The stability of the coral sand foundation is a critical factor in ensuring the safety and integrity of ocean engineering structures. However, the deposition characteristics of coral sand significantly influence its mechanical properties. This study conducts a series of undrained shear tests to investigate the undrained response of saturated coral sand under various deposition directions (φ). Under monotonic shear conditions, the effective internal friction angle ( \(\phi^{\prime}\) ) increases with deviatoric stress (q) and eventually stabilizes, while the peak effective internal friction angle ( \(\phi^{\prime}_{{\text{p}}}\) ) varies significantly across different deposition directions. As φ increases, \(\phi^{\prime}_{{\text{p}}}\) initially decreases and then increases, ranging from 56.8° to 66.5° and reaching its minimum at φ = 60°. Under cyclic shear conditions, a fitting model with parameters A and B is developed to predict the generation trend of the generalized shear strain amplitude (γga) based on the excess pore water pressure ratio (ru) for various φ, in which A ranges from 0.318 to 0.355, and B is constant to 0.93. A linear relationship exists between parameter A and \(\phi^{\prime}_{{\text{p}}}\) for coral sand with different φ values. The physical properties of the soil play a significant role in the development of γga. The relationships between ru and γga for saturated coral sand, Nantong silt, and Fujian sand exhibit distinct differences but can all be described by the proposed fitting model, and it is more appropriate to use γga = 2.5% as the failure criterion for these three types of sandy soils. In addition, linear relationships are established between parameters A, and B and the physical property index which is correlated with the void ratio (e) and the average grain size (d50) regardless of soil types.
