Abstract:

Coral gravel soil (CGS) is a biogenic composite material composed of coral gravels (CG) and clay mineral matrix, commonly found in tropical coastal and reef island regions. This type of soil typically exhibits high porosity, irregular particle shapes, and a high breakage potential, making its engineering behavior significantly different from that of terrigenous coarse-grained soils. Traditional soil mechanics theories often fail to accurately describe the deformation and seepage behavior of CGS. To better understand its compression-induced deformation and permeability characteristics, one-dimensional constant rate of strain consolidation tests were conducted on CGS specimens with varying CG content. The compressibility, permeability, and consolidation characteristics were investigated. Parameters such as skeletal void ratio and particle breakage index were introduced to quantify particle breakage during compression and evaluate its effects on deformation and permeability. Microstructural changes before and after compression were observed using scanning electron microscopy, enabling the development of a micro-mechanistic model of CGS compression–permeability behavior. The results indicate that both CG content and effective stress jointly govern the compression–permeability response of CGS. Under high effective stress and high CG content (greater than 30% by volume), significant particle breakage occurs, leading to a notable increase in compressibility and a significant reduction in both permeability and the coefficient of consolidation. The compression behavior of CGS essentially represents a microstructural evolution from a “skeletal framework” to “densely packed” configuration. The skeletal void ratio e₂ effectively captures this transformation, with a critical value of e₂ = 5 proposed as a threshold to distinguish different microstructural characteristics and their impact on compression–permeability behavior. This study provides mechanical parameters and technical insights valuable for engineering applications on coral reef islands.

Key words: coral gravel soil, compression, permeability, skeleton void ratio, particle breakage