To assess the stability of coral sand foundation in complex environments, the undrained monotonic and cyclic shear tests were conducted in the laboratory. The test results indicate that the coral sand exhibits pronounced inherent anisotropy in the vertical direction. Under complex consolidation conditions, significant stress-induced anisotropy can also be observed. With increasing generalized shear strain (gamma g), both the generalized monotonic and cyclic shear modulus (Ggm, Ggd) exhibit a decreasing trend irrespective of consolidation ratio (kc) and inclinations of major principal stress (alpha c). Additionally, a strong linear relationship is evident between Ggm and Ggd, suggesting a consistent reduction pattern of Gg for various loading modes. The investigation on the inclination of the failure line (phi FL) for monotonic and cyclic shear is also conducted. The test results show that consolidation conditions have minimal influence on phi FL during monotonic shear, but exert a significant impact on phi FL during cyclic shear. A novel index called the consolidation parameter (eta) is proposed to quantitatively assess the relationship between kc, alpha c and phi FL. The average values of phi FL for cyclic shear increase with increasing eta, indicating the non-failure zone of coral sand during undrained cyclic shear will shrink with higher values of kc and alpha c.

Coarse particle shape in slip zone soil influences the mesoscopic structure of the soil, which in turn affects soil shear strength and failure behavior. In order to investigate the effect of particle shape on the shear characteristics of coarse-fine-grained mixed slip zone soil, three types of coarse particles (spheroidal, rounded, and angular) were selected for mixing and matching, and a total of 10 sets of medium-scale shear tests were designed for this paper. To quantify the shear deformation and failure process of slip zone soils, particle image velocimetry (PIV) technology and the hanging hammer method were used to obtain mesoscopic data of the soil (displacement vector data of soil particles and elevation data of the shear failure surface), which were used to calculate shear band thickness, shear dilatation, and roughness coefficient of the shear failure surface. The results indicate that coarse particle shape can considerably affect the macroscopic mechanical properties (internal friction angle and shear strength) and mesoscopic deformation characteristics (shear band thickness, shear dilatation, and shear surface morphology) of soils. Angular coarse particles have higher interlocking strength than spheroidal and rounded coarse particles, allowing angular coarse-grained slip zone soils to develop large shear band thickness and rough shear failure surfaces. In addition, mesoscopic damage analysis suggests that the damage rate of slip zone soils decreases with increasing coarse particle shape complexity. These findings enhance comprehension of the failure characteristics of soil-rock mixture slopes and serve as a good reference for the stability analysis of similar slopes.