The static fatigue properties and acoustic emission characterization of calcareous sand under multi-contact loading

Calcareous sand is a common biogenic carbonate material in tropical and subtropical oceans. Its static fatigue characteristics govern the long-term stability of offshore and island reef engineering. However, the static fatigue mechanism of calcareous sand under natural multi-contact conditions, the coupled effect of particle morphology and coordination number (CN) on fatigue performance, and the acoustic emission (AE) multi-parameter characterization method for damage evolution remain unclear. In this study, static compression tests, X-ray CT imaging, AE monitoring and high-speed photography were conducted to investigate the static fatigue behavior of flaky, blocky, and spindle-shaped calcareous sand particles under CN=2∼8. Results show that particle morphology determines the fatigue performance: blocky particles present multi-stage progressive damage with the longest fatigue life, spindle-shaped particles exhibit quasi-continuous stable damage, while flaky particles are dominated by brittle fracture with the fastest failure. CN regulates fatigue behavior via stress dispersion: high CN significantly improves the strength stability of blocky and spindle-shaped particles, while flaky particles show minimal sensitivity to CN variation due to inherent structural defects. AE multi-parameters can quantitatively characterize the whole process of damage evolution: the hit rate surges and b-value drops abruptly in the late damage stage, which can be used as robust failure precursors of static fatigue. High-speed photography confirms that high CN can mitigate the transition from brittle failure to ductile damage of calcareous sand particles. This study provides a critical micromechanical basis for the long-term stability evaluation and real-time early warning system development of calcareous sand engineering.