# Effect of Fines Content on the Compression Behavior of Calcareous Sand
## 1 Introduction
Most studies have concentrated on the compressibility of terrigenous sand-silt mixtures [13; 16; 17; 18], whereas researches on the compressibility of calcareous sand-silt mixtures are limited. Soil compression generally results from the reduction in pore volume, encompassing the compression of porous gas and the expulsion of pore water and gas. The deformation primarily stems from the compression of pore volume between soil particles. However, calcareous sand-silt mixtures may exhibit markedly different compressibility characteristics compared to quartz sand-silt mixtures. [PERSON] and [PERSON] conducted compression tests on calcareous sand from Dogs Bay in western Ireland and the South China Sea, respectively, and found that calcareous sand exhibited a higher compression coefficient [2; 3]. The compression process in calcareous sand involves consolidation, particle rearrangement, and particle breakage, with this deformation being largely irreversible plastic deformation. This is because calcareous sand particles have a high internal porosity and tend to break at low stress levels [19; 20]. Thus, it is crucial to account for both pore compression and particle breakage when evaluating the compressibility of calcareous sand. Through extensive laboratory testing, [PERSON] observed that calcareous sand initially underwent relative position changes between particles during compression, with volume reduction becoming increasingly dominated by particle breakage as pressure rose [21]. Other studies have similarly shown that particle breakage substantially enhanced the compressibility of calcareous sand [22; 23; 24]. Further research has explored the effects of various particle components on the compressibility of calcareous sand. [PERSON] et al. conducted triaxial compression tests on coarse-fine particle mixtures with fines contents of 0%, 40%, and 60%, revealing that under low confining pressures, the sample volume decreased with increasing deviatoric stress, and higher fine content intensified volume shrinkage [25]. [PERSON] et al. found that coarse particles can improve the compressibility of calcareous sand to some extent, concluding that the relatively most stable particle size during breakage was 0.25 mm [26]. Using quartz sand as a reference, [PERSON] et al. examined the influence of gradation on calcareous sand's compressibility, showing that the compressibility of calcareous sand was significantly greater, and the coarse fraction (i.e., 1-5 mm) content has the most pronounced impact [5]. When the coarse fraction content is below 25%, and the mass ratio of medium to fine grains is constant, the most unfavorable coarse fraction content is reached, making calcareous sand highly compressible. [PERSON] et al. studied the effects of gradation characteristics and morphology on the compressibility and breakage of calcareous sand under high stress levels, finding that the compression curves of differently graded calcareous sands converge on the \(e\)-log \(p\) plane as vertical stress increased [6]. However, research on the impact of fines content on the compressibility of calcareous sand, particularly in the 0-40% range, remains limited. In this study, laboratory experiments and mechanistic analyses on clean sand and calcareous silty sand with varying fines contents were conducted. The aim of this study was to explore the effects of fines content and initial relative density on the compressibility behavior of calcareous sand-fines mixture, and further investigate the particle breakage characteristic of calcareous silty sand and clean sand. Using a soil classification system based on inter-particle contacts, this study analyzed the intrinsic mechanism of how fines content affects the compressibility behavior of calcareous sand from a microstructural perspective. In addition, the threshold fines content (\(f_{\text{c-th}}\)) that signifies changes in the compressibility of calcareous sand is identified.