# Effect of Fines Content on the Compression Behavior of Calcareous Sand
## 1 Introduction  
Coral reef sediments dominated by calcareous sand are found throughout the world. In recent decades, with the progressive extension of construction works into the oceanic areas, marine projects involving calcareous sand have developed rapidly (e.g., filling by hydraulic suction dredgers). Calcareous sand, the primary foundation material for marine construction and renovation projects, is a unique geotechnical medium formed by marine organisms, with a calcareous carbonate content exceeding 50%. Due to its unique formation process, calcareous sand exhibits special engineering characteristics such as irregular shape, high intra-grain porosity, high angularity, low particle hardness, and significant particle breakage, which leads to considerable differences in the basic mechanical properties compared to general terrestrial sands [1; 2; 3; 4; 5; 6]. In addition, the hydraulic sorting effect during the blow-fill process causes uneven particle distribution in calcareous sand foundation, often resulting in the formation of fine-grained aggregation layers predominantly composed of fine sand with varying fines content. Consequently, calcareous sand foundation always has issues with insufficient bearing capacity and uneven settlement. Scholars, both domestically and internationally, have explored the mechanisms by which fines content affects the mechanical properties of sandy soils. [PERSON] et al. introduced the concepts of intergranular void ratio and inter-fine void ratio and proposed that significant microstructural changes occurred once the fines content in sandy soil surpasses a certain threshold, resulting in noticeable macroscopic mechanical differences among clean sands, silty sands, and sandy silts [7; 8]. [PERSON] et al. systematically examined how varying fines content affected the static liquefaction potential under monotonic compressionloading for Nevada sand and Ottawa sand. Their experimental findings demonstrated that increasing fines content enhanced the liquefaction potential of clean sand, with fine particles increasingly dominating the overall behavior of undrained soils after a certain fines content threshold was reached [9]. [PERSON] et al. conducted dynamic triaxial tests on undisturbed soil samples with varying fines content, finding that liquefaction resistance initially decreased and then increased as fines content rose. This shift was attributed to changes in the role of fine particles in the soil's load-bearing structure [10]. Based on the inter-grain contact index, [PERSON] et al. proposed an elastoplastic model to effectively predict the mechanical behavior of sand-silt mixtures [11]. [PERSON] et al. introduced a compression model for sand-marine clay mixtures within a homogenization framework [12]. Later, [PERSON] et al. developed a simple and effective compression model for clay and silty sand, based on the concept of an equivalent void ratio [13]. [PERSON] et al. examined the effect of fines content on the compressibility of high-density Chelf sand (at relative densities of 65% and 80%), finding that the fines content transition depended on the initial state of the samples--such as relative density, preparation method, and maximum void ratio--and can be calculated using a hypoplastic model [14]. Recently, [PERSON] et al. expanded the intergranular contact state classification for sand-fines mixtures proposed by [PERSON], introducing critical intergranular contact state parameters (\(F\)cin-min and \(F\)cin-max) to characterize in-transition soils [15]. In summary, extensive research has been conducted on the influence of fines content on the mechanical properties of terrigenous sand, but studies in this area for calcareous sand remain insufficient. However, the above theoretical foundation of the microstructure of terrestrial sand-fine particles can provide a good reference for the study of mechanical properties of calcareous sand-fines mixtures.