Pre-compaction-triggered polymeric capsules for mitigating triaxial creep of coral sand in coastal defense applications

Coral sand, commonly used in coastal defense foundations, exhibits high porosity, compressibility, and creep, compromising both short- and long-term structural stability of infrastructure. This study introduced a novel stabilization method using calcium alginate capsules loaded with 85% tung oil (4% by sand weight), synthesized via ionic gelation. Pre-compaction (applied during specimen preparation) ruptured the capsules and released tung oil, which subsequently hardened and bonded the sand particles, forming a stabilized sand–capsule composite. Multi-stage drained triaxial creep tests investigated the influence of deviator stress ( = 0–1600 kPa), effective confining stress ( = 50–200 kPa), and initial dry density ( ≈ 1.365 g/cm 3 and 1.435 g/cm 3 ) on both untreated and capsule-stabilized coral sand. Results showed that capsule inclusion substantially reduced axial creep strain and volumetric creep contraction at low stress levels, due to inhibited particle rearrangement by tung oil bonds. At higher stresses, bond breakage limited stabilization benefits. Particle size analysis revealed that creep in the untreated sand was primarily due to particle rearrangement, while visual observation through the triaxial cell indicated that it was caused by bond breakage in the sand–capsule composites. The use of commercially available materials and a simple manufacturing process suggested a viable path for large-scale implementation. This approach highlighted polymeric capsules as a promising and sustainable strategy for mitigating creep deformation, a key factor affecting the long-term serviceability of coastal defenses built on coral sand.