The effect of relative density, water content, and fractal dimension on the oedometer compression behavior of calcareous sand in the South China Sea

• The effects of D r , w i and D on the compression of calcareous sand were clarified. • The compression behavior of calcareous sand exhibited linear under p below 200 kPa. • Robust empirical compressive models for calcareous sand demonstrated high accuracy. Calcareous sand, primarily derived from coral reef debris and marine shells, is widely distributed across islands and reefs in the South China Sea. It exhibits irregular morphology, high porosity, and inherent fragility. A series of Oedometer compression tests were conducted to investigate the coupled effects of relative density ( D r , 0.2–0.8), initial water content ( w i , 0%–30%), and fractal dimension ( D , 0.12–1.02) on its compressive behavior. Test results show that calcareous sand behaves linearly elastically under vertical pressures p ≤200 kPa, with a distinct yield point at p >200 kPa, marking the onset of nonlinear deformation dominated by particle breakage and rearrangement. Elevated w i , D r , and D reduce the difference between the initial porosity ratio ( e i ) and post–cycle residual void ratio ( e r ), owning to enhanced particle interlock, pore water damping, and improved particle packing. The relative breakage potential ( B r ) is strongly correlated with D r , w i , and D , confirming their key roles in particle crushing. An empirical model incorporating w i , D r , and D is proposed to characterize the primary loading–unloading compression behavior by relating B r to stress–strain responses. Subsequent loading–unloading cycles reveal that intensified particle breakage significantly lowers the compression modulus ( λ ) and vertical yield strength ( σ y ), while exerting little effect on the resilience modulus ( κ ). Notably, the integrated model effectively predicts the compression behavior of calcareous sand under cyclic loading–unloading conditions, offering a reliable framework for geotechnical design in marine carbonate environments.