Monotonic triaxial testing and hypoplastic modelling of calcareous sand: A focus on particle breakage and initial relative density

The stress-strain behavior of calcareous sand is significantly influenced by particle breakage ( B ) and initial relative density ( D ri ), but few constitutive models consider their combined effects. To bridge this gap, we conducted a series of triaxial tests on calcareous sand with varying D ri and stress paths, examining particle breakage and critical state behavior. Key findings include: (1) At a constant stress ratio ( η ), B follows a hyperbolic relationship with mean effective stress ( p' ), and for a given p' , B increases proportionally with η ; (2) The critical state line (CSL) moves downward with increasing D ri , whereas the critical state friction angle ( φ cs ) decreases with increasing B . Based on these findings, we propose a unified breakage evolution model to quantify particle breakage in calcareous sand under various loading conditions. Integrating this model with the Normal Consolidation Line (NCL) and CSL equations, we successfully simulate the steepening of NCL and CSL slopes as B increases with the onset of particle breakage. Furthermore, we quantitatively evaluate the effect of B on φ cs . Finally, within the framework of Critical State Soil Mechanics and Hypoplasticity theory, we develop a hypoplastic model incorporating B and D ri . The model is validated through strong agreement with experimental results across various initial relative densities, stress paths and drainage conditions.