared with different relative densities and subjected to various effective confining pressures and cyclic stress ratios to study the flowability of viscous liquefied sand. The cyclic shear stress-strain rate relationship for calcareous and silica sands transitioned from an elliptical shape to an asymmetric Lame curve shape as excess pore pressures accumulated under cyclic loading. The asymmetric Lame curve-shaped relationship demonstrates that the saturated sand exhibited low shearing resistance and high fluidity under elevated excess pore pressures for the conditions evaluated. The average flow coefficient, kappa over bar , defined as the maximum shear strain rate triggered by the unit average cyclic shear stress, and the flow curve defining the variation in kappa over bar with the number of loading cycles, describes the flowability of the saturated sand and is used to quantify the cyclic failure potential of the saturated sand under a proposed viscous fluid flow failure criterion. The effect of relative density, effective confining pressure, and cyclic stress ratio on the flow curves and the number of cycles to failure under the proposed viscous fluid flow failure criterion is discussed and compared with the cyclic resistance determined from widely used excess pore pressure- and strain-based cyclic failure criteria. The viscous fluid flow cyclic failure criterion is more stringent than these alternative criteria, and the corresponding axial strains are consistent with those associated with liquefaction triggering under cyclic strain approach.