Self-healing in saturated sand: mechanical response and rupture behavior of polymeric core-shell capsules

Capsule-based self-healing techniques offer a promising approach to autonomously mitigate soil deformations (e.g., landslides and differential settlement) in geotechnical engineering. However, existing capsule-based solutions face limitations in saturated soils, such as immiscibility of water with oil-based healing agents or the inability to encapsulate alkaline agents such as sodium silicate. This study develops core-shell sodium silicate-ethyl cellulose capsules via extrusion-spheronization, specifically engineered for saturated soil conditions. Advanced characterization (SEM, TGA, FTIR) confirmed their spherical morphology with sphericity of 0.92 \,± \, ± 0.03, thermal stability with decomposition onset > 100 \,∘ ∘ C, and chemical integrity. The ethyl cellulose (EC) shell’s hydrophobic properties (contact angle: 116.1°± 0.6°; WDPT > 600 s) protected reactive cores and improved stiffness. In sand-capsule mixtures, post-healing direct shear tests showed capsules can enhance the shear strength of saturated sand by 52%, supported by SEM/EDS-identified calcium silicate binding. Mechanical tests for single capsules revealed: (1) sodium silicate plasticization reduces non-coated capsule stiffness by 63% at 30% SS dosage, promoting ductile failure modes (strain increase from 7.6 to 13.7%). (2) EC increases the tensile strength (or stiffness) of coated capsules than non-coated capsules at 2–3 times. The tensile strength closely follows the Weibull distribution model with a Weibull modulus of m = 4.550 and m = 2.823 for coated and non-coated capsules. (3) A unified hyperbolic model yields a superior fit for both non-coated and coated capsules, effectively capturing the transition from the initial elastic regime to the plastic regime, which classical Hertz/Reissner theories can only fit at small deformation (0–5%). These findings demonstrate the potential of the capsules for self-healing of saturated soil, with performance governed by tunable composition-structure–property relationships.