Mechanical Properties and Micro-Mechanism of Polyurethane-Stabilized Calcareous Sand: An Approach for Island Reef Construction

This study investigates the mechanical behavior of polyurethane (PU)-stabilized calcareous sand with varying PU contents and relative sand densities using unconfined compression and direct shear tests. The results demonstrate that PU stabilization significantly enhances compressive and shear strength and induces a transition from brittle to ductile failure with increasing PU content. Strength and stiffness exhibit nonlinear growth as an interconnected polymer bonding network develops. Relative density controls the timing and efficiency of strength mobilization, with dense specimens strengthening earlier and loose specimens exhibiting accelerated strength development at higher PU contents. SEM and XRD analyses confirm that stabilization is dominated by a bonding–solidification mechanism, without altering the mineralogical composition. Overall, PU stabilization provides an effective approach for achieving rapid strength development and stable mechanical performance in calcareous sand.