Multiscale observations of the behavior of composite soil containing biogenic crushable carbonate gravel from ring shear test

Coral gravel soil (CGS) is widely encountered in civil and coastal engineering in coastal areas but receives significantly less attention than its counterpart, carbonate sand, which has similar mineral compositions. The unique behavior of this soil results from its complex particle composition (coral gravel and fines), highly irregular particle shape, and high potential for particle crushing. However, the shear behavior of this soil subjected to large deformation remains poorly understood. This study investigates the soil responses using a ring shear test, which enables the attainment of unlimited shear strain levels. It clarifies the effects of effective vertical stress (50–400 kPa), shear rate (0.1–50 mm/min), and ultimate shear strain (exceeding 2500%), with the latter two parameters scarcely addressed in the literature. In addition, this study quantifies the particle crushing characteristics under various shear conditions and correlates this information with the soil’s shear behavior, including stress–strain and dilatancy-strain relationships, as well as the shear strength parameters. The most novel contribution of this study over previous investigations is the establishment of the microstructural evolution of CGS during shear, which is utilized to interpret the soil behavior. These findings improve the current understanding of this soil and provide an important data set for further studies.