Natural biogenic coral gravel soils in coastal areas in southern China: Sampling, mechanical characterization, and microstructure observation

The dominant presence of coral gravels (CGs) with biological origins, irregular shapes, and abundant internal porosity fundamentally differentiates coral gravel soil (CGS)'s geotechnical behavior from terrestrial soils. The properties of natural CGS remain largely enigmatic, a situation exacerbated by the extreme difficulty in sampling. This study upgrades the Mazier sampling technique by applying a chemical drilling additive, enabling the collection of high-quality, undisturbed CGS specimens in Hainan Province in South China. The quality of these specimens is assessed through in situ and laboratory tests, as well as microstructural identification via computed tomography (CT) scanning. The study establishes physical, mineralogical, chemical, and shear strength profiles for CGS at 2–16 m deep. The relative proportion of coral gravel to soil matrix emerges as a critical indicator of soil behavior in shearing. In addition, this research investigates how the unique structure of coral gravel influences the geotechnical behavior of the studied soil. This study provides foundational datasets to unveil the mechanisms of soil shear strength, thus contributing to a comprehensive understanding of the geotechnical behavior of coral-bearing biogenic soils.