Polymeric microcapsules for self-healing applications in geotechnical engineering

Abstract

Microencapsulation approaches, initially developed for self-healing applications emerge as a potential solution to enhance, switch or prolong the longevity of granular materials and ground infrastructure. These techniques have demonstrated success in cementitious construction materials, particularly in concrete, where the healing mechanism involves the rupture of microcapsules, releasing healing agents, and filling the cracks with crystalline products. As geohazards and climate change increasingly threaten human activities worldwide, the adverse impacts of these phenomena highlighted the urgent need for resilient infrastructure capable of adapting to evolving environmental conditions. This thesis investigates the application of microencapsulation techniques as an novel approach to address critical challenges in soil engineering, including (1) imparting and recovering hydrophobicity, (2) enhancing oil contamination remediation, (3) facilitating soil strength recovery, and (4) mitigating soil desiccation cracking. Notably, this study marks the first instance of microcapsules being applied to granular soils, demonstrating their effectiveness in addressing these geotechnical challenges. Hydrophobized soils have functional hydrophobic coatings to delay or restrict water infiltration. However, the exposure to the external environment leads to the degradation of the hydrophobic properties over time. To address this challenge, microcapsules with hydrophobic cargo were produced and added into sand. The results showed the effectiveness of the microcapsules in imparting and recovering hydrophobicity in granular materials and thus prolonging the functional lifespan of hydrophobized soils. Oil-contaminated soil presents substantial environmental issues. Leveraging the controlled release properties of capsules, polymeric capsules loaded with surfactant cargo were produced for enhanced oil remediation in soil. Laboratory washing tests show that the capsules enhanced the efficiency of oil removal during the washing process by controlled and sustained release of surfactant cargo. II Microcapsule-based self-healing methods present a promising method for ground-related challenges, such as landslides and differential settlement. This study developed microcapsules with cementing cargo, a hardening oil. The performance of this capsule in sandy soil was investigated. The capsules can be ruptured by the sand particle movement and release cargo. As cargo hardened, it bonded the sand particles and enhanced sand shear strength. This research shows the potential of a capsules-based self-healing system to actively recover the strength of soils by a ‘smart’ response to soil failure. Building upon the initial success of applying microcapsules to granular materials to enhance their strength, microcapsules with tung oil, sodium silicate, and colloidal silica cargos were developed. Their performance in shear strength recovery was evaluated, which further broadens the scope of practical application of this approach. Desiccation cracking presents a considerable obstacle in soil-based infrastructure, posing threats to both soil strength and hydraulic properties. This research synthesized microcapsules containing cement powder as cargo and investigated their efficacy in mitigating desiccation cracking in clay soils. The desiccation experimental results demonstrate a significant decrease in cracking penetration depth and width. This research designed and synthesized polymeric microcapsules by matching shell and cargo systems suitable for self-healing purposes in geotechnical applications. The findings demonstrated the microcapsules use in soil hydrophobicity recovery, enhanced soil remediation, soil shear strength recovery, and the arrest for soil desiccation cracking. Future research ought to investigate their fundamental properties including microcapsules' durability, mechanical strength, release behaviour, and rupture characteristics.